Mask inspection method and apparatus

Abstract

The present invention relates to a method and an apparatus for inspecting a mask, the mask being designed for reflective operation at an operating wavelength of less than 30 nm and intended to be illuminated in a lithography process of a projection exposure apparatus for exposing a wafer. In the method according to the present invention, an object field (1260) located in an object plane (OP) and illuminated with EUV radiation having a wavelength of less than 30 nm via an illumination system (1310) is imaged by a projection lens (1220, 1320) onto an image field (230, ..., 1130) located in an image plane (IP), a sensor arrangement (200, ..., 1340) having a plurality of sensors (201, 202, ..., 301, 302, ...) is located in the image plane, a mask (1330) is guided over the object field in the object plane in a scanning motion, and an image of the mask is formed by combining sensor images captured by each of the individual sensors in the scanning motion.

Description

[Technical Field]

[0001] This application claims the priority of German patent application no. 102022133829.0, filed on December 19, 2022, the contents of which are incorporated by reference into the body of the present application.

[0002] The present invention relates to a method and apparatus for mask inspection. [Background technology]

[0003] Microlithography is used to manufacture finely structured components, e.g., in integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus, which comprises an illumination system and a projection lens. In this case, the image of a mask (= reticle) illuminated by the illumination system is projected by the projection lens onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist) and placed in the image plane of the projection lens, transferring the mask structure into the photosensitive coating of the substrate.

[0004] In lithography processes, unwanted defects on the mask have a particularly detrimental effect because they can reappear with each exposure step, potentially rendering the entire production of semiconductor components unusable. Therefore, it is very important to test masks for sufficient imaging capabilities before using them in mass production.

[0005] Therefore, it is necessary to test masks quickly and easily, in particular under conditions as similar as possible to those actually present in a projection exposure apparatus. For this purpose, it is known to use a mask inspection apparatus with an illumination system and a projection lens, by which an illuminated area of ​​the mask is imaged onto a sensor arrangement, such as a CCD camera. In practice, this can lead to problems, particularly in that the imaging result finally obtained as a result of the lithography process on the wafer or on its photosensitive layer (photoresist) in the projection exposure apparatus still differs from the result predicted on the basis of intensity measurements performed with the sensor arrangement in the mask inspection apparatus.

[0006] In particular, avoiding or mitigating this problem is a difficult task when inspecting masks designed for operation at EUV (i.e., wavelengths below 30 nm, especially below 15 nm). In this regard, characterization of relevant (EUV) masks at higher wavelengths in the DUV range (e.g., about 248 nm or about 193 nm), while possible in principle, deviates significantly from the actual operating wavelength of the projection exposure apparatus, which leads to losses in terms of the reliability of the mask inspection, for example, that certain particles or defects on the mask are not recognized at all, that defects are imaged in the projection exposure apparatus at the deviated wavelength due to their optical effects and are therefore represented differently, or that defects are incorrectly recognized where no defects are present at all, and that the achieved resolution also decreases as a result of the transition to higher wavelengths in mask inspection.

[0007] On the other hand, the shift to lower wavelengths (especially EUV) in mask inspection, while desirable in principle for the above reasons, fundamentally brings about the further problem that the EUV radiation typically generated via plasma light sources in mask inspection apparatuses cannot be reduced to a sufficiently small image field or to an image field that corresponds to the typical dimensions of available sensors. This problem arises from the fact that the plasma light sources required for the generation of EUV radiation, in contrast to excimer lasers used in the DUV range, are initially emitted in all spatial directions, but in order to maintain the etendue, it is not easily possible to concentrate the generated EUV radiation onto a sufficiently small image field without simultaneously accepting light losses. Summary of the Invention

[0008] In light of the above background, it is an object of the present invention to provide a method and apparatus for mask inspection that is able to predict as accurately as possible the imaging results that will result from a lithography process on a wafer, while at least partially avoiding the problems mentioned above.

[0009] This object is achieved by a method according to the features of independent claim 1 and by an apparatus according to alternative independent claim 31.

[0010] According to one aspect, the invention relates to a method for inspecting a mask, the mask being designed for reflective operation at an operating wavelength of less than 30 nm and intended to be illuminated in a lithography process in a projection exposure apparatus for exposing a wafer, an object field located in an object plane and illuminated via an illumination system with EUV radiation having a wavelength of less than 30 nm is imaged by a projection lens onto an image field located in an image plane, the image plane being occupied by a sensor arrangement having a plurality of sensors; the mask is guided in a scanning motion over the object field in the object plane, An image of the mask is formed by combining the sensor images captured by each of the individual sensors in a scanning motion.

[0011] The present invention is based in particular on the idea of ​​carrying out inspection of masks designed for operation with EUV or intended for use in EUV projection exposure apparatus likewise using EUV radiation (i.e. in particular "actinically"), while at the same time overcoming the fundamental problem explained in the introduction of the relatively large image fields that have to be processed in mask inspection due to the fact that in the method and apparatus for mask inspection according to the invention, each image of the mask is formed by combining multiple sensor images captured by each of the multiple sensors of the sensor arrangement according to the invention in a scanning operation.

[0012] In this case, the present invention further comprises the principle, realized below based on various embodiments, that the highest possible light-emitting efficiency is achieved using a skillful geometrical arrangement of the individual sensors in the sensor arrangement, as long as the largest possible proportion of the image field is effectively filled with active sensor areas or active sensor pixels. Here, the present invention preferably further comprises the concept of maximizing the "line fill factor," defined below, in the sense that the active sensor areas constitute the largest possible percentage of the exposed image field length in a scanning operation. Furthermore, advantageous geometrical arrangements of the individual sensors in embodiments of the present invention include, in particular, relative arrangements of the individual sensors, for example, such that a certain minimum number of sensors swept in a scanning operation is ensured in each case and / or a predetermined number of sensor rows not swept in a scanning operation is not exceeded in each case.

[0013] As a result, according to the present invention, firstly, the fundamental advantages of actinic mask inspection (i.e., mask inspection carried out using an "inspection wavelength" that corresponds to the actual operating wavelength of the mask in a projection exposure apparatus) are achieved, and secondly, particularly accurate and reliable mask inspection is thus realized due to the fact that the problems inherently associated with the transition to the EUV wavelength range in mask inspection, due to the relatively large image fields that must be managed, and in this case overcome, are overcome.

[0014] According to one embodiment, TDI sensors (TDI = "Time Delay and Integration") with sensor areas are used as sensors of the sensor arrangement, only a part of each sensor area being embodied as an active sensor area with active sensor pixels. For the construction of TDI sensors known per se, see DE 197 14 221 A1, purely by way of example.

[0015] According to one embodiment, in a scanning operation, the projections of different areas of the mask sweep across the sensor arrangement along different scan lines.

[0016] According to one embodiment, the line filling factor, defined for each of said scan lines as the ratio between the distance in each case covered by active sensor pixels in a scan operation and the image field length exposed in the scan direction in a scan operation, is in each case 25% or more for each of the scan lines, in particular 35% or more for each of the scan lines and more particularly 50% or more for each of the scan lines. Here and below, "scan direction" is understood to mean the direction in which the projection of the mask moves in the image plane during a scan operation.

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

[0018] According to one embodiment, the sensors of the sensor arrangement are arranged adjacent to each other in the scanning direction and form a number of rows of sensors extending transversely to the scanning direction.

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

[0020] According to one embodiment, adjacent rows of sensors are offset relative to one another in a direction extending transversely to the scanning direction, whereby the offset can in particular be selected such that at least one sensor of a row of sensors partially overlaps two sensors of adjacent rows of sensors.

[0021] According to one embodiment, the active sensor areas of the sensors are asymmetrically arranged on the respective sensor areas, with the asymmetry of different sensors pointing in different directions.

[0022] According to one embodiment, combining sensors to form a sensor arrangement includes performing classification based on a prior determination of defect areas of each sensor. This takes into account the situation where, due to manufacturing guidelines, individual sensors typically have so-called "deadlines," and these deadlines are always defective because they produce a zero or maximum sensor signal. Instead of completely classifying such sensors, they can be appropriately "classified," as will be explained in more detail below, in which case, for example, deadlines are avoided or not allowed in places where only a relatively small number of sensors are located in the sensor arrangement.

[0023] According to one embodiment, the image field of the projection lens has obscurations in the form of areas that are shadowed during imaging.

[0024] According to one embodiment, the obscuration is at least partially located within the image field. In this case, parts of the image field can be located in particular on both sides of the optical axis. Furthermore, parts of the image field can be located on both sides of the obscuration. More particularly, the obscuration can be arranged symmetrically around the axis of rotation of the projection lens.

[0025] According to one embodiment, the readout of the data captured by each individual sensor during the scanning operation is synchronized with the guidance of the mask over the object field. In this case, "synchronized" is understood to mean that the mask movement speed (in millimeters / second) multiplied by the imaging magnification of the projection lens corresponds to the sensor readout frequency (in kHz) multiplied by the size of the sensor pixel measured in the scanning direction. In other words, the sensor readout speed must be faster than the mask movement by a factor, this factor being the imaging magnification of the projection lens.

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

[0027] According to one embodiment, at least two sensors or sensor regions of the sensor arrangement are read out at different readout frequencies. This configuration can take into account the situation where the effective imaging magnification of each sensor may be slightly different if not all sensors are mounted exactly in one plane, or if some sensors are not mounted exactly parallel to this plane, or if the optical unit used is distorted. If the readout of the data captured by each individual sensor in a scanning operation is synchronized with the guidance of the mask over the object field, the associated effect of the imaging magnification will result in an undesirable blurring of the image and therefore a reduction in contrast as a result of the TDI process, even when all sensors are read out at the same readout frequency. This effect can be avoided or at least reduced by reading out each sensor at its optimal readout frequency and / or by reading out different sensor regions of one and the same sensor at different readout frequencies.

[0028] According to one embodiment, the sensor images of the sensors are pre-processed before being combined. This pre-processing can include, for example, low-pass filtering. Furthermore, before the sensor images are added together, the sensor images of the individual sensors can be displaced and / or enlarged, or reduced and / or distorted with sub-pixel accuracy. As a result, the above-mentioned differences in magnification can be at least partially compensated for.

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

[0030] According to one embodiment, the sensor is cooled for noise reduction purposes. In this case, the sensor can be cooled, in particular to a temperature lower than the average temperature of the projection lens (for example, a temperature of 10°C, 0°C, or -20°C). Cooling can be performed, for example, by means of a cooling fluid and / or a Peltier element. This can take into account situations in which so-called dark current noise can increase over the service life of the sensor arrangement, and such cooling or adaptation of the operating temperature can reduce the dark current noise (for example, to a value initially given for the "new" sensor arrangement).

[0031] The invention further relates to a sensor arrangement comprising a plurality of sensors arranged adjacent to one another in a predetermined direction and forming a plurality of sensor rows extending transversely to the predetermined direction, wherein for each line extending parallel to the predetermined direction across a predetermined image field that is at least partially covered by the sensor arrangement, a line filling factor, defined as the ratio between the distance covered in each case by active sensor pixels and the length of the entire image field in the predetermined direction, is in each case 25% or more for each of the lines.

[0032] According to one embodiment, the line fill factor is 35% or greater for any of said lines, and more particularly 50% or greater for any of said lines.

[0033] According to one embodiment, the sensor is designed for an operating wavelength of less than 30 nm.

[0034] The present invention further relates to a sensor arrangement comprising a plurality of sensors arranged adjacent to one another in a predetermined direction to form a plurality of sensor rows extending transversely to the predetermined direction, the sensors being designed for an operating wavelength of less than 30 nm.

[0035] In one embodiment, the sensor is configured as a TDI sensor.

[0036] According to one embodiment, adjacent rows of sensors are offset relative to each other in a direction extending transverse to the predetermined direction.

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

[0038] According to one embodiment, each sensor has a sensor area, and for each sensor, only a portion of the respective sensor area is embodied as an active sensor area having active sensor pixels.

[0039] According to one embodiment, the active sensor areas are asymmetrically arranged on each sensor area, with the asymmetry of different sensors pointing in different directions.

[0040] According to one embodiment, the sensor arrangement is formed by combining sensors such that the sensors are classified based on the defect areas present on each sensor.

[0041] According to one embodiment, the sensor arrangement comprises a cooling device.

[0042] The sensor arrangement may be specifically designed for use in a method having the features described above.

[0043] The present invention further relates to an apparatus for inspecting masks, the masks being designed for reflective operation at an operating wavelength of less than 30 nm and intended to be illuminated in a lithography process in a projection exposure apparatus for exposing a wafer, the apparatus comprising an illumination system, a projection lens and a sensor arrangement, the apparatus being located in an object plane, an object field illuminated via the illumination system with EUV radiation having a wavelength of less than 30 nm is imaged by the projection lens onto an image field located in an image plane, a sensor arrangement having a plurality of sensors being located in the image plane, the apparatus comprising a sensor arrangement having the above-mentioned characteristics.

[0044] The apparatus may be particularly designed to carry out a method having the above-mentioned features.

[0045] For advantages and advantageous configurations of the sensor arrangement and device, please refer to the above embodiments relating to the method according to the invention.

[0046] Further configurations of the invention can be seen from the present description and the dependent claims.

[0047] The invention is explained in more detail below on the basis of preferred exemplary embodiments with reference to the accompanying drawings. [Brief explanation of the drawings]

[0048] [Figure 1] 1 is a schematic diagram illustrating the problem addressed by the present invention; [Figure 2] 1 is a schematic diagram illustrating one possible embodiment of the method according to the invention and the sensor arrangement used in the method; [Figure 3] 5A-5C are schematic diagrams for explaining further possible embodiments of the method according to the invention and of the sensor arrangement used in the method; [Figure 4] 5A-5C are schematic diagrams for explaining further possible embodiments of the method according to the invention and of the sensor arrangement used in the method; [Figure 5]5A-5C are schematic diagrams for explaining further possible embodiments of the method according to the invention and of the sensor arrangement used in the method; [Figure 6] 5A-5C are schematic diagrams for explaining further possible embodiments of the method according to the invention and of the sensor arrangement used in the method; [Figure 7] 5A-5C are schematic diagrams for explaining further possible embodiments of the method according to the invention and of the sensor arrangement used in the method; [Figure 8] 5A-5C are schematic diagrams for explaining further possible embodiments of the method according to the invention and of the sensor arrangement used in the method; [Figure 9] 5A-5C are schematic diagrams for explaining further possible embodiments of the method according to the invention and of the sensor arrangement used in the method; [Figure 10] 5A-5C are schematic diagrams for explaining further possible embodiments of the method according to the invention and of the sensor arrangement used in the method; [Figure 11] 5A-5C are schematic diagrams for explaining further possible embodiments of the method according to the invention and of the sensor arrangement used in the method; [Figure 12] 2 is a schematic diagram of a basic possible setup of a projection lens used in an apparatus according to the invention for mask inspection; [Figure 13] 1 is a schematic diagram of a basic possible setup of an apparatus for mask inspection used in the method according to the invention; DETAILED DESCRIPTION OF THE INVENTION

[0049] As shown merely schematically in Figure 13, an apparatus 1300 for mask inspection that can be used in a method according to the present invention includes an illumination system 1310 and a projection lens 1320, where light from a light source (not shown in Figure 13) enters the illumination system 1310 and is incident on a mask 1330 positioned in the object plane of the projection lens 1320, and the illuminated area of ​​the mask 1330 is imaged onto a sensor arrangement 1340 via the projection lens.

[0050] Here and hereinafter, the photomask may have an aspect ratio, particularly, in the range of 1:1 to 1:3, preferably in the range of 1:1 to 1:2, and more preferably 1:1 or 1:2. The mask may have a substantially rectangular shape. The mask may preferably have a length and width in the range of 5 inches to 7 inches, more preferably 6 inches. Alternatively, the mask may have a length in the range of 5 inches to 7 inches and a width in the range of 10 inches to 14 inches, preferably 6 inches long and 12 inches wide.

[0051] In order to make a prediction of the imaging result that will be achieved on the mask when performing a lithographic process in a projection exposure apparatus, the intensity distribution that is obtained on the mask is first measured in a mask inspection apparatus or by a sensor arrangement according to Fig. 13. In this case, it is preferable to use the same wavelength in the mask inspection apparatus as is also used in the lithographic process in the projection exposure apparatus.

[0052] In the schematic diagram of Figure 1, "180" denotes an image field that can be imaged in its entirety via an imaging optical unit or projection lens of an apparatus for mask inspection, "120" denotes an obscuration (in the form of an area that is shadowed during imaging) that will be described in further detail, and "130" denotes a rectangular image field onto which an object field located in the object plane of the projection lens is imaged. A sensor arrangement (described below) comprising a plurality of sensors is located in the image plane of the projection lens.

[0053] In the following, various embodiments of the method according to the invention and the sensor arrangements used therein for mask inspection will be described with reference to the schematic diagrams of FIGS.

[0054] Common to the embodiments described below is that in the apparatus according to the invention for mask inspection, EUV radiation is likewise used to inspect masks designed for operation in the EUV wavelength range, and in order to manage the relatively large image fields that are essentially involved during the imaging of a mask in a projection exposure apparatus, a scanning process is realized in which the image of the mask is formed by combining sensor images that are respectively captured by individual sensors of the sensor arrangement in a scanning operation.Here, the embodiments described below with reference to Figure 2 onwards correspond in each case to various particularly "clever" or advantageous sensor geometries, with the aim of achieving the highest possible luminous efficiency.

[0055] Here, the actual scanning movement can be performed such that the image field of the mask inspection apparatus is guided over the sensor arrangement at a constant speed, for example of the order of a few millimeters per second.

[0056] As sensors of the sensor arrangement, TDI sensors are used here, whose images are added together with a corresponding time offset. As is known per se, each of these TDI sensors on a carrier (typically made from a ceramic material) has active sensor pixels only on a part of the total sensor surface area (forming the "active sensor area"). By way of example only (and without the invention being limited in this respect), the dimensions of the carrier for an individual sensor in the exemplary embodiment shown in FIG. 2 and described below can be 68 mm x 30 mm, the sensor surface area can be 64 mm x 26 mm, and the active sensor area can be 60 mm x 18 mm.

[0057] In purely schematic terms, Figure 2 shows a sensor arrangement 200 comprising a total of 12 sensors 201, 202, ..., whose active sensor areas are designated "201a", "202a", .... The carriers of sensors 201, 202, ... are typically made from a ceramic material and are designated "201b", "202b", ... respectively.

[0058] The image field guided over the sensor arrangement 200 in the above-described scanning operation is designated "230." For comparison, "280" denotes the image field (with an exemplary diameter of 308 mm) that can be imaged in its entirety via the imaging optical unit or projection lens of the mask inspection apparatus, and "220" denotes any obscuration (in the form of a shadowed area during imaging) that is provided in the illustrated exemplary embodiment (but again, without limiting the invention in this respect) and will be described in more detail below.

[0059] As shown schematically by the double-headed arrows in Fig. 2, during the scanning operation during imaging of the respective illuminated areas of the mask onto the image field, the projection of different areas of the mask sweeps over the sensor arrangement along different scan lines (three of which are depicted in Fig. 2 merely by way of example and designated "235a", "235b", and "235c"). In the exemplary embodiment according to Fig. 2, the geometric arrangement of the sensors 201, 202, ... is selected such that for each of said scan lines, the active sensor areas 201a, 202a, ... of at least three sensors 201, 202, ... are swept. In this case, scanning can be performed both from left to right and from right to left.

[0060] A particularly suitable criterion for maximally efficient utilization of the active sensor area in the scanning process according to the present invention is the ratio of the distance covered in each case by the active sensor pixels in the scanning operation to the image field length exposed in the scanning operation, said ratio being referred to here and below as the "line filling factor." Preferably, the geometric arrangement of the individual sensors relative to one another in the sensor arrangement according to the present invention is selected here so that the line filling factor as defined above is 25% or more for any scan line, in particular 35% or more for any scan line, and more particularly 50% or more for any scan line. In the particular exemplary embodiment of Figure 2, this line filling factor is 51% for a scan line sweeping over the active sensor areas of three sensors and 68% for a scan line or trajectory sweeping over four sensors.

[0061] According to FIG. 2, the sensors 201, 202, ... of the sensor arrangement 200 are arranged adjacent to one another in the scanning direction and form a plurality of sensor rows extending transversely to the scanning direction. In this case, adjacent sensor rows are offset from one another in the direction extending transversely to the scanning direction. Furthermore, the active sensor areas 201a, 201b, ... of the sensors 201, 202, ... according to FIG. 2 are arranged asymmetrically on the respective sensor areas, and the asymmetry of the different sensors is partially oriented in different directions. In particular, in the specific exemplary embodiment of FIG. 2, the two sensor rows shown on the left are arranged rotated 180° with respect to their respective active sensor areas relative to the two sensor rows shown on the right. This allows the achieved line-filling factor to be achieved with a relatively small size of the image field 230 (which in this example has dimensions of 178 mm × 106 mm). A further advantageous effect of this geometric arrangement of sensors 201, 202 with respect to their respective active sensor areas 201 a, 202 a, ... is that the thermal load acting during operation is more evenly distributed over the entire area of ​​the sensor arrangement, which is advantageous with regard to the design of a corresponding cooling device. Furthermore, one advantage of this asymmetric arrangement is that it compensates for or eliminates possible differences in the forward and reverse modes of the sensors during scanning operation (e.g., due to two sensors "operating" in the forward direction and two sensors "operating" in the reverse direction along the scan line).

[0062] Preferably, as can also be seen in the exemplary embodiment of Fig. 2, the offset of adjacent sensor rows in a direction extending transverse to the scanning direction is selected so that at least one sensor of a sensor row partially overlaps two sensors of adjacent sensor rows. This overlap must in each case be greater than twice the position tolerance of the individual sensors (plus, if appropriate, any tolerances required by the image processing algorithm). As a result, even if the position tolerance of the individual sensors is large, sufficient overlap is ensured to avoid situations in which the scan lines of the relevant area do not sweep the active sensor area.

[0063] FIG. 3 shows a schematic diagram of a sensor arrangement according to the present invention for illustrating a further embodiment. In this case, similar or substantially functionally identical components are designated by reference numerals with an additional "100" compared to FIG. 2 . In addition to the image field 380 (with an exemplary diameter of 308 mm) already shown in FIG. 2 , which can be maximally imaged by the imaging or projection optical unit, FIG. 3 shows an additional image field 380a with a relatively small diameter (279 mm in this example). Here, it is assumed, by way of example, that the imaging optical unit of the mask inspection apparatus provides sufficiently good image quality only for the relatively small image field 380a. This situation can be taken into account according to FIG. 3 by not reading the hatched area 350 of the active sensor area. In this case, it is further ensured that each scan line in the scanning operation still sweeps at least three sensors (thus, the line fill factor defined above is still at least 51%). In the example of FIG. 3 , the entire active area of ​​sensor 301 is hatched. Therefore, sensor 301 can also be omitted to save manufacturing costs.

[0064] FIG. 4 shows a schematic diagram for illustrating a further possible embodiment, where, compared to FIG. 3, similar or substantially functionally identical components are designated by reference numerals increased by "100."

[0065] According to FIG. 4, the image field 430 (larger compared to FIGS. 2-3 ) (having exemplary dimensions of 242 mm × 106 mm) is filled with 16 sensors 401, 402, ..., still ensuring that in a scanning operation each scan line or each imaging area of ​​the mask still sweeps over at least three sensors (corresponding to a minimum line fill factor of 51%). In this case, the relatively large image field 430 has the advantage that, depending on the light source used, light from the light source can be coupled with a higher efficiency or yield into the illumination system of the mask inspection apparatus. In this case, the meaning of area 450 corresponds to the meaning of area 350 in FIG. 3.

[0066] 5 shows a schematic diagram for illustrating a further embodiment of the present invention, in which, compared to FIG. 3, similar or substantially functionally identical components are designated by reference numerals increased by "200." In this case, the invention is based on the consideration that individual sensors of a sensor arrangement according to the present invention have so-called "deadlines" that are defective insofar as they always produce a zero signal or a maximum sensor signal. Instead of completely classifying such sensors, in the context of the present invention, these sensors are preferably "classified" appropriately after determining the position of the relevant deadline before final assembly.

[0067] Specifically, in the illustrated exemplary embodiment, deadlines can be avoided or not allowed on scan lines covered by only three sensors of the sensor arrangement 500. In the exemplary embodiment of FIG. 5, this means that deadlines are not allowed in the three regions 540 with dashed borders. In contrast, deadlines can be omitted in regions where four sensors are located, but in that case, two deadlines on the same scan line must be avoided. Any number of deadlines can be located in regions located outside the image field 530. The locations of the deadlines can then be stored in a database, e.g., the corresponding deadlines are not considered for subsequent image processing. In the region where the deadlines are located, only three sensors are available for image evaluation, e.g., three.

[0068] Figure 6 shows a schematic diagram of a further embodiment, in which similar or substantially functionally identical components compared to Figure 5 are designated by reference numerals increased by "100." According to Figure 6, an image field having the same dimensions compared to Figure 5 is filled with a total of 36 sensors, and each scan line or imaging mask area sweeps at least 8 of these sensors. In this case, a line fill factor of 75% is achieved for scan lines that each sweep 8 sensors, and a line fill factor of 85% is achieved for scan lines that each sweep 9 sensors.

[0069] FIG. 7 illustrates a further exemplary embodiment, in which, compared to FIG. 6, similar or substantially functionally identical components are designated by reference numerals with an additional "100." The embodiment of FIG. 7 differs from the embodiment of FIG. 6 in the specific dimensions of the image field 730 (having values ​​of 173 mm × 130 mm), resulting in an aspect ratio close to 1:1. Depending on the light source used, this has the advantage of allowing light from the light source to be more efficiently coupled into the illumination system of the mask inspection apparatus. Here, it is intentionally allowed that a portion of the image field 730 is located outside the diameter of the maximum image field 780 that can be imaged by the imaging optical unit, while another portion of the image field 730 is shaded by the shading portion 720. In the illustrated exemplary embodiment, each scan line or each imaged mask area sweeps over at least nine sensors and up to eleven sensors. The line fill factor is 69% for a scan line sweeping over nine sensors and 85% for a scan line sweeping over eleven sensors. As a result, although a high in-coupling efficiency is possible due to the preferably square image field 730, the minimum line fill factor is somewhat lower compared to the embodiment of Figure 6. In this case, the meaning of region 750 corresponds to that of region 350 in Figure 3.

[0070] In particular, embodiments of the present invention allow for vertical imaging of the mask in the direction of mask inspection, where the image field 830 extends symmetrically around the optical axis designated "OA" according to the schematic diagram of Figure 8. In this case, the obscuration 820 is located in the center of the image field 830.

[0071] 9 shows an exemplary geometric arrangement of sensors suitable for this purpose, which achieves the effect that each scan line or each imaging mask area sweeps over the active sensor areas of at least four sensors. To this end, the diameter of the shielding portion must not extend beyond a predetermined maximum value (38 mm in this example). The minimum line fill factor achieved in the illustrated exemplary embodiment is 43% (for a scan line sweeping over the active sensor areas of four sensors). In this case, the meaning of area 950 corresponds to the meaning of area 350 in FIG. 3.

[0072] Using the same sensor arrangement, it is also possible to capture a slightly larger image field, for example having dimensions of 168 mm x 178 mm (if the maximum image field that can be produced by the imaging optical unit is 245 mm in diameter), again achieving a minimum line fill factor of 43%.

[0073] Figure 10 shows a further exemplary embodiment, again with an image field arranged symmetrically with respect to the optical axis and a shielding portion located in the center of the image field, and again each scan line or each imaging area of ​​the mask sweeps over the active sensor area of ​​each of at least four sensors.

[0074] FIG. 11 shows an exemplary embodiment with a smaller image field compared to FIG. 10, where nevertheless, due to clever geometric arrangement of the sensors, the effect is achieved that each imaging mask area or each scan line sweeps over the active area of ​​at least four sensors, achieving a higher line fill factor of 53% here compared to FIG. 10.

[0075] 12 shows a schematic diagram of a basic possible setup of a projection lens 1220 used in an apparatus according to the invention for mask inspection, which, as mentioned above, generates an image field 1230 arranged symmetrically about an optical axis OA and consists of four mirrors M1-M4 in the so-called Schwarzschild design. In this case, in FIG. 12, the object field located in the object plane OP of the projection lens 1220 is designated by "1260" and the image field located in the image plane IP of the projection lens 1220 is designated by "1230".

[0076] In the case of an image field arranged symmetrically about the optical axis OA and a shielding part located in the center of this image field, according to the embodiment described above, the advantages according to the invention are particularly pronounced insofar as a suitable geometric arrangement of the sensor in combination with the scanning operation according to the invention makes it possible to avoid or at least reduce the disruptive influence of the shielding part on mask inspection. However, the invention is not limited to applications with an image field arranged symmetrically with respect to the optical axis and a shielding part located in the center of this image field, but can also be advantageously implemented in situations with image fields that are not arranged symmetrically with respect to the optical axis.

[0077] In addition to the sensors of the sensor arrangement present in the above-described embodiments, the device according to the present invention can also include additional sensors capable of determining the relative movement of the sensor arrangement and the (EUV) optical unit with respect to each other in at least two degrees of freedom. This can take into account situations where it is advantageous in principle not to directly couple the sensor arrangement or the camera to the EUV optical unit, so that undesired effects from the camera or sensor arrangement (e.g., heat or vibrations caused by the flow of a cooling fluid) do not directly affect the sensitive optical unit. The relative movement of the sensor arrangement with respect to the optical unit that occurs in this regard can be measured in at least two degrees of freedom by a measurement system (e.g., a laser interferometer, a capacitive sensor, or an optical position sensor (PSD sensor, PSD = "position sensing device")). This movement can be taken into account computationally during image processing and image evaluation, or it can be actively compensated for by a movement system that can move the sensor arrangement or a mask stage carrying the mask. The corresponding additional movement can ideally accurately compensate for the movement of the sensor arrangement (taking into account the imaging magnification).

[0078] While the present invention has been described with reference to particular embodiments, numerous variations and alternative embodiments will be apparent to those skilled in the art, e.g., by combining and / or substituting features of the particular embodiments. Accordingly, it will be apparent to those skilled in the art that all such variations and alternative embodiments are concomitantly encompassed by the present invention, the scope of which is limited only within the meaning of the appended claims and equivalents thereof.

[0079] The present invention further includes embodiments defined in the following clauses, which form part of this specification but are not claimed, in accordance with European Patent Office Litigation Review Board decision J15 / 88.

[0080] Clause 1 A method for mask inspection, the method comprising: a mask designed for reflective operation at an operating wavelength shorter than 30 nm and intended to be illuminated in a lithography process of a projection exposure apparatus for exposing a wafer; an object field located in an object plane and illuminated via an illumination system with EUV radiation having a wavelength shorter than 30 nm is imaged by a projection lens onto an image field located in an image plane; a sensor arrangement having a plurality of sensors is located within the image; the mask is guided over the object field in the object plane in a scanning motion; and an image of the mask is formed by combining sensor images captured by each of the individual sensors in the scanning motion.

[0081] Clause 2 2. The method according to clause 1, characterized in that as sensors of the sensor arrangement, TDI sensors are used, in which only a part of each sensor area is embodied as an active sensor area with active sensor pixels.

[0082] Clause 3 3. The method according to clause 1 or 2, characterized in that in a scanning operation the projection of different areas of the mask sweeps over the sensor arrangement along different scan lines.

[0083] Clause 4 The method described in clause 3, characterized in that for each of the scanning lines, the line filling factor, defined as the ratio of the distance covered in each case by active sensor pixels in the scanning operation to the image field length exposed in the scanning direction in the scanning operation, is in each case 25% or more for each of the scanning lines, in particular 35% or more for each of the scanning lines, and more particularly 50% or more for each of the scanning lines.

[0084] Clause 5 5. The method according to clause 3 or 4, characterized in that in the scanning operation for each of the scanning lines the number of sensors swept in each case is at least one, in particular at least two, more particularly at least three.

[0085] Clause 6 6. The method according to any one of clauses 1 to 5, characterized in that the sensors of the sensor arrangement are arranged adjacent to each other in the scanning direction and form a plurality of sensor rows extending transversely to the scanning direction.

[0086] Clause 7 7. The method according to clause 6, characterized in that in the scanning operation for each of the scan lines the number of sensor rows not swept in each case is at most two, in particular at most one.

[0087] Article 8 8. The method according to clause 6 or 7, characterized in that adjacent rows of sensors are offset relative to each other in a direction extending transversely to the scanning direction.

[0088] Article 9 9. The method according to clause 8, characterized in that the offset is selected such that at least one sensor of a sensor row partially overlaps two sensors of an adjacent sensor row.

[0089] Article 10 10. The method according to any one of clauses 6 to 9, characterized in that the active sensor areas of the sensors are asymmetrically arranged on the respective sensor areas, the asymmetries of the different sensors pointing in different directions.

[0090] Article 11 11. The method according to any one of clauses 1 to 10, characterized in that forming a sensor arrangement by combining sensors includes performing classification based on a prior determination of defect areas of each sensor.

[0091] Article 12 12. The method according to any one of clauses 1 to 11, characterized in that the projection lens generates an obscuration in the form of a shadowed area during imaging.

[0092] Article 13 13. The method according to clause 12, characterized in that the occlusion is located at least partially in the image field.

[0093] Article 14 14. The method according to any one of clauses 1 to 13, characterized in that the readout of the data captured by each of the individual sensors during the scanning operation is synchronized with the guidance of the mask over the object field.

[0094] Article 15 15. The method according to any one of clauses 1 to 14, characterized in that the calibration of the brightness of each of the sensor images captured by each individual sensor in the scanning operation is performed based on intensity measurements performed using an intensity sensor.

[0095] Article 16 16. The method according to any one of clauses 1 to 15, characterized in that at least two sensors or sensor areas of the sensor arrangement are read out with mutually different readout frequencies.

[0096] Article 17 17. The method according to any one of clauses 1 to 16, characterized in that before synthesizing the sensor images of the sensors, the sensor images are pre-processed.

[0097] Article 18 18. The method according to any one of clauses 1 to 17, characterized in that the sensor is cooled for the purpose of noise reduction, in particular to a temperature lower than the average temperature of the projection lens.

[0098] Article 19 A sensor arrangement comprising a plurality of sensors, the sensors being arranged adjacent to one another in a predetermined direction and forming a plurality of sensor rows extending transversely to the predetermined direction, wherein for each line extending parallel to the predetermined direction across a predetermined image field that is at least partially covered by the sensor arrangement, a line filling factor, defined as the ratio of the distance covered in each case by active sensor pixels to the length of the entire image field in the predetermined direction, is in each case 25% or more for any of the lines.

[0099] Article 20 20. The sensor arrangement of clause 19, characterized in that the line filling rate is greater than or equal to 35% for any of said lines, more particularly greater than or equal to 50% for any of said lines.

[0100] Article 21 21. The sensor arrangement according to clause 19 or 20, characterized in that the sensor is designed for an operating wavelength of less than 30 nm.

[0101] Article 22 1. A sensor arrangement comprising a plurality of sensors, the sensors being arranged adjacent to one another in a predetermined direction to form a plurality of sensor rows extending transversely to the predetermined direction, the sensors being designed for an operating wavelength of less than 30 nm.

[0102] Article 23 23. The sensor arrangement according to any one of clauses 19 to 22, characterized in that the sensor is configured as a TDI sensor.

[0103] Article 24 24. The sensor arrangement according to any one of clauses 19 to 23, characterized in that adjacent rows of sensors are offset relative to each other in a direction extending transversely to the predetermined direction.

[0104] Article 25 25. A sensor arrangement according to clause 24, characterized in that the offset is selected such that at least one sensor of a sensor row partially overlaps two sensors of an adjacent sensor row.

[0105] Article 26 A sensor arrangement according to any one of clauses 19 to 25, characterized in that in each case of the sensors only a part of the respective sensor area is embodied as an active sensor area with active sensor pixels.

[0106] Article 27 27. A sensor arrangement according to clause 26, characterized in that the active sensor areas are arranged asymmetrically on the respective sensor area, the asymmetries of the different sensors pointing in different directions.

[0107] Article 28 28. A sensor arrangement according to any one of clauses 19 to 27, characterized in that the sensor arrangement is formed by combining sensors such that the sensors are classified based on the defect areas present on each sensor.

[0108] Article 29 29. The sensor arrangement according to any one of clauses 19 to 28, characterized in that the sensor arrangement comprises a cooling device.

[0109] Article 30 A sensor arrangement according to any one of clauses 19 to 29, characterized in that it is designed for use in a method according to any one of clauses 1 to 18.

[0110] Article 31 An apparatus for inspecting masks, the mask being designed for reflective operation at an operating wavelength of less than 30 nm and intended to be illuminated in a lithography process in a projection exposure apparatus for exposing a wafer, the apparatus comprising an illumination system, a projection lens and a sensor arrangement, located in an object plane, an object field illuminated via the illumination system with EUV radiation having a wavelength of less than 30 nm is imaged by the projection lens onto an image field located in an image plane, a sensor arrangement having a plurality of sensors located in the image plane, the sensor arrangement being configured in accordance with any one of clauses 19 to 30.

[0111] Article 32 32. An apparatus according to clause 31, designed to carry out the method according to any one of clauses 1 to 18.

Claims

1. 1. A method for mask inspection, wherein the mask is designed for reflective operation at an operating wavelength of less than 30 nm and is intended to be illuminated in a lithography process of a projection exposure apparatus for exposing a wafer, an object field (1260) located in an object plane (OP) and illuminated via an illumination system (1310) with 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), a sensor arrangement (200, 300, 400, 500, 600, 700, 900, 1000, 1100, 1340) having a plurality of sensors (201, 202, . . . , 301, 302, . . . ) located in said image plane (IP), said mask (1330) is guided over said object field (1260) in said object plane (OP) in a scanning motion, the image of said mask (1330) is formed by combining the sensor images captured by each of said individual sensors (201, 202,..., 301, 302,...) in said scanning operation; method.

2. 2. The method according to claim 1, wherein the sensors (201, 202, ...) of the sensor arrangement (200, 300, 400, 500, 600, 700, 900, 1000, 1100) are TDI sensors having sensor areas, only a portion of each sensor area being embodied as an active sensor area (201a, 202a, ...) having active sensor pixels.

3. 3. The method of claim 1, wherein in the scanning operation, the projections of different areas of the mask (1330) are swept over the sensor arrangement (200, 300, 400, 500, 600, 700, 900, 1000, 1100) along different scan lines.

4. 4. A method according to claim 3, characterized in that for each of the scan lines, the line filling factor, defined as the ratio of the distance covered in each case by active sensor pixels in the scan operation to the image field length exposed in the scan direction in the scan operation, is in each case 25% or more for each of the scan lines, in particular 35% or more for each of the scan lines, and more particularly 50% or more for each of the scan lines.

5. 5. A method according to claim 3 or 4, characterized in that in the scanning operation for each of the scan lines the number of sensors swept in each case is at least one, in particular at least two, more particularly at least three.

6. 6. The method according to claim 1, wherein the sensors (201, 202, ...) of the sensor arrangement (200, 300, 400, 500, 600, 700, 900, 1000, 1100) are arranged adjacent to one another in the scanning direction and form a plurality of rows of sensors extending transversely to the scanning direction.

7. 7. A method according to claim 6, characterized in that in the scanning operation for each of the scan lines the number of sensor rows not swept in each case is at most two, in particular at most one.

8. 8. A method according to claim 6 or 7, characterized in that adjacent rows of sensors are offset relative to each other in a direction extending transversely to the scanning direction.

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

10. 10. The method according to any one of claims 6 to 9, characterized in that the active sensor areas (201a, 202a, ...) of the sensors are asymmetrically arranged on the respective sensor area, the asymmetry of different sensors pointing in different directions.

11. 11. The method according to claim 1, wherein combining the sensors to form the sensor arrangement comprises performing a classification based on a prior determination of defect areas of the respective sensors.

12. Method according to any one of the preceding claims, characterized in that the projection lens generates an obscuration (120, 220, 320, 420, 520, 620, 720, 920, 1020, 1120) in the form of a shadowed area during the imaging.

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

14. 14. The method according to claim 1, wherein the readout of data captured by each of the individual sensors (201, 202, ...) during the scanning operation is synchronized with the guidance of the mask (1330) over the object field (1260).

15. 15. The method according to claim 1, wherein calibration of the brightness of each of the sensor images captured by each of the individual sensors (201, 202, ...) in the scanning operation is performed on the basis of intensity measurements performed using an intensity sensor.

16. 16. The method according to any one of the preceding claims, characterized in that at least two sensors or sensor areas of the sensor arrangement (200, 300, 400, 500, 600, 700, 900, 1000, 1100, 1340) are read out at mutually different readout frequencies.

17. The method according to any one of claims 1 to 16, characterized in that before combining the sensor images of the sensors, the sensor images are pre-processed.

18. Method according to any one of the preceding claims, characterized in that the sensor is cooled for noise reduction, in particular to a temperature lower than the average temperature of the projection lens.

19. 1. A sensor arrangement (200, 300, 400, 500, 600, 700, 900, 1000, 1100, 1340) comprising a plurality of sensors (201, 202, ..., 301, 302, ...), the sensors being arranged adjacent to one another in a predetermined direction and forming a plurality of sensor rows extending transversely to the predetermined direction, wherein for each line extending parallel to the predetermined direction across a predetermined image field that is at least partially covered by the sensor arrangement, a line filling factor, defined as the ratio in each case of the distance covered by active sensor pixels to the length of the entire image field in the predetermined direction, is in each case 25% or more for each of the lines.

20. 20. The sensor arrangement of claim 19, wherein the line fill factor is greater than or equal to 35% for any of the lines, and more specifically greater than or equal to 50% for any of the lines.

21. 21. The sensor arrangement according to claim 19 or 20, characterized in that the sensor is designed for an operating wavelength of less than 30 nm.

22. 1. A sensor arrangement (200, 300, 400, 500, 600, 700, 900, 1000, 1100, 1340) comprising a plurality of sensors (201, 202, ..., 301, 302, ...), the sensors being arranged adjacent to one another in a predetermined direction to form a plurality of sensor rows extending transversely to the predetermined direction, the sensors being designed for an operating wavelength of less than 30 nm.

23. The sensor arrangement according to any one of claims 19 to 22, characterized in that the sensor is configured as a TDI sensor.

24. A sensor arrangement according to any one of claims 19 to 23, characterized in that adjacent rows of sensors are offset relative to each other in a direction extending transversely to the predetermined direction.

25. 25. The sensor arrangement of claim 24, wherein the offset is selected so that at least one sensor in a row of sensors partially overlaps two sensors in an adjacent row of sensors.

26. 26. The sensor arrangement according to any one of claims 19 to 25, characterized in that each of the sensors has a sensor area, and in the case of each of the sensors only a part of the respective sensor area is embodied as an active sensor area (201a, 202a, ...) with active sensor pixels.

27. 27. The sensor arrangement according to claim 26, characterized in that the active sensor areas (201a, 202a, ...) are asymmetrically arranged on the respective sensor area, the asymmetry of different sensors pointing in different directions.

28. 28. The sensor arrangement of any one of claims 19 to 27, characterized in that the sensor arrangement is formed by combining the sensors such that the sensors are classified based on the defect areas present on the respective sensor.

29. The sensor arrangement according to any one of claims 19 to 28, characterized in that the sensor arrangement comprises a cooling device.

30. A sensor arrangement according to any one of claims 19 to 29, characterized in that it is designed for use in a method according to any one of claims 1 to 18.

31. An apparatus for inspecting a mask, the mask being designed for reflective operation at an operating wavelength of less than 30 nm and intended to be illuminated in a lithography process in a projection exposure apparatus for exposing a wafer, the apparatus comprising an illumination system (1310), a projection lens (1320) and a sensor arrangement (200, 300, 400, 500, 600, 700, 900, 1000, 1100, 1340), the apparatus comprising an object field (OP) located in an object plane (OP) and illuminated with EUV radiation having a wavelength of less than 30 nm via the illumination system (1310).

31. An apparatus, comprising: an imaging lens (1220, 1320) for imaging a lens (1260) onto an image field (230, 330, 430, 530, 630, 730, 930, 1030, 1130) located at an image plane (IP); and a sensor arrangement (200, 300, 400, 500, 600, 700, 900, 1000, 1100, 1340) having a plurality of sensors (201, 202, ..., 301, 302, ...) located at said image plane, said sensor arrangement being configured according to any one of claims 19 to 30.

32. Apparatus according to claim 31, designed to carry out the method according to any one of claims 1 to 18.

Images