Patent Publication Number: US-2002001919-A1

Title: Method of forming partial reverse active mask

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
     [0001] This application claims priority benefit of Taiwan application Serial no. 87105966, filed Apr. 18, 1998, the full disclosure of which is incorporated herein by reference. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The invention relates to a chemical-mechanical polishing (CNP) method applied in shallow trench isolation (STI), and more particular, to a chemical-mechanical polishing method incorporated with a partial reverse active mask applied in shallow trench isolation.  
       [0004] 2. Description of the Related Art  
       [0005] For a very large scale integration (VLSI) or even an ultra large scale integration (ULSI), chemical-mechanical polishing is the only technique that provides global planaration. Since this technique greatly reduces feature size of an integrated circuit, the manufacturers rely on this technique for planarization process. A great interest to further develop this technique is evoked for further reduction in feature size and fabrication cost.  
       [0006] As the dimension of semiconductor devices becomes smaller and smaller, deep sub-half micron technique, for example, a line width of 0.25 μm, or even 0.8 μm, is used. To planarize the wafer surface by chemical-mechanical polishing, especially to planarize the oxide layer within in a trench, becomes more and more important. To prevent the formation of a recess on the surface of the oxide layer within a shallow trench isolation of a larger area, a reverse tone active mask is used in process. An etch back process is also performed to obtain a better chemical-mechanical polishing uniformity. However, a misalignment often occurs.  
       [0007] In a conventional process of forming a shallow trench isolation, since the active regions have different dimensions, the dimensions of shallow trench between active regions are different. In FIG. 1A to FIG. 1E, a cross sectional view of the process for forming a shallow trench isolation by chemical-mechanical polishing is shown. In FIG. 1A, a pad oxide layer  15  and a silicon nitride layer  16  are formed on a substrate  10 . Using photolithography and anisotropic etching, a shallow trench  14  and an active region  12  are formed. The dimensions of the shallow trench  14  are various according to the various dimensions of the active region  12 .  
       [0008] In FIG. 1B, using atmosphere pressure chemical vapor deposition (APCVD), an oxide layer  18  is formed over the substrate  10  and fills the shallow trench  14 . Due to the topography of the shallow trench  14  within the substrate  10  and the characteristics of step coverage of the oxide layer  18 , the surface of the deposited oxide layer  18  is undulating but smooth. A photo-resist agent is coated on the oxide layer  18 . Using photolithography, a reverse tone active mask  20  is formed. The reverse tone active mask  20  covers the surface of the shallow trench  14  and becomes complementary to the active regions  20 . It is known that during the formation of the reverse tone mask  20 , a misalignment often occurs. Consequently, the reverse tone active mask  20  covers a range of the oxide layer  18  beyond the shallow trench  14 .  
       [0009] In FIG. 1C, the exposed part of the oxide layer  18 , that is, the part which is not covered by the oxide layer  18 , is etched away until the silicon nitride layer  16  is exposed. The resultant structure of the oxide layer is denoted as  18   a . As shown in the figure, the oxide layer  18   a  covers most of the shallow trench  14  and a small part of the silicon nitride layer  16  on the active region . In FIG. 1D, the reverse tone active mask  20  is removed. It is found that a recess  22  is formed since the oxide layer  18   a  does not covered the shallow trench  14  completely.  
       [0010] In FIG. 1E, the oxide layer  18   a  is polished by chemical-mechanical polishing until the oxide layer  18   a  has a same level as the silicon nitride layer  16 . Since the oxide layer  18   a  formed by APCVD has a smooth profile, so that it is difficult to be planarized. In addition, it is obvious that the recess  22  is formed since the oxide layer  18   a  does not fill the shallow trench  14  completely. A kink effect is thus easily occurs by the recess  22 . That is, a current leakage or a short circuit is caused. The yield of the wafer is affected.  
       SUMMARY OF THE INVENTION  
       [0011] It is therefore an object of the invention to provide a method of forming a shallow trench isolation by chemical-mechanical polishing incorporating a high density plasma chemical vapor deposition (HDP-CVD) with a partial reverse active mask. The shallow trench isolations have various dimensions in accordance with the dimensions of the active regions therebetween. An oxide layer formed by HDP-CVD has a pyramid-like profile on the active region. Therefore, this oxide layer is easier to be planarized by chemical-mechanical polishing than an oxide layer form by conventional APCVD. The central part of an oxide layer on an active region of a large area is removed. Whereas the oxide layer on an active region of a small area is remained. A uniformity is thus obtained for chemical-mechanical polishing. Consequently, the recess and misalignment caused by reverse tone effect are avoided.  
       [0012] To achieve these objects and advantages, and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention is directed towards a method of forming a partial reverse active mask. A mask pattern comprising a large active region pattern with an original dimension and a small active region pattern is provided. The large active region pattern and the small active region pattern are shrunk until the small active region pattern disappears. The large active region pattern enlarged to a dimension slightly smaller than the original dimension.  
       [0013] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014]FIG. 1A to FIG. 1E are cross sectional views, on which a conventional method of forming a shallow trench isolation by reverse tone active mask is shown;  
     [0015]FIG. 2A to FIG. 2E are cross sectional views, on which a method of forming a shallow trench isolation by partial reverse active mask according to the invention is shown; and  
     [0016]FIG. 3A to FIG. 3D show a method of forming a partial reverse active mask according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0017] In the invention, using HDP-CVD incorporating with partial reverse active mask and chemical-mechanical polishing, a shallow trench isolation is formed. The formation of a recess due to misalignment of reverse tone active mask and a short circuit or a leakage current caused by a kink effect caused are avoided.  
     [0018] In FIG. 2A to FIG. 2E, a method of forming a shallow trench isolation in a preferred embodiment according to the invention is shown. In FIG. 2A, active regions  42   a ,  42   b ,  42   c  and  42   d  are formed on a substrate  40 . A pad oxide layer  45  and a silicon nitride layer  46  are formed on the substrate  40 . Using photolithography and etching, the pad oxide layer  45 , the silicon nitride layer  46 , and a part of the substrate  40  are defined to form a shallow trench  44  between the active regions  42   a ,  42   b ,  42   c  and  42   d . The dimension of the shallow trench  44  is variable corresponding to the active regions  42   a    42   b ,  42   c  and  42   d . In FIG. 2B, using HDP-CVD, an oxide layer  48  is formed over the substrate  40 . Due to the shallow trench  44 , the oxide layer  48  formed by HDP-CVD has a profile, of which a pyramid-like structure is formed on the active regions  42 .  
     [0019] In FIG. 2C, a photo-resist layer is formed on the oxide layer  48 . Using photolithography and etching, the photo-resist layer is defined into a partial reverse active mask  50 . In addition, an opening  52  formed on a large active region  42   a  to expose the oxide layer  48  thereon. Since only the oxide layer  48  on the central part of the active region  42   a  is exposed within the opening  52 , even a misalignment occurs to cause a shift of the partial reverse active mask  50 , the oxide layer  46  on the shallow trench  44  is not exposed.  
     [0020] In FIG. 2D, the exposed oxide layer  48  within the opening  52  is etched back until the silicon nitride layer  46  is exposed. The partial reverse active mask  50  is stripped. The remaining oxide layer on the small active region  42   b ,  42   c  and  42   d  is denoted as oxide layer  48   b , whereas the remaining oxide layer on the large active region  42   a  is denoted as  48   a . As mentioned above, the oxide layer  48  is formed by HDCVD, so that the remaining oxide layer  48   a  and  48   b  tend to have a pyramid-like profile.  
     [0021] In FIG. 2E, using chemical-mechanical polishing, the oxide layer  48   b  and the oxide layer  48   a  are planarized with the silicon nitride layer  46  as an etch stop, so that the oxide layer  48  within the shallow trench  44  has a same level as the silicon nitride layer  46 .  
     [0022] In the above embodiment, a partial reverse active mask is employed for forming a shallow trench isolation. In FIG. 3A to FIG. 3D, a method of forming a partial reverse active mask is shown. As shown in FIG. 3A, to define a photo-mask pattern, active regions are formed first. The active regions include a large active region pattern  60  and a small active region pattern  62 . As will be appreciated by persons skilled in the art, before the partial reverse active mask is actually fabricated, the large active pattern  60  and small active pattern  62  are first designed by computer program is those skilled in this art, the program (and its use) for designing such a pattern is well known. Therefore one can use the program to simulate, shrinking or enlarging pattern without increasing cost. After completing the shrinking and enlarging simulation process, a partial reverse active mask pattern is obtained.  
     [0023] In FIG. 3B, the large active region pattern  60  and the small active pattern region  62  are shrunk as shown in the figure. The shrinking large active region pattern and the shrinking small active region pattern are denoted as  60   a  and  62   a  respectively.  
     [0024] In FIG. 3C, the shrinking process is continued until the shrinking small active region pattern  62   a  (as shown in FIG. 3B) disappears. The shrinking distance is about 0.5 μm to 2 μm each side. At this time, only the shrinking large active region pattern [ 62 ] 60   a  is left.  
     [0025] In FIG. 3D, the shrinking large active region pattern [ 62 ] 60   a  (as in FIG. 3C) is enlarged with a dimension of about 0.2 μm to 2 μm each side. This enlarged dimension is smaller than the shrinking distance mentioned above. The resultant active region pattern is shown as the figure and denoted as  60   b . It is seen that the resultant active region pattern  60   b  is slightly smaller than the original active region pattern  60 . The shrinking step and the enlarging step is performed by computer simulation. The resultant active region pattern obtained from the computer simulation is then used to form a mask.  
     [0026] By applying this photo-mask pattern in forming a shallow trench isolation, the central part of an active region is exposed, whereas the edge part of the active region is covered by a photo-resist. A partial reverse active mask pattern is thus obtained.  
     [0027] The advantages of the invention are:  
     [0028] (1) Using a partial reverse active mask to etch away the oxide layer on the central part of an active region, only the oxide layer on the edge part of the active region and on a small active region is remained. The profile of the remaining oxide layer is pyramid-like and has a better uniformity. Therefore, a recess formed while polishing a large trench is avoided.  
     [0029] (2) Since only the oxide layer on the central part of an active region is etched away by using a partial reverse active mask, even when a misalignment occurs, the oxide layer within the trench is not etched. The kink effect is prevented. As a consequence, the current leakage and the short circuit caused by kink effect are avoided, so that the yield of wafer is enhanced.  
     [0030] Other embodiment of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.