DE102022202839A1 - Measuring arrangement and method for detecting a contamination state of an EUV mirror surface, optical device - Google Patents

Abstract

The invention relates to a measuring arrangement (5) for detecting a contamination state of an EUV mirror surface (10) of an optical device, in particular a lithography device (1), which has a first optical element (6) with an EUV mirror having the mirror surface (10). having an at least partially reflecting second optical element (7) and having a light beam source (3) and having a light beam sensor (8), the light beam source (3), the light beam sensor (8) and the optical elements (6, 7) being arranged in relation to one another in such a way are arranged to form a Fabry-Perot resonator (9).

Description

The invention relates to a measuring arrangement for detecting a contamination state of an EUV mirror surface of an optical device, in particular a lithography device, which has a first optical element with an EUV mirror having the mirror surface. The invention also relates to an optical device that has such a measuring arrangement.

Furthermore, the invention relates to a method for detecting a contamination state of an EUV mirror surface of an optical device, in particular a lithography device, which has a first optical element with an EUV mirror having the mirror surface, the method using in particular the above-mentioned measuring arrangement.

EUV mirrors are used, for example, for EUV lithography devices. EUV lithography devices are used for lithography, in particular of semiconductor components, and use reflective optical elements for the extreme ultraviolet wavelength range (EUV), which has wavelengths between 5 nm and 20 nm, for example. EUV lithography devices generally have a number of reflective optical elements which have the highest possible reflectivity in order to ensure a sufficiently high overall reflectivity. However, the reflectivity of these optical elements can be reduced over their service life by contamination of the optically used surfaces of the optical elements. Since several reflective optical elements are often arranged one behind the other in the beam path in an EUV lithography device, even slight contaminations on individual reflective optical surfaces have a greater effect on the overall reflectivity of the system. The contamination can arise due to the short-wave radiation or EUV radiation together with residual gases in the operating atmosphere. For example, contamination can also occur due to moisture residues, where water molecules are split by the EUV radiation and the resulting oxygen radicals oxidize in the optically active surfaces of the optical element. Other sources of contamination are, for example, hydrocarbons or materials that act in conjunction with a laser-based EUV plasma light source.

In order to avoid lithography productivity drops, it is important to be able to monitor the contamination status of such surfaces in lithography facilities.

The present invention is therefore based on the object of creating an advantageous measuring arrangement for detecting a contamination state of an EUV mirror surface in, for example, a lithography device.

The object on which the invention is based is achieved by a measuring arrangement having the features of claim 1 . This has the advantage that the contamination state of an EUV mirror surface can be precisely detected with the aid of components that are known per se. In particular, the measurement arrangement according to the invention allows the contamination state to be detected in situ even when the EUV mirror is installed in a lithography device, for example.

The object on which the invention is based is achieved with the features of claim 1 in that the measuring arrangement has the first optical element with the EUV mirror surface, ie includes the test specimen or the optical element to be tested itself. In addition, the measuring arrangement has an at least partially reflecting second optical element, as well as a light beam source and a light beam sensor, the light beam source, the light beam sensor and the two optical elements mentioned being arranged relative to one another in such a way that they form a Fabry-Perot resonator.

A typical Fabry-Perot resonator or interferonmeter has two partially reflecting mirrors with high reflectivity, which together form an optical resonator. The transmission spectrum of this arrangement shows small transmission maxima for wavelengths that meet the resonator criterion, while other wavelengths are canceled. This is done through constructive and destructive interference. In particular, a laser beam source is used as the light beam source, with which the frequency of the laser beam can be adjusted. To form the Fabry-Perot cavity, the laser beam frequency is tuned as a function of cavity length using, for example, the Pound-Drever-Hall laser frequency stabilization technique. The contamination or the contamination state of the optical element to be tested can be determined precisely by detecting the reflection image using the light sensor and evaluating the transmission maxima. A contamination state of the first optical element or the test object is determined in particular by comparing the detected actual resonator frequency of the Fabry-Perot resonator with a reference resonator frequency, ie with an expected resonator frequency that is present when the surface is free of contamination. For this purpose, in particular, a deviation of the actual resonator frequency from the reference resonator nator frequency is determined and the contamination status is determined as a function of the deviation.

According to a preferred embodiment of the invention, the first optical element has an effective surface of the EUV mirror surface exposed to contamination and a contamination-free reference surface. In particular, the above-mentioned reference resonator frequency can be determined with the aid of the reference surface. A contamination-free reference surface is created, for example, by lying at a point on the measuring arrangement or the optical device where it does not suffer from contamination, which is the case, for example, if it does not receive EUV radiation and/or EUV-induced plasma is exposed. The effective area is, for example, the entire EUV mirror surface or only part of the EUV mirror surface that is exposed to the EUV radiation and thus to possible contamination during operation.

Furthermore, it is preferably provided that the reference surface is a contamination-free area of the EUV mirror next to the effective surface. The reference surface is thus a section of the EUV mirror or the EUV playing surface that lies away from or next to the effective surface, ie the area of the EUV mirror surface that lies in the EUV beam path during operation of the device. This has the advantage that there is no further mirror element for the reference surface. Instead, the EUV mirror that is already present is divided into an effective area (effective area) and a reference area (reference area), which results in particular in cost and installation space advantages.

Furthermore, according to an alternative embodiment, it is preferably provided that the reference surface is formed on a reference mirror arranged next to the EUV mirror or the EUV mirror surface. The reference surface is thus arranged on a separate mirror element and can, for example, be arranged at a distance from or close to the EUV mirror. The reference mirror can also lie directly on the side of the EUV mirror. The reference mirror is preferably designed as a separate component or as a region or section of the same optical element that is separate from the EUV mirror. In the last case, the reference mirror and the EUV mirror have a common carrier element, for example.

The light beam source is preferably designed to selectively guide the light beam onto the active surface or onto the reference surface. As a result, the actual resonator frequency and the reference resonator frequency can be determined with one and the same light beam source. In particular, a metrology crosshair (metrology reticle) is used as the light beam source, which can be inserted or inserted into the device, in particular a lithography device. Several light beams can be generated using the metrology crosshairs so that the reference resonator frequency and the actual resonator frequency can be determined simultaneously. In this case, the light sensor is arranged in particular at a distance from the light beam source, for example at the end of a beam path of the optical device, the light beam source being arranged at one end and the light beam sensor at the other end of the beam path of the device.

According to a preferred development of the invention, the second optical element is also designed as an EUV mirror. This forms the Fabry-Perot interferometer or resonator between the two EUV mirrors.

Furthermore, it is preferably provided that, according to an alternative embodiment, the second optical element is a simple mirror or a lens. The measuring arrangement can thus also be integrated elsewhere in a device, in particular a lithography device, in which there is no second EUV mirror.

Furthermore, according to a further embodiment, it is preferably provided that the light beam source and the light beam sensor are designed as a manageable sensor unit. The light beam source and the light beam sensor thus form a sensor unit which can be arranged in the device or in relation to the testing optical element detached from the other design of a device. In particular, the light beam source and light beam sensor then have a common housing. The unit can also be permanently integrated into a lithography device or can be used for an ex-situ examination of a test piece or the first optical element.

The lithography device according to the invention with the features of claim 9 is distinguished by the measuring arrangement according to the invention. This results in the advantages already mentioned.

The method according to the invention with the features of claim 10 is characterized in that the first optical element is assigned a partially reflecting second optical element, a light beam from a light beam source being directed through the second optical element onto the first optical element, and the optical elements , the light beam and a light beam sensor are arranged and aligned with one another in such a way that they form a Fabry-Perot resonator form, and wherein a contamination state of the first optical element is determined as a function of a deviation of a detected actual resonator frequency of the Fabry-Perot resonator from a reference resonator frequency. This results in the advantages already mentioned above.

The reference resonator frequency is preferably determined and stored in advance. As a result, an actual resonator frequency recorded later is always compared with a previously stored reference resonator frequency in order to determine the current contamination status.

According to an alternative embodiment, the resonant frequency is determined by means of a contamination-free reference surface or reference mirror surface assigned to the EUV mirror. As a result, the reference resonator frequency is detected during operation, which is then compared with the actual resonator frequency. This has the advantage that the contamination of the first optical element can be determined, in particular in situ, without prior tests.

• 1 eine Lithographie-Einrichtung in einer vereinfachten Darstellung,
• 2 eine schematische Darstellungserfassung einer vorteilhaften Messanordnung für die Lithographie-Einrichtung,
• 3 ein weiteres Ausführungsbeispiel der Messanordnung.

Further advantages and preferred features and feature combinations result in particular from what has been described above and from the claims. The invention will be explained in more detail below with reference to the drawing. to show

• 1 a lithography facility in a simplified representation,
• 2 a schematic representation of an advantageous measurement arrangement for the lithography device,
• 3 another embodiment of the measuring arrangement.

1 shows a simplified representation of an advantageous lithography device 1, which has a housing 2 in which a large number of optical elements M1 to M6 are arranged, which form a radiation path from a light beam source 3 to a substrate 4, such as a wafer, to process the wafer using lithography processes. In particular, the light beam source 3 is designed as an EUV radiation source, preferably with a reticle, in order to generate an EUV light beam and in particular in order to image a structure formed on the reticle on the substrate 4 or wafer. To this extent, the device 1 is an EUV lithography device. The optical elements M1 to M6 are in the form of mirrors or lenses which are at least partially reflective. Of course, the device 1 can also have more or fewer than six optical elements. In the present case, at least the optical element M6 is also designed to be translucent at least in regions. The optical elements are designed as EUV light elements, in particular as EUV mirrors, and are therefore exposed to contamination in normal operation when they are exposed to EUV light. The contamination state of one or more of the optical elements M1 to M6 is advantageously detected or monitored with the measuring arrangement described below.

2 1 shows, in a simplified representation, an advantageous measuring arrangement 5 which can be used in the device 1. The advantageous measuring arrangement 5 is characterized in that a test object, or an optical element 6 to be tested, is assigned a second optical element 7, which is also designed to be at least partially reflective. The optical elements 6 and 7, the light beam source 3 and a light beam sensor 8 presently arranged in the area of the plane of the substrate are arranged and aligned relative to one another in such a way that a Fabry-Perot resonator 9 is formed.

For this purpose, the light beam source 3 is designed and aligned, for example, in such a way that a light beam generated by it penetrates the second optical element 7, is reflected on the first optical element 6 and is reflected again on the second optical element 7, so that an optical resonance body in the sense of a Fabry -Perot interferometer is created.

The optical element 6, which is, for example, the optical element M6 of the device 1, has a coating that forms an EUV mirror surface 10.

The second optical element 7 is optionally formed by one of the further optical elements M1 to M5, in particular in a suitable position and design.

The surface 10 is exposed to contamination during normal operation, as previously described. A resonator frequency f act , which occurs between the optical elements 6 and 7, _is detected by means of the Fabry-Perot resonator. This resonator frequency f actual _is the actual resonator frequency. In order to determine the degree or state of contamination of the optical element 6, this actual resonator frequency is measured using a reference resonator frequency f _ref . The reference resonator frequency is in particular one that is present when the first optical element 6 is in a contamination-free state. For this purpose, for example when the device 1 is put into operation, that is to say when no EUV contamination has yet been able to take place, the current actual resonator frequency is recorded and stored as the reference frequency. At a later point in time, at which contamination can be expected, the check is carried out again and the current actual resonator frequency f _is recorded. The actual resonator frequency f actual _is then compared with the reference resonator frequency f _ref . If the deviation Δf of the actual resonator frequency _f actual from the reference resonator frequency f _ref deviates by a definable limit value, it is recognized that the first optical element is contaminated or critically contaminated, so that continued operation of the device 1 is not recommended without previously replace the optical element 6 or to clean the EUV mirror surface 10.

For illustration shows 2 in addition, at A the initial measurement for detecting the reference value f _ref and at B the detection at a later point in time at which the mirror surface was contaminated. The contamination changes the distance between the optical elements 6 and 7 and thus the resonator length L by the amount ΔL that results from the contamination of the mirror surface. If the reference measurement is carried out under the same environmental conditions as the later measurement, changes in the reflection index of the mirror surface caused by atmospheric pressure, for example, are compensated for. The resonator structure can also be designed in such a way that the frequency is a radio frequency that can be detected with conventional sensors.

As an alternative to a time-separated reference measurement at the same measuring point, provision is made in accordance with the further exemplary embodiment for a reference measurement to be carried out at the same time or almost at the same time. This can be done, for example, according to a measurement arrangement as in 2 is shown, take place in that the first optical element 6 is assigned a reference element 11 or a reference resonator 14 . The reference element 11 has, in particular, a reference mirror surface 12, which lies outside the beam path of the device 1, but in particular next to the first optical element 6. By directing the light beam from the light beam source 3 into the reference resonator 14, a Fabry-Perot resonator is used for the reference measurement process formed between the second optical element 7 and the reference element 11 or the reference mirror surface 12 . If the mirror surface 10 is sufficiently contaminated, a comparison of the measurements with the reference element 11 and the first optical element 6 then results in a detectable deviation, which allows the contamination to be identified. In this case, the Fabry-Perot resonator 9 and the reference resonator 14 use the same optical element 7.

Alternatively, a further optical element 13 is assigned to the reference element 11, so that a complete second Fabry-Perot resonator 14 is formed, which acts in parallel with the Fabry-Perot resonator 9 but has independent optical elements. By controlling the light beam source 3, the light beam can be assigned to one or the other Fabry-Perot resonator 9, 14, or both Fabry-Perot resonators each have their own light beam source 3 assigned so that they can be operated independently of one another.

The measuring arrangement 5 can be placed at different points of the device 1 in order to test different mirror surfaces. The contamination of different optical elements M1 to M6 in the beam path of the lithography device 1 can then be detected by beam deflection using only one light sensor 8 .

According to a further embodiment, as in 3 is shown, the light beam source 3 and the light beam sensor 8 are designed as a sensor unit 15 in a common housing 16 . In particular, the second optical element 7 is also part of the sensor unit 15. This embodiment of the measuring arrangement 5 also makes it possible to detect the contamination outside of the device 1, ie outside of the EUV optics. In this case, too, a time-separated reference measurement for detecting a deviation is possible.

The optical element 6 preferably has an effective surface 17 on its EUV mirror surface 10, which is exposed to the EUV radiation during normal operation, such as by an incident beam cone 19 and an emerging beam cone 20 of the EUV operating beam path in 3A implied. The EUV mirror surface also has the reference surface 12 adjacent to the effective surface 17, which is characterized in that it is not exposed to the EUV light beams during normal operation of the device 1 or a similar device and should therefore not show any signs of contamination.

Alternatively, the reference surface 12 is formed on a reference mirror 18 which is arranged at a distance and is arranged at a distance from the EUV mirror surface 10 . Several such reference surfaces 12, in particular reference mirrors 18, are optionally available. The reference surfaces 12 are thus also spaced apart from the effective surface 17. 3A shows a simplified side view of the measuring arrangement 5 and 3B shows a simplified plan view.

The optical element 6 is in particular designed aspherically, whereby the contamination can nevertheless be reliably determined by the advantageous measuring arrangement 5 .

The sensor unit 15 is now controlled to detect the actual resonator frequency _f actual between the optical element 7 and the effective surface 17 on the one hand and the reference resonator frequency f _ref between one of the reference surfaces 12 of the optical element 7 on the other hand, in order to then determine the deviation Δf of the Detect actual resonator frequency from the reference resonator frequency in order to determine the contamination of the active surface 17 of the EUV mirror surface 10 as described above.

Due to the integration into the device 1, the Fabry-Perot resonator 9 or interferometer can also be used to detect the alignment of the optical elements M1 to M6 with respect to one another, so that they can be realigned or repositioned if necessary.

In particular, the light beam source is a metrology reticle that is used in the lithography device 1 . It is thus possible to generate a plurality of light beams, in particular EUV light beams, by means of which the lithography can be carried out and contamination states can also be detected. Typically, up to 3 times 13 field points can be illuminated using the metrology crosshairs. In this way, in particular, a homogeneous illumination of a light cone can be achieved, through which all mirror surfaces are illuminated at the same time. Alternatively, several individual light beams can be used in order to capture only selected mirror surfaces.

The resonance, in particular the resonance frequency, is detected by means of the light sensor 8 via the distribution of the points of light, in particular via the angular distribution.

The obtained wavefront or wavefront distribution can be compared to the measured EUV wavefront, which is dominated by the multilayer structure and less affected by surface contamination, to distinguish between mirror surface shape changes, contamination and decarbonization phenomena. The lithography device 1 is preferably equipped with a multi-wavelength option in order to detect changes in the shape of mirror surfaces with higher sensitivity and, depending on the type of contamination, to detect material contrasts, such as in the case of silicon contamination through a Si membrane ( DGLm), which is used, for example, for the vacuum-technical separation between test object and optical device or optical volume, or pellicle pieces. The integration of the advantageous measuring arrangement with the Fabry-Perot resonator makes it possible to monitor the mirror positions of the optical elements, to detect changes in the shape of the mirror surfaces and to detect contamination of individual mirrors, in particular the EUV mirror.

Claims (12)

Measuring arrangement (5) for detecting a contamination state of an EUV mirror surface (10) of an optical device, in particular lithography device (1), which has a first optical element (6) with an EUV mirror having the mirror surface (10), which has a at least partially reflecting second optical element (7) and having a light beam source (3) and a light beam sensor (8), the light beam source (3), the light beam sensor (8) and the optical elements (6, 7) being arranged in relation to one another in such a way that they form a Fabry-Perot resonator (9). measurement arrangement claim 1 , characterized in that the first optical element (6) has a contamination-exposed effective surface (17) of the EUV mirror surface (10) and a contamination-free reference surface (12). measurement arrangement claim 2 , characterized in that the reference surface (12) is a contamination-free area of the mirror surface (10) next to the active surface (17). measurement arrangement claim 2 , characterized in that the reference surface (12) is formed on a reference mirror (18) arranged next to the EUV mirror surface (10). Measuring arrangement according to one of the preceding claims, characterized in that the light beam source (3) is designed to guide the light beam selectively onto the active surface (17) or onto the reference surface (12). Measuring arrangement according to one of the preceding claims, characterized in that the second optical element (7) is an EUV mirror. Measuring arrangement according to one of Claims 1 until 6 , characterized in that the second optical element (7) is a mirror or a lens. Measuring arrangement according to one of the preceding claims, characterized in that the light beam source (3) and the light beam sensor (8) are designed as a manageable sensor unit (15). Optical device, in particular lithography device (1), characterized by at least one measuring arrangement (5) according to one of Claims 1 until 8th . Method for detecting contamination of an EUV mirror surface (10) of an optical device, in particular lithography device (1), which has at least one first optical element (6) which has the EUV mirror surface (10), in particular by means of a measuring arrangement ( 5) according to one of Claims 1 until 9 , wherein a partially reflecting second optical element (7) is assigned to the first optical element (6), wherein a light beam is directed through the second optical element (7) onto the first optical element (6), and wherein the optical elements (6, 7), the light beam source (3) and a light beam sensor (8) are arranged and aligned relative to one another in such a way that they form a Fabry-Perot resonator (9), and wherein, depending on a deviation (Δf) of a detected actual resonator frequency ( f _ist ) of the Fabry-Perot resonator (9) from a reference resonator frequency (f _ref ) a contamination state of the EUV mirror surface (10) is determined. procedure after claim 10 , characterized in that the reference resonator frequency (f _ref ) is determined in advance and stored. procedure after claim 11 , characterized in that the reference resonator frequency (f _ref ) is determined by means of a contamination-free reference surface (12) assigned to the EUV mirror surface (10).

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