Photolithography reticle and overlay alignment method

Abstract

The present application relates to a kind of photolithography and overlay alignment method, the photolithography includes alignment mark, the alignment mark includes: main alignment pattern;At least two rotation discriminant patterns, each rotation discriminant pattern is close to the main alignment pattern and is located in first area, the at least two rotation discriminant patterns include the absolute value of two rotation discriminant patterns of deflection angle same, direction opposite;The deflection angle is the center of each rotation discriminant pattern and the center of main alignment pattern The included angle formed by the line and main alignment pattern, the direction opposite indicates that one of two deflection angles is located in the clockwise rotation direction of main alignment pattern, another is located in the counterclockwise rotation direction of main alignment pattern;Wherein, the first area is the area obtained by the main alignment pattern rotating 360 degrees around the center of main alignment pattern except the main alignment pattern in the area.The present application can avoid alignment to adjacent shot when overlay alignment, improve the success rate of overlay alignment.

Description

Technical Field

[0001] This invention relates to the field of semiconductor manufacturing, and in particular to a photomask and an overlay alignment method. Background Technology

[0002] In the manufacturing of MEMS (Micro-Electro-Mechanical Systems) wafer products, due to the special nature of the process, SUSS (SUSS MicroTec's equipment) proximity exposure equipment is typically used instead of the original Stepper or Scanner equipment for photolithography exposure. When using standard SUSS alignment marks for exposure, occasional severe misalignment of the lithographic pattern (lithographic misalignment) may occur. Summary of the Invention

[0003] Therefore, it is necessary to provide a photomask that can improve the success rate of overlay alignment.

[0004] A photomask includes alignment marks, the alignment marks comprising: a master alignment pattern; and at least two rotation discrimination patterns, each rotation discrimination pattern being disposed close to the master alignment pattern and located in a first region, the at least two rotation discrimination patterns comprising two rotation discrimination patterns having the same absolute value of deflection angle and opposite directions; the deflection angle being the angle formed by the line connecting the center of each rotation discrimination pattern and the center of the master alignment pattern and the master alignment pattern, the opposite directions meaning that one of the two deflection angles is located in the clockwise rotation direction of the master alignment pattern and the other is located in the counterclockwise rotation direction of the master alignment pattern; wherein, the first region is the region other than the master alignment pattern obtained by rotating the master alignment pattern 360 degrees around its center.

[0005] If the wafer is deflected during overlay alignment, causing the alignment mark of the photomask to move above the alignment mark of an adjacent shot (exposure area) on the wafer, the alignment mark of the adjacent shot will be deflected relative to the alignment mark of the target shot. Therefore, it can be determined whether the alignment between the photomask and the wafer has been rotated by checking whether the rotation discrimination pattern on the wafer is blocked by the main alignment pattern of the photomask. This can prevent the photomask from being aligned with an adjacent shot during overlay alignment and improve the success rate of overlay alignment.

[0006] In one embodiment, the main alignment pattern is a cross shape, the main alignment pattern includes two mutually perpendicular bars, and the deflection angle is the angle formed by the line connecting the center of the rotation discrimination pattern and the center of the main alignment pattern and the nearest bar.

[0007] In one embodiment, the number of rotation discrimination patterns of the alignment mark is two, one of which is located near one end of the bar and the other is located near the other end.

[0008] In one embodiment, each rotation discrimination graphic is a square.

[0009] In one embodiment, the length of the two strips is 72 micrometers and the width is 8 micrometers, and the side length of the two rotation discrimination patterns is 4 micrometers.

[0010] In one embodiment, the absolute value of the deflection angle of each of the rotation discrimination patterns is arctan(X / D1), where D1 is the distance from the wafer center when the alignment mark is formed on the wafer, and X is the X-direction dimension of a single exposure area of ​​the photomask during exposure, the X-direction being perpendicular to the wafer diameter direction on the wafer plane.

[0011] In one embodiment, the absolute value of the deflection angle of each of the rotation discrimination patterns is arctan(X / 4Y), where X is the X-direction dimension of a single exposure area of ​​the photomask during exposure, Y is the Y-direction dimension of a single exposure area of ​​the photomask during exposure, and the alignment mark is located at the position of the fourth exposure area from the center of the wafer when it is formed on the wafer.

[0012] In one embodiment, the master alignment pattern is cross-shaped, comprising two mutually perpendicular bars, and when the alignment mark is formed on the wafer, the bars of different alignment marks on the wafer are parallel to each other.

[0013] In one embodiment, the width of the bar of the master alignment pattern is greater than the width of each rotation discrimination pattern.

[0014] In one embodiment, the alignment mark is located in the dicing channel when it is formed on the wafer.

[0015] It is also necessary to provide an overlay alignment method.

[0016] An overlay alignment method includes: forming alignment marks on a wafer; placing a photomask including the alignment marks above the wafer, and aligning the photomask and the wafer by vertically aligning the alignment marks on the photomask and the wafer; wherein the photomask is any one of claims 1-7, and the alignment marks on the wafer are identical in pattern to the alignment marks on the photomask; wherein, in the step of vertically aligning the alignment marks on the photomask and the wafer, if the main alignment pattern of the photomask obscures any rotation discrimination pattern of the wafer, it is determined that the alignment of the photomask and the wafer has been rotated.

[0017] In one embodiment, after the step of aligning the photomask and the wafer, the method further includes a step of exposing the photoresist on the wafer through the photomask, wherein the exposure equipment is a SUSS machine.

[0018] In one embodiment, the wafer is used to form a microelectromechanical system (MEMS).

[0019] In one embodiment, if it is determined that the alignment of the photomask and the wafer has been rotated, the photomask and the wafer are rotated relative to each other to align them.

[0020] In one embodiment, the step of forming alignment marks on the wafer includes forming a plurality of alignment marks, wherein different alignment marks on the wafer are located in different exposure areas. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application or the conventional technology, the drawings used in the description of the embodiments or the conventional technology will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is an example alignment mark for a SUSS machine;

[0023] Figure 2 This is a schematic diagram of the distribution of shot and alignment marks on the wafer;

[0024] Figure 3 This is an exemplary side view of the SUSS machine's photomask and wafer overlay alignment;

[0025] Figure 4 This is a schematic diagram of normal and abnormal alignment;

[0026] Figure 5 This is a schematic diagram showing the movement of adjacent alignment markers to the position of the target alignment marker;

[0027] Figure 6 This is a graphic representation of a positioning mark in one embodiment of this application;

[0028] Figure 7 This is a schematic diagram showing the rotational offset that occurs during wafer alignment.

[0029] Figure 8 This is a flowchart of an overlay alignment method in one embodiment;

[0030] Figure 9a This is a graphic representation of the alignment marker in another embodiment. Figure 9b This is a graphic representation of the alignment marker in yet another embodiment. Detailed Implementation

[0031] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0033] It should be understood that when an element or layer is referred to as "on," "adjacent to," "connected to," or "coupled to" other elements or layers, it may be directly on, adjacent to, connected to, or coupled to other elements or layers, or there may be intervening elements or layers. Conversely, when an element is referred to as "directly on," "directly adjacent to," "directly connected to," or "directly coupled to" other elements or layers, there are no intervening elements or layers. It should be understood that although the terms first, second, third, etc., may be used to describe various elements, components, areas, layers, and / or portions, these elements, components, areas, layers, and / or portions should not be limited by these terms. These terms are only used to distinguish one element, component, area, layer, or portion from another element, component, area, layer, or portion. Therefore, without departing from the teachings of this invention, the first element, component, area, layer, or portion discussed below may be referred to as the second element, component, area, layer, or portion.

[0034] Spatial relation terms such as “below,” “under,” “below,” “under,” “above,” “above,” etc., are used herein for convenience of description to describe the relationship between one element or feature shown in the figure and other elements or features. It should be understood that, in addition to the orientation shown in the figure, spatial relation terms are intended to also include different orientations of the device in use and operation. For example, if the device in the figure is flipped, then the element or feature described as “below” or “under” the other element or feature will be oriented “above” the other element or feature. Therefore, the exemplary terms “below” and “under” can include both upper and lower orientations. The device may be otherwise oriented (rotated 90 degrees or otherwise) and the spatial descriptive terms used herein will be interpreted accordingly.

[0035] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. When used herein, the singular forms “a,” “an,” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “comprising” and / or “including,” when used in this specification, identify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups. When used herein, the term “and / or” includes any and all combinations of the associated listed items.

[0036] Embodiments of the invention are described herein with reference to cross-sectional views that serve as schematic diagrams of ideal embodiments (and intermediate structures). Thus, variations in the shape shown can be anticipated due to, for example, manufacturing techniques and / or tolerances. Therefore, embodiments of the invention should not be limited to the specific shapes of the regions shown herein, but include shape deviations due to, for example, manufacturing processes. For example, implantation regions shown as rectangular typically have rounded or curved features at their edges and / or implantation concentration gradients, rather than a binary change from implantation regions to non-implantation regions. Similarly, the buried regions formed by implantation can result in some implantation in the region between the buried region and the surface traversed during implantation. Therefore, the regions shown in the figures are substantially schematic, and their shapes are not intended to show the actual shapes of the regions of the device and are not intended to limit the scope of the invention.

[0037] Example alignment marks for SUSS machines are as follows: Figure 1 As shown, the alignment marks are cross-shaped, with the width of the strips within the cross being 30 micrometers and the length being 40 + 30 + 40 = 110 micrometers. The alignment marks are formed on the photomask and the wafer, and are repeatedly distributed on the wafer according to the shot (exposure area), as illustrated in the diagram. Figure 2 As shown. Figure 2 A square on the wafer represents a shot, and a cross-shaped mark is a alignment marker. For example... Figure 2 As shown, alignment marks are formed at the vertices where two shots meet. The basic principle of the exemplary alignment process is as follows: two cameras positioned above the photomask capture the alignment marks on the photomask and the alignment marks on the wafer, respectively; as... Figure 3 As shown, the alignment marks on the photomask and the wafer are aligned vertically by moving the wafer on the stage, thereby achieving the purpose of overlay alignment between the photomask and the wafer.

[0038] Because the SUSS machine uses a graphic comparison method for alignment, the exemplary alignment marks show little difference even with slight rotation. However, in actual production, this can easily lead to mis-grabbing, causing the markings on the photomask and wafer to shift. (See diagram below.) Figure 4 As shown. Specifically, see [link to relevant documentation]. Figure 5 , Figure 5 The upper center shows a properly aligned wafer, while the lower center shows a wafer that has shifted. The wafer has two adjacent alignment marks, m1 and m2, representing the alignment marks for two adjacent shots. After the shift, alignment mark m2 moves to the original position of the target alignment mark m1. The alignment mark on the photomask, which should have been aligned with target m1, will now be aligned with alignment mark m2. Because the changes in the alignment marks are not obvious when slightly rotated, they are easily misidentified by the equipment; the equipment will still align the photomask's alignment mark with alignment mark m2, resulting in an alignment error.

[0039] This application proposes a novel graphic design for photolithography alignment marks, used to align the photomask and wafer before photolithography exposure. The alignment mark incorporates a rotation-detection graphic. When the wafer rotates relative to the photomask, the rotation-detection graphic is obscured, causing the graphic comparison to fail. This identifies a rotational misalignment between the photomask and the wafer, preventing anomalies.

[0040] Figure 6 This is a graphic representation of an alignment mark in one embodiment of this application. In this embodiment, the alignment mark includes a main alignment mark 10 and at least two rotation discrimination marks 20. Each rotation discrimination mark 20 is slightly offset from the main alignment mark 10 and is located in a first region. The first region is the region obtained by rotating the main alignment mark 10 360 degrees around its center, excluding the main alignment mark 10. That is, after rotating the main alignment mark 10 around its center by a suitable angle, it can obscure any of the rotation discrimination marks 20. To ensure that the wafer can be identified regardless of whether it is rotated clockwise or counterclockwise, each alignment mark has at least two rotation discrimination patterns 20 set in pairs. The absolute values ​​of the deflection angles θ of the paired rotation discrimination patterns 20 (the deflection angle θ is the angle formed by the line connecting the center of each rotation discrimination pattern 20 and the center of the main alignment pattern 10 and the main alignment pattern 10) are the same, but the directions are opposite. The opposite directions of the two deflection angles θ mean that the main alignment pattern 10 will exactly block one of the rotation discrimination patterns 20 after rotating clockwise by θ / exactly block the other rotation discrimination pattern 20 after rotating counterclockwise by θ. Figure 6 The two rotation discriminant figures 20 in the figure are a pair of rotation discriminant figures 20 set in pairs.

[0041] If the wafer is deflected during overlay alignment of the patterned photomask with the above alignment marks, causing the alignment marks of the photomask to move above the alignment marks of an adjacent shot on the wafer, the alignment marks of the adjacent shot will be deflected relative to the alignment marks of the target shot. Therefore, it can be determined whether the alignment between the photomask and the wafer has been rotated by whether the rotation discrimination pattern 20 on the wafer is blocked by the main alignment pattern of the photomask. This avoids alignment with an adjacent shot during overlay alignment, which can improve the success rate of overlay alignment and reduce the rework rate and defect rate of online products.

[0042] exist Figure 6 In the illustrated embodiment, the main alignment pattern 10 is cross-shaped, comprising two mutually perpendicular stripes, and the deflection angle θ is the angle formed by the line connecting the center of the rotation discriminant pattern 20 and the center of the main alignment pattern 10, and the strip closest to the rotation discriminant pattern 20. Figure 6 In the embodiment shown, the width of the strip in the cross of the master alignment pattern 10 is greater than the width of each rotation discrimination pattern 20.

[0043] exist Figure 6 In the illustrated embodiment, there are two rotation discrimination patterns 20 for the alignment markers, one positioned near one end of a strip in the cross and the other near the other end of that strip. In other embodiments, the paired rotation discrimination patterns 20 may also be two different stripes positioned near the main alignment pattern 10, such as... Figure 9a As shown; or, the two pairs of rotational discriminant patterns 20 can also be set on the left and right sides of a strip of the main alignment pattern 10. The number of rotational discriminant patterns 20 in each alignment mark can also be odd, for example, a pair of rotational discriminant patterns 20 + a single rotational discriminant pattern 20; if the number of rotational discriminant patterns 20 is even, it can also be a pair of rotational discriminant patterns 20 + an even number of unpaired rotational discriminant patterns 20.

[0044] In one embodiment of this application, each rotation discrimination graphic 20 is a square.

[0045] exist Figure 6 In the embodiment shown, the width of the bar of the master alignment pattern 10 is greater than the width of each rotation discrimination pattern 20.

[0046] exist Figure 6 In the embodiment shown, the two bars of the main alignment pattern 10 have a length of 72 micrometers and a width of 8 micrometers. The rotation discrimination pattern 20 is a square with a side length of 4 micrometers.

[0047] In one embodiment of this application, the patterned photomask with the above-mentioned alignment marks is used in the photolithography process for manufacturing MEMS products.

[0048] In one embodiment of this application, the alignment marks on the wafer are formed by photolithography followed by etching using a patterned photomask with the aforementioned alignment marks. Furthermore, multiple alignment marks can be formed at different locations on the wafer, with different alignment marks located in different shots. In one embodiment of this application, the alignment marks are cross-shaped, and the alignment marks on the wafer are parallel to each other (i.e., one strip of the cross is parallel to one of the two stripes of another alignment mark). In one embodiment of this application, the two stripes of the alignment marks on the wafer are parallel to the scribe lines in the X and Y directions, respectively.

[0049] Figure 7 This diagram illustrates the rotational shift that occurs during wafer alignment. The wafer has two adjacent alignment marks, m1 and m2, representing the alignment marks for two adjacent shots. After the shift, alignment mark m2 moves to the original position of the target alignment mark m1. The distance between alignment marks m1 and m2 and the wafer center is D1, and the distance between the target alignment mark m1 and alignment mark m2 is D2 (meaning the target alignment mark m1 shifts by a distance of D2). Therefore, the wafer's shift angle is approximately arctan(D2 / D1). In one embodiment of this application, the absolute value of the deflection angle θ of the rotation discrimination pattern 20 is also set to arctan(D2 / D1), where arctan(D2 / D1) ≈ arctan(X / D1). Therefore, the absolute value of the deflection angle θ can be set to arctan(X / D1). In this embodiment, X is the dimension of the X direction (i.e., the X direction of the shot) of a single exposure area of ​​the photomask during exposure, and the X direction is perpendicular to the wafer diameter direction on the wafer plane. In other embodiments, X is the sum of the dimensions of the X direction (i.e., the X direction of the shot) of two or more exposure areas of the photomask during exposure, and the X direction is perpendicular to the wafer diameter direction on the wafer plane. In one embodiment of this application, the alignment mark is located at the position of the fourth exposure area from the wafer center when it is formed on the wafer. Therefore, the absolute value of the deflection angle θ is set to arctan(X / 4Y), where Y is the dimension of the Y direction of a single exposure area of ​​the photomask during exposure, and the Y direction is parallel to the wafer diameter direction on the wafer plane.

[0050] In one embodiment of this application, the alignment marks are located in the scribe line when formed on the wafer. The exemplary scribe line width is about 60 micrometers or 80 micrometers, which is an order of magnitude different from the side length of the shot (exemplarily about 22 mm). Therefore, when estimating the dimensions D1 and D2 when designing the deflection angle θ, the size of the scribe line can be ignored.

[0051] An 8-inch wafer has a diameter of approximately 200 mm, and an exemplary shot side length of 22 mm (for non-SUSS exposure layers in MEMS, Nikon exposure equipment is typically used, with a maximum shot side length of 22 mm). Therefore, a maximum of nine shots can be arranged within the diameter of an 8-inch wafer. In one embodiment of this application, the alignment mark used during alignment is generally located four shots from the center of the wafer.

[0052] This application provides a corresponding overlay alignment method, which uses the alignment marks proposed in this application for overlay alignment. See also Figure 8 The overlay alignment method includes the following steps:

[0053] S810 forms alignment marks on the wafer.

[0054] The alignment mark can be any of the alignment marks described in the foregoing embodiments. In one embodiment of this application, a photomask with alignment marks can be used to photolithographically etch and etch the wafer to form alignment marks on the wafer. After etching, multiple alignment marks can be formed on the wafer at different locations, and different alignment marks on the wafer are located in different shots.

[0055] S820 aligns the photomask and the wafer by vertically aligning the alignment marks on the photomask and the wafer.

[0056] A photomask with alignment marks is placed above the wafer (the pattern of the alignment marks on the photomask is the same as each alignment mark formed on the wafer in step S810). The wafer (or photomask) is moved by the stage to make the alignment marks on the photomask and the alignment marks on the wafer coincide in the vertical direction, thus aligning the photomask and the wafer. Two cameras located above the photomask can capture the alignment marks on the photomask and the wafer respectively to determine whether the alignment marks on the photomask and the wafer coincide in the vertical direction. If the main alignment pattern 10 in the photomask alignment marks obscures any rotation discrimination pattern 20 in the wafer alignment marks, it is determined that the alignment of the photomask and the wafer has been rotated, and an alarm can be triggered.

[0057] In the above overlay alignment method, if the wafer is deflected, causing the alignment mark of the photomask to move above the alignment mark of an adjacent shot on the wafer, the alignment mark of the adjacent shot will be deflected relative to the alignment mark of the target shot. Therefore, it can be determined whether the alignment between the photomask and the wafer has been rotated by whether the rotation discrimination pattern on the wafer is blocked by the main alignment pattern of the photomask. This avoids alignment with an adjacent shot during overlay alignment and can improve the success rate of overlay alignment.

[0058] In one embodiment of this application, if it is determined that the alignment between the photomask and the wafer has been rotated, the photomask and the wafer are aligned by rotating them relative to each other.

[0059] In one embodiment of this application, after step S820, a step of exposing the photoresist on the wafer through the photomask is further included, and the exposure equipment used is a SUSS machine.

[0060] In one embodiment of this application, the wafer is used to form a MEMS product.

[0061] It should be understood that although the steps in the flowchart of this application are shown sequentially as indicated by the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the flowchart of this application may include multiple steps or multiple stages, which are not necessarily completed at the same time, but may be executed at different times, and the execution order of these steps or stages is not necessarily sequential, but may be performed alternately or in turn with other steps or at least some of the steps or stages of other steps.

[0062] In the description of this specification, references to terms such as "some embodiments," "other embodiments," and "ideal embodiments" indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative descriptions of the above terms do not necessarily refer to the same embodiments or examples.

[0063] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features of the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0064] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A photomask, comprising alignment marks, characterized in that, The alignment markers include: Main alignment graphics; At least two rotation discrimination patterns are provided, each rotation discrimination pattern being positioned close to the main alignment pattern and located in the first region. The at least two rotation discrimination patterns include two rotation discrimination patterns with the same absolute value of the deflection angle and opposite directions. The deflection angle is the angle formed by the line connecting the center of each rotation discrimination pattern and the center of the main alignment pattern and the main alignment pattern. The opposite directions mean that one of the two deflection angles is located in the clockwise rotation direction of the main alignment pattern and the other is located in the counterclockwise rotation direction of the main alignment pattern. The first region is the region other than the main alignment pattern obtained by rotating the main alignment pattern 360 degrees around its center; the main alignment pattern is cross-shaped and includes two mutually perpendicular bars; the deflection angle is the angle formed by the line connecting the center of the rotation discrimination pattern and the center of the main alignment pattern and the nearest bar; the width of the bars of the main alignment pattern is greater than the width of each of the rotation discrimination patterns.

2. The photomask according to claim 1, characterized in that, The alignment marks are located in the dicing channels when formed on the wafer.

3. The photomask according to claim 2, characterized in that, The number of rotation discrimination graphics for the alignment mark is two, one of which is set near one end of the bar and the other is set near the other end.

4. The photomask according to claim 3, characterized in that, The length of the two strips is 72 micrometers and the width is 8 micrometers. The two rotation discrimination patterns are squares with a side length of 4 micrometers.

5. The photomask according to claim 1, characterized in that, The absolute value of the deflection angle of each of the rotation discrimination patterns is arctan(X / D1), where D1 is the distance between the alignment mark formed on the wafer and the wafer center, and X is the X-direction dimension of a single exposure area of ​​the photomask during exposure, and the X-direction is perpendicular to the wafer diameter direction on the wafer plane.

6. The photomask according to claim 1, characterized in that, The absolute value of the deflection angle of each of the rotation discrimination patterns is arctan(X / 4Y), where X is the X-direction dimension of a single exposure area of ​​the photomask during exposure, Y is the Y-direction dimension of a single exposure area of ​​the photomask during exposure, and the alignment mark is located at the position of the fourth exposure area from the center of the wafer when it is formed on the wafer.

7. The photomask according to claim 5 or 6, characterized in that, The main alignment pattern is cross-shaped, comprising two mutually perpendicular bars. When the alignment mark is formed on the wafer, the bars of different alignment marks on the wafer are parallel to each other.

8. A method for overlay alignment, comprising: Form alignment marks on the wafer; A photomask including alignment marks is placed above the wafer, and the photomask and the wafer are aligned by aligning the alignment marks of the photomask and the wafer in the vertical direction; the photomask is the photomask of any one of claims 1-7, and the alignment marks of the wafer are the same as the alignment marks of the photomask. In the step of aligning the alignment marks of the photomask and the alignment marks of the wafer in the vertical direction, if the main alignment pattern of the photomask obscures any rotation discrimination pattern of the wafer, it is determined that the alignment of the photomask and the wafer has been rotated.

9. The overlay alignment method according to claim 8, characterized in that, After the step of aligning the photomask and the wafer, the method further includes a step of exposing the photoresist on the wafer through the photomask, and the exposure equipment used is a SUSS machine.

10. The overlay alignment method according to claim 8 or 9, characterized in that, The wafer is used to form microelectromechanical systems.

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