# Patent

## Title
Method for forming photomasks

## Abstract

A method for forming photomasks includes the following steps. A first photomask including a first target pattern and a first unprintable dummy pattern is provided. A second photomask including a second target pattern and a second printable dummy pattern are provided, wherein at least part of the second printable dummy pattern overlapping the first unprintable dummy pattern exposure limit, such that the second printable dummy pattern can not be printed in a wafer.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method for forming photomasks, and more specifically to a method for forming photomasks that applies to photomasks having a sacrificial pattern with a dimension larger than the exposure limit, and that is paired with a self-aligned via process to prevent the sacrificial pattern from being printed on the wafer.
2. Description of the Prior Art
In the semiconductor fabrication process, lithography processes are important steps to transfer integrated circuit layouts to semiconductor wafers. Generally, a wafer manufacturing company designs a mask layout according to an integrated circuit layout; and then fabricates a mask having the designed mask layout. Afterwards, by way of lithography processes, the pattern on the mask (i.e. the mask pattern) is transferred to a photoresist layer on the surface of a semiconductor wafer with a specific scale.
As the complexity and integration of integrated circuit continue to progress, the size of every segment of a mask pattern is designed to be smaller. However, the exposure limit of every segment fabricated by exposure is limited to the resolution limit of the optical exposure tool used during the transfer step of the mask pattern. One problem that easily arises during the exposure of a mask pattern with high-density arranged segments to form a pattern on a photoresist is the optical proximity effect. Therefore, resolution losses occur because of overexposure or underexposure, which brings deviations of the original pattern on the photoresist. Many saving methods have been used to improve the deviation caused by the optical proximity effect in order to improve the quality of the transferred pattern. The most popular method is the optical proximity correction (OPC). And there has been a variety of commercial optical proximity correction softwares that can theoretically correct the mask pattern in order to acquire more accurate pattern on a wafer.
SUMMARY OF THE INVENTION
The present invention provides a method for forming photomasks, which forms a sacrificial pattern with a dimension larger than the exposure limit of a photomask, to increase the luminous flux of the photomask during the exposure step, and that is paired with a self-aligned via process to prevent the sacrificial pattern from being printed on the wafer.
The present invention provides a method for forming photomasks including the following steps. A first photomask including a first target pattern and a first unprintable dummy pattern is provided. A second photomask including a second target pattern and a second printable dummy pattern is provided, wherein at least part of the second printable dummy pattern overlaps the first unprintable dummy pattern, such that the second printable dummy pattern can not be printed in a wafer.
According to the above, the present invention provides a method for forming photomasks, which forms a second printable dummy pattern with a dimension larger than the exposure limit in a second photomask so as to increase the luminous flux of the second photomask during the exposure step. Besides, due to a first target pattern of a first photomask not overlapping the second printable dummy pattern, the printing of a second printable dummy pattern on a wafer can be prevented. This way, the luminous flux of the second target pattern can be increased through forming the second printable dummy pattern with a dimension larger than the exposure limit next to the second target pattern, and the second printable dummy pattern can be prevented to be printed on the wafer by also performing a self-aligned via method, thereby shrinking the exposure limit of the second target pattern, which allows both the physical targets and the wafer layout to be achieved.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-3 schematically depict top views of a method for forming photomasks according to an embodiment of the present invention.
FIGS. 4-8 schematically depict cross-sectional views of a method for forming photomasks according to an embodiment of the present invention.



DETAILED DESCRIPTION
Patterns described in the following may be printable or unprintable, which is related to exposure conditions, such as wavelengths of sources. A first mask 10 is exposed by using a first source and under a first exposure condition, while a second mask 20 is exposed by using a second source and under a second exposure condition, wherein the first source may be common with or different from the second source, and the first exposure condition may be common with or different from the second exposure condition. For example, when using a 193 nanometers (nm) immersion source and exposure condition, the mask dimension of via patterns that can be exposed is about 60 nanometers (nm).
FIGS. 1-3 schematically depict top views of a method for forming photomasks according to an embodiment of the present invention. FIGS. 4-8 schematically depict cross-sectional views of a method for forming photomasks according to an embodiment of the present invention.
As shown in FIG. 1, the top diagram of FIG. 1 is a top view of a first photomask 10, and the bottom diagram of FIG. 1 is a top view of a wafer 40 corresponding to the first photomask 10, and a mask 30 located on the wafer 40. First, the first photomask 10 including a first target pattern 12 and a first unprintable dummy pattern 14 is provided. Then, a pattern printing process is performed on the wafer 40 by using the first photomask 10. The first target pattern 12 needs to be printed on the wafer 40 or on the mask 30 that is on the wafer 40, while the first unprintable dummy pattern 14 serves as a virtual pattern for adjusting the luminous flux of the first target pattern 12 during the exposure step, wherein the dimension of the first unprintable dummy pattern 14 is smaller than the first exposure limit under the first exposure condition and using the first source (that represents the minimal resolving dimension under the first exposure condition and using the first source), so that the first unprintable dummy pattern 14 will not be printed on the wafer 40 or on the mask 30.
As shown in FIG. 1, as a first process P1 is performed, only the first target pattern 12 of the first photomask 10 can therefore be printed on the mask 30 on the wafer 40, and a first corresponding pattern Q is therefore formed on the mask 30, while the first unprintable dummy pattern 14 is not printed on the mask 30 since the dimension of the first unprintable dummy pattern 14 is smaller than the first exposure limit. In general, the mask 30 may include at least a hard mask (not shown), such as an isolation material or a metal material etc, and at last a photosensitive material, such as a photoresist, may also be further formed on the mask 30. The first process P1 may be lithographic and etching procedures: the photoresist on the mask 30 is exposed and developed, and then the mask 30 is etched by using the patterned photoresist. By doing this, as the first corresponding pattern Q is formed on the mask 30, a corresponding part N of the wafer 40 is exposed. In another embodiment, other target material layers may be further formed under the mask 30 and the first corresponding pattern Q of the mask 30 may be further printed on the target material layers, or the target material layers can serve as buffers for buffering the mask 30 and the wafer 40 without exposing the wafer 40 directly.
In this embodiment, the first target pattern 12 is used to define the pattern of metal wires of a structure such as a dual damascene structure, so that the first corresponding pattern Q formed on the mask 30 is a trench pattern having a plurality of trenches; and, the first unprintable dummy pattern 14 is used to increase the luminous flux of the first target pattern 12, so that the first unprintable dummy pattern 14 is not printed on the mask 30 and the wafer 40. In this embodiment, the first process P1 just exposes and develops the mask 30 to expose the wafer 40; in another embodiment, apart from exposing and developing the mask 30, the corresponding part N of the wafer 40 is also etched and a plurality of trenches are therefore formed in the wafer 40 as the first process P1 is performed. Moreover, the first unprintable dummy pattern 14 is located beside the first target pattern 12, and the first unprintable dummy pattern 14 may also be a trench pattern, especially a trench pattern parallel to the first target pattern 12, so as to increase the luminous flux of the first target pattern 12, but it is not limited thereto. In another embodiment, the first target pattern 12 may also be a conductive pattern used for defining a gate structure, so that the first corresponding pattern Q formed in the mask 30 is a strip pattern having a plurality of wires, and the first unprintable dummy pattern 14 is used to increase the luminous flux of the first target pattern 12 but is not printed on the mask 30 and the wafer 40. Additionally, the first unprintable dummy pattern 14 may not be formed close to the first target pattern 12, depending upon the needs. To sum up, the first unprintable dummy pattern 14 will not be printed on the mask 30 and the wafer 40 whether the first unprintable dummy pattern 14 is formed or not.
To illustrate the wafer 40 and the mask 30 located on the wafer 40 clearly, FIGS. 4-6 are presented, which schematically depict cross-sectional views of a method for forming photomasks according to an embodiment of the present invention. As shown in FIGS. 4-6, the right diagrams are cross-sectional views along an x direction of the bottom diagram of FIG. 1, and the left diagrams of FIGS. 4-6 are cross-sectional views along an y direction of the bottom diagram of FIG. 1.
As shown in FIG. 1 and FIG. 4, the wafer 40 and the mask 30 thereon are provided. A photoresist layer R1 is located on the mask 30 on the wafer 40 for performing processes such as later performed lithographic and etching processes. The mask 30 may include a stacked material layer. For example, the mask 30 may include an oxide layer and a metal layer or etc. In this embodiment, the mask 30 may include a first material layer 32 and a second material layer 34 located on the first material layer 32. The second material layer 34 may include an oxide layer and the first material layer 32 may include a metal layer serving as a hard mask, but it is not limited thereto. A sacrificial oxide layer B or etc may be formed between the wafer 40 and the mask 30 serving as an etch stop layer for later etching processes, in order to prevent the wafer 40 from being damaged during etching. The wafer 40 may include a substrate 44 and a target material layer 42 located on the substrate 44. The substrate 44 may be a semiconductor substrate such as a silicon substrate, a silicon containing substrate, a III-V group-on-silicon (such as GaN-on-silicon) substrate, a graphene-on-silicon substrate or a silicon-on-insulator (SOI) substrate. The target material layer 42 may be an interdielectric layer, an ultra-low-k material layer or a nitrogen-doped SiC (NDC) layer, and the first corresponding pattern Q in the mask 30 is further printed in the target material layer 42 to form a trench pattern of structures, such as a dual damascene structure, but it is not limited thereto.
A first process P1 is performed. As shown in FIG. 5, the photoresist layer R1 is patterned through a lithography process to form a patterned photoresist layer R1′, which means at that time that the first target pattern 12 of FIG. 1 has been transferred to the photoresist layer R1. Thus, the corresponding pattern of the first target pattern 12 is formed on the patterned photoresist layer R1′ while the corresponding pattern of the first unprintable dummy pattern 14 is not formed on the patterned photoresist layer R1′. Then, the second material layer 34 is etched by applying the patterned photoresist layer R1′, and a patterned second material layer 34′ is formed. Thereafter, the patterned photoresist layer R1′ is removed. As shown in FIG. 6, the first material layer 32 is etched by applying the patterned second material layer 34′, and a patterned first material layer 32′ is formed, which means the first corresponding pattern Q of FIG. 1 is formed at that time.
The pattern of the first photomask 10 of FIG. 1 is therefore transferred to the mask 30. It is emphasized that, only the first target pattern 12 of FIG. 1 is transferred to the mask 30 (including the first material layer 32 and the second material layer 34), and the first unprintable dummy pattern 14 is not transferred to the mask 30.
As shown in FIG. 2, wherein the top diagram of FIG. 2 is a top view of a second photomask 20, and the bottom diagram of FIG. 2 is a top view of the wafer 40 and the mask 30 on the wafer 40. The first photomask 10 in the top diagram of FIG. 1 is also depicted by dotted lines for disclosing the relative positions of the second photomask 20 and the first photomask 10, so that the printed locations of the patterns of the first photomask 10 and the second photomask 20 in the mask 30 and the wafer 40 can be disclosed clearly. However, only a pattern transferring process using the second photomask 20 is actually performed to the wafer 40.
As shown in FIG. 2, the second photomask 20 includes a second target pattern 22 and a second printable dummy pattern 24, and at least part of the second target pattern 22 will be printed on the wafer 40 but all of the second printable dummy pattern 24 will not be printed in the wafer 40. More precisely, the second printable dummy pattern 24 disposed surrounding the second target pattern 22 is used to increase the luminous flux of the second target pattern 22 during exposure, thereby shrinking the exposure limit of the second target pattern 22. The second printable dummy pattern 24 is just transferred to the photosensitive material (not shown), such as a photoresist, on the mask 30 by a lithography process, but it will not be transferred to the wafer 40 or the mask 30 by an etching process using the patterned photoresist. In the present invention, the dimension of the second printable dummy pattern 24 of the second photomask 20 is larger than the second exposure limit so as to effectively increase the luminous flux of the second target pattern 22 and shrink the exposure limit of the second target pattern 22. The second printable dummy pattern 24 does not overlap the first target pattern 12 of the first photomask 10, and at least a part of the second printable dummy pattern 24 overlaps the first unprintable dummy pattern 14. Thus, since the first unprintable dummy pattern 14 is not transferred onto the mask 30, the second printable dummy pattern 24 can not be printed on the wafer 40, even if the dimension of the second printable dummy pattern 24 is larger than the exposure limit. More precisely, the patterned first material layer 32′ is included in the mask 30 to serve as a hard mask, and the part of the hard mask corresponding to the second printable dummy pattern 24 can not be etched to expose the wafer 40, so as to prevent the second printable dummy pattern 24 from being printed on the wafer 40 as the second target pattern 22 is printed on the wafer 40. In this embodiment, the patterned second material layer 34′ on the patterned first material layer 32′ is etched and a part of the second printable dummy pattern 24 is transferred as the second target pattern 22 is printed on the wafer 40, and the patterned first material layer 32′ will stop the etching process so that the second printable dummy pattern 24 will not printed on the wafer 40, but it is not limited thereto.
Furthermore, the second mask 20 includes a second unprintable dummy pattern 26, which overlaps the first target pattern 12 of the first mask 10 but can not be printed in the wafer 40.
More precisely, as the second process P2 is performed, only the part of the second target pattern 22 overlapping the first corresponding pattern Q is transferred to the wafer 40, and a printed pattern K is therefore formed on the wafer 40, as shown in the bottom diagram of FIG. 2. Therefore, as the width of the second target pattern 22 along the X direction is w1, the width of the printed pattern K along the X direction is also w1 because the second target pattern 22 in the X direction is entirely transferred to the wafer 40; as the width of the second target pattern 22 along the y direction is w1, the width of the printed pattern K along the y direction shrinks to w because the second target pattern 22 sticking out the first corresponding pattern Q will be stopped by the mask 30 and will not be printed on the wafer 40. Therefore, by applying the self-aligned via (SAV) method, the luminous flux of the second target pattern 22 can be increased by increasing the dimension of the second target pattern 22, and a smaller dimension for the printed pattern K can be obtained, thereby shrinking the exposure limit of the second target pattern 22 and achieving the physical targets. In a semiconductor process, at least a (printed) pattern is formed in a dense region and an isolation (ISO) region of the wafer 40, wherein the luminous flux of the pattern in the dense region is larger than the luminous flux of the pattern in the isolation (ISO) region, thereby giving to the pattern in the dense region a lower exposure limit. Thus, in a preferred embodiment, the exposure limit of the pattern in the isolation (ISO) region can be reduced and a suitable luminous flux can be obtained, enabling the exposure limit of the pattern in the isolation (ISO) region to be similar to the exposure limit of the pattern in the dense region
The second process P2 may include a lithographic and an etching procedure, which exposes and develops the second target pattern 22 and the second printable dummy pattern 24 on at least a part of the mask (it is preferred to be a photoresist formed on the mask), and then forms the printed pattern K corresponding to the second target pattern 22 on the wafer 40. The second target pattern 22 may be a via pattern of a structure, such as a dual damascene structure, enabling the printed pattern K being a via in the wafer 40, but it is not limited thereto. In another embodiment, the second target pattern 22 may be another pattern, and the printed pattern K may be a plurality of vias or another pattern, depending upon the needs. In a preferred embodiment, the second printable dummy pattern 24 surrounds the second target pattern 22 so as to increase the luminous flux of the second target pattern 22 effectively. In this embodiment, the second printable dummy pattern 24 is a via pattern as shown in FIG. 2; in other words, the second printable dummy pattern 24 is a plurality of vias surrounding the second target pattern 22. In another embodiment, the second printable dummy pattern 24 may be a U-shaped pattern surrounding the second target pattern 22 as shown in FIG. 3, wherein the U-shaped pattern can have a larger luminous flux of the second target pattern 22 than that of the via pattern of FIG. 2. It is worth noting that, the second printable dummy pattern 24 does not overlap the first target pattern 12 of the first photomask 10, but at least part of the second printable dummy pattern 24 overlaps the first unprintable dummy pattern 14. Likewise, the part of the second target pattern 22 in FIG. 3 sticking out the first corresponding pattern Q can increase the luminous flux of the second target pattern 22. Besides, since the second target pattern 22 sticking out the first corresponding pattern Q can not be printed on the target material layer 42 or the wafer 40 because of the second target pattern 22 sticking out the first corresponding pattern Q being blocked off by the first material layer 32′ (used as a hard mask) in the mask 30. In other words, only the part of the second target pattern 22 overlapping the first corresponding pattern Q can be printed on the target material layer 42 or on the wafer 40.
To illustrate more clearly the wafer 40 and the mask 30 thereon in the bottom diagram of FIG. 2, FIG. 7 is presented, wherein FIG. 7 schematically depicts a cross-sectional view of a method for forming photomasks according to an embodiment of the present invention. The right diagram of FIG. 7 is a cross-sectional view along the x direction of FIG. 2, and the left diagram of FIG. 7 is a cross-sectional view along the y direction of FIG. 2.
As shown in FIG. 2 and FIG. 7, the second process P2 is performed. First, a photoresist layer (not shown) is formed to entirely cover the wafer 40 and the mask 30, and the photoresist layer (not shown) is patterned through a lithography process to form a patterned photoresist layer R2, wherein the corresponding pattern of the second target pattern 22 and the second printable dummy pattern 24 are formed on the patterned photoresist layer R2. Then, a patterned second material layer 34′′ is formed by using the patterned photoresist layer R2. And, the exposed target material layer 42 or the exposed wafer 40 is etched, and the printed pattern K is therefore formed in the wafer 40. It is emphasized that, only the second material layer 34′, the target material layer 42 or the wafer 40 can be etched by the second process P2, but the patterned first material layer 32′ can not be etched because the corresponding pattern of the first unprintable dummy pattern 14 can not be formed in the mask 30 when a suitable etching selectively material is chosen. Thus, the second printable dummy pattern 24 can only be transferred to the patterned second material layer 34′′ through the second process P2, but can not be transferred to the wafer 40 due to the patterned first material layer 32′ on the wafer 40, as shown in the left diagram of FIG. 7 and FIG. 2. In other words, only the part of the second target pattern 22 corresponding to the exposed target material layer 42 or the exposed wafer 40 can be transferred to the target material layer 42 or the wafer 40, and even if the second printable dummy pattern 24 is transferred to the photoresist layer R2, all of the second printable dummy pattern 24 will not be printed to the first material layer 32′ of the mask 30. Thus, the width of the patterned second material layer 34′′ along the y direction is w1, but the width of the target material layer 42 or the wafer 40 along the y direction shrinks to w, which is similar to the width of the patterned first material layer 32′. As shown in the right diagram of FIG. 7, the second target pattern 22 along the x direction can be entirely transferred to the target material layer 42 or the wafer 40 because the second target pattern 22 is not on the patterned first material layer 32′. Thereafter, later semiconductor processes, such as removing the patterned photoresist layer R2 and the mask 30, can be performed.
The part of the second target pattern 22 of the second photomask 20 overlapping the first corresponding pattern Q of the first photomask 10 transferred to the target material layer 42 or the wafer 40 is now complete. Besides, as the second process P2 is achieved, only the part of the second target pattern 22 of the second photomask 20 overlapping the first corresponding pattern Q of the first photomask 10 is transferred to the target material layer 42 or the wafer 40, even though the corresponding pattern of the second printable dummy pattern 24 is formed in the pattern photoresist layer R2 during the second process P2.
Thereafter, the pattern photoresist layer R2 may be removed; the patterns of the patterned second material layer 34′′ and the patterned first material layer 32′ may be transferred into the target material layer 42 or the wafer 40; and, the patterned second material layer 34′′ and the patterned first material layer 32′ are removed to form a via D so as to form a dual damascene as shown in FIG. 8. Then, metals may be filled into the via D and be planarized right after.
To summarize, the present invention provides a method for forming photomasks, which forms a patterned mask, especially forms a patterned hard mask which usually is a metal layer, having a first target pattern from a first photomask, while a first unprintable dummy pattern of the first photomask is formed neither in the photoresist nor on the patterned mask during a first lithographic and etching procedure using the first photomask; then, part of a second target pattern of a second photomask overlapping the first target pattern is transferred into the wafer, while a second printable dummy pattern on the second photomask is just formed in the photoresist without being transferred into the mask, the target material layer or the wafer during a second lithographic and etching procedure using the second photomask. By doing this, the luminous flux of the second target pattern can be increased during the exposure step through increasing the dimension of the second target pattern, and a printed pattern with a smaller size can be obtained in the wafer, thereby shrinking the exposure limit of the second target pattern.
Moreover, the first photomask of the present invention includes the first unprintable dummy pattern with a dimension smaller than the exposure limit so as to increase the luminous flux of the first target pattern during the exposure step, meaning that the first unprintable dummy pattern can not be transferred by lithographing or etching. Besides, the second photomask of the present invention includes the second printable dummy pattern with a dimension larger than the exposure limit, so as to increase the luminous flux of the second target pattern during the exposure step. Since the first target pattern of the first photomask does not overlap the second printable dummy pattern, the hard mask on the wafer will prevent the second printable dummy pattern from being printed on the wafer as an etching process is performed by a second photomask, meaning that the second printable dummy pattern can be exposed and developed but can not be transferred by etching. Moreover, even if the first photomask allows the first unprintable dummy pattern to increase the luminous flux of the first target pattern during the exposure step, the first unprintable dummy pattern still can not be formed in the mask because the dimension of the first unprintable dummy pattern is smaller than the exposure limit, such that the second printable dummy pattern overlapping the first unprintable dummy pattern still can not be printed in the wafer. In this way, the luminous flux of the second target pattern can be increased by forming the second printable dummy pattern with a dimension larger than the exposure limit next to the second target pattern, and the second printable dummy pattern can be prevented from being printed on the wafer by coupling it with the self-aligned via method, thereby shrinking the exposure limit of the second target pattern, which allows both the physical targets and the wafer layout to be achieved.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.

## Claims

1. A method for forming photomasks, comprising:
providing a first photomask comprising a first target pattern and a first unprintable dummy pattern, and the dimension of the first unprintable dummy pattern is smaller than the exposure limit; and
providing a second photomask comprising a second target pattern and a second printable dummy pattern, wherein the dimension of at least part of the second printable dummy pattern overlapping the first unprintable dummy pattern is larger than the exposure limit, such that the second printable dummy pattern can not be printed in a wafer.

2. The method for forming photomasks according to claim 1, wherein the first target pattern is printed on a mask on the wafer to form a first corresponding pattern in the mask.

3. The method for forming photomasks according to claim 2, wherein as the first corresponding pattern is formed in the mask, a corresponding part of the wafer is exposed.

4. The method for forming photomasks according to claim 2, wherein the first corresponding pattern comprises a trench pattern.

5. The method for forming photomasks according to claim 2, wherein the part of the second target pattern overlapping the first corresponding pattern is printed in the wafer so that a printed pattern is formed in the wafer, and the part of the second target pattern not overlapping the first corresponding pattern is not printed in the wafer.

6. The method for forming photomasks according to claim 5, wherein the printed pattern comprises at least a via.

7. The method for forming photomasks according to claim 5, wherein the printed pattern is in an isolation (ISO) region of the wafer.

8. The method for forming photomasks according to claim 5, wherein the printed pattern is formed by a self-aligned via method.

9. The method for forming photomasks according to claim 2, wherein the mask comprises a first material layer and a second material layer located on the first material layer.

10. The method for forming photomasks according to claim 9, wherein the first corresponding pattern is formed in the first material layer and the second material layer, the second target pattern is just transferred in the second material layer, and the part of the second target pattern overlapping the first corresponding pattern is directly printed on the wafer.

11. The method for forming photomasks according to claim 10, further comprising:
performing a first process to form the first corresponding pattern in the first material layer and the second material layer; and
performing a second process to transfer the second target pattern in the second material layer but not in the first material layer and to etch the part of the wafer corresponding to the first corresponding pattern overlapping the second target pattern to form a printed pattern in the wafer.

12. The method for forming photomasks according to claim 11, wherein the first material layer and the second material layer can be patterned by the first process and only the second material layer and the wafer can be patterned by the second process.

13. The method for forming photomasks according to claim 12, wherein the second material layer comprises an oxide layer, and the first material layer comprises a metal layer.

14. The method for forming photomasks according to claim 1, wherein the first unprintable dummy pattern is located beside the first target pattern to increase the luminous flux of the first target pattern during an exposure step.

15. The method for forming photomasks according to claim 1, wherein the first unprintable dummy pattern comprises a trench pattern.

16. The method for forming photomasks according to claim 1, wherein the second target pattern comprises a via pattern.

17. The method for forming photomasks according to claim 1, wherein the second printable dummy pattern surrounds the second target pattern.

18. The method for forming photomasks according to claim 1, wherein the second printable dummy pattern comprises a via pattern or a U-shaped pattern.

19. The method for forming photomasks according to claim 1, wherein the second mask further comprises a second unprintable dummy pattern overlapping the first target pattern.