CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 18174565.4 which was filed on May 28, 2018 and EP application 18187515.4 which was filed on August 6, 2018 which are incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to lithographic apparatus. The present invention has particular, but not exclusive, use in connection with EUV lithographic apparatus and EUV lithographic tools. The present invention relates to a system for providing a controlled amount of gas to an apparatus.

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 at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[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] Known methods of providing a controlled amount of gas may be unsuitable for providing the relatively small amounts of oxygen required in a lithographic apparatus and/or for maintaining a reliably accurate and precise amount of gas within the lithographic apparatus. For example, known mass flow controllers may provide a minimum amount of oxygen gas that is between about 100 and about 1000 times greater than the amount of oxygen required in the EUV lithographic apparatus. That is, known mass flow controller may introduce too much oxygen to the lithographic apparatus and damage optical components of the lithographic apparatus. Furthermore, known mass flow controllers may not offer adequate accuracy in maintaining a steady flow of the relatively small amount of oxygen.

[0006] One method of mitigating the reduced transmission of EUV radiation involves introducing oxygen gas to the lithographic apparatus. However, introducing too much oxygen to the lithographic apparatus may risk damaging one or more optical components of the lithographic apparatus, e.g. mirrors configured to reflect the EUV radiation beam. The amount of oxygen to be introduced to the lithographic apparatus may be relatively small. The balance between providing too much oxygen and too little oxygen may be delicate and require accurate and reliable control. It is desirable to provide a method of accurately and reliably controlling the relatively small amount of oxygen gas that is to be provided to the lithographic apparatus.

SUMMARY

[0007] According to an aspect of the invention there is provided a gas flow control system for providing oxygen (O2) to an EUV lithographic apparatus, the gas flow control system comprising: a first inlet configured to be connected to a first gas source, the first gas comprising oxygen (O2) gas; a second inlet configured to be connected to a second gas source, the second gas not containing any (O2) gas; the gas flow control system being configured to mix the first and second gas to obtain a mixed gas comprising diluted oxygen (O2) gas; the gas flow control system further comprising:

a first outlet configured to output a first amount of the mixed gas to an interior of the EUV lithographic apparatus and a second outlet configured to output a second amount of the mixed gas to a dump, exterior to the EUV lithographic apparatus.

[0008] In an embodiment the gas flow control system has a mixed gas comprising an amount of the second gas that is at least a 100 times more, preferable a 1000 times more, that an amount of the first gas.

[0009] In an embodiment the gas flow control system has a first outlet comprising an outlet mass flow controller for controlling the first amount of the mixed gas. The first outlet comprises a first outlet mass flow controller for controlling a first portion of the first amount of the mixed gas and a second outlet mass flow controller for controlling a second portion of the first amount of the mixed gas.

[00010] In an embodiment the first inlet comprises a first inlet mass flow controller for controlling an amount of the first gas and a second inlet mass flow controller for controlling an amount of the second gas.

[00011] The gas flow control system according to any of the preceding embodiments may further comprise a pressure gauge or pressure controller for controlling a pressure of the mixed gas. The gas flow control system is configured for example to supply the first portion of the first amount of the mixed gas to an area of the EUV lithographic apparatus in use comprising a reticle and to supply the second portion of the first amount of the mixed gas to an area of the EUV lithographic apparatus in use comprising a substrate. The first gas may comprise for example clean dry air (CDA) or extreme clean dry air (XCDA). The the second gas substantially consists for example of nitrogen (N2).

[00012] According to another aspect of the invention relates to an EUV lithographic apparatus comprising a gas flow control system according to any of the preceding embodiments. The EUV lithographic apparatus further comprises a vacuum compartment. The vacuum compartment comprises for example a first vacuum compartment for in use holding a reticle and a second vacuum compartment for holding a substrate when in use. BRIEF DESCRIPTION OF THE DRAWINGS

[00013] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[00014] Figure 1 is a schematic illustration of a lithographic system comprising a lithographic apparatus and a radiation source;

[00015] Figure 2 is a schematic illustration of a gas flow control system for providing oxygen to a lithographic apparatus.

DETAILED DESCRIPTION

[00016] Figure 1 is a schematic illustration of a lithographic system. 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, 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 patterning device 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.

[00017] 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 the 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.

[00018] The radiation source SO shown in Figure 1 is of a type that may be referred to as a laser produced plasma (LPP) source. A laser 1, which may for example be a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) that is provided from a fuel emitter 3. 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 3 may comprise a nozzle configured to direct tin, for example, in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of ions of the plasma.

[00019] The EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes referred to more generally as a normal incidence radiation collector). The collector 5 may have a multilayer structure that is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.

[00020] In other embodiments of a laser produced plasma (LPP) source the collector 5 may be a so- called grazing incidence collector that is configured to receive EUV radiation at grazing incidence angles and focus the EUV radiation at an intermediate focus. A grazing incidence collector may, for example, be a nested collector, comprising a plurality of grazing incidence reflectors. The grazing incidence reflectors may be disposed axially symmetrically around an optical axis O.

[00021] The radiation source SO may include one or more contamination traps (not shown). For example, a contamination trap may be located between the plasma formation region 4 and the radiation collector 5. The contamination trap may for example be a rotating foil trap, or may be any other suitable form of contamination trap.

[00022] The laser 1 may be separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser 1 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 1 and the radiation source SO may together be considered to be a radiation system.

[00023] Radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point 6 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 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source.

[00024] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. 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 (which may for example be a mask) 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.

[00025] 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 that are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The mirrors 13, 14 which form the projection system may be configured as reflective lens elements. 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).

[00026] The lithographic apparatus may, for example, be used in a scan mode, wherein the support structure (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a substrate W (i.e. a dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the demagnification and image reversal characteristics of the projection system PS. The patterned radiation beam that is incident upon the substrate W may comprise a band of radiation. The band of radiation may be referred to as an exposure slit. During a scanning exposure, the movement of the substrate table WT and the support structure MT may be such that the exposure slit travels over an exposure field of the substrate W.

[00027] The radiation source SO and/or the lithographic apparatus that is shown in Figure 1 may include components that are not illustrated. For example, a spectral filter may be provided in the radiation source SO. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[00028] In other embodiments of a lithographic system the radiation source SO may take other forms. For example, in alternative embodiments the radiation source SO may comprise one or more free electron lasers. The one or more free electron lasers may be configured to emit EUV radiation that may be provided to one or more lithographic apparatuses.

[00029] In a lithographic apparatus an illumination system conditions a beam of radiation and a patterned mask is illuminated with the conditioned beam of radiation. The patterned mask imparts a pattern to the conditioned radiation beam to form a patterned radiation beam. The patterned radiation beam is projected onto a substrate via a projection system in order to transfer the pattern onto the substrate. The dose of radiation that is provided to the substrate is an important consideration when performing a lithographic exposure. The dose of radiation reaching the substrate may vary over time due to unwanted absorption of EUV radiation. For example, surfaces within the lithographic apparatus may release one or more chemicals (e.g. silanes) into an internal environment of the lithographic apparatus. The presence of said chemicals may reduce a transmission of EUV radiation through the internal environment of the lithographic apparatus, thus reducing the dose of radiation provided to the substrate. In order to mitigate the reduced transmission of EUV radiation, a gas may be introduced to the lithographic apparatus. The gas may comprise oxygen (O2). The lithographic apparatus may, for example, be an extreme ultraviolet (EUV) lithographic apparatus. The present invention relates to a system for providing a controlled amount of gas to an apparatus such as a lithographic apparatus.

[00030] One method of accurately controlling the relatively small amount of oxygen that is to be provided to the lithographic apparatus includes mixing a first gas comprising oxygen (O2) gas with one or more other gases, the one or more other gases not containing oxygen (O2) gas, before introducing a controlled flow of the mixture of gases to the lithographic apparatus. For example, clean dry air, CD A, (e.g. extreme clean dry air, XCDA) may be mixed with a gas such as nitrogen gas or hydrogen gas so as to dilute the amount of oxygen (O2) gas in the clean dry air to a lower concentration. The mixture of gases may be such that it comprises a suitably low concentration of oxygen that is suitable for use in a lithographic apparatus. The mixture of gases may be distributed amongst one or more desired locations within the lithographic apparatus. The mixture of gases may be introduced to the lithographic apparatus using known mass flow controllers since the concentration of oxygen in the mixture of gases is suitably low. That is, the mixing of gases enables the provision of low partial pressures of oxygen gas proximate the optical components of the lithographic apparatus with suitable accuracy and stability.

[00031] A system 100 for providing a controlled amount of gas to an apparatus is described in Figure 2. Figure 2 schematically depicts a gas flow control system 100 for providing oxygen to a lithographic apparatus. A gas source 110 provides a flow 120 of an oxygen containing gas such as clean dry air or extreme clean dry air and another flow 130 of a gas which does not contain oxygen, e.g. a flow of nitrogen gas or hydrogen gas. A first mass flow controller 140 is used to control the flow 120 of extreme clean dry air and another mass flow controller 150 is used to control the flow 130 of the non-oxygen containing gas, e.g. the nitrogen gas or hydrogen gas. The gas flows 120 and 130 are combined and mixed after passing through the mass flow controllers 140, 150 to form a gaseous mixture 160 having a low concentration of oxygen.

[00032] A pressure gauge and/or pressure controller 170 may be provided to monitor and/or control a pressure of the gaseous mixture 160. The gaseous mixture 160 is then split between three separate branches 180.1, 180.2, 180.3. A first branch 180.1 comprises a valve 190 and a dump 200. The valve 190 is configured to control the flow of the gaseous mixture through the first branch 180.1 and the dump 200 is configured to receive any excess gaseous mixture that is not needed in an internal environment 210 of the lithographic apparatus. The second 180.2 and third 180.3 branches each comprise a mass flow controller 220, 230 and a valve 240, 250. The second 180.2 and third 180.3 branches are configured to supply a controlled amount of the gaseous mixture 160 having a low concentration of oxygen to different parts of the internal environment 210 of the lithographic apparatus. For example, the second branch 180.2 may provide a flow of the gaseous mixture 160 proximate the reticle and the third branch 180.3 may provide a flow of the gaseous mixture 160 proximate the substrate. A larger or smaller number of branches may be used. Alternative or additional branches may e.g. be applied to provide a flow of the gaseous mixture 160 to the illumination system IL or the projection system PS, in particular to one or more mirrors of those systems, e.g. facetted field mirror device 10 and facetted pupil mirror device 11, or mirrors 13, 14, as shown in Figure 1.

[00033] Another system 300 for providing a controlled amount of a gas to an apparatus is schematically shown in Figure 3. Figure 3 schematically depicts a gas flow control system 300 for providing oxygen to a lithographic apparatus. A gas source 310 provides a flow 320 of an oxygen containing gas such as clean dry air or extreme clean dry air and another flow 330 of a gas which does not contain oxygen, e.g. a flow of nitrogen gas or hydrogen gas. A first mass flow controller 340 is used to control the flow 320 of extreme clean dry air and another mass flow controller 350 is used to control the flow 330 of the non-oxygen containing gas, e.g. the nitrogen gas or hydrogen gas. In the embodiment as shown, the flow 330 of the non-oxygen containing gas is split in two branches 330.1 and 330.2. In the embodiment as shown, the gas flows 320 is combined, i.e. mixed, with the fraction or part 330.1 of the gas flow 330 to form a gaseous mixture 360 having a low concentration of oxygen.

[00034] A pressure gauge and/or pressure controller (not shown) may be provided to monitor and/or control a pressure of the gaseous mixture 360. The gaseous mixture 360 is then split between two branches 360.1 and 360.2. The first branch 360.1 comprises a valve 390 and a dump 400. The second branch comprises a mass flow controller 355. The valve 390 is configured to control the flow of the gaseous mixture through the first branch 360.1 and the dump 400 is configured to receive any excess gaseous mixture that is not needed in an internal environment 410 of the lithographic apparatus. The second branch 360.2 of the gaseous mixture 360 having the low concentration of oxygen is first all mixed with the fraction or part 330.2 of the gas flow 330, gas flow 330 being the non-oxygen containing gas. By doing so, a further dilution of the oxygen content of the branch 360.2 is obtained. The combined gas flow of branch 360.2 and branch 330.2, referred as branch 360.3, is then applied to provide a controlled amount of a gaseous mixture having a low concentration of oxygen to different parts of the internal environment 410 of the lithographic apparatus.

[00035] In the embodiment as shown, the gaseous mixture of branch 360.3 is split in a first branch 360.31 and a second branch 360.32. The first 360.31 and second 360.32 branches each comprise a mass flow controller 420, 430 and a valve 440, 450. Said branches thus contain a gaseous mixture having a low concentration of oxygen which can be applied to different parts of the internal environment 410 of the lithographic apparatus. For example, the first branch 360.31 may provide a flow of the gaseous mixture 360.3 proximate the reticle and the second branch 360.32 may provide a flow of the gaseous mixture 360.3 proximate the substrate. A larger or smaller number of branches may be used. Alternative or additional branches may e.g. be applied to provide a flow of the gaseous mixture 360.3 to the illumination system 1L or the projection system PS, in particular to one or more mirrors of those systems, e.g. facetted field mirror device 10 and facetted pupil mirror device 11, or mirrors 13, 14, as shown in Figure 1.

[00036] The gaseous mixture 160 may be provided to any desired part of the lithographic apparatus. The gas flow control system 100 may be operable to tune the gaseous mixture 160 in any desired way. The gas flow control system 100 may offer a greater range across which the contents of the gaseous mixture 160 may be tuned. For example, known systems may be able to vary concentrations of gas within the gaseous mixture by a factor of about 10 whereas the gas flow control system 100 disclosed herein may be able to vary concentrations of gas within the gaseous mixture by a factor of about 1000. The gas flow control system is versatile and may be used to control any desired mixture of gases. The gas flow control system is more accurate, reliable and has a longer operational lifetime than known systems.

[00037] 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.

[00038] The term "EUV radiation" may be considered to encompass electromagnetic radiation having a wavelength within the range of 4-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 4-10 nm such as 6.7 nm or 6.8 nm.

[00039] 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.

[00040] 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 set out below.