Abstract:
A mask and a method of forming the mask obviate optical proximity effects. The mask includes a light-shielding layer on a transparent substrate. The light-shielding layer is patterned to form a main pattern and a phantom pattern. The main and phantom patterns each have a light shielding portion and a light-transmitting portion. The pitch of the features constituting the phantom pattern is identical to the pitch of the features constituting the main pattern. The shape of the light-transmitting features of the phantom pattern region is identical to the shape of the light-transmitting features of the main pattern region.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the process of photolithography used in fabricating semiconductor devices and the like. More specifically, the present invention relates to a mask used to carry out a photolithographic process and to a method for forming the same.  
         [0003]     2. Description of the Related Art  
         [0004]     Photolithography is a known process essential to the fabricating of memory devices such as DRAMs, SRAMs and flash memory devices. Photolithography comprises pattern transcription from mask to resist. More specifically, the pattern transcription is carried out by arranging a mask over a resist layer on a wafer, and exposing the resist to light of a specific wavelength through the mask. As a result, the pattern of the mask is transferred to the resist. Ideally, the pattern of the mask is transcribed accurately onto the resist.  
         [0005]     However, photolithography is prone to pattern transference problems referred to as “optical proximity effects.” Optical proximity effects occur when forming the very fine patterns necessary to produce a highly integrated circuit. The light waves passing through the closely spaced pattern features of the mask produce interference, thereby distorting the final transferred pattern. These distortions manifest themselves as variations in the dimensions of the patterned resist or as a rounding of ends of the patterns. The optical proximity effects become more pronounced the finer the pattern features are and the thicker the resist is.  
         [0006]     When optical proximity effects occur, the dimensions of dense and fine patterns transferred to the wafer are different from each other even though the dimensions of the corresponding features of the mask pattern are the same. In some instances, pattern features are not transferred at all from the mask to the wafer. For example, as shown in  FIG. 1 , feature  10  at the edge of the pattern is smaller than the same features of the main part of the pattern. As illustrated in  FIG. 2 , the intensity of the exposure light at the edge α of the pattern is lower than that of the light used to expose the main part of the pattern adjacent the edge α.  
         [0007]     A number of techniques have been studied in an attempt to overcome the significant problems of optical proximity effects.  
         [0008]     One approach, as is illustrated in  FIG. 3 , is to design the mask so that the feature  20  at the edge of the pattern is enlarged by an amount considered beforehand, whereby the features of the resultant pattern all have approximately the same dimensions. However, this approach can still be subject to problems resulting from interference such that such as incomplete patterns are formed.  
         [0009]     Another approach, shown in  FIG. 4 , is to form a line and space type of auxiliary pattern  30  around a main pattern (desired pattern) on the mask. As is illustrated in  FIG. 5 , the intensity of light transmitted through the region E of the mask containing the auxiliary pattern region is lower than that of light transmitted through the region M containing the main pattern. Specifically, the intensity of the light transmittable through the auxiliary pattern region E is less than the critical intensity (0.20 in this example) required for the patterning of the resist, i.e., required for facilitating the photochemical reaction in the resist.  
         [0010]     However, referring to  FIG. 6 , the change in light intensity at the edge II of the pattern is larger than change in light intensity at the main part I of the pattern. This means that the focus margin at the edge part II is lower than the focus margin at the main part I.  
         [0011]     Referring to  FIG. 7 , complete patterning is obtained in the main part of the pattern within a range of focus of −0.1 to +0.3 μm, and analogous patterning is obtained outside this range of focus. However, complete pattering is obtained in the edge part of the pattern only within a range of focus of from 0.0 to +0.1 μm, and incomplete patterning occurs outside this range of focus of 0.0 to +0.1 μm. Thus, securing a range of focus acceptable for all of the photolithographic processes necessary to fabricate a semiconductor device revisions requires revising a mask having a line and space pattern. That is, a number of such masks are required, which incurs increased manufacturing costs.  
       SUMMARY OF THE INVENTION  
       [0012]     An object of the present invention is to overcome the above-mentioned problems of the prior art.  
         [0013]     In particular, one object of the present invention to provide a mask and a method of forming the same that effectively eliminate optical proximity effects.  
         [0014]     According to one aspect of the present invention, the mask comprises a main pattern corresponding to the desired pattern for transcription, and a phantom pattern provided alongside the main pattern at a peripheral region of the mask to minimize the intensity of light at the edge of the transcribed pattern. The shape and pitch of respective features of the phantom pattern are identical to those of the main pattern, respectively. However, the light-transmitting features of the phantom pattern are under-sized (narrower) compared to those of the main pattern and/or the light-transmitting features of the phantom pattern scatter incident light to such an extent that the light transmitted by the portion of the mask containing the phantom pattern will not expose the resist, i.e., will not facilitate the patterning of the wafer.  
         [0015]     According to another aspect of the present invention, a method of forming the mask comprises steps of: providing a transparent substrate, forming a light-shielding layer on the transparent substrate, and patterning the light-shielding layer to form the main pattern and phantom pattern.  
         [0016]     The main pattern and the phantom pattern each comprise portions of the light-shielding layer that absorb the exposure light and thereby shield the substrate, and exposed portions of the transparent substrate that transmit the exposure light. The patterning is carried out so that the pitch of the light-shielding and exposed portions constituting the phantom pattern are identical to the pitch of the light-shielding and exposed portions constituting the main pattern, and so that the shape of the exposed portions of the substrate constituting the phantom pattern are identical to the shape of the exposed portions of the substrate constituting the main pattern.  
         [0017]     The exposed portions of the substrate constituting the phantom pattern may be the same size as the exposed portions of the substrate constituting the main pattern. In this case, part of the transparent substrate is removed, i.e., etched way, at each of the exposed portions of the substrate constituting the phantom pattern so that light is scattered by the sidewalls of the substrate defining the resulting trenches. Alternatively, the exposed portions of the substrate constituting the phantom pattern may be narrower than the exposed portions of the substrate constituting the main pattern. In this case as well, each of the exposed portions of the substrate can be etched to form trenches in the substrate.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     These and other objects, features and advantages of the present invention will become better understood from the following detailed description of the preferred embodiments made with reference to the accompanying drawings, of which:  
         [0019]      FIG. 1  is a photograph of a wafer patterned using a conventional mask;  
         [0020]      FIG. 2  is a graph of the profile of light intensity when carrying out an exposure process using the conventional mask;  
         [0021]      FIG. 3  is a photograph of a wafer patterned using another conventional type of mask;  
         [0022]      FIG. 4  is a top view of the conventional mask used in the process shown in  FIG. 3 ;  
         [0023]      FIG. 5  is a graph of a profile of light intensity when carrying out an exposure process using the conventional mask shown in  FIG. 4 ;  
         [0024]      FIG. 6  shows a correlation between light intensity and focus at the main and edge parts of the pattern formed using the conventional mask shown in  FIG. 4 ;  
         [0025]      FIG. 7  shows main and edge parts of a pattern formed using the conventional mask shown in  FIG. 4 ;  
         [0026]      FIGS. 8 and 9  are sectional views of a substrate illustrating a first embodiment of a method of forming a mask according to the present invention;  
         [0027]      FIG. 10  is a top view of the mask;  
         [0028]      FIG. 11  is a graph of a profile of light intensity when carrying out an exposure process using the mask according to the present invention;  
         [0029]      FIG. 12  shows a correlation between light intensity and focus at the main and edge parts of the pattern formed using the first embodiment of the mask according to the present invention;  
         [0030]      FIGS. 13 through 15  are sectional views of a substrate illustrating a method of forming a second embodiment of a mask according to the present invention; and  
         [0031]      FIGS. 16 through 18  are sectional views of a substrate illustrating a third embodiment of a method of forming a mask according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     The First Embodiment  
         [0033]     Referring to  FIGS. 8 and 9 , the mask  120  comprises a transparent substrate  100 , such as a quartz substrate. The mask is divided into a main pattern region A 1  and a phantom pattern region B 1 . A light shielding layer  110  is formed on the transparent substrate  100 . The light shielding layer  110  comprises a material selected from the group consisting of Cr and Mo. Furthermore, the light shielding layer  110  is also formed of a low reflective material, e.g., a polymer.  
         [0034]     Referring to  FIG. 9 , the light shielding layer  110  is patterned to form a main pattern consisting of a main light-shielding portion  112   a  and a main light-transmitting portion  112   b  in the main pattern region A 1 . The main pattern refers to the pattern that is desired to be transferred to the wafer. The shape of the features of and pitch P A1  of the main pattern depend on the pattern desired to be transferred to the wafer.  
         [0035]     Simultaneously, a phantom pattern comprising a phantom light-shielding portion  114   a  and a phantom light-transmitting portion  114   b  is formed in phantom pattern region B 1  at the periphery of the main pattern region A 1 . The phantom pattern refers to a pattern that should not be transferred to the wafer. However, the shape of the features of and pitch P B1  of the phantom pattern are identical to those of the main pattern, respectively. On the other hand, the line widths of the main and phantom light-transmitting portions  112   b  and  114   b  depend on the design of the pattern, etc.  
         [0036]     The phantom light-transmitting portion  114   b  is under-sized as compared to the main light-transmitting portion  112   b . The phantom light-transmitting portion  114   b  is so narrow that intensity of light which can be transmitted through the phantom light-transmitting portion  114   b  is less than the critical intensity. In other words, the intensity of light that is transmittable through the main light-transmitting portion  112   b  is great enough to facilitate the patterning of a resist exposed to such light, but the intensity of light that is transmittable through the phantom light-transmitting portion  114   b  can not facilitate the patterning of the same resist.  
         [0037]     Referring to  FIG. 10 , the phantom light-transmitting portion  114   b  in the phantom pattern region B 1  comprises features that are narrower than the features making up the main light-transmitting portion  112   b  in the main pattern region A 1 . However, the shape and pitch of the light-transmitting features in the phantom pattern region B 1  are identical to those of the light-transmitting features in the main pattern region A 1 , respectively.  
         [0038]      FIG. 11  shows the results of an exposure process using the mask  120  having the main pattern region A 1  and the phantom pattern region B 1 . In this case, the critical intensity of the light for patterning the resist was 0.2. As can be seen from the figure, the intensities of the wavefronts of the light transmitted through the main pattern region A 1  are all over the critical intensity, and the intensities of the wavefronts of the light transmitted through phantom pattern region B 1  are all under the critical intensity. Therefore, a complete patterning is carried out by light transmitted through the main pattern region A 1  but no patterning occurs in that portion of the photoresist exposed to the light transmitted through the phantom pattern region B 1 .  
         [0039]     Also, referring back to  FIG. 2 , the intensity of light transmitted from the conventional mask at the periphery of the mask pattern is so close to the critical intensity that a complete patterning does not occur, i.e., the intensity of the light at the edge α of the exposure region of the resist is not great enough to produce a coherent feature. On the other hand, the intensity of light transmitted from the mask  120  of the present invention, at the edge of the main pattern region A 1 , is sufficiently high compared to critical intensity, as to form a complete and coherent feature at the corresponding peripheral part α′ of the exposed field of the resist. This is because optical proximity effects at the edge part α′ are suppressed. Accordingly, light transmitted through the main pattern region A 1  facilitates a complete patterning.  
         [0040]     Referring to  FIG. 12 , with respect to the main pattern region A 1 , there is not much variation in the intensity in the edge part (II) of the main pattern that adjoins the phantom pattern region B 1  and variation from the maximum intensity in the main part (I) of the main pattern surrounded the edge part (II). Accordingly, a relatively wide focus margin is secured for the edge part (II) of the main pattern.  
         [0041]     The Second Embodiment  
         [0042]     Referring to  FIGS. 13-15 , the mask  220  comprises a transparent substrate  200  and a light shielding layer  210  formed on the transparent substrate  200 . The mask  220  is divided into a main pattern region A 2  and a phantom pattern region B 2 .  
         [0043]     Referring to  FIG. 14 , the light shielding layer  210  is patterned to form a main pattern comprising a main light-shielding  212   a  portion and a main light-transmitting portion  212   b  in main pattern region A 2 , and a phantom pattern comprising a phantom light-shielding portion  214   a  and a phantom light-transmitting portion  214   b  in phantom pattern region B 2 . The shape and pitch of the features of the phantom pattern in phantom pattern region B 2  are identical to those of the main pattern in main pattern region A 2 , respectively. However, the pitch P A1  of the features of the main pattern in main pattern region A 2  and the pitch P B1  of the features of the phantom pattern in the phantom pattern region B 2  depending on the design of the pattern to be transcribed onto the wafer.  
         [0044]     Referring to  FIG. 15 , a part of the transparent substrate  200  corresponding to the phantom light-transmitting portion  214   b  is removed by an etching process. This etching process forms a trench-type of phantom light-transmitting portion  214   c  in the transparent substrate  200 . Light scattering occurs at sidewalls of the substrate  200 , defining the sides of the trench-type of phantom light-transmitting portion  214   c . The intensity of the light transmitted through the phantom light-transmitting portion  214   c  becomes less than the intensity of light that is transmittable through the main light-transmitting portion  212   b . In this respect, the intensity of light that can be transmitted through the phantom light-transmitting portion  214   c  should be less than the critical intensity. To this end, the etching process is performed until the trenches in the substrate  200  are so deep that the light which will be transmitted through the phantom light-transmitting portion  214   c  will not to facilitate the patterning of the resist exposed to such light. This may be accomplished by satisfying the following equation: 
 
 d=λ/ 2( n− 1) 
 
 (wherein d is the depth of the trench in the substrate, λ is the wavelength of the transmitted light, n is the refractive index of the mask substrate). 
 
         [0046]     The light scattering phantom light-transmitting portion  214   b  acts like the under-sized of phantom light-transmitting portion  212   b  of the first embodiment. Therefore, the same results occur when using the first and second embodiments of the mask to pattern a wafer.  
         [0047]     The Third Embodiment  
         [0048]     Referring to  FIGS. 16-18 , a light shielding layer  310  is formed on a transparent substrate  300 . The mask  320  is divided into a main pattern region A 3  and a phantom pattern region B 3 .  
         [0049]     Referring to  FIG. 17 , the light shielding layer  310  is patterned to form a main pattern comprising a main light-shielding portion  312   a  and a main light-transmitting portion  312   b  in main pattern region A 3 . The exact shape and pitch P A3  of the features constituting the main pattern depend on the design of the pattern to be transcribed onto the wafer.  
         [0050]     Simultaneously, a phantom pattern comprising a phantom light-shielding portion  314   a  and a phantom light-transmitting portion  314   b  is formed in the phantom pattern region B 3  at the periphery of the main pattern region A 3 . The pitch P B3  of the features of the phantom pattern is identical to that of the features of the main pattern. However, the features constituting the phantom light-transmitting portion  314   b  are smaller (narrower) than those constituting the main light-transmitting portion  312   b . The features of the phantom light-transmitting portion  314   b  are so narrow that the intensity of light transmitted through the phantom light-transmitting portion  314   b  is less than the critical intensity of the resist. In other words, the intensity of the light transmitted through the main light-transmitting portion  312   b  is great enough to facilitate the patterning of the resist, but the intensity of the light transmitted through the phantom light-transmitting portion  314   b  is not great enough to do so. The widths of the features constituting the main and phantom light-transmitting portions  312   b  and  314   b  are set based on the design of the pattern to be transcribed onto the wafer.  
         [0051]     Referring to  FIG. 18 , a part of the transparent substrate  300  corresponding to the phantom light-transmitting portion  314   b  is removed by an etching process. This etching process forms trenches in the transparent substrate  300 . Light scattering occurs at sidewalls of the trenches that define the features of the phantom light-transmitting portion  314   c.    
         [0052]     The intensity of light transmitted through the phantom light-transmitting portion  314   c  is less than that of the intensity of light transmitted through the main light-transmitting portion  312   b  due to the scattering of the light at the sidewalls of the substrate  300 . In addition, the features constituting the phantom light-transmitting portion  314   b  are narrower than those constituting the main light-transmitting portion  312   b . Therefore, the entirety of the main pattern can be transcribed onto a wafer using the mask  320 .  
         [0053]     The phantom pattern according to the present invention is applicable to all masks used in photolithography, such as binary masks, halftone attenuated phase shift masks and chrome-less masks. Furthermore, the present invention is applicable to not only positive type photolithographic processes but also to negative type photolithographic processes.  
         [0054]     As described above, the present invention provides a mask in which the variations in intensity and focus margin at the edge of the transcribed pattern are improved. Also, the present invention makes it easy to predict the outcome of simulations of the photolithographic process, especially at the edge of the transcribed pattern. Thus, the number of revisions of the mask are few throughout the course of the photolithographic process—from design to completion.  
         [0055]     Finally, the present invention was described above in connection with the preferred embodiments thereof. However, those of ordinary skill in the art will recognize that many modifications and variations are possible in light of the above teachings. Therefore, the present invention is not limited to the preferred embodiments. Rather, the true spirit and scope of the invention is defined by the appended claims.