DE102021212993A1 - Method for measuring an absorption of an electromagnetic radiation by an optical element, device for measuring a transformation property of an optical element and lithography system - Google Patents

Abstract

The invention relates to a method for measuring an absorption of electromagnetic radiation by an optical element (2), in particular by an optical element (116, 118, 119, 120, 121, 122, Mi, 207) of a lithography system, in particular a projection exposure system (100, 200 ), In a defined wavelength range, after which the optical element (2) is irradiated with a test radiation (13) and is heated at least locally by the absorption of the test radiation (13), after which prior to irradiation with the test radiation (13) one of one The measuring beam (5) generated by the radiation source (3) is fed to the optical element (2) and reshaped by a reshaping property of the optical element (2) and a first output wavefront (7a) of the reshaped measuring beam (5) is measured, after which, after irradiation with the Test radiation (13), the measuring beam (5) fed to the optical element (2), by means of a shaping property changed by the heating of the optical element (2) is reshaped and a second output wavefront (7b) of the reshaped measuring beam (5) is measured, after which the first output wavefront (7a) and the second output wavefront (7b) are subtracted from one another and an output wavefront deviation is determined and after which from the output wave front deviation on the change in the conversion property and thus on the heating and thus on the absorption of the optical element (2) is closed. It is also provided that a first input wavefront (6a) of the measuring beam (5) is measured at least approximately synchronously with the determination of the first output wavefront (7a) before it is fed to the optical element (2) and at least approximately synchronously with the determination of the second output wavefront (7b ) a second input wavefront (6b) of the measuring beam (5) is measured before it is fed to the optical element (2), after which the first input wavefront (6a) and the second input wavefront (6b) are subtracted from one another and an input wavefront deviation is determined, after which the output wave front deviation and the input wave front deviation are subtracted from one another and a corrected output wave front deviation is determined.

Description

The invention relates to a method for measuring absorption of electromagnetic radiation by an optical element, in particular by an optical element of a lithography system, in a defined wavelength range, after which the optical element is irradiated with test radiation and is heated at least locally by the absorption of the test radiation , according to which a measuring beam generated by a radiation source is supplied to the optical element before the irradiation with the test radiation and is shaped by a shaping property of the optical element and a first output wavefront of the shaped measuring beam is measured, after which the measuring beam supplied to the optical element is measured after the irradiation with the test radiation is reshaped by a change in heating by the reshaping property of the optical element and a second output wavefront of the reshaped measuring beam is measured, after which the first output wavefront and the second output wave fronts are subtracted from one another and an output wave front deviation is determined and after which the change in the conversion property and thus the heating and thus the absorption of the optical element are inferred from the output wave front deviation.

The invention also relates to a device for measuring a transformation property of an optical element, in particular an optical element of a lithography system, with a radiation source and a wavefront sensor, with a measurement beam emitted by the radiation source having an input wavefront and an output wavefront shaped by the transformation property, and with the wavefront sensor being set up is to determine the output wavefront.

The invention also relates to a lithography system, in particular a projection exposure system for microlithography, with an illumination system that has a radiation source, illumination optics and projection optics, the illumination optics and/or the projection optics having at least one optical element.

Optical elements for guiding and shaping electromagnetic waves are known from practice. Examples of optical elements that can be mentioned are planar mirrors, concave mirrors, convex mirrors, facet mirrors, convex lenses, concave lenses, convex-concave lenses, plano-convex lenses and plano-concave lenses. Glass and silicon, among others, are known as materials for optical elements, in particular mirrors.

In order to avoid undesired structures of the wavefronts of the electromagnetic waves formed by optical elements, it is advantageous to have an exact control and precise formation of the properties of the optical elements interacting with the electromagnetic waves.

In particular, the property of the optical elements to be able to reshape wave fronts should be defined.

Projection exposure systems have a large number of optical elements. In particular when using the optical elements in a microlithographic DUV (deep ultraviolet) projection exposure system and especially when using them in a microlithographic EUV (extreme ultraviolet) projection exposure system, precise knowledge and control of the wavefront-influencing properties of the optical elements used is of particular importance Meaning.

The optical elements are exposed to a variety of external influences that change their properties and thus impair their functionality. Furthermore, even before they are used in the projection exposure system, the optical elements can have undesired properties which reduce and/or impair the functionality of the projection exposure system.

For example, it is known from practice that optical elements also absorb the light waves that they are only intended to guide and shape. Such absorption of the light leads both to an unfavorable reduction in the intensity available at the end of the projection exposure system and to an unfavorable heating of the optical elements themselves.

Thermal expansion of the materials forming the optical elements, which is associated with the heating, can change the geometry of the optical elements and thus a deformation property of the optical elements that forms the electromagnetic wave fronts. especially rehearsal Such a change in the optical properties is lematic if it occurs dynamically. For example, at the start of operation when all the optical elements are cool, the transformation property can be formed correctly, while at a later point in time after absorption of light energy, the optical elements are heated and the transformation properties are therefore changed. This can lead to a reduced functionality of a projection exposure system during operation.

For the reasons mentioned, it is of particular advantage if the optical elements have the lowest possible degree of absorption.

In order to optimize the degree of absorption of an optical element, it is known from practice to coat the optical element with an absorption-reduced layer, for example an absorption-reduced reflection layer and/or absorption-reduced anti-reflection layer. A determination of the suitability of an absorption-reduced layer used is of particular importance if such a suitability determination allows a systematic search for optimized coatings.

From the publication "Photo-thermal Measurement of Absorption Losses, Temperature induced Wavefront Deformation and Compaction in DUV-Optics", Bernd Schäfer, Jonas Glober, Uwe Leinhost and Klaus Mann, OPTICS EXPRESS Volume 17, No. 25, December 7, 2009, pages 23025-23036, a measuring system for the quantitative detection of transient and irreversible lens effects in DUV optics, which were induced by absorbed UV laser radiation, is known.

The system known from the prior art is based on a Hartmann-Shack wavefront sensor for precise, continuous monitoring of wavefront deformations of a collimated test laser beam, which is transmitted through a laser-irradiated point on a sample. Due to the temperature dependence of the refractive index and the thermal expansion, the initially flat wavefront of the test laser is disturbed. The shape of the wavefront change depends on a sign and an amount of change in refractive index and extension. This transient disturbance of the wavefront provides a quantitative measure of absorption losses in the sample. In the case of quartz glass, an additional permanent change in the wavefront indicates irreversible material compaction. In the prior art, an excimer laser, which irradiates the sample, is used to heat the sample. The light from the excimer laser is absorbed by the sample and heats it up.

A disadvantage of the prior art is that the absorption losses determined according to the prior art cannot be reliably determined since the measurements can sometimes be faulty.

The present invention is based on the object of creating a method which avoids the disadvantages of the prior art, in particular enabling a further improved, correct and precise determination of an absorption factor of an optical element.

According to the invention, this object is achieved by a method for measuring an absorption of electromagnetic radiation by an optical element, in particular by an optical element of a lithography system, in a defined wavelength range with the features mentioned in claim 1.

The present invention is also based on the object of creating a device which avoids the disadvantages of the prior art, in particular which enables a further improved, correct and precise determination of a deformation property of an optical element.

This object is achieved by a device for measuring a transformation property of an optical element having the features mentioned in claim 6.

The present invention is also based on the object of creating a lithography system which avoids the disadvantages of the prior art, in particular having optical elements with precisely measured reshaping properties.

This object is achieved by a lithography system having the features specified in claim 16.

In the method according to the invention for measuring absorption of electromagnetic radiation by an optical element, in particular by an optical element of a lithography system in a defined wavelength range, the optical element is irradiated with a test radiation and is heated at least locally by the absorption of the test radiation. Before being irradiated with the test radiation, a measuring beam generated by a radiation source is fed to the optical element and shaped by a shaping property of the optical element, and a first output wave front of the shaped measuring beam is measured. Furthermore, after the irradiation with the test radiation, the measuring beam supplied to the optical element is reshaped by a reshaping property of the optical element that has been changed by the heating. A second output wavefront of the reshaped measurement beam is measured. Furthermore, the first and second output wave fronts are subtracted from one another and an output wave front deviation is determined. Furthermore, the change in the conversion property and thus the heating and thus the absorption of the optical element are inferred from the output wave front deviation. According to the invention, a first input wavefront of the measuring beam is measured before it is fed to the optical element, at least approximately synchronously with the determination of the first output wavefront, after which the first input wavefront and the second input wavefront are subtracted from one another and an input wavefront deviation is determined, and then the output wavefront deviation and subtracting the input wavefront excursion and determining a corrected output wavefront excursion.

The measurement of the second output wave front after the irradiation is preferably carried out in such a way that the irradiation has already had a sufficient effect on the optical element at the time of the measurement. This includes a course of the method in which the irradiation is continued after the beginning of the irradiation before, during and after the measurement of the second output wavefront with and in particular also without interruptions. Accordingly, the measurement can also take place during the irradiation, provided that a non-zero period of time has elapsed between the beginning of the irradiation and the measurement.

Furthermore, subtracting the first and the second input wavefront or the first and the second output wavefront is to be understood in such a way that an amount of a difference between the first and the second input wavefront or the first and the second output wavefront is determined.

When determining the corrected output wavefront deviation, a difference between the first output wavefront and the first input wavefront and a difference between the second output wavefront and the second input wavefront can be determined in a mathematically equivalent manner and instead of the described procedure. In a second step, the corrected output wavefront is determined by subtracting the difference between the second output wavefront and the second input wavefront from the difference between the first output wavefront and the first input wavefront. As already described, the absolute value of the result of the subtraction can be considered here.

In the method according to the invention, the measuring beam can preferably be fed to the optical element in such a way that the measuring beam interacts with the optically relevant components of the optical element that form the deformation property. If the optical element is designed as a mirror, for example, then the measuring beam is to be fed to a reflecting area of the mirror and is reflected by it. If the optical element is designed as a lens, the measuring beam should preferably be fed to the lens in such a way that it passes through it in an area that is relevant for the use and operation of the lens.

The inventors have recognized that changes in a wavefront incident on the optical element to be examined, which occur between the measurement of the unheated and the heated optical element, can falsify the measurement results.

In particular, changes in the incident wavefront can be erroneously attributed to a change in the transformation property of the optical element under study due to the induced heating. A layer that is actually suitable for avoiding absorption can therefore be incorrectly classified as unsuitable, since changes in the incident wavefront are attributed to heating that did not occur and thus to non-existent absorption.

The method according to the invention has the advantage that a change in the measuring beam, in particular the shape of the wavefront of the measuring beam before and after the irradiation, is also taken into account. If the shape of the measuring beam changes during the period of irradiation or between the determination of the first and the second output wavefront, this change would be incorrectly attributed to a change in the transformation property. However, such a change in the measurement beam can be quantified very precisely using the method according to the invention by determining the first and the second input wavefront. The difference between the first and the second input wave front, the input wave front deviation, therefore provides information about the temporal stability of the measuring beam.

If, for example, a very small input wave front deviation is determined at all points, it can possibly be assumed that the measurement beam is only subject to very small fluctuations. If a large input wave front deviation is determined, this indicates a large instability of the measuring beam. By determining the input wavefront deviation, such instabilities of the measuring beam can be quantified and the output wavefront deviation can be corrected accordingly in order to determine that difference in the output wavefronts as the corrected output wavefront deviation, which is actually a change in the deformation property of the optical element, for example due to a temperature-dependent change in a refractive index of a material a lens.

In an advantageous development of the method according to the invention, it can be provided that the output wave fronts are measured with a first Hartmann-Shack sensor and the input wave fronts are measured with a second Hartmann-Shack sensor.

The use of Hartmann-Shack sensors to determine the wavefronts has the advantage that Hartmann-Shack sensors are known to be precise and reliable wavefront sensors in practice. The use of a second Hartmann-Shack sensor for determining the input wavefronts has the advantage that the first output wavefront and the first input wavefront or the second output wavefront and the second input wavefront can be determined almost simultaneously. Furthermore, no repositioning or modifications to a measuring stand used are required.

Advantageously, care can be taken that the first Hartmann-Shack sensor and the second Hartmann-Shack sensor each detect the same area of the wavefront of the measuring beam or the output wavefronts and the input wavefronts. As a result, the deformation property can advantageously be determined accurately and precisely. If the image sections of the first and the second Hartmann-Shack sensor were different, different areas of the measuring beam, which are subject to different dynamics, could under certain circumstances be viewed, and thus an incorrect result could be achieved.

It is advantageous if the input wave fronts are measured at least approximately directly on the optical element.

It is advantageous to measure the input wavefront as close as possible to the optical element, since changes and instabilities in the measuring beam or the input wavefronts, for example due to density fluctuations in the gas atmosphere through which the measuring beam can run between the radiation source and the optical element, lead to a change in the wavefront be able.

If such fluctuations or changes in the measuring beam occur after the point at which the input wavefront is measured, then these can no longer be taken into account when determining the wavefront and may falsify the result of the output wavefront deviation.

On the other hand, if the optical path length between a branching point of the measuring beam in a part that is measured by the second Hartmann-Shack sensor and a part that is supplied to the optical element is too long, the branched part of the measuring beam along this path length can cause further interference experienced, which can lead to greater errors in the determination of the input wavefronts.

The input wavefront is thus preferably determined immediately before the measurement beam is fed to the optical element.

It is advantageous if a diode laser is used as the radiation source.

Using a diode laser as a radiation source has the advantage that a diode laser is a cost-effective radiation source that can be obtained in many different wavelengths.

However, a disadvantage of a diode laser is its fluctuating beam quality, in particular the fact that the beam quality or the shape of the wave fronts emitted by the laser can fluctuate over time, in particular on a time scale of seconds, minutes and/or hours. Fluctuations in the beam quality include fluctuations in the wave fronts of the measuring beam.

Such fluctuations can, however, be determined particularly efficiently and advantageously by the method according to the invention, and the output wavefront deviation can be corrected in such a way that these fluctuations are taken into account. The method according to the invention is therefore particularly suitable for measurement setups that use a diode laser and are therefore particularly inexpensive.

It is advantageous if the measuring beam before and/or after the optical element is received at least approximately completely by at least one beam guiding device.

If the measuring beam is guided in a beam line device, only very small or no density fluctuations of the surrounding gas atmosphere occur within the beam line device.

This further reduces the instability of the measuring beam. Furthermore, instabilities of the measuring beam after the optical element, which are caused by density fluctuations in the surrounding gas atmosphere, are suppressed. This is of particular advantage since changes in the measuring beam which occur after the interaction of the measuring beam with the optical element has ended and which are variable in particular over time cannot be distinguished from a change in the deformation property.

For example, a glass body and/or in particular one or more fiber optics can be provided as the beam line device.

In order to take account of disturbances in the output wavefront which act on the output wavefront between the optical element and the wavefront sensor, provision can be made for a reference output wavefront to be measured at least approximately immediately after the optical element. For this purpose, for example, a third and/or further wavefront sensors, preferably Hartmann-Shack sensors, can be provided.

The invention also relates to a device for measuring a transformation property of an optical element.

The device according to the invention for measuring a transformation property of an optical element comprises a radiation source and a wavefront sensor, with a measurement beam emitted by the radiation source having an input wavefront and an output wavefront shaped by the transformation property, with the wavefront sensor being set up to determine the output wavefront. According to the invention, a reference measuring device is provided in order to determine the input wavefront.

By determining the input wavefront, the conversion property of the optical element can be determined in absolute terms, since the conversion property results from the effect of the optical element on the input wavefront.

It is advantageous if a device is provided and set up to determine the transformation property from the output wavefront and the input wavefront.

The transformation property can advantageously be determined by comparing the output wavefront with the input wavefront, in particular by subtraction.

For example, the device can be set up to subtract a three-dimensional representation of the input wavefront from a three-dimensional representation of the output wavefront.

The wavefront sensors provided within the scope of the invention are preferably Hartmann-Shack sensors.

If the wavefront is measured using a Hartmann-Shack sensor, an angle of incidence of the wavefront on the subaperture and thus the course of the wavefront can be reconstructed for each subaperture. A change in the course of the wavefront, ie the reshaping of the input wavefront to the output wavefront, therefore characterizes the reshaping property of the optical element. The device can therefore preferably be set up to compare the wavefront course with the resolution of the subapertures between the output wavefront and the input wavefront.

It can be advantageous if the device is set up to determine a first output wavefront and a first input wavefront in a first time period and to determine a second output wavefront and a second input wavefront in a second time period and to determine a change in the transformation property from this.

If the device according to the invention is used in at least two different periods of time, it can be determined to what extent the deformation property changes between a first period of time and a second period of time that lies after the first period. This can always be an advantage when dynamic changes in a physical property of the optical element are to be expected. Such dynamic changes can occur in particular with temperature fluctuations, which cause changes in the refractive index and/or a distortion and/or a different expansion of different areas of the optical element.

It is advantageous if the reference measuring device is formed from at least one beam splitter and a second wavefront sensor.

If a beam splitter is positioned in the beam path of the measuring beam as close as possible in front of the optical element, so that part of the measuring beam is decoupled before it strikes the optical element and before the transformation property affects the measuring beam, the decoupled part of the measuring beam can be easily be measured with a second wavefront sensor, in particular a Hartmann-Shack sensor.

It is particularly important to ensure that the sections of the wavefront that are imaged by the wavefront sensor and the second wavefront sensor are congruent.

In this way, the output wavefronts and the input wavefronts can be determined approximately at the same time. As a result, fluctuations and fluctuations in the measurement beam, caused, for example, by fluctuations in the beam quality of the radiation source and/or by density fluctuations in a surrounding gas atmosphere, can be advantageously corrected or minimized.

Changes in the wave fronts, which are caused by the beam splitter, can be regarded as approximately constant over time and therefore do not significantly impair the functionality of the device.

Alternatively, it can be provided that the reference measuring device is formed from at least one beam splitter, at least one deflection mirror, at least one closure device and the wavefront sensor.

In order to implement the device according to the invention in a simple and cost-effective manner, it can be provided that the input wave front is determined with the same wave sensor as the output wave front.

For this purpose, for example, a part of the measurement beam can be decoupled by means of a beam splitter in front of the optical element and then cast obliquely onto the wave front sensor by means of a mirror. Oblique incidence appears in a Hartmann-Shack sensor as a constant offset of a lateral deflection of the individual foci of the sub-apertures of the Shack-Hartmann sensor, with the lateral offset caused by the constant oblique angle of incidence for calibration either being determined empirically and/or based on the angle can be calculated. In this way, a constant correction factor for determining the input wavefront can be determined, and the input wavefront can be determined using the wavefront sensor.

A closure device or shutter device can be provided in order to determine the output wave front and the input wave front with the shortest possible time interval.

To determine the input wave front, the shutter device covers the beam path of the measuring beam between the optical element and the wave front sensor in such a way that only the decoupled part of the measuring beam carrying the input wave front impinges on the wave front sensor. The wavefront sensor measures the input wavefront. After the input wavefront has been measured, the closure device covers that part of the measuring beam that carries the input wavefront and releases a part of the measuring beam that carries the output wavefront, so that the wavefront sensor can determine the output wavefront. If this happens in rapid succession, for example within milliseconds, the influence of fluctuations that vary over time on the shape of the wavefronts can be minimized, so that it can be assumed that the input wavefront and output wavefront are determined approximately synchronously.

Alternatively, it can be provided that instead of an oblique incidence, a beam combiner is provided in order to couple the part of the measurement beam carrying the input wave front back into the beam path of the measurement beam after the optical element. In this case, the closure device is provided and set up to release and/or interrupt both parts of the measuring beam.

Furthermore, it can be provided that, for example in order to realize a cost-effective implementation of the device and to enable an actually simultaneous recording of the wavefront, the sensor area of the wavefront sensor is divided in such a way that the output wavefront is determined in a first partial area and the input wavefront is determined in a second partial area . For this purpose, the branches of the measuring beam, which carry the output wavefront or the input wavefront, can be adjusted with the aid of optics and mirrors such that only the specified partial areas of the sensor area detect the intended beam branches.

It is advantageous if a heating device is provided in order to heat the optical element.

In order to determine the temperature stability of an optical element and in particular the functionality of an optical element under temperature fluctuations, provision can be made for the optical element to be heated and for the deformation property to be determined in the cold and heated or heated state. In particular, it can be provided to identify temperature ranges in which the deformation property is almost unchangeable. From this it can be derived in which temperature range the optical element is to be kept during operation.

It is advantageous if the heating device is designed as a heating radiation source for irradiating the optical element with a test radiation.

A frequent source of heat, which leads to the heating of an optical element, is the radiation itself guided and shaped by the optical element. It can therefore be expedient to design the heating device as a radiation source for irradiation with a test radiation.

It can be advantageous if the test radiation is designed in such a way that the test radiation is absorbed by the optical element.

Since the above-mentioned heating of the optical element by the radiation guided and shaped through the optical element is itself mediated by an absorption process, the optical element can be heated particularly efficiently by the test beam if the test radiation is absorbed by the optical element.

In particular, it can be provided that the test radiation has that wavelength which corresponds to the wavelength of that radiation for the guidance and shaping of which the optical element is provided.

Of course, test radiation that is not absorbed by the optical element can also be advantageous insofar as this allows conclusions to be drawn about a very low degree of absorption of the optical element in the wavelength range of the test radiation.

It can be provided that the test radiation has a wavelength of 190 nm to 195 nm, preferably 193 nm and/or 245 nm to 250 nm, preferably 248 nm, and/or a wavelength of 5-30 nm, preferably 13.5 nm, having.

It can be provided that the measuring beam has a wavelength of 500 nm to 700 nm, preferably 631 nm.

It is advantageous if the device is set up to infer a degree of absorption of the optical element from a change in the conversion property after the optical element has been irradiated with the test radiation compared to a conversion property before the irradiation.

In particular, a very low degree of heating after irradiation of the optical element with the test radiation and thus a very low degree of absorption, or the presence of a very low degree of absorption, can be determined from the fact that a change in the shaping property after irradiation compared to the Forming property is very low before irradiation.

In this way, the device according to the invention can be used, for example, to determine the degree of absorption of different coatings of the optical element at a specific wavelength of the test beam.

If the test beam is selected in such a way that its wavelength corresponds to the wavelength of the light that interacts with the optical element during regular use, very low absorption can indicate that a coating tested is particularly suitable for such a use.

It is advantageous if at least one beam line device is provided in order to receive the measuring beam at least almost completely.

Furthermore, provision can be made for arranging the device in a vacuum environment in order to reduce density fluctuations in a surrounding gas atmosphere and to prevent too rapid cooling of the optical element through interaction and heat transfer to the surrounding gas atmosphere. Under certain circumstances, such a premature cooling can make the degree of absorption appear too low, in particular if the optical element is used regularly under vacuum conditions and heat transfer to the gas is low.

Provision can also be made for arranging the device in an inert gas environment, in particular a xenon environment. In this way, for example, light-induced molecular vibrations in gas molecules can be avoided, which can lead to refractive index fluctuations.

If the measuring beam is guided in a beam line device before and/or after the optical element, only very small or no density fluctuations occur within the beam line device due to the surrounding gas atmosphere.

This suppresses instabilities caused by density fluctuations in the surrounding gas atmosphere. This is of particular advantage since changes in the measuring beam which occur after the interaction of the measuring beam with the optical element has ended and which are variable in particular over time cannot be distinguished from a change in the deformation property.

If the beam line device consists, for example, of hollow and/or solid glass bodies, a beam splitter can be easily implemented in that the glass body has an inclined exit surface on which a part of the measuring beam exits the glass body to the optical element in the direction of the second shack Hartmann sensor is reflected.

For this purpose, it is of particular advantage if the beam line device extends to the second Shack-Hartmann sensor or is designed in such a way in an area in which the part of the measuring beam reflected in the direction of the second Shack-Hartmann sensor exits the beam line device that a determination of the input wavefronts can be done unhindered.

To measure heating of the optical element or the absorption of the measurement radiation by the optical element, a reference measurement is therefore first carried out in the method according to the invention, which is why this period or this phase of the measurement is referred to as the reference phase.

In this case, the first input wavefront of the measuring beam is recorded by the second wavefront sensor and the first output wavefront is recorded by the wavefront sensor. Each wavefront sensor preferably carries out a number of measurements per second or measures a number of wavefronts per second. Within the scope of the invention, a recording rate of 5 to 60, preferably 20 to 30, in particular 20 recordings per second has proven to be particularly suitable.

The measurements by the wavefront sensor and the second wavefront sensor take place at least approximately simultaneously, so that individual recordings w _R,i take place within a short time of one another. In particular, it has proven to be advantageous in this context if the time interval between the individual recordings of the two wavefront sensors is small compared to the time interval between the next recording and the same wavefront sensor. Preferably, the interval between the measurements of the wavefront sensor and the second wavefront sensor can be 0.02 seconds, while the interval between the measurements of the wavefront sensor can be 0.05 seconds.

According to the prior art, an averaged wavefront over N individual recordings is determined to determine the first output wavefront W _R : $$W_R=\frac{1}{N}{\displaystyle \sum _{i=1}^N{w}_{R,i}}$$

In contrast, in the method according to the invention, individual recordings w _R2,i of the second wavefront sensor are subtracted from the individual recordings w _R,i of the wavefront sensor, and the N individual recordings determined in this way are averaged. The wavefront determined and averaged in this way is a corrected first output wavefront w′ _R,i which has been corrected for the input wavefronts w _R2,i measured by the second wavefront sensor. $$W{\text{'}}_R=\frac{1}{N}{\displaystyle \sum _{i=1}^N\left(w_{R,i}-{\mathrm{<figure-callout\; id="2"\; label="w\; R"\; filenames="DE102021212993A1\_0014.png,DE102021212993A1\_0015.png"\; state="\{\{state\}\}">w</figure-callout>}}_R\right)}$$

Here, the corrected first output wavefront W' _R is shown in the formula (2). While the first output wavefront W _R determined using a prior art method may contain a randomly changing signature due to instabilities of the radiation source, this signature has been removed from the corrected first output wavefront W' _R in the method according to the invention.

As an alternative, but mathematically equivalent to the aforementioned calculation method proposed in formula (2), the signature, which can be caused by fluctuations in the radiation source, can also be corrected for the averaging instead of for the individual recordings. Such a procedure is formulated in formula (3). $$W{\text{'}}_R=W_R-{\mathrm{<figure-callout\; id="2"\; label="W\; R"\; filenames="DE102021212993A1\_0014.png,DE102021212993A1\_0015.png"\; state="\{\{state\}\}">W</figure-callout>}}_R=\frac{1}{N}{\displaystyle \sum _{i=1}^N{w}_{R,i}}-\frac{1}{N}{\displaystyle \sum _{i=1}^{\mathrm{<figure-callout\; id="2"\; label="N\; w\; R"\; filenames="DE102021212993A1\_0014.png,DE102021212993A1\_0015.png"\; state="\{\{state\}\}">N</figure-callout>}}{w}_R{\mathrm{2,}i}_{}}$$

As long as the aforementioned at least approximate simultaneity of the detection of the individual first output wavefronts and first input wavefronts is given, both calculations are equivalent. However, it can be advantageous to use the calculation method specified in formula (3), since this can sometimes be carried out with less effort.

In the second time period, which can also be referred to as the heating phase, the optical element is then irradiated with the test radiation in order to determine the corrected first output wavefront. In the interior or on the surface of the optical element, increasing heating occurs with increasing irradiation time due to the test radiation, i. H. a wavefront of the measuring beam leading from the optical element to the wavefront sensor is increasingly changed with the irradiation time compared to the first corrected output wavefront.

To determine the second corrected output wavefront in the heating phase, the wavefront of the measuring beam that changes due to the irradiation with the test radiation is measured using the wavefront sensor and the second wavefront sensor, similar to the first corrected output wavefront.

According to the prior art, a certain number M of individual recordings w _H,i of the second output wave front are recorded by means of the wave front sensor for a second time period of length t _H . The second output wavefront averaged over the heating phase and recorded by the wavefront sensor is then given by formula (4). $$W_H=\frac{1}{M}{\displaystyle \sum _{i=1}^M{w}_{H,i}}$$

In the method according to the invention, not only are the individual recordings w _H,i of the second output wavefront recorded by the wavefront sensor during the heating phase, but also the individual recordings of the second input wavefront are recorded by the second wavefront sensor.

The averaged second input wavefront is therefore given by formula (5). $${\mathrm{<figure-callout\; id="2"\; label="W\; H"\; filenames="DE102021212993A1\_0014.png,DE102021212993A1\_0015.png"\; state="\{\{state\}\}">W</figure-callout>}}_H=\frac{1}{M}{\displaystyle \sum _{i=1}^{\mathrm{<figure-callout\; id="2"\; label="M\; w\; H"\; filenames="DE102021212993A1\_0014.png,DE102021212993A1\_0015.png"\; state="\{\{state\}\}">M</figure-callout>}}{w}_H{\mathrm{2,}i}_{}}$$

A corrected second output wavefront _W′H can thereby be determined. The mean corrected second output wavefront W' _H is given by formula (6). $$W{\text{'}}_H=W_H-{\mathrm{<figure-callout\; id="2"\; label="W\; H"\; filenames="DE102021212993A1\_0014.png,DE102021212993A1\_0015.png"\; state="\{\{state\}\}">W</figure-callout>}}_H$$

According to the prior art, a change in the transformation property is determined by determining an output wave front deviation. According to the prior art, the mean wavefront curvature in the measuring beam W _LH , which is also called lens heating, caused by the irradiation of the optical element with the test radiation, is the mean second output wavefront W _H measured during the heating phase minus the first output wavefront W measured in the reference phase _R. A formula for the output wave front deviation is given in formula (7). $$W_{LH}=W_H-W_R$$

In the method according to the invention, it is advantageous over the method according to the prior art that the output wavefront deviation, in addition to the lens heating in the optical element, also includes noise components due to an input wavefront of the measuring beam that is not constant over time, as emitted by the radiation source.

By detecting the input wavefronts by the second wavefront sensor or by the reference measuring device, this noise component can be removed from the measurement and the corrected output wavefront deviation can be determined according to formula (8). $$W{\text{'}}_{LH}=W{\text{'}}_H-W{\text{'}}_R$$

By removing the noise component that comes from the temporally unstable input wavefront of the measuring beam, it is possible to improve the sensitivity of absorption measurements or lens heating measurements using wavefront sensors, so that they can also detect low degrees of absorption of modern fused silica materials, for example in DUV lithography optics can resolve.

By minimizing the optical path length that the measuring beam has to travel through air or gas, the influence of density fluctuations of the gas on the measured wave fronts can be minimized. However, the beam line device used for this purpose has a reshaping property which acts on the shape of the wave fronts in addition to the reshaping property of the optical element. In addition to the influences on the shape of the wave fronts mentioned above, there are now influences on the shape of the wave fronts due to possible inhomogeneities in a refractive index and/or a pass of the beam line device used. However, since these influences are static, they are eliminated in a first approximation by the method according to the invention always using a reference phase and a heating phase. A corrected first output wavefront with static influences due to the beamline device is given by formula (9). $$\begin{array}{l}W\text{'}{\text{'}}_R=W\text{'}{\text{'}}_R-W\text{'}{\text{'}}_{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="DE102021212993A1\_0014.png,DE102021212993A1\_0015.png"\; state="\{\{state\}\}">R</figure-callout>}2}=\frac{1}{N}{\displaystyle \sum _{i=1}^N{w}_{R,i}}+{\overline{W}}_{AC}-\left(\frac{1}{N}{\displaystyle \sum _{i=1}^{\mathrm{<figure-callout\; id="2"\; label="N\; w\; R"\; filenames="DE102021212993A1\_0014.png,DE102021212993A1\_0015.png"\; state="\{\{state\}\}">N</figure-callout>}}{w}_R{\mathrm{2,}i}_{}}+{\overline{W}}_{AB}\right)=W_R-{\mathrm{<figure-callout\; id="2"\; label="W\; R"\; filenames="DE102021212993A1\_0014.png,DE102021212993A1\_0015.png"\; state="\{\{state\}\}">W</figure-callout>}}_R+{\overline{W}}_{AC}-{\overline{W}}_{AB}\\ =W{\text{'}}_R+{\overline{W}}_{AC}-{\overline{W}}_{AB}\end{array}$$

A corrected second output wavefront with static influences due to the beamline device is given by formula (10). $$\begin{array}{l}W\text{'}{\text{'}}_H=W\text{'}{\text{'}}_H-W\text{'}{\text{'}}_{\mathrm{<figure-callout\; id="2"\; label="H"\; filenames="DE102021212993A1\_0014.png,DE102021212993A1\_0015.png"\; state="\{\{state\}\}">H</figure-callout>}2}=\frac{1}{M}{\displaystyle \sum _{i=1}^M{w}_{H,i}}+{\overline{W}}_{AC}-\left(\frac{1}{M}{\displaystyle \sum _{i=1}^{\mathrm{<figure-callout\; id="2"\; label="M\; w\; H"\; filenames="DE102021212993A1\_0014.png,DE102021212993A1\_0015.png"\; state="\{\{state\}\}">M</figure-callout>}}{w}_H{\mathrm{2,}i}_{}}+{\overline{W}}_{AB}\right)=W_H-{\mathrm{<figure-callout\; id="2"\; label="W\; H"\; filenames="DE102021212993A1\_0014.png,DE102021212993A1\_0015.png"\; state="\{\{state\}\}">W</figure-callout>}}_H+{\overline{W}}_{AC}-{\overline{W}}_{AB}\\ =W{\text{'}}_H+{\overline{W}}_{AC}-{\overline{W}}_{AB}\end{array}$$

Here, the contributions of the individual components of the beam line device are identified by the indices AB and AC. The index AB describes the influence of the at least one beam line device on the input wavefront, i.e. on that branch of the measuring beam which is fed to the beam splitter (part A) and then fed by the beam splitter from the second wavefront sensor (part B). The index AC describes the influence of the at least one beam line device on the output wavefront, i.e. on that branch of the measuring beam which is fed to the beam splitter (part A) and then fed from the beam splitter from the optical element and from the optical element from the Wavefront sensor is supplied (Part C). In a first approximation, these influences are constant over time.

The determination of the corrected output wave front deviation results from formula (11). $$W\text{'}{\text{'}}_{LH}=W\text{'}{\text{'}}_H-W\text{'}{\text{'}}_R=\left(W{\text{'}}_H+{\overline{W}}_{AC}-{\overline{W}}_{AB}\right)-\left(W{\text{'}}_R+{\overline{W}}_{AC}-{\overline{W}}_{AB}\right)=W{\text{'}}_H-W{\text{'}}_R=W{\text{'}}_{LH}$$

Due to the static nature of the influences of the beam line device, these are eliminated when forming the difference.

Further possibilities for minimizing the influence of the beam line device on the wave fronts can consist, for example, in that

• - eine Passe an den Enden der Strahlleitungseinrichtung möglichst eben ist,
• - zur Minimierung thermischer Drift ein Ultra-Low-Expansion (ULE)-Glas anstatt Quarzglas verwendet wird,
• - ein anderes Material mit minimaler thermischer Ausdehnung verwendet wird.

- the second wavefront sensor is positioned as close as possible to the beam splitter so that the materials used to form the beam line device are as homogeneous as possible,

• - a yoke at the ends of the jet line device is as flat as possible,
• - to minimize thermal drift, an ultra-low expansion (ULE) glass is used instead of quartz glass,
• - another material with minimal thermal expansion is used.

Furthermore, the influence of the beam line device can be minimized by stabilizing a measurement chamber in which the method is carried out, for example to a temperature range of 22°C +/- 1°C in order to minimize thermal drift or by the method being carried out under a vacuum atmosphere instead of being carried out in air.

In order to determine the deformation property as error-free as possible, provision can be made for further optical parameters to be examined.

For example, it can be provided that the radiation source can emit measuring beams of different wavelengths. Radiation can react differently to the fluctuations in the refractive index in a gas atmosphere at different wavelengths. However, a change in the conversion property as a function of heating can be theoretically and/or empirically predetermined for the different wavelengths. If the conversion properties at different wavelengths do not behave as previously determined, the wavelength-dependent deviation may provide information about influences on the output wavefronts that are not due to the change in the conversion properties due to absorption of the test radiation and subsequent heating are due. A corrected output wave front deviation and thus a precise degree of absorption can also be determined as a result.

Provision can also be made to use measuring beams of different polarizations instead of or in addition to the use of measuring beams of different wavelengths.

The invention also relates to a lithography system, in particular a projection exposure system.

The lithography system according to the invention, in particular a projection exposure system for microlithography, comprises an illumination system which has a radiation source, illumination optics and projection optics, the illumination optics and/or the projection optics having at least one optical element.

The degree of absorption of at least one of the optical elements of the lithography system is determined using the method according to the invention and/or at least one transformation property of at least one of the optical elements is determined using the device according to the invention.

Features that have been described in connection with one of the objects of the invention, specifically given by the method according to the invention, the device according to the invention and the lithography system according to the invention, can also be advantageously implemented for the other objects of the invention. Likewise, advantages that were mentioned in connection with one of the objects of the invention can also be understood in relation to the other objects of the invention.

In addition, it should be noted that terms such as "comprising", "having" or "with" do not exclude any other features or steps. Furthermore, terms such as "a" or "that" which indicate a singular number of steps or features do not exclude a plurality of features or steps - and vice versa.

In a puristic embodiment of the invention, however, it can also be provided that the features introduced in the invention with the terms “comprising”, “having” or “with” are listed exhaustively. Accordingly, one or more listings of features may be considered complete within the scope of the invention, e.g. considered for each claim. The invention can consist exclusively of the features mentioned in claim 1, for example.

It should be mentioned that designations such as “first” or “second” etc. are primarily used for reasons of distinguishing the respective device or method features and are not necessarily intended to indicate that features are mutually dependent or related to one another.

Exemplary embodiments of the invention are described in more detail below with reference to the drawing.

The figures each show preferred exemplary embodiments in which individual features of the present invention are shown in combination with one another. Features of an exemplary embodiment can also be implemented separately from the other features of the same exemplary embodiment and can accordingly easily be connected by a person skilled in the art to further meaningful combinations and sub-combinations with features of other exemplary embodiments.

Elements with the same function are provided with the same reference symbols in the figures.

• 1 eine EUV-Projektionsbelichtungsanlage im Meridionalschnitt;
• 2 eine DUV-Projektionsbelichtungsanlage;
• 3 eine schematische Darstellung eines Ausführungsbeispiels einer Vorrichtung gemäß dem Stand der Technik;
• 4 eine schematische Darstellung eines Ausführungsbeispiels der erfindungsgemäßen Vorrichtung;
• 5 eine schematische Darstellung eines weiteren Ausführungsbeispiels der erfindungsgemäßen Vorrichtung;
• 6 eine schematische Darstellung eines weiteren Ausführungsbeispiels der erfindungsgemäßen Vorrichtung;
• 7 eine schematische Darstellung eines weiteren Ausführungsbeispiels der erfindungsgemäßen Vorrichtung;
• 8 eine schematische Darstellung einer ersten Eingangswellenfront und einer ersten Ausgangswellenfront;
• 9 eine schematische Darstellung einer zweiten Eingangswellenfront und einer zweiten Ausgangswellenfront;
• 10 eine blockdiagrammmäßige Darstellung eines weiteren Ausführungsbeispiels des erfindungsgemäßen Verfahrens;
• 11 eine blockdiagrammmäßige Darstellung eines weiteren Ausführungsbeispiels des erfindungsgemäßen Verfahrens;

Show it:

• 1 an EUV projection exposure system in the meridional section;
• 2 a DUV projection exposure system;
• 3 a schematic representation of an embodiment of a device according to the prior art;
• 4 a schematic representation of an embodiment of the device according to the invention;
• 5 a schematic representation of a further embodiment of the device according to the invention;
• 6 a schematic representation of a further embodiment of the device according to the invention;
• 7 a schematic representation of a further embodiment of the device according to the invention;
• 8th a schematic representation of a first input wavefront and a first output wavefront;
• 9 a schematic representation of a second input wavefront and a second output wavefront;
• 10 a block diagram representation of a further embodiment of the method according to the invention;
• 11 a block diagram representation of a further embodiment of the method according to the invention;

The following are first with reference to 1 the essential components of an EUV projection exposure system 100 for microlithography are described as an example of a lithography system. The description of the basic structure of the EUV projection exposure system 100 and its components should not be understood as limiting here.

In addition to a radiation source 102 , an illumination system 101 of the EUV projection exposure system 100 has illumination optics 103 for illuminating an object field 104 in an object plane 105 . In this case, a reticle 106 arranged in the object field 104 is exposed. The reticle 106 is held by a reticle holder 107 . The reticle holder 107 can be displaced via a reticle displacement drive 108, in particular in a scanning direction.

In 1 a Cartesian xyz coordinate system is drawn in for explanation. The x-direction runs perpendicularly into the plane of the drawing. The y-direction is horizontal and the z-direction is vertical. The scan direction is in 1 along the y-direction. The z-direction runs perpendicular to the object plane 105.

The EUV projection exposure system 100 includes projection optics 109. The projection optics 109 are used to image the object field 104 in an image field 110 in an image plane 111. The image plane 111 runs parallel to the object plane 105. Alternatively, there is also an angle other than 0° between the object plane 105 and the image plane 111 is possible.

A structure on the reticle 106 is imaged onto a light-sensitive layer of a wafer 112 arranged in the region of the image field 110 in the image plane 111. The wafer 112 is held by a wafer holder 113. The wafer holder 113 can be displaced via a wafer displacement drive 114, in particular along the y-direction. The displacement of the reticle 106 via the reticle displacement drive 108 on the one hand and the wafer 112 on the other hand via the wafer displacement drive 114 can be synchronized with one another.

The radiation source 102 is an EUV radiation source. The radiation source 102 emits in particular EUV radiation 115, which is also referred to below as useful radiation or illumination radiation. The useful radiation 115 has, in particular, a wavelength in the range between 5 nm and 30 nm a DPP (Gas Discharged Produced Plasma) source. It can also be a synchrotron-based radiation source. The radiation source 102 can be a free-electron laser (FEL).

The illumination radiation 115 emanating from the radiation source 102 is bundled by a collector 116 . The collector 116 can be a collector with one or more ellipsoidal and/or hyperboloidal reflection surfaces. The at least one reflection surface of the collector 116 can be used in grazing incidence ("Grazing Incidence", GI), i.e. with angles of incidence greater than 45°, or in normal incidence ("Normal Incidence", NI), i.e. with angles of incidence smaller than 45° of the illumination radiation 115 are applied. The collector 116 can be structured and/or coated on the one hand to optimize its reflectivity for the useful radiation 115 and on the other hand to suppress stray light.

After the collector 116, the illumination radiation 115 propagates through an intermediate focus in an intermediate focal plane 117. The intermediate focal plane 117 can represent a separation between a radiation source module, comprising the radiation source 102 and the collector 116, and the illumination optics 103.

The illumination optics 103 includes a deflection mirror 118 and a first facet mirror 119 downstream of this in the beam path. The deflection mirror 118 can be a plane deflection mirror or alternatively a mirror with an effect that influences the bundle beyond the pure deflection effect. Alternatively or additionally, the deflection mirror 118 can be designed as a spectral filter, which separates a useful light wavelength of the illumination radiation 115 from stray light of a different wavelength. If the first facet mirror 119 is arranged in a plane of the illumination optics 103 which is optically conjugate to the object plane 105 as a field plane, it is also referred to as a field facet mirror. The first facet mirror 119 includes a multiplicity of individual first facets 120, which are also referred to below as field facets. Of these facets 120 are in the 1 only a few shown as examples.

The first facets 120 can be embodied as macroscopic facets, in particular as rectangular facets or as facets with an arcuate or part-circular edge contour. The first facets 120 can be embodied as planar facets or alternatively as convexly or concavely curved facets.

Like for example from the DE 10 2008 009 600 A1 is known, the first facets 120 themselves can each also be composed of a multiplicity of individual mirrors, in particular a multiplicity of micromirrors. The first facet mirror 119 can be embodied in particular as a microelectromechanical system (MEMS system). For details refer to the DE 10 2008 009 600 A1 referred.

The illumination radiation 115 runs horizontally between the collector 116 and the deflection mirror 118, ie along the y-direction.

A second facet mirror 121 is arranged downstream of the first facet mirror 119 in the beam path of the illumination optics 103. If the second facet mirror 121 is arranged in a pupil plane of the illumination optics 103, it is also referred to as a pupil facet mirror. The second facet mirror 121 can also be arranged at a distance from a pupil plane of the illumination optics 103 . In this case, the combination of the first facet mirror 119 and the second facet mirror 121 is also referred to as a specular reflector. Specular reflectors are known from US 2006/0132747 A1 , the EP 1 614 008 B1 and the U.S. 6,573,978 .

The second facet mirror 121 includes a plurality of second facets 122. In the case of a pupil facet mirror, the second facets 122 are also referred to as pupil facets.

The second facets 122 can also be macroscopic facets, which can have round, rectangular or hexagonal borders, for example, or alternatively facets composed of micromirrors. In this regard, also on the DE 10 2008 009 600 A1 referred.

The second facets 122 can have plane or alternatively convexly or concavely curved reflection surfaces.

The illumination optics 103 thus forms a double-faceted system. This basic principle is also known as the "Fly's Eye Integrator".

It can be advantageous not to arrange the second facet mirror 121 exactly in a plane which is optically conjugate to a pupil plane of the projection optics 109 .

The individual first facets 120 are imaged in the object field 104 with the aid of the second facet mirror 121 . The second facet mirror 121 is the last beam-forming mirror or actually the last mirror for the illumination radiation 115 in the beam path in front of the object field 104.

In a further embodiment of the illumination optics 103 that is not shown, transmission optics can be arranged in the beam path between the second facet mirror 121 and the object field 104 be, which contributes in particular to the imaging of the first facets 120 in the object field 104. The transmission optics can have exactly one mirror, but alternatively also have two or more mirrors, which are arranged one behind the other in the beam path of the illumination optics 103 . The transmission optics can in particular comprise one or two mirrors for normal incidence (NI mirror, "normal incidence" mirror) and/or one or two mirrors for grazing incidence (GI mirror, "gracing incidence" mirror).

The illumination optics 103 has the version in which 1 shown, exactly three mirrors after the collector 116, namely the deflection mirror 118, the field facet mirror 119 and the pupil facet mirror 121.

In a further embodiment of the illumination optics 103, the deflection mirror 118 can also be omitted, so that the illumination optics 103 can then have exactly two mirrors downstream of the collector 116, namely the first facet mirror 119 and the second facet mirror 121.

The imaging of the first facets 120 by means of the second facets 122 or with the second facets 122 and transmission optics in the object plane 105 is generally only an approximate imaging.

The projection optics 109 includes a plurality of mirrors Mi, which are numbered consecutively according to their arrangement in the beam path of the EUV projection exposure system 100 .

At the in the 1 example shown, the projection optics 109 includes six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or another number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 115. The projection optics 109 are doubly obscured optics. The projection optics 109 has an image-side numerical aperture which is greater than 0.5 and which can also be greater than 0.6 and which can be 0.7 or 0.75, for example.

Reflection surfaces of the mirrors Mi can be designed as free-form surfaces without an axis of rotational symmetry. Alternatively, the reflection surfaces of the mirrors Mi can be designed as aspherical surfaces with exactly one axis of rotational symmetry of the reflection surface shape. Just like the mirrors of the illumination optics 103, the mirrors Mi can have highly reflective coatings for the illumination radiation 115. These coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon.

The projection optics 109 has a large object-image offset in the y-direction between a y-coordinate of a center of the object field 104 and a y-coordinate of the center of the image field 110. This object-image offset in the y-direction can be something like this be as large as a z-distance between the object plane 105 and the image plane 111.

The projection optics 109 can in particular be anamorphic. In particular, it has different image scales βx, βy in the x and y directions. The two image scales βx, βy of the projection optics 109 are preferably at (βx, βy)=(+/−0.25, +/-0.125). A positive image scale β means an image without image reversal. A negative sign for the imaging scale β means imaging with image inversion.

The projection optics 109 thus leads to a reduction in the ratio 4:1 in the x-direction, ie in the direction perpendicular to the scanning direction.

The projection optics 109 lead to a reduction of 8:1 in the y-direction, ie in the scanning direction.

Other imaging scales are also possible. Image scales with the same sign and absolutely the same in the x and y directions, for example with absolute values of 0.125 or 0.25, are also possible.

The number of intermediate image planes in the x-direction and in the y-direction in the beam path between the object field 104 and the image field 110 can be the same or, depending on the design of the projection optics 109, can be different. Examples of projection optics with different numbers of such intermediate images in the x and y directions are known from U.S. 2018/0074303 A1 .

In each case one of the pupil facets 122 is assigned to precisely one of the field facets 120 in order to form a respective illumination channel for illuminating the object field 104 . In this way, in particular, lighting can result according to Köhler's principle. The far field is broken down into a large number of object fields 104 with the aid of the field facets 120 . The field facets 120 generate a plurality of images of the intermediate focus on the pupil facets 122 respectively assigned to them.

The field facets 120 are each imaged onto the reticle 106 by an associated pupil facet 122 in a superimposed manner in order to illuminate the object field 104 . In particular, the illumination of the object field 104 is as homogeneous as possible. It preferably has a uniformity error of less than 2%. Field uniformity can be achieved by superimposing different illumination channels.

The illumination of the entrance pupil of the projection optics 109 can be geometrically defined by an arrangement of the pupil facets. The intensity distribution in the entrance pupil of the projection optics 109 can be set by selecting the illumination channels, in particular the subset of the pupil facets that guide light. This intensity distribution is also referred to as an illumination setting.

A likewise preferred pupil uniformity in the area of defined illuminated sections of an illumination pupil of the illumination optics 103 can be achieved by redistributing the illumination channels.

Further aspects and details of the illumination of the object field 104 and in particular the entrance pupil of the projection optics 109 are described below.

The projection optics 109 can in particular have a homocentric entrance pupil. This can be accessible. It can also be inaccessible.

The entrance pupil of the projection optics 109 cannot regularly be illuminated exactly with the pupil facet mirror 121 . When imaging the projection optics 109, which telecentrically images the center of the pupil facet mirror 121 onto the wafer 112, the aperture rays often do not intersect at a single point. However, a surface can be found in which the distance between the aperture rays, which is determined in pairs, is minimal. This surface represents the entrance pupil or a surface conjugate to it in position space. In particular, this surface shows a finite curvature.

The projection optics 109 may have different positions of the entrance pupil for the tangential and for the sagittal beam path. In this case, an imaging element, in particular an optical component of the transmission optics, should be provided between the second facet mirror 121 and the reticle 106 . With the help of this optical component, the different positions of the tangential entrance pupil and the sagittal entrance pupil can be taken into account.

At the in the 1 In the arrangement of the components of the illumination optics 103 shown, the pupil facet mirror 121 is arranged in a surface conjugate to the entrance pupil of the projection optics 109 . The first field facet mirror 119 is arranged tilted to the object plane 105 . The first facet mirror 119 is tilted relative to an arrangement plane that is defined by the deflection mirror 118 .

The first facet mirror 119 is tilted relative to an arrangement plane that is defined by the second facet mirror 121 .

In 2 an exemplary DUV projection exposure system 200 is shown. The DUV projection exposure system 200 has an illumination system 201, a device known as a reticle stage 202 for receiving and precisely positioning a reticle 203, by means of which the later structures on a wafer 204 are determined, a wafer holder 205 for holding, moving and precisely positioning the wafer 204 and an imaging device, namely projection optics 206, with a plurality of optical elements, in particular lenses 207, which are held in an objective housing 209 of the projection optics 206 via mounts 208.

As an alternative or in addition to the lenses 207 shown, various refractive, diffractive and/or reflective optical elements, including mirrors, prisms, end plates and the like, can be provided.

The basic functional principle of the DUV projection exposure system 200 provides that the structures introduced into the reticle 203 are imaged onto the wafer 204 .

The illumination system 201 provides a projection beam 210 in the form of electromagnetic radiation that is required for imaging the reticle 203 onto the wafer 204 . A laser, a plasma source or the like can be used as the source for this radiation. The radiation is shaped in the illumination system 201 via optical elements in such a way that the projection beam 210 has the desired properties in terms of diameter, polarization, shape of the wave front and the like when it strikes the reticle 203 .

An image of the reticle 203 is generated by means of the projection beam 210 and transmitted to the wafer 204 in a correspondingly reduced size by the projection optics 206 . The reticle 203 and the wafer 204 can be moved synchronously, so that areas of the reticle 203 are imaged onto corresponding areas of the wafer 204 practically continuously during a so-called scanning process.

Optionally, an air gap between the last lens 207 and the wafer 204 can be replaced by a liquid medium that has a refractive index greater than 1.0. The liquid medium can be, for example, ultrapure water. Such a structure is also referred to as immersion lithography and has an increased photolithographic resolution.

The use of the invention is not limited to use in lithography systems and also not to use in projection exposure systems 100, 200, in particular also not with the structure described. Nevertheless, the invention is particularly suitable for lithography systems, in particular projection exposure systems, in particular with the structure described. Furthermore, the invention and the following exemplary embodiments are not to be understood as being restricted to a specific design. The following figures represent the invention only by way of example and in a highly schematic manner.

3 shows an embodiment according to the prior art. A device 1 for measuring a transformation property of an optical element 2 is shown, which can preferably be designed or used as one of the optical elements 116, 118, 119, 120, 121, 122, Mi, 207. According to the prior art, a radiation source 3 and a wavefront sensor 4 are provided, with a measuring beam 5 emitted by the radiation source 3 having an input wavefront 6 and an output wavefront 7 formed by the conversion property of the optical element 2 (see 8th ) having. The wavefront sensor 4 is set up to determine the output wavefront 7 .

4 shows an embodiment of a device 1 according to the invention for measuring the deformation property of the optical element 2, which can preferably be designed or used as one of the optical elements 116, 118, 119, 120, 121, 122, Mi, 207, wherein compared to the in 3 The device 1 shown is additionally provided with a reference measuring device 8 in order to determine the input wave front 6 .

Furthermore, in the in 4 illustrated embodiment of the device 1 a device 9 is provided. The device 9 is set up to determine the transformation property from the output wave front 7 and the input wave front 6 .

The device 9 is also set up to determine a first output wavefront 7a and a first input wavefront 6a in a first time period and to determine a second output wavefront 7b and a second input wavefront 6b in a second time period and to determine a change in the conversion property from this.

Furthermore, in the in 4 illustrated embodiment, the reference measuring device 8 has a beam splitter 10 and a second wavefront sensor 11 .

in the in 4 In the exemplary embodiment illustrated, a heating device 12 is also provided in order to heat the optical element 2 .

In this case, the heating device is designed as a heating radiation source 12 for irradiating the optical element 2 with a test radiation 13 .

In this case, the test radiation 13 is designed in such a way that it is absorbed by the optical element 2 .

in the in 4 In the exemplary embodiment illustrated, the device 9 is set up to infer an absorptance of the optical element 2 from a change in the conversion property after the irradiation of the optical element 2 with the test radiation 13 compared to a conversion property before the irradiation.

5 shows a further exemplary embodiment of the device 1, the reference measuring device 8 having a beam splitter 10, a deflection mirror 14, a closure device 15 and the wavefront sensor 4.

A part of the measuring beam 5 is decoupled by the beam splitter 10 and deflected by the deflection mirror 14 onto the wave front sensor 4 . The closure device 15 enables the detection of the input wavefronts 6, 6a, 6b and the output wavefronts 7, 7a, 7b to be separated in the time domain by covering that branch of the measuring beam 5 whose wavefront is not to be measured. The input wavefronts 6, 6a, 6b and the output wavefronts 7, 7a, 7b can be detected at very short time intervals by means of a fast-acting locking device 15, so that it can be assumed that the detection is approximately synchronous.

6 shows an embodiment of the device 1, wherein a beam line device 16 is provided in order to record the measuring beam 5 at least approximately completely.

in the in 6 In the exemplary embodiment illustrated, an input beam line device 16a is provided in order to at least approximately completely receive the measuring beam in a region between the radiation source 3 and the optical element 2 .

Furthermore, an output beam line device 16b is provided in order to at least approximately completely receive the measuring beam 5 between the optical element 2 and the wave front sensor 4 .

In the in the 4 until 6 illustrated embodiment of the device 1, the measuring beam 5 passes the optical element 2 in a transmission configuration and then impinges on the wavefront sensor 4.

The test radiation 13 preferably hits the optical element 2 at an angle other than zero degrees to the measuring beam 5. However, in an exemplary embodiment that is not shown, it can also be provided that the test radiation 13 hits the optical element 2 parallel and centered to the measuring beam , in which case a filter device, preferably designed as a spectral filter and/or as a dichroic mirror, can be provided in front of the wavefront sensor 4 in order to avoid direct incidence of the test radiation 13 on the wavefront sensor 4 by the test radiation 13 being filtered out of the measuring beam 5 .

in the in 7 illustrated embodiment, the measuring beam 5 is reflected on the optical element 2 and thrown onto the wave front sensor 4 . In this case, the test radiation 13 preferably strikes the optical element 2 perpendicularly.

8th shows a basic representation of an effect of the conversion property of the optical element 2 on a first input wavefront 6a, which is converted into a first output wavefront 7a by the conversion property.

9 shows a basic representation of a deformation property of the optical element 2, which is compared to that of the deformation in 8th underlying changed deformation property of the optical element 2 is changed. A second input wavefront 6b is in this case reshaped by the changed reshaping property of the optical element 2 into a second output wavefront 7b that is changed in comparison to the first output wavefront 7a. The change in the deformation property results in the in 9 illustrated example from a local heating of the optical element 2, which is caused by an absorption of the test radiation 13.

In the 4 The device 1 shown is suitable, for example, for carrying out a method for measuring an absorption of electromagnetic radiation by an optical element 2, in particular by an optical element 116, 118, 119, 120, 121, 122, Mi, 207 of a lithography system 100,200, in one Defined wavelength range, after which the optical element 2 is irradiated with a test radiation 13 and experiences at least locally by the absorption of the test radiation 13 heating, after which before Irradiation with the test radiation 13, a measuring beam 5 generated by a radiation source 3 is fed to the optical element 2 and reshaped by a conversion property of the optical element 2 and a first output wavefront 7a is measured, after which, after irradiation with the test radiation 13, the wavefront fed to the optical element 2 Measuring beam 5 is reshaped by a reshaping property of optical element 2 that is changed by heating, and a second output wavefront 7b of reshaped measuring beam 5 is measured, after which first output wavefront 7a and second output wavefront 7b are subtracted from one another and an output wavefront deviation is determined, and after which the change in the conversion property and thus the heating and thus the absorption of the optical element 2 is inferred from the output wave front deviation.

In the case of the in 4 The method carried out in device 1 shown provides that a first input wavefront 6a of the measuring beam 5 is measured at least approximately synchronously with the determination of the first output wavefront 7a before it is fed to the optical element 2, and a second input wavefront 6b of the Measuring beam 5 is measured before it is fed to the optical element 2, after which the first input wavefront 6a and the second input wavefront 6b are subtracted from one another and an input wavefront deviation is determined, and after which the output wavefront deviation and the input wavefront deviation are subtracted from one another and a corrected output wavefront deviation is determined becomes.

by means of the in 4 Device 1 shown, the method can preferably be carried out in such a way that the output wavefronts 7a, 7b are measured with the wavefront sensor 4, which is preferably designed as a first Hartmann-Shack sensor, and the input wavefronts 6a, 6b with a second wavefront sensor 11, which is preferably designed as the second Hartmann-Shack sensor is designed to be measured.

By positioning the beam splitter 10 close to the optical element 2, the aforementioned method can be implemented in such a way that the input wave fronts 6a, 6b are measured at least approximately directly on the optical element 2.

Particularly suitable for carrying out the method mentioned is in 4 device 1 shown when a diode laser is used as the radiation source 3 .

in the in 6 The exemplary embodiment of the device 1 illustrated in the embodiment can be carried out in such a way that the measuring beam 5 before and/or after the optical element 2 is received at least approximately completely by the beam guiding device 16 . This can accordingly also in the embodiments according to the 4 , 5 and or 7 be provided

10 shows a block diagram representation of a possible embodiment of a method known from the prior art for measuring absorption of electromagnetic radiation by an optical element 2.

In a first period of time, the first output wavefront 7a is determined in a first output wavefront block 20 . For this purpose, the measuring beam 5 is generated in a first measuring beam generating block 21 by the radiation source 3 .

The measuring beam 5 emitted by the radiation source 3 propagates in the direction of the optical element 2 in a first input path block 22 . In this case, the wave front of the measuring beam 5 is exposed to density fluctuations and thus to refractive index fluctuations in a gas atmosphere through which the measuring beam 5 runs.

Then the measuring beam 5 impinges on the optical element 2 . Here the measuring beam 5 is reshaped by the optical element 2 in a first reshaping block 23 . Then the measuring beam 5, from here as a shaped measuring beam 5, spreads out in a first output path block 24 in the direction of the wavefront sensor 4 and is exposed to refractive index fluctuations of the gas atmosphere in the first output path block 24, through which the shaped measuring beam 5 runs.

The fluctuations mentioned in the path blocks 24 and 22 lead to wave fronts of the measuring beam 5 that change over time, regardless of the shaping property of the optical element 2 which acts on the measuring beam 5 in the first shaping block 23 .

The first output wavefront 7a is then determined by the wavefront sensor 4 in a first wavefront determination block 25 .

In a second period of time, the second output wavefront 7b is determined in a second output wavefront block 26 . In a second measuring beam generation block 27, a measuring beam 5 is again generated in the second time period mentioned, which is then exposed in a second input path block 28 to changed density or refractive index conditions.

In a heating beam block 29, the optical element 2 is irradiated with test radiation 13 in such a way that the optical element 2 heats up, as a result of which the measuring beam 5 impinges on a second shaping block 30, which has now changed compared to the first shaping block 23. The measuring beam 5 then propagates in a second output path block 31 in the direction of the wavefront sensor 4 and is measured by the wavefront sensor 4 in a second wavefront determination block 32 .

From the measurement results of the first wavefront determination block 25 and the second wavefront determination block 32, in 10 illustrated method according to the prior art on the heating of the optical element 2, which was caused by the radiant heater block 29 and has led to different transformation blocks 23 and 30, in a first conclusion block 33 closed. In this case, the heating was brought about in the radiant heater block 29 by absorption of the test radiation 13 on the optical element 2 .

The conclusion made in the first conclusion block 33 , to which heating the optical element 2 was subjected by the radiant heater block 28 , is falsified by a first error block 34 . In this case, the first error block 34 contains differences in the measuring beam 5 which can be explained by differences between the first measuring beam generation block 21 and the second measuring beam generation block 27 .

Since the first output wavefront block 20 was carried out in a first period and the second output wavefront block 26 was carried out at a second period after the first period, a temporal instability of the radiation source 3 leads to differences in the measuring beam generation blocks 21 and 27.

Furthermore, the first error block 34 contains differences between the first output wavefront 7a and the second output wavefront 7b, which are not caused by changes in the transformation property but by a change in the wavefronts of the measuring beam 5 along the path blocks 22,24,28,31 due to density or refractive index fluctuations the gas atmosphere surrounding the beam paths.

The aforementioned fluctuations can originate, for example, in a temperature dependence of the refractive index, i. H. in a derivative dn/dT of the refractive index n with respect to the non-zero temperature T. As a result, the refractive index of the gas atmosphere can also change, for example due to an effect of the heating beam.

Accordingly, a wavefront is determined in the first wavefront determination block 25, the shape of which depends on the wavefront generated by the radiation source 3 in the first measurement beam generation block 21, with this being changed by the first input path block 22 and by the first shaping block 23 and by the first output path block 24.

In the second wavefront determination block 32, a wavefront is determined which depends on the wavefront emitted by the radiation source 3 in the second measuring beam generation block 27 during the second period, this wavefront being reshaped by the second input path block 28, the second shaping block 30 and the second output path block 31.

If the wavefronts emitted by the radiation source 3 differ in the generation blocks 21 and 27 and in the path blocks 22,24,28,31, then there is a difference between the first output wavefront 7a determined in the first wavefront determination block 25 and the second output wavefront 7a determined in the second wavefront determination block 32 Output wavefront 7b is attributable to a change in the transformation property, as well as changes in a beam profile of the radiation source 3 and changes in a refractive index distribution along the beam paths. This makes it difficult to precisely determine the change in the deformation property.

In 11 a block diagram representation of a possible embodiment of the method according to the invention is shown. In addition to those already in 10 illustrated steps of the method known from the prior art for measuring an absorption of an electromagnetic radiation by an optical element 2, further blocks are provided in the present embodiment. In a first input wavefront determination block 35, the first input wavefront 6a is determined, which the measuring beam 5 has after it was generated in the first measuring beam generation block 21 by the radiation source 3 and then passed the first input path block 22.

In a second input wavefront determination block 36, the second input wavefront 6b is determined, which the measuring beam 5 has after it was generated in a second measuring beam generation block 27 by the radiation source 3 in a second time period and the second input path block 27 has passed. Arrows indicate that both the properties of the radiation source 3 and the path blocks 21 and 28 influence the shape of the input wavefronts 6a and 6b determined in the input wavefront determination blocks 35 and 36 .

The first input wavefront 6a determined in the first wavefront determination block 35 is now calculated in a first calculation block 37 with the output wavefront 6a determined in the first wavefront determination block and a first corrected output wavefront is determined.

Similarly, the second input wavefront 6b determined in the second input wavefront determination block 36 is offset against the second output wavefront 7b determined in the second wavefront determination block 32 in a second calculation block 38 and a second corrected output wavefront is determined.

The corrected output wave fronts are then used in a second conclusion block 39 to determine whether the optical element 2 has been heated. By correcting the ascertained output wavefronts 7a and 7b by the ascertained input wavefronts 6a and 6b, the heating of the optical element 2 ascertained in the second conclusion block compared to that in 10 illustrated embodiment of a method known from the prior art more precisely.

The second error block 40, which falsifies the conclusion made in the second conclusion block 39, therefore only contains those deviations which are caused by changes in the wave fronts in the first output path block 24 and in the second output path block 31, for example by fluctuations in the ambient atmosphere.

The error sources mentioned in the second error block 40 can be further minimized by the measurement beam 5 being received between the optical element 2 and the wavefront sensor 4, for example in an output beam line device 16b.

Reference List

1

contraption

2

optical element

3

radiation source

4

wavefront sensor

4a

first Hartmann-Shack sensor

5

measuring beam

6

input wavefront

6a

first input wavefront

6b

second input wavefront

7

output wavefront

7a

first output wavefront

7b

second output wavefront

8th

reference measuring device

9

furnishings

10

beam splitter

11

second wavefront sensor

11 a

second Hartmann-Shack sensor

12

Heating device / heat radiation source

13

test radiation

14

deflection mirror

15

locking device

16

beam line device

16a

input beamline device

16b

output beamline device

20

first output wavefront block

21

first measuring beam generation block

22

first input path block

23

first reshaping block

24

first output path block

25

first wavefront determination block

26

second output wavefront block

27

second measuring beam generation block

28

second input path block

29

radiant heater block

30

second reshaping block

31

second output path block

32

second wavefront determination block

33

first conclusion block

34

first error block

35

first input wavefront determination block

36

second input wavefront determination block

37

first billing block

38

second billing block

39

second conclusion block

40

second error block

100

EUV projection exposure system

101

lighting system

102

radiation source

103

lighting optics

104

object field

105

object level

106

reticle

107

reticle holder

108

reticle displacement drive

109

projection optics

110

image field

111

picture plane

112

wafers

113

wafer holder

114

Wafer displacement drive

115

EUV / useful / illumination radiation

116

collector

117

intermediate focal plane

118

deflection mirror

119

first facet mirror / field facet mirror

120

first facets / field facets

121

second facet mirror / pupil facet mirror

122

second facets / pupil facets

200

DUV projection exposure system

201

lighting system

202

reticle stage

203

reticle

204

wafers

205

wafer holder

206

projection optics

207

lens

208

version

209

lens body

210

projection beam

wed

mirror

QUOTES INCLUDED IN DESCRIPTION

This list of the documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102008009600 A1 [0141, 0145]
• US 2006/0132747 A1 [0143]
• EP 1614008 B1 [0143]
• US6573978 [0143]
• US 2018/0074303 A1 [0162]

Claims (16)

Method for measuring an absorption of electromagnetic radiation by an optical element (2), in particular by an optical element (116, 118, 119, 120, 121, 122, Mi, 207) of a lithography system, in particular a projection exposure system (100,200), in one defined wavelength range, after which the optical element (2) is irradiated with a test radiation (13) and is at least locally heated by the absorption of the test radiation (13), after which, before irradiation with the test radiation (13), a radiation source (3) generated measuring beam (5) is fed to the optical element (2) and reshaped by a reshaping property of the optical element (2) and a first output wavefront (7a) of the reshaped measuring beam (5) is measured, after which after irradiation with the test radiation (13) the measuring beam (5) fed to the optical element (2) by a deformation property of the optical element (2) changed by the heating and mformed and a second output wavefront (7b) of the converted measuring beam (5) is measured, after which the first output wavefront (7a) and the second output wavefront (7b) are subtracted from one another and an output wavefront deviation is determined and after which the output wavefront deviation indicates the change the conversion property and thus the heating and thus the absorption of the optical element (2), characterized in that a first input wavefront (6a) of the measuring beam (5) is measured at least approximately synchronously with the determination of the first output wavefront (7a) before it is fed is measured to the optical element (2) and at least approximately synchronously with the determination of the second output wavefront (7b), a second input wavefront (6b) of the measuring beam (5) is measured before it is fed to the optical element (2), after which the first input wavefront ( 6a) and the second input wavefront (6b) subtracted from each other w ground and an input wave front deviation is determined, after which the output wave front deviation and the input wave front deviation are deducted from one another and a corrected output wave front deviation is determined. procedure after claim 1 , characterized in that the output wavefronts (7a, 7b) are measured with a first Hartmann-Shack sensor (4a) and the input wavefronts (6a, 6b) with a second Hartmann-Shack sensor (11, 11a). procedure after claim 1 or 2 , characterized in that the input wave fronts (6a, 6b) are measured at least approximately directly on the optical element (2). Procedure according to one of Claims 1 until 3 , characterized in that a diode laser is used as the radiation source (3). Procedure according to one of Claims 1 until 4 , characterized in that the measuring beam (5) before and/or after the optical element (2) is received at least approximately completely by at least one beam guiding device (16, 16a, 16b). Device (1) for measuring a transformation property of an optical element (2), in particular an optical element (116, 118, 119, 120, 121, 122, Mi, 207) of a lithography system, having a radiation source (3) and a wavefront sensor (4 ), wherein a measuring beam (5) emitted by the radiation source (3) has an input wavefront (6) and an output wavefront (7) shaped by the transformation property, the wavefront sensor (4) being set up to determine the output wavefront (7), characterized in that a reference measuring device (8) is provided in order to determine the input wave front (6). Device (1) after claim 6 , characterized in that a device (9) is provided and set up to determine the transformation property from the output wave front (7) and the input wave front (6). Device (1) after claim 7 , characterized in that the device (9) is set up to generate a first output wavefront (7a) and a first input wavefront (6a) in a first period and to generate a second output wavefront (7a) and a second input wavefront (6a) in a second period to determine and from this to determine a change in the deformation property. Device (1) according to one of Claims 6 until 8th , characterized in that the reference measuring device (8) is formed from at least one beam splitter (10) and a second wavefront sensor (11). Device (1) according to one of Claims 6 until 8th , characterized in that the reference measuring device (8) is formed from at least one beam splitter (10), at least one deflection mirror (14), at least one closure device (15) and the wavefront sensor (4). Device (1) according to one of Claims 6 until 10 , characterized in that a heating device (12) is provided to heat the optical element (2). Device (1) after claim 11 , characterized in that the heating device is designed as a heating radiation source (12) for irradiating the optical element (2) with a test radiation (13). Device (1) after claim 12 , characterized in that the test radiation (13) is formed such that the test radiation (13) is absorbed by the optical element (2). Device (1) according to one of Claims 7 until 13 , characterized in that the device (9) is set up, from a change in the conversion property after the irradiation of the optical element (2) with the test radiation (13) compared to a conversion property before the irradiation, to an absorptivity of the optical element (2 ) close. Device (1) according to one of Claims 6 until 14 , characterized in that at least one beam line device (16, 16a, 16b) is provided in order to receive the measuring beam (5) at least approximately completely. Lithography system, in particular projection exposure system (100, 200) for microlithography, with an illumination system (101, 201) which has a radiation source (102), illumination optics (103) and projection optics (109, 206), the illumination optics (103) and/or the projection optics (109, 206) have at least one optical element (116, 118, 119, 120, 121, 122, Mi, 207), characterized in that an absorption coefficient of at least one of the optical elements (116, 118, 119 , 120, 121, 122, Mi, 207) at least partially by means of a method according to any one of Claims 1 until 5 was determined and / or wherein at least one transformation property of at least one of the optical elements (116, 118, 119, 120, 121, 122, Mi, 207) at least partially by means of a device according to one of Claims 6 until 15 is determined.

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