CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP/US application 21154767.4 which was filed on February 2, 202 land which is incorporated herein in its entirety by reference.

[0002] The present invention relates to a lithographic apparatus, a membrane assembly for a lithographic apparatus, and a use of a lithographic apparatus or a membrane in a lithographic apparatus or method. The present invention also relates to methods of extending the lifespan of a membrane in a lithographic apparatus, and the use of such methods in a lithographic apparatus or method. The present has particular, but not exclusive, application to membranes included in a dynamic gas lock assembly.

BACKGROUND

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

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

[0005] A lithographic apparatus includes a patterning device (e.g. a mask or reticle). Radiation is provided through or reflected off the patterning device to form an image on a substrate. A membrane assembly, also referred to as a pellicle, may be provided to protect the patterning device from airborne particles and other forms of contamination. Contamination on the surface of the patterning device can cause manufacturing defects on the substrate.

[0006] Since EUV radiation is strongly absorbed by matter, within a lithographic apparatus using EUV radiation the optical path of the EUV radiation is under near vacuum conditions, i.e. at pressures significantly below atmospheric pressure, with a small amount of hydrogen present. In particular, the projection system, which comprises a system of optical elements for projecting the EUV radiation onto a substrate, may be held under near vacuum conditions. The EUV radiation is projected through an opening defined by the projection system onto the substrate. It is desirable to limit ingress of contaminants into the projection system. It is known to provide a dynamic gas lock for this purpose. The dynamic gas lock may comprise a membrane. The membrane is configured to provide a barrier to contaminants passing from one portion of the lithography apparatus to another section and is primarily configured to filter out out-of-band radiation, particularly deep ultraviolet (DUV) and infrared radiation. This is because some EUV sources, such as those which generate EUV radiation using a plasma, do not only emit desired ‘in-band’ EUV radiation, but also undesirable (out-of-band) radiation. This out-of- band radiation is most notably in the deep UV (DUV) radiation range (100 to 400 nm). Moreover, in the case of some EUV sources, for example laser produced plasma EUV sources, the radiation from the laser, usually at 10.6 microns, presents a significant out-of-band radiation.

[0007] In a lithographic apparatus, spectral purity is desired for several reasons. One reason is that the resist is sensitive to out of-band wavelengths of radiation, and thus the image quality of patterns applied to the resist may be deteriorated if the resist is exposed to such out-of-band radiation. Furthermore, out-of-band radiation infrared radiation, for example the 10.6 micron radiation in some laser produced plasma sources, leads to unwanted and unnecessary heating of the patterning device, substrate, and optics within the lithographic apparatus. Such heating may lead to damage of these elements, degradation in their lifetime, and/or defects or distortions in patterns projected onto and applied to a resist-coated substrate.

[0008] The membrane is intended to be highly transmissive to radiation passing through the membrane in order to provide a high throughput of wafers imaged by the lithography apparatus. Over time, the transmissivity of the membrane in the dynamic gas lock can change, which can reduce the throughput of the lithography apparatus, which is clearly undesirable.

[0009] The present invention has been devised in an attempt to address at least some of the problems identified above.

SUMMARY OF THE INVENTION

[00010] According to a first aspect of the present invention, there is provided a lithographic apparatus including a membrane assembly and a radiation beam path, wherein the membrane assembly includes a membrane, the membrane being disposed in the radiation beam path, wherein the membrane assembly is configured to move the membrane from a first position in the radiation beam path to a second position in the radiation beam path.

[00011] Lithographic apparatuses project a beam of radiation onto a substrate in order to form a pattern on the substrate. The radiation beam follows a radiation beam path through the lithographic apparatus from where it is generated to the substrate being patterned. Since it is desirable to prevent contamination from the portion of the lithographic apparatus which creates the radiation, which in the case of an extreme ultraviolet (EUV) lithographic apparatus is created via a tin plasma, from the portion of the lithographic apparatus which conditions the radiation beam, a membrane is provided in the path of the radiation beam. The membrane is transmissive to the radiation beam at the desired wavelength used for imaging, but is able to prevent particulate contaminants from passing through the apparatus and is also configured to filter out out-of-band radiation. The membrane is exposed to the radiation beam in a plasma environment. The plasma may include hydrogen, water, oxygen and other species. Over time, the membrane is degraded and the rate at which the degradation takes places is a function of the dose of radiation received by the membrane. As such, in areas of the membrane which experience the highest dose of radiation, there is the greatest rate of degradation. In existing lithographic apparatuses, the membrane of the dynamic gas lock is removed and replaced with a fresh membrane when the membrane has started to degrade, and is always held in a fixed position relative to the radiation beam path within the lithographic apparatus. Replacement of the whole membrane is only possible when the machine is not operating and it takes around 10 hours of downtime to replace the membrane and bring the machine back into an operational state and begin exposing wafers again. This decreases the number of wafers which can be imaged and reduces throughput of the apparatus. Since the intensity of the radiation beam is not uniform, that degradation of the membrane is also not uniform. As such, in the present invention, rather than swapping out the entire membrane, it is possible to move the membrane within the apparatus so that the radiation beam illuminates a different area of the same membrane. As such, moving the membrane relative to the radiation beam path means that when the lithographic apparatus is in operation the radiation beam path intersects a different portion of the membrane. Since the degradation of the area of the membrane which was not previously exposed to the highest dose of radiation is lower than the area of the membrane which was previously exposed to the highest dose of radiation, by moving the membrane, the radiation beam passes through a less degraded area of the membrane. In this way, it is possible to avoid the need to disassemble the lithography apparatus to swap out the membrane. This is particularly advantageous for membrane in the dynamic gas lock since these are replaced less frequently than other membranes, such as pellicles which protect the reticle, and are less easily replaced. Being able to extend the lifetime of membranes, such as in the dynamic gas lock, allows for time and cost savings. In addition, by moving the same membrane in situ, rather than by swapping the membrane with a different membrane, there is a lower risk of contaminants being introduced into the lithographic apparatus. It will be appreciated that there may be more than two operational positions to which the membrane may be moved. An operational position is one in which the membrane is located when the lithographic apparatus is in use. As such, where the membrane is totally removed from the apparatus and/or totally outside of the radiation beam path, it is not in an operational position. In this way a single membrane can be moved to a number of different positions within the lithographic apparatus such that a greater proportion of the surface area of the membrane can be utilised Non uniform degradation of the membrane can occur. Some parts of the membrane see more light over time than other parts. This can lead to a non-uniform degradation profile which can results undesired effects of the image on wafer. As such, by moving the membrane, non-uniform degradation is prevented. In this way, the degradation is smeared out by moving the higher degraded part of the membrane into a position where it will see less degradation and moving the lower degraded part into a position where it will see more degradation. This results in an improvement in imaging quality. The membrane may be moved in any axis. [00012] The apparatus may include a control system configured to control the movement of the membrane relative to the radiation beam path. The control system can control the movement of the membrane from the first operational position to the second (or further) operational position. The radiation beam path is the path which the radiation beam takes when the lithographic apparatus is in use. The present invention is not particularly limited by the exact way in which the membrane is moved relative to the radiation beam path. One way in which the membrane could be moved is via one or more electric motors.

[00013] The apparatus may be configured to rotate the membrane. Additionally or alternatively, the apparatus may be configured to move the membrane in one or more directions. The directions may be linear directions, but could also be in a non-linear direction. Rotation of the membrane may be achieved by rotating part or all of the membrane assembly. The membrane assembly may include a frame which supports the membrane. The cross-sectional shape of the radiation beam is elongate and so by rotating and/or moving the membrane relative to the radiation beam, a different area of the membrane is exposed to the area of the radiation beam providing the highest dose of radiation. It will be appreciated that movement in this context does not relate to complete removal of the membrane from the apparatus, but rather movement of the membrane so that a fresh area is exposed to the radiation beam, particularly the portion of the radiation beam which provides the highest dose. The membrane may be moved in any direction, although primarily the membrane will be rotated and/or moved in a direction which is perpendicular to the direction the radiation travels along the radiation beam path.

[00014] The membrane assembly, and therefore the membrane, may be configured to move continuously or in a stepwise manner. In order to mitigate the degradation of the membrane due to exposure to the radiation beam and plasma, the membrane can be moved continuously so that the degradation is spread evenly across the membrane. The membrane can also be moved in a stepwise manner, with one area of the membrane being exposed to the radiation beam for a period of time before being moved to a different position to allow another area of the membrane to be exposed to the radiation beam.

[00015] The membrane assembly may be configured to move the membrane up to 10 mm, up to 20 mm, up to 30 mm, up to 40 mm, or up to 50 mm at a time. The exact distance moved may be selected depending on the size of the area of the membrane which has been degraded. Similarly, the membrane may be rotated through any angle of rotation as required.

[00016] The membrane may be a dynamic gas lock assembly or a component thereof. Of course, other membranes, such as a pellicle membrane protecting a reticle, are also contemplated and so the membrane assembly may be a pellicle membrane assembly.

[00017] According to a second aspect of the present invention, there is provided a membrane assembly for a lithographic apparatus, said membrane assembly including a membrane, wherein the membrane assembly is configured to move the membrane from a first position in a radiation beam path to a second position in the radiation beam path of the lithographic apparatus. [00018] Membrane assemblies have previously been removed from lithographic apparatuses and swapped out for assemblies with new membranes as required. However, existing membrane assemblies do not provide for the possibility of positioning the membrane in different positions within the lithographic apparatus without needing to dismantle the lithographic apparatus or physically swap one membrane for a different membrane. The present invention provides for moving the membrane between different operational positions within the lithographic apparatus. An operational position is a position which the assembly may be located whilst the assembly is in use, which is in contrast to positions which are outside of the lithographic apparatus of which the assembly may be a component. In existing apparatuses, there is only a single position for the membrane, particularly when part of the dynamic gas lock.

[00019] The membrane assembly may include a control system configured to control the movement of the membrane relative to the lithographic apparatus.

[00020] The membrane assembly may be configured to move the membrane in a continuous or stepwise manner.

[00021] The membrane assembly may be configured to move the membrane up to 10 mm, up to 20 mm, up to 30 mm, up to 40 mm, or up to 50 mm at a time.

[00022] The membrane assembly may be a dynamic gas lock assembly or component thereof. The membrane assembly may be a pellicle assembly or component thereof.

[00023] According to a third aspect of the present invention, there is provided a method of extending the lifespan of a membrane of a lithography apparatus, said method including providing a membrane in a radiation beam path of the lithography apparatus, and moving the membrane relative to the radiation beam path.

[00024] As described, by moving the membrane within the lithography apparatus such that, in use, the radiation beam passes through a different portion of the membrane, the degradation caused by exposure to the radiation beam and the plasma environment can be distributed over the membrane and the lifetime of the membrane can thereby be extended. This avoids the need to have extended periods of downtime to replace the membrane and also avoids the risk of inadvertently introducing additional contamination into the lithography apparatus. The membrane can be selectively moved relative to the radiation beam path to expose a different portion of the membrane to the portion of the radiation beam with the highest intensity.

[00025] Moving the membrane may include rotating the membrane and/or moving the membrane in one or more directions.

[00026] According to a fourth aspect of the present invention, there is provided the use of a lithographic apparatus according to the first aspect of the present invention, a membrane assembly according to the second aspect of the present invention, or a method according to the third aspect of the present invention in a lithography apparatus or process. [00027] It will be appreciated that features described in respect of one embodiment may be combined with any features described in respect of another embodiment and all such combinations are expressly considered and disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[00028] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawing in which corresponding reference symbols indicate corresponding parts, and in which:

[00029] Figure 1 depicts a lithographic apparatus according to an embodiment of the invention;

[00030] Figure 2 is an image showing the difference in transmissivity of a membrane after irradiation and exposure to plasma in a lithographic apparatus;

[00031] Figure 3 is a schematic depiction of a method of extending the lifespan of a membrane by moving the membrane in siti , and

[00032] Figure 4 is a schematic depiction of a method of extending the lifespan of a membrane by rotating the membrane.

[00033] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION

[00034] Figure 1 shows a lithographic apparatus according to an embodiment of the present invention. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithographic apparatus LA comprises an illumination system IL, 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. The illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.

[00035] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS. [00036] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source. A laser, which may for example be a CO2 laser, is arranged to deposit energy via a laser beam into a fuel, such as tin (Sn) which is provided from a fuel emitter. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may for example be in liquid form, and may for example be a metal or alloy. The fuel emitter may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region. The laser beam is incident upon the tin at the plasma formation region. The deposition of laser energy into the tin creates a plasma at the plasma formation region. Radiation, including EUV radiation, is emitted from the plasma during de-excitation and recombination of ions of the plasma.

[00037] The EUV radiation is collected and focused by a near normal incidence radiation collector (sometimes referred to more generally as a normal incidence radiation collector). The collector may have a multilayer structure which is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region, and a second focal point may be at an intermediate focus, as discussed below.

[00038] The laser may be separated from the radiation source SO. Where this is the case, the laser beam may be passed from the laser to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser and the radiation source SO may together be considered to be a radiation system.

[00039] Radiation that is reflected by the collector forms a radiation beam B. The radiation beam B is focused at a point to form an image of the plasma formation region, which acts as a virtual radiation source for the illumination system IL. The point at which the radiation beam B is focused may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus is located at or near to an opening in an enclosing structure of the radiation source.

[00040] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. A dynamic gas lock may be provided between the radiation source SO and the illumination system IL and is configured to filter out any out-of-band radiation created in the radiation source SO and allow radiation of the desired wavelength to pass through. 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 radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. 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.

[00041] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors 13, 14 in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors).

[00042] The radiation sources SO shown in Figure 1 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation. Such a filter may include a membrane assembly according to the present invention which provides for movement of the membrane relative to the radiation beam such that a fresh part of the same membrane can be exposed to the EUV radiation and plasma conditions.

[00043] Figure 2 illustrates the reduced transmissivity of a membrane with the areas exposed to the highest dose of EUV radiation shown in darker grey, which indicates a reduction of transmissivity of the membrane.

[00044] Figure 3 depicts comparison of a membrane 15a which is not moved within the lithographic apparatus and a membrane 15b which has been moved within the lithographic apparatus. The darker shading denotes areas of the membrane which have reduced transmissivity, which is indicative of degradation. By moving the membrane relative to the radiation beam (not shown), the degradation can be spread out over more of the area of the membrane 15a and so the lifespan of the membrane can be increased as fresh areas can be used when the transmissivity of the membrane drops below a predetermined amount, which is something which can be readily measured via metrology.

[00045] Figure 4 is similar to Figure 3, except that the membrane 15c is rotated relative to the radiation beam rather than translated. Again, by rotating the membrane 15c, the degradation of the membrane can be spread out across the surface of the membrane 15c and thereby extend its operational life span.

[00046] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[00047] 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 set out below.