This application claims the priority benefit of Taiwan application serial no. 90110906, filed May 8, 2001.
1. Field of the Invention
The invention relates in general to a photolithography process, and more particularly, to a method of optical proximity correction (OPC).
2. Description of the Related Art
As the integration of semiconductor devices becoming higher and higher, the resolution of photolithography process becomes more and more demanding. The analyzable minimum dimension (R) is defined as: R=k1xcex/NA (xcex is the wavelength, and NA is the numerical aperture of the optical system). From the above equation, it is known that the larger the numerical aperture is, the higher the resolution is. The numerical aperture of the exposure optical system used in the current photolithography process is thus gradually increased.
When the numerical aperture increases over 0.7, pattern deformation is caused by the following two reasons.
1. The exposure light adopted for exposure process is the polarized light. The polarized light includes the P-polarized and S-polarize lights perpendicular to each other in electromagnetic polarization direction. When the numerical aperture is smaller than 0.7, the transmission coefficient of these two polarized lights is the same. However, when the numerical aperture reaches 0.7, the transmission coefficient of the P-polarized light is larger than that of the S-polarized light. The difference is even larger as the numerical aperture increases further.
2. For a pattern with a certain orientation, the P-polarized light and S-polarized light through the photomask cause different intensity profiles of the photoresist layer. The total intensity profile of the photoresist pattern is thus determined by a sum of the intensity profiles of the P- and S-polarized lights.
When the numerical aperture is smaller than 0.7, the transmission coefficients for the P-polarized light and the S-polarized light are the same. Whatever the orientation of the pattern is, the pitch and size of obtained photoresist pattern are not changed accordingly. However, when the numerical aperture is larger than 0.7, the transmission coefficient of the P-polarized light is larger than that of the S-polarized light, so that the pitch or size of the photoresist pattern are changed while the orientation of the pattern is changed.
FIG. 1 shows a polarization direction of P-/S-polarized light and Y-directional pattern 102 and X-directional pattern 104 on a photomask 100. The electric polarization directions of the P-polarized light and the S-polarized light are X-direction and Y-direction, respectively. The Y-directional pattern 102 has a same pitch a as that of the X-directional pattern.
In FIG. 2A, the intensity profile of a photoresist layer (not shown) caused by the Y-directional pattern 102 is shown. As shown in FIG. 2A, the Y-directional pattern 102 is in the same direction as the polarization direction of the S-polarized light, so that the distribution of the intensity profile 202s of the S-polarized light 202s is narrower than distribution of the intensity profile 202p of the P-polarized light. As a result, the integration of the intensity profile 202p is larger than that of the intensity profile 202s. That is, the total intensity profile 212 of the Y-directional pattern 102 is determined by the wider intensity profile 202p. 
In FIG. 2B, the intensity profile of the photoresist layer caused by the X-directional pattern 104 is shown. As shown in FIG. 2B, since the X-directional pattern is in the polarization direction of the P-polarized light, the distribution of intensity profile 204p of the P-polarized light is narrower than the distribution of the intensity profile 204s of the S-polarized light. On the words, since the transmission coeffient of the P-polarized light is larger than that of the S-polarized light, the integration of the intensity profile 204p is thus larger than that of the intensity profile 204s. Simply speaking, the total intensity profile 214 is determined by the intensity profile 204p with a narrower distribution.
Referring to FIGS. 2A and 2B, the total intensity profile 212 of the Y-directional pattern 102 is determined by the wider intensity profile 202p, and the total intensity profile 214 of the X-directional pattern 104 is determined by the narrower intensity profile 204p. Therefore, the total intensity profile 212 is larger than the total intensity profile 214. As a result, when a positive photoresist is used, under a certain threshold exposure intensity Eth, of the photoresist pattern pitch bX of the X-directional pattern 104 is smaller than the photoresist pattern pitch bY of the Y-directional pattern 102.
To resolve the above deviation, an optical system with a high numerical aperture is used to correction before performing the photolithography process. However, the current optical proximity correction model is designed to calculate the scalar of the incident only. The vector of the incident light (P/S polarized light) is not considered. Therefore, the difference in intensity profile caused by difference of transmission coefficient for P-/S-polarized light and pattern orientation cannot be compensated. The pitch and size of the resultant pattern is varied by the orientation change, so that deviation of different ratio occurs.
The invention provides a method of optical proximity correction, applicable to a photolithography process with a larger numerical aperture. The transmission coefficient of the P-polarized light is larger than that of the S-polarized light. According to the different pattern orientations of different patterns, different optical proximity correction model is used to correct. While correcting any pattern, the ratio of transmission coefficient of the P-polarized light to the S-polarized light, the pattern orientation of the pattern, and the polarization directions of the P-polarized light and the S-polarized light are considered.
The invention provides a method of optical proximity correction applicable to a photolithography process that employs a light source having P-polarized light and S-polarized light and a photomask comprising a plurality of patterns, wherein the P-polarized light has a transmission coefficient larger than that of the S-polarized light, and two patterns with different orientations are selected. Different optical proximity correction models are used to correct the patterns according to a ratio of the transmission coefficient between the P-polarized light and S-polarized light, the orientations of each pattern, and an angle between polarization directions of the P-polarized light and S-polarized light.
The invention provides a photolithography process. After a photoresist layer is formed on a substrate, an exposure step is performed using a photomask corrected by the above method of optical proximity correction. The photoresist layer is then developed to obtain the photoresist pattern.
In the above optical proximity correction method and the photolithography process, a hammerhead or a serif can be used for the correction. Or alternatively, the pattern linewidth can be adjusted for correction. In addition, the correction model includes either one of an optics model and an experiment model, or a combination of these two models. The former is obtained by optical algorithm to calculate, while the latter is achieved by the experimental error test-correction method.
As mentioned above, the invention considers the orientation of each pattern to adopt different optical proximity correction model. With a high numerical aperture, when the P-polarized light has a transmission coefficient larger than that of the S-polarized light, the influence by the P- and S-polarized light to each pattern with different orientation can be precisely calculated, and the correction can be performed.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.