Abstract:
A method for forming a shallow trench isolation (STI) structure with reduced stress is described. An amorphous silicon layer is deposited on a trench surface of a shallow trench isolation structure, and the amorphous silicon is then oxidized by thermal oxidation to form a liner oxide. The thickness of the liner oxide is uniform to reduce stress caused by a liner oxide having non-uniform thickness in the prior art, and the leakage risk between the semiconductor devices can thus be prevented.

Description:
RELATED APPLICATIONS 
   The present application is based on, and claims priority from, Taiwan Application Serial Number 93138139, filed Dec. 9, 2004, the disclosure of which is hereby incorporated by reference herein in its entirety. 
   BACKGROUND 
   1. Field of Invention 
   The present invention relates to a method for forming a shallow trench isolation (STI) structure in the semiconductor device process. More particularly, the present invention relates to a method for forming an STI structure with reduced stress. 
   2. Description of Related Art 
   In the field of the semiconductor technologies, many semiconductor devices are often integrated and manufactured on identical semiconductor wafer. Therefore, an appropriate isolation structure is necessary between all semiconductor devices on the semiconductor wafer for preventing an undesired electronic leakage or short, which results in a bad semiconductor. 
   One of the conventional methods for forming the isolation structure is Local Oxidation of Silicon (LOCOS). The fundamental of LOCOS is utilizing a mask to cover a semiconductor device at first, in which only some silicon substrate regions of the semiconductor device where the isolation structures are to be formed in are exposed, and then a thermal oxide (SiO 2 ) layer is grown on the exposed silicon substrate regions by thermal oxidation. The grown SiO 2  layers can directly be the isolation structures between the semiconductor devices due to the perfect electrical isolation capability of SiO 2 . 
   Both the process and equipment used to form an isolation structure by LOCOS are uncomplicated, but the disadvantages of LOCOS are greater and more apparent in the advanced semiconductor process. For an example, a longer heating time is needed by LOCOS and it involves a higher thermal budgets. Additionally, in the advanced semiconductor process, the feature size of a semiconductor and the distances between devices are smaller and smaller, and the size of a SiO 2  isolation structure is also harder and harder to control. Thus, the SiO 2  isolation structure will easily extend into the active region of the semiconductor device and cause faults in the semiconductor device. 
   Therefore, the shallow trench isolation (STI) technique is often utilized to form an isolation structure at present. The fundamental of STI technique is forming the trenches by etching a silicon substrate, and the SiO 2  is directly deposited into the trenches in place of the step of forming the SiO 2  layer by thermal oxidizing the silicon substrate. Because the size of the trench is very similar to the size of the isolation structure, the size of isolation structure will almost never exceed an expected size, and the problem of the thermal budget with excessive heat is also solved because the heating step is omitted. 
     FIG. 1  shows a trench structure in an STI structure without the SiO 2  layer. It can be seen that a silicon substrate  102  is covered by a pad oxide  104  formed by SiO 2 , and the pad oxide  104  is covered by a mask layer  106  formed by silicon nitride (Si 3 N 4 ). The mask layer  106  is used as an etching mask for forming a trench  108  on the silicon substrate  102 . Then, a SiO 2  layer is grown on the surface of the trench  108  by oxidation in a typical STI process with a high temperature, that is, a liner oxide  110 . 
   In general, the material of a semiconductor wafer substrate is the silicon with single crystal orientation, such as the silicon substrate  102  shown in  FIG. 1 . Therefore, the oxidation rate is limited by the crystal orientation; that is to say, portions of the silicon substrate with different orientations have different oxidation rates during the thermal oxidation process. According to the foregoing description, when the trench structure shown in  FIG. 1  is thermally oxidized, each of the surfaces of the trench  108 , such as the lateral and the bottom surfaces, will have different oxidation rates; thus, the SiO 2  layer grows with different thicknesses on each corner  112 . Therefore, high stress is readily generated in the corner  112 , and then a leakage risk appears in the semiconductor devices cause by the high stress in the corner  102  of the trench structure. 
   According to the foregoing description, when a STI process within a semiconductor process is performed, a method for reducing or eliminating the stress is really needed to reduce or prevent the leakage risk between the semiconductor devices. 
   SUMMARY 
   It is therefore an objective of the present invention to provide a method for forming a STI structure within a semiconductor device process. 
   It is another objective of the present invention to provide a method for reducing or eliminating the stress in an STI structure within a semiconductor device process. 
   It is still another objective of the present invention to provide a method for reducing or preventing the leakage risk between the devices within a semiconductor device process. 
   According to the foregoing objectives of the invention, a pad oxide is formed on a silicon substrate, a mask layer is formed on the pad oxide and a photoresist is formed on the mask layer, first. Then, the photoresist is utilized in the mask layer etching process and then the photoresist is removed. Next, the patterned mask layer is utilized in the pad oxide and silicon substrate etching process for forming a trench on the silicon substrate. After the trench is formed, an amorphous silicon layer is deposited on the surface of whole structure. Then, the amorphous silicon layer is oxidized to a SiO 2  layer by a high temperature, and becomes a liner oxide. After the liner oxide is formed, the trench is filled with SiO 2  as a fill layer. Next, portions of the SiO 2  over the mask are removed by polishing. Finally, portions of whole structure over the surface of the silicon substrate are removed by etching to complete a STI structure. 
   The purposes of the STI structure completed by the foregoing steps are identical to those of the conventional STI structure, but the former provides better isolation. Because the liner oxide adhering at the surface of the trench is formed from the oxidized amorphous silicon material, the thickness of the whole liner oxide is uniform, stress does not appear in the corners of the liner oxide, and the leakage risk can be further prevented. 
   It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
       FIG. 1  is a lateral, cross-sectional view of a conventional STI structure; and 
       FIG. 2A-2H  are lateral, cross-sectional views of an STI structure accompanied by each steps of the method according to a preferred embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
   A SiO 2  layer formed by thermal oxidation a single crystal of silicon in a trench structure results in different oxidation rates at the corners because the orientations of the crystal at the corners are different. This results in uneven thickness of the SiO 2  layer. Therefore, a general idea of the present invention is to deposit an amorphous silicon layer on the surface of the trench structure before the step of thermal oxidation the silicon substrate of the trench structure. The amorphous silicon naturally lacks the problem resulting from the different orientations of crystal because the amorphous silicon is not crystalline. Consequently, a SiO 2  layer with a uniform thickness can be grown by an uniform oxidation rate from the amorphous silicon layer in the trench structure, and the stress at the trench structure can be further prevented. 
     FIGS. 2A-2H  separately show each step of the process according to a preferred embodiment of the present invention. In  FIG. 2A , a process such as thermal oxidation is used to form a pad oxide  204  formed by SiO 2  on a silicon substrate  202 . Then, a deposition process such as Low Pressure Chemical Vapor Deposition (LPCVD) is utilized to deposit a material such as Si 3 N 4  on the pad oxide  204  as a mask layer  206 . Finally, a photoresist is coated on the mask layer  206 , and the regions predetermined to form the trenches are exposed. 
   In  FIG. 2B , the portions of the pad oxide  204  and mask layer  206  not protected by the photoresist  208  are removed from the surface of silicon substrate  202  by a process such as general photolithography to form a mask used to etch the silicon substrate  202 . Then, the photoresist  202  is removed. 
   In  FIG. 2C , a trench  210  is formed by an anisotropic etching process such as reactive ion etching to etch silicon substrate  202  via the mask comprising the pad oxide  204  and mask layer  206 . In general, the side walls of the trench, that is, the trench  210 , formed by this kind of process are approximately perpendicular. Thus, corner structures exist between the side walls and the bottom of the trench  210 . 
   In,  FIG. 2D , a conformal amorphous silicon layer  212  is deposited on the whole structure by a deposition process such as Low Pressure Chemical Vapor Deposition (LPCVD). That is to say, the portion of the amorphous silicon layer  212  deposited into the trench  210  has side walls and corners structure identical to those of the trench  210 . In this embodiment, the thickness of the amorphous silicon layer  212  is between about 1 nm and 30 nm. 
   In  FIG. 2E , a liner oxide  214  is formed from the amorphous silicon layer  212  by a process such as furnace oxidation or Rapid Thermal Process (RTP) at a high temperature. The oxidation rate of the whole amorphous silicon layer  212  is very uniform in the duration of the thermal oxidation because the amorphous silicon material is not crystalline and therefore does not have the orientation problem as single crystal. Consequently, the thickness of the whole liner oxide  214  is uniform, even at the side walls and the corners of the trenches. 
   In  FIG. 2F , a fill layer is formed on the whole structure by utilizing a deposition process such as LPCVD to fill the trench  210 . The material used to fill the trench  210  may be SiO 2  produced by a Chemical Vapor Deposition (CVD) process wherein the deposition vapor source is Tetraethylorthosilicate (TEOS). 
   In  FIG. 2G , the portion of the SiO 2  over the mask layer  206  is removed by a process such as Chemical Mechanical Polishing (CMP); that is, the portions of the liner oxide  214  and the fill layer  216  over the mask layer  206  are removed. The mask layer  206  can be the polish stop layer in the CMP process. The portions of the liner oxide  214  and the fill layer  216  in the trench  210  can be regarded as an isolation layer  218  herein. 
   In  FIG. 2H , an STI structure according to the embodiment of the present invention is completed by utilizing an etching process such as wet etching to remove the portions of the mask  206 , the pad oxide  204  and the isolation layer  218  over the silicon substrate  202 . 
   A STI structure according to the embodiment of the present invention is showed in  FIG. 2H , the isolating property of which is better. Because the liner oxide  214  adhered inside of the trench  210  is formed by oxidizing the amorphous silicon material, the thickness of the whole liner oxide  214  is very uniform, and the stress does not appear at the corners of the liner oxide  214 . Thus, the leakage risk can be further prevented. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.