Lithographic apparatus

A lithographic apparatus has: a conduit through which a gas can flow; a gas mover configured to cause the gas to flow in the conduit; a wall in contact with the gas in the conduit and defining a membrane aperture therein; and an acoustic filter including a flexible membrane fixed in the membrane aperture. The acoustic filter reduces transmission of acoustic disturbances without adding any flow resistance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase entry of PCT patent application no. PCT/EP2016/056758, which was filed on Mar. 29, 2016, which claims the benefit of priority of EP application no. 15166563.5, which was filed on May 6, 2015, and which applications are incorporated herein in its entirety by reference.

BACKGROUND

Field of the Invention

The present invention relates to a lithographic apparatus.

Description of the Related Art

In a lithographic apparatus, acoustic disturbances, e.g. noise, can cause imaging errors because the mask, projection system or substrate are momentarily displaced from their optimal positions due to the noise or because the noise causes an error in a measurement. There are many sources of noise within a lithographic apparatus, for example the movements of components of the apparatus—such as the mask table and wafer table—and the movements of fluids—for example immersion liquid, purge gas and temperature conditioning gas. As well as efforts to reduce the sensitivity of the lithographic apparatus to noise, measures to reduce the generation of noise at source have been made. However, the desire to further reduce the sizes of imaged features and to increase throughput mean that further measures to reduce the effects of noise in a lithographic apparatus are desirable.

SUMMARY

It is desirable to provide an approach to the mitigation or amelioration of low frequency pressure pulses in a lithographic apparatus.

According to an aspect of the invention, there is provided a lithographic apparatus configured for imaging a pattern onto a substrate, the apparatus comprising:

a conduit through which a gas can flow;

a gas mover configured to cause the gas to flow in the conduit;

a wall in contact with the gas in the conduit and defining a membrane aperture therein; and

an acoustic filter comprising a flexible membrane fixed in the membrane aperture.

DETAILED DESCRIPTION

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables or “substrate supports” (and/or two or more mask tables or “mask supports”). In such “multiple stage” machines the additional tables or supports may be used in parallel, or preparatory steps may be carried out on one or more tables or supports while one or more other tables or supports are being used for exposure.

The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the mask support structure (e.g., mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning device PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B.

Similarly, the first positioning device PM and another position sensor (which is not explicitly depicted inFIG.1) can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioning device PM. Similarly, movement of the substrate table WT or “substrate support” may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW.

In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.

In a lithographic apparatus, a purge gas (e.g. nitrogen or pressurized and/or filtered or purified air) is used to ensure a consistent environment for radiation beams and for temperature conditioning. For example, the projection system may be continually flushed with purge gas at high rates of flow in order to ensure that refractive optical elements, e.g. lenses, are maintained at a constant temperature. High flow rates are necessary because the refractive elements absorb energy from the projection beam. The absorption of energy from the beam is not uniform and resulting temperature changes can change the shape of a refractive element, leading to imaging errors. In some lithographic apparatus this phenomenon is employed to effect wavefront corrections.

A wavefront corrector comprises a transparent plate in the beam path and is provided with an array of individually controllable heaters. By selectively heating different parts of the transparent plate, it can be controllably distorted to introduce desired corrections to the wavefront of the projection beam. It is desirable that the average temperature of the wavefront corrector does not deviate from the target temperature of the remainder of the projection system. Therefore a temperature conditioning gas (e.g. nitrogen or pressurized and/or filtered or purified air) is caused to flow over or past the plane plate to remove the heat imparted by the heaters. A high flow rate of the temperature conditioning gas may be required.

The present inventors have determined that the high rate of flow of temperature conditioning gas required to cool the wavelength corrector is a significant source of noise within the lithographic apparatus. In particular, in order to achieve the high flow rates required without a large pressure different—which might distort components of the projection system—it is necessary to provide an active exhaust for the temperature conditioning gas. Thus, a gas mover, e.g. a fan, is provided in the gas conduit downstream of the wavefront converter. The gas mover generates noise which can disturb the lithographic apparatus and cause imaging errors.

The present inventors therefore propose to include an acoustic filter in or adjacent to the gas flow path between a noise source, e.g. a gas mover, and a functional subsystem that is cooled by the gas flow but is sensitive to noise, e.g. the wavefront converter.

An example of a suitable acoustic filter100is depicted schematically inFIG.2. The acoustic filter100is provided in line in the gas flow conduit200. Acoustic filter100comprises a flexible membrane101which forms a part of the wall of the gas flow conduit200and filter walls102which define a cavity103behind membrane101. Desirably, in its rest state flexible membrane101has the same cross sectional shape and area as the walls of conduit200. Therefore, acoustic filter100does not impart any additional resistance to the mass flow of gas through gas flow conduit200. However, acoustic filter100presents an acoustic impedance to acoustic disturbances propagating along the conduit200.

Pressure variations in the gas in conduit200due to the acoustic disturbances induce vibrations in the flexible membrane101and the acoustic impedance is caused by the interactions between the vibrating flexible membrane101and the gas flowing in the conduit. Some acoustic energy is absorbed and dissipated in the flexible membrane101but a greater effect is that the abrupt change in acoustic impedance between the relatively rigid walls of gas conduit200and the flexible membrane101causes propagating acoustic disturbances to be reflected back towards their source. This is illustrated inFIG.2showing acoustic disturbance propagating from the left of the figure. Some disturbance is transmitted and some disturbance is reflected back to the source.

The reflection, transmission and absorption coefficients of acoustic filter100depend on its resonant frequencies. In an embodiment, the acoustic filter has a resonant frequency in the range of from 10 Hz to 1000 Hz, preferably 10 Hz to 100 Hz or 200 Hz to 500 Hz.

The resonant frequencies of the acoustic filter depend on several properties thereof, for example: the length L of flexible membrane101; the width h of conduit200; the width h_c of cavity103; the thickness, density and elastic modulus of flexible membrane101; the tension of flexible membrane101; and the pressure and density of filter gas (e.g. nitrogen or pressurized and/or filtered or purified air) in cavity103. In an embodiment the membrane has a length parallel to the direction of flow of the gas in the range of from 10 mm to 3 m, preferably 50 mm to 2 m.

FIG.3depicts the effect of varying one of these parameters, specifically the tension in the membrane, in an acoustic filter having a length L of 700 mm in a conduit of diameter h=100 mm and with a cavity height h_c of 20 mm. The three graphs inFIG.3depict transmission loss TL in decibels as a function of non-dimensional frequency f. Non-dimensional frequency is defined as f=ƒ·h/c0, where ƒ is frequency in Hz, h is the duct height in m and c0is the speed of sound in air. It can be seen that with zero tension in the membrane (top graph) there is relatively little transmission loss (less than 3 dB) and the frequency dependence of the transmission loss is quite gentle. When a tension T of 650 N (middle graph) or 750 N (bottom graph) is applied to the membrane, a number of sharp peaks appear in the transmission loss TL, with peak values exceeding 30 dB. With increasing tension, additional transmission loss peaks appear at higher frequencies.

FIG.4depicts a practical embodiment of acoustic filters in the temperature conditioning system for a wavefront corrector201. The wavefront corrector201is located in the projection system PS of a lithographic apparatus and comprises a plane plate provided with a plurality of individually controllable heaters disposed across its surface. The heaters, e.g. electrical resistive heaters, are selectively energized to locally heat the plane plate so as to create a desired shape change of the plane plate due to thermal expansion of the material thereof. The shape change of the plane plate is calculated to provide a desired wavefront correction for the projection beam. The wavefront correction may, for example, be to compensate for lens heating effects elsewhere in the projection system PS. To effect the desired shape change of the plane plate and to avoid introducing a thermal disturbance into the remainder of the projection system PS, it is desirable that the average temperature of the plane plate does not deviate too far from the target temperature at which the remainder of the projection system PS is maintained. Therefore, a flow of temperature conditioning gas (e.g. nitrogen or pressurized and/or filtered or purified air) over the plane plate is provided. The flow of temperature conditioning gas desirably has a high flow rate in order to transfer away the heat generated in the wavefront converter.

The gas flow conduit200which guides the gas over wavelength converter201has a supply side202and an exhaust side204. The gas is supplied to the supply side under pressure. An orifice plate203is positioned between the supply side of the conduit and the wavelength converter201. Orifice plate203introduces a flow restriction to create a pressure drop by so that the temperature conditioning gas flowing over wavefront corrector201is at a low pressure. In the exhaust side204a gas mover205e.g. a fan, is provided to maintain a flow of gas away from the wavefront corrector201in spite of the low gas pressure. Acoustic filter100ais provided in the exhaust section204between wavefront corrector201(which is an example of a functional subsystem) and gas mover205.

Acoustic filter100acomprises flexible membrane101aand filter walls102awhich define filter cavity103a. In embodiments described below, a lower case suffix letter in a reference for a component indicates that the respective component belongs to an acoustic filter referenced by a reference having the same lower case suffix letter.

The dimensions and other parameters of acoustic filter100aare selected so that acoustic filter100aexhibits a low transmission coefficient and a high reflection coefficient to frequencies of acoustic disturbances that are generated by gas mover205and to which wavefront corrector201, or another nearby functional subsystem, is sensitive. Multiple acoustic filters can be provided between gas mover205and wavefront converter201if desirable to ensure that wavefront converter201is protected from all undesirable frequencies of acoustic disturbances.

A second acoustic filter100bis provided on the supply side202of the conduit200. Again, acoustic filter100bcomprises flexible membrane101band filter walls102bdefining filter cavity103b. Acoustic filter100bhas its dimensions and other parameters selected so as to reduce transmission to wavefront corrector201, and/or any other nearby functional subsystems, of noise generated in the gas supply and in the upstream gas supply path. Whilst a gas mover may generate most noise at particular frequencies, flow noise is likely to be spread more uniformly across a range of frequencies. Although an acoustic filter according to an embodiment of the present invention provides peak transmission losses at relatively narrow frequency bands, multiple acoustic filters can be deployed in series so that the total transmission loss extends across a wider range of frequencies. Alternatively or in addition, the acoustic filter(s) can be selected to provide a high transmission loss at certain frequencies to which the functional subsystem(s) being protected is(are) most sensitive.

It will be noted that an acoustic filter according to an embodiment of the present invention can be used both when the direction of propagation of acoustic disturbances is with the gas flow direction and when the direction of propagation of acoustic disturbances is against the gas flow direction.

Another advantage of an acoustic filter according to the present invention is that it does not generate flow noise in itself. Conventional mufflers involving baffles can generate significant flow noise.

Another advantage of an acoustic filter according to the present invention is that it causes no or minimal flow resistance.

Another advantage of an acoustic filter according to the present invention is that it does not introduce a contamination risk since the filter cavity103is sealed from the conduit by flexible membrane101which can readily be made of cleanroom-compatible materials. Conventional mufflers involving fibrous material, such as wool, can introduce a contamination risk.

An acoustic filter according to an embodiment of the present invention can be used anywhere in a lithographic apparatus where there is a gas flow. The example of the flow of temperature conditioning gas for a wavefront corrector described above is especially advantageous because of the high gas flow rates involved. Other parts of a lithographic apparatus that might involve high gas flows and to which the present invention is particularly applicable include: air mounts (gas bearings), purge gas flows for optical systems, gas showers and wafer load/unload locks.

FIG.5depicts a further acoustic filter100cwhich can have its resonant behavior, and hence transmission profile adjusted. Acoustic filter100cis provided with an adjuster comprising a clamp member105which is movable in a direction parallel to the gas flow direction, i.e. along the length of membrane101c. In effect, clamp105divides membrane101cinto two sub-membranes101c-1,101c-2having respective lengths L1and L2. Each of the sub-membranes101c-1,101c-2will have its own resonant frequencies determined by the respective lengths L1and L2and the other parameters of the acoustic filter. The transmission loss of acoustic filter100cis substantially equivalent to the sum of the transmission losses of two separate acoustic filters having membranes of lengths L1and L2respectively.

Clamp member105can be configured so that it is adjustable in set up of the apparatus or provided with an actuator so that it is adjustable during operation of the lithographic apparatus. Clamp105can be fixedly connected to the membrane101so that moving it in the direction parallel to the gas flow changes the respective tensions in the sub-membranes101c-1,101c-2. Alternatively, clamp105can be arranged so that it slides relative to membrane101cin which case the resonant behavior of the acoustic filter100cis changed by varying the lengths L1, L2of the two sub-membranes101c-1,101c-2. Clamp105can also be configured so that it allows only vibrational modes of the membrane that have a node at the clamp position.

In an embodiment of the present invention an acoustic filter can be provided with an adjuster comprising multiple clamps spaced along the flexible membrane101. Alternatively or in addition, clamp105can be configured as a telescoping member inside or outside of flexible membrane101so that by changing the length as well as the position of the telescoping clamp, independent control of lengths L1, L2can be achieved. In an embodiment clamp105is configured as a telescoping member extending inward parallel to the flow direction from one end of acoustic filter100so that the free length of membrane101can be adjusted without creation of a second sub-membrane.

FIG.6depicts another acoustic filter100daccording to an embodiment of the present invention that is adjustable in use. A pressure adjuster, e.g. comprising a gas supply106and valve107, is provided to enable the pressure of the gas in cavity103d. Changing the pressure in the cavity103dalters the resonant behavior of the acoustic filter and therefore the frequency dependence of the transmission loss, e.g. through changing the tension in flexible membrane101d.

FIG.7depicts another acoustic filter100eaccording to an embodiment of the present invention that is adjustable in use. In acoustic filter100e, an adjuster comprises movable piston107which is connected to actuator108and forms at least a part of one of the filter walls102e. The resonant properties, and therefore the frequency dependence of the transmission loss, of acoustic filter101ecan be varied by moving piston107so as to change the effective height of cavity103eand/or the pressure of gas in cavity103e.

FIG.8depicts another acoustic filter100faccording to an embodiment of the present invention that is adjustable in use. Acoustic filter100fhas an adjuster comprising a magnetic member109fixed to membrane101fand an electromagnet110driven by drive circuit111, which is provided within cavity103fnear magnetic member109. Magnetic member109may be a permanent magnet or an unmagnetised but magnetic, e.g. ferromagnetic, material. By varying the current through electromagnet110, magnetic member109can be attracted and/or repelled thus exerting a controllable local force on membrane101f. The resonant properties thereof are thereby adjusted and hence the frequency dependence of the transmission loss of the acoustic filter100f. Acoustic filter100fmay be provided with a plurality of magnetic members109spaced apart on the membrane101falong with respective independently controllable electromagnets110.

In a particularly simple variant of the embodiment ofFIG.8, magnetic member109is a permanent magnet and electromagnet110is provided with a switch or switches so that the coils thereof can be short circuited or varied in effective length. In this way, a controllable damping effect can be created by varying the impedance of the coil and hence back electro-magnetic forces exerted on magnet member109when it vibrates.

FIG.9depicts another acoustic filter100gaccording to an embodiment of the present invention that is adjustable in use. In acoustic filter100gthe adjuster comprises a part of cavity wall102g, which is constructed as a bellows112, and an actuator113, which is provided to move the cavity wall102gso as to change the effective volume of cavity103gand/or the effective gas pressure therein.

It will be appreciated that a lithographic apparatus according to an embodiment of the present invention can include multiple acoustic filters according to one or more of the different variants described above. Also, a single acoustic filter can be provided with multiple adjusters according to the different principles described above for adjusting the properties thereof.

An adjustable acoustic filter according to an embodiment of the invention can be configured to be adjustable at the time of construction and/or servicing or calibration. An adjuster of an acoustic filter according to an embodiment of the present invention can be configured to be adjustable during use of the lithographic apparatus for the exposure of substrates, e.g. in synchronism with other events occurring in the apparatus. For example, an acoustic filter according to an embodiment of the present invention can be adjusted in synchronism with, or in response to, changes in gas flow rate in the protected conduit and/or changes in speed of a gas mover such as a fan.

An acoustic filter100according to an embodiment of the invention may take various forms in cross-section, some examples of which are illustrated inFIGS.10to13.

In its simplest form, the membrane101atakes the form of a hollow tube or cylinder having the same diameter as the conduit200and concentric therewith. This ensures that no additional flow resistance is imparted. The filter walls102acan also be cylindrical in form and concentric with the membrane101a. Such an arrangement is simple to manufacture and provides a cavity with constant height h_c so that the acoustic behavior of the acoustic filter is simple and readily predictable.

An alternative acoustic filter100his illustrated inFIG.11. The flexible membrane101his again cylindrical and concentric with the conduit200. However, cavity walls102hform a cuboid, which may be square in cross-section perpendicular to the direction of flow of gas in the conduit. Such an arrangement can be simple to manufacture and can make most effective use of available volume within the lithographic apparatus. However, the acoustic behavior of the acoustic filter is more complex due to the variation in effective height h_c of the cavity. Such complex behavior can be advantageous in providing a broader peak of transmission loss.

A further alternative acoustic filter100iis depicted inFIG.12. This arrangement is suitable for use where conduit200is rectangular, e.g. square in cross section. Membrane101iis made up of four flat membranes101i-1,101i-2,101i-3,101i-4which are joined at longitudinal edges to form a square prism structure that preferably matches in shape and size the conduit200. Cavity walls102i-1to102i-4are provided to form respective cavities behind each flat membrane101i. The flat membranes101ican be anchored to the cavity walls102iat the edges. Such an arrangement can provide predictable behavior of the acoustic filter and allow separate optimization of the individual parts thereof.

Another acoustic filter100jis depicted inFIG.13. This again is suitable for use with a conduit having a rectangular cross section. Similarly to the embodiment ofFIG.12, membrane100jis made up for four separate flat membranes100j-1to100j-4. These are anchored to support members114along their longitudinal edges to form a sealed flow path. Cavity walls102jtake the form of a cuboid completely surrounding the membrane101jand support members114to form a single cavity around the flexible membrane101j. Such an arrangement can be simple to manufacture and can make most effective use of available volume within the lithographic apparatus.

FIG.14depicts a conventional orifice plate203that is usable as a flow restrictor in the supply side of a conduit for a flow of temperature conditioning gas used to cool a wavefront corrector. Orifice plate203consists of a simple plate with a central circular aperture203a. The size of aperture203ais determined to impart a desired flow resistance.

The present inventors have determined that the orifice plate can be improved by incorporation of an acoustic filter therein.

FIG.15depicts an orifice plate210according to an embodiment of the invention that incorporates an acoustic filter. Orifice plate210comprises a plate211that has a flow restriction aperture212equivalent to the aperture203aof the conventional orifice plate. The size of flow restriction aperture212is determined to impart a desired flow resistance. In addition a plurality of filter apertures are provided in plate211, each filter aperture being sealed by a flexible membrane213. A mass may be provided on each flexible membrane213, e.g. in the center thereof.

Acoustic disturbances propagating towards orifice plate210excite flexible membranes213to vibrate. Flexible membranes213can exhibit different vibrational modes. In some vibrational modes, the average displacement of the flexible membrane is zero, i.e. different parts of the flexible membrane vibrate out-of-phase with each other. At frequencies corresponding to these vibrational modes, the orifice plate presents a very low transmission Tr and a very high reflectance R. At other frequencies the orifice plate may have a high transmission Tr and a low reflectance R. This is illustrated inFIG.16which depicts measured transmission Tr (solid line) and reflectance R (dot chain line) as a function of frequency F. For comparison the transmission Tr of a conventional orifice plate is also shown (dashed line).

The frequencies of the transmission minima of an orifice plate210can be selected by selecting the parameters of the flexible membranes213, for example tension, thickness, elastic modulus and the mass of any attached mass. In an embodiment of the invention an adjuster is provided to adjust a parameter of one or more of the flexible membranes. The adjuster can operate according to any of the principles described above. An orifice plate211can be configured to have multiple transmission minima, e.g. by using different membranes in different filter apertures.FIG.17shows a measured transmission coefficient Tr for a dual frequency orifice plate (solid line) along with the transmission Tr of a conventional orifice plate (dashed line) for comparison.

In an embodiment of the invention, the orifice plate211is configured to have one or more transmission minima at a frequency that is either prevalent in the lithographic apparatus or to which a functional subsystem to be protected is particularly sensitive.

FIGS.18to24depict several variants of the orifice plate211, illustrating various changes that can be made to achieve a desired frequency dependence.

FIG.18depicts a simple orifice plate210awith four equivalent flexible membranes213aspaced evenly around the flow restriction aperture212.

FIG.19depicts an orifice plate210bwith four flexible membranes213bspaced evenly around the flow restriction aperture212. Two flexible membranes213b-1are small and two flexible membranes213b-2are large. Orifice plate210bis a dual frequency orifice plate.

FIG.20depicts an orifice plate with six equivalent flexible membranes213cspaced evenly around the flow restriction aperture212. Compared to orifice plate210a. orifice plate210cmay have a lower transmission at the transmission minima.

FIG.21depicts an orifice plate210dwith four flexible membranes213dspaced evenly around the flow restriction aperture212. Two flexible membranes213d-1have a different membrane material and/or tension and/or a different attached mass than the other two flexible membranes213d-2. Orifice plate210bis a dual frequency orifice plate.

FIG.22depicts an orifice plate210ewith four equivalent flexible membranes213espaced evenly around the flow restriction aperture212. Flexible membranes213eare rectangular in shape and may demonstrate a more complex frequency dependence than circular membranes.

FIG.23depicts an orifice plate210fwith four equivalent flexible membranes213fspaced evenly around the flow restriction aperture212. Flexible membranes213fare positioned closer to the flow restriction aperture than are the flexible membranes213ain orifice plate210a. The location of the flexible membrane can be a useful parameter to optimize for performance and/or constructional reasons.

FIG.24depicts an orifice plate210gwith four equivalent flexible membranes213gspaced evenly around the center thereof. The single centrally-located flow restriction aperture212of previously described embodiments is replaced by a plurality of smaller flow restriction apertures214spaced apart in between the flexible membranes213g. A plurality of small flow restriction apertures can generate less flow noise than a single larger flow restriction aperture of equivalent effect.

In an embodiment of the invention a plurality of orifice plates incorporating acoustic filters can be provided in series in a gas flow path. The total transmission loss of such a series is substantially equal to the sum of the transmission losses of each orifice plate, provided a space is provided between adjacent orifice plates. A broadband filtering effect can thereby be achieved.

In an embodiment, an orifice plate incorporating an acoustic filter is used with one or more acoustic filters of the type described above with reference toFIGS.2to13. In an embodiment, an orifice plate incorporating an acoustic filter is used without an acoustic filter of that type.

In an embodiment, there is provided a lithographic apparatus configured for imaging a pattern onto a substrate, the apparatus comprising: a conduit through which a gas can flow; a gas mover configured to cause the gas to flow in the conduit; a wall in contact with the gas in the conduit and defining a membrane aperture therein; and an acoustic filter comprising a flexible membrane fixed in the membrane aperture.

In an embodiment, the acoustic filter has a resonant frequency in the range of from 10 Hz to 1000 Hz, desirably in the range of from 10 Hz to 100 Hz or in the range of from 200 Hz to 500 Hz. In an embodiment, the acoustic filter further comprises an adjuster configured to adjust a resonant frequency of the flexible membrane. In an embodiment, the adjuster is configured to adjust a tension of the membrane. In an embodiment, the adjuster is configured to adjust a free length of the membrane. In an embodiment, the adjuster is configured to adjust a position of a node of the membrane. In an embodiment, the adjuster comprises a magnetic member mounted on the membrane and a magnetic field generator. In an embodiment, the wall is a side wall of the conduit and the acoustic filter further comprises a filter wall defining a cavity adjacent the flexible membrane and outside the conduit. In an embodiment, the lithographic apparatus further comprises a pressure adjuster configured to adjust a pressure of gas in the cavity. In an embodiment, the membrane extends substantially completely around the conduit. In an embodiment, the membrane has a length parallel to the direction of flow of the gas in the range of from 10 mm to 3 m, desirably in the range of from 50 mm to 2 m. In an embodiment, the wall extends across the conduit and has a flow aperture configured to allow the gas to flow therethrough. In an embodiment, the wall has a plurality of membrane apertures therein and a corresponding plurality of membranes, each membrane being fixed in a respective one of the membrane apertures. In an embodiment, a first one of the plurality of membranes differs from a second one of the plurality of membranes in at least one parameter selected from the group consisting of: size, tension, density, modulus of elasticity, and shape. In an embodiment, the lithographic apparatus comprises a plurality of acoustic filters as described herein. In an embodiment, the lithographic apparatus further comprises a functional subsystem, wherein the gas flows past or through the functional subsystem to control the temperature of the functional subsystem and the acoustic filter is located between the functional system and the gas mover. In an embodiment, the gas mover causes the gas to move away from the functional system towards the gas mover. In an embodiment, the functional subsystem is a wavefront adjuster having a selectively heatable plane plate and wherein the gas flowing in the conduit cools the selectively heatable plane plate. In an embodiment, the functional subsystem is an optical system for imaging the pattern on the substrate. In an embodiment, the functional subsystem is an alignment system.

Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In a lithographic apparatus that employs very short wavelength radiation to expose substrates, parts of the lithographic apparatus traversed by the radiation beam, e.g. the substrate stage compartment, may be filled with a low pressure of gas, e.g. hydrogen or helium, so as to minimize absorption of the very short wavelength radiation. The low pressure may be referred to as a “vacuum” environment but the present invention is applicable if the gas pressure in a part of the lithographic apparatus is sufficient to transmit acoustic disturbances.