Method and device for mask inspection

Abstract

This invention relates to a method and apparatus for inspecting photomasks, wherein the photomask is designed for reflective operation at a working wavelength less than 30 nm, and is used to be irradiated to expose a wafer in a lithography process of a projection exposure apparatus. In the method according to the invention, an object field (1260) located in an object plane (OP) and illuminated by an illumination system (1310) using EUV radiation having a wavelength less than 30 nm is imaged using a projection lens (1220, 1320) onto an image field (230, 330, 430, 530, 630, 730, 930, 1030, 1130) located in an image plane (IP), wherein a sensor configuration (200, 300, 400, 500, 6...) having a plurality of sensors (201, 202, ..., 301, 302, ...) is used. The photomask (1330) is located in the image plane (IP), wherein in a scanning operation the photomask (1330) is guided above the object field (1260) in the object plane (OP), and wherein an image of the photomask (1330) is formed by combining multiple sensor images captured by each of the sensors (201, 202, ..., 301, 302, ...) in the scanning operation.

Description

Technical Field

[0001] This invention relates to a method and apparatus for photomask inspection. [Interactive Reference]

[0002] This application claims priority to German patent application DE 10 2022 133 829.0 (filed December 19, 2022). The contents of the German application are incorporated herein by reference. Prior Technology

[0003] Lithography is used to fabricate microstructure components, such as integrated circuits or LCDs. The lithography process is performed in a so-called projection exposure apparatus, which includes an illumination device and a projection lens. In this case, the image of a photomask (reduced photomask) illuminated by the illumination device is projected through the projection lens onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist), and positioned on the image plane of the projection lens to transfer the photomask structure onto the photosensitive coating of the substrate.

[0004] In lithography, unwanted defects on the photomask can have particularly adverse effects because they reproduce with each exposure step, potentially rendering the entire semiconductor device unusable in the worst-case scenario. Therefore, it is crucial to test the photomask for sufficient imaging capability before mass production.

[0005] Therefore, there is a need for rapid and simple testing of photomasks, especially under conditions similar to those actually present in projection exposure apparatuses, to test as many photomasks as possible. For this purpose, apparatus for photomask inspection is known, which includes an illumination system and a projection lens through which the illuminated area of ​​the photomask is imaged onto a sensor configuration, such as, for example, a CCD camera. In this case, a particular problem arises because the final image produced, due to the lithography process on the wafer or in its photosensitive layer (photoresist) in the projection exposure apparatus, is therefore still different from the result predicted based on intensity measurements performed using the sensor configuration in the photomask inspection apparatus.

[0006] Avoiding or mitigating this problem is a significant challenge, especially when inspecting masks designed for operation in EUV (i.e., wavelengths less than 30 nm, particularly less than 15 nm). For example, in this case, while it is theoretically possible to characterize the relevant (EUV) mask at higher wavelengths in the DUV range (e.g., about 248 nm or about 193 nm), significant deviations from the actual operating wavelengths of projection exposure apparatuses lead to a loss of reliability in mask inspection. For instance, defects caused by using imaging at off-wavelengths may be represented differently relative to their optical effects in the projection exposure apparatus, making it impossible to identify specific particles or defects on the mask. Alternatively, the shift to higher wavelengths in mask inspection may reduce the achieved resolution, leading to the misidentification of defects at locations where they are not actually present.

[0007] On the other hand, the shift to lower (especially EUV) wavelengths in photomask inspection (ideally in the context described above) leads to further problems. Fundamentally, the EUV radiation generated by plasma sources in equipment typically used for photomask inspection cannot be reduced to a sufficiently small image field or an image field corresponding to the typical size of an available sensor. This problem can be attributed to the fact that, compared to excimer lasers used in the DUV range, the plasma sources required to generate EUV radiation initially emit in all spatial directions. Due to the maintenance of optical spread, it is impossible to directly concentrate the generated EUV radiation onto a sufficiently small image field without simultaneously accepting light loss. Summary of the Invention

[0008] Against the background described above, an object of the present invention is to provide a method and apparatus for photomask inspection, which can predict the resulting imaging results as accurately as possible as the results of lithography processes on wafers, while at least partially avoiding the aforementioned problems.

[0009] This objective is achieved by the method of the features of Independent Request 1 and the apparatus according to Selective Independent Request 31.

[0010] According to one embodiment, the present invention relates to a method for inspecting a photomask, wherein the photomask is designed for reflective operation at an operating wavelength of less than 30 nm, and the photomask is used to be irradiated to expose a wafer in a lithography process of a projection exposure apparatus; - An object field located in an object plane and illuminated by an illumination system using EUV radiation with a wavelength less than 30 nm is imaged onto an image plane using a projection lens, wherein a sensor having a plurality of sensors is disposed on the image plane; - In one scanning operation, the photomask is guided above the object field in the object plane; and - An image of the photomask is formed by combining multiple sensor images captured by each of the sensors during the scanning operation.

[0011] The present invention is particularly based on the concept that EUV radiation (i.e., especially "photochemical") is also used to perform photomask inspection for EUV operation or for EUV projection exposure apparatus, while overcoming the fundamental problem mentioned in the preamble, namely that a relatively large image field must be handled in photomask inspection, because in the method and apparatus for photomask inspection according to the present invention, the corresponding image of the photomask is formed by combining the images of the plurality of sensors captured by each of the plurality of sensors in the sensor configuration according to the present invention during the scanning operation.

[0012] In this context, the invention further includes the principle, implemented based on various embodiments, of using a clever geometrical arrangement of the individual sensors in a sensor configuration to actively fill the active sensor region or active sensor pixel with the highest possible luminous efficiency within the maximum possible proportion of the image field. The invention also preferably includes the concept of maximizing the "line fill rate" as defined below, in the sense of the maximum possible percentage proportion of the exposed image field length constituted by the active sensor region in the scanning operation. Furthermore, the advantageous geometrical arrangement of the individual sensors in embodiments of the invention particularly includes their relative arrangement, the method of which, for example, ensures a specific minimum number of sensors swept during the scanning operation in each case and / or does not exceed a predetermined number of sensor series not swept during the scanning operation in each case.

[0013] Therefore, the present invention achieves particularly accurate and reliable photomask inspection through the following facts: firstly, it achieves the fundamental advantage of photochemical photomask inspection (i.e., photomask inspection using an "inspection wavelength" corresponding to the actual operating wavelength of the photomask in the projection exposure apparatus); secondly, it overcomes the problem that arises in principle with the shift to the EUV wavelength range in photomask inspection due to the relatively large image field to be managed in this case.

[0014] According to one embodiment, as a sensor configuration, a TDI sensor (TDI = "Time Delay and Integration") with a sensor region is used, wherein only a portion of the individual sensor region is implemented as an active sensor region having a plurality of active sensor pixels. Regarding configurations of TDI sensors that are inherently known, only the example of patent DE 197 14 221 A1 is cited for reference.

[0015] According to one embodiment, in the scanning operation, multiple projections of multiple different regions of the photomask are scanned across the sensor configuration along multiple different scan lines.

[0016] According to one embodiment, the line fill rate is, in each case, not less than 25%, particularly not less than 35%, and even more particularly not less than 50% for any of the scan lines. The line fill rate for any of the scan lines is defined as a ratio between the distance covered by a plurality of active sensor pixels during the scanning operation and the length of the image field exposed along the scanning direction during the scanning operation. Herein and hereinafter, "scanning direction" will be understood as referring to the direction in which the projection of the photomask moves in the image plane during the scanning operation.

[0017] According to one embodiment, in a scanning operation for each of the scan lines, the number of sensors scanned in each case is at least one, particularly at least two, and more particularly at least three.

[0018] According to one embodiment, the sensors are configured to form a plurality of sensor series arranged adjacent to each other along the scanning direction and extend laterally relative to the scanning direction.

[0019] According to one embodiment, in the scanning operation for each of the scan lines, the number of sensor series that are not scanned in each case is at most two, and in particular at most one.

[0020] According to one embodiment, adjacent sensor series are offset relative to each other in a direction extending laterally relative to the scanning direction. In this case, this offset can be selected, in particular, such that at least one sensor in a sensor series partially overlaps with two sensors in an adjacent sensor series.

[0021] According to one embodiment, the plurality of active sensor regions of the sensors are asymmetrically arranged on a single sensor region, wherein the asymmetry of the different sensors is oriented differently.

[0022] According to one embodiment, forming the sensor configuration by combining these sensors involves performing classification based on a prior determination of multiple defective regions of the individual sensors. This is based on the consideration that, generally, due to process requirements, individual sensors have so-called "dead lines," which are defective because they consistently produce zero or maximum sensor signals. These sensors can be appropriately "classified" rather than completely classified, as described in more detail below, in cases where, for example, only a relatively small number of sensors are present in the sensor configuration, dead lines are avoided or not permitted.

[0023] According to one embodiment, the image field of the projection lens has a region-like shielding that is shielded during imaging.

[0024] According to one embodiment, the shield is at least partially located within the image field. In this case, a portion of the image field may be located specifically on each side of the optical axis. Furthermore, a portion of the image field may be located on each side of the shield. More specifically, the shield may be symmetrically configured about the rotation axis of the projection lens.

[0025] According to one embodiment, in this scanning operation, the readout of data captured by each of the sensors is synchronized with the guidance of the photomask over the object field. In this case, "synchronization" should be understood as the photomask's moving speed (in millimeters per second) multiplied by the imaging scale of the projection lens corresponding to the sensor's readout frequency (in kHz) multiplied by the size of the sensor pixels measured along the scanning direction. In other words, the sensor's readout speed must be a factor faster than the photomask's movement, where this factor is the imaging scale of the projection lens.

[0026] According to one embodiment, a calibration of the individual brightness of the sensor images captured by each of the sensors during the scanning operation is performed based on an intensity measurement performed using an intensity sensor.

[0027] According to one embodiment, the configuration of reading at least two sensors or sensor regions of the sensor configuration at multiple different readout frequencies takes into account that not all sensors are precisely mounted in a plane, or some sensors are not mounted perfectly parallel to the plane, or the optical units used have distortion, and the active imaging ratio of each sensor may be slightly different. Since the imaging ratio is affected by the synchronization of data readout from each individual sensor during the scanning operation with the guidance of the photomask on the object field, unwanted image blurring occurs during readout from all sensors at the same readout frequency due to the TDI process, thus leading to reduced contrast. This effect can be avoided or at least reduced by reading each sensor at an optimal readout frequency, and / or by reading individual different sensor regions of the same sensor at different readout frequencies.

[0028] According to one embodiment, the sensor images of the sensors are preprocessed before being combined. This preprocessing may include, for example, low-pass filtering. Furthermore, before being added, the sensor images of each sensor can be shifted and / or magnified or reduced and / or distorted with sub-pixel precision. Therefore, the aforementioned scale differences can be at least partially compensated.

[0029] According to one embodiment, the dark current of the sensor is measured and then subtracted from the measurement result.

[0030] According to one embodiment, the sensors are cooled for the purpose of reducing noise. In this case, the sensors are specifically cooled to a temperature below the average temperature of the projection lens (e.g., 10°C, 0°C, or -20°C). Cooling can be achieved, for example, through a cooling fluid and / or through a Peltier element. This takes into account the possibility that so-called dark current noise may increase during the lifespan of the sensor configuration, and the aforementioned cooling or adjustment of the operating temperature enables the reduction of dark current noise (e.g., to the value initially provided for a "new" sensor configuration).

[0031] The present invention further relates to a sensor configuration comprising a plurality of sensors, wherein the sensors form a plurality of sensor series, the sensor series being arranged adjacent to each other along a predetermined direction and extending laterally relative to the predetermined direction, wherein the line fill rate is not less than 25% for any of the plurality of lines in each case, the line fill rate being defined for each line extending parallel to the predetermined direction above a predetermined image field as a ratio between the distance covered by the plurality of active sensor pixels in each case and the entire image field length along the predetermined direction, wherein the predetermined image field is at least partially covered by the sensor configuration.

[0032] According to one embodiment, the line fill rate is not less than 35% for any of the lines, and more particularly not less than 50%.

[0033] According to one embodiment, these sensors are designed for an operating wavelength of less than 30 nm.

[0034] The present invention relates more to a sensor configuration comprising a plurality of sensors, wherein the sensors form a plurality of sensor series, the sensor series being arranged adjacent to each other along a predetermined direction and extending laterally relative to the predetermined direction, wherein the sensors are designed for an operating wavelength of less than 30 nm.

[0035] According to one embodiment, the sensors are configured as a plurality of TDI sensors.

[0036] According to one embodiment, the series of sensors that are adjacent to each other are offset relative to each other in a direction that extends laterally relative to the predetermined direction.

[0037] According to one embodiment, this offset is selected such that at least one sensor in a sensor series partially overlaps with two sensors in an adjacent sensor series.

[0038] According to one embodiment, each of the sensors has a sensor region, wherein in the case of each of the sensors, only a portion of the individual sensor region is implemented as an active sensor region having a plurality of active sensor pixels.

[0039] According to one embodiment, the active sensor regions are asymmetrically arranged on individual sensor regions, wherein the asymmetry of the different sensors is oriented differently.

[0040] According to one embodiment, the sensors are combined to form the sensor configuration by classifying the sensors based on multiple defect regions present on the individual sensors.

[0041] According to one embodiment, the sensor configuration includes a cooling device.

[0042] This sensor configuration can be designed, particularly for methods with the features described above.

[0043] The present invention relates more particularly to a photomask inspection apparatus, wherein a photomask is designed for reflective operation at a working wavelength less than 30 nm and for illumination in a lithography process in a projection exposure apparatus for exposing a wafer, wherein the apparatus includes an illumination system, a projection lens and a sensor configuration, wherein an object field located in an object plane and illuminated by EUV radiation having a wavelength less than 30 nm through the illumination system is imaged by the projection lens onto an image field located in an image plane, wherein the sensor configuration having a plurality of sensors is located in the image plane, and wherein the apparatus includes the sensor configuration having the features described above.

[0044] The device can be designed to perform methods having the features described above.

[0045] For the advantages and favorable configurations of the sensor configuration and device, please refer to the foregoing embodiments relating to the method according to the present invention.

[0046] Further configurations of the present invention can be found in the specification and the scope of the patent application at the end of the document.

[0047] The present invention will be explained in more detail below with reference to the accompanying drawings and preferred exemplary embodiments. Simple Explanation of the Diagram

[0048] In these diagrams:

[0049] Figure 1 shows a schematic diagram illustrating the problem solved by the present invention;

[0050] Figure 2 shows a schematic diagram illustrating a possible embodiment of the method according to the present invention and the sensor configuration used in the method;

[0051] Figures 3 to 11 illustrate schematic diagrams for explaining the method according to the invention and another possible embodiment of the sensor configuration used in the method;

[0052] Figure 12 shows a schematic diagram of a basic possible arrangement of the projection lens used in the apparatus for photomask inspection of the present invention; and

[0053] Figure 12 shows a schematic diagram of a basic possible setup of the apparatus for photomask inspection of the method according to the present invention. Implementation

[0054] As shown only schematically in FIG13, an apparatus 1300 for photomask inspection that can be used according to the method of the present invention includes an illumination system 1310 and a projection lens 1320, wherein light from a light source (not shown in FIG13) enters the illumination system 1310 and is incident on a photomask 1330 disposed in the object plane of the projection lens 1320, and wherein the illuminated area of ​​the photomask 1330 is imaged onto a sensor configuration 1340 through the projection lens.

[0055] In this and the following, the photomask may have an aspect ratio in the range of 1:1 to 1:3, preferably in the range of 1:1 to 1:2, and more preferably in the range of 1:1 or 1:2. The shape of the photomask may be substantially rectangular. The length and width of the photomask are preferably in the range of 5 inches to 7 inches, more preferably 6 inches. Alternatively, the length of the photomask may be in the range of 5 inches to 7 inches, and the width may be in the range of 10 inches to 14 inches, preferably 6 inches in length and 12 inches in width.

[0056] In order to predict the image obtained by using a photomask during the lithography process in a projection exposure apparatus, the intensity distribution of the photomask to be measured is first obtained from the apparatus for photomask inspection shown in Figure 13 or through a sensor configuration. In this case, the same wavelength used in the lithography process of the projection exposure apparatus is preferably used in the photomask inspection apparatus.

[0057] In the schematic diagram of Figure 1, "180" represents the image field that can be imaged as a whole through the imaging optical unit or projection lens of the device used for photomask inspection; "120" represents occlusion (in the form of the area occluded during imaging), which will be described in more detail; "130" represents the rectangular image field that is imaged from the object field located on the object plane of the projection lens. A sensor configuration containing a plurality of sensors (described below) is located in the image plane of the projection lens.

[0058] The following description, with reference to the schematic diagrams in Figures 2 to 11, illustrates various embodiments of the method according to the invention and the sensor configuration therein for photomask inspection.

[0059] The common feature of the embodiments described below is that, in order to design photomask inspection for operation in the EUV wavelength range in the apparatus for photomask inspection according to the invention, EUV radiation is also used, and in order to manage a relatively large image field, it is substantially consistent with it during photomask imaging in the projection exposure apparatus, wherein the inter-scan process is carried out in a manner in which the image of the photomask is formed by combining the sensor images captured separately by the individual sensors in the sensor configuration during the scanning operation. Hereinafter, the embodiments described with reference to FIG2 and the following figures correspond to different, in each case, particularly to the “skilled” or advantageous geometric configuration of the sensors for maximizing luminous efficiency.

[0060] In this paper, the actual scanning operation is performed in a manner that, for example, guides the image field of the device for photomask inspection at a constant speed on the order of millimeters per second in the sensor configuration.

[0061] As the sensor configuration, a TDI sensor is used here, whose image is added with a corresponding time offset. In a manner known in nature, each of these TDI sensors has active sensor pixels on a carrier (typically made of ceramic material) only on a portion of the entire sensor surface area (forming an "active sensor area"). By way of example only (and the invention is not limited thereto), as shown in Figure 2, and in the exemplary embodiments described below, the carrier dimensions of each sensor may be 68 mm × 30 mm, the sensor surface area may be 64 mm × 26 mm, and the active sensor area may be 60 mm × 18 mm.

[0062] For illustration purposes only, Figure 2 shows a sensor configuration 200 containing a total of 12 sensors 201, 202, ..., whose active sensor areas are indicated by "201a", "202a", ... The carriers of sensors 201, 202, ... are indicated by "201b", "202b", ... respectively, and the carriers are typically made of ceramic material.

[0063] In the above scanning operation, the image field guided on sensor configuration 200 is indicated by "230". "280" indicates the image field (with an exemplary diameter of 308 mm) that can be imaged as a whole by the imaging optical unit or projection lens of the device for photomask inspection for comparison, and "220" indicates the presence of occlusion (in the form of a shadow area during imaging), which is provided in the exemplary embodiment shown (but again does not limit the invention in this respect) and will be discussed in more detail below.

[0064] As illustrated by the double-headed arrows in Figure 2, during the scanning operation that images individual illumination areas of the photomask onto the image field, the projections of different areas of the photomask sweep across the sensor configuration along different scan lines (the three scan lines are shown in Figure 2 by way of example only and are indicated by "235a", "235b", and "235c"). In the exemplary embodiment according to Figure 2, the geometry of the sensors 201, 202, ... is selected such that for each of the scan lines, the respective active sensor regions 201a, 202a, ... of at least three sensors 201, 202, ... are swept. In this case, the scanning can be performed from left to right and from right to left.

[0065] A criterion particularly suitable for the most active utilization of the active sensor region during scanning according to the present invention is the ratio of the distance covered by the active sensor pixels in each case during the scanning operation to the exposed image field length during the scanning operation; this ratio is referred to herein and hereinafter as the "line fill rate". Preferably, the geometric configuration of the sensors in the sensor configuration of the present invention relative to each other is chosen such that the line fill rate defined above is not less than 25% for any scan line, specifically not less than 35% for any scan line, and more particularly not less than 50% for any scan line. In the specific exemplary embodiment of FIG. 2, the line fill rate is 51% for those scan lines that scan through the individual active sensor regions of three sensors; and 68% for those scan lines or tracks that scan through four sensors.

[0066] According to Figure 2, the sensors 201, 202... of the sensor configuration 200 are formed as a plurality of sensor series arranged adjacent to each other in the scanning direction and extending laterally relative to the scanning direction. In this case, the adjacent sensor series are offset relative to each other in the direction of lateral extension relative to the scanning direction. Furthermore, the respective active sensor regions 201a, 201b,... of the sensors 201, 202,... according to Figure 2 are asymmetrically arranged on each sensor region, wherein the asymmetrical portions of other different sensors are oriented differently. In a specific exemplary embodiment of Figure 2, specifically, the two sensor series shown on the left are arranged in a manner that rotates 180° relative to their respective active sensor regions relative to the two sensor series shown on the right. This allows the obtained line fill rate to be achieved using a relatively small image field 230 (in this example, having dimensions of 178 mm × 106 mm). Another advantage of this geometrical configuration of sensors 201, 202 relative to their respective active sensor regions 201a, 202a… is a more uniform distribution of the heat load acting on the entire area of ​​the sensor configuration during operation, which is advantageous for the design of cooling devices. Furthermore, one advantage of this asymmetrical configuration is that it compensates for or eliminates the differences that may exist in the forward and backward modes of the sensors during scanning operation (e.g., both sensors “operate” forward along the scan line, both sensors “operate” backward, etc.).

[0067] Preferably, and further, as can also be seen in the exemplary embodiment of FIG2, adjacent sensor series are selected with relative offsets to each other in a direction extending laterally relative to the scanning direction, such that at least one sensor in one sensor series partially overlaps two sensors in an adjacent sensor series. In each case, the overlap should be greater than twice the positional tolerance of each sensor (plus a certain tolerance required by the image processing algorithm, if appropriate). Therefore, even with large positional tolerances of the individual sensors, sufficient overlap can be ensured to avoid situations where scan lines in the relevant region do not sweep across the active sensor area.

[0068] Figure 3 illustrates a schematic diagram of a sensor configuration according to the invention, used to illustrate another example in which similar or substantially identical components to those in Figure 2 are indicated by reference numerals with the addition of "100". In Figure 3, in addition to the image field 380 (exemplary diameter 308 mm) already indicated in Figure 2, which can be imaged to the maximum extent by the imaging or projection optics unit, another image field 380a with a relatively small diameter (279 mm in the example) is shown. Here, by way of example, it is assumed that the imaging optics unit of the apparatus for photomask inspection only provides sufficiently good image quality for the relatively small image field 380a. According to Figure 3, this situation can be considered through the shaded area 350 that does not read the active sensor area. In this case, it is also ensured that each scan line still sweeps across at least three sensors during the scanning operation (therefore the line fill rate defined above is still at least 51%). In the example of Figure 3, the entire active area of ​​sensor 301 is shown in shaded lines. Therefore, sensor 301 can also be omitted to save production costs.

[0069] Figure 4 shows a schematic diagram for illustrating another possible embodiment, in which similar or essentially functionally identical components are indicated by reference numerals with the addition of "100" compared to Figure 3.

[0070] According to Figure 4, (larger than Figures 2 and 3) the image field 430 (with an exemplary size of 242 mm × 106 mm) is filled with 16 sensors 401, 402, ..., while still ensuring that each scan line or each imaging area of ​​the mask still scans through at least three sensors during the scanning operation (corresponding to a minimum line fill rate of 51%). In this case, the advantage of the relatively large image field 430 is that, depending on the light source used, light from the light source can be coupled into the illumination system of the apparatus used for mask inspection to achieve higher efficiency or throughput. In this case, the importance of region 450 corresponds to the importance of region 350 in Figure 3.

[0071] Figure 5 shows a schematic diagram illustrating another aspect of the invention, in which similar or essentially functionally identical components are indicated by reference numerals with the addition of "200" compared to Figure 3. In this case, the invention is based on the consideration that individual sensors in the sensor configuration according to the invention have so-called "dead zones," which are defective because they consistently produce zero or maximum sensor signals. Such sensors have not been fully sorted out; preferably, in the context of the invention, after determining the location of the relevant dead zones before final assembly, they are now appropriately "classified."

[0072] Specifically, in the exemplary embodiments shown, for example, dead boundaries can be avoided or not permitted on those scan lines covered only by the three sensors of sensor arrangement 500. In the exemplary embodiment of FIG. 5, this means that dead boundaries are not permitted in the three regions 540 with dashed boundaries. In contrast, dead boundaries can be omitted in the regions where four sensors are located, but two dead boundaries on the same scan line must be avoided. Regions outside the image field 530 may have any number of dead boundaries. The locations of the dead boundaries can then be stored in a database, for example, without considering the corresponding dead boundaries in subsequent image processing. Then, in the regions with dead boundaries, for example, only three sensors are available for image evaluation.

[0073] Figure 6 illustrates a schematic diagram of another embodiment, in which components similar to or substantially identical in function to those in Figure 5 are indicated by reference numerals with the addition of "100". According to Figure 6, a total of 36 sensors are filled in an image field of the same size as in Figure 5, with each scan line or each imaging mask region sweeping across at least eight of these sensors. In this case, the line fill rate for a scan line scanning eight sensors is 75%, and the line fill rate for a scan line scanning nine sensors is 85%.

[0074] Figure 7 illustrates another exemplary embodiment, in which similar or essentially functionally identical components are indicated by reference numerals with the addition of "100" compared to Figure 6. The embodiment in Figure 7 differs from the embodiment in Figure 6 in the specific dimensions of the image field 730 (with a value of 173 mm × 130 mm), making the aspect ratio closer to 1:1. Depending on the light source used, this has the advantage that light from the light source can be coupled into the illumination system of the apparatus for more efficient photomask inspection. It is intentionally accepted here that a portion of the image field 730 lies outside the diameter of the image field 780, which is capable of being imaged to the maximum extent by the imaging optics unit, and another portion of the image field 730 is shielded by the photoresist 720. In the exemplary embodiment shown, each scan line or each imaging photomask area scans at least 9 sensors and at most 11 sensors. For a scan line scanning 9 sensors, the line fill rate is 69%; for a scan line scanning 11 sensors, the line fill rate is 85%. Therefore, although the preferred square image field 730 achieves higher input coupling efficiency, the minimum line fill rate is slightly lower compared to the embodiment in Figure 6. In this case, the importance of region 750 corresponds to the importance of region 350 in Figure 3.

[0075] In an embodiment of the invention, specifically, vertical imaging of the photomask can be performed in the direction of photomask inspection. In this case, according to the schematic diagram in FIG8, the image field 830 extends symmetrically around the optical axis represented by "OA". In this case, the mask 820 is located at the center of the image field 830.

[0076] Figure 9 illustrates an exemplary geometric arrangement of sensors for a suitable sensor configuration, achieving the effect that each scan line or each imaging mask region sweeps across the individual active sensor regions of at least four sensors. For this purpose, the diameter of the photoresist must not exceed a predefined maximum value (38 mm in an example). The minimum line fill ratio achieved in the illustrated exemplary embodiment is 43% (for a scan line sweeping across the active sensor regions of four sensors). In this case, the importance of region 950 corresponds to the importance of region 350 in Figure 3.

[0077] Using the same sensor configuration, a slightly larger image field can also be captured, such as 168 mm × 178 mm (the imaging optics unit can produce an image field diameter of up to 245 mm), again achieving a minimum line fill rate of 43%.

[0078] Figure 10 illustrates another exemplary embodiment, which again features an image field symmetrically arranged about the optical axis and a photoresist located at the center of the image field, wherein each scan line of the photomask or each imaging region again scans across the respective active sensor regions of at least four sensors.

[0079] Figure 11 illustrates an exemplary embodiment with a smaller image field compared to Figure 10. In this case, a clever geometric configuration of the sensors achieves the effect that each imaging mask region or each scan line sweeps through the active regions of at least four sensors, resulting in a higher line fill rate of 53% compared to Figure 10.

[0080] Figure 12 shows a schematic diagram of a basic possible configuration of the projection lens 1220 used in the apparatus for photomask inspection of the present invention. As mentioned above, this projection lens produces an image field 1230 symmetrically arranged around the optical axis OA and is composed of four mirrors M1-M4 in the so-called Schwarzschild design. In this case, in Figure 12, the object field located in the object plane OP of the projection lens 1220 is indicated by "1260", and the image field located in the image plane IP of the projection lens 1220 is indicated by "1230".

[0081] According to the above embodiments, when the image field is symmetrically arranged about the optical axis OA and the photoresist is located at the center of the image field, the advantages of the present invention are particularly well achieved through the appropriate geometric configuration of the sensor combined with the scanning operation of the present invention, which can avoid or at least reduce the interference of the photoresist on the photomask inspection. However, the present invention is not limited to the application of having an image field symmetrically arranged about the optical axis and a photoresist located at the center of the image field, but can also be advantageously implemented when the image field is not symmetrically arranged around the optical axis.

[0082] In addition to the sensors in the sensor configurations present in the above embodiments, the device according to the invention may also include additional sensors that can be used to determine the relative motion of the sensor configuration and the (EUV) optical unit with respect to each other in at least two degrees of freedom. This is conceivable in the principle that it is advantageous for the sensor configuration or camera not to be directly coupled to the EUV optical unit, so that adverse effects from the camera or sensor configuration (e.g., heat or vibration caused by coolant flow) do not directly affect the highly sensitive optical unit. The relative motion of the sensor configuration with respect to the optical unit (e.g., a laser interferometer, capacitive sensor, or optical position sensor (PSD sensor, PSD = "position-sensitive element")) occurring in this state can then be measured in at least two degrees of freedom using a measurement system. The motion can then be calculated and taken into account during image processing and image evaluation, or it can be actively compensated for by a motion system, wherein this displacement system can displace the sensor configuration or the photomask platform carrying the photomask. Through corresponding additional displacement, the displacement of the sensor configuration (considering the imaging scale) can then ideally be accurately compensated.

[0083] Although the invention has been described based on specific embodiments, those skilled in the art will understand various variations and alternative embodiments, for example, through combinations and / or substitutions of features of the various embodiments. Therefore, it is self-evident to those skilled in the art that such variations and alternative embodiments are also included in the invention, and the scope of the invention is limited only to the meaning of the claims made below and their equivalents.

[0084] According to the decision J15 / 88 of the European Patent Office’s Board of Legal Appeals, the present invention further includes the forms defined in the following claims, which form part of this specification but are not within the scope of the claims.

[0085] 1. A method for inspecting a photomask, wherein the photomask is designed for reflective operation at an operating wavelength of less than 30 nm, and the photomask is used to be irradiated to expose a wafer in a lithography process of a projection exposure apparatus; wherein an object field located in an object plane and illuminated by an illumination system using EUV radiation having a wavelength of less than 30 nm is imaged onto an image field located in an image plane by a projection lens, wherein a sensor having a plurality of sensors is disposed in the image plane; wherein in a scanning operation the photomask is guided above the object field in the object plane; and wherein an image of the photomask is formed by combining a plurality of sensor images captured by each of the sensors in the scanning operation.

[0086] 2. The method as described in Clause 1, characterized in that a plurality of TDI sensors having a sensing region are used as a plurality of sensors configured as such sensors, wherein only a portion of each individual sensing region is implemented as an active sensing region having a plurality of active sensor pixels.

[0087] 3. The method as described in clause 1 or 2, characterized in that, during the scanning operation, multiple projections of multiple different regions of the photomask are scanned across the sensor configuration along multiple different scan lines.

[0088] 4. The method as described in Clause 3, characterized in that the line fill rate is not less than 25%, particularly not less than 35%, and even more particularly not less than 50% for any of the scan lines in each case, the line fill rate for any of the scan lines being defined as a ratio between the distance covered by a plurality of active sensor pixels in the scanning operation in each case and the length of the image field exposed along the scanning direction in the scanning operation.

[0089] 5. The method as described in Clause 3 or 4, characterized in that, in the scanning operation for each of the scan lines, the number of sensors scanned in each case is at least one, particularly at least two, and more particularly at least three.

[0090] 6. The method as described in any of the preceding clauses, characterized in that the sensors are configured to form a plurality of sensor series arranged adjacent to each other along the scanning direction and extend laterally relative to the scanning direction.

[0091] 7. The method as described in Clause 6, characterized in that, in the scanning operation for each of the scan lines, the number of sensor series not scanned in each case is at most two, and in particular at most one.

[0092] 8. The method as described in Clause 6 or 7, characterized in that the series of sensors adjacent to each other are offset relative to each other in a direction extending laterally relative to the scanning direction.

[0093] 9. The method as described in Clause 8, characterized in that the offset system is selected such that at least one sensor in a sensor series is partially overlapping two sensors in an adjacent sensor series.

[0094] 10. The method of any one of clauses 6 to 9, characterized in that a plurality of active sensor regions of the sensors are asymmetrically arranged on individual sensor regions, wherein the asymmetry of the different sensors is oriented differently.

[0095] 11. The method as described in any of the preceding clauses, characterized in that forming the sensor configuration by combining the sensors involves performing classification based on a prior determination of a plurality of defective regions of the individual sensors.

[0096] 12. The method as described in any of the preceding clauses, characterized in that the projection lens produces a region of masking that is masked during imaging.

[0097] 13. The method as described in Clause 12, wherein the shielding is at least partially located within the image field.

[0098] 14. The method as described in any of the preceding clauses, characterized in that the readout of data captured by each of the sensors during the scanning operation is synchronized with guiding the photomask over the object field.

[0099] 15. The method as described in any of the preceding clauses, characterized in that a calibration of the individual brightness of the sensor images captured by each of the sensors during the scanning operation is performed based on an intensity measurement performed using an intensity sensor.

[0100] 16. The method as described in any of the preceding clauses, characterized in that at least two sensors or sensor regions of the sensor configuration are read at a plurality of mutually different readout frequencies.

[0101] 17. The method as described in any of the preceding clauses, characterized in that the sensor images of the sensors are preprocessed before being combined with the sensor images.

[0102] 18. The method as described in any of the preceding clauses, characterized in that, for the purpose of reducing noise, the sensors are cooled, in particular, to a temperature below the average temperature of the projection lens.

[0103] 19. A sensor configuration comprising a plurality of sensors, wherein the sensors form a plurality of sensor series, the sensor series being arranged adjacent to each other along a predetermined direction and extending laterally relative to the predetermined direction, wherein a line fill rate is not less than 25% for any of the plurality of lines in each case, the line fill rate being defined for each line extending parallel to the predetermined direction over a predetermined image field as a ratio between the distance covered by the plurality of active sensor pixels in each case and the entire image field length along the predetermined direction, wherein the predetermined image field is at least partially covered by the sensor configuration.

[0104] 20. The sensor configuration as described in Clause 19, characterized in that the line fill rate is not less than 35% for any of the lines, and more particularly not less than 50%.

[0105] 21. The sensor configuration as described in Clause 19 or 20, characterized in that such sensors are designed for an operating wavelength of less than 30 nm.

[0106] 22. A sensor configuration comprising a plurality of sensors, wherein the sensors form a plurality of sensor families arranged adjacent to each other along a predetermined direction and extending laterally relative to the predetermined direction, wherein the sensors are designed for an operating wavelength of less than 30 nm.

[0107] 23. The sensor configuration as described in any one of clauses 19 to 22, characterized in that the sensor configuration is a plurality of TDI sensors.

[0108] 24. The sensor configuration as described in any one of clauses 19 to 23, characterized in that the adjacent sensor series are offset relative to each other in a direction extending laterally relative to the predetermined direction.

[0109] 25. The sensor configuration as described in Clause 24, characterized in that the offset system is selected such that at least one sensor in a sensor series partially overlaps with two sensors in an adjacent sensor series.

[0110] The sensor configuration as described in any one of Clauses 19 to 25 is characterized in that each of the sensors has a sensor region, wherein only a portion of the individual sensor region of each of the sensors is implemented as an active sensor region having a plurality of active sensor pixels.

[0111] 27. The sensor configuration as described in Clause 26, characterized in that the active sensor regions are asymmetrically arranged on individual sensor regions, wherein the asymmetry of the different sensors is oriented differently.

[0112] 28. The sensor configuration as described in any one of Clauses 19 to 27, characterized in that the sensors are combined by classifying the sensors based on a plurality of defective regions present on the individual sensors to form the sensor configuration.

[0113] 29. The sensor configuration as described in any one of clauses 19 to 28, characterized in that the sensor configuration includes a cooling device.

[0114] 30. The sensor configuration as described in any one of Clauses 19 to 29, characterized in that the sensor configuration is designed for use in the method according to any one of Clauses 1 to 18.

[0115] 31. A photomask inspection apparatus, wherein a photomask is designed for reflective operation at an operating wavelength less than 30 nm and for illumination in a lithography process in a projection exposure apparatus for exposing a wafer, wherein the apparatus includes an illumination system, a projection lens and a sensor configuration, wherein an object field located in an object plane and illuminated by EUV radiation having a wavelength less than 30 nm through the illumination system is imaged by the projection lens onto an image field located in an image plane, wherein the sensor configuration having a plurality of sensors is located in the image plane, wherein the sensor configuration is configured as described in any of clauses 19 to 30.

[0116] 32. The apparatus as described in Clause 31, wherein the apparatus is designed to perform the method as described in any one of Clauses 1 to 18.

[0117] 120: Concealment 130: Rectangular Image Field 180: Image Field 200: Sensor Configuration 201: Sensor 201a: Active sensor area 201b: Carrier 202: Sensor 202a: Active sensor area 202b: Carrier 220: Cover 230: Image Field 235a: Scan line 235b: Scan line 235c: Scan line 280: Image Field 300: Sensor Configuration 301: Sensor 302: Sensor 320: Cover 330: Image Field 350: Shaded area 380: Image Field 400: Sensor Configuration 401: Sensor 402: Sensor 420: Cover 430: Image Field 450: Area 500: Sensor Configuration 520: Cover 530: Image Field 540: Area 600: Sensor Configuration 620: Cover 630: Image Field 700: Sensor Configuration 720: Optical Resist 730: Image Field 750: Area 780: Image Field 820: Cover 830: Image Field 900: Sensor Configuration 920: Cover 930: Image Field 950: Area 1000: Sensor Configuration 1020: Shading 1030: Image Field 1100: Sensor Configuration 1120: Cover 1130: Image Field 1220: Projection Lens 1230: Image Field 1260: Object Field 1300: Device 1310: Lighting System 1320: Projection Lens 1330: Photomask 1340: Sensor Configuration IP: Image Plane M1: Reflector M2: Reflector M3: Reflector M4: Reflector OA: Optical Axis OP: Object plane

Claims

1. A method for inspecting a photomask, wherein the photomask is designed for reflective operation at an operating wavelength of less than 30 nm, and the photomask is used to be irradiated to expose a wafer in a lithography process of a projection exposure apparatus; wherein an object field (1260) located in an object plane (OP) and illuminated by an illumination system (1310) using EUV radiation having a wavelength of less than 30 nm is imaged by a projection lens (1220, 1320) onto an image field (230, 330, 430, 530, 630, 730, 930, 1030, 1130) located in an image plane (IP), wherein a sensor configuration (200, 300, 400, 500, 600, 700, 900, 1000, 1100, 1340) having a plurality of sensors (201, 202, ..., 301, 302, ...) is located in the image plane (IP); In one scanning operation, the photomask (1330) is guided above the object field (1260) in the object plane (OP); and an image of the photomask (1330) is formed by combining multiple sensor images captured by each of the sensors (201, 202, ..., 301, 302, ...) in the scanning operation.

2. The method of claim 1, wherein multiple sensors (201, 202, ...) with a plurality of TDI sensors having a sensor region are used as the sensor configuration (200, 300, 400, 500, 600, 700, 900, 1000, 1100), wherein only a portion of each individual sensor region is implemented as an active sensor region (201a, 202a, ...) having a plurality of active sensor pixels.

3. The method as described in claim 1 or 2, wherein during the scanning operation, multiple projections of multiple different regions of the photomask (1330) are scanned along multiple different scan lines across the sensor configuration (200, 300, 400, 500, 600, 700, 900, 1000, 1100).

4. The method of claim 3, wherein the line fill rate is not less than 25% for any of the scan lines in each case, and the line fill rate for any of the scan lines is defined as a ratio between the distance covered by a plurality of active sensor pixels in the scan operation in each case and the image field length exposed along the scan direction in the scan operation.

5. The method as described in claim 3, wherein in a scanning operation for each of the scan lines, the number of sensors scanned in each case is at least one.

6. The method as claimed in claim 1 or 2, wherein the sensors (201, 202, ...) in the sensor configuration (200, 300, 400, 500, 600, 700, 900, 1000, 1100) form a plurality of sensor series arranged adjacent to each other along the scanning direction and extend laterally relative to the scanning direction.

7. The method as described in claim 6, wherein in the scanning operation for each of the scan lines, the number of sensor series not scanned in each case is at most two.

8. The method of claim 6, wherein the series of sensors that are adjacent to each other are offset relative to each other in a direction extending laterally relative to the scanning direction.

9. The method of claim 8, wherein the offset system is selected such that at least one sensor in a sensor series partially overlaps with two sensors in an adjacent sensor series.

10. The method of claim 6, wherein a plurality of active sensor regions (201a, 202a, ...) of the sensors are asymmetrically arranged on individual sensor regions, wherein the asymmetry of the different sensors is oriented differently.

11. The method as claimed in claim 1 or 2, wherein forming the sensor configuration by combining the sensors involves performing classification based on a prior determination of multiple defect regions of the individual sensors.

12. The method as claimed in claim 1 or 2, wherein a region-shaped shield (120, 220, 320, 420, 520, 620, 720, 920, 1020, 1120) is generated in the projection lens, which is shielded during imaging.

13. The method as described in claim 12, wherein the shield (120, 220, 320, 420, 520, 620, 720, 920, 1020, 1120) is at least partially located within the image field (730, 930, 1030, 1130).

14. The method as claimed in claim 1 or 2, wherein the readout of data captured by each of the sensors (201, 202, ...) during the scanning operation is synchronized with guiding the photomask (1330) over the object field (1260).

15. The method of claim 1 or 2, wherein a calibration of the individual brightness of the sensor images captured by each of the sensors (201, 202, ...) in the scanning operation is performed based on an intensity measurement performed using an intensity sensor.

16. The method as claimed in claim 1 or 2, wherein at least two sensors or sensor regions of the sensor configuration (200, 300, 400, 500, 600, 700, 900, 1000, 1100, 1340) are read at multiple readout frequencies that are different from each other.

17. The method of claim 1 or 2, wherein the sensor images are preprocessed before being combined with the sensor images.

18. The method as claimed in claim 1 or 2, wherein the sensors are cooled, in particular, to a temperature below the average temperature of the projection lens, for the purpose of reducing noise.

19. A sensor configuration (200, 300, 400, 500, 600, 700, 900, 1000, 1100, 1340) comprising a plurality of sensors (201, 202, ..., 301, 302, ...), wherein the sensors form a plurality of sensor series arranged adjacent to each other along a predetermined direction and extending laterally relative to the predetermined direction, wherein a line fill rate is not less than 25% for any of the plurality of lines in each case, the line fill rate being defined for each line extending parallel to the predetermined direction above a predetermined image field as a ratio between the distance covered by the plurality of active sensor pixels in each case and the entire image field length along the predetermined direction, wherein the predetermined image field is at least partially covered by the sensor configuration.

20. The sensor configuration as described in claim 19, wherein the line fill rate is not less than 35% for any of the lines.

21. The sensor configuration as described in claim 19 or 20, wherein the sensors are designed for an operating wavelength of less than 30 nm.

22. A sensor configuration (200, 300, 400, 500, 600, 700, 900, 1000, 1100, 1340) comprising a plurality of sensors (201, 202, ..., 301, 302, ...), wherein the sensors form a plurality of sensor series, the sensor series being arranged adjacent to each other along a predetermined direction and extending laterally relative to the predetermined direction, wherein the sensors are designed for an operating wavelength less than 30 nm.

23. The sensor configuration as described in any one of claims 19 to 20 and 22, wherein the sensors are configured as a plurality of TDI sensors.

24. The sensor configuration as claimed in any one of claims 19 to 20 and 22, wherein the series of sensors adjacent to each other are offset relative to each other in a direction extending laterally relative to the predetermined direction.

25. The sensor configuration as described in claim 24, wherein the offset system is selected such that at least one sensor in a sensor series partially overlaps with two sensors in an adjacent sensor series.

26. The sensor configuration as claimed in any one of claims 19 to 20 and 22, wherein each of the sensors has a sensor region, wherein in the case of each of the sensors, only a portion of the individual sensor region is implemented as an active sensor region (201a, 202a, ...) having a plurality of active sensor pixels.

27. The sensor configuration as described in claim 26, wherein the active sensor regions (201a, 202a, ...) are asymmetrically arranged on individual sensor regions, wherein the asymmetry of the different sensors is oriented differently.

28. The sensor configuration as claimed in any one of claims 19 to 20 and 22, wherein the sensors are combined by classifying the sensors based on a plurality of defect regions present on the individual sensors to form the sensor configuration.

29. The sensor configuration as claimed in any one of claims 19 to 20 and 22, wherein the sensor configuration includes a cooling device.

30. The sensor configuration as described in any one of claims 19 to 20 and 22, wherein the sensor configuration is designed for use in the method according to any one of claims 1 to 18.

31. A photomask inspection apparatus, wherein a photomask is designed for reflective operation at an operating wavelength less than 30 nm and is used for illumination in a lithography process in a projection exposure apparatus for exposing a wafer, wherein the apparatus includes an illumination system (1310), a projection lens (1320), and a sensor configuration (200, 300, 400, 500, 600, 700, 900, 1000, 1100, 1340), wherein the photomask is located in an object plane (OP) and illuminated by the illumination system (1310) with a wavelength less than 30 nm. An object field (1260) of one nm EUV radiation illumination is imaged by the projection lens (1220, 1320) onto an image field (230, 330, 430, 530, 630, 730, 930, 1030, 1130) located in an image plane (IP), wherein the sensor configuration (200, 300, 400, 500, 600, 700, 900, 1000, 1100, 1340) having a plurality of sensors (201, 202, ..., 301, 302, ...) is located in the image plane, wherein the sensor configuration is configured as requested in any of items 19 to 30.

32. The apparatus of claim 31, wherein the apparatus is designed to perform the method of any one of claims 1 to 18.

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