Method for operating an immersion lithography apparatus

Cross-reference to related applications

This application claims the benefit under 35 U.S. C. 119(e)(l) of U.S. Provisional Application No. 60/886,780, filed January 26, 2007, the disclosure of which is considered as part of and is incorporated by reference in the disclosure of this application.

Technical field

The present invention relates to a method for operating an immersion lithography apparatus. Further, it relates to a method for producing an array of individually controllable piezo elements, which can be used in one of the above methods.

Background art

A lithography apparatus is used to produce semiconductor devices and other finely structured components, such as integrated circuits or LCDs. Such a lithography apparatus, usually designed as a projection exposure apparatus, contains not only a light source and an illumination system for illumination of a photomask or a reticle, but also a projection objective, which projects the pattern of the reticle onto a light-sensitive substrate, also called lithography substrate, for example a silicon wafer which has been coated with a photoresist.

So far, three approaches in particular have been adopted in order to produce ever smaller structures in the order of magnitude of less than 100 run: Firstly, attempts are made to enlarge to an ever greater extent the image-side numerical aperture NA of the projection objectives. Secondly, the wavelength of the illumination light is reduced ever further, preferably to wavelengths of below 250 nm, for example 248 nm, 193 nm, 157 nm or even less. Finally, further measures are used to improve the resolution, such as phase-shifting masks, multipole illumination or oblique illumination.

Another approach to increase the resolution capability is based on the idea of introducing an immersion liquid into the intermediate space which remains between the last optical element on the image side of the projection objective, in particular a lens, and the photoresist or another light-sensitive layer that is to be exposed. This technique is referred to as immersion lithography. Projection objectives which are designed for immersion operation are for this reason also referred to as immersion objectives.

The advantages of immersion lithography are due to the higher refractive index of the immersion liquid with respect to that of vacuum or air. This results in an increase in the resolution and the depth of focus and allows for a value of the numerical aperture (NA) of even more than 1.0. For an operating wavelength of 193 nm ultra pure water can be used as an immersion liquid. The refractive index of water at a wavelength of 193 nm is 1.437.

In order to provide an immersion liquid with a higher refractive index than water at 248 or 193 nm the use of hydrocarbons has been proposed in WO 2006/045748 and WO

2005/119371. For use at even lower wavelength immersion liquids comprising fluorinated hydrocarbons can be used such as those given in the article "Immersion lithography at 157 nm" by M. Switkes and M. Rothschild, J. Vac. Sci. Technol. B 19(6), Nov/Dec 2001, pages 1 ff, or as proposed in WO 2005/013009.

While performing immersion lithography the immersion liquid is in contact with at least part of the optical surface of the last optical element of the projection objective. The last optical element of the projection objective is the optical element which is positioned adjacent to the light sensitive substrate. The surface of this last optical element can be contaminated by deposits out of the immersion liquid. Such deposits are mainly due to species which are present in the immersion liquid or molecules of the immersion liquid itself which decompose due to irradiation with the operating wavelength during the operation of the projection exposure apparatus and which undergo further chemical reactions to form an insoluble product which is deposited on the surface of the last lens element. Another possible source of contamination can be debris from the photoresist coated wafer surface.

For cleaning the optical surface of the last optical element with a cleaning liquid, the wafer can be demounted to apply the cleaning liquid to the last optical element. It is also known to use a cloth impregnated with the cleaning liquid.

It is also known to clean the surface of the last optical element in a lithography apparatus by applying a plasma activated gas to the optical surface of the last optical element. Corresponding means and methods are shown in US application 60/697,632 and EP 1 429 189.

It is also known to clean the surface of the last optical element by means of pressure waves generated by an ultrasonic transducer, as shown in US 2006/0023185.

Summary of the invention It is an object of the invention to provide an improved method for operating an immersion lithography apparatus including efficient cleaning of the last optical element.

This object is solved by a method for operating an immersion lithography apparatus according to claim 1. By a method comprising a first step of providing an immersion lithography apparatus comprising at least one optical element and a lithography substrate, wherein a gap is located between the optical element and the lithography substrate, and a second step of providing an immersion liquid into the gap, wherein at least part of a surface of the optical element is in contact with the immersion liquid; and by a third step of performing a cleaning process for removing a deposit formed on at least said part of the surface of the optical element by supplying a cleaning fluid into the gap, wherein the cleaning is effected by a chemical reaction between a chemical species in said cleaning fluid and the deposit, it is possible to clean away deposits in a simple and efficient way.

This kind of cleaning by effecting a chemical reaction between a deposit and a reactive species in the cleaning fluid is much more efficient than by rinsing with conventional cleaning solvents like ethanol or water, especially when the immersion fluid comprises

hydrocarbons, since the use of such an immersion liquid may lead to deposition of a degradation product of these hydrocarbons on the optical surface of the last optical element under irradiation with deep ultraviolet light. As a result, a deposit which comprises mainly carbon will develop on the optical surface of the last lens element. In the following this deposit will also be referred to as carbon based deposit. Such a carbon based deposit is usually not soluble in the aforementioned cleaning solvents.

The described method for operating an immersion apparatus is particularly effective when an immersion liquid comprising an alkane, in particular a cyclic or a polycyclic alkane, such as cyclohexane, cyclooctane, decahydronaphthalene or a bridged alkane like adamantane, is used. It is also well suited for immersion liquids comprising a fluorinated hydrocarbon. The refractive index of these kinds of immersion fluids is 1.5 or more, in particular 1.6 or more for a wavelength of 193 nm.

For cleaning a carbon based deposit formed out of these immersion liquids, it is advantageous to provide a chemical species, which is an oxidizing species, so that the chemical reaction is an oxidation. In this case, the carbon containing degradation product of the hydrocarbons will be transformed to oxidation products of carbon, which are gaseous or volatile products, such as carbon dioxide. These oxidation products can either be pumped away from the gap, if the cleaning fluid is a gas, or will be dissolved, in case the cleaning fluid is a liquid.

It is advantageous to remove the immersion liquid from the gap, for example during exposure breaks, and to replace it by the cleaning liquid. By introducing the cleaning fluid into the gap between the last lens element and lithography substrate, the wafer, without removing the last optical element and keeping said last optical element mounted in the immersion lithography apparatus during cleaning, as preferred, a subsequent adjustment or recalibration of the last optical element can be avoided or at least reduced with respect to time and effort.

It is advantageous to provide a cleaning liquid having a viscosity between 50% and 200 % of the viscosity of the immersion liquid. Usually the immersion liquid is applied to and removed from the gap between the last optical element and the wafer by means of a system of tubes. The geometry of this system and the width of the gap between the last optical element and the wafer is adapted to the viscosity of the immersion liquid. The cleaning liquid is advantageously applied to the last optical element by the same means as the immersion liquid. Thus it is preferred to employ a cleaning liquid exhibiting a viscosity between 50% and 200 % of the viscosity of the immersion liquid. Even more preferred is a viscosity between 75% and 150% of the immersion liquid. Highly convenient is employment of a cleaning liquid having essentially the same viscosity as the immersion liquid.

The efficiency of the cleaning can further be enhanced by generating the oxidizing species by irradiation of the cleaning liquid with electromagnetic radiation and/or by generating the oxidizing species in a catalytically supported way.

If the oxidizing species is generated by irradiation, it is particularly advantageous fo use as the electromagnetic radiation for generating the oxidizing species the exposure light of the immersion lithography apparatus. Typically the exposure light of the immersion lithography apparatus has a wavelength of 193 nm or 157 nm, called operating wavelength. The oxidizing species can be chosen to be a radical comprising hydroxide. In this case, water can be used as a cleaning liquid. By adding a catalyst comprising titanium dioxide and by providing irradiation with ultra violet light of the operating wavelength water can be decomposed to form hydroxide radicals which are highly reactive oxidation agents and which will allow cleaning with a very high efficiency. Titanium dioxide is also a particularly effective catalyst for decomposing hydrogen peroxide and oxygen. For the decomposition of hydrogen peroxide manganese dioxide can be used as a catalyst alternatively.

Further oxidizing species in a cleaning liquid can be oxidizing acids, oxidizing bases, peroxides, oxidizing gases, and oxidizing salts. An oxidizing acid can be nitric acid, an

oxidizing base can be sodium peroxide, a peroxide can be hydrogen peroxide, an oxidizing gas can be oxygen, ozone, halgene containing gases, in particular chlorine, hydrogen peroxide, and compounds of noble gases with halogens, in particular with fluorides, e.g. xenon fluorides, and an oxidizing salt can be potassium permanganate. Oxidizing gases and oxidizing salts are typically dissolved in the cleaning liquid. A cleaning liquid comprising a base and hydrogen peroxide is especially preferred since hydrogen peroxide has a stronger oxidizing effect in an alkaline solution.

Alternatively, the cleaning fluid can be a gas or a gas mixture. In this case, it is advantageous while introducing the cleaning gas to the gap between the last lens element and the wafer to provide in addition a low pressure device connected to the gap for sucking said gas or gas mixture from said gap with said low pressure device.

In this case a cleaning gas or gas mixture ozone, a gas mixture comprising a halogen, e.g. chlorine, or a gas mixture comprising oxygen and nitrogen can be used. When a gas mixture comprising oxygen or nitrogen is used, it is advantageous to irradiate the surface of the last lens element with ultraviolet light during the cleaning step. This will lead to an activation of the cleaning gas, which in turn will provide for a more efficient cleaning.

In this case, it is preferred to use the exposure light of the lithography apparatus such that no additional light source will be needed.

It is preferred to provide a cleaning substrate during the cleaning process and to mount said cleaning substrate such that a gap is located between the optical element and the cleaning substrate, and to provide the cleaning fluid into the gap between the cleaning substrate and the optical element in such a way, that at least part of the surface of the optical element comes into contact with the cleaning fluid. The cleaning substrate is preferably chosen in such a way that it adsorbs the oxidation products from the cleaning process. As a consequence, such oxidation products are removed from the cleaning liquid and the cleaning is facilitated. If a cleaning substrate is used, the gap between the cleaning

substrate and the last optical element may be of the same width as the gap between the last optical element and the wafer while performing lithography.

When the cleaning liquid is irradiated as described above in order to generate or to activate oxidizing species, it is preferred to employ a cleaning substrate which reflects more than 90%, even more preferred 95%, of the exposure light of the immersion lithography apparatus. Thus, most of the exposure light is not absorbed by the cleaning substrate but reflected into the cleaning fluid again.

When a cleaning substrate is used, it is advantageous to replace the lithography substrate, the wafer, by the cleaning substrate. In this case, no additional devices have to be provided for inserting the cleaning substrate into the gap and the same inlets as for the immersion fluid can be used for introducing the cleaning fluid.

In an advantageous embodiment, the cleaning substrate is coated with a catalyst, which promotes the generation of the oxidizing species in the cleaning fluid as described above.

When a liquid is used for cleaning, it is advantageous to support retaining of the cleaning liquid within the gap between the last optical element and the cleaning substrate, by providing a coating of the cleaning substrate with a hydrophobic layer or with a hydrophilic layer, depending on the nature of the cleaning liquid.

Preferably, a wear layer is provided on at least part of the surface of the optical element, and at least part of the wear layer is dissolved in the cleaning fluid by bringing the wear layer into contact with the cleaning fluid. In such a way, in particular when the deposit covers large parts or the entire surface of the wear layer, the chemical species in the cleaning fluid can be used to at least partially remove the deposit from the surface of the wear layer, thus increasing the contact area of the wear layer with the cleaning fluid. It is understood that by dissolving at least part of the wear layer in the cleaning fluid, the deposit formed on the surface of the wear layer will be removed from the optical element.

According to a further aspect of the invention a method for operating an immersion lithography apparatus is provided comprising the steps: providing an immersion lithography apparatus comprising at least one optical element and a lithography substrate, providing a wear layer on at least part of the surface of the optical element, providing an immersion liquid into a gap located between the optical element and the lithography substrate, wherein at least said part of the surface of the optical element is in contact with the immersion liquid, performing a cleaning process for removing a deposit formed on at least part of the surface of the wear layer by supplying a cleaning fluid into the gap for bringing the wear layer into contact with the cleaning fluid for dissolving at least part of the wear layer in the cleaning fluid.

In this case, cleaning of the optical element can be performed by dissolving the wear layer, in particular when the deposit does not cover the entire surface or large areas of the surface of the wear layer, such that a sufficiently large contact area between the cleaning fluid and the wear layer may be provided for the cleaning process.

During a single cleaning process usually only part of the wear layer is dissolved in the cleaning fluid, thus allowing the execution of a plurality of cleaning processes before the wear layer is used up. The wear layer may be designed e.g. as an exchangeable plate which is arranged on the surface of the optical element, thus allowing the wear layer to be exchanged when a considerable portion of the material of the wear layer has been dissolved in the cleaning fluid.

In a preferred variant, the material of the wear layer has a refractive index in close proximity to the refractive index of the immersion liquid, a refractive index difference with respect to the refractive index of the immersion liquid being preferably less than 0.05, more preferably less than 0.01, in particular less than 0.005. In such a case the gap between the surface of the optical element and the substrate is filled with media having an identical refractive index during the exposure process. Therefore, the location at which the solid/liquid interface between the surface of the wear layer and the immersion liquid is situated and the surface form of this interface have no adverse effect on the exposure

process. Thus, the dissolution of the protective layer material under the action of the cleaning fluid cannot impair the optical properties of the system. The person skilled in the art will appreciate that the refractive index of the wear layer may be an average or effective refractive index resulting from the use of mixed materials having different refractive indices for forming the wear layer. The use of mixed materials for the wear layer as well as suitable deposition methods for such materials are described in US 2005/0225737 Al due to the applicant, the disclosure of which is incorporated herein by reference.

Preferably, the material of the wear layer is selected from the group comprising: lithium fluoride, sodium fluoride, magnesium fluoride, and calcium fluoride. These and other fluoride or oxide materials as well as mixtures thereof are preferred for the wear layer. In particular, these materials or mixtures thereof may be chosen such that their refractive indices essentially coincide with the refractive index of the immersion liquid. Suitable layer materials for high-index immersion liquids with a refractive index at or above 1.5 are disclosed e.g. in WO 2006 133884 Al due to the applicant, the disclosure of which is incorporated herein by reference.

According to a further aspect of the invention a method for operating an immersion lithography apparatus is provided comprising the steps: providing an immersion lithography apparatus comprising a last optical element and an immersion liquid, wherein while performing lithography with said apparatus on a wafer said immersion liquid is in contact with a surface of said last optical element, providing an ultrasonic transducer for generating pressure waves, and checking for a deposit out of said immersion liquid on said surface by detecting reflections of said pressure waves with said transducer.

If an ultrasonic transducer is used for checking for a deposit out of the immersion liquid on the last optical element, it is preferred to employ the ultrasonic transducer also for cleaning the last optical element with pressure waves.

Usually a wafer is positioned on a wafer support. It is practical to position the ultrasonic transducer on the wafer support. Then either the wafer support or the last optical element

can be positioned such that the transducer faces the optical surface of the last optical element via a gap. So the pressure waves generated by the ultrasonic transducer hit the surface of the optical element directly. The gap between the ultrasonic transducer and the last optical element can be filled with a liquid, e.g. water. As a consequence, the pressure waves can hit a deposit on the last optical element with a higher intensity than without such a liquid. While doing so, it is beneficial to provide a steady flow of the just mentioned liquid for removing the debris of the deposit.

In addition to checking for a deposit by means of the ultrasonic transducer a light emitter and a light detector can be used. The light emitter can be a laser diode and the light detector can be a CCD detector. Both are positioned facing the surface of the last optical element such that the light emitted by the light emitter is reflected by the surface of the last optical element and the detector detects the reflected light. Employing two independent systems for checking for a deposit on the last optical element improves the performance (in terms of hit rate and false positives).

It has proven to be appropriate to focus the light emitter on the surface while its central emission direction displays an angle of 15° to 40° to the surface normal of the surface of the last optical element.

Since the ultrasonic transducer generates pressure waves and also detects them, the ultrasonic transducer can be used as a means for measuring the travelling time of the pressure waves from the ultrasonic transducer to the last optical element and back. If there is a deposit on the surface of the last optical element the travelling time will be shorter. Thus, it is preferred to employ the ultrasonic transducer for measuring the propagation time of the pressure waves for checking for a deposit on the last optical element.

It is preferred to use an ultrasonic transducer comprising an array of individually controllable piezo elements preferably individually controllable with respect to phase and amplitude, to generate pressure waves which can display spatial variations in the plane parallel to the ultrasonic transducer.

It is also preferred to employ such an array of individually controllable piezo elements to generate pressure waves which are focused on a deposit on the last optical element. The energy applicable for removing the deposit via pressure waves is thus increased.

Alternatively, it is preferred to actuate all piezo elements of an array in phase. Thus a large section of the surface of the last optical element can be exposed to the pressure waves at once, thereby increasing checking speed.

For checking for deposit, it is useful to compare the actual pressure wave distribution to a standard reflection, i.e. a reference measurement in the absence of any deposit. It is also preferred to employ standard reflections as a reference for the light reflections.

Description of the preferred embodiments Hereunder, preferred embodiments of the invention will be described in more detail. These embodiments are merely illustrative and not meant to limit the scope of the invention as defined in the claims. The features disclosed could also be relevant in other combinations.

Figure 1 a shows a schematic cross-sectional view of a last lens element as part of an immersion lithography apparatus adapted for cleaning with a cleaning liquid and a cleaning substrate Figure Ib shows a variation of figure Ia

Figure 2 shows a schematic top view of a cleaning substrate as shown in figure Ia

Figure 3 shows a schematic cross-sectional view of a last lens element as part of an immersion lithography apparatus adapted for use of a cleaning liquid comprising hydrogen peroxide

Figure 4 shows a schematic cross-sectional view of a last lens element as part of an immersion lithography apparatus adapted for use of pure water as a cleaning liquid Figure 5 shows a schematic cross-sectional view of a last lens element as part of an immersion lithography apparatus optimized for use of a cleaning gas

Figure 6 shows a variation of the embodiment of fig. 5

Figure 7 shows a schematic perspective view of a projection objective and a wafer support provided with an array of piezo elements

Figure 8 shows a schematic cross-sectional view of a last lens element as part of an immersion lithography apparatus comprising a light source for detecting a deposit on the last lens element Figure 9 shows a schematic cross-sectional view of a detail of figure 8 in a first mode of operation

Figure 10 shows a schematic cross-sectional view of a detail of figure 8 in a second mode of operation.

Figures 1 Ia to l ie show three stages during the production of an array of individually controllable piezo elements

Figure 12 shows an array of individually controllable piezo elements produced according to the method demonstrated by means of figures 1 1 a to l ie

Reference numerals which refer to corresponding parts in the different examples are retained for all subsequent figures

The method of operating an immersion lithography apparatus comprises the steps: performing lithography, i.e. process a wafer using exposure light, and cleaning the last lens element of such an immersion lithography apparatus.

While performing lithography on a wafer, a last lens element, i.e. the last lens element in front of the wafer, is immersed in an immersion liquid, which is in contact with at least part of the surface of the last lens element and at least part of the surface of the wafer. In the present examples the immersion liquid comprises hydrocarbons. Alternatively, the immersion liquid could comprise fluorinated hydrocarbons, which are especially suited for use with operating wavelengths of less than 190 nm.

The exposure light of the lithography apparatus, at a wavelength of 193 nm interacts with the immersion liquid and leads to decomposition of a small part of the fluorinated hydrocarbon molecules of the immersion liquid. As a result a carbon based deposit will develop on the optical surface of the last lens element.

Figure 1 a shows a part of an immersion lithography apparatus during cleaning. The before mentioned wafer is replaced by a cleaning substrate 4 positioned on a wafer support 6. Between a last lens element 2 and a cleaning substrate 4 a cleaning liquid 3 is provided. The optical surface of the lens element 2 is immersed in cleaning liquid 3. Cleaning liquid 3 is applied by tubes 5, which are also used to remove cleaning liquid 3. Tubes 5 are connected to a corresponding reservoir (not shown). The same tubes are used to apply the immersion liquid for performing lithography. Cleaning liquid 3 is based on water and comprises nitric acid. The nitric acid oxidizes the carbon based deposit on the surface of last lens element 2. The reaction products are mainly gaseous oxides of carbon, such as carbon monoxides and carbon dioxide. These oxides will partly escape from the gap between the last lens element 2 and the cleaning substrate 4 into the surrounding atmosphere and partly be dissolved in the cleaning liquid. Other oxidation products which are not soluble in the cleaning liquid 3 are partly adsorbed on the cleaning substrate 4.

The cleaning substrate 4 is preferably made of a porous and hydrophobic material which absorbs hydrophobic compounds like hydrocarbons.

Cleaning liquid 3 has substantially the same viscosity as the immersion liquid used before for performing lithography. As a consequence, the configuration of tubes 5 with respect to the last lens element 2 and with respect to the cleaning substrate 4 corresponds to the configuration during performing lithography on a wafer. Further, the width of the gap between last lens element 2 and cleaning substrate 4 corresponds to the width of the gap between last lens element 2 and the wafer to be processed.

In figure 1 b cleaning substrate 4 of figure Ia is replaced by a different cleaning substrate 7. Here, cleaning substrate 7 corresponds to a vessel containing the cleaning liquid. Last lens element 2 is immersed in the cleaning liquid (not shown, disguised by vessel 7) in vessel 7.

Figure 2 shows a top view of cleaning substrate 4 from figure Ia. An outer ring surface 8 surrounding an inner surface region 9 is coated with a hydrophobic material (hatched). As a consequence, when an aqueous solution is used as a cleaning liquid a column of the cleaning liquid of some millimetres height will be enclosed on the inner surface region 9 of cleaning substrate 4 without crossing outer ring surface 8. On the other hand, if the cleaning liquid comprises as a main component a non-polar solvent, a hydrophilic material is used for the outer ring surface region 9.

Figure 3 shows a schematic cross-sectional view of a last lens element as part of an immersion lithography apparatus, which differs from the embodiment shown in figures Ia and Ib in that cleaning liquid 3 consists of an aqueous solution of hydrogen peroxide. The oxidizing effect of hydrogen peroxide is increased by irradiating it with exposure light 10 of the lithography apparatus. Exposure light 10 is applied to the hydrogen peroxide solution through the last lens element 2. The person skilled in the art will appreciate that instead of using an aqueous solution of hydrogen peroxide, a cleaning liquid may be used in which hydrogen peroxide or other suitable gases such as ozone, halogen gases, in particular chlorine, oxygen and compounds of noble gases with halogenides, in particular xenon fluorides, are dissolved.

The surface of cleaning substrate 4 is coated with a reflecting layer suitable for reflecting light with a wavelength of 193 nm such that basically all, but at least 90%, of the exposure light 10 is reflected from the surface of cleaning substrate 4.

In another embodiment, cleaning substrate 4 is coated with titanium dioxide, which acts as a catalyst for further increasing the oxidizing effect of the cleaning liquid by generating reactive hydroxide species from hydrogen peroxide. Such a catalyst coating can be

combined with the hydrophobic/hydrophilic layer and/or with the reflecting layer of the previously described embodiments.

Figure 4 shows another embodiment, in which the cleaning liquid 3 consists of pure water. The surface of the cleaning substrate 4 is covered with a catalyst coating 41, for example a coating comprising titanium dioxide. When the surface of the coating substrate 4 is irradiated with exposure light 10 of an ultra violet wavelength, such as 193 nm, the chemical bonds of water molecules can be broken leading to the formation of OH- and H- radicals. These radicals are highly reactive species which will react with the carbon based deposit on the last lens element 2 and thus provide a very efficient cleaning.

When the generation of highly reactive oxidizing species in the cleaning liquid 3 is effected by means of a catalyst coating 41 on the cleaning substrate 4, it is highly preferably to arrange the cleaning substrate 4 very close to the surface of the last lens element, because highly reactive species like OH-radicals do not have a long lifetime. For this reason, there is a high probability that these radicals will react with any other chemical species present in the cleaning liquid before they will reach the surface of the optical element 2 to be cleaned. In order to make sure that enough reactive species can be provided close to the optical element 1 surface, the gap between the cleaning substrate 4 and the last lens element 2 should be kept small.

Although cleaning can usually be performed efficiently in the way described above, certain kinds of deposits may be difficult to remove from the surface of the optical element 2 without using cleaning fluids which at the same time attack the material of the optical element 2. For this reason, a wear layer made e.g. of lanthanum fluoride (LaF ₃ ) may be provided on the planar exit surface of the last lens element 2. In such a way, the deposit can be removed from the optical element 2 by bringing e.g. water as a cleaning liquid 3 into contact with the material of the wear layer during the cleaning, such that a gradual dissolution of the lanthanum fluoride material of the wear layer occurs.

The lanthanum fluoride material of the wear layer has a refractive index of approximately 1.7 which essentially coincides with the refractive index of the immersion liquid, the refractive index difference being less than 0.05. In such a way, during the lithography process, the gap between the last lens element 2 and the substrate is filled with media having almost identical refractive index, such that a gradual dissolution of the wear layer does not impair the optical properties of the lithography system during the exposure process.

In the preferred embodiment of Fig. 4, the wear layer 2a and the formation of the OH- and H-radicals produced by irradiating the cleaning substrate 4 are advantageously combined, as the OH- and H-radicals accelerate the dissolution of the wear layer 2a and at the same time react with the deposit, thus increasing the contact area of the cleaning liquid 3 with the surface of the wear layer 2a.

The wear layer 2a was applied to the surface of the last lens element 2 by chemical vapour deposition. Alternatively, an exchangeable element, e.g. an exchangeable plate, may be used as a wear layer, thus allowing simple replacement when the wear layer is used up. The person skilled in the art will appreciate that additionally, an antireflection layer system may be inserted between the surface of the optical element and the wear layer.

Figure 5 shows another embodiment, in which cleaning is effected by introducing a cleaning gas instead of a cleaning liquid into the gap between the last lens element 2 and the wafer. In general, the same setup as for introducing the immersion liquid or for introducing the cleaning liquid can be used. Cleaning gas, e.g. ozone or chlorine is introduced into the gap via tubes 5 thereby filling the gap between the last lens element 2 and the wafer with cleaning gas. Ozone gas is provided by an ozone generating device (not shown). It is possible to introduce ozone gas alone or a gas mixture which comprises apart from a certain amount of ozone in addition nitrogen gas and/or oxygen gas. In order to provide for an effective cleaning process, however, ozone should be present in the gas mixture at a concentration of at least 10 ppm. Ozone will then oxidize any carbon based deposit on the surface of the last lens element 2 to volatile oxidation products, such as

carbon dioxide. Tubes 5 for introducing the cleaning gas or a gas mixture can be the same tubes 5 that are used for introducing the immersion liquid. Alternatively, separate tubes of a similar construction can be used for the cleaning gas. This might be advantageous if a special corrosion resistant tube material has to be used with the cleaning gas.

Both chlorine and ozone are poisonous gases, which might harm the environment. In order to avoid that the cleaning gas will escape into the ambient atmosphere, the gap between the last lens element 2 and the wafer is sealed from the ambient atmosphere by a surrounding vessel (not shown). The escape of cleaning gas can also be avoided by using a device for introducing a cleaning gas as described with respect to figure 6.

Figure 6 shows another device for introducing a cleaning gas into the gap between the last lens element 2 and the wafer. In this case, the wafer support is removed from the lithography apparatus and replaced by the gas introducing device. As a cleaning gas ozone or halogen gas can be used for oxidizing a carbon based deposit on the optical surface of the last lens element 2.

A first vessel 11 is positioned near the optical surface of last lens element 2 such that it covers a substantial part of the optical surface of last lens element 2. As a result, the first vessel 11 constitutes a space 13 together with last lens element 2. A gap 14 of less than 0,1 mm is left between the wall of first vessel 11 and the surface of last lens element 2. Via a tube 15 belonging to the first vessel 11 space 13 is filled with chlorine. As a consequence, a deposit on the optical surface of last lens element 2 is oxidized by the chlorine. The chlorine leaves space 13 via gap 14 between first vessel 11 and last lens element 2.

First vessel 11 is embraced by a second outer vessel 12 positioned relative to last lens element 2 in basically the same way as first vessel 11. There is also a gap between the wall of outer vessel 12 and last lens element 2. The gap has the same width as gap 14 between first vessel 11 and last lens element 2. The gap has the same width as gap 14 between first vessel 11 and last lens element 2. Outer vessel 12 s connected via a tube 16 to a low pressure device for removing the chlorine (not shown).

The low pressure device generates low pressure between first vessel 1 1 and outer vessel 12 such that the chlorine and reaction products are safely removed from the last lens element 2. Due to the low pressure, air is sucked through the gap between outer vessel 12 and lens element 2 into the outer vessel 12, so no chlorine can leave through this gap. Outer vessel 12 is supported by a mounting 17.

The inner surface of first vessel 11 adsorbs reaction products of the halogen and the deposit.

Figure 7 shows a projection objective 18, a wafer support 19 and an array of individually controllable piezo elements. While performing lithography, a wafer (not shown) positioned on wafer support 19 is processed by means of a projection objective 18.

For cleaning a last lens element (not shown) at the lower end of the projection objective 18 the wafer support is positioned such that an array of piezo elements 20 is arranged below the last lens element. A gap between array 20 and the last lens element (not shown) is filled with an immersion liquid, similar as described with respect to figure Ia.

The piezo elements of array 20 are acutated to generate pressure waves. These pressure waves travel through the liquid in the gap between array 20 and the last lens element and are reflected by the last lens element or a deposit on the last lens element respectively.

The piezo elements of array 20 couple to the reflected pressure waves and thus detect the reflection. For checking for a deposit on the last lens element the propagation time of the pressure waves is measured via the piezo elements and evaluated by a control device, e.g. a computer (not shown).

Since array 20 comprises several spatially distributed piezo elements, the reflected pressure waves can be laterally resolved. In addition to measuring the propagation time of the

pressure waves the laterally resolved reflections are compared with a reference (by a control device, now shown), which was measured in the absence of a deposit.

Once a deposit is detected, it is removed by exhibiting it to pressure waves generated by array 20. It could, however, also be removed by other means as shown in figures Ia to 5, e-g-

In this embodiment no scanning is necessary since array 20 covers the whole lateral extension of the optical surface of the last lens element.

In another embodiment shown in figure 8, detection of a deposit is supported by an additional independent detection system consisting of a light source 21, which can be an LED or a laser diode and a CCD detector 22. Light source 21 and CCD detector 22 are both positioned facing the optical surface of last lens element 2 in such a way that the light emitted by light source 21 is reflected by the optical surface of last lens element 2, and detector 22 can detect the reflected light. Light source 21 is focused on the surface of last lens element 2 while its central emission direction displays an angle of 22° to the surface normal of last lens element 2.

For checking for a deposit, wafer support 19 is moved such that the optical surface of last lens element 2 is effectively scanned by means of the light emitted by light source 21 and the pressure waves generated with array 20. For detecting a deposit, the reflection of the light is compared to reference reflections measured in the absence of a deposit.

Figure 9 shows individual piezo elements 20a to 2Oe of the array 20 shown in figures 6 and 7. For generating high impact pressure waves for removing a deposit from the surface of the last lens element the individual piezo elements 20a to 2Oe are individually controlled with respect to phase and amplitude such that the generated pressure waves are focused on the surface of last lens element 2.

Figure 10 shows the individual piezo elements from figure 9 actuated in phase for checking for a deposit.

Checking for a deposit and cleaning employing pressure waves is performed iteratively until no deposit is detected anymore.

Figures 1 Ia to l ie show different stages during the production of an array of individually controllable piezo elements. Figure 10a shows a slab of a piezo-electric ceramic 24 positioned on a non-conductive support 23 for the piezo-electric ceramic. Slab 24 is glued to support 23 by means of a silver-containing conductive lacquer 25. Figure 10b shows drill holes 26 which are drilled through support 23 and through conductive lacquer 25 into the slab of pizeo-electric ceramic 24. Through drilling holes 26 supply lines 27, i.e. copper wires, are inserted such that they are in contact with the piezo-electric ceramic 24.

Figure 1 Ic shows array 20 after sawing through the slab of piezo-electric ceramic 24 and support 23. The sawing is performed such that the slab of piezo-electric ceramic 24 is divided into unconnected parts, wherein each part comprises one supply line 27. The unconnected parts of piezo-electric ceramic 24, i.e. the piezo elements, can be electrically contacted via their supply lines.

Figure 12 shows an array 20 of individually controllable piezo elements 28 produced according to the method presented in figures 1 Ia to l ie.

The following geometric properties have proven to be appropriate for a practical array of piezo-electric elements 28: an area covered with individual piezo elements 28 of 3 x 5 cm ² , the slab of piezo-electric material 24 having a thickness o 1 - 2 mm, the support 23 having a thickness of 1 -3 mm, conductive lacquer 25 displaying a thickness of 0.05 - 0.1 mm, drill holes 26 having a diameter of 0.1 — 0.4 mm, the width of the sawed trenches being 0.1 - 0.3 mm, individual piezo elements 28 having a lateral length of 0.5 - 1.5 mm.