Photomask and method for inspecting same

Abstract

The purpose of the present invention is to provide: a photomask that can be easily and accurately inspected even if a device pattern of the photomask has a curved shape; and a method for inspecting the photomask.　This photomask has a device region in which a device pattern is formed, and an inspection region (122) which is provided in a region other than the device region and in which an inspection pattern is formed. A Lissajous figure of a closed curve obtained by synthesizing displacement amounts (x, y) of a single vibration period in two directions orthogonal to each other, which are represented by the following equations (1) and (2), is formed in the inspection region (122) as an inspection pattern (122C).　(1): x = A cos (at) (2): y = B sin (bt + δ) where A and B are amplitudes, a and b are frequencies, t is time, and δ is an initial phase difference.

Description

Photomask and inspection method thereof

[0001] The present invention relates to a photomask and an inspection method thereof.

[0002] Photomasks used in manufacturing semiconductors are designed using CAD so that the design of the device pattern to be formed on the semiconductor is parallel to the X-axis and Y-axis that make up the horizontal plane. However, if the device pattern is formed on the photomask using the designed design, the wafer will not be exposed to the exact shape of the device pattern during semiconductor manufacturing due to the optical proximity effect, and the semiconductor will not be able to be manufactured with the designed design.

[0003] Therefore, optical proximity correction (OPC) is performed to correct the device pattern to be formed on the photomask in advance by taking the influence of the optical proximity effect into consideration in advance and simulating a device pattern that can be exposed on a wafer with the designed design.

[0004] In this case, the device pattern of the photomask that can expose the wafer according to the designed design tends to be curved in many parts. On the other hand, the electron beam lithography system that forms the device pattern on the photomask mainly uses a rectangular variable shaped beam (VSB) method, which forms the device pattern with a shape of an array of rectangles, i.e., a shape of connected straight lines. Therefore, the device pattern formed on the photomask is one in which the curves of the device pattern obtained by the OPC process are replaced with straight lines to approximate them.

[0005] In such photomasks, during manufacturing, an inspection pattern consisting only of straight lines parallel to the X-axis and Y-axis, such as a square, rectangle, L-shape, or cross, is formed in an extra-device area other than the device area where the device pattern is to be formed, at the same time as the device pattern is formed. Then, by measuring the inspection pattern with a critical dimension scanning electron microscope (CD-SEM), the dimensional accuracy of device patterns with complex shapes can be easily inspected.

[0006] JP 2015-161834 A U.S. Pat. No. 9,557,655

[0007] In recent years, there has been a shift in the imaging system for forming device patterns on photomasks from the VSB system described above to a multi-beam (MB) system. The MB system first rasterizes the OPC-processed device pattern, converting it into pixels (dots) to create a lithography mask pattern. Then, a large number of electron beams (approximately 260,000) corresponding to this lithography mask pattern are irradiated at once to write and form the OPC-processed device pattern.

[0008] With this MB method, even if many parts of the OPC-processed device pattern have curved shapes, it is easy to write and form the device pattern using a lithography mask pattern that corresponds to the curved shapes, making it possible to manufacture semiconductors with device patterns with higher precision.

[0009] Incidentally, when inspecting the dimensional accuracy of a photomask on which a curved device pattern is formed, the following method has been considered: In this method, the device pattern formed on the photomask is photographed with a CD-SEM to extract a contour image, and the finished shape is predicted from the lithography mask pattern using a simulation model, and the contour image and the predicted shape are superimposed for inspection.

[0010] However, the contour images captured and extracted using a CD-SEM are noisy and very unstable, so it is necessary to average multiple images or calculate the true contour position using machine learning, which makes accurate inspection extremely time-consuming.

[0011] SUMMARY OF THE INVENTION In view of the above, an object of the present invention is to provide a photomask and an inspection method thereof that can easily and accurately inspect even device patterns having curved shapes.

[0012] In order to solve the above-mentioned problems, the photomask of the present invention is a photomask having a device region in which a device pattern is formed, and an inspection region provided outside the device region in which an inspection pattern is formed, and is characterized in that a Lissajous figure of a closed curve obtained by combining displacement amounts x and y of the periods of simple harmonic motion in two mutually perpendicular directions, expressed by the following equations (1) and (2), is formed in the inspection region as the inspection pattern.

[0013] x=A cos(at) (1) y=B sin(bt+δ) (2) where A and B are amplitudes, a and b are frequencies, t is time, and δ is the initial phase difference.

[0014] Furthermore, in the photomask according to the present invention, it is preferable that the inspection region is sized to fit within a field of view (FOV) at a measurement magnification of a critical dimension scanning electron microscope (CD-SEM) that is set based on the line width of the device pattern in the device region.

[0015] Furthermore, the photomask according to the present invention is preferably the above-described photomask, in which a plurality of the inspection regions are provided, and the inspection patterns in the plurality of the inspection regions are Lissajous figures having shapes different from each other.

[0016] Furthermore, the photomask according to the present invention is preferably the above-described photomask, in which a plurality of inspection areas are provided, the inspection pattern of at least one of the inspection areas is the Lissajous figure, and the inspection pattern of at least one other of the inspection areas is a figure consisting of only straight lines.

[0017] On the other hand, a photomask inspection method according to the present invention for solving the above-mentioned problems is the above-mentioned photomask inspection method, characterized by comprising: an imaging step of imaging the inspection pattern within the inspection area of ​​the photomask as a bitmap image; a coordinate calculation step of determining, from data of the bitmap image, pixel coordinates of a center position of the inspection pattern and pixel coordinates of each of maximum positions in the X-axis direction and the Y-axis direction of the inspection pattern; a reference pattern creation step of determining, from the obtained pixel coordinates, values ​​of A and B constituting equations (1) and (2) of the inspection pattern and creating a bitmap image of a reference pattern corresponding to the inspection pattern based on equations (1) and (2); a pattern synthesis step of superimposing the reference pattern on the inspection pattern so that pixel coordinates of a center position of data of the bitmap image of the reference pattern coincide with pixel coordinates of a center position of data of the bitmap image of the inspection pattern; and an evaluation step of comparing the superimposed reference pattern and the inspection pattern to perform evaluation.

[0018] Furthermore, in the photomask inspection method according to the present invention, it is preferable that the photographing step is a step of photographing an entire image of the inspection pattern collectively as a single data item.

[0019] Furthermore, in the photomask inspection method according to the present invention, it is preferable that in the above-described photomask inspection method, the evaluation step includes a distance calculation step of calculating the distance between the superimposed reference pattern and the inspection pattern at specified intervals, a variation calculation step of calculating the variation in the calculated distances, and a determination step of determining the pattern as pass if the calculated variation is within a predetermined range, and determining the pattern as fail if the calculated variation is outside the predetermined range.

[0020] Furthermore, in the photomask inspection method according to the present invention, it is preferable that, in the above-described photomask inspection method, the distance calculation step is a step of calculating the slope of the reference pattern at each specified interval by a differential method, determining perpendicular lines to the reference pattern based on each calculated slope, and calculating the distance of each perpendicular line to an edge of a line of the inspection pattern.

[0021] In addition, the present invention also provides a method for manufacturing a photomask, which is characterized by manufacturing the photomask by adjusting the processing conditions of the photomask so that the variation that is judged to be a failure in the judgment step of the above-mentioned photomask inspection method falls within the predetermined range.

[0022] According to the photomask and inspection method thereof of the present invention, the Lissajous figure expressed by the above formulas (1) and (2) is formed as an inspection pattern in the inspection area, so that an inspection pattern and a reference pattern that have a central intersection at the origin and are line-symmetric (mirror-symmetric) about the X-axis and Y-axis can be created based on the formulas and superimposed for inspection. Therefore, even device patterns having curved shapes with various curvatures and directions can be easily inspected using an inspection pattern and a reference pattern that correspond to the curved shapes of the device patterns, and therefore accurate inspection can be easily performed.

[0023] 1 is a plan view showing a schematic configuration of a main embodiment of a photomask according to the present invention. It is a cross-sectional view taken along line II-II of FIG. 1 . It is a plan view of an inspection pattern of a Lissajous figure formed in an inspection area when a=1 and b=2. It is a plan view of an inspection pattern of a Lissajous figure formed in an inspection area when a=3 and b=2. It is a plan view of an inspection pattern of a Lissajous figure formed in an inspection area when a=3 and b=4. It is a plan view of an inspection pattern of a Lissajous figure formed in an inspection area when a=5 and b=4. It is an explanatory diagram of the steps of a manufacturing method of the photomask of FIG. 1 . It is a flow chart explaining the steps of a main embodiment of an inspection method of a photomask according to the present invention. It is an explanatory diagram of the observation field when the inspection pattern in the inspection area of ​​the photomask of FIG. 1 is measured by a critical dimension scanning electron microscope. It is an explanatory diagram of a reference pattern corresponding to the inspection pattern in the inspection area of ​​the photomask of FIG. 1 . It is an explanatory diagram of the inspection pattern of FIG. 6 superimposed on the reference pattern of FIG. 7. It is an explanatory diagram of the inspection method of FIG. 8. It is a plan view showing a schematic configuration of another embodiment of a photomask according to the present invention. 11A and 11B are enlarged views of an extracted inspection area where a rectangular (line) inspection pattern is formed in Fig. 10. Fig. 11A and 11B are enlarged views of an extracted inspection area where a square (space) inspection pattern is formed in Fig. 10.

[0024] The present invention will be described with reference to the accompanying drawings, which are provided to explain the principles of the present invention. The present invention is not limited to the following embodiments, but may be substituted or combined with other embodiments as needed.

[0025] [Main Embodiment] Main embodiments of a photomask and a method for inspecting the same according to the present invention will be described with reference to FIGS.

[0026] 1 and 2, a light-shielding film 120 made of chromium or the like is provided on a substrate 110 made of glass or the like. The light-shielding film 120 is provided with a device region 121 where a device pattern is formed and an inspection region 122 where an inspection pattern is formed. In the device region 121, a device pattern is formed that has been subjected to optical proximity correction (OPC) for a design to be formed on a device such as a semiconductor.

[0027] The inspection area 122 is provided in an area other than the device area 121, and the inspection pattern is formed as a closed curve Lissajous figure obtained by combining the displacement amounts x and y of the period of simple harmonic motion in two mutually perpendicular directions (X-axis direction and Y-axis direction), as expressed by the following equations (1) and (2).

[0028] x=A cos(at) (1) y=B sin(bt+δ) (2) where A and B are amplitudes, a and b are frequencies, t is time, and δ is the initial phase difference.

[0029] Here, for example, when A and B are 1, δ is 0, and a=1, b=2, the result is an inspection pattern 122A of a Lissajous figure of a symmetrical closed curve with an intersection at the center as shown in FIG. 3A; when a=3, b=2, the result is an inspection pattern 122B of a Lissajous figure of a symmetrical closed curve with an intersection at the center as shown in FIG. 3B; when a=3, b=4, the result is an inspection pattern 122C of a Lissajous figure of a symmetrical closed curve with an intersection at the center as shown in FIG. 3C; and when a=5, b=4, the result is an inspection pattern 122D of a Lissajous figure of a symmetrical closed curve with an intersection at the center as shown in FIG. 3D.

[0030] Note that, since the above formula (1) representing the displacement amount x in the X-axis direction uses cosine (cos), and the above formula (2) representing the displacement amount y in the Y-axis direction uses sine (sin), a phase difference of π / 2 is originally generated between the X-axis direction and the Y-axis direction. Therefore, by setting the initial phase difference δ to 0 (or π) and the actual phase difference to π / 2, a Lissajous figure of a symmetrical closed curve is obtained.

[0031] On the other hand, when both the displacement amount x in the X-axis direction and the displacement amount y in the Y-axis direction are expressed using sine (sin) or cosine (cos), the X-axis direction and the Y-axis direction are in phase. Therefore, by setting the initial phase difference δ to π / 2 and the actual phase difference to π / 2, a Lissajous figure that is a symmetrical closed curve can be obtained.

[0032] In the above formulas (1) and (2), the values ​​of A and B and the values ​​of a and b are set appropriately depending on the shape of the device pattern obtained by OPC, i.e., conditions such as the curvature and direction of the curved portion.

[0033] Furthermore, the inspection region 122 is sized to fit within the observation field (FOV) at the measurement magnification of a critical dimension scanning electron microscope (CD-SEM) that is set based on the line width of the device pattern in the device region 121. Specifically, for example, if the line width of the device pattern in the device region 121 is 60 nm, the appropriate measurement magnification of the CD-SEM is about 50,000 times (50k times), and therefore the inspection region 122 is set to a rectangular shape with a side length of about 2.8 to 3 μm.

[0034] The photomask 100 according to this embodiment can be manufactured in the same manner as in the conventional method. For example, as shown in Fig. 4, a photomask blank is produced by depositing chromium or the like on a substrate 110 such as glass to provide a light-shielding film 120 (see Fig. 4A), and then a photosensitive resin solution is uniformly applied to the light-shielding film 120 and dried to form a resist film 130 (see Fig. 4B).

[0035] Next, MB is irradiated onto the resist film 130 to draw a lithography mask pattern in which all of the inspection patterns and OPC-processed device patterns have been converted into pixels (dots), and then a development process is performed to remove the MB-irradiated portions of the resist film 130 (see Figure 4C).

[0036] Next, the exposed portion of the light-shielding film 120 that is not covered by the resist film 130 is removed by dry etching using a reactive gas or the like (see FIG. 4D), and then the resist film 130 is completely removed and washed (see FIG. 4E). This allows for the production of a photomask 100 that includes, on the substrate 110, the light-shielding film 120 on which the device region 121 and the inspection region 122 are formed.

[0037] A method for inspecting the photomask 100 according to this embodiment manufactured in this manner will be described below with reference to FIG.

[0038] First, the manufactured photomask 100 is set in a CD-SEM (setting step S1 in FIG. 5). Next, the CD-SEM is set to a measurement magnification (e.g., 50,000 times (50k times)) corresponding to the line width (e.g., 60 nm) of the device pattern in the device region 121, and the CD-SEM is adjusted so that the inspection region 122 is located within the field of view (FOV). At this time, the inspection region 122 has a length (e.g., 2.8-3 μm) corresponding to the measurement magnification of the CD-SEM, so that the entire interior of the region fits within the FOV (adjustment step S2 in FIG. 5).

[0039] Specifically, in the above formulas (1) and (2), for example, if A and B are 1 μm, a=3, b=4, δ is 0, and the line width is 60 nm, then the inspection area 122 formed with the inspection pattern 122C as shown in Fig. 6 can be accommodated within the FOV. Then, by photographing the inspection area 122, the entire inspection pattern 122C within the inspection area 122 is acquired collectively as a single (identical) bitmap image data (photographing step S3 in Fig. 5).

[0040] Next, from the obtained bitmap image data, the pixel coordinates of the center position Oc of the check pattern 122C are calculated, as well as the pixel coordinates of each maximum position in the X-axis direction and the Y-axis direction of the check pattern 122C (coordinate calculation step S4 in FIG. 5).

[0041] From the obtained pixel coordinates, the values ​​of A and B constituting the formulas (1) and (2) of the test pattern 122C on the bitmap image are calculated, and a reference pattern 122Cs as shown in FIG. 7 corresponding to the test pattern 122C is created based on the formulas (1) and (2) (reference pattern creation step S5 in FIG. 5).

[0042] The reference pattern 122Cs is then superimposed on the check pattern 122C so that the pixel coordinates of the center position Os of the bitmap image data of the created reference pattern 122Cs coincide with the pixel coordinates of the center position Oc of the bitmap image data of the check pattern 122C (pattern combining step S6 in FIG. 5). This allows a center line defined by the reference pattern 122Cs to be provided on the check pattern 122C, as shown in FIG.

[0043] Next, an evaluation step is performed in which the superimposed reference pattern 122Cs and the check pattern 122C are compared for evaluation. Specifically, the slope of the reference pattern 122Cs at each specified interval is calculated using a differential method. Based on each calculated slope, perpendicular lines Lv to the reference pattern 122Cs are set as shown in Fig. 9, and the distances of each perpendicular line Lv to the edge of the line of the check pattern 122C are calculated. This allows the distance between the reference pattern 122Cs and the check pattern 122C to be determined at each specified interval (distance calculation step S7 in Fig. 5).

[0044] Then, the variation in the distance for each determined specified interval is calculated (variation calculation step S8 in FIG. 5), and it is determined whether the variation is within a preset range (determination step S9 in FIG. 5). If it is within the range, it is deemed to be a pass, and if it is outside the range, it is deemed to be a fail.

[0045] In other words, the test pattern and reference pattern are created and superimposed based on a mathematical formula that can draw a closed curve that has an intersection point at the origin, is line-symmetric (mirror-symmetric) about the X-axis and Y-axis, and is expandable and contractible in the X-axis and Y-axis directions, i.e., a Lissajous figure. This allows device patterns with curved shapes of various curvatures and directions to be easily inspected using the test pattern 122C and reference pattern 122Cs that correspond to the curved shapes of the device patterns.

[0046] Therefore, according to the photomask 100 and the inspection method thereof according to this embodiment, even device patterns having curved shapes can be easily inspected with high precision.

[0047] Furthermore, since the inspection pattern 122C and the reference pattern 122Cs can be enlarged or reduced in the X-axis direction and the Y-axis direction, the entire inspection pattern 122Cs in the inspection region 122C can also be accommodated within the FOV by setting the measurement magnification of the CD-SEM corresponding to the line width of the device pattern in the device region 121. Therefore, it is not necessary to photograph the inspection pattern 122Cs in divided parts, which reduces the time and effort required for inspection and also prevents a decrease in inspection accuracy due to misalignment of the divided images.

[0048] Furthermore, since the test pattern 122C and the reference pattern 122Cs are closed curves with their centers at the origin, the center position Oc of the test pattern 122C obtained by photographing and the center position Os of the reference pattern 122Cs obtained by the formula can be easily superimposed on each other by aligning them on the bitmap image, which makes it easy to combine the photographed image and the formula image.

[0049] Furthermore, the processing conditions are adjusted so that the variations judged as failure in judgment step S9 are within the predetermined range, i.e., by feeding back the inspection results to the manufacturing conditions, the photomask 100 can always be manufactured with high precision.

[0050] Specifically, since a Lissajous figure is line-symmetric (mirror-symmetric) about the X-axis and Y-axis, the degree of eccentricity of the MB can be determined by inspecting the symmetry of the inspection pattern 122C. The degree of eccentricity thus determined is fed back to the angle control of the MB irradiation head of the drawing device when manufacturing the next photomask. This allows photomasks to be manufactured with consistently high precision.

[0051] [Other Embodiments] In the above-described embodiment, the photomask 100 is described as having one inspection region 122 in which the inspection pattern 122C is formed, but the present invention is not limited to this. As other embodiments, for example, it is also possible to use a photomask having a plurality of inspection regions 122 in which the inspection pattern 122C is formed, or a photomask having a plurality of inspection regions 122 in which inspection patterns 122A-122D of Lissajous figures with different shapes are respectively formed.

[0052] Furthermore, for example, as shown in Fig. 10, it is possible to provide not only a plurality of inspection regions 122 in which a Lissajous figure inspection pattern is formed, but also a plurality of inspection regions 123, 124 in which an inspection pattern of a figure (linear figure) consisting only of straight lines parallel to the X-axis and Y-axis, such as a square, rectangle, L-shape, or cross shape, is formed. Specifically, for example, it is possible to provide a plurality of inspection regions 123 in which a rectangular (line) inspection pattern 123A is formed as shown in Fig. 11, and a plurality of inspection regions 124 in which a square (space) inspection pattern 124A is formed as shown in Fig. 12.

[0053] The photomask and the inspection method thereof according to the present invention can be used extremely advantageously in various industries because it allows for easy and accurate inspection.

[0054] 100 Photomask 110 Substrate 120 Light-shielding film 121 Device area 122 Inspection area 122A-122D Inspection pattern (Lissajous figure) 122Cs Reference pattern (Lissajous figure) 123 Inspection area 123A Inspection pattern (line) 124 Inspection area 124A Inspection pattern (space)

Claims

1. A photomask having a device region in which a device pattern is formed, and an inspection region provided outside the device region in which an inspection pattern is formed, characterized in that a Lissajous figure of a closed curve obtained by combining displacement amounts x and y of the period of simple harmonic motion in two mutually perpendicular directions, expressed by the following formulas (1) and (2), is formed in the inspection region as the inspection pattern: x = A cos(at) (1) y = B sin(bt + δ) (2) where A and B are amplitudes, a and b are frequencies, t is time, and δ is an initial phase difference.

2. The photomask according to claim 1, characterized in that the inspection area is sized to fit within a field of view (FOV) at a measurement magnification of a critical dimension scanning electron microscope (CD-SEM) that is set based on the line width of the device pattern in the device area.

3. The photomask according to claim 1, characterized in that a plurality of said inspection regions are provided, and the inspection patterns in the plurality of said inspection regions are Lissajous figures of mutually different shapes.

4. The photomask according to claim 1, characterized in that a plurality of the inspection areas are provided, the inspection pattern in at least one of the inspection areas is the Lissajous figure, and the inspection pattern in at least one other of the inspection areas is a figure consisting of straight lines only.

5. A photomask inspection method according to claim 1, comprising the steps of: an imaging step of imaging the inspection pattern within the inspection area of ​​the photomask as a bitmap image; a coordinate calculation step of determining, from the data of the bitmap image, pixel coordinates of the center position of the inspection pattern and pixel coordinates of each maximum position in the X-axis and Y-axis directions of the inspection pattern; a reference pattern creation step of determining, from the obtained pixel coordinates, the values ​​of A and B constituting equations (1) and (2) of the inspection pattern and creating a bitmap image of a reference pattern corresponding to the inspection pattern based on equations (1) and (2); a pattern synthesis step of superimposing the reference pattern on the inspection pattern so that the pixel coordinates of the center position of the data of the bitmap image of the reference pattern coincide with the pixel coordinates of the center position of the data of the bitmap image of the inspection pattern; and an evaluation step of comparing the superimposed reference pattern and the inspection pattern to perform an evaluation.

6. The photomask inspection method according to claim 5, wherein said photographing step is a step of photographing an entire image of said inspection pattern in a batch as a single data item.

7. The photomask inspection method according to claim 5, characterized in that the evaluation step comprises the steps of: a distance calculation step of determining the distance between the superimposed reference pattern and the inspection pattern at specified intervals; a variation calculation step of calculating the variation of the determined distance; and a judgment step of determining that the pattern is pass when the calculated variation is within a preset range, and that the pattern is fail when the calculated variation is outside the preset range.

8. The photomask inspection method according to claim 7, characterized in that the distance calculation step is a step of: calculating the slope of the reference pattern at each specified interval by a differential method; determining perpendicular lines to the reference pattern based on each calculated slope; and calculating the distance of each perpendicular line to an edge of a line of the inspection pattern.

9. A method for manufacturing a photomask, comprising the steps of: manufacturing the photomask by adjusting processing conditions of the photomask so that the variation evaluated as failing in the judgment step of the photomask inspection method described in claim 7 falls within the predetermined range.

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