Abstract:
A method of forming a photoresist layer free from a side-lobe is disclosed. A mask consists of a side-lobe region, a pattern region, and an intermediate region, wherein the side-lobe region is the corresponding area of the side-lobe produced in the photoresist layer, the pattern region is the corresponding area of the pattern formed in the photoresist layer, and the intermediate region is the area between the side-lobe region and the pattern region. The method characterized in that the transmittance of the side-lobe region is set lower than that of the intermediate region.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates in general to a method of forming a photoresist pattern free from side-lobe phenomenon, and more particularly to a method of forming a photoresist pattern free from side-lobe phenomenon by improving the design of the mask.  
           [0003]    2. Description of the Related Art  
           [0004]    Optical lithography has advanced in recent years in its ability to produce very small features. The most important recent development in lithography was the introduction of a photomask structure called a phase shift mask (PSM). Phase shift masks enable compensation for the diffraction effects which otherwise limit the size of the smallest features imaged by optical lithography method.  
           [0005]    Optical lithography refers generally to the technology which enables etching patterns on a substrate through use images being developed onto the substrate by a mask. Generally, the process involves directing light (such as ultraviolet light) through a photomask to expose a light-sensitive film previously deposited on the substrate. If the lightsensitive film is a so-called onto the substrate by a mask. Generally, the process involves directing light (such as ultraviolet light) through a photomask to expose a light-sensitive film previously deposited on the substrate. If the lightsensitive film is a so-called positive resist and the resist is located beneath a clear area in the photomask, the resist undergoes a physical and chemical change that renders it soluble in a development solution. This process results in the transfer of an image from the photomask to the resist film. Finally, the application of an acid to the surface of the resist film transfers the image on the resist film to the surface of the substrate.  
           [0006]    [0006]FIG. 1( a ) to FIG. 1( d ) illustrate how the mask pattern, the light transmittance, the light intensity, and the developed photoresist related to each other according to a conventional method. A mask  10 , as shown in FIG. 1 ( a ), is used to selectively expose portions of a wafer  20  covered with a photoresist layer  26  shown in FIG. 1( d ). The mask  10  includes a dark region  12 , a partial-clear region  14 , and a clear region  16 . Light  30  transmits through the mask  10  and exposes the photoresist layer  26  on the wafer  20 .  
           [0007]    The transmittance of the mask  10  is plotted in FIG. 1( b ), wherein the transmittances of the dark region  12 , the partial-clear region  14 , and the clear region  16  are 0%, 6%, and 100%, respectively.  
           [0008]    [0008]FIG. 1( c ) illustrates the intensity of the light exposed onto the photoresist layer  26 . The light exposure profile has a main peak  42  corresponding to the center of the clear region  16 . Away from the main which is indicated by the horizontal dashed line.  
           [0009]    [0009]FIG. 1( d ) illustrates the developed photoresist layer  26  after exposure by using the mask  10  of FIG. 1( a ). The undesired side-lobe hole  28  is formed in the photoresist layer  26 , along with the predetermined hole  24 . The predetermined hole  24 , which corresponds to the clear region  16  on the mask  10 , is used to create a contact hole in the layer under the photoresist layer  26 . The undesired side-lobe hole  28  causes a side-lobe phenomenon in the layer under the photoresist layer  26 , which usually impedes the process window.  
           [0010]    It is necessary to propose a method or design on the mask to suppress the side-lobe phenomenon.  
         SUMMARY OF THE INVENTION  
         [0011]    It is therefore an object of the invention to provide a method of solving the side-lobe phenomenon.  
           [0012]    The present invention provides a method to form a photoresist layer free from side-lobe. The photoresist layer is exposed with a mask. The mask consists of a side-lobe region, a pattern region, and an intermediate region, wherein the side-lobe region is the corresponding area of the side-lobe possibly produced in the photoresist layer, the pattern region is the corresponding area of the pattern formed in the photoresist layer, and the intermediate region is the area between the side-lobe region and the pattern region. The present invention suppresses the side-lobe phenomenon by setting the transmittance of the side-lobe region lower than the transmittance of the intermediate region.  
           [0013]    The side-lobe region, the pattern region, and the intermediate region are formed by etching a native mask. The native mask, the side-lobe region, the pattern region, and the intermediate region have the transmittance of 0%, 0 to 4%, 100%, and 4 to 100%, respectively. The photoresist layer can be composed of I-line photoresist material or the deep-UV photoresist material.  
           [0014]    Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1( a ) to FIG. 1( d ) illustrates the correspondence between the mask, the light transmittance, the light intensity, and the developed photoresist according to a conventional method.  
         [0016]    [0016]FIG. 2( a ) to FIG. 2( d ) illustrates the correspondence between the mask, the light transmittance, the light intensity, and the developed photoresist according to the method of the present invention.  
         [0017]    [0017]FIG. 3( a ) and FIG. 3( b ) are respectively the top view of FIG. 2( a ) and  2 ( d ). 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    The present invention provides a method to solve the side-lobe phenomenon, such that the process window of the photolithography manufacturing can be improved. FIG. 2( a ) to FIG. 2( d ) illustrate the relation between the mask, the light transmittance, the light intensity, and the developed photoresist according to the method of the present invention. FIG. 2( a ) shows the cross-sectional view of a mask  210 , which is proposed to form a plurality of contact holes disposed in a square-matrix arrangement.  
         [0019]    Referring to FIG. 2( d ), a photoresist layer  226  is coated on a wafer  220 . The photoresist layer  226  was developed according to the pattern on the mask  210  of FIG. 2( a ). The mask  210  is used to selectively expose portions of a wafer  220 , which is covered with the photoresist layer  226 . The mask  210  includes a dark region  212 , a partial-clear region  214 , and a clear region  216 , wherein the dark region  212  is located at the periphery of the mask  210 . Light  230  transmits through the mask  210  and exposes the photoresist layer  226  on the wafer  220 .  
         [0020]    The mask  210  was originally all dark region. The partial-clear region  214  and the clear region  216  of the mask  210  are formed by etching, leaving the dark regions  212 , as shown in FIG. 2( a ). The etching process is particularly designed so that the transmittance of the dark region  212 , the region  214  and the clear region  216  of the mask  210  are formed by etching, leaving the dark regions  212 , as shown in FIG. 2( a ). The etching process is particularly designed so that the transmittance of the dark region  212 , the partial-clear region  214 , and the clear region  216  are 0%, 0 to 20%, and 100%, respectively. Herein, the etching is taken as an example of forming the mask  210  with three predetermined regions. However, this invention is not limited hereto, other methods able to form the desired mask  210  of the invention are within the scope of the invention.  
         [0021]    The partial-clear region  214  further includes an outer region  214 ( o ) and an inner region  214 ( i ). The-outer region  214 ( o ) has a transmittance in a range from 4 to 20%, and the transmittance of the inner region  214 ( i ) ranges from 0 to 4%. The inner region  214 ( i ) corresponds to the site where side-lobe hole  28  (shown in FIG. 1( d )) is formed. The side-lobe hole  28  is formed owing to the diffraction effect, which is evidenced by the minor peak  46  of the intensity curve in FIG. 1( c ). According to the invention, the transmittance at the corresponding position of the side-lobe hole  28  is reduced by forming an inner region  214 ( i ) in order to impair the minor peak  46 . In this embodiment, the preferred transmittance of the outer region  214 ( o ) is about 6% transmittance, and the preferred transmittance of the inner region  214 ( i ) is about 0%.  
         [0022]    The transmittance of the whole mask  210  is plotted in FIG. 2( b ), wherein the transmittances of the dark region  212 . and the clear region  216  are about 0% and 100%, respectively. The transmittance of the inner region  214 ( i ) of the partial-clear region  214  can be about 0% and the transmittance of the outer region  214 ( o ) of the partial-clear region  214  can be about 6%.  
         [0023]    [0023]FIG. 2( c ) illustrates the intensity of the light exposed onto the photoresist layer  226 . The light exposure profile has main peaks  242 , which correspond to the clear region  216 . Away from the main peaks  242 , there are minor peaks  246  corresponding to the center of the partial-clear region  214 . According to the preferred embodiment of the present invention, the minor peaks  246  are much lower than the threshold required to trigger the photo-reaction of material for the photoresist layer  226 , which is indicated by the horizontal dashed line.  
         [0024]    [0024]FIG. 2( d ) illustrates the developed photoresist layer  226  after exposure by using the mask  210  of FIG. 2( a ). The photoresist layer  226  can be composed of I-Line photoresist material or the deep-UV photoresist material. In the photoresist layer  226 , the holes  224  for defining a contact hole are formed in a condition without side-lobe phenomenon. It is clearly shown in FIG. 2( c ) that the undersired minor peak  246  is much lower than the triggering threshold and no undesired concave is formed on the photoresist layer  226 . We can therefore conclude that, according to the present invention, the side-lobe phenomenon is successfully impeded.  
         [0025]    [0025]FIG. 3( a ) and FIG. 3( b ) are the top view of the mask  210  and the photoresist layer  226 , which correspond to FIG. 2( a ) and  2 ( d ), respectively.  
         [0026]    In FIG. 3( a ), the mask  210  has base including a partial-clear regions  214  and a dark region  212  located around the partial-clear region. The partial-clear region  214 , represented by the dash line, includes an inner region  214 ( i ) and an outer region  214 ( o ) of rectangular shape. The inner region  214 ( i ) is positioned in the center of the partial-clear region  214  and the outer region  214 ( o ) is the part of the partial-clear region  214  other than the inner region  214 ( i ). The mask  210  further includes a plurality of clear regions  216  located at the corners of the rectangular shape.  
         [0027]    In FIG. 3( b ), the hole  224 , corresponding to the clear region  216 , are also disposed in square-matrix. And the side-lobe phenomenon has been suppressed proved by the 0% transmittance of the inner region  214 ( i ).  
         [0028]    By reducing the transmittance of the mask corresponding to the position of side-lobe, the present invention successfully reduces the side-lobe phenomenon. As a result, the exposing energy for the hole  224  as well as the DOF (depth of focus) can be increased. Therefore, the process window of the photolithography is consequently improved.  
         [0029]    While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.