Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
MEMBRANE CLEANING APPARATUS
Document Type and Number:
WIPO Patent Application WO/2021/078534
Kind Code:
A1
Abstract:
A membrane cleaning apparatus for removing particles from a membrane comprises: a membrane support; a controlled environment chamber and cleaning substance delivery mechanism. The particles of the cleaning substance delivered to membrane sublime in the controlled environment without leaving residue. The combination of masks, apertures and gas drag in the controlled environment allow to provide cleaning substance particles with speed in the range about 1-100 m/s (preferably about 3-30 m/s) and size in the range about 1-100 um (preferably 1-10 um) to the membrane, such that two-phase flow cleans but does not rupture the thin and fragile membrane.

Inventors:
NIKIPELOV, Andrey (5500 AH Veldhoven, NL)
BANINE, Vadim, Yevgenyevich (5500 AH Veldhoven, NL)
BLOM, Sander (5500 AH Veldhoven, NL)
DE VRIES, Gosse, Charles (5500 AH Veldhoven, NL)
NIENHUYS, Han-Kwang (5500 AH Veldhoven, NL)
SBRIZZAI, Fabio (5500 AH Veldhoven, NL)
Application Number:
EP2020/078350
Publication Date:
April 29, 2021
Filing Date:
October 09, 2020
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ASML NETHERLANDS B.V. (5500 AH Veldhoven, NL)
International Classes:
G03F1/62; G03F1/82; G03F7/20; B08B7/02
Attorney, Agent or Firm:
ASML NETHERLANDS B.V. (5500 AH Veldhoven, NL)
Download PDF:
Claims:
CLAIMS

1. A membrane cleaning apparatus for removing particles from a surface of a membrane, the apparatus comprising: a membrane support for supporting the membrane; and a cleaning substance delivery mechanism for delivering a cleaning substance to a membrane surface opposite to the surface to be cleaned.

2. A membrane cleaning apparatus for removing particles from a membrane, the apparatus comprising: a chamber; a control system for controlling a pressure and/or a temperature within the chamber; a membrane support for supporting the membrane within the chamber; and a cleaning substance delivery mechanism for delivering a cleaning substance to the membrane; wherein the control system is configured to control the pressure and/or the temperature within the chamber such that at least a portion of the cleaning substance undergoes a phase transition into a gas in the chamber.

3. A membrane cleaning apparatus for removing particles from a membrane, the apparatus comprising: a membrane support for supporting the membrane; and a cleaning substance delivery mechanism for delivering a cleaning substance to the membrane, wherein the cleaning substance comprises carbon dioxide.

4. The apparatus of any of claim 1 or claim 2, wherein the cleaning substance comprises carbon dioxide.

5. The apparatus of any preceding claim, wherein the cleaning substance delivery mechanism comprises a mechanism configured to provide a first jet of the cleaning substance in the form of a snow, such that at least a portion of the first jet is incident on a surface of a membrane when supported by the membrane support.

6. The apparatus of claim 5, wherein the cleaning substance delivery mechanism further comprises a mechanism configured to provide a supplementary flow, wherein at least a portion of the supplementary flow is provided in a vicinity of the first jet.

7. The apparatus of claim 6, wherein the apparatus comprises an enclosure which confines at least a portion of the supplementary flow to a vicinity of the first jet.

8. The apparatus of claim 7, wherein the apparatus further comprises a first jet expansion assembly configured to direct the supplementary flow towards the first jet such that at least part of the supplementary flow mixes with at least part of the first jet so as to increase a cross-section of the first jet transverse to a main direction of travel of the first jet.

9. The apparatus of claim 8, wherein the first jet expansion assembly comprises a plurality of coaxially aligned, axially spaced flow shaping members, each flow shaping member having an aperture, wherein the flow shaping members are arranged: such that the first jet propagates through the apertures of the flow shaping members; and to direct the supplementary flow through one or more gaps between adjacent flow shaping members so as to mix the supplementary flow with the first jet.

10. The apparatus of claim 8, wherein the first jet expansion assembly comprises one or more flow shaping members, each flow shaping member comprising a plurality of apertures arranged such that the first jet propagates through the plurality of apertures.

11. The apparatus of any of claim 6 to claim 10, wherein the supplementary flow comprises a second jet of the cleaning substance in the form of a snow, wherein the second jet is arranged to mix with the first jet prior to at least a portion of the first jet being incident on a surface of a membrane when supported by the membrane support.

12. The apparatus of claim 6, wherein the supplementary flow comprises a jet of dry gas arranged to mix with the first jet prior to at least a portion of the first jet being incident on a surface of a membrane when supported by the membrane support.

13. The apparatus of any preceding claim, when dependent either directly or indirectly on claims 2 or 7, wherein the apparatus further comprises a first mask having an aperture, and wherein the first mask is arranged so as to form part of a wall of the chamber and is arranged between the cleaning substance delivery mechanism and a membrane when supported by the membrane support.

14. The apparatus of claim 13 wherein the first mask is configured to: block at least a portion of the first jet from propagating towards a membrane when supported by the membrane support; and provide a stagnation point of the first jet proximate to the aperture of the first mask. 15. The apparatus of any preceding claim, wherein the apparatus further comprises a second mask, and wherein the second mask is arranged between the cleaning substance delivery mechanism and a membrane when supported by the membrane support, and wherein the second mask is configured to block at least a portion of the first jet from propagating towards a membrane when supported by the membrane support.

16. The apparatus of claim 15, wherein the second mask is substantially aligned with the first jet so as to block a central portion of the first jet from propagating towards a membrane when supported by the membrane support.

17. The apparatus of any preceding claim in which the apparatus comprises a chamber, wherein the apparatus further comprises a third mask having an aperture, and wherein the third mask is disposed within a cavity defined by the chamber, and wherein the third mask is configured to block an outer portion of the first jet from propagating towards a membrane when supported by the membrane support.

18. The apparatus of any of claim 1 to claim 4, wherein the cleaning substance delivery mechanism comprises: a block of solid cleaning substance; and a stimulation mechanism operable to cause at least a portion of the cleaning substance in the block of cleaning substance to sublime, such that at least a portion of the cleaning substance propagates away from the block of cleaning substance and towards a membrane when supported by the membrane support.

19. The apparatus of claim 18, wherein the stimulation mechanism comprises a radiation source configured to provide radiation to a surface of the block of cleaning substance.

20. The apparatus of claim 19, wherein the stimulation mechanism comprises a laser operable to produce laser radiation which is provided to the surface of the block of cleaning substance in one or more pulses.

21. The apparatus of any of claim 19 or 20, wherein the apparatus is configured such that radiation is provided to the surface of the block of solid cleaning substance after first being transmitted by a membrane when supported by the membrane support.

22. The apparatus of any of claim 19 or 20, wherein the apparatus is configured such that radiation is provided to the surface of the block of solid cleaning substance without first being transmitted by a membrane when supported by the membrane support. 23. The apparatus of any of claim 20 to 22, wherein the stimulation mechanism is configured to provide laser radiation such that a spatial power intensity of the radiation incident on the block of solid cleaning substance is non-uniform.

24. The apparatus of any preceding claim, wherein the apparatus further comprises a contaminant collection plate arranged proximate to the membrane.

Description:
MEMBRANE CLEANING APPARATUS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 19204455.0 which was filed on October 22, 2019 and EP application 19210882.7 which was filed on November 22, 2019 which are incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a membrane cleaning apparatus and an associated method. The apparatus and method may have particular application for cleaning a pellicle used in a lithographic apparatus.

BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus may, for example, project a pattern provided on a patterning device (e.g., a mask) onto a layer of radiation-sensitive material provided on a substrate (e.g., a silicon wafer). A lithographic apparatus can be used in the manufacture of integrated circuits.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] Unwanted particles present on a patterning device may contribute to a pattern imparted to a beam of radiation. In a lithographic apparatus, this can lead to errors in the pattern applied to the substrate. It is therefore important to prevent particles from reaching, and thereby contaminating, the patterning device. It is known to provide a membrane between a patterning device and sources of particles in a lithographic apparatus to prevent particles from reaching the patterning device. A membrane used for such a purpose is known in the art as a pellicle.

[0006] The pellicle is spaced from the patterning device such that it is not in a field plane and therefore any particles disposed on the pellicle should not be imaged (and therefore should not contribute to errors in the pattern applied to the substrate). However, particles on the pellicle may result in increased absorption of the radiation during exposure of a substrate and therefore result in local hot spots on the pellicle that may lead to failure of the pellicle. In addition, particles on a surface of the pellicle which faces the patterning device may be transferred to the patterning device whereby they can cause errors in the pattern applied to the substrate. [0007] It may be desirable to provide an apparatus and associated method for cleaning a membrane (e.g., a pellicle).

SUMMARY

[0008] According to a first aspect of the invention there is provided a membrane cleaning apparatus for removing particles from a surface of a membrane, the apparatus comprising: a membrane support for supporting the membrane; and a cleaning substance delivery mechanism for delivering a cleaning substance to a membrane surface opposite to the surface to be cleaned.

[0009] It may be desirable to remove contaminant particles from a particular surface of a membrane. Such a surface of the membrane may be referred to as a critical surface or, alternatively, a first surface. A membrane generally has only two surfaces. A surface of the membrane opposite the critical/first surface may be referred to as a non-critical/second surface. The apparatus according to the first aspect of the invention provides an arrangement wherein particles may be removed from a first surface of the membrane by delivering a cleaning substance to a second surface of the membrane which is opposite the first surface.

[00010] In particular, the apparatus according to the first aspect of the present invention exploits the fact that membranes may generally be thin and/or deformable. In use, when the cleaning substance is incident on the second surface of the membrane, the cleaning substance may transfer momentum to the membrane. At least a portion of this momentum may subsequently be transferred to one or more contaminant particles disposed on the first surface of the membrane. This may result in said one or more contaminant particles being ejected from the first surface of the membrane (away from the membrane). [00011] The cleaning substance may be a substance which can be delivered to the membrane (e.g., in the form of one or more particles), and may be liable to undergo a phase transition into gas form after transferring momentum to the membrane. The cleaning substance may be a substance which can be delivered to the membrane in gas form, in liquid form, and/or in solid form. The cleaning substance may be a substance which can be delivered to the membrane in multi-phase form. Using such cleaning substances to clean the membrane may, advantageously, leave no residue on the membrane.

[00012] According to a second aspect of the invention there is provided a membrane cleaning apparatus for removing particles from a membrane, the apparatus comprising: a chamber; a control system for controlling a pressure and or a temperature within the chamber; a membrane support for supporting the membrane within the chamber; and a cleaning substance delivery mechanism for delivering a cleaning substance to the membrane; wherein the control system is configured to control the pressure and or the temperature within the chamber such that at least a portion of the cleaning substance undergoes a phase transition into a gas in the chamber.

[00013] The phase transition may comprise a transition from a solid into a gas.

[00014] The phase transition may comprise a transition from a liquid into a gas. [00015] In use, a membrane, when supported by the membrane support, may be disposed within the chamber. This may allow the control system to control an environment (particularly the pressure and/or temperature) surrounding the membrane. This environment (which may refer to an environment within the chamber) may be controlled such that the cleaning substance is liable to undergo a phase transition into a gas. In particular, the control system may control the environment surrounding the membrane such that the cleaning substance undergoes a phase transition into a gas after being delivered to the membrane.

[00016] The apparatus according to the second aspect of the present invention may remove contaminant particles from the membrane by explosive sublimation of the cleaning substance. Advantageously, such explosive sublimation may create a flow of gas which may remove contaminant particles from a surface of the membrane to which the cleaning substance is delivered. Additionally or alternatively, the apparatus according to the second aspect of the present invention may remove contaminant particles from the membrane by transfer of momentum from the cleaning substance to the contaminant particles (as described above with reference to the first aspect of the present invention). Advantageously, sublimation of the cleaning product may result in no residue being left on the membrane by the cleaning product.

[00017] According to a third aspect of the invention there is provided a membrane cleaning apparatus for removing particles from a membrane, the apparatus comprising: a membrane support for supporting the membrane; and a cleaning substance delivery mechanism for delivering a cleaning substance to the membrane, wherein the cleaning substance comprises carbon dioxide.

[00018] In any of the first, second and third aspects, the cleaning substance delivery mechanism may be arranged to provide a cleaning substance comprising a gas jet, a two-phase flow, or a multi-phase flow. Such a two-phase flow may comprise a jet of gas and solid, a jet of gas and liquid or a jet of liquid and solid. Such a multi-phase flow may comprise a jet of gas, liquid and solid.

[00019] Within the apparatus according to the first, second or third aspects, the cleaning substance may comprise carbon dioxide.

[00020] Carbon dioxide may be delivered, by the cleaning substance delivery mechanism, in the form of one or more particles. Advantageously, carbon dioxide particles may be generated to be of comparable size to contaminant particles which may be disposed on a critical surface of a membrane. This may be particularly useful for transferring momentum to contaminant particles and subsequently ejecting said particles from the membrane.

[00021] Carbon dioxide may be particularly well suited to subliming into gas form in an environment within the membrane cleaning apparatus. Carbon dioxide may sublime from solid form to gas form shortly after being incident on the non-critical surface of the membrane. Therefore, advantageously, carbon dioxide may leave no residue on a surface of the membrane.

[00022] Carbon dioxide may be relatively cheap to purchase. As the cleaning substance may be consumed during use of the membrane cleaning apparatus, using carbon dioxide for the cleaning substance may, advantageously, result in relatively low running costs of the membrane cleaning apparatus.

[00023] The apparatus according to the first or third aspects may further comprise: a chamber within which the membrane support is provided; and a control system for controlling a pressure and/or a temperature within the chamber.

[00024] The control system may be arranged to maintain the pressure within the chamber below a pressure of an environment in which the chamber is disposed.

[00025] For example, the environment within the chamber may be maintained at a pressure between approximately 1 kPa and 10 kPa. Maintaining the environment within the chamber at a low pressure may result in a relatively low number of gas molecules (e.g., from air) present in a vicinity of the membrane. This may result in a relatively low amount of air resistance experienced by anything moving within the environment. Therefore, advantageously, by maintaining the environment within the chamber at a low pressure, the cleaning substance may propagate a longer distance prior to being incident on the membrane (compared with the environment not being maintained at a low pressure). Further, it may be easier to (at least partially) deform the membrane when the membrane is disposed in a low pressure environment. This may be particularly advantageous for embodiments of the present invention in which momentum is transferred from the cleaning substance to the membrane, and subsequently to contaminant particles, so as to eject any contaminant particles from the membrane. [00026] The membrane, when supported by the membrane support, may be disposed within the chamber of the membrane cleaning apparatus. The cleaning substance delivery mechanism may be disposed outside of this chamber.

[00027] The cleaning substance may be accelerated towards the membrane by a difference in pressure between an inside of the chamber and an outside of the chamber, for example from a stagnation point. As explained below, such stagnation may be provided using a first mask having an aperture. The cleaning substance produced by a nozzle (which may form part of the cleaning substance delivery mechanism) may have a stagnation pressure in the range 1-100 Bar, which may be too intense to be applied directly to a (fragile) membrane. A pressure difference between an interior of the chamber and the environment in which the chamber is disposed allows the cleaning substance to be partially extracted and accelerated towards the membrane in a controlled manner.

[00028] Further, a flow of particles and/or molecules of the cleaning substance, provided by the cleaning substance delivery mechanism, may be otherwise advantageously perturbed upon entry into the chamber as a result of a difference in pressure between an inside of the chamber and an outside of the chamber. In particular, if the interior of the chamber is maintained at lower pressure, for embodiments wherein the cleaning substance comprises a multiphase flow comprising liquid and or solid particles entrained in a gas flow, these particles can more easily partially decouple from the gas flow lines so as to impinge on the membrane. (It will be appreciated that the gas flow lines will flow around the membrane). [00029] In any of the first, second and third aspects, the cleaning substance delivery mechanism may comprise a mechanism configured to provide a first jet of the cleaning substance in the form of a snow, such that at least a portion of the first jet is incident on a surface of a membrane when supported by the membrane support.

[00030] It will be appreciated that “snow”, as used herein, may be taken to refer to a mixture of solid particles and gas. This may be described as a two-phase mixture. The solid particles and the gas may be different states of matter of the same substance. For example, when the cleaning substance comprises carbon dioxide, the cleaning substance delivery mechanism may comprise a mechanism configured to deliver a jet of carbon dioxide gas mixed with carbon dioxide particles. That is, the cleaning substance delivery mechanism may comprise a mechanism configured to deliver a first jet of carbon dioxide snow. [00031] A jet may be a particularly advantageous mechanism by which to deliver the cleaning substance as it may be relatively easy to control a main direction of the first jet. Further, it may be relatively easy to generate a jet of carbon dioxide snow. For example, compressed, liquid carbon dioxide may be expelled through a nozzle. Commercially available apparatus may be used to generate a jet of carbon dioxide snow.

[00032] It will be appreciated that in some embodiments only a single jet of the cleaning substance may be provided. Use of the term “first jet” should not be taken to imply that there is more than one jet, nor should use of the term “first jet” be taken to imply that there is only one jet. The term “first jet” is a non-limiting label.

[00033] In any of the first, second and third aspects, the cleaning substance delivery mechanism may further comprise a mechanism configured to provide a supplementary flow, wherein at least a portion of the supplementary flow is provided in a vicinity of the first jet.

[00034] The supplementary flow may refer to anything which can be provided in the form of a fluid flow. For example, the supplementary flow may comprise one or more gases. The supplementary flow may comprise one or more particles carried in the one or more gases. The supplementary flow may comprise dry air. The supplementary flow may comprise dry nitrogen gas. The supplementary flow may comprise a mixture of dry air and dry nitrogen gas. The supplementary flow may comprise any dry gas or mixture of dry gases. The supplementary flow may be generally chemically inert within the membrane cleaning apparatus.

[00035] The supplementary flow may be relatively cold. This may insulate solid particles of the first jet from a relatively warm environment surrounding the first jet. Advantageously, this may prevent solid particles of the first jet from subliming before such particles reach the membrane. That is, the supplementary flow may facilitate the efficient propagation of the cleaning substance in solid form. In some embodiments, this may result in a relatively high efficiency of transfer of momentum from the first jet to contaminant particles (via the membrane).

[00036] In any of the first, second and third aspects, the apparatus may comprise an enclosure which confines at least a portion of the supplementary flow to a vicinity of the first jet. [00037] The enclosure may at least partially surround the first jet and the supplementary flow. The enclosure may be particularly useful for shaping the supplementary flow. The enclosure may, advantageously, generally maintain the supplementary flow in a vicinity of the first jet. This may help to achieve the advantages of providing the supplementary flow as described above.

[00038] In any of the first, second and third aspects, the apparatus may further comprise a first jet expansion assembly configured to direct the supplementary flow towards the first jet such that at least part of the supplementary flow mixes with at least part of the first jet so as to increase a cross-section of the first jet transverse to a main direction of travel of the first jet.

[00039] The first jet expansion assembly may be provided within the enclosure. The first jet expansion assembly, in combination with the supplementary flow, may expand (and thereby reduce a density of) the first jet. The first jet expansion assembly, in combination with the supplementary flow, may reduce a velocity of the first jet in a main direction of travel thereof. In particular, the first jet expansion assembly, in combination with the supplementary flow, may reduce a velocity of solid particles of the cleaning substance in the first jet.

[00040] As described above, membranes may generally be fragile. Advantageously, the velocity of solid particles of the cleaning substance in the first jet may be reduced to such an extent that the membrane is not damaged by the first jet. However, the velocity of solid particles of the cleaning substance in the first jet may still be high enough such that, in some embodiments, momentum which is transferred to any contaminant particles on a surface of the membrane is enough to eject said particles from said surface.

[00041] In any of the first, second and third aspects, the first jet expansion assembly may comprise a plurality of coaxially aligned, axially spaced flow shaping members, each flow shaping member having an aperture, wherein the flow shaping members are arranged: such that the first jet propagates through the apertures of the flow shaping members; and to direct the supplementary flow through one or more gaps between adjacent flow shaping members so as to mix the supplementary flow with the first jet. [00042] Each flow shaping member may have a different size. The plurality of flow shaping members may be arranged in order of size.

[00043] The aperture of each flow shaping member may have a different size. The apertures may be arranged in order of size. The apertures may be arranged such that the first jet propagates from a smallest aperture to a largest aperture.

[00044] The one or more flow shaping members may be generally disc-shaped.

[00045] In general, the of each flow shaping member may be of the form of a generally planar component having one or more apertures.

[00046] The jet expansion assembly may comprise one or more flow shaping members. Each flow shaping member may comprise a plurality of apertures arranged such that the first jet propagates through the plurality of apertures. [00047] A flow shaping member may be of the form of a generally planar component with an array of apertures arranged in a regular pattern, such that this flow shaping member may be described as a grid. A flow shaping member may comprise a mesh.

[00048] A flow shaping member heating mechanism may be provided. The flow shaping member heating mechanism may be arranged to heat one or more flow shaping members. Advantageously, this may this may prevent the cleaning substance from building up around the one or more flow shaping members in use. Such build up of the cleaning substance may limit and/or block a flow path of the cleaning substance.

[00049] In any of the first, second and third aspects, the supplementary flow may comprise a second jet of the cleaning substance in the form of a snow. The second jet may be arranged to mix with the first jet prior to at least a portion of the first jet being incident on a surface of a membrane when supported by the membrane support.

[00050] The second jet, being arranged to mix with the first jet, may expand (and thereby reduce a density of) the first jet. The second jet, being arranged to mix with the first jet, may reduce a velocity of a portion of the first jet which propagates towards the membrane. The second jet, being arranged to mix with the first jet, may reduce a velocity of solid and liquid particles of the cleaning substance in the first jet. In particular, the second jet, being arranged to mix with the first jet, may result in a reduced density and or reduced speed of the mixed jets proximate to a stagnation plane. This may result in a reduced pressure applied to the membrane (which is generally fragile). Advantageously, the velocity of particles of the cleaning substance in the first jet may be reduced to such an extent that the membrane is not damaged by the first jet. However, the velocity of particles of the cleaning substance in the first jet may still be high enough such that, in some embodiments, momentum which is transferred to any contaminant particles on a surface of the membrane is enough to eject said particles from said surface. [00051] In any of the first, second and third aspects, the supplementary flow may comprise a jet of dry gas arranged to mix with the first jet prior to at least a portion of the first jet being incident on a surface of a membrane when supported by the membrane support.

[00052] The dry gas may comprise dry air. The dry gas may comprise dry nitrogen gas. The dry gas may comprise a mixture of dry air and dry nitrogen gas. The dry gas may comprise any dry gas or mixture of dry gases. The dry gas may be generally chemically inert within the membrane cleaning apparatus.

[00053] Condensation of water vapour on the membrane may increase adhesion between contaminant particles and the membrane. This may reduce cleaning efficacy of the membrane cleaning apparatus, which may be disadvantageous. The dry gas, advantageously, may reduce an amount of water vapour present in the membrane cleaning apparatus (compared with the dry gas not being provided). This may reduce a likelihood of condensation of water vapour on the membrane.

[00054] The jet of dry gas, being arranged to mix with the first jet, may expand (and thereby reduce a density of) the first jet. The jet of dry gas, being arranged to mix with the first jet, may reduce a velocity of a portion of the first jet which propagates towards the membrane. In particular, the jet of dry gas, being arranged to mix with the first jet, may reduce a velocity of solid particles of the cleaning substance in the first jet. Advantageously, the velocity of solid particles of the cleaning substance in the first jet may be reduced to such an extent that the membrane is not damaged by the first jet. However, the velocity of solid particles of the cleaning substance in the first jet may still be high enough such that, in some embodiments, momentum which is transferred to any contaminant particles on a surface of the membrane is enough to eject said particles from said surface.

[00055] In any of the first, second and third aspects, the apparatus may further comprise a first mask having an aperture. The first mask may be arranged so as to form part of a wall of the chamber and may be arranged between the cleaning substance delivery mechanism and a membrane when supported by the membrane support.

[00056] Advantageously, such an arrangement comprises both: a chamber and a control system for controlling a pressure and/or a temperature within the chamber; and an aperture that forms part of a wall of the chamber. With such an arrangement, a supply of cleaning substance having a large stagnation pressure can be attenuated such that it will not damage a membrane supported by the membrane support. The wall of the chamber and the aperture can be used to cause stagnation of a flow of cleaning substance and control of the relative pressure inside and outside of the chamber can allow part of this stagnated flow to be directed towards a membrane supported by the membrane support. [00057] The first mask may be configured to: block at least a portion of the first jet from propagating towards a membrane when supported by the membrane support; and provide a stagnation point of the first jet proximate to the aperture of the first mask.

[00058] The first mask having an aperture may be referred to as an orifice.

[00059] Typical available cleaning substance delivery mechanisms, for example carbon dioxide snow guns, may provide a jet having a pressure between 1 and 100 bar. A membrane is generally fragile. Direct application of a jet from a known cleaning substance delivery mechanism may therefore damage a membrane. Advantageously, a membrane cleaning apparatus comprising a first mask with an aperture provides a mechanism by which a jet from a known cleaning substance delivery mechanism may be attenuated and or controlled. In particular, it may be that a two-phase flow generated by such a delivery mechanism does not damage a relatively fragile membrane.

[00060] In use, the first mask having an aperture may constitute an obstacle for a flow of the first jet. The flow of the first jet may therefore at least partially stagnate proximate to the aperture. The cleaning substance may be drawn into a chamber of the membrane cleaning apparatus, and may be accelerated towards the membrane, by a difference in pressure between an inside of the chamber and an outside of the chamber. Advantageously, the flow of the first jet may be attenuated by the first mask, whilst at least partially preserving particles of the cleaning substance generated by the cleaning substance delivery mechanism. [00061] It will be appreciated that “stagnation”, as used herein, may be taken to refer to partial or complete stagnation of a flow.

[00062] Further, a flow of particles and/or molecules of the cleaning substance, provided by the cleaning substance delivery mechanism, may be otherwise advantageously perturbed upon entry into the chamber as a result of a difference in pressure between an inside of the chamber and an outside of the chamber.

[00063] Several parameters may be selected in order to control characteristics of the cleaning substance which is delivered to the membrane. For example, a difference in pressure between an inside and an outside of the chamber, a distance between a source of the first jet and the first mask, and/or a distance between the first mask and the membrane may be selected in order to control characteristics of the cleaning substance which is delivered to the membrane. Selecting such parameters may allow for the size and or speed of particles of the cleaning substance incident on the membrane to be controlled. [00064] Typical particles of the cleaning substance may have dimensions less than 100 um, or preferably less than 10 um. Typical particles of the cleaning substance may at least partially decouple from gas flow lines inside the chamber and therefore be directed to the membrane, even if such gas flow lines are parallel to the membrane.

[00065] A portion of the first mask proximate to the aperture may be heated. Advantageously, this may prevent the cleaning substance (which may be delivered via the first jet) from building up around, and consequently blocking, the aperture.

[00066] In any of the first, second and third aspects, the apparatus may further comprise a second mask, wherein the second mask is arranged between the cleaning substance delivery mechanism and a membrane when supported by the membrane support, and wherein the second mask is configured to block at least a portion of the first jet from propagating towards a membrane when supported by the membrane support.

[00067] The second mask may be positioned and/or dimensioned so as to determine a size of solid particles of cleaning substance which may or may not propagate towards the membrane. In particular, the second mask may be positioned and/or dimensioned so as to prevent solid particles of cleaning substance of a certain size from propagating towards the membrane.

[00068] The second mask may be substantially aligned with the first jet so as to block a central portion of the first jet from propagating towards a membrane when supported by the membrane support. [00069] The second mask may be substantially aligned with the first jet so as to block a central portion of the first jet from propagating towards a membrane when supported by the membrane support. In such an arrangement, the second mask may prevent relatively large and/or relatively fast solid particles of cleaning substance from propagating towards the membrane.

[00070] A central portion of the first jet (proximate to a stagnation plane, coincident with the membrane in the chamber in the absence of a second mask) may comprise the largest solid particles of cleaning substance in the first jet. This may be due to the largest inertia of such particles, decoupling the particles most from the flow lines. Particles with sufficiently large inertia cannot follow the flow lines and will decouple from the flow and impinge on the membrane, risking damage to the membrane. These largest solid particles of cleaning substance may be travelling at a relatively high velocity and/or with a relatively high energy. If these largest solid particles of cleaning substance are incident on the membrane, the membrane may be damaged. The second mask may therefore be particularly advantageous for preventing a central portion of the first jet from propagating towards the membrane. When the second mask is present, the stagnation plane is coincident with the second mask and these potentially problematic larger particles will impinge on the second mask (rather than the membrane). [00071] In any of the first, second and third aspects, the apparatus may further comprise a third mask having an aperture. The third mask may be disposed within a cavity defined by the chamber. The third mask may be configured to block an outer portion of the first jet from propagating towards a membrane when supported by the membrane support.

[00072] A position of the aperture of the third mask with respect to the first jet may be selected. A dimension of the aperture may be selected. Solid particles of the cleaning substance in the first jet (or, as it may be, in a second jet) may generally follow flow lines. A flow line followed by a particular particle of the cleaning substance may be determined at least in part by a mass of said particle. Hence, by positioning and or dimensioning the aperture appropriately, solid particles of the cleaning substance in the first jet of only a specific mass may be allowed to propagate through the aperture and towards the membrane.

[00073] In any of the first, second and third aspects, the cleaning substance delivery mechanism may comprise: a block of solid cleaning substance; and a stimulation mechanism operable to cause at least a portion of the cleaning substance in the block of cleaning substance to sublime, such that the at least a portion of the cleaning substance propagates away from the block of cleaning substance and towards a membrane when supported by the membrane support.

[00074] The at least a portion of the cleaning substance which propagates away from the block of cleaning substance and towards a membrane may comprise a gas jet or a two-phase jet. For example, said at least a portion may comprise a mixture of solid and gaseous carbon dioxide. The at least a portion of the cleaning substance which propagates away from the block of cleaning substance and towards a membrane may comprise a mulit-phase jet (for example a mixture of gas, liquid and solid).

[00075] The stimulation mechanism may be configured to provide energy to the block of solid cleaning substance. The stimulation mechanism may be tuned so as to eject solid particles of the cleaning substance alongside gas of the cleaning substance. The stimulation mechanism may be tuned so as to create a desired: quantity of cleaning substance to sublime; and or size and/or speed of solid particles of cleaning substance. Advantageously, this provides an easily controllable arrangement which can be tailored to the removal of contaminant particles of various sizes from the membrane.

[00076] Further, the arrangement of the block of solid cleaning substance and the membrane (in particular, a pressure of the environment between the block of solid cleaning substance and the membrane and/or a distance between the block of solid cleaning substance and the membrane) may at least partially decelerate particles of the cleaning substance when propagating towards the membrane. This may advantageously reduce kinetic energy of such particles and therefore prevent damage to the membrane. It will be appreciated that the arrangement of the block of solid cleaning substance and the membrane may be configured so as to achieve a desired deceleration of particles of the cleaning substance.

[00077] The stimulation mechanism may comprise a radiation source configured to provide radiation to a surface of the block of cleaning substance.

[00078] The stimulation mechanism may comprise a laser and may be operable to produce laser radiation. Radiation from the laser may be provided to the surface of the block of cleaning substance in one or more pulses.

[00079] The wavelength of the laser may be selected such that the absorption depth in the solid cleaning substance is 0.1-10 times an optimal size of the cleaning substance particles.

[00080] The stimulation mechanism may be configured such that a wavelength of the laser radiation is between 4.0 and 4.5 um. The stimulation mechanism may be configured such that a wavelength of the laser radiation is between 4.1 and 4.4 um

[00081] The stimulation mechanism may be configured such that energy provided in each pulse of radiation is greater than 0.1 mJ.

[00082] The stimulation mechanism may be configured such that energy provided in each pulse of radiation is greater than 1 mJ. A

[00083] The stimulation mechanism may be configured such that an absorption depth of the laser radiation is between 1 and 100 um in the block of solid cleaning substance.

[00084] The stimulation mechanism may be configured such that an absorption depth of the laser radiation is between 5 and 50 um in the block of solid cleaning substance.

[00085] The apparatus may be configured such that radiation is provided to the surface of the block of solid cleaning substance either directly or indirectly via a membrane when supported by the membrane support.

[00086] The apparatus may be configured such that radiation is provided to the surface of the block of solid cleaning substance after first being transmitted by a membrane when supported by the membrane support.

[00087] The apparatus may be configured such that radiation is provided to the surface of the block of solid cleaning substance without first being transmitted by a membrane when supported by the membrane support.

[00088] The stimulation mechanism may be configured to provide laser radiation such that a spatial power intensity of the radiation incident on the block of solid cleaning substance is non-uniform. For example, the spatial power intensity of the radiation incident on the block of solid cleaning substance may be modulated or periodic. [00089] The spatial power intensity may comprise a pattern of repeated shapes. One or more of the repeated shapes may have dimensions between 1 and 100 um.

[00090] For example, an interference pattern of the laser radiation may be generated on the surface of the block of solid cleaning substance. By having repeated shapes of the spatial power intensity of a particular dimension, particles of the cleaning substance of that particular dimension may be preferentially generated. Advantageously, this may allow for generation of particles of the cleaning substance which are particularly effective at removing particles form the membrane.

[00091] The radiation source may be particularly well suited for stimulating a selected amount of the block of cleaning substance.

[00092] The radiation source may be disposed outside of an environment in which, in use, the membrane is disposed.

[00093] In any of the first, second and third aspects, the apparatus may further comprise a heating mechanism configured to heat a membrane when supported by the membrane support.

[00094] The heating mechanism may comprise a radiative heating mechanism. The heating mechanism may comprise an inductive heating mechanism. The heating mechanism may comprise a resistive heating mechanism. The heating mechanism may comprise a convective heating mechanism. The heating mechanism may be considered to constitute part of a control system of the apparatus that is for controlling a pressure and/or a temperature within a chamber of the apparatus.

[00095] Delivering the cleaning substance to the membrane may cool a temperature of the membrane. This may result in condensation of water vapour on the membrane. Condensation of water vapour on the membrane may reduce a cleaning efficacy of the membrane cleaning apparatus, as described above. Advantageously, providing a heating mechanism configured to heat the membrane may reduce water vapour condensation on the membrane.

[00096] Further, a layer of what may be described as “ice” (which may be formed from the cleaning substance) may be liable to form on the membrane. Such a layer of ice would reduce a cleaning efficacy of the membrane cleaning apparatus. Providing a heating mechanism configured to heat the membrane may advantageously promote sublimation of solid particles of the cleaning substance after said solid particles of the cleaning substance are incident on the membrane. Therefore, advantageously, providing a heating mechanism configured to heat the membrane may prevent such a layer of ice from forming on the membrane.

[00097] In any of the first, second and third aspects, the apparatus may further comprise a contaminant collection plate arranged proximate to the membrane.

[00098] Contaminant particles which are removed from the membrane may be incident on the contaminant collection plate. Contaminant particles may remain on a surface of the contaminant collection plate. For example, intermolecular forces may prevent such contaminant particles from leaving a surface of the contaminant collection plate. Advantageously, the contamination collection plate may prevent contaminant particles which have been removed from the membrane from propagating back towards the membrane.

[00099] In any of the first, second and third aspects, the cleaning substance delivery mechanism may be operable to generate particles of the cleaning substance having dimensions smaller than 100 um. [000100] In any of the first, second and third aspects, the cleaning substance delivery mechanism may be operable to generate particles of the cleaning substance having dimensions smaller than 10 um. [000101] In any of the first, second and third aspects, the cleaning substance delivery mechanism may be operable to generate cleaning substance particles with speeds in the range about 1-100 m/s, for example speeds in the range 3-30 m/s.

[000102] In any of the first, second and third aspects, the cleaning substance delivery mechanism may be operable to generate cleaning substance particles with sizes in the range about 1-100 um, for example sizes in the range 1-10 um.

BRIEF DESCRIPTION OF THE DRAWINGS

[000103] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 schematically depicts a lithographic system, demonstrating a pellicle in use;

Figure 2 schematically depicts an embodiment of a membrane cleaning apparatus according to the present invention, and a membrane;

Figure 3 schematically depicts a further embodiment of a membrane cleaning apparatus according to the present invention, and a membrane;

Figure 4 schematically depicts a further embodiment of a membrane cleaning apparatus according to the present invention, and a membrane;

Figure 5 schematically depicts a further embodiment of a membrane cleaning apparatus according to the present invention, and a membrane;

Figure 6 schematically depicts a further embodiment of a membrane cleaning apparatus according to the present invention, and a membrane; and

Figure 7 schematically depicts a further embodiment of a membrane cleaning apparatus according to the present invention, and a membrane.

DETAIFED DESCRIPTION

[000104] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus FA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus FA. The lithographic apparatus FA comprises an illumination system IF, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W. [000105] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[000106] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[000107] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[000108] A relative vacuum, i.e., a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

[000109] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[000110] Some lithographic apparatus (e.g., EUV and DUV lithographic apparatus) comprise a pellicle 15. The pellicle 15 may be attached to the support structure MT or, alternatively, the pellicle 15 may be attached directly to the patterning device MA. The pellicle 15 comprises a thin membrane 16 of transmissive film (typically less than about 70 nm) mounted on a frame 17. The pellicle membrane 16 is spaced a few mm (typically less than 10 mm, for example 2 mm) away from the patterning device MA. A particle which is received on the pellicle membrane 16 is in the far field with respect to the pattern of the patterning device MA, and consequently does not have a significant impact upon the quality of image which is projected by the lithographic apparatus LA on to a substrate W. If the pellicle 15 were not present, such particles may lie on the patterning device MA and would obscure a portion of the pattern on the patterning device MA, thereby preventing the pattern from being projected correctly on to the substrate W. The pellicle 15 thus plays an important role in preventing particles from adversely affecting the image formed on a substrate W by the lithographic apparatus LA.

[000111] Before the pellicle 15 is attached to the support structure MT or the patterning device MA for use in a lithographic apparatus LA, the pellicle membrane 16 may become dirty. That is, particles may be incident on the pellicle membrane 16 before the pellicle 15 is used in a lithographic apparatus LA as described above. Activities such as transporting the pellicle 15, packaging the pellicle 15, and mounting the pellicle membrane 16 to a frame 17 may result in particles being incident upon the pellicle membrane 16.

[000112] It has been found that some particles that are present on the pellicle membrane 16 detach and travel from the pellicle membrane 16 to the patterning device MA during a lithographic exposure, and thereby negatively affect the pattern projected onto the substrate W. Particles with a dimension between 0.5 um and 5 um have been reported to move. It will be appreciated that, in other setups, particles with one or more dimensions outside of this range may move.

[000113] A pellicle 15 may be formed from one or more layers, which may be formed on a support substrate. The support substrate allows the thin membrane 16 of the pellicle 15 to be formed without risking the membrane 16 rupturing. Once the layers of the membrane 16 have been formed, the support substrate can be removed (for example by etching) to form the final thickness of the membrane 16. Pellicles 15 with membranes 16 that are found to be too dirty for use may be discarded. Whilst there exists some methods for cleaning pellicles 15, these are typically used before the final thickness of the membrane 16 has been achieved, that is when the membrane 16 is still disposed on the support substrate. These known methods for pellicle cleaning include wet cleaning or applying heat. However, the known methods are unsuitable for use once the final thickness of the membrane 16 has been achieved since they risk rupturing the thin pellicle membrane 16. Furthermore, cleaning methods that involve applying heat may also contribute to a weakening of the pellicle membrane 16, thereby reducing the operational lifetime of the pellicle 15, mostly due to stress at interfaces of materials with different coefficients of thermal expansion and/or due to temperature inhomogeneity translating to mechanical stress.

[000114] Embodiments of the present invention relate to apparatus and associated methods for removing particles from a membrane. In particular, embodiments of the present invention relate to apparatus and associated methods for removing particles from a membrane using a cleaning substance which is delivered to the membrane. Embodiments of the present invention may be particularly well suited and adapted to cleaning relatively thin membranes (such as, for example, pellicle membranes), which are fragile.

[000115] An embodiment of a membrane cleaning apparatus 200 according to an embodiment of the present invention is now described with reference to Figure 2.

[000116] Figure 2 shows a cross-section through the membrane cleaning apparatus 200, which comprises: a support 202; a chamber 204; a vacuum pump 206; a window 208; a heating apparatus 210; an orifice 212; a membrane shield 216; an enclosure 218; an inlet 220; and a nozzle 222. [000117] The orifice 212 may be referred to as a first mask. The orifice 212 comprises an opening in a wall of the chamber 204. The opening may be referred to as an aperture. The opening may have an area between 0.1 and 10 mm 2 . The orifice 212 is arranged so as to form part of a wall of the chamber 204. The orifice 212 is arranged between the nozzle 222 and the support 202. The orifice 212 is further defined by the orifice surround 214. The orifice surround 214 is disposed on an outside of the chamber 204, proximate to, and generally surrounding, the orifice 212. The orifice surround 214 may have a generally toroidal shape in cross section. The orifice 212 and orifice surround 214 provide fluid communication between an environment 205 within the chamber 204 and an environment in which the membrane cleaning apparatus 200 is disposed.

[000118] Also shown in Figure 2 is a pellicle, comprising a membrane 16 mounted to a frame 17. The support 202 is disposed within the chamber 204. The support 202 is arranged to support the frame 17. In use, the support 202 supports the frame 17 of the pellicle.

[000119] The vacuum pump 206 is connected to a wall of the chamber 204. Only a connection of the vacuum pump 206 to the chamber 204 is shown in Figure 2, for simplicity. The vacuum pump 206 is operable to control a pressure of the environment 205 within the chamber 204. The vacuum pump 206 may be operable to lower the pressure of the vacuum chamber 204 to near vacuum conditions. During operation of the membrane cleaning apparatus 200, the pressure of the environment 205 may be maintained between approximately 1 kPa and 10 kPa.

[000120] The nozzle 222 is disposed outside of the chamber 204. The nozzle 222 may contain a cleaning substance 224. The nozzle 222 is operable to provide a jet J1 of the cleaning substance 224. The nozzle 222 is provided within a wall of the enclosure 218. The enclosure 218 substantially, but not entirely, surrounds a space between the nozzle 222 and orifice 212 of the chamber 204.

[000121] The nozzle 222 of the membrane cleaning apparatus 200 contains liquid, compressed carbon dioxide 224. In use, the nozzle 222 provides a jet J1 of carbon dioxide snow. It will be appreciated that “snow”, as used herein, may be taken to refer to a mixture of solid particles and gas. This may be described as a two-phase mixture, or a multi-phase mixture. The solid particles and the gas may be different states of matter of the same substance. For example, the jet J 1 may comprise carbon dioxide gas mixed with carbon dioxide particles. A jet may be a particularly advantageous mechanism by which to deliver the cleaning substance 224 (i.e., carbon dioxide) as it may be relatively easy to control a main direction of the jet Jl. The jet J1 propagates towards the orifice 212. Further, it may be relatively easy to generate a jet of carbon dioxide snow. Commercially available apparatus may be used to generate the jet Jl of carbon dioxide snow.

[000122] The orifice 212 is configured to: block at least a portion of the jetJl from propagating towards the membrane 16 when supported by the support 202; and provide a stagnation point of the first jet jl proximate to the orifice 212. “Stagnation”, as used herein, may be taken to refer to partial or complete stagnation of a flow. The flow of cleaning substance 224 in the jet Jl slows down or stops proximate to the orifice 212. [000123] At least part of the jet J1 which stagnates at the stagnation point may be drawn into the chamber 204 by a difference in pressure between an inside of the chamber 204 (which may be between approximately 1 kPa and 10 kPa) and an outside of the chamber 204. Particles of the cleaning substance 224 may then be accelerated towards the membrane 16 by this difference in pressure between the inside of the chamber 204 and the outside of the chamber 204. Further, a flow of particles and/or molecules of the cleaning substance 224, provided by the cleaning substance delivery mechanism, may be otherwise advantageously perturbed upon entry into the chamber 204 as a result of a difference in pressure between an inside of the chamber 204 and an outside of the chamber 204.

[000124] The membrane 16 is generally thin and or deformable. In use, when particles of the cleaning substance 224 are incident on the membrane 16, the cleaning substance 224 may transfer momentum to the membrane 16. At least a portion of this momentum may subsequently be transferred to one or more contaminant particles 232 disposed on the membrane 16. This may result in said one or more contaminant particles 232 being ejected from the membrane 16 (away from the membrane 16). The membrane 16 may therefore be cleaned.

[000125] Typical available cleaning substance delivery mechanisms, for example carbon dioxide snow guns, may provide a jet having a pressure between 1 and 100 bar. A membrane is generally fragile. Direct application of a jet from a known cleaning substance delivery mechanism may therefore damage a membrane. Advantageously, the membrane cleaning apparatus 200, comprising the orifice 212, provides a mechanism by which the jet J 1 from a known cleaning substance delivery mechanism may be attenuated and or controlled. In particular, a two-phase flow generated by such a delivery mechanism may no longer damage a relatively fragile membrane (as a result of this attenuation and or control). Advantageously, the flow of the jet J1 may be attenuated by the orifice 212, whilst at least partially preserving particles of the cleaning substance generated by the cleaning substance delivery mechanism. [000126] The cleaning substance 224 (such as carbon dioxide snow) may be a substance which can be delivered to the membrane 16 (e.g., in the form of one or more particles), and may be liable to undergo a phase transition into gas form after transferring momentum to the membrane 16 to remove contaminant particles 232 from the membrane 16. The cleaning substance 224 may be a substance which can be delivered to the membrane 16 in gas form, in liquid form, and/or in solid form (or any mixture therof). Using such cleaning substances to clean the membrane 16 may, advantageously, leave no residue on the membrane 16.

[000127] The membrane shield 216 may be referred to as a second mask. The membrane shield 216 is smaller than the membrane 16. The membrane shield 216 is disposed within the chamber 204. The membrane shield 216 is disposed between the membrane 16 and the orifice 212. The membrane shield 216 is configured to block at least a portion of the jet J1 from propagating towards the membrane 16. [000128] The membrane shield 216 is positioned and dimensioned so as to determine a size of solid particles of cleaning substance 224 which may or may not propagate towards the membrane 16. In particular, the membrane shield 216 is positioned and dimensioned so as to prevent solid particles of cleaning substance 224 of a certain size from propagating towards the membrane 16. The membrane shield 216 is arranged so as to be substantially aligned with the jet Jl. The membrane shield 216 is arranged so as to block a central portion of the jet J 1 from propagating towards the membrane 16. [000129] The membrane shield 216 perturbs flow lines of particles of the cleaning substance 224 within the chamber. Relatively small snow particles 228a, 228b (with smaller inertia) may generally follow the flow lines within the chamber 204, whereas relatively large snow particles 230a, 230b may at least partially decouple from the flow lines due to inertia (at a stagnation plane which, in the absence of membrane shield 216 would coincide with the membrane 16). The relatively small snow particle 228a, 228b is incident on the membrane 16, consequently removing contaminant particles 232 from the membrane 16 through the mechanism described above. The relatively large snow particle 230a, 230b is instead intercepted by the membrane shield 216. The relatively large snow particle 230a, 230b may be travelling at a relatively high velocity and/or with a relatively high energy. If such a particle 230a, 230b were to be incident on the membrane 16, the membrane 16 may be damaged. The membrane shield 216 is therefore particularly advantageous for preventing a central portion of the jet Jl from propagating towards the membrane 16.

[000130] Several parameters may be selected in order to control characteristics of the cleaning substance which is delivered to the membrane. For example, a difference in pressure between an inside and an outside of the chamber, a distance between a source of the first jet and the first mask, a distance between the first mask and the membrane, and or a size and position of the membrane shield 216 may be selected in order to control characteristics of the cleaning substance which is delivered to the membrane. Selecting such parameters may allow for the size and/or speed of particles of the cleaning substance incident on the membrane to be controlled.

[000131] Typical particles of the cleaning substance 224 may have dimensions less than 100 um, or preferably less than 10 um. Typical particles of the cleaning substance 224 (including relatively small particles 228a, 228b) may at least partially decouple from flow lines inside the chamber 204. Typical particles of the cleaning substance 224 (including relatively small particles 228a, 228b) may therefore be incident on the membrane 16, even if flow lines are parallel to the membrane 16.

[000132] The enclosure 218 comprises the inlet 220. The inlet 220 is operable to provide a supplementary flow 226. The supplementary flow 226 may refer to anything which can be provided in the form of a fluid flow. For example, the supplementary 226 flow may comprise one or more gases. The supplementary flow 226 may comprise one or more particles carried in the one or more gases. The supplementary flow 226 may comprise dry air. The supplementary flow 226 may comprise dry nitrogen gas. The supplementary flow 226 may comprise a mixture of dry air and dry nitrogen gas. The supplementary flow 226 may comprise any dry gas or mixture of dry gases. The supplementary flow 226 may be generally chemically inert within the membrane cleaning apparatus 200.

[000133] The supplementary flow 226 may be relatively cold. This may insulate solid particles of the jet Jl from a relatively warm environment surrounding the jet Jl. Advantageously, this may prevent solid particles of the jet J1 from subliming before such particles reach the membrane 16. That is, the supplementary flow 226 may facilitate the efficient propagation of particles of the jet J1 in solid form. In some embodiments, this may result in a relatively high efficiency of transfer of momentum from the jet J 1 to contaminant particles 232 (via the membrane 16).

[000134] The enclosure 218 at least partially surrounds the jet J1 and the supplementary flow 226. The enclosure 218 may be particularly useful for shaping the supplementary flow 226. The enclosure 218 may, advantageously, generally maintain the supplementary flow 226 in a vicinity of the first jet. This may help to achieve the advantages of providing the supplementary flow 226 as described above. [000135] The window 208 is disposed within a wall of the chamber 204. The window is disposed within a wall of the chamber 204 opposite the orifice 212. The heating apparatus 210 is disposed outside of the chamber 204. The heating apparatus 210 is disposed proximate to the window 208. The heating apparatus 210 is operable to provide radiation (such as infrared radiation). The window 208 is at least partially transparent to radiation provided by the heating apparatus 210.

[000136] The heating apparatus 210 may be described as a radiative heating mechanism. In variations of the current embodiment, the heating apparatus 210 may comprise one or more of an inductive heating mechanism, a resistive heating mechanism, or a convective heating mechanism. The heating apparatus 210 may be considered to constitute part of a control system of the membrane cleaning apparatus 200 that is for controlling a pressure and/or a temperature within the chamber 204.

[000137] Delivering the cleaning substance 224 to the membrane 16 may cool a temperature of the membrane 16. This may result in condensation of water vapour on the membrane 16. Condensation of water vapour on the membrane 16 may reduce a cleaning efficacy of the membrane cleaning apparatus 200. Advantageously, providing a heating apparatus 210 configured to heat the membrane 16 may reduce water vapour condensation on the membrane 16.

[000138] Further, a layer of what may be described as “ice” (which may be formed from the cleaning substance 224) may be liable to form on the membrane 16. Such a layer of ice would reduce a cleaning efficacy of the membrane cleaning apparatus 200. Providing a heating apparatus 210 configured to heat the membrane 16 may advantageously promote sublimation of solid particles of the cleaning substance 224 after said solid particles are incident on the membrane 16. Therefore, advantageously, providing a heating apparatus 210 configured to heat the membrane 16 may prevent such a layer of ice from forming on the membrane 16.

[000139] A portion of the orifice 212 may also be heated. In particular, the orifice surround 214 may be heated. Advantageously, this may prevent the cleaning substance 224 (delivered via the jet Jl) from building up around, and consequently blocking, the opening defined by the orifice 212.

[000140] A membrane cleaning apparatus 300 according to an embodiment of the present invention is now described with reference to Figure 3, which shows a cross-section through the membrane cleaning apparatus 300. [000141] The membrane cleaning apparatus 300 shown in Figure 3 shares several features in common with other embodiments of membrane cleaning apparatus described herein. Any features of the membrane cleaning apparatus 300 shown in Figure 3 which generally correspond to, and may be generally the same as, features of other embodiments share common reference numerals therewith. The following description of the membrane cleaning apparatus 300 shown in Figure 3 will focus on features of this membrane cleaning apparatus 300 which differ from previously described embodiments of membrane cleaning apparatus. It will be appreciated that, where components or the functionality thereof is not described below, it shall be assumed that said components or the functionality thereof is as described with reference to previously described embodiments.

[000142] A principal difference between the membrane cleaning apparatus 300 shown in Figure 3 and the membrane cleaning apparatus 200 shown in Figure 2 is that, in Figure 3, the jet J1 of snow is provided within the chamber 204. Also, a pressure of an environment 305 within the chamber 204 may be higher than in the previously described embodiment, for example at 1 bar or lower. The environment 305 may contain dry air, dry nitrogen, or any dry gas. A temperature of environment 305 may be less than 30 °C, preferably less than -10 °C.

[000143] Another principal difference between the membrane cleaning apparatus 300 shown in Figure 3 and the membrane cleaning apparatus 200 shown in Figure 2 is that, in Figure 3, the membrane cleaning apparatus 300 comprises a plurality of coaxially aligned, axially spaced flow shaping members 302, each flow shaping member 302 having an aperture. The flow shaping members 302 are generally disc-shaped. These flow shaping members 302 may be considered to form part of a first jet expansion assembly. The flow shaping members 302 are arranged such that the first jet J1 propagates through the apertures of the flow shaping members 302. The flow shaping members 302 are also arranged to direct the supplementary flow 226 (from inlet 220) through one or more gaps between adjacent flow shaping members 302 so as to mix the supplementary flow 226 with the first jet Jl.

[000144] Each flow shaping member 302 has a different size and the plurality of flow shaping members 302 are arranged in order of size (from smallest to largest in the propagation direction of the first jet Jl. The aperture of each flow shaping member 302 has a different size, and the apertures are arranged in order of size. In particular, the apertures are arranged such that the first jet Jl propagates from a smallest aperture to a largest aperture.

[000145] A diameter d2 of the first jet J 1 after propagating through the flow shaping members 302 may be of the order of ten times a diameter dl of the first jet Jl before propagating through the flow shaping members 302. A density of the first jet Jl after propagating through the flow shaping members 302 may be approximately 0.1 -0.5 times a density of the first jet Jl before propagating through the flow shaping members 302. A velocity of the first jet J 1 after propagating through the flow shaping members 302 may be approximately 5-10 times smaller than a velocity of the first jet Jl before propagating through the flow shaping members 302. Hz may be less than 1 cm, Hy may be less than 1 mm, Wz may be of the order of 1-10 mm. [000146] The plurality of flow shaping members 302 is provided within the enclosure 218. The plurality of flow shaping members 302, in combination with the supplementary flow 226, may expand (and thereby reduce a density of) the jet Jl. The plurality of flow shaping members 302, in combination with the supplementary flow 226, may reduce a velocity of the jet Jl in a main direction of travel thereof. In particular, the plurality of flow shaping members 302, in combination with the supplementary flow 226, may reduce a velocity of solid particles of the cleaning substance 224 in the jet Jl.

[000147] As described above, membranes 16 may generally be fragile. Advantageously, the velocity of solid particles of the cleaning substance 224 in the jet Jl may be reduced to such an extent that the membrane 16 is not damaged by the jet Jl. However, the velocity of solid particles of the cleaning substance 224 in the jet J 1 may still be high enough such that, in some embodiments, momentum which is transferred to any contaminant particles 232 on a surface of the membrane 16 is enough to eject said particles 232 from said surface.

[000148] Another principal difference between the membrane cleaning apparatus 300 shown in Figure 3 and the membrane cleaning apparatus 200 shown in Figure 2 is that, in Figure 3, the membrane cleaning apparatus 300 comprises a third mask 304 having an aperture 306. The third mask 304 is disposed in a cavity defined by the chamber 204. The third mask 304 is disposed between the jet Jl and the membrane 16 when supported by the support 202. The third mask 304 is configured to block an outer portion of the jet J 1 from propagating towards the membrane 16.

[000149] A position of the aperture 306 of the third mask 304 with respect to the jet J 1 may be selected. A dimension of the aperture 306 may be selected. Smaller particles of the cleaning substance in the jet Jl may generally follow flow lines (of a gas flow). However, larger particles (with greater inertia) may at least partially decouple from such flow lines, particularly in locations where the flow lines undergo a significant change in direction (and such larger particles have too much inertia to follow the flow lines) such as, for example, at obstacles in the flow. Therefore, a trajectory followed by a particular particle of the cleaning substance may be determined at least in part by a mass of said particle. Hence, by positioning and/or dimensioning the third mask 304 and its aperture 306 appropriately, solid particles of the cleaning substance in the jet Jl of only a specific mass may be allowed to propagate through the aperture 306 and towards the membrane. This may enable the membrane cleaning apparatus 300 to be configured to remove contaminant particles 232 of a specific size range from the membrane 16 (as snow particles may be particularly effective at removing contaminant particles from the membrane 16 if the snow particles are of a similar size to the contaminant particles).

[000150] Size selectivity of snow particles impinging on the membrane 16 is achieved by a combination of: the shaping of the jet Jl of snow by the plurality of flow shaping members 302 using the supplementary flow 226; and the mask 304 with an aperture 306.

[000151] In this embodiment, the heating apparatus 210 is provided inside the chamber 204 (and therefore no window is provided). [000152] As described above, the plurality of flow shaping members 302 constitute a jet expansion assembly. However, variations may be made to the exact embodiment shown in Figure 3 and substantially similar advantages may be achieved. For example, only one flow shaping member may be provided. Regardless of whether one or more flow shaping members are provided, each flow shaping member may comprise a single aperture (as shown in Figure 3), or one or more flow shaping members may comprise a plurality of apertures. A flow shaping member may be of the form of a generally planar component having one or more apertures. A flow shaping member may be of the form of a generally planar component having many apertures. A flow shaping member may be of the form of a generally planar component with an array of apertures arranged in a regular pattern, such that this flow shaping member may be described as a grid. Alternatively, a flow shaping member may comprise a mesh. [000153] A flow shaping member heating mechanism may be provided (though this is not shown in Figure 3). The flow shaping member heating mechanism may be arranged to heat one or more flow shaping members. Advantageously, this may this may prevent the cleaning substance (delivered via the jet J 1) from building up around one or more flow shaping members 302, which may limit and/or block a flow path of the jet Jl.

[000154] Snow particles smaller than 10 um may follow flow lines shown in Figure 3 and shoot off to the side. Snow particles having dimensions between 10 and 30 um may decouple from these flow lines and pass through the aperture 306 to hit the pellicle 16.

[000155] It will be appreciated that providing the cleaning substance to the membrane 16 using membrane cleaning apparatus 300 of Figure 3 removes particles from (i.e., cleans) the membrane through a substantially similar mechanism to that described above with reference to membrane cleaning apparatus 200 of Figure 2.

[000156] A membrane cleaning apparatus 400 according to an embodiment of the present invention is now described with reference to Figure 4, which shows a cross-section through the membrane cleaning apparatus 400.

[000157] The membrane cleaning apparatus 400 shown in Figure 4 shares several features in common with other embodiments of membrane cleaning apparatus described herein. Any features of the membrane cleaning apparatus 400 shown in Figure 4 which generally correspond to, and may be generally the same as, features of other embodiments share common reference numerals therewith. The following description of the membrane cleaning apparatus 400 shown in Figure 4 will focus on features of this membrane cleaning apparatus 400 which differ from previously described embodiments of membrane cleaning apparatus. It will be appreciated that, where components or the functionality thereof is not described below, it shall be assumed that said components or the functionality thereof is as described with reference to previously described embodiments.

[000158] The membrane cleaning apparatus 400 comprises: a support 202; a third mask 304; two nozzles 402, 404; heating apparatus 210; and a chamber 204. The support 202, third mask 304 nozzles 402, 404, and the heating apparatus 210 are all disposed within the chamber 204. A first nozzle 402 is operable to produce a first jet J1 and a second nozzle 404 is operable to produce a second jet J2. The first and second jets Jl, J2 both comprise the cleaning substance (in the form of a snow). The second jet J2 may be considered to be a supplementary flow. The two nozzles 402, 404 are arranged such that the second jet J2 mixes with the first jet Jl prior to at least a portion of the first jet Jl being incident on a surface of a membrane 16 when supported by the membrane support 202. Mixing of the two jets Jl, J2 forms a stagnation plane 406.

[000159] The second jet J2, being arranged to mix with the first jet Jl, may expand (and thereby reduce a density of) the first jet Jl. The second jet J2, being arranged to mix with the first jet Jl, may reduce a velocity of a portion of the first jet Jl which propagates towards the membrane 16. The second jet J2, being arranged to mix with the first jet Jl, may reduce a velocity of solid particles of the cleaning substance in the first jet Jl. In particular, the second jet J2, being arranged to mix with the first jet Jl, may result in a reduced density and/or reduced speed of the mixed jets Jl, J2 proximate to the stagnation plane 406. This may result in a reduced pressure applied to the membrane 16 (which is generally fragile). Advantageously, the velocity of particles of the cleaning substance in the first jet Jl (and the second jet J2) may be reduced to such an extent that the membrane 16 is not damaged by the first jet Jl (or the second jet J2). However, the velocity of particles of the cleaning substance in the first jet Jl (and the second jet J2) may still be high enough such that, in some embodiments, momentum which is transferred to any contaminant particles 232 on a surface of the membrane 16 is enough to eject said particles 232 from said surface.

[000160] In this embodiment, size selectivity of snow particles incident on the membrane 16 is achieved by a combination of: the shaping of the first jet Jl of snow by the second jet J2 of snow; and the third mask 304 with an aperture 306 (in a similar method to how the third mask 304 of membrane cleaning apparatus 300 of Figure 3 is used).

[000161] With reference to Figure 4, a separation between the membrane 16 and the third mask 304, h, may be of the order of 1 to 10 mm. A diameter of the aperture 306, W, may be of the order of 1 to 10 mm. A separation, D, of the two jets Jl, J2, a separation, H, of the jets Jl, J2 from the membrane 16 and an orientation, A, of the jets Jl, J2 relative to the plane of the membrane 16 may each be configured so as to achieve a desirable mixing of the two jets Jl, J2.

[000162] Smaller snow particles may follow the flow lines shown in Figure 4 (and are directed towards the stagnation plane 406) and may be incident on the pellicle 16 via the aperture 306 of the third mask 304. In contrast, larger snow particles tend to follow flow lines less, be farther from the stagnation plane 406, and tend to be blocked by the third mask 304.

[000163] A membrane cleaning apparatus 500 according to an embodiment of the present invention is now described with reference to Figure 5, which shows a cross-section through the membrane cleaning apparatus 500.

[000164] The membrane cleaning apparatus 500 shown in Figure 5 shares many features in common with the membrane cleaning apparatus 400 shown in Figure 4. [000165] It will be appreciated that Figure 5 only shows a portion of the apparatus 500 (for example the chamber 204, support 202 and heating apparatus 210 are not shown in Figure 4). However, the membrane cleaning apparatus 500 shown in Figure 5 may comprise any of the features of the membrane cleaning apparatus 400 shown in Figure 4.

[000166] The membrane cleaning apparatus 500 shown in Figure 5 may be substantially identical to the membrane cleaning apparatus 400 shown in Figure 4 with the exception that, although it comprises two nozzles 502, 504 only one of these provides a jet of cleaning substance. A first nozzle 502 is operable to produce a first jet J1 comprising the cleaning substance (in the form of a snow). A second nozzle 504 is operable to produce a second jet J2 comprising dry air, dry nitrogen, and/or any dry gas or mixture of dry gases. The second jet J2 may be cooled, so as to preserve the particles from the two- phase jet J 1 , which is typically cold. The dry gas or gases may be generally chemically inert within the membrane cleaning apparatus. The second jet J2 produced from nozzle 504 may be used to attenuate the first jet J1 produced from nozzle 502 (Figure 5) in substantially the same way that the second jet J2 produced from nozzle 404 may be used to attenuate the first jet J1 produced from nozzle 402 (Figure 4). That is, a stagnation plane 506 may be formed, and size selectivity of snow particles incident on the membrane 16 is achieved by a combination of: the shaping of the first jet J 1 of snow by the second jet J2 of dry gas or gases; and the third mask 304 with an aperture 306.

[000167] A membrane cleaning apparatus 600 according to an embodiment of the present invention is now described with reference to Figure 6, which shows a cross-section through the membrane cleaning apparatus 600.

[000168] The membrane cleaning apparatus 600 shown in Figure 6 shares several features in common with other embodiments of membrane cleaning apparatus described herein. Any features of the membrane cleaning apparatus 600 shown in Figure 6 which generally correspond to, and may be generally the same as, features of other embodiments share common reference numerals therewith. The following description of the membrane cleaning apparatus 600 shown in Figure 6 will focus on features of this membrane cleaning apparatus 600 which differ from previously described embodiments of membrane cleaning apparatus. It will be appreciated that, where components or the functionality thereof is not described below, it shall be assumed that said components or the functionality thereof is as described with reference to previously described embodiments.

[000169] The membrane cleaning apparatus 600 comprises: a block of solid cleaning substance 602; and a radiation source 604 (for example a laser) configured to provide radiation 606 to a surface of the block of cleaning substance 602 through a window 608. Together, the block of solid cleaning substance 602 and the radiation source 604 may be considered to be, or form part of, a cleaning substance delivery mechanism. The radiation source 604 may be considered to be, or form part of, a stimulation mechanism. The radiation source 604 is operable to cause at least a portion of the cleaning substance in the block 602 of cleaning substance to sublime, such that at least a portion 610 of the cleaning substance propagates away from the block of cleaning substance 602 and towards a membrane 16 when supported by the membrane support 202.

[000170] The at least a portion 610 of the cleaning substance which propagates away from the block of cleaning substance and towards a membrane may comprise a gas jet or a two-phase jet. For example, said at least a portion may comprise a mixture of solid and gaseous carbon dioxide.

[000171] The stimulation mechanism may comprise a laser (i.e., the radiation source 604 may be a laser). Radiation 606 from the laser may be provided to the surface of the block 602 in one or more pulses. The laser may be configured such that particles 610 of the cleaning substance are ejected from the block 602 by the pulsed laser radiation 606. In particular, the laser may be configured such that particles 610 of a particular size or size range (such as between 1 and 100 um) are ejected from the block 602.

[000172] The laser may provide laser radiation 606 having a wavelength between 4.1 and 4.4 um. The laser may be configured such that the energy provided in each pulse of radiation 606 is greater than 0.1 mJ, though it may preferably be greater than 1 mJ.

[000173] A wavelength of the laser radiation 606 may be selected so as to result in an absorption depth of between 1 and 100 um in the block of solid cleaning substance, though the wavelength may preferably be selected so as to result in an absorption depth of between 5 and 50 um. The laser may be configured such that the absorption depth in the block 602 is between 0.1 and 10 times a desired size of the particles of cleaning substance to be ejected from the block 602.

[000174] The stimulation mechanism may be configured to provide laser radiation 606 such that a spatial power intensity of the radiation 606 incident on the block 602 is non-uniform. For example, the spatial power intensity of the radiation 606 incident on the block 602 may be modulated or periodic. The spatial power intensity may comprise a pattern of repeated shapes. One or more of the repeated shapes may have dimensions between 1 and 100 um. For example, an interference pattern of the laser radiation 606 may be generated on the surface of the block 602. By having repeated shapes of the spatial power intensity of a particular dimension, particles 610 of the cleaning substance of that particular dimension may be preferentially generated. Advantageously, this may allow for generation of particles 610 of the cleaning substance which are particularly effective at removing contaminant particles 232 from the membrane 16 when incident on the membrane 16.

[000175] The radiation source 604 may be configured to provide energy to the block 602 of solid cleaning substance. The radiation source 604 may be tuned so as to eject solid particles 610 of the cleaning substance alongside gas of the cleaning substance. The radiation source 604 may be tuned so as to create a desired: quantity of cleaning substance 610 to sublime; and/or size and/or speed of solid particles of cleaning substance. Advantageously, this provides an easily controllable arrangement which can be tailored to the removal of contaminant particles 232 of various sizes from the membrane 16. [000176] The arrangement of the block 602 and the membrane 16 (in particular, a pressure of the environment between the block 602 and the membrane 16 and/or a distance, L, between the block 602 and the membrane 16) may at least partially decelerate particles 610 of the cleaning substance when propagating towards the membrane 16. This may advantageously reduce kinetic energy of such particles 610 and therefore prevent damage to the membrane 16. It will be appreciated that the arrangement of the block 602 and the membrane 16 may be configured so as to achieve a desired deceleration of particles 610 of the cleaning substance.

[000177] The radiation source 604 may be particularly well suited for stimulating a selected amount of the block 602 of cleaning substance. The radiation source 604 is disposed outside of an environment 605 within the chamber 204. A window 608 is provided in a wall of the chamber 204 through which the radiation 606 propagates to the block of solid cleaning substance 602. As can be seen in Figure 6, laser radiation 606 propagates through the window 608 and is then directly incident on the block 602. [000178] It will be appreciated that providing the cleaning substance to the membrane 16 using membrane cleaning apparatus 600 of Figure 6 may remove particles from (i.e., cleans) the membrane 16 through a substantially similar mechanism to that described above with reference to membrane cleaning apparatus 200 of Figure 2 (which applies equally to other previously described embodiments). [000179] A membrane cleaning apparatus 700 according to an embodiment of the present invention is now described with reference to Figure 7, which shows a cross-section through the membrane cleaning apparatus 700.

[000180] The membrane cleaning apparatus 700 shown in Figure 7 shares many features in common with the membrane cleaning apparatus 600 shown in Figure 6. Any features of the membrane cleaning apparatus 700 shown in Figure 7 which generally correspond to, and may be generally the same as, features of other embodiments share common reference numerals therewith. It will be appreciated that, where components or the functionality thereof is not described below, it shall be assumed that said components or the functionality thereof is as described with reference to previously described embodiments.

[000181] Similarly to the membrane cleaning apparatus 600 shown in Figure 6, the membrane cleaning apparatus 700 also comprises: a block of solid cleaning substance 602; a radiation source 604 (for example a laser) configured to provide radiation 606 to a surface of the block of cleaning substance 602 via a window 608 in a wall of a chamber 204; and a membrane support 202. The membrane cleaning apparatus 700 shown in Figure 7 shows an alternative arrangement of these features to the membrane cleaning apparatus 600 shown in Figure 6.

[000182] A principal difference between the membrane cleaning apparatus 700 shown in Figure 7 and the membrane cleaning apparatus 600 shown in Figure 6 is that, in Figure 7, laser radiation 606 propagates through the window 608 and is transmitted by the membrane 16 before being incident on the block 602. This may heat the membrane 16 and achieve similar advantages as described above with reference to previous embodiments that use a separate heating apparatus 210. Also, when supported by the membrane support 202, the membrane 16 is generally in close proximity to the surface of the block 602 on which the laser radiation 606 is incident. For example, the separation K may be approximately 1 mm.

[000183] It will be appreciated that providing the cleaning substance to the membrane 16 using membrane cleaning apparatus 700 of Figure 7 may remove particles from (i.e., cleans) the membrane 16 through a substantially similar mechanism to that described above with reference to membrane cleaning apparatus 200 of Figure 2 (which applies equally to other previously described embodiments). [000184] However, as the membrane 16 is generally in close proximity to the block 602, a gaseous form of the cleaning substance (generated via the incidence of laser radiation 606 on the block 602) may transfer momentum to contaminant particles 232 (via the membrane 16) as well as solid particles on the cleaning substance. That is, incidence of laser radiation 606 on the block 602 may generate a gas puff 710 which: is directly incident on the membrane 16; and carries solid particles of the cleaning substance to the membrane 16. Both states of matter of the cleaning substance may advantageously transfer momentum to contaminant particles 232 on the membrane 16, thereby ejecting said contaminant particles 232, and cleaning the membrane 16. The radiation source 604 may be configured so as to create a puff of gas 710 having a diameter, N, of approximately 1 mm.

[000185] Although carbon dioxide snow cleaning is a known cleaning technique, known carbon dioxide snow cleaning methods are not suitable for cleaning pellicles (which are relatively thin and liable to rupture). Known techniques relate to arrangements whereby a carbon dioxide jet is focused, and generally propagates, without any significant obstacles or attenuation of the jet. In contrast, a number of the above-described embodiments employ new arrangements that slow down carbon dioxide jets and/or additionally limit the area of the membrane subject to any (attenuated) carbon dioxide jet (or other flow of particles or gas), in order to reduce the force and prevent possible rupture of the membrane. [000186] It will be appreciated that the terms “particle size” and “particle mass” may have been used interchangeably above. A particle which is more massive will generally correspond to a particle which is larger, and no distinction between these terms is generally intended.

[000187] It will be appreciated that in any of the above-described membrane cleaning apparatus 200, 300, 400, 500, 600, 700 the membrane assembly may be a pellicle for use in an EUV lithographic apparatus. In particular, the membrane assembly may comprise the pellicle 15 (which comprises a thin membrane 16 mounted on a frame 17) shown in Figure 1.

[000188] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid- crystal displays (LCDs), thin-film magnetic heads, etc.

[000189] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[000190] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims and clauses set out below.

1. A membrane cleaning apparatus for removing particles from a surface of a membrane, the apparatus comprising: a membrane support for supporting the membrane; and a cleaning substance delivery mechanism for delivering a cleaning substance to a membrane surface opposite to the surface to be cleaned.

2. A membrane cleaning apparatus for removing particles from a membrane, the apparatus comprising: a chamber; a control system for controlling a pressure and/or a temperature within the chamber; a membrane support for supporting the membrane within the chamber; and a cleaning substance delivery mechanism for delivering a cleaning substance to the membrane; wherein the control system is configured to control the pressure and/or the temperature within the chamber such that at least a portion of the cleaning substance undergoes a phase transition into a gas in the chamber.

3. The apparatus of clause 2, wherein the phase transition comprises a transition from a solid into a gas.

4. The apparatus of clause 2 or clause 3, wherein the phase transition comprises a transition from a liquid into a gas.

5. A membrane cleaning apparatus for removing particles from a membrane, the apparatus comprising: a membrane support for supporting the membrane; and a cleaning substance delivery mechanism for delivering a cleaning substance to the membrane, wherein the cleaning substance comprises carbon dioxide.

6. The apparatus of any of clause 1 to clause 4, wherein the cleaning substance comprises carbon dioxide.

7 The apparatus of any of clause 1, 5, or 6, further comprising: a chamber within which the membrane support is provided; and a control system for controlling a pressure and/or a temperature within the chamber.

8. The apparatus of any of clauses 2, 3, 4, or 7, wherein the control system is arranged to maintain the pressure within the chamber below a pressure of an environment in which the chamber is disposed.

9. The apparatus of any preceding clause, wherein the cleaning substance delivery mechanism comprises a mechanism configured to provide a first jet of the cleaning substance in the form of a snow, such that at least a portion of the first jet is incident on a surface of a membrane when supported by the membrane support.

10. The apparatus of clause 9, wherein the cleaning substance delivery mechanism further comprises a mechanism configured to provide a supplementary flow, wherein at least a portion of the supplementary flow is provided in a vicinity of the first jet.

11. The apparatus of clause 10, wherein the apparatus comprises an enclosure which confines at least a portion of the supplementary flow to a vicinity of the first jet.

12. The apparatus of clause 11, wherein the apparatus further comprises a first jet expansion assembly configured to direct the supplementary flow towards the first jet such that at least part of the supplementary flow mixes with at least part of the first jet so as to increase a cross-section of the first jet transverse to a main direction of travel of the first jet.

13. The apparatus of clause 12, wherein the first jet expansion assembly comprises a plurality of coaxially aligned, axially spaced flow shaping members, each flow shaping member having an aperture, wherein the flow shaping members are arranged: such that the first jet propagates through the apertures of the flow shaping members; and to direct the supplementary flow through one or more gaps between adjacent flow shaping members so as to mix the supplementary flow with the first jet.

14. The apparatus of clause 13, wherein each flow shaping member has a different size, the plurality of flow shaping members being arranged in order of size.

15. The apparatus of clause 13 or clause 14, wherein the aperture of each flow shaping member has a different size, the apertures being arranged in order of size, and wherein the apertures are arranged such that the first jet propagates from a smallest aperture to a largest aperture.

16. The apparatus of any of clause 13 to clause 15, wherein one or more flow shaping members are generally disc-shaped.

17. The apparatus of clause 12, wherein the first jet expansion assembly comprises one or more flow shaping members, each flow shaping member comprising a plurality of apertures arranged such that the first jet propagates through the plurality of apertures.

18. The apparatus of any of clause 12 to clause 17, further comprising a flow shaping member heating mechanism arranged to heat one or more flow shaping members.

19. The apparatus of any of clause 10 to clause 18, wherein the supplementary flow comprises a second jet of the cleaning substance in the form of a snow, wherein the second jet is arranged to mix with the first jet prior to at least a portion of the first jet being incident on a surface of a membrane when supported by the membrane support.

20. The apparatus of clause 10, wherein the supplementary flow comprises a jet of dry gas arranged to mix with the first jet prior to at least a portion of the first jet being incident on a surface of a membrane when supported by the membrane support.

21. The apparatus of any preceding clause, when dependent either directly or indirectly on clauses 2 or 7, wherein the apparatus further comprises a first mask having an aperture, and wherein the first mask is arranged so as to form part of a wall of the chamber and is arranged between the cleaning substance delivery mechanism and a membrane when supported by the membrane support.

22. The apparatus of clause 21 wherein the first mask is configured to: block at least a portion of the first jet from propagating towards a membrane when supported by the membrane support; and provide a stagnation point of the first jet proximate to the aperture of the first mask.

23. The apparatus of any preceding clause, wherein the apparatus further comprises a second mask, and wherein the second mask is arranged between the cleaning substance delivery mechanism and a membrane when supported by the membrane support, and wherein the second mask is configured to block at least a portion of the first jet from propagating towards a membrane when supported by the membrane support.

24. The apparatus of clause 23, wherein the second mask is substantially aligned with the first jet so as to block a central portion of the first jet from propagating towards a membrane when supported by the membrane support.

25. The apparatus of any preceding clause in which the apparatus comprises a chamber, wherein the apparatus further comprises a third mask having an aperture, and wherein the third mask is disposed within a cavity defined by the chamber, and wherein the third mask is configured to block an outer portion of the first jet from propagating towards a membrane when supported by the membrane support.

26. The apparatus of any of clause 1 to clause 8, wherein the cleaning substance delivery mechanism comprises: a block of solid cleaning substance; and a stimulation mechanism operable to cause at least a portion of the cleaning substance in the block of cleaning substance to sublime, such that at least a portion of the cleaning substance propagates away from the block of cleaning substance and towards a membrane when supported by the membrane support.

27. The apparatus of clause 26, wherein the stimulation mechanism comprises a radiation source configured to provide radiation to a surface of the block of cleaning substance.

28. The apparatus of clause 27, wherein the stimulation mechanism comprises a laser operable to produce laser radiation which is provided to the surface of the block of cleaning substance in one or more pulses. 29. The apparatus of clause 28, wherein the stimulation mechanism is configured such that a wavelength of the laser radiation is between 4.0 and 4.5 um.

30. The apparatus of clause 28 or clause 29, wherein the stimulation mechanism is configured such that energy provided in each pulse of radiation is greater than 0.1 mJ.

31. The apparatus of any of clause 28 to 30, wherein the stimulation mechanism is configured such that an absorption depth of the laser radiation is between 1 and 100 um in the block of solid cleaning substance.

32. The apparatus of any of clause 28 to 31, wherein the stimulation mechanism is configured such that an absorption depth of the laser radiation is between 5 and 50 um in the block of solid cleaning substance.

33. The apparatus of any of clause 27 to 32, wherein the apparatus is configured such that radiation is provided to the surface of the block of solid cleaning substance after first being transmitted by a membrane when supported by the membrane support.

34. The apparatus of any of clause 27 to 32, wherein the apparatus is configured such that radiation is provided to the surface of the block of solid cleaning substance without first being transmitted by a membrane when supported by the membrane support.

35. The apparatus of any of clause 28 to 34, wherein the stimulation mechanism is configured to provide laser radiation such that a spatial power intensity of the radiation incident on the block of solid cleaning substance is non-uniform.

36. The apparatus of clause 35, wherein the spatial power intensity comprises a pattern of repeated shapes, and wherein one or more of the repeated shapes have dimensions between 1 and 100 um.

37. The apparatus of any preceding clause, wherein the apparatus further comprises a heating mechanism configured to heat a membrane when supported by the membrane support.

38. The apparatus of any preceding clause, wherein the apparatus further comprises a contaminant collection plate arranged proximate to the membrane.

39. The apparatus of any preceding clause, wherein the cleaning substance delivery mechanism is operable to generate particles of the cleaning substance having dimensions smaller than 100 um.