Photomask and pattern forming method

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2012-059983, filed on Mar. 16, 2012; the entire contents of which are incorporated herein by reference.

Embodiments described herein relate generally to a photomask and a pattern forming method.

As the scaling of circuit patterns for semiconductor devices progresses, so does the scaling of mask patterns on photomasks. When the width of a mask pattern is reduced so that it is near the film thickness of the mask pattern, the focus position (best focus) at which the sensitivity to a focus shift is minimized becomes different depending on the shape of the mask pattern. Thus, it has become difficult to make the best focus uniform over the entire photomask, therefore, it has become difficult to reduce the dimension error of a resist pattern to fall within the allowable range over the entire pattern region of the photomask.

FIG. 1A is a plan view illustrating a schematic configuration of a photomask according to a first embodiment, FIG. 1B is a cross-sectional view illustrating a schematic configuration of the photomask in FIG. 1A , and FIG. 1C is a cross-sectional view illustrating a schematic configuration of resist patterns formed when exposure is performed through the photomask in FIG. 1A ;

FIG. 2 is a diagram illustrating the relationship between the dimension of a resist pattern formed when exposure is performed through the photomask in FIG. 1A and the amount of substrate defocus;

FIG. 3A is a plan view illustrating a schematic configuration of a photomask in which the film thickness of light shielding patterns is constant, FIG. 3B is a cross-sectional view illustrating a schematic configuration of the photomask in FIG. 3A , and FIG. 3C to FIG. 3E are cross-sectional views illustrating schematic configurations of resist patterns formed when exposure is performed through the photomask in FIG. 3A ;

FIG. 4 is a diagram illustrating the relationship between the dimension of a resist pattern formed when exposure is performed through the photomask in FIG. 3A and the amount of substrate defocus;

FIG. 5A is a plan view illustrating a modified example of the photomask according to the first embodiment and FIG. 5B is a cross-sectional view illustrating a schematic configuration of the photomask in FIG. 5A ;

FIG. 6A is a plan view illustrating a schematic configuration of a photomask according to a second embodiment, FIG. 6B is a cross-sectional view illustrating a schematic configuration of the photomask in FIG. 6A , and FIG. 6C to FIG. 6E are cross-sectional views illustrating schematic configurations of resist patterns formed when exposure is performed through the photomask in FIG. 6A ;

FIG. 7A is a plan view illustrating a schematic configuration of the entire wafer in which the resist patterns, which are formed when exposure is performed through the photomask in FIG. 6A , are arranged and FIG. 7B is a plan view illustrating part of the resist patterns formed on the wafer in FIG. 7A in an enlarged scale;

FIG. 8 is a diagram illustrating the relationship between the center-to-center distance of opening patterns formed in the resist pattern when exposure is performed through the photomask in FIG. 6A and substrate defocus;

FIG. 9A is a plan view illustrating a modified example of the photomask according to the second embodiment and FIG. 9B is a cross-sectional view illustrating a schematic configuration of the photomask in FIG. 9A ;

FIG. 10 is a flowchart illustrating a pattern forming method according to the third embodiment; and

FIG. 11 is a cross-sectional view illustrating a schematic configuration of a photomask according to the fourth embodiment.

In general, according to one embodiment, a photomask includes a mask substrate transparent to light, a light shielding pattern formed on the mask substrate, and a thin film portion that is provided at a part of the light shielding pattern and is thinned to have a higher light transmittance than the light shielding pattern.

A photomask and a pattern forming method according to the embodiments will be explained below with reference to the drawings. The present invention is not limited to these embodiments.

(First Embodiment)

FIG. 1A is a plan view illustrating a schematic configuration of a photomask according to the first embodiment, FIG. 1B is a cross-sectional view illustrating a schematic configuration of the photomask in FIG. 1A , and FIG. 1C is a cross-sectional view illustrating a schematic configuration of resist patterns formed when exposure is performed through the photomask in FIG. 1A .

In FIG. 1A and FIG. 1B , light shielding patterns 12 and 13 A to 13 E are formed on a mask substrate 11 . For example, a transparent substrate, such as quartz, can be used as the material for the mask substrate 11 and MoSi, Cr, or the like can be used as the material for the light shielding patterns 12 and 13 A to 13 E. The light shielding patterns 13 A to 13 E can form a periodic pattern in which the line width and the line spacing are set periodically. Examples of this periodic pattern may include a line and space. Moreover, for example, the half pitch of this periodic pattern can be set to 160 nm. Moreover, the light shielding patterns 13 A and 13 B among the light shielding patterns 13 A to 13 E can form a periodic end pattern arranged in the end portion of the periodic pattern.

The light shielding patterns 13 A and 13 B are provided with thin film portions 14 A and 14 B that are thinned to have higher light transmittance than the light shielding patterns 13 A and 13 B. The thin film portions 14 A and 14 B can be arranged at the edges of the light shielding patterns 13 A and 13 B along the longitudinal direction of the light shielding patterns 13 A and 13 B. The film thickness of the thin film portions 14 A and 14 B is set such that the phase difference between transmitted light L 1 that is transmitted through the photomask and partially-transmitted light L 2 that is partially transmitted through the thin film portions 14 A and 14 B is made different from an integral multiple of 0 degrees and 180 degrees. Particularly, a large focus shift can occur when the phase difference is set to a value near 90 degrees. For example, the film thickness of the thin film portions 14 A and 14 B can be set to ½ of the film thickness of the light shielding patterns 13 A and 13 B. Specifically, if the half pitch of the light shielding patterns 13 A to 13 E is 160 nm, the material of the light shielding patterns 13 A to 13 E is MoSi, and the wavelength of the transmitted light L 1 and the partially-transmitted light L 2 is 193 nm, the film thickness of the light shielding patterns 13 A to 13 E can be set to 70 nm and the film thickness of the thin film portions 14 A and 14 B can be set to 35 nm. When the light shielding patterns 13 A to 13 E are a halftone phase shift mask formed of MoSi, the phase difference becomes approximately 90 degrees by setting the film thickness of the thin film portions 14 A and 14 B to ½. The film thickness of the light shielding patterns 12 and 13 A to 13 E does not necessarily need to be set to completely shield the light and may be set such that approximately a few percent of the light is transmitted therethrough.

Moreover, as a method of forming the thin film portions 14 A and 14 B, it is possible to use a method of selectively etching the light shielding patterns 13 A and 13 B. In this process, for example, an electron beam defect repair apparatus (MeRiT MG) manufactured by Carl Zeiss can be used. Moreover, the film thickness and the range of the thin film portions 14 A and 14 B may be determined by using a mask for setting conditions or may be calculated by using exposure simulation. When exposure simulation is used, it is desirable to use what is called mask 3D model with which the electromagnetic field near the mask is accurately calculated while taking into consideration the three-dimensional structure of the mask. The film thickness and the range of the thin film portions 14 A and 14 B can be changed depending on the neighboring pattern layout shape, the material of a mask light shielding film, the illumination condition (illumination shape and NA of a projector lens), the state of residual aberration of a projector lens, and the like.

On the other hand, a resist film is formed on an underlying layer 21 . The underlying layer 21 may be a semiconductor substrate, a dielectric layer formed on the semiconductor substrate, or the like, and is not specifically limited thereto. After the resist film is exposed through the photomask in FIG. 1A and FIG. 1B , the resist film is developed, whereby resist patterns 22 and 23 A to 23 E corresponding to the light shielding patterns 12 and 13 A to 13 E are formed on the underlying layer 21 as shown in FIG. 10 . For this exposure, for example, an ArF immersion exposure apparatus whose NA is 1.35 and projection magnification is ¼ can be used.

The transmitted light L 1 is regularly transmitted in the periodic pattern, however, a disturbance occurs in the regularity of the transmitted light L 1 in the periodic end pattern. Under the condition that the width of the light shielding patterns 13 A to 13 E is approximately as fine as the film thickness of the light shielding patterns 13 A to 13 E, if there is a disturbance in the regularity of the transmitted light L 1 , the best focus shifts compared with a case where there is no disturbance.

When the light shielding patterns 13 A and 13 B are provided with the thin film portions 14 A and 14 B, the transmitted light L 1 and the partially-transmitted light L 2 , whose phase is shifted, interfere with each other, therefore, the best focus in the periodic end pattern can be shifted. Therefore, the best focuses in the periodic pattern and in the periodic pattern can be made close to each other, enabling the dimension error of the resist patterns 23 A to 23 E to fall within the allowable range over the entire pattern region of the photomask.

FIG. 2 is a diagram illustrating the relationship between the dimension of a resist pattern formed when exposure is performed through the photomask in FIG. 1A and the amount of substrate defocus. In FIG. 2 , P 1 indicates a dimension H 1 between the resist patterns 23 A and 23 B in FIG. 1C and P 2 indicates a dimension H 2 between adjacent resist patterns of the resist patterns 23 C to 23 E in FIG. 1C .

In FIG. 2 , it is possible to match the focus range in which the sensitivity to a focus shift of the light shielding patterns 13 A and 13 B is stable with the focus range in which the sensitivity to a focus shift of the light shielding patterns 13 C to 13 E is stable by providing the light shielding patterns 13 A and 13 B with the thin film portions 14 A and 14 B. Thus, if the dimension error allowable range of the dimensions H 1 and H 2 is A 1 , the necessary focus accuracy becomes F 1 , therefore, the error of the dimensions H 1 and H 2 can fall within the error range determined by a focus control capability F 2 of the exposure apparatus.

FIG. 3A is a plan view illustrating a schematic configuration of a photomask in which the film thickness of light shielding patterns is constant, FIG. 3B is a cross-sectional view illustrating a schematic configuration of the photomask in FIG. 3A , and FIG. 3C to FIG. 3E are cross-sectional views illustrating schematic configurations of resist patterns formed when exposure is performed through the photomask in FIG. 3A .

In FIG. 3A and FIG. 3B , light shielding patterns 32 and 33 A to 33 E are formed on a mask substrate 31 . The photomask in FIG. 3A and FIG. 3B is the same as the photomask in FIG. 1A and FIG. 1B except that the film thickness of the light shielding patterns 32 and 33 A to 33 E is constant. The light shielding patterns 33 A and 33 B can form a periodic end pattern and the light shielding patterns 33 C to 33 E can form a periodic pattern.

Then, as shown in FIG. 3C to FIG. 3E , resist patterns 42 and 43 A to 43 E corresponding to the light shielding patterns 32 and 33 A to 33 E are formed on an underlying layer 41 by using the photomask in FIG. 3A and FIG. 3B . The exposure condition at this time is the same as the exposure condition when the resist patterns 22 and 23 A to 23 E in FIG. 1C are formed.

The transmitted light L 1 is regularly transmitted in the periodic pattern, however, a disturbance occurs in the regularity of the transmitted light L 1 in the periodic end pattern. Then, if there is a disturbance in the regularity of the transmitted light L 1 , the sensitivity of the resist dimension change to a focus shift becomes high compared with a case where there is no disturbance and the best focus shifts.

Therefore, as shown in FIG. 3C to FIG. 3E , although the dimension H 4 between adjacent resist patterns of the resist patterns 43 C to 43 E does not substantially change due to occurrence of substrate defocus, the resist patterns 43 A and 43 B are tilted toward each other and the dimension H 3 between the resist patterns 43 A and 43 B changes.

FIG. 4 is a diagram illustrating the relationship between the dimension of a resist pattern formed when exposure is performed through the photomask in FIG. 3A and the amount of substrate defocus. In FIG. 4 , P 1 ′ indicates the dimension H 3 between the resist patterns 43 A and 43 B in FIG. 3C to FIG. 3E and P 2 ′ indicates the dimension H 4 between adjacent resist patterns of the resist patterns 43 C to 43 E in FIG. 3C to FIG. 3E .

In FIG. 4 , when the light shielding patterns 33 A and 33 B are not provided with the thin film portions 14 A and 14 B, a focus range R 2 in which the sensitivity to a focus shift of the light shielding patterns 33 A and 33 B is stable and a focus range R 1 in which the sensitivity to a focus shift of the light shielding patterns 33 C to 33 E is stable are different from each other. At this time, the focus range R 1 becomes sensitive to a focus shift of the light shielding patterns 33 A and 33 B. Thus, if the dimension error allowable range of the dimensions H 3 and H 4 is A 1 , the necessary focus accuracy in the dimension H 3 becomes F 3 , therefore, the error of the dimension H 3 cannot fall within the error range determined by the focus control capability F 2 of the exposure apparatus.

When the light shielding patterns 33 A and 33 B are provided with the thin film portions 14 A and 14 B, it is possible to shift the focus range R 2 in which the sensitivity to a focus shift of the light shielding patterns 33 A and 33 B is stable. Therefore, as shown in FIG. 2 , the focus range R 2 can be made to match the focus range R 1 , enabling the dimension error of the resist patterns to fall within the allowable range over the entire pattern region of the photomask.

The thin film portions 14 A and 14 B are preferably arranged along the inner edges of the light shielding patterns 33 A and 33 B when the resist patterns 43 A and 43 B tilt. The film thickness and the width of the thin film portions 14 A and 14 B may be adjusted in accordance with the tilt angle of the resist patterns 43 A and 43 B. Moreover, if the resist patterns 43 C to 43 E also tilt due to the exposure condition in addition to the resist patterns 43 A and 43 B, the light shielding patterns 33 C to 33 E may also be provided with a thin film portion.

FIG. 5A is a plan view illustrating a modified example of the photomask according to the first embodiment and FIG. 5B is a cross-sectional view illustrating a schematic configuration of the photomask in FIG. 5A .

In FIG. 5A and FIG. 5B , a similar effect can be obtained by selectively depositing a film forming material instead of forming the thin film portions by selectively etching the light shielding patterns. In this process, for example, an electron beam defect repair apparatus (MeRiT MG) manufactured by Carl Zeiss can be used. The film forming material may be the same as or different from the light shielding pattern of the mask. For example, silicon dioxide (SiO 2 ) can be used. Moreover, in this case, the thickness of the film forming material may be thinner or thicker than the thickness of the mask light shielding portion as long as it is set such that the phase difference between the transmitted light L 1 that is transmitted through the photomask and the transmitted light L 2 that is transmitted or partially transmitted through thin film portions 15 A and 15 B is made different from an integral multiple of 0 degrees and 180 degrees.

(Second Embodiment)

FIG. 6A is a plan view illustrating a schematic configuration of a photomask according to the second embodiment, FIG. 6B is a cross-sectional view illustrating a schematic configuration of the photomask in FIG. 6A , and FIG. 6C to FIG. 6E are cross-sectional views illustrating schematic configurations of resist patterns formed when exposure is performed through the photomask in FIG. 6A . FIG. 7A is a plan view illustrating a schematic configuration of the entire wafer in which the resist patterns, which are formed when exposure is performed through the photomask in FIG. 6A , are arranged and FIG. 7B is a plan view illustrating part of the resist patterns formed on the wafer in FIG. 7A in an enlarged scale.

In FIG. 6A and FIG. 6B , a light shielding pattern 52 A is formed on a mask substrate 51 . Then, opening patterns 53 A and 53 B are formed in the light shielding pattern 52 A, thereby forming a light shielding pattern 52 B between the opening patterns 53 A and 53 B. For example, a transparent substrate, such as quartz, can be used as the material for the mask substrate 51 and MoSi, Cr, or the like can be used as the material for the light shielding patterns 52 A and 52 B.

The light shielding pattern 52 B is provided with thin film portions 54 A and 54 B that are thinned to have higher light transmittance than the light shielding pattern 52 B. The thin film portions 54 A and 54 B can be arranged such that the light shielding pattern 52 B becomes more sensitive to a focus shift than the light shielding pattern 52 A. The thin film portions 54 A and 54 B can be arranged to face each other with the light shielding pattern 52 B therebetween. The film thickness of the thin film portions 54 A and 54 B is set such that the phase difference between the transmitted light L 1 that is transmitted through the photomask and the partially-transmitted light L 2 that is partially transmitted through the thin film portions 54 A and 54 B is made different from an integral multiple of 0 degrees and 180 degrees. Particularly, a large focus shift can occur when the phase difference is set to a value near 90 degrees. For example, the film thickness of the thin film portions 54 A and 54 B can be set to ½ of the film thickness of the light shielding pattern 52 B. The film thickness of the light shielding patterns 52 A and 52 B does not necessarily need to be set to completely shield the light and may be set such that approximately a few percent of the light is transmitted therethrough.

On the other hand, a resist film is formed on an underlying layer 61 . The underlying layer 61 may be a semiconductor wafer W in FIG. 7A , a dielectric layer formed on the semiconductor wafer W, or the like, and is not specifically limited thereto. After the resist film is exposed through the photomask in FIG. 6A and FIG. 6B , the resist film is developed, whereby resist patterns 62 A and 62 B corresponding to the light shielding patterns 52 A and 52 B are formed on the underlying layer 61 as shown in FIG. 6C to FIG. 6E . As shown in FIG. 6C , opening patterns 63 A and 63 B are formed in the resist pattern 62 A, whereby the resist pattern 62 B is formed between the opening patterns 63 A and 63 B.

In the light shielding pattern 52 A in which the opening patterns 53 A and 53 B are formed, the focus range is set to the focus range in which the sensitivity to a focus shift is stable. At this time, if the light shielding pattern 52 B is provided with the thin film portions 54 A and 54 B, in the light shielding pattern 52 B, the focus range shifts to the focus range that is sensitive to a focus shift. Therefore, if defocus of the wafer occurs, although the position of the edge of the resist pattern 62 A does not substantially change, the edge of the resist pattern 62 B changes and a center-to-center distance HD of the opening patterns 63 A and 63 B changes.

FIG. 8 is a diagram illustrating the relationship between the center-to-center distance of the opening patterns formed in the resist pattern when exposure is performed through the photomask in FIG. 6A and defocus of the wafer.

In FIG. 8 , it is found that the amount of change of the center-to-center distance HD of the opening patterns 63 A and 63 B is substantially proportional to defocus of the wafer. Therefore, the amount of defocus (magnitude and direction) of the wafer can be obtained by measuring the center-to-center distance HD of the opening patterns 63 A and 63 B. The center-to-center distance HD may be measured by using an optical measurement instrument or may be measured by using a scanning electron microscope (SEM). When the amount of defocus of the wafer is obtained, the exposure condition is adjusted so that the amount of defocus of the wafer approaches zero, therefore the dimension accuracy of the resist patterns can be improved in the subsequent wafer exposure. If the center-to-center distance HD is measured at a plurality of locations on the semiconductor wafer W and is converted into the amount of defocus of the wafer, a focus map of the entire surface of the semiconductor wafer W can be generated.

In the focus control of the exposure apparatus, the surface shape of the wafer is measured by using autofocus light and the position and posture of a wafer stage are controlled based on the measurement result to set the height of the wafer to the best focus surface of a projector lens. At this time, the focus accuracy is deteriorated because of the change of the best focus surface of the projector lens due to the change in temperature and atmospheric pressure, the focus correction residue due to irregularities on the wafer surface, the focus illusion due to the reflectance distribution in the layer lower than the resist layer of the wafer, and the like. In order to improve the focus accuracy, it is effective to obtain the focus map of the actually exposed wafer and feed back the focus map to the wafer stage control of the exposure apparatus in addition to improving the exposure apparatus. In order to obtain this focus map, a pattern (focus monitor mark) sensitive to the amount of defocus of the wafer needs to be transferred onto the wafer. Furthermore, it is desirable to use the focus monitor mark with which the direction of focus (for example, direction in which the wafer approaches the projector lens is plus and direction in which the wafer is away from the projector lens is minus) can be recognized. The photomask in FIG. 6A and FIG. 6B can be used for generating such a focus monitor mark. This focus monitor mark may be generated as a TEG pattern on a scribe line of the wafer W or the like.

FIG. 9A is a plan view illustrating a modified example of the photomask according to the second embodiment and FIG. 9B is a cross-sectional view illustrating a schematic configuration of the photomask in FIG. 9A .

In FIG. 9A and FIG. 9B , a similar effect can be obtained by selectively depositing a film forming material instead of forming the thin film portions by selectively edging the light shielding pattern. The film forming material may be the same as or different from that of the light shielding pattern of the mask. For example, silicon dioxide (SiO 2 ) can be used. Moreover, in this case, the thickness of the film forming material may be thinner or thicker than the thickness of the mask light shielding portion as long as it is set such that the phase difference between the transmitted light L 1 that is transmitted through the photomask and the transmitted light L 2 that is transmitted or partially transmitted through deposited portions 55 A and 55 B is made different from an integral multiple of 0 degrees and 180 degrees.

(Third Embodiment)

FIG. 10 is a flowchart illustrating a pattern forming method according to the third embodiment. In FIG. 10 , resist patterns used as a focus monitor mark are formed on the wafer by using the photomask in FIG. 6A and FIG. 6B (S 1 ). Next, the dimension of this focus monitor mark is measured (S 2 ). Then, the amount of defocus of the wafer is obtained based on this dimension measurement result (S 3 ). At this time, for example, the amount of defocus of the wafer can be obtained by referring to the relationship in FIG. 8 . Next, focus of exposure is adjusted based on this amount of defocus (S 4 ).

(Fourth Embodiment)

FIG. 11 is a cross-sectional view illustrating a schematic configuration of a photomask according to the fourth embodiment.

In FIG. 11 , a multilayer reflective film 71 B is formed on a mask substrate 71 A. Light absorbing patterns 72 and 73 A to 73 E are formed on the multilayer reflective film 71 B. For example, a substrate, such as quartz, can be used as the material for the mask substrate 71 A, a stacked structure of Mo and Si can be used as the material for the multilayer reflective film 71 B, and oxide, such as TaBO, or nitride, such as TaBN, can be used as the material for the light absorbing patterns 72 and 73 A to 73 E. The light absorbing pattern 73 A to 73 E can form a periodic pattern in which the line width and the line spacing are set periodically, and examples of this periodic pattern may include a line and space. Moreover, the light absorbing patterns 73 A and 73 B among the light absorbing pattern 73 A to 73 E can form a periodic end pattern arranged in the end portion of the periodic pattern.

The light absorbing patterns 73 A and 73 B are provided with thin film portions 74 A and 74 B that are thinned to have higher reflectance than the light absorbing patterns 73 A and 73 B. The thin film portions 74 A and 74 B can be arranged at the edges of the light absorbing patterns 73 A and 73 B along the longitudinal direction of the light absorbing patterns 73 A and 73 B. The film thickness of the thin film portions 74 A and 74 B is set such that the phase difference between reflected light LO 1 reflected from the photomask with respect to incident light LI and partially-reflected light LO 2 partially reflected from the thin film portions 74 A and 74 B is made different from an integral multiple of 0 degrees and 180 degrees. Particularly, a large focus shift can occur when the phase difference is set to a value near 90 degrees. For example, the film thickness of the thin film portions 74 A and 74 B can be set to ½ of the film thickness of the light absorbing patterns 73 A and 73 B. The film thickness of the light absorbing patterns 72 and 73 A to 73 E does not necessarily need to be set to completely absorb the light and may be set such that approximately a few percent of the light is reflected. Moreover, as the incident light LI, for example, extreme ultraviolet light with a wavelength of 13.5 nm can be used.

The reflected light LO 1 regularly reflects in the periodic pattern, however, a disturbance occurs in the regularity of the reflected light LO 1 in the periodic end pattern. Then, if there is a disturbance in the regularity of the reflected light LO 1 , the sensitivity to a focus shift increases compared with a case where there is no disturbance and the best focus shifts.

When the light absorbing patterns 73 A and 73 B are provided with the thin film portions 74 A and 74 B, the partially-reflected light LO 2 can interfere with the reflected light LO 1 , therefore, it is possible to shift the focus position at which the sensitivity to a focus shift of the periodic end pattern is stable. Therefore, the focus positions, at which the sensitivity to a focus shift is stable, can be made close to each other in the periodic pattern and the periodic end pattern, enabling the dimension error of the resist patterns to fall within the allowable range over the entire pattern region of the photomask.

In the example in FIG. 11 , explanation is given for the method of providing the thin film portions 74 A and 74 B thinned to have higher reflectance than the light absorbing patterns 73 A and 73 B as an adjusting portion adjusted to have a lower optical absorptance than the light absorbing patterns 73 A and 73 B, however, a material having a lower optical absorptance than the light absorbing patterns 73 A and 73 B may be used.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.