WO2021160463A1

WO2021160463A1 - Projection lens of a lithography system and method for monitoring a projection lens of a lithography system

Info

Publication number: WO2021160463A1
Application number: PCT/EP2021/052375
Authority: WO
Inventors: Carsten Marzok; Julian Kaller
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2020-02-11
Filing date: 2021-02-02
Publication date: 2021-08-19

Abstract

The invention relates to a projection lens of a lithography system comprising a housing (11), optical elements (12) that are disposed within the housing (11) and a mask (5) imaging onto a substrate (8), and at least one sensor (14) that is disposed within the housing (11) and detects photoacoustic signals.

Description

Projection objective of a lithography system and method for monitoring a projection objective of a lithography system

The invention relates to a projection objective of a lithography system and a method for monitoring it. The invention also relates to an illumination system of a lithography system, a lithography system and a method for producing a projection lens of a lithography system as well as a method for predictive compensation of imaging errors of a projection lens of a lithography system and a method for operating a lithography system.

A lithography system generally has an illumination system and a projection lens. The lighting system uses the light from a light source to generate a desired light distribution for illuminating a mask. With the projection objective, the mask is imaged onto a light-sensitive material that is applied, for example, to a wafer or another substrate. In this way, the photosensitive material is exposed in a structured manner with a pattern predetermined by the mask. Since the mask has tiny structural elements and the exposure process is part of a production process and is continuously repeated, it is of great importance that the projection lens guarantees an absolutely precise image over a long period of time.

However, the projection lens is exposed to a large number of influences that can have a negative effect on the image quality. The spatial arrangement or the shape of the optical elements of the projection lens can be influenced in particular by mechanical or thermal effects, which in turn can lead to the creation of image errors. Even with perfect shielding from external influences, image defects can be caused by the heat input of the light required to expose the photosensitive material. This effect is known as lens heating.

Expected lens heating effects are already taken into account during the design and manufacture of the projection lens. In addition, manipulators can be provided which at least partially compensate for the effects of the lens heating effects on the image. An effective compensation presupposes that the lens heating effects are known. A determination of lens heating effects by means of simulation models and the metrological recording of lens heating effects are, however, comparatively difficult, especially when quick lens heating effects are also to be taken into account, ie lens heating effects that are significant, for example on a time scale of less than 10 seconds vary.

From DE 103 21 806 A1 a method and an arrangement for foreign gas detection in the beam path of optical imaging or beam guidance systems are known, which are filled and / or flushed with a protective gas. The protective gas is fed to a detection chamber and there exposed to an electromagnetic analysis wave field in order to determine any foreign gas components with the aid of the photoacoustic effect.

The invention is based on the object of enabling the metrological detection of rapid effects in a lithography system which have an influence on the function of the lithography system.

This object is achieved by the combinations of features in the independent claims.

The projection objective according to the invention of a lithography system has a housing and optical elements which are arranged within the housing and which image a mask onto a substrate. Furthermore, the projection lens according to the invention has at least one sensor which is arranged inside the housing and detects photoacoustic signals.

The invention has the advantage that it enables rapid effects to be recorded which have an influence on the function of the projection objective and thus of the lithography system.

The optical elements can include, for example, one or more lenses and / or one or more mirrors and / or one or more diffractive optical elements. In particular, several sensors for detecting photoacoustic signals can also be arranged within the housing. As a result, more information can be obtained via the projection lens than with a sensor. The sensor can be designed as a structure-borne sound sensor or as a microphone. The structure-borne sound sensor can be used to record photoacoustic signals that propagate in a solid body. The microphone can be used to record photoacoustic signals that propagate in a gas. The design of the sensor thus offers the option of selecting which type of photoacoustic signals are to be detected.

The sensor can be arranged on one of the optical elements or on a holder of one of the optical elements. The former leads to a particularly good coupling of the sensor to the photoacoustic signals. The latter reduces the risk of damaging the optical element by attaching the sensor.

The housing can have at least one cavity which is filled with a gas and the sensor can be arranged within the cavity. This enables the detection of photoacoustic signals that were generated in the gas and / or that propagate in the gas. The cavity can serve as an acoustic resonator to amplify the photoacoustic signals. This leads to a better signal strength and thus easier detectability.

The housing can be formed by the holders of the optical elements. This enables an efficient and stable construction of the projection lens.

The invention also relates to an illumination system of a lithography system. The lighting system according to the invention has a housing and optical elements which are arranged within the housing and generate a lighting setting. Furthermore, the lighting system according to the invention has at least one sensor which is arranged inside the housing and detects photoacoustic signals.

Since the lighting system according to the invention is also a component of the lithography system, it is advantageous to enable the metrological detection of rapid effects in the lighting system as well, since these can also influence the function of the lithography system. The invention further relates to a lithography system with a projection objective according to the invention and / or an illumination system according to the invention.

The invention also relates to a method for monitoring a projection objective of a lithography system. In the method according to the invention, photoacoustic signals are generated within the projection lens by irradiating light, and these photoacoustic signals are detected by a sensor.

The photoacoustic signals can be generated by irradiating a region of an optical element or a cavity filled with gas within the projection objective. This opens up the possibility of monitoring the optical element and / or the gas-filled cavity. The irradiated area can have a diameter below a maximum value. The concentration of the light enables a good signal yield of photoacoustic signals. The maximum value can be 20 mm, preferably 10 mm. The optical element or the cavity filled with gas can be arranged at a shorter distance from a pupil plane than from a field plane of the projection objective.

The position of the irradiated area of the optical element or of the gas-filled cavity can be varied. This enables a locally selective generation of photoacoustic signals and thus also the determination of spatially resolved information. In particular, the size and / or the position of the irradiated area can be adjusted via an illumination setting generated by an illumination system. For this purpose, an illumination system with a diffractive optical element or with a micromirror arrangement can be used. The latter is particularly advantageous because it offers a high degree of flexibility.

The photoacoustic signals can be generated by excitation with light of the working wavelength of the projection lens. This makes it possible to use the light source which is already present in the lithography system and no expenditure for an additional light source is required. In particular, the photoacoustic signals can be generated during the exposure process. This has the advantage that permanent monitoring of the projection lens is also possible during the exposure process and errors can thus be recognized as quickly as possible. Another benefit is there in that no pauses in exposure are required for any measurements with the aid of the photoacoustic signals and therefore the exposure operation is not impaired by the measurements.

The photoacoustic signals can be generated with the light that is used for exposure. In particular, the light can be directed onto the mask after the photoacoustic signals have been generated. Accordingly, the light can first be directed onto the optical element or into the gas-guided cavity for generating the photoacoustic signals and then passed on to the mask in order to expose the mask. This enables a seamless integration of measurements with the aid of the photoacoustic signals in the exposure mode and the measurements can be carried out in real time under operating conditions.

The captured photoacoustic signals can be compared with reference signals that were determined during a proper functional state of the projection lens. This enables a very reliable evaluation of the photoacoustic signals even with a comparatively low signal strength.

Information relating to the optical absorption behavior of one of the optical elements of the projection objective can be determined from the recorded photoacoustic signals. In particular, the detected photoacoustic signals can be used to check whether the optical element is degraded to an inadmissible degree. Information relating to the optical absorption behavior of a plurality of optical elements of the projection objective can also be determined.

An action can be initiated if there is an impermissibly severe degradation of the optical element. This has the advantage that it is possible to react quickly and adequately to the degradation.

The action can consist in controlling at least one manipulator of at least one optical element. This is particularly useful if the degradation has not yet progressed too far and has the advantage that the manipulator can be controlled quickly and with little effort. The action can also consist of replacing the optical element. Replacing the optical element is particularly useful when it is not possible to compensate for the effects caused by the degradation with the aid of a manipulator.

The photoacoustic signals can be recorded at different times or after different irradiations of the optical element and from the course of the recorded photoacoustic signals or a related variable it can be determined at which future point in time or after which further irradiation an exchange of the optical element will be necessary . This procedure has the advantage that a required exchange of the optical element can be planned in good time and the required spare parts can be provided so that the exchange can be carried out quickly.

The exchange of the optical element can be carried out within the framework of a system maintenance that is already provided before the determined future point in time or the determined further irradiation is exceeded. This has the advantage that no additional shutdown of the lithography system is required to replace the optical element.

With the captured photoacoustic signals, a spatially resolved check can be made as to whether the optical element has undergone an impermissibly severe degradation. This enables a very reliable assessment of whether the optical element can still be used or has to be replaced.

In the event of an impermissibly strong degradation in a region of the optical element, it is possible to switch to an illumination setting in which a lower light intensity occurs in this region of the optical element than in the previous illumination setting. This opens up the possibility of further use of the optical element in the case of locally limited or locally varying degradation.

From the recorded photoacoustic signals it can be determined whether optical absorption occurs primarily in the area of the surface of the optical element. Furthermore, it can be determined from the recorded photoacoustic signals whether cleaning of the surface of the optical element is necessary and / or whether cleaning of the surface of the optical element has delivered a desired result. This has the advantage that the optical element can be cleaned as required and, as a result, unnecessarily early and too late cleaning can be avoided. Another advantage is that you can be certain of the actual cleaning result. In addition, it can be ensured that cleaning is only carried out for as long as is necessary to achieve the desired cleaning result. As a result, the load on other components caused by cleaning can be reduced to a necessary minimum and the time required for cleaning can be kept short.

It is also possible to determine information relating to the optical absorption behavior of the gas-filled cavity from the recorded photoacoustic signals. Information relating to the optical absorption behavior of several cavities filled with gas can also be determined. In addition, it is possible to determine information relating to the optical absorption behavior of a subsystem consisting of at least one optical element and at least one cavity filled with gas from the recorded photoacoustic signals. This has the advantage that the optical element, the gas-filled cavity or the subsystem can be permanently monitored during the exposure operation.

The signal transit times of the photoacoustic signals can be used to determine whether the signals are primarily assigned to the optical element or to the gas-filled cavity. This makes it possible to use a single sensor to obtain information both with regard to the optical element and with regard to the gas-filled cavity.

Information on the state of the projection lens on a time scale of less than 10 seconds can be determined from the recorded photoacoustic signals. The projection lens can be operated with laser pulses and information on the state of the projection lens can be determined from the photoacoustic signals on a time scale corresponding to the repetition frequency of the laser pulses. Signals can be output from the sensor to a lock-in amplifier. The use of the lock-in amplifier enables the evaluation of very weak signals. The lock-in amplifier can be mixed with a frequency which corresponds to a cavity resonance of the gas-filled cavity or a structural resonance of the optical element. In this way, the signal-amplifying effect of resonance effects can be used, so that originally very weak signals can also be detected.

The invention also relates to a method for producing a projection objective of a lithography system. In the method according to the invention, photoacoustic signals are generated within the projection objective by irradiating light and, on the basis of these photoacoustic signals, a check is carried out to determine whether a predetermined specification is being met.

In this way, reliable quality control is possible, including precise localization of any faults. The signals recorded during the production of the projection lens can be stored as reference values. This has the advantage that future measurements can be carried out relative to the initial state documented in this way and, in this way, even slight changes compared to the initial state can be reliably recognized.

The invention further relates to a method for predictive compensation of imaging errors of a projection lens of a lithography system that are generated by heating parts of the projection lens with exposure light with which the projection lens images a mask onto a substrate. In this case, data about a local optical absorption in the projection lens or parts thereof are collected by means of photoacoustic measurement. From the data on the local optical absorption and further data on the illumination of the mask and / or structures present on the mask, a time-dependent heating that is expected during the exposure and the imaging errors resulting therefrom are calculated numerically. The calculated imaging errors are at least partially compensated for by controlling at least one manipulator of an optical element of the lithography system in a time-dependent manner.

In this way, imaging errors caused by heating can be kept within an acceptable range. The manipulation of the optical element can take place by a rigid body movement of the optical element and / or by deformation of the surface of the optical element.

The imaging errors of the projection objective can be corrected at least once for each exposure of a substrate with the projection objective by means of predictive compensation. It can thereby be achieved that each substrate is exposed while maintaining defined imaging properties.

The invention further relates to a method for operating a lithography system, information being determined by photoacoustic measurements and the imaging properties of the lithography system being corrected on the basis of this information by manipulating at least one optical element of the lithography system.

The invention is explained in more detail below with reference to the exemplary embodiments shown in the drawing.

Show it

1 shows an exemplary embodiment of a lithography system designed according to the invention in a schematic representation,

2 shows an embodiment of the invention in a schematic representation,

3 shows a block diagram of an exemplary embodiment of the evaluation device,

4 shows a further embodiment of the invention in a schematic representation,

5 shows a further exemplary embodiment of the invention in a schematic representation,

6 shows a further exemplary embodiment of the invention in a schematic representation,

7 shows a further exemplary embodiment of the invention in a schematic representation, 8 shows a diagram for the time course of various signals,

9 shows a further diagram for the time course of various signals,

10 is a diagram showing the dependence of the temperature increase generated by the laser pulses on the size of the irradiated area of the optical element,

11 is a diagram showing the dependence of the sound pressure level generated by the laser pulses on the size of the irradiated area of the optical element,

12 shows a diagram to illustrate the dependence of the signal / noise ratio of the signals detected by the sensor on the size of the irradiated area of the optical element,

13 shows a diagram to illustrate the dependence of the temperature increase generated by the laser pulses on the size of the irradiated area of the optical element,

14 shows a diagram to illustrate the dependency of the sound pressure level generated by the laser pulses on the size of the irradiated area of the optical element and FIG

15 shows a diagram to illustrate the dependence of the signal / noise ratio of the signals detected by the sensor on the size of the irradiated area of the optical element.

FIG. 1 shows an exemplary embodiment of a lithography system designed according to the invention in a schematic representation. The lithography system has an illumination system 1 and a projection objective 2. The light required to operate the lithography system is generated by a light source 3. A reticle stage 4, on which a mask 5, also referred to as a reticle, is fixed, is arranged between the illumination system 1 and the projection objective 2. The Reticlestage 4 has a Drive 6, viewed in the direction of light, after the projection lens 2 is arranged a substrate stage 7 which carries a substrate 8 such as a wafer and has a drive 9. Furthermore, a control device 10 is shown in FIG. 1, which is connected to the light source 3, the lighting system 1, the reticle stage 4, the projection objective 2 and the substrate stage 7. In addition, the control device 10 is connected to a manipulator 20, which is a component of the projection lens 2.

The light source 3 can in particular be a laser, for example an excimer laser, which generates light with a wavelength of 193 nm. The light source 3 can, for example, generate short laser pulses that are 100 ns long and have a repetition frequency of 6 kHz.

The lighting system 1 forms the light generated by the light source 3 by means of a series of optical components in a precisely defined manner and deflects it onto the mask 5. Depending on the embodiment, the lighting system 1 can be designed so that it contains the entire mask 5 or just one Part of the mask 5 is illuminated. The lighting system 1 is able to illuminate the mask 5 in such a way that almost identical light conditions prevail at each illuminated point of the mask 5. In particular, the light intensity and the angular distribution of the incident light are almost identical for each illuminated point of the mask 5.

The lighting system 1 is able to generate a large number of different angular distributions, which are also referred to below as lighting settings. The desired lighting setting is generally selected as a function of the structural elements formed on the mask 5. For example, dipole or quadrupole illumination settings are used relatively frequently, in which the light hits each illuminated point from two or four different directions. Depending on the design of the lighting system 1, the different lighting settings can be generated, for example, by means of different diffractive optical elements in conjunction with a zoom axicon lens or by means of mirror arrays, each of which has a large number of small mirrors arranged next to one another.

The mask 5 can be designed, for example, as a glass plate which is transparent to the light supplied by the lighting system 1 and on which opaque structures, for example in the form of a chrome coating. If only a partial area of the mask 5 is illuminated by the illumination system 1 at the same time, the drive 6 of the reticle stage 4 is controlled by the control device 10 in such a way that the mask 5 is moved relative to the illumination system 1 during the exposure of the substrate 8 and thereby the illuminated Partial area migrates over the entire mask 5. The substrate 8 is moved synchronously by a coordinated control of the drive 9 of the substrate stage 7, in which the imaging properties of the projection objective 2 are also taken into account. This is also referred to as scanning in the following.

The projection objective 2 images the illuminated mask 5 or the illuminated partial area of the mask 5 onto the substrate 8. In order to be able to convert the latent image thus formed onto the substrate 8 into a physical structure, a light-sensitive layer is applied to the substrate 8. The image of the mask 5 is exposed into this light-sensitive layer and a permanent structure can be produced therefrom on the substrate 8 with the aid of subsequent chemical processes. The drive 9 of the substrate stage 7 is controlled by the control device 10 in such a way that the mask 5 is imaged one after the other on different areas of the substrate 8. The mask 5 can be imaged as a whole or sequentially by scanning.

The projection objective 2 has a large number of optical elements, in particular lenses and / or mirrors. By acting at least one manipulator 20 on at least one of the optical elements, the imaging properties of the projection objective 2 can be influenced and, in particular, imaging errors can be reduced. Since lithography systems are used in particular in the production of semiconductors that have tiny structures, the projection lens 2 is an absolute precision instrument and immense effort is required to keep negative effects on the image quality away from the projection lens 2 or, for example, with the aid of the manipulators 20 to compensate.

With increasing miniaturization in semiconductor production, more and more influencing factors have to be taken into account and the individual influencing factors have to be compensated with ever greater precision. One of the influencing factors that increasingly cause problems is lens heating, that is to say the heating of the optical elements of the projection lens 2 by the light used for the imaging and the resulting light associated influencing of the imaging properties. For example, the imaging properties of lenses and mirrors change due to thermal expansion and thereby cause imaging errors.

Quasi-static lens heating effects, which show constant behavior after a certain settling time, are relatively easy to control, since they can be precisely analyzed in advance and suitable countermeasures can be provided. Even non-static but comparatively slow varying lens heating effects can still be controlled to a certain extent, since there is usually time available for measurements, for example when changing substrates. The shorter the time scale, the more difficult it becomes to measure or simulate the lens heating effects. Compensation is then ruled out due to a lack of knowledge of what is to be compensated. For lens heating effects that take place on a time scale of less than 10 seconds, there are currently no practicable measurement options in a lithography system.

With the aid of the invention, it is now possible to record fast lens heating effects by measurement, for example those that take place on a time scale of less than 10 seconds. The hardware used for this and the procedure are described in more detail using the following figures.

Fig. 2 shows an embodiment of the invention in a schematic representation. A section of a projection objective 2 designed according to the invention is shown.

The projection objective 2 has a housing 11, which can be designed as a hollow cylinder, for example. Optical elements 12, for example lenses and / or mirrors, are arranged inside the housing 11. In the section shown, there is only a single optical element 12, which is designed as a lens. The optical element 12 can have a coating on its surface, for example an anti-reflective coating. The optical element 12 is fastened to a holder 13, which fixes the optical element 12 with high precision in a desired position and, in the exemplary embodiment shown, is designed as a lens mount. The attachment can be implemented, for example, in the form of an adhesive or clamp connection. The holder 13 is attached to the housing 11. Alternatively, the housing 11 can also have several Brackets 13 are formed. In this case, the brackets 13 can be designed, for example, ring-shaped and stacked one on top of the other.

A sensor 14 is attached to the optical element 12, for example by gluing. The bond is to be produced very carefully so that this does not produce any impermissibly strong deformations of the optical element 12 or mechanical stresses are formed in the optical element 12 which influence its optical properties in an intolerable manner. The sensor 14 can be a structure-borne sound sensor, which is designed, for example, as an acceleration sensor or as a strain gauge. The sensor 14 is attached outside a region of the optical element 12 in which light strikes the optical element 12 during the exposure of the substrate 8. This irradiated area is not shown explicitly in FIG. It is only indicated schematically by a dashed arrow that light strikes the optical element 12. The sensor 14 is connected to an evaluation device 15.

The evaluation device 15 can be designed as an integral part of the control device 10 or as a separate unit. The structure and the mode of operation of the evaluation device 15 are explained below with reference to FIG. 3.

The embodiment of the invention shown in Fig. 2 is based on the following functional principle:

The light passing through the optical element 12 consists of very short laser pulses of high energy. Part of the energy is absorbed by the optical element 12 (including its coating) and converted into thermal energy. As a result of the very short pulse duration of the laser pulses, the thermal energy is suddenly deposited in the optical element 12 and leads to a rapid local expansion of the material of the optical element 12 structure-borne sound waves generated in this way, which are illustrated in FIG. 2 by concentrically arranged circular lines. The discrete frequencies are the resonance frequencies of the optical element 12. The structure-borne sound waves propagate in the optical element 12 and are finally detected by the sensor 14. Such a measuring method is also referred to as photoacoustic spectroscopy. Since the thermal energy deposited in the optical element 12 by the laser pulses depends on the optical absorption behavior of the optical element 12, conclusions can be drawn about the optical absorption behavior from the structure-borne sound waves that are generated with the aid of this thermal energy. The optical absorption behavior in turn has a considerable influence on lens heating effects, so that the lens heating effects can be estimated on the basis of the determined optical absorption behavior. Since the laser pulses hit the optical element 12 with a repetition frequency in the kilohertz range, the lens heating effects can be determined with a high time resolution. In addition, the determination can take place in situ during the exposure of the substrate 8, since the exposure light can be used to generate the structure-borne sound waves. This means that with this procedure a monitoring of lens heating effects is possible on a very short time scale well below 10 seconds and that this monitoring is possible during the exposure of the substrate 8.

In a modification of the embodiment shown in FIG. 2, direct mechanical contact between the sensor 14 and the optical element 12 is dispensed with and the surface of the optical element 12 is scanned with a light beam, for example, in order to detect the structure-borne sound waves. In particular, in this way a speed measurement can take place via the Doppler effect, from which information about the sound waves can in turn be determined.

3 shows a block diagram of an exemplary embodiment of the evaluation device 15. The evaluation device 15 has a lock-in amplifier 16 into which the signal output by the sensor 14 is fed. The lock-in amplifier 16 is mixed with one of the resonance frequencies of the optical element 12. The resonance frequency can be stored in a memory of the lock-in amplifier 16 or can be entered from the outside. The lock-in amplifier 16 can achieve a good signal / noise ratio and even very small signal amplitudes can be reliably detected. In addition, the signal level at the resonance frequencies is significantly increased due to the resonance effect, which has a positive effect on the signal strength. Since the optical elements 12 used in lithography for transmissive applications generally have extremely low optical absorption, and thus the excitation of the structure-borne sound waves is comparatively weak, the utilization of the resonance effect is of great importance. The output signal of the lock-in amplifier 16 is fed to a low-pass filter 17 in order to reduce the electronic noise. The filtered signal is a measure of the optical absorption of the optical element 12 (including the coating). The filtered signal is fed to an evaluation unit 18. The evaluation unit 18 compares the filtered signal with a reference value for the initial optical absorption of the optical element 12, which was determined immediately after the production of the projection objective 2. Since the masks 5 used later for the exposure are usually not available to the manufacturer of the projection lens 2, the reference value can be determined with the aid of a reference mask which, for. B. is used for calibration tasks anyway. It is also possible to determine several reference values with several reference masks. In particular, it is also possible to determine the reference value when the lithography system is first started up in the semiconductor factory using precisely the mask 5 that is provided for the exposure operation. Reference values for a plurality of masks 5 can also be determined in this case. The reference values determined in this way can always be used when the exposure is carried out with one of these masks 5. Alternatively, a comparison can also be made with a reference value determined by simulation or in some other way. The reference value can be stored in a memory of the evaluation unit 18 or can be entered externally.

If the comparison results in a deviation below a predefined threshold value, it is assumed that the optical absorption of the optical element 12 is in a permissible range. In this case, the exposure operation can be continued without additional measures. Otherwise, it is concluded that the optical absorption of the optical element 12 has an impermissible value and a warning message can be output and / or measures can be initiated to at least partially compensate for the associated optical effects. These measures can consist in the control of manipulators 20 (see FIG. 1) with which, for example, the position of one or more of the optical elements 12 can be influenced. However, it may also be necessary to replace the optical element 12, the optical absorption of which has an impermissible value. To implement the measures, the evaluation unit 18 can output a signal and process it further in the control device 10. It is also possible to determine the deviation repeatedly over long periods of time, for example over years, and in this way to detect the gradual degradation of the optical element 12. The deviation can be determined at predetermined time intervals, for example every quarter or every six months. A determination can also take place after a given irradiation of the optical element 12. A course of the deviation can be sketched from the determined values. From an extrapolation of the course into the future, it can be determined at which future point in time or at which irradiation the threshold value of the deviation is to be expected to be exceeded in a way that can no longer be compensated by actuating the manipulators 20, but rather one Requires replacement of the optical element 12. This information can be included in the planning of maintenance work, so that the replacement of the optical element 12 takes place within the framework of maintenance that is already provided before the threshold value of the deviation is no longer compensated for. In this way, on the one hand, the optical element 12 can be used almost to the end of its service life and, on the other hand, an unscheduled shutdown of the projection objective 2 to replace the optical element 12 can be avoided.

For example, a linear extrapolation can be carried out to predict the future course of the deviation. Such a procedure is suitable in the case of a linear increase in the optical absorption, for example in the case of an optical element 12 made of calcium fluoride.

There are also materials such as, for example, quartz glass, in which the optical absorption increases massively from a certain point in time and the optical element 12 then becomes unusable. This point in time is reached when the hydrogen present in the quartz glass has diffused out. In this case, no linear extrapolation of the deviation is carried out; instead, it is estimated from the deviations determined when the increase in the deviation is to be expected.

It is also possible to determine the deviation for different lighting settings. For example, several annular lighting settings with different radii can be used. Likewise, it is also possible to use dipole or quadrupole settings that rotate relative to one another by different angles about the optical axis are. In this way it is possible to scan different areas of the optical element 12 and to determine a local distribution of the degradation. In the event of progressive degradation of the optical element 12, this opens up the possibility of converting to the use of areas that have not been or only slightly degraded, if such areas are present. For this purpose, lighting settings are switched to, in which high light intensities occur in the less degraded areas. This changeover can also be used, for example, to bridge the remaining time until the next scheduled system maintenance.

The parameters used in the excitation of the structure-borne sound waves and the positioning of the sensor 14 or sensors 14 can be used to influence whether the optical absorption is primarily detected in the area of the surface of the optical element 12 or in the area of the volume of the optical element 12. Since a contamination of the optical element 12 as a rule primarily influences the surface absorption, a high surface absorption indicates a contamination. Such a measurement can thus be used to trigger a cleaning cycle, for example with the aid of a gas, and the result of the cleaning can also be monitored, i. H. Use the measurement to check whether the cleaning was successful. It is also possible with the aid of such a measurement to ensure that cleaning is only carried out for as long as is necessary to achieve a desired cleaning result. As a result, the stress on other components caused by cleaning can be reduced to a necessary minimum. In addition, the time required for cleaning can be kept short. In particular, an oxidizing gas can be used for cleaning.

From an evaluation of the degradation behavior of various optical elements in a large number of lithography systems, improved degradation models can be developed and particularly susceptible optical elements 12 can be identified and improvements can be made.

Fig. 4 shows a further embodiment of the invention in a schematic representation. The exemplary embodiment shown in FIG. 4 largely corresponds to the exemplary embodiment in FIG. 2. However, there is a difference with regard to the attachment of the sensor 14. In contrast to the exemplary embodiment in FIG. 2, in the exemplary embodiment in FIG. 4, the sensor 14 is not on the optical element 12, but on the bracket 13 of the optical element 12 attached. In an embodiment of the holder 13 not explicitly shown, which is fastened to the optical element 12 by means of thin spring elements, the sensor 14 can in particular be attached to an area of the holder 13 that is located downstream of the spring elements on the side facing away from the optical element 12. As a result, the risk of mechanical stresses or deformations of the optical element 12 can be reduced considerably. However, this also leads to a certain dynamic decoupling of the sensor 14 from the optical element 12, so that a suitable compromise has to be found in each individual case.

Fig. 5 shows a further embodiment of the invention in a schematic representation. The exemplary embodiment shown in FIG. 5 largely corresponds to the exemplary embodiment in FIG. 4. However, the exemplary embodiment according to FIG. Analogously to the exemplary embodiment in FIG. 4, each sensor 14 is attached to the holder 13 of an optical element 12 and connected to an evaluation device 15. The evaluation devices 15 are each designed as shown in FIG. 3 and have a corresponding mode of operation. Thus, in the exemplary embodiment in FIG. 5, a large number of optical elements 12, in the extreme case all optical elements 12, can be monitored with regard to their lens heating behavior. Depending on the results determined, a degraded optical element 12 could then be replaced, for example, or the optical effects could be compensated for by controlling suitable manipulators 20. These manipulators 20 can, for example, be devices known per se for moving or tilting one or more of the optical elements 12 or other devices known per se with which the imaging properties of the projection objective 2 can be influenced.

An analogous modification is also possible in the exemplary embodiment in FIG. 2, in which the sensor 14 is not attached to the holder 13, but rather directly to the optical element 12.

In the manner described above, not only optical elements 12 in the form of lenses can be monitored, but also mirrors, etc., for example. For this purpose, sensors 14 are attached to these optical elements 12 or to their holders 13. Fig. 6 shows a further embodiment of the invention in a schematic representation. Again, only a section of the projection lens 2 is shown. The section shown comprises an area with two optical elements 12, which are designed as lenses and are held by holders 13 designed as lens mounts. The optical elements 12 can each have a coating and are arranged at a distance from one another in the housing 11, so that a cavity 19 is enclosed by the optical elements 12 and the housing 11. The cavity 19 is filled with a gas, for example nitrogen. Inside the cavity 19, a sensor 14 designed as a microphone is attached to the housing 11. The sensor 14 is connected to the evaluation device 15, which can be designed according to FIG. 3.

When light, indicated by the dashed arrow, passes through the optical elements 12 and through the cavity 19, sound waves are excited by the photoacoustic effect in the gas which fills the cavity 19. In addition, structure-borne sound waves are excited in the optical elements 12 including the coating, but these will not be discussed in more detail for the time being. The sound waves propagate in the gas and are reflected on the adjoining surfaces of the housing 11 and the optical elements 12, so that sound waves are superimposed and, as a result, resonances are formed. The cavity 19 thus acts as a resonator. At the resonance frequencies of the cavity 19, which are predetermined by its geometry, the sound waves are considerably amplified and thus generate a significantly stronger signal when they strike the sensor 14 than at other frequencies. By suitably placing the sensor 14 at a point in the cavity 19 at which particularly high pressure fluctuations are generated in the gas by the sound waves, the signal strength can be further optimized.

The signal from the sensor 14 is fed to the lock-in amplifier 16 in accordance with the block diagram in FIG. In order to achieve a good signal yield, the lock-in amplifier 16 is mixed with one of the resonance frequencies of the cavity 19. The signal from the lock-in amplifier 16 is fed to the evaluation unit 18 via the low-pass filter 17. The evaluation unit 18 compares the filtered signal with a reference value for the correct gas mixture in this cavity 19. The reference value was determined at an earlier point in time. If the comparison results in a deviation below a predetermined threshold value, it is assumed that the optical absorption of the gas in the cavity 19 is in a permissible range and the exposure operation can be continued without additional measures. If the deviation exceeds the permissible threshold value, a possible cause may be that the optical absorption of the gas in the cavity 19 has an impermissible value and the gas is contaminated, for example as a result of a leak, or does not have the intended pressure. A warning message can then be output and / or measures can be initiated to at least partially compensate for the associated visual effects. These measures can consist, for example, in activating manipulators 20 or in restoring the correct gas composition and / or the correct gas pressure.

To implement the measures, the evaluation unit 18 can output a signal and process it further in the control device 10. Depending on the output signal, a leak search can also be started and after the leak has been localized and eliminated, the original gas mixture can be restored, for example by means of a suitable gas supply.

As a rule, however, a reliable determination of the cause of the exceeding of the threshold value requires further measures, since it cannot be avoided that when the light passes through the optical elements 12 as a result of the pulse-like expansion of the optical elements 12, additional pressure pulses and thus also sound waves in the adjacent gas can be generated. These additional sound waves are superimposed on the sound waves originally generated in the gas and are also detected by the sensor 14. For the interpretation of the signals detected by the sensor 14, it is helpful if, in addition to the sensor 14 designed as a microphone, there is a sensor 14 designed as a structure-borne sound sensor, which is arranged according to one of FIGS. 2, 4 or 5. If the signals from both sensors 14 vary in an analogous manner, a variation in the optical absorption of at least one of the optical elements 12 can be assumed to be the cause of the variation in the signals. If only the signal from the sensor 14 designed as a microphone varies, but not the signal from the sensor 14 designed as a structure-borne sound sensor, a variation in the optical absorption of the gas can be assumed to be the cause of the variation in the signal. Fig. 7 shows a further embodiment of the invention in a schematic representation. The exemplary embodiment illustrated in FIG. 7 largely corresponds to the exemplary embodiment in FIG. 6. However, the exemplary embodiment in FIG. Accordingly, further optical elements 12 are also shown in FIG. 7, which delimit the cavities 19. The sensors 14 are each connected to an evaluation device 15. The evaluation devices 15 can each be constructed as shown in FIG. 3 and operated as described in FIG. 6. Since the resonance frequencies depend on the geometry of the cavities 19, which as a rule differ from one another, the lock-in amplifiers 16 are mixed in a corresponding manner for different resonance frequencies.

In the exemplary embodiment in FIG. 7, several cavities 19 are thus monitored. In principle, all cavities 19 between successive optical elements 12 can be monitored in this way. However, it can happen that not all cavities 19 are equally well suited for such monitoring, for example because of their geometry. As a rule, it is therefore advisable to monitor only a suitable subset of cavities 19.

As already mentioned in FIG. 6, in addition to the sound waves through photoacoustic excitation of the gas in the cavity 19, structure-borne sound waves are also generated through photoacoustic excitation of the optical elements 12 including their coatings. The structure-borne sound waves can be detected with additional sensors 14 which are designed as structure-borne sound sensors. As already explained, by comparing the signals output by the individual sensors 14, conclusions can be drawn as to the extent to which changes in the signals output by the sensors 14 designed as microphones are caused by changes in the gas. Typical signal profiles are explained with reference to FIGS. 8 and 9.

8 shows a diagram for the time course of various signals. The signal strength is plotted in any units over time in milliseconds.

The diagram relates to a situation in which rectangular laser pulses with a wavelength of 193 nm, a pulse length of 100 ns and a pulse rate of 6 kHz are centered the optical element 12 designed as a lens or hit the gas in the cavity 19 and generate structure-borne sound waves or sound waves through the photoacoustic effect, which are detected by a sensor 14 designed as a structure-borne sound sensor or a microphone. The sensor 14 designed as a structure-borne sound sensor is arranged on the edge of the optical element 12. The sensor 14 designed as a microphone is arranged directly next to it on the housing 11, which delimits the cavity 19, so that the paths of the structure-borne sound waves or the sound waves to the respective sensor 14 are approximately the same.

The speeds of sound in the optical element 12 are approx. 6000 m / s and in the gas approx. 340 m / s. The optical element 12 has a diameter of 200 mm. The laser pulses have a diameter of 10 mm at the point of impingement on the optical element 12 or on the gas, so that in each case an approximately circular irradiated area with a diameter of 10 mm is present. The representation is greatly simplified and does not show any real signals, but the calculated behavior over time of the various signals, which are each represented as a sequence of square-wave pulses.

Three signal curves are plotted on top of one another. The upper signal curve relates to the laser pulses generated by the light source 3 and shows the first six pulses. The middle signal profile relates to the structure-borne sound waves generated in the optical element 12 and shows the first six pulses. The lower signal curve relates to the sound waves generated in the gas and shows the first five pulses.

The laser pulses are very narrow and have the time width of 100 ns mentioned at the beginning. The first laser pulse defines the zero point of the time scale. As can be seen from the pulse rate of 6 kHz, the laser pulses follow one another at a time interval of 167 ps.

The pulses of the structure-borne sound waves are still very narrow, but already considerably wider than the laser pulses. Their pulse width is typically about 1 to 10 ps and is particularly dependent on the diameter of the laser pulses when they strike the optical element 12, as long as this diameter is smaller than the thickness of the optical element 12. The pulse width results from the structure-borne sound waves as a result the spatial extent of the excitation area from the place of its origin to the place of detection cover different distances and consequently arrive at different times. In the present case, the pulse width is just under 2 ps. The pulses of the structure-borne sound waves follow one another at the same distance (start of pulse to start of pulse) as the laser pulses, da the sequence is directly predetermined by the laser pulses. However, the first pulse of the structure-borne sound waves is delayed in relation to the first laser pulse, since the structure-borne sound waves need a certain time to propagate from the place of their origin to the place of their detection. The time delay between the first pulse of the structure-borne sound waves and the first laser pulse thus depends on the location at which the laser pulses impinge and on the diameter of the optical element 12 and in the present case is approximately 16 ps. The same time delay to the respective causing laser pulse can also be observed in all subsequent pulses of the structure-borne sound waves.

The pulses of the sound waves (lower signal curve) are significantly wider than the laser pulses and also than the pulses of the structure-borne sound waves and can have pulse durations of approx. 10 to 200 ps. In the present case, the pulse duration is approx. 29 ps. This is based on the significantly lower propagation speed of the sound waves compared to the structure-borne sound waves with a comparable size of the excitation area. For the same reason, the time delay between the pulses of the sound waves and the respective associated laser pulses is significantly greater than that of the structure-borne sound waves and in the present case is approx. 280 ps.

It can thus be seen from the signal curves in FIG. 8 that the signals that are output by the sensors 14 designed as structure-borne sound sensors and the signals that are output by the sensors 14 that are designed as microphones differ considerably.

9 shows a further diagram for the time course of various signals. The representation corresponds to the representation in FIG. 8. In FIG. 9, only a few parameters have been changed compared to FIG. 8. The changed parameters are the diameter of the area irradiated with the laser pulses and the diameter of the optical element 12. In the diagram of FIG. 9, the irradiated area has a diameter of 40 mm and the optical element 12 has a diameter of 400 mm . The other parameters are selected to be identical to FIG. 8; in particular, the arrangement of the sensors 14 is also selected to be identical.

The time course of the laser pulses is unchanged compared to FIG. 8 (upper signal course). The larger diameter of the irradiated area compared with FIG. 8, however, leads to a larger excitation area both in the optical element 12 and in the gas. Consequently, in the diagram of FIG. 9, both the pulses of the structure-borne sound waves generated by optical absorption in the optical element 12 (middle signal curve) and the pulses of the sound waves generated by optical absorption in the gas (lower signal curve) are significantly wider than the corresponding pulses in the diagram of FIG Fig. 8 and have a width of just under 7 pm and just under 118 ps. If the diameter of the irradiated area is increased further, there is even the risk that the pulses generated by optical absorption in the gas will overlap.

The larger diameter of the optical element 12 compared to FIG. 8 leads to a greater distance between the location of the excitation and the location of the detection and thus to longer transit times and accordingly to the greater time delays in the optical element 12 (which can be seen in FIG. 9) middle signal curve) and pulses generated in the gas (lower signal curve) relative to the corresponding laser pulses (upper signal curve).

A problem with the use of the photoacoustic effect in lithography optics is the extremely high material quality of the optical elements 12 in this area and thus their low optical absorption, through which the structure-borne sound waves and sound waves are ultimately excited. This leads to comparatively weak signals which, depending on the selected parameters, can even be below the detection limit.

One way of increasing the signal strengths is to increase the intensity of the laser pulses used for the excitation. However, the pulse energy of the laser is usually specified and cannot be increased significantly. However, it is possible to irradiate only a small area and thereby concentrate the energy of the laser pulses in a small excitation area and in this way generate stronger signals. This is explained in more detail with reference to FIG. 10.

10 shows a diagram to illustrate the dependence of the temperature increase generated by the laser pulses on the size of the irradiated area of the optical element 12. For this purpose, FIG. 10 shows the temperature increase DT of the material of the optical element 12 over the diameter Ds _{pot of} the irradiated area applied. A logarithmic representation was chosen. The diagram relates to an optical element 12, which is designed as a lens made of quartz, which has a lens diameter of 300 mm, a lens thickness of 20 mm, an optical absorption of 2 × 10 ⁴ / cm and a relative thermal expansion of 5 × 10 ⁷ has. The laser pulses have a pulse energy of 5 mJ.

It can be seen from the diagram that the temperature increase of the optical element 12 decreases very sharply as the diameter of the irradiated area increases. This temperature increase occurs on a very short time scale in the order of magnitude of the pulse duration of the laser pulse and causes a sudden expansion of the material of the optical element 12. With a horizontal arrangement of the optical element 12, the vertical expansion speed in the interface with the gas is within the area irradiated by the laser pulse transformed into a sound velocity of the gas. The sound pressure thus generated in the gas is detected by means of the sensor 14. A very low-noise microphone is suitable as sensor 14 for this purpose. Whether the sound pressure can be measured depends on whether it is above the detection limit of sensor 14. This is explained in more detail with reference to FIGS. 11 and 12.

11 shows a diagram to illustrate the dependence of the sound pressure level generated by the laser pulses on the size of the irradiated area of the optical element 12. FIG on the size of the irradiated area of the optical element 12.

In FIG. 11, the sound pressure level Lp generated by the laser pulses is plotted over the diameter Dspot of the irradiated area. This course is shown by a solid line. As is to be expected from the temperature profile shown in FIG. 10, the sound pressure level decreases sharply as the diameter of the irradiated area increases.

In the same diagram, the full-band detection limit of the sensor 14 is shown by a horizontal dashed line. The sensor 14 can only detect photoacoustic signals whose sound pressure level is above this line. The diagram shows that this condition is met when the diameter of the irradiated area is no greater than approx. 10 mm. In FIG. 12, the signal / noise ratio S / R of the signals output by the sensor 14 is _{plotted over the diameter Ds pot of} the irradiated area (solid line). The full-band detection limit of the sensor 14 is again shown by a horizontal dashed line. The course of the signal / noise ratio corresponds to that in Fig.

11 shown curve of the sound pressure level. Accordingly, it can also be seen from the diagram in FIG. 12 that photoacoustic signals can only be detected with the sensor 14 if the diameter of the irradiated area is not greater than approximately 10 mm.

The signal / noise ratio of the signals output by the sensor 14 can be improved by suitable measures, in particular by using the lock-in amplifier 16. In the diagram in FIG. 12, this is illustrated by two additional curves. A dashed curve represents a signal / noise ratio increased by 10 dB. A dash-dotted curve represents a signal / noise ratio increased by 20 dB. Corresponding curves are also entered in the diagram in FIG. 11 (dashed and dash-dotted curve). For this purpose, the increased signal / noise ratios were converted into correspondingly increased sound pressures which without lock-in amplifier 16 etc. would deliver signals with correspondingly increased signal / noise ratios.

From the diagrams in FIGS. 11 and 12 it can be seen that with a signal / noise ratio increased by 10 dB, detection is still possible up to a diameter of the irradiated area of approx. 20 mm and with a signal / noise increased by 20 dB - Ratio detection is still possible up to a diameter of the irradiated area of just under 40 mm.

The diagrams in FIGS. 10 to 12 are based on an optical element 12 which, viewed in absolute terms, has a low optical absorption. In relation to the use in applications in lithography, however, the optical absorption is comparatively high. If instead an optical element 12 with an even lower optical absorption is used, the detection of the photoacoustically generated sound waves is even more difficult. This is explained below with reference to FIGS. 13 to 15. With the exception of the optical absorption, these figures are based on the same set of parameters as in FIG. 10 to 12. The optical absorption of the optical element 12, which in turn consists of quartz, has a value of 5 × 10 ⁵ / cm in FIGS. 13 to 15.

13 shows a diagram to illustrate the dependence of the temperature increase generated by the laser pulses on the size of the irradiated area of the optical element 12. The representation is selected analogously to FIG. The temperature increase in FIG. 13 has a course similar to that in FIG. 10. However, the values of the temperature increase for a given diameter of the irradiated area in FIG. 13 are significantly lower than in FIG. 10. This is also to be expected since the optical element 12 according to FIG. 13 generates less thermal energy as a result of the lower optical absorption will.

FIG. 14 shows a diagram to show the dependence of the sound pressure level generated by the laser pulses on the size of the irradiated area of the optical element 12, analogous to FIG. 11. FIG Signals output by the sensor 14 of the size of the irradiated area of the optical element 12 analogous to FIG. 12. The full-band detection limit of the sensor 14 is again shown in each case by a horizontal dashed line. The solid curve shows the course of the sound pressure level generated by the laser pulses (FIG. 14) or the signal / noise ratio S / R of the signals output by the sensor 14 (FIG. 15). The dashed curve and the dash-dotted curve in FIG. 15 represent the course of a signal / smoke ratio improved by 10 dB and 20 dB, respectively, and in FIG. 14 the sound pressure level calculated therefrom.

The curves in FIGS. 14 and 15 correspond qualitatively to the curves in FIGS. 11 and 12. However, due to the lower optical absorption in the curves in FIG. 14 compared to FIG. 11, there is a shift to lower sound pressure levels and in the curves 15 compared with FIG. 12 to a shift to lower signal-to-noise ratios. Ultimately, this has the consequence that, without additional measures, a meaningful measurement requires a diameter of the irradiated area below approx. 6 mm. If additional measures are used that increase the signal / noise ratio by 10 dB, measurements below a diameter of the irradiated area of approx. 10 mm are possible. When using additional measures that increase the signal / noise With a ratio of 20 dB, measurements below a diameter of the irradiated area of just under 20 mm are possible. The maximum possible diameters of the irradiated area are thus roughly halved at the transition from a material of the optical element 12 with an optical absorption of 2 × 10 ⁴ / cm to a material with an optical absorption of 5 × 10 ⁵ / cm.

As a central statement, it can be seen from the above considerations that the detection of the photoacoustically generated sound waves is facilitated with a given laser power with decreasing diameter of the irradiated area and below a maximum diameter of the irradiated area, which is determined by the optical absorption of the material of the optical element 12 and by the Measurement arrangement depends, is even possible in the first place. Using a material with a strong optical absorption for the optical element 12 in order to facilitate the detection only makes sense in special applications, since in lithography it is preferred to use materials with the lowest possible optical absorption. Optimizing the measurement arrangement is certainly useful. However, the improvements that can be achieved are limited. Thus, after the optimization of the measuring arrangement or as an alternative to optimizing the measuring arrangement, the reduction in the diameter of the irradiated area remains as a means for improving the detection of the photoacoustically generated sound waves.

In the case of a measurement during exposure operation, however, the optical beam path is designed for the most efficient exposure operation possible. A modification to optimize the measurement would have a detrimental effect on the efficiency of the exposure and is therefore usually ruled out. However, areas of the individual optical elements 12 of the projection objective 2 of different sizes are irradiated during the exposure operation. It is therefore possible to select optical elements 12 for the measurement in which the irradiated areas have the smallest possible diameter. This is usually the case with optical elements 12 close to the field, ie with optical elements 12 at a short distance from the object plane in which the mask 5 is arranged or from the image plane in which the substrate 8 is arranged or from any intermediate image planes that may be present. In the case of the optical elements 12 close to the object plane, however, the irradiated area is at least as large as the area of the object imaged at the same time. This also applies analogously to the optical elements 12 near the image plane or an intermediate image plane, the effective imaging scale also having to be taken into account in each case. Depending on the illumination of the mask 5, further optical elements 12 can be present in which the diameter of the irradiated areas is comparatively small. These optical elements 12 are optical elements 12 close to the pupil, ie optical elements 12 at a short distance from a pupil plane in which an aperture stop of the projection objective 2 is usually arranged. This applies in any case when an illumination setting is used in which the light strikes the mask 5 only in narrowly limited angle of incidence ranges. Typical such lighting settings are, for example, dipole and quadrupole settings.

The above considerations were based on detection of the photoacoustically generated sound waves or structure-borne sound waves during the exposure operation. However, it is also possible to carry out the detection in a separate measuring mode. Since the light is not required for the imaging of the mask 5 in the measurement mode, the restrictions associated therewith do not apply and an optimization for the best possible detection can be carried out. For this purpose, a locally highly concentrated lighting can be aimed for, particularly within the scope of the machine possibilities. With a given power of the light source 3, this can be achieved by a small diameter of the irradiated area.

Furthermore, there is the possibility of scanning one or more optical elements 12 during the measuring operation, i. H. to vary the position of the irradiated area on the respective optical element 12 in order to determine spatially resolved information regarding the optical absorption behavior.

Depending on the design of the lighting system 1, the size and position of the irradiated area can be influenced in different ways. This is explained in more detail below for two common types of lighting systems 1:

In a first type of lighting system 1, the lighting setting is set by means of a diffractive optical element (DOE) in conjunction with a zoom axicon. DOEs are available that create a lighting setting with two or more poles. Accordingly, two or more irradiated areas can be generated in the vicinity of the pupil plane of the projection objective 2. For this purpose, the mask 5 is from the Beam path removed. In principle, it is also possible to design a DOE in such a way that an illumination setting is generated with only one pole. Accordingly, only an irradiated area would be generated in the vicinity of the pupil plane. The diameter of the irradiated area can be varied with the help of the zoom axicon. In principle, there is also the possibility of influencing the irradiated area via a diaphragm in the pupil plane of the illumination system 1. But this is associated with a loss of light.

A variation of the irradiated area in the vicinity of the object plane and the conjugate planes of the projection objective 2 is possible with the aid of a field stop, i.e. H. a diaphragm, which is arranged in one of these planes, is possible. It is also possible to arrange such a diaphragm in the lighting system 1 and to image it in the object plane of the projection lens 2. Such a diaphragm in the lighting system 1 is also referred to as a ReMa diaphragm. Since the diaphragms do not concentrate the light, but only partially hide it, the diaphragm solution is primarily useful for influencing the position of the irradiated areas.

In a second type of lighting system 1, the lighting setting is set by means of mirror arrays. This allows almost any lighting settings to be set by appropriately controlling the mirror arrays. In particular, very small poles can be generated with it. When the projection system 2 is operated with such an illumination setting, one or more poles are accordingly generated in the vicinity of the pupil plane of the projection objective 2. Since almost any desired lighting settings can be generated with this lighting system 1, it is also possible to use it to generate lighting settings that have the same lighting pole, which, however, is each arranged at a different point.

The irradiated area in the pupil plane of the projection objective 2 and in the vicinity thereof can thus be scanned laterally without further aids. Since the light is only deflected slightly differently in each case, but not masked out, the irradiated area can be generated and scanned with almost no loss. The variation of the illuminated area in the vicinity of the object plane or a conjugate plane of the projection objective 2 is also only possible in this illumination system 1 by means of diaphragms and thus with losses. In the exemplary embodiments described above, one or more optical elements 12 in the form of lenses and / or one or more gas-filled cavities 19 of the projection objective 2 were monitored with the aid of the photoacoustic effect. In addition or as an alternative, the projection objective 2 can also have reflective, diffractive or other optical elements 12 with or without refractive power. In addition or as an alternative to the optical elements 12 designed as lenses and / or the gas-filled cavities 19, there is also the possibility of monitoring reflective, diffractive or other optical elements 12 with the aid of the photoacoustic effect.

It is also possible, in addition to the projection lens 2 or instead of the projection lens 2, to equip the lighting system 1 with one or more sensors 14 which detect structure-borne noise or sound waves generated by means of the photoacoustic effect. This can take place in a manner analogous to that described above for the projection objective 2.

The sensors 14 built into the projection lens 2 or the lighting system 1 can also be used to support the adjustment of the projection lens 2 or the lighting system 1 during manufacture or at a later point in time and / or to a quality inspection of the projection lens 2 or the lighting system 1 undergo. The measured values determined during production can be saved as reference values for subsequent measurements or for monitoring.

The photoacoustic measurements described above can be used during the operation of the lithography system in such a way that the measurements are used to determine information on the basis of which imaging errors of the lithography system are corrected. The correction can take place by manipulating at least one optical element 12 of the lithography system with the manipulators 20.

In particular, the photoacoustic measurements enable a predictive compensation of imaging errors of the projection lens 2. These imaging errors can be generated by heating parts of the projection lens 2 by the light used to expose the substrate 8. As already explained, with the photoacoustic measurements, data about a local optical absorption in the projection lens 2 or parts thereof, e.g. B. one or more optical elements 12 if necessary including one or more gas-filled cavities 19. The time-dependent heating expected during the exposure of the substrate 8 and the resulting imaging errors of the projection objective 2 can be calculated numerically from the data on the local optical absorption. The calculations can be carried out on the basis of models for the heating of the optical elements 12, which are also referred to as lens heating models.

Further information can be incorporated into the calculations. In particular, information about the illumination of the mask 5, such as, for example, the illumination setting used, can be incorporated. The lighting setting has an influence on the light distribution in the individual optical elements 12 and thus on their heating behavior. In addition, information about the mask 5, in particular about the structures present on the mask, can flow in. These structures also influence the light distribution in the optical elements 12 and thus their heating behavior.

The calculated imaging errors can be at least partially compensated for by controlling a manipulator 20 or a plurality of manipulators 20 of an optical element 12 in a time-dependent manner. With the manipulators 20, the position of the optical element 12 can be influenced by a rigid body movement. For example, the optical element 12 can be shifted or tilted. It is also possible to deform the optical element 12 with the aid of manipulators 20. All of these effects on the optical element 12 serve to influence the imaging properties of the projection objective 2 in a desired manner. Several optical elements 12 can also be manipulated.

The compensation of imaging errors can be carried out again and again in accordance with a predetermined scheme in order to achieve an imaging quality that is as constant as possible. For example, it can be provided that the compensation is carried out at least once for each exposure of a substrate 8 with the projection objective 2. Reference numbers

1 lighting system

2 projection lens

3 light source

4 reticle days

5 mask

6 drive

7 substrate days

8 substrate

9 drive

10 control device

11 housing

12 Optical element

13 Bracket

14 sensor

15 Evaluation device

16 lock-in amplifiers

17 low pass filter

18 Evaluation Unit

19 cavity

20 manipulator

Claims

1. Proj ektionsobj ektiv with a lithography system

- a housing (11),

- Optical elements (12) which are arranged within the housing (11) and image a mask (5) on a substrate (8) and

- At least one sensor (14) which is arranged within the housing (11) and detects photoacoustic signals.

2. Projection lens according to claim 1, wherein the sensor (14) is designed as a structure-borne sound sensor or as a microphone.

3. Projection objective according to one of the preceding claims, wherein the sensor (14) is arranged on one of the optical elements (12) or on a holder (13) of one of the optical elements (12).

4. Projection objective according to one of the preceding claims, wherein the housing (11) has at least one cavity (19) which is filled with a gas and the sensor (14) is arranged within the cavity (19).

5. Illumination system with a lithography system

- a housing (11),

- optical elements (12) which are arranged within the housing (11) and generate a lighting setting,

6. lithography system with a projection lens (2) and / or an illumination system (1) according to one of the preceding claims.

7. A method for monitoring a projection lens (2) of a lithography system, photoacoustic signals being generated within the projection lens (2) by irradiation of light and these photoacoustic signals being detected by a sensor (14).

8. The method according to claim 7, wherein the photoacoustic signals are generated by irradiating a region of an optical element (12) or a gas-filled cavity (19) within the projection lens (2).

9. The method according to claim 8, wherein the position of the irradiated area of the optical element (12) or of the gas-filled cavity (19) is varied.

10. The method according to any one of claims 7 to 9, wherein the photoacoustic signals are generated by excitation with light of the working wavelength of the projection lens (2).

11. The method according to any one of claims 7 to 10, wherein the photoacoustic signals are generated during the exposure process.

12. The method according to any one of claims 7 to 11, wherein the captured photoacoustic signals are compared with reference signals that were determined during a proper functional state of the projection lens (2).

13. The method according to any one of claims 7 to 12, wherein the detected photoacoustic signals are used to check whether there is an impermissibly strong degradation of the optical element.

14. The method according to claim 13, wherein an action is initiated if there is an impermissibly strong degradation of the optical element.

15. The method according to claim 14, wherein the action consists in controlling at least one manipulator (20) of at least one optical element (12).

16. The method of claim 14, wherein the action is to replace the optical element (12).

17. The method according to any one of claims 7 to 12, wherein the photoacoustic signals are recorded at different times or after different irradiations of the optical element (12) and from the course of the recorded photoacoustic signals or a related variable is determined at what future time or after which further irradiation an exchange of the optical element (12) will be necessary.

18. The method according to claim 17, wherein the replacement of the optical element (12) is carried out within the framework of a system maintenance that is provided anyway before the determined future point in time or the determined further irradiation is exceeded.

19. The method according to any one of claims 13 to 18, wherein the detected photoacoustic signals are used to check in a spatially resolved manner whether there is an impermissibly strong degradation of the optical element (12).

20. The method according to claim 19, wherein in the event of an impermissibly strong degradation in an area of the optical element (12) is switched to a lighting setting in which a lower light intensity occurs in this area of the optical element (12) than in the previous lighting setting.

21. The method according to any one of claims 7 to 20, wherein it is determined from the captured photoacoustic signals whether there is optical absorption primarily in the area of the surface of the optical element (20).

22. The method according to any one of claims 7 to 21, wherein it is determined from the captured photoacoustic signals whether cleaning of the surface of the optical element (20) is necessary and / or whether cleaning the surface of the optical element (20) is a desired result has delivered.

23. The method according to any one of claims 7 to 22, wherein signals are output from the sensor (14) to a lock-in amplifier (16).

24. The method according to claim 23, wherein the lock-in amplifier (16) is mixed with a frequency which corresponds to a cavity resonance of the gas-filled cavity (19) or a structural resonance of the optical element (12).

25. A method for producing a projection lens (2) of a lithography system, photoacoustic signals being generated within the projection lens (2) by irradiation of light and a check being carried out on the basis of these photoacoustic signals to determine whether a predetermined specification is being met.

26. A method for predictive compensation of imaging errors of a projection lens (2) of a lithography system, which are generated by heating parts of the projection lens (2) with exposure light, with which the projection lens (2) images a mask (5) on a substrate (8) , whereby

- By means of photoacoustic measurement, data are collected about a local optical absorption in the projection lens (2) or parts thereof,

- from the data on the local optical absorption and further data on the illumination of the mask (5) and / or structures present on the mask (5), a time-dependent heating expected during the exposure and the resulting imaging errors are calculated numerically and

- the calculated imaging errors are at least partially compensated by time-dependent control of at least one manipulator (20) of an optical element (12) of the lithography system.

27. The method according to claim 26, wherein the manipulation of the optical element (12) takes place by a rigid body movement of the optical element (12) and / or by deformation of the surface of the optical element (12).

28. The method according to any one of claims 26 or 27, wherein the imaging errors of the projection lens (2) are corrected at least once for each exposure of a substrate (8) with the projection lens (2) by predictive compensation.

29. A method for operating a lithography system, wherein information is determined by photoacoustic measurements and the imaging properties of the lithography system are corrected on the basis of this information by manipulating at least one optical element (12) of the lithography system.