Procedure for qualifying a mask of a lithography system

Abstract

Method (100) for qualifying a mask (13) for a lithography system (1), wherein the mask (13) has a plurality of predetermined measurement points (22) for capturing one or more critical dimensions (20) of the mask (13) at each measurement point (22), comprising: - Initial acquisition (101) of at least one critical dimension (20) of the mask (13) at the measurement points (22), wherein the initial acquisition (101) is sequential and the duration of the initial acquisition (101) defines a measurement period; - Determining (102) a plurality of reference measurement points (24) from the plurality of measurement points (22), wherein the number of reference measurement points (24) is less than the number of measurement points (22); - Second acquisition (103) of at least one critical dimension (20) of the mask (13) at the reference measurement points (24) at a time interval from the first acquisition (101) at the measurement points (22) determined as reference measurement points (24); - Determining (104) a deviation (21) between the first and the second recorded critical dimension (20) at each of the reference measurement points (24); - Determine (105), depending on the determined deviation (21), a time course of a correction factor over the measurement period; and - Applying (106) the determined time course of the correction factor to at least one critical dimension (20) to obtain a corrected critical dimension of the mask (13).

Description

[0001] The present invention relates to a method for qualifying a mask of a lithography system.

[0002] Microlithographic lithography systems are used in the production of microstructured components, such as integrated circuits, with particularly small structures. In a lithography system, the image of a mask (also called a "reticule") illuminated by a light source with very short-wavelength, deep ultraviolet or extreme ultraviolet radiation (DUV or EUV radiation) is projected onto a lithography object using a projection device, in order to transfer the mask structure onto the lithography object.

[0003] To achieve high image quality, the mask structure itself must exhibit high accuracy. To ensure that a mask meets these accuracy requirements and that a lithographic object produced with it possesses the desired properties and functionality, the mask is checked for deviations from the intended structure using qualification procedures before being used in a lithography system. For the highest possible precision in the qualification process, the mask structure must be tested at as many measurement points as possible, for example, using interferometric methods.Furthermore, the mask structure is tested multiple times in succession to determine deviations between the results obtained in the measurement runs and thus to account for any influence of the device used on the test, for example, due to drift occurring during the test (such as temperature drift or damper drift), by means of a suitable correction. Therefore, such a qualification procedure is associated with a considerable time expenditure. Document US 2005 / 0190381 A1 describes a method for qualifying a mask for a lithography system in which a predetermined mask thickness is regularly determined during the measurement of the mask, and if a deviation of the parameter from an initial value is detected, the entire metrology device is recalibrated.Document US 2013 / 0044205 A1 concerns a mask defect inspection by comparing a pattern generated by illuminating the mask with a reference pattern.

[0004] One object of the present invention is therefore to provide, in light of the aforementioned problems, an improved method with which a mask of a lithography system can be qualified as precisely as possible with less time expenditure.

[0005] The inventive solution lies in the features of independent claim 1. Advantageous further developments are the subject of dependent claims.

[0006] According to the invention, a method for qualifying a mask for a lithography system is disclosed, wherein the mask has a plurality of predetermined measuring points for capturing one or more critical dimensions of the mask at each measuring point, comprising: - Initial acquisition of at least one critical dimension of the mask at the measurement points, wherein the initial acquisition is sequential and the duration of the initial acquisition defines a measurement period; - Determining a plurality of reference measurement points from the plurality of measurement points, where the number of reference measurement points is less than the number of measurement points; - Second measurement of at least one critical dimension of the mask at the reference measurement points, each time spaced apart from the first measurement at the measurement points designated as reference measurement points; - Determining a deviation between the first and second recorded critical dimension at each of the reference measurement points; - Determine, depending on the specified deviation, a correction factor over the measurement period; and - Applying the specific time course of the correction factor to at least one critical dimension to obtain a corrected critical dimension of the mask.

[0007] The following is an explanation of some terms used in connection with the invention:

[0008] Critical dimensions are defined as the dimensions of critical features of the mask's structure, such as the minimum line width of individual lines, the minimum line width of closely spaced lines, and the minimum size of circular areas. The uniformity of these critical dimensions significantly impacts the image quality of the lithographic process and, consequently, the quality of the structures transferred to the lithographed object. Critical dimensions can be determined, for example, using interferometric methods. A critical dimension can be represented, for instance, as a value with the unit [nm].

[0009] A "measuring point" is defined as an area of ​​the mask where at least one critical dimension to be measured can be determined. For example, if the minimum line width of a single line is to be measured, the measuring point can have an extent that covers the entire line width. Similarly, to measure the minimum line width of closely spaced lines, the measuring point can also cover the area between two lines.

[0010] In particular, "capturing at least one critical dimension" can also include capturing multiple critical dimensions. The specific critical dimensions to be captured may depend, for example, on the nature and suitability of the specified measurement points. For instance, the measurement points may be defined in such a way that all critical dimensions of the mask required for its qualification can be captured.

[0011] The method according to the invention is, for example, part of a registration method for a mask for an EUV lithography system.

[0012] The invention relates to an advantageous method for qualifying a mask of a lithography system, in which the temporal evolution of a correction factor for at least one critical dimension of the mask is taken into account to obtain a corrected critical dimension. To determine the correction factor, the critical dimension under consideration is sequentially measured first once at all measurement points of the mask and then a second time only at selected reference measurement points, which represent a subset of the measurement points. Based on the critical dimensions thus acquired, a deviation between the first and second acquired critical dimension can be determined for the reference measurement points, and from this, the temporal evolution of the correction factor over the measurement period of the first acquisition can be determined, which can then be applied to the critical dimension under consideration.By requiring only a subset of the measurement points to be considered in the subsequent acquisition process necessary for determining the deviation, the time required for the second acquisition can be significantly reduced in the inventive method. This, in turn, can considerably reduce the overall time required for the qualification procedure.

[0013] In one embodiment, determining the plurality of reference measurement points depends on the temporal distribution of measurement times of the measurement points within the measurement period. Preferably, the measurement times of the measurement points to be determined as reference measurement points are equally spaced from one another, so that they are uniformly distributed within the measurement period. This enables a uniform determination of the deviation between the first and the second measured critical dimension across the majority of measurement points, and thus a particularly precise determination of the temporal profile of the correction factor.

[0014] Preferably, determining the time course of the correction factor includes determining a reference curve using the deviations determined for each of the reference measurement points. In this way, a deviation of the critical dimension under consideration can also be estimated with sufficient precision for measurement points not designated as reference points, thus enabling the determination of the time course of the correction factor with greater accuracy.

[0015] For example, the procedure further includes a third acquisition of at least one critical dimension of the mask at at least some of the reference measurement points, spaced out from the second acquisition, whereby the determination of the deviation is additionally dependent on the additionally acquired critical dimensions. For example, an average is calculated from the values ​​of the respective critical dimension determined during the second and third acquisitions, and the deviation is determined between the first acquired critical dimension and the average of the second and third acquired critical dimensions. The third acquisition and the consideration of the third acquired critical dimension when determining the deviation allow for a more precise determination of the correction factor over time.

[0016] Advantageously, the mask has at least 500 measurement points, preferably at least 700. Alternatively or additionally, the majority of reference measurement points comprise a maximum of 10%, preferably a maximum of 5%, of the measurement points. With these numbers of measurement points and reference measurement points, the method represents a particularly efficient way to qualify a mask for a lithography system with sufficient precision.

[0017] In particular, the first, second, and / or third data collection can each involve multiple collections of at least one critical dimension. The critical dimension to be collected in each instance can be determined by averaging the multiple collections of critical dimensions.

[0018] According to one embodiment, the method further includes outputting a signal if the deviation determined at a reference measurement point exceeds a predetermined threshold. For example, a threshold for permissible deviation is defined for a critical dimension, preferably for each of the detectable critical dimensions. If determining the deviation of the respective critical dimension under consideration for a reference measurement point, or for a minimum number of reference measurement points, yields a value above the predetermined threshold, a signal can be output via a suitable user interface. The signal can be an acoustic signal, such as a warning tone, or a visual signal, such as a message on a display.In this way, a possible malfunction of a device used to perform the procedure for qualifying a mask of a lithography system, such as an incorrect setting of the environmental conditions during the execution of the procedure or an incorrect setting of an image acquisition unit of the device, can be signaled.

[0019] Furthermore, a device for qualifying a mask for a lithography system is disclosed, wherein the mask has a plurality of predetermined measuring points for capturing one or more critical dimensions of the mask at each measuring point, comprising a mask holder, an image acquisition unit, a processing unit and an evaluation unit, wherein the evaluation unit is configured to carry out the method according to the invention by means of the mask holder, the image acquisition unit and the processing unit.

[0020] The device is, for example, a suitably designed mask metrology device for registering a mask for an EUV lithography system.

[0021] For a more detailed explanation of further advantageous embodiments of the device, reference is made to the embodiments of the method described above. Likewise, the method can be further developed with additional features that are described in connection with the device.

[0022] The embodiments and configurations described above are to be understood as merely exemplary and are not intended to limit the present invention in any way.

[0023] The invention is explained in more detail below with reference to the accompanying drawings and by way of example of advantageous embodiments. The drawings show: Fig. 1 a schematic representation of an exemplary embodiment of a lithography system; Fig. 2 a schematic flowchart of a method according to the invention in an exemplary embodiment; Fig. 3 an illustration of critical dimensions recorded at a plurality of measuring points and reference measuring points according to an exemplary embodiment; Fig. 4 an illustration of deviations of a critical dimension determined according to an exemplary embodiment for a plurality of reference measurement points; Fig. 5 an illustration of a local distribution of deviations of a critical dimension determined according to an exemplary embodiment; and Fig. 6 A schematic representation of a device according to an exemplary embodiment.

[0024] In Fig. Figure 1 schematically illustrates an EUV lithography system as an embodiment of a lithography system. The EUV lithography system comprises a lighting device 10 and a projection device 11. The lighting device 10 illuminates a mask 13 arranged in the object field in an object plane 12.

[0025] The illumination device 10 comprises an illumination radiation source 14 that emits electromagnetic radiation in the EUV range, specifically with a wavelength between 5 nm and 100 nm. The illumination radiation emitted by the illumination radiation source 14 is first focused by a collector 15 into an intermediate focal plane 16.

[0026] The illumination device 10 comprises a deflecting mirror 17, which deflects the illumination radiation emitted by the illumination radiation source 14 onto a first faceted mirror 18. A second faceted mirror 19 is arranged downstream of the first faceted mirror 18. The first faceted mirror 18 and the second faceted mirror 19 each comprise a plurality of micromirrors that can be individually pivoted about two axes perpendicular to each other. The individual facets of the first faceted mirror 18 are imaged onto the mask 13 by means of the second faceted mirror 19.

[0027] Using the projection device 11, the mask 13 is imaged onto a photosensitive layer of a wafer arranged in an image plane 9 via a plurality of mirrors 8. The various mirrors of the EUV lithography system 1, which reflect the illumination radiation, are designed as EUV mirrors. The EUV mirrors are provided with highly reflective coatings, for example in the form of multilayer coatings, in particular with alternating layers of molybdenum and silicon. The method according to the invention relates to the qualification of such a mask 13 in order to determine whether the quality of the mask 13 is sufficient for use in the lithography system 1.

[0028] Fig. Figure 2 shows a schematic flowchart of an embodiment of a method 100 according to the invention for qualifying a mask 13 for a lithography system 1. The mask 13 has a plurality of predetermined measuring points 22 for detecting one or more critical dimensions 20 at each measuring point 22 of the mask 13.

[0029] In step 101, an initial acquisition of at least one critical dimension 20 of the mask 13 is carried out at the measuring points 22. This is done sequentially using interferometric methods. The duration of the initial acquisition 101 defines a measurement period.

[0030] In Fig. Figure 3 shows the critical dimensions 20 recorded for each of the measurement points 22 in step 101, with values ​​between 47.6 nm and 48.4 nm. The number of measurement points 22 is 736. The duration of the first measurement step 101 is 7 hours.

[0031] In step 102, a plurality of reference measurement points 24 are determined from the plurality of measurement points 22. The number of reference measurement points 24 is less than the number of measurement points 22 and, in this example, amounts to 30, which is less than 5% of the number of measurement points 22. The reference measurement points 24 determined from the plurality of measurement points 22 are also in Fig. 3 indexed.

[0032] In the present example, the determination of 102 depends on a temporal distribution of the measurement times of the measurement points 22 within the measurement period. In this way, reference measurement points 24 can be determined from the measurement points 22, taking into account their respective measurement times within the duration of the first acquisition 101. In the example described, the reference measurement points 24 have been determined such that the measurement times of the measurement points 22 to be determined as reference measurement points 24 have an equal temporal interval between them and are thus evenly distributed within the measurement period.

[0033] At the reference measurement points 24, a second measurement 103 of at least one critical dimension 20 of the mask 13 is carried out, each time at a time interval from the first measurement 101 at the measurement points 22 designated as reference measurement points 24. The values ​​of these second recorded critical dimensions 20 are also in Fig. 3 for the individual reference measurement points 24. The duration of the second acquisition 103 is 20 min and is therefore considerably shorter than that of the first acquisition 101.

[0034] In step 104, a deviation 21 between the first and second recorded critical dimension 20 is determined at each of the reference measurement points 24. For this purpose, for each measurement point 22 designated as a reference measurement point 24, a deviation 21 of the value of the second recorded critical dimension 20 is determined from the value of the critical dimension 20 previously recorded there in step 101. In the example shown, the deviation 21 is determined by subtracting the value of the first critical dimension 20 recorded at the measurement point 22 designated as a reference measurement point 24 from the value of the second critical dimension 20 recorded at this reference measurement point 24.

[0035] The deviations 21 thus determined between the first and the second recorded critical dimension 20 are in Fig. Figure 4 shows the value of the deviation 21 determined in step 104 between the first and second recorded critical dimension 20 for each of the reference measurement points 24 (from the majority of measurement points 22). In this example, the deviations 21 shown take on values ​​between -0.03 nm and 0.15 nm.

[0036] In step 105, depending on the deviation 21 determined in step 104 between the first and the second recorded critical dimension 20, a temporal course of a correction factor is determined over the measurement period.

[0037] In the presented example, determining the time course of the correction factor (105) involves determining a reference curve using the deviations (21) of the respective critical dimension (20) determined for the reference measurement points (24). For determining the reference curve, a curve fitting using a polynomial of degree n can be provided for the deviation (21) determined for the reference measurement points (24) in step (104).

[0038] Since the initial measurement 101 at measuring points 22 was carried out sequentially within a measurement period defined by the duration of the initial measurement 101, in this example 7 hours, a temporal profile of the deviations 21 determined at the reference measuring points 24 can be determined. Based on the change in the determined deviations 21 over this temporal period, the temporal profile of the correction factor can then be determined.

[0039] In the example described, the application of the specific temporal profile of the correction factor to at least one critical dimension 20 in step 106 is carried out such that the correction factor for the considered critical dimension 20 of the mask 13 and its temporal profile are taken into account in a registration procedure of the mask 13 for use in the lithography system 1. Registration of the mask 13 can then be performed depending on a critical dimension of the mask 13 corrected in this way.

[0040] Furthermore, the procedure 100 can also provide for a third acquisition of at least one critical dimension 20 of the mask 13 at the reference measurement points 24 or at at least a part of the reference measurement points 24 as a further step 303, which is temporally offset from the second acquisition 103. The determination 104 of the deviation 21 of the at least one critical dimension 20 can then additionally depend on the at least one critical dimension 20 additionally acquired in step 303.

[0041] Analogous to the critical dimensions recorded in step 103, a deviation 21 can also be determined for the critical dimensions additionally recorded at each measurement point 22 designated as a reference measurement point 24 in step 303. This deviation is calculated between the first critical dimension 20 recorded at this measurement point 22 (step 101) and the third critical dimension 20 recorded at this measurement point 22 (step 303). These additionally determined deviations 21 are also shown as a trend 304 across the reference measurement points 24 in Fig. 4 shown.

[0042] Taking into account the deviations 21 of at least one critical dimension 20 determined at each of the reference measurement points 24, an average can be calculated and an average deviation 21 of the respective critical dimension 20 can be determined for each of the reference measurement points 24. A trend 306 of such an averaged deviation 21 across the reference measurement points 24 is also possible. Fig. Figure 4 is shown. Accordingly, the determination of the time course of the correction factor can be carried out depending on the average deviation 21 thus determined.

[0043] Procedure 100 also provides for the output of a signal 401 if the deviation 21 determined for a reference measurement point 24 exceeds a predefined threshold value. In this case, depending on an available user interface, a warning tone is emitted and / or a corresponding warning message is displayed.

[0044] Fig. Figure 5 shows an illustration of a deviation 21, determined according to an embodiment, between the first and the second measured critical dimension 20 above a predetermined threshold value, in a local distribution over the area of ​​a mask 13, which extends 140 mm in both the x-direction and the y-direction. The x-axis and the y-axis indicate the respective coordinates of a measuring point 22 and a reference measuring point 24, respectively, on the area of ​​the mask 13. In the areas shaded black, the respective determined deviation 21 lies above the threshold value.

[0045] The initial acquisition 101 of the critical dimensions 20 at the measuring points 22 was performed row by row, starting from the bottom left and moving upwards to the top right. In the area of ​​mask 13 in the bottom left, there are larger areas of reference measuring points 24 where the determined deviation 21 is above the threshold value than in the other areas of mask 13. It can be concluded that the deviation 21 is larger at the beginning of acquisition 101 than towards the end of acquisition 101, reflecting a reduction of the deviation 21 over time. Accordingly, the correction factor and its temporal profile are also determined in step 105 and applied to the critical dimension 20 under consideration in such a way that its influence decreases with increasing duration of the lithography process.

[0046] In Fig.Figure 6 shows a device 30 for qualifying a mask 13 for a lithography system 1, wherein the mask 13 has a plurality of predetermined measuring points 22 for capturing one or more critical dimensions 20 of the mask 13 at each measuring point 22. The device 30 comprises a mask holder 32, a processing unit 34, an image acquisition unit 36, and an evaluation unit 38, wherein the evaluation unit 38 is operatively connected to the mask holder 32, the processing unit 34, and the image acquisition unit 36. The evaluation unit 38 is configured to perform the method 100 described above using the mask holder 32, the image acquisition unit 36, and the processing unit 34.

[0047] The mask holder 32 is controlled such that the image acquisition unit 36 ​​can perform an initial acquisition 101 of at least one critical dimension 20 of a mask 13 held by the mask holder 32 at the measuring points 22. Subsequently, the image acquisition unit 36 ​​performs such an initial acquisition 101, whereby the initial acquisition 101 is sequential and the duration of the initial acquisition 101 defines a measurement period.

[0048] Furthermore, the processing unit 34 determines a plurality of reference measurement points 24 from the plurality of measurement points 22 (step 102), wherein the number of reference measurement points 24 is less than the number of measurement points 22.

[0049] Subsequently, the image acquisition unit 36 ​​performs a second acquisition 103 which can carry out at least one critical dimension 20 of the mask 13 at the reference measurement points 24, each with a time difference compared to the first acquisition 101 at the measurement points 22 designated as reference measurement points 24, for which purpose the mask 13 is positioned accordingly by the mask holder 32.

[0050] The processing unit 34 determines a deviation 21 between the first and second recorded critical dimension 20 at each of the reference measurement points 24 (step 104) and, depending on the determined deviation 21, a temporal profile of a correction factor over the measurement period (step 105). Subsequently, the evaluation unit 38 applies 106 the determined temporal profile of the correction factor to at least one critical dimension 20 to obtain a corrected critical dimension of the mask 13 by the processing unit 34.

Claims

[1] Method (100) for qualifying a mask (13) for a lithography system (1), wherein the mask (13) has a plurality of predetermined measurement points (22) for capturing one or more critical dimensions (20) of the mask (13) at each measurement point (22), comprising: - Initial acquisition (101) of at least one critical dimension (20) of the mask (13) at the measurement points (22), wherein the initial acquisition (101) is sequential and the duration of the initial acquisition (101) defines a measurement period; - Determining (102) a plurality of reference measurement points (24) from the plurality of measurement points (22), wherein the number of reference measurement points (24) is less than the number of measurement points (22); - Second acquisition (103) of at least one critical dimension (20) of the mask (13) at the reference measurement points (24) at a time interval from the first acquisition (101) at the measurement points (22) determined as reference measurement points (24); - Determining (104) a deviation (21) between the first and the second recorded critical dimension (20) at each of the reference measurement points (24); - Determine (105), depending on the determined deviation (21), a time course of a correction factor over the measurement period; and - Applying (106) the determined time course of the correction factor to at least one critical dimension (20) to obtain a corrected critical dimension of the mask (13). [2] Method (100) according to claim 1, wherein the determination (102) of the plurality of reference measurement points (24) depends on a temporal distribution of measurement times of the measurement points (22) within the measurement period, wherein preferably the reference measurement points (24) have measurement times that are evenly distributed over time within the measurement period. [3] Method (100) according to one of the preceding claims, wherein determining (105) the time course of the correction factor comprises determining a reference curve by the deviations (21) determined for the reference measurement points (24) respectively. [4] Method (100) according to any one of the preceding claims, further comprising: - Third acquisition (303) of at least one critical dimension (20) of the mask (13) at at least one part of the reference measurement points (24) at a time interval from the second acquisition (103), wherein the determination (104) of the deviation (21) is additionally dependent on the third acquired critical dimensions (20). [5] Method (100) according to one of the preceding claims, wherein the mask (13) has at least 500 measuring points (22), preferably at least 700 measuring points (22). [6] Method (100) according to one of the preceding claims, wherein the plurality of reference measuring points (24) comprises a maximum of 10%, preferably a maximum of 5%, of the measuring points (22). [7] Method (100) according to one of the preceding claims, wherein the first acquisition (101), the second acquisition (103) and / or the third acquisition (303) each comprises multiple acquisitions of the at least one critical dimension to form an average. [8] Method (100) according to one of the preceding claims, wherein the measuring points (22) are specified such that all critical dimensions of the mask (13) required for the qualification of the mask (13) can be detected. [9] Method (100) according to any one of the preceding claims, further comprising: - Output (401) a signal if the deviation (21) determined at a reference measurement point (24) is above a specified threshold.

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