Lithographic apparatus and device manufacturing method

Field

0001The present invention relates to a lithographic apparatus and a method for manufacturing a device.

Background

0002A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the "scanning"-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

0003It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective NA of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein.

0004However, submersing the substrate or substrate and substrate table in a bath of liquid (see for example <patcit id="pcit0001" dnum="US4509852A"><text>US 4,509,852</text></patcit> ) means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.

0005One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in <patcit id="pcit0002" dnum="WO9949504A"><text>WO 99/49504</text></patcit>. As illustrated in <figref idref="f0002">Figures 2 and 3</figref>, liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a -X direction, liquid is supplied at the +X side of the element and taken up at the -X side. <figref idref="f0002">Figure 2</figref> shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of <figref idref="f0002">Figure 2</figref> the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible, one example is illustrated in <figref idref="f0002">Figure 3</figref> in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.

0006Another solution which has been proposed is to provide the liquid supply system with a seal member which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. Such a solution is illustrated in <figref idref="f0002">Figure 4</figref>. The seal member is substantially stationary relative to the projection system in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis). A seal is formed between the seal member and the surface of the substrate. Preferably the seal is a contactless seal such as a gas seal. Such as system with a gas seal is disclosed in <patcit id="pcit0003" dnum="EP03252955A"><text>European Patent Application No. 03252955.4</text></patcit>.

0007In <patcit id="pcit0004" dnum="EP1420300A"><text>European Patent Application EP1420300</text></patcit> the idea of a twin or dual stage immersion lithography apparatus is disclosed. Such an apparatus is provided with two stages for supporting the substrate. Leveling measurements are carried out with a stage at a first position, without immersion liquid, and exposure is carried out with a stage at a second position, where immersion liquid is present. Alternatively, the apparatus has only one stage.

0008<patcit id="pcit0005" dnum="EP0605103A1"><text>EP-A1-0,605,103</text></patcit> discloses an immersion lithographic apparatus in which a plane parallel plate is covered in a material with a high affinity to the immersion liquid to reduce the chances of bubbles forming on the plate. <patcit id="pcit0006" dnum="WO2004053956A"><text>WO 2004/053956</text></patcit> says that the final element may be coated in a hydrophilic or hydrophobic material, dependant on desired function. <patcit id="pcit0007" dnum="WO03077034A"><text>WO 03/077034</text></patcit> discloses an organic coating for a CaF₂ lens.

0009The use of immersion liquid in the space between the final element of the projection system and the substrate means that the final element of the projection system (e.g. an 'abschlussplatte' which seals the projection system, or the final optical element of the projection system) and substrate table are in contact with the immersion liquid. This can lead to problems with reaction or dissolution in the immersion liquid of the components of the projection system or substrate table.

0010<patcit id="pcit0008" dnum="EP1429190A2"><text>EP-A2-1,429,190</text></patcit> discloses an immersion lithographic apparatus wherein the final element of the projection system has a protective material or a surface in contact with the immersion liquid.

0011<patcit id="pcit0009" dnum="WO2004053959A"><text>WO 2004/053959</text></patcit> discloses an immersion lithographic apparatus wherein the final element of the projection system has a SiO₂ layer or its surface in contact with said immersion liquid.

0012<patcit id="pcit0010" dnum="EP1670038A"><text>EP 1670038</text></patcit> which belongs to the prior art according to Article 54(3) EPC discloses an immersion lithographic apparatus wherein the final element of the projection system has a fused silica thin plate attached to the final element by contact bonding without glue.

SUMMARY

0013It is desirable to provide a lithographic projection apparatus in which degradation of components, because of contact with immersion liquid, is reduced.

0014According to an aspect of the invention, there is provided a lithographic projection apparatus according to claim 1.

0015In this way the final element of the projection system may be made of a material which is selected due to superior optical properties and considerations regarding the activity between the material of the element and the immersion liquid do not need to be taken into count.

0016According to an aspect of the invention, there is provided a device manufacturing method according to claim 5.

BRIEF DESCRIPTION OF THE DRAWINGS

0017Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: <ul id="ul0001" list-style="none" compact="compact"><li>- <figref idref="f0001">Figure 1</figref> depicts a lithographic apparatus;</li><li>- <figref idref="f0002">Figures 2 and 3</figref> depict a liquid supply system used in a prior art lithographic projection apparatus;</li><li>- <figref idref="f0002">Figure 4</figref> depicts a liquid supply system according to another prior art lithographic projection apparatus;</li><li>- <figref idref="f0003">Figure 5</figref> depicts another liquid containment system;</li><li><figref idref="f0003">Figure 6</figref> depicts a final element of the projection system with a protective coating;</li><li><figref idref="f0004">Figure 7</figref> depicts a protective plate applied to the final element of a projection system according to the invention;</li><li><figref idref="f0004">Figure 8</figref> depicts a protective plate and liquid layer applied to the final element of a projection system; and</li><li><figref idref="f0004">Figure 9</figref> depicts a two layer protective coating applied to the final element of a projection system.</li></ul>

DETAILED DESCRIPTION

0018<figref idref="f0001">Figure 1</figref> schematically depicts a lithographic apparatus. The apparatus comprises: <ul id="ul0002" list-style="dash" compact="compact"><li>an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or DUV radiation).</li><li>a support structure (e.g. a mask table) MT constructed to support <u>a</u> patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters;</li><li>a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and</li><li>a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.</li></ul>

0019The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

0020The support structure supports, i.e. bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device."

0021The term "patterning device" used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

0022The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

0023The term "projection system" used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered as synonymous with the more general term "projection system".

0024As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

0025The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

0026Referring to <figref idref="f0001">Figure 1</figref>, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

0027The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

0028The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the 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 positioner 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 positioner PM and another position sensor (which is not explicitly depicted in <figref idref="f0001">Figure 1</figref>) 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 positioner PM. Similarly, movement of the substrate table WT 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, M2 and 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.

0029The depicted apparatus could be used in at least one of the following modes: <ol id="ol0001" compact="compact"><li>1. In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.</li><li>2. In scan mode, the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.</li><li>3. In another mode, the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.</li></ol>

0030Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

0031<figref idref="f0003">Figure 5</figref> shows a liquid reservoir 10 between the projection system PL and a substrate W which is positioned on the substrate stage WT. The liquid reservoir 10 is filled with a liquid 11 having a relatively high refractive index, e.g. water, provided via inlet/outlet ducts 13. The liquid has the effect that the radiation of the projection beam is a shorter wavelength in the liquid than in air or in a vacuum, allowing smaller features to be resolved. It is well known that the resolution limit of a projection system is determined, inter alia, by the wavelength of the projection beam and the numerical aperture of the system. The presence of the liquid may also be regarded as increasing the effective numerical aperture. Furthermore, at fixed numerical aperture, the liquid is effective to increase the depth of field.

0032The reservoir 10 forms a preferably contactless seal to the substrate W around the image field of the projection lens PL so that the liquid is confined to fill the space between the substrate's primary surface, which faces the projection system PL, and the final element (e.g. an 'abschlussplatte' which seals the projection system, or the final optical element of the projection system) of the projection system PL. The reservoir is formed by a seal member 12 positioned below and surrounding the final element of the projection lens PL. Thus, the liquid containment system LCS provides liquid on only a localized area of the substrate. The seal member 12 forms part of the liquid containment system LCS for filling the space between the final element of the projection system and the sensor 10 (or substrate W) with a liquid. This liquid is brought into the space below the projection lens and within the seal member 12. The seal member 12 extends a little above the bottom element of the projection lens and the liquid rises above the final element so that a buffer of liquid is provided. The seal member 12 has an inner periphery that at the upper end closely conforms to the shape of the projection system or the final element thereof and may, e.g. be round. At the bottom the inner periphery forms an aperture which closely conforms to the shape of the image field, e.g. rectangular, though this is not necessarily so. The projection beam passes through this aperture.

0033The liquid 11 is confined in the reservoir 10 by a seal device 16. As illustrated in <figref idref="f0003">Figure 5</figref>, the seal device is a contactless seal, i.e. a gas seal. The gas seal is formed by gas, e.g. air or synthetic air, provided under pressure via inlet 15 to the gap between seal member 12 and substrate W and extracted by first outlet 14. The over pressure on the gas inlet 15, vacuum level on the first outlet 14 and the geometry of the gap are arranged so that there is a high-velocity air flow inwards towards the optical axis of the apparatus that confines the liquid 11. As with any seal, some liquid is likely to escape, for example up the first outlet 14.

0034<figref idref="f0002">Figures 2 and 3</figref> also depict a liquid reservoir defined by inlet(s) IN, outlet(s) OUT, the substrate W and the final element of projection lens PL. Like the liquid containment system of <figref idref="f0003">Figure 5</figref> the liquid containment system illustrated in <figref idref="f0002">Figures 2 and 3</figref>, comprising inlet(s) IN and outlet(s) OUT, supplies liquid to a space between the final element of the projection system and a localized area of the primary surface of the substrate.

0035Both of the liquid containment systems LCS of <figref idref="f0002">Figures 2 &amp; 3 and Figure 4</figref> as well as other solutions, such as a bath in which the substrate W or whole substrate table WT is immersed, can be used with the present invention described below.

0036<figref idref="f0003">Figure 6</figref> illustrates in detail the final element 20 of the projection system PL. In the example illustrated in <figref idref="f0003">Figure 6</figref> the final element is a final optical element 20 which comprises a first component 25 and a second component 27. The final element 20 of the projection system PL is comprised of first and second components 25, 27 so that the element may be made of a material which exhibits birefringence. A preferred material for irradiation at 157 nm is CaF2 which is transmissive but exhibits birefringence properties at this wavelength. Quartz is barely transmissive at 157nm. CaF2 is also preferred for 193nm although quartz can also be used at this wavelength. However, quartz lenses suffer from compaction at these wavelengths which can cause radiation to be focused on to small bits of the lens which discolor (go dark) and absorb more heat and so a channel can get cut.

0037By providing the final element 20 as first and second components or parts, the birefringence exhibited by CaF2 at 157 nm can be compensated for by ensuring that the crystal orientations of the first and second components are aligned such that the intrinsic birefringence exhibited by the first component 25 is cancelled by the intrinsic birefringence exhibited by the second component 27. In this way, the projection PB which passes first through the first component 25 and then through the second component 27 exits the second component 27 substantially free of birefringence phenomena.

0038The remaining optical elements of the projection system PL may be comprised of materials other than CaF2. The intensity of the projection beam is highest at the last element which is also the smallest so that it is this element which is most likely to suffer from compaction if made of quartz.

0039As illustrated in <figref idref="f0003">Figure 6</figref>, the final element 20 of the projection system PL is substantially hemispherical in shape. Thus, the second component 27 is in the shape of a hemisphere and is positioned in a recess of the first component 25 which has an outer surface of the shape of a hemisphere with a recess in its non-curved surface.

0040It is has been found that CaF2 may dissolve or react with immersion liquid 11 used in an immersion liquid lithographic projection apparatus. Presently the immersion liquid is envisaged as comprising substantially water for 248 and 193nm. For 157nm, perfluouro-hydrocarbons are envisaged.

0041One way of protecting the final element 20 of the projection system from attack by the immersion liquid 11 (which is not in accordance with the present invention) is to provide a protective coating 40 on a surface of the final element 20 which is in contact with the immersion liquid. The material of the protective coating 40 is preferably inert in the immersion liquid 11 and does not dissolve. As is illustrated in <figref idref="f0003">Figure 6</figref>, the protective coating 40 is attached to the bottom (as illustrated) surface of the second component 27 of the projection system PL.

0042It is preferable to make the protective layer as thin as possible while still providing protection to the final optical element 20 of the projection system PL. The protective coating is preferably between 5 and 500 nm thick, more preferably between 10 and 200 nm thick. Preferably material of the protective coating 40 is a metal, a metal oxide or nitride or SiO2 with a low contact angle with the immersion fluid to limit bubble inclusion. The layer may be deposited on the element 20 by e.g. evaporation, sputtering etc.

0043The use of a protective coating 40 is not limited to the case where the final element 20 of the projection system PL is comprised of CaF2. For example, if the final element is comprised of quartz (as typically in the case of an abschlussplatte being the final element), there may also be problems due to the dissolution or reaction of quartz with the immersion liquid 11. In this case a protective layer 40 may also be required.

0044The protective coating 40 should be as thin as possible to minimize transmission losses. The refractive index of the protective coating 40 can be partially varied by the deposition process and the deposition parameters. Experience gained in the deposition of EUV coatings might be usefully harnessed to optimize this process.

0045In an embodiment of the invention, shown in <figref idref="f0004">Figure 7</figref>, the final element 20 of the projection system is protected by a fused silica plate 45. This plate has a thickness in the range of from 50µm to 5mm and is contact bonded to the final element 20. In contact bonding, no glue is used - the bonding surfaces are made smooth and clean enough to directly bond together. After bonding to the final element, the fused silica plate may be ground and polished to the desired thickness, avoiding difficulties inherent in handling a very thin plate. A liquid-tight seal 46 may be provided around the perimeter of the join.

0046A seal 46 around the join of the final element and the fused silica protective plate is especially desirable where the final element and fused silica plate are contact bonded together. Although this form of bonding can provide an exceptionally strong bond, where dissimilar materials, such as CaF and fused silica, are bonded temperature changes and thermal gradients can cause the bond to "breathe" - differential thermal expansion or contraction of the two materials causes them to separate until the stress is relieved. Although the bond usually reforms very quickly in the case of thermal separation, if this occurs when the final element is in contact with a liquid, e.g. during polishing or grinding of the protective layer or use of the apparatus, liquid can be drawn into the gap.

0047One form of seal that may be used is a layer of SiO formed by applying a suitable precursor (such as silicone fluids (i.e. comprising Si-O chains of various lengths with various hydrocarbon side-chains), tetraethyl orthosilicate, decamethyl tetrasiloxane and tetrabutyl orthosilicate) and irradiating it with DUV light to photo-convert the precursor to SiO. This form of seal has the advantage that it has a similar hardness to the fused silica plate and so polishes at a similar rate.

0048Another form of seal that is useful is a silicon caulk provided over a layer of titanium oxide. The titanium oxide is applied by painting a precursor onto the seal and photo-converting it to titanium oxide and acts to protect the silicone caulk from UV light during operation of the apparatus.

0049A further form of seal is formed by painting tetraethyl orthosilicate around the join, which then decomposes at room temperature to form a thin layer of fused silica which forms a seal. This seal is however rather brittle so that careful handling is required.

0050In a variant of the <figref idref="f0004">Figure 7</figref> arrangement shown in <figref idref="f0004">Figure 8</figref>, a liquid 47, such as oil, is provided between the last lens element 20 and the protective plate 45. The liquid 47 preferably has a refractive index as close as possible to that of the immersion fluid 11, which may be water, but is not damaging to the material of the final lens element 20, which may be CaF₂. This enables the protective plate to be interchangeable by substantially reducing the requirements on the accuracy with which the protective plate 45 must be positioned as the fluids above and below it have similar refractive indexes.

0051A further variant, shown in <figref idref="f0004">Figure 9</figref>, uses a two layer protective coating, made up of inner layer 48 and outer layer 49. It can be very difficult to form a layer of protective coating, without pinholes. Even the smallest pinhole in a protective coating applied to a CaF body allows dissolution of the CaF body when immersed in water, causing cavitation which is extremely deleterious to the optical properties of the element. By the use of a two layer protective coating, it can be arranged that the pinholes in one layer do not match up with the pinholes in the other layer so that there is no through path in the protective layer. It can best be ensured that the pinholes of the two layers do not match up by applying the two protective layers by different methods.

0052In an example a first layer 48 of SiO is applied by sputtering and a second layer 49 is applied by spin coating a precursor and photo-converting the precursor to SiO. This method has been found to be more effective than sputtering two layers of SiO since the pinholes in the second sputtered layer have a tendency to line up with those in the first layer. Surprisingly, it has been found that the spin coating and photo-conversion method provides a layer of bulk SiO rather than porous SiO. A layer formed by spin-coating a precursor and then photo-converting it to SiO may also be used on its own as a seal layer.

0053The precursor used to form protective layer 49 may be any suitable fluid of, or containing, organo-silicon compounds. Suitable examples are silicone fluids (i.e. comprising Si-O chains of various lengths with various hydrocarbon side-chains), tetraethyl orthosilicate, decamethyl tetrasiloxane and tetrabutyl orthosilicate. The material may be chosen to have a desired viscosity to enable a suitably even layer to be provided by spin coating. Solvents, preferably volatile organic solvents, may be used to adjust the viscosity if necessary.

0054Photo-conversion of the precursor to SiO can be achieved with irradiation by DUV light of various wavelengths, e.g. 184nm or 172nm, at a rate determined to avoid any deleterious effects that might be induced by thermal gradients in the element.

0055Each of the two layers of the protective coating may have a thickness in the range of 50 to 200nm.

0056Although 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, such as 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. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

0057The terms "radiation" and "beam" used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 248, 193, 157 or 126 nm).

0058The term "lens", where the context allows, may refer to any one or combination of various types of optical components, including refractive and reflective optical components.

0059While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

0060The present invention can be applied to any immersion lithography apparatus, in particular, but not exclusively, those types mentioned above.

0061The 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 which define the invention.