Abstract:
A trench MOSFET with high cell density is disclosed where there is a heavily doped contact region on the top surface of mesas between a pair of gate trenches. The present invention can prevent the degradation of avalanche capability when shrinking the device in prior art.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the cell structure, device configuration and manufacture method of semiconductor devices. More particularly, this invention relates to an improved device configuration with high cell density and the manufacture method to produce the same. 
       BACKGROUND OF THE INVENTION 
       [0002]    In order to shrink the mesa width in a trench device, many structures were disclosed in prior art, referring to  FIG. 1  for a typical one, where a trench MOSFET includes a plurality of trenches  110  encompassed by N+ source regions  112  formed in P body regions  114 . P+ contact region  116  is formed between N+ source region  112  in mesa to contact source metal  120  with N+ source region  112  and P body region  114 . Furthermore, the source metal  120  is extending into gate trenches to contact N+ source region  112  on the top sidewalls of gate trenches to enlarge the contact area, and said source metal  120  is isolated from the doped poly filled in gate trenches by an insulation layer. 
         [0003]    The disclosed structure in  FIG. 1  shrank the mesa width and enhanced the source-body contact capability by enlarging the contact area of said source metal  120  to said source regions  112 , however, as further shrink the device, the P+ contact region  116  will become smaller, causing poor contact to P+ contact region hence resulting in degradation of avalanche capability by turning on a parasitic bipolar N+ (Source region)/P(body region)/N(epitaxial region). 
         [0004]    Accordingly, it would be desirable to provide new and improved device configuration to enhance the avalanche capability of semiconductor devices while shrinking the device. 
       SUMMARY OF THE INVENTION 
       [0005]    It is therefore an object of the present invention to provide new and improve device configuration to solve the problem discussed above by forming a heavily-doped contact region on top surface of source regions and second body region in said mesa, said heavily-doped contact region has body region dopant type and a heavier doping concentration than said second body region. For example, in an N-channel trench MOSFET, a P++ contact region is formed on top surface of N+ source region and second P+body region in  FIG. 2  which is P+ contact region in the prior art. By employing this structure, trench MOSFET with high cell density can be achieved without degrading the avalanche capability when shrinking the mesa width. 
         [0006]    Another aspect of the present invention is that, in some preferred embodiment, the source metal is not extending into the gate trenches, but connected to the W metal plug filled into the upper portion of the gate trenches to further enhance the contact performance to source region. 
         [0007]    Another aspect of the present invention is that, in some preferred embodiment, gate insulation layer is thicker at trench bottom than along the sidewalls of gate trenches to further reduce the charge between trenched gate and drain region. 
         [0008]    Another aspect of the present invention is that, in some preferred embodiment, a doped region with epitaxial layer dopant type and heavier concentration is formed wrapping the bottom of each gate trench to further reduce the resistance between source and drain. 
         [0009]    Briefly, in a preferred embodiment, as shown in  FIG. 2 , the present invention discloses a trench MOSFET formed on a substrate heavily doped with a first conductivity doping type (N+ source region in  FIG. 2 ). Onto said substrate, an epitaxial layer of said first conductivity doping type is grown with a lower doping concentration than said substrate. A plurality of gate trenches with doped poly filled in lower portion over a gate oxide layer is formed within said epitaxial layer, forming mesa between the upper portions of every two adjacent gate trenches over a first body region which is doped with a second conductivity doping type (P body region in  FIG. 2 ). Inside said mesa, source regions heavily doped with said first conductivity doping type are formed adjacent to the upper sidewalls of each gate trench while a second body region (P+ body region in  FIG. 2 ) of said second conductivity doping type formed between a pair of said source regions with doping concentration higher than the first body region. On top of each mesa, a heavily-doped contact region of said second conductivity doping type is formed covering top surface of said source region and said second body region with a higher doping concentration than said second body region. Onto a barrier layer of Ti/TiN or Co/TiN or Ta/TiN, which is covering the upper sidewalls of each gate trench and the top surface of each mesa, front metal of Al alloys or Cu is deposited and extending into each gate trench to contact said source region and said heavily-doped contact region. Within each gate trench, an insulation layer is formed on top of said doped poly filled in the lower portion of the gate trench to isolate said doped poly from the front metal. 
         [0010]    Briefly, in another preferred embodiment, as shown in  FIG. 3 , the present invention discloses a trench MOSFET which is similar to that in  FIG. 2 , except that, the upper portion of each gate trench is filled with W metal plug padded with a barrier layer over an insulation layer to isolate from said doped poly below. And front metal is deposited over a resistance-reduction layer of Ti or Ti/TiN covering each mesa and each W metal plug. 
         [0011]    Briefly, in another preferred embodiment, as shown in  FIG. 4 , the present invention discloses a trench MOSFET which is similar to that in  FIG. 3 , except that, each gate trench has a thick gate oxide at the gate trench bottom, which means that, the gate oxide layer at the bottom of each gate trench is thicker than that along the sidewalls of each gate trench to further reduce the charge between gate and drain region. 
         [0012]    Briefly, in another preferred embodiment, as shown in  FIG. 5 , the present invention discloses a trench MOSFET which is similar to that in  FIG. 4 , except that, around the bottom of each gate trench, a doped region of said first conductivity doping type (n* area as shown in  FIG. 5 ) is formed with a heavier doping concentration than said epitaxial layer to further reduce the resistance between source and drain. 
         [0013]    The present invention further discloses a method for making trench MOSFET with high cell density. The method further comprises process to form source regions by lateral diffusion of PSG (Phosphorus-doped silicon glass) filled within said gate trenches; and process to make a heavily-doped contact region on top of mesa defined by two adjacent gate trenches. 
         [0014]    These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The present invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein: 
           [0016]      FIG. 1  is a cross-sectional view of a trench MOSFET of prior art. 
           [0017]      FIG. 2  is a cross-sectional view of a preferred embodiment according to the present invention. 
           [0018]      FIG. 3  is a cross-sectional view of another preferred embodiment according to the present invention. 
           [0019]      FIG. 4  is a cross-sectional view of another preferred embodiment according to the present invention. 
           [0020]      FIG. 5  is a cross-sectional view of another preferred embodiment according to the present invention. 
           [0021]      FIGS. 6A-61  are a serial of side cross-sectional views for showing the processing steps for fabricating the trench MOSFET with high cell density as shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0022]    Please refer to  FIG. 2  for a preferred embodiment of this invention where an N-channel trench MOSFET is formed on an N+ substrate  200  with metal layer  290  on the rear side as drain. Onto said substrate  200 , an N epitaxial layer  202  is grown with a plurality of gate trenches formed wherein. To fill the lower portion of each gate trench  204 , doped poly  210  is deposited padded with a gate oxide layer  218 , onto which an insulation layer, for example, PSG layer  206  is deposited. Between every two adjacent gate trenches  204 , a first P body region  214  is formed within said epitaxial layer  202 . Inside a mesa over said first P body region  214 , N+ source regions  212  are formed encompassing the upper sidewalls of said gate trenches  204  with a second P+ body region  216  formed wherebetween. On top of each mesa, a P++ heavily-doped contact region  208  is formed covering the top surfaces of said N+ source regions  212  and said second P+ body region  216 . After deposition of a barrier layer  222  of Ti/TiN or Co/TiN or Ta/TiN, front metal layer  220  is formed covering the top surface of said mesa to contact said P++ heavily-doped contact region  208 , while extending into the upper portion of said gate trenches  204  to contact N+ source regions  212  along the upper sidewalls of the gate trench, and said front metal  220  is isolated from the doped poly  210  by said PSG layer  206 . 
         [0023]      FIG. 3  shows another preferred embodiment of the present invention where the disclosed trench MOSFET has a similar structure to that in  FIG. 2  except that, to fill the upper portion of each gate trench, W metal plug  324  padded with a barrier layer  322  is deposited to contact with N+ source regions  312 , and said W metal plugs is isolated from doped poly  310  by PSG layer  306 . Over a resistance-reduction layer  326  of Ti or Ti/TiN which covering the top surface of mesas and the W metal plugs  324 , front metal  320  such as Al alloys, Copper, Ti/Ni/Ag or Ti/Ni/Au is deposited to contact with P++ heavily-doped contact region  308  and N+ source regions  312  via W metal plugs  324 . 
         [0024]      FIG. 4  shows another preferred embodiment of the present invention where the disclosed trench MOSFET has a similar structure to that in  FIG. 3  except that, the gate oxide layer  418  at the bottom of each gate trench is thicker than that along the sidewalls of each gate trench. 
         [0025]      FIG. 5  shows another preferred embodiment of the present invention where the disclosed trench MOSFET has a similar structure to that in  FIG. 4  except that, there is an n* area  580  around the bottom of each gate trench. Said n* area  580  has a heavier doping concentration than epitaxial layer  502 . 
         [0026]      FIGS. 6A to 6I  show a series of exemplary steps that are performed to form the inventive trench MOSFET with high cell density shown in  FIG. 2 . In  FIG. 6A , an N doped epitaxial layer  202  is grown on an N+ doped substrate  200 . A trench mask (not shown) is applied onto said epitaxial layer  202  for the formation of a plurality of gate trenches  204  by dry silicon etching. In  FIG. 6B , a sacrificial oxide (not shown) is first grown and then removed to eliminate the plasma damage introduced during opening those gate trenches  204 . After that, a gate oxide layer  218  is formed along the inner surface of said gate trenches  204  and the top surface of mesas defined by two adjacent gate trenches, onto which doped poly  210  is deposited and then etched back or CMP (Chemical Mechanical Polishing) to fill said gate trenches. Then, an ion implantation of P type dopant is carried out to form said first P body region  214  within epitaixal layer  202  followed by a P dopant diffusion, and another ion implantation of P type dopant is carried out to form said second P+ body region  216  over said first P body region  214  followed by a P+ dopant diffusion. Said second P+ body region  216  has a heavier doping concentration than said first P body region  214 . 
         [0027]    In  FIG. 6C , said doped poly  210  is etched to remain within lower portion of said gate trenches. In  FIG. 6D , said gate oxide layer  218  is removed from the front surface of said second P+ body region  216  and from the upper sidewalls of gate trenches in the area without having doped poly. 
         [0028]    In  FIG. 6E , a PSG layer  206  is deposited on top of said doped poly  210  and said gate oxide  218  within upper portion of said gate trenches, and then etch back to make top surface of the PSG below the top surface of said second P+ body region  216  as shown in  FIG. 6F , then RTA (Rapid Thermal Anneal) is sequentially performed to form N+ source region  212  by a lateral diffusion process. Said N+ source regions  212  has a heavier doping concentration than said epitaxial layer  202  and is located along sidewalls of the upper portion of the gate trench but below the top surface of said mesas. Therefore, said second P+ body region  216  is compressed to be located between a pair of said N+ source region  212  and near the top surface of said mesas. In  FIG. 6G , said PSG layer  206  is etched back to leave a thinner layer than in  FIG. 6F  to expose N+ source region  212 , and in  FIG. 6H , an ion implantation of P type dopant is carried out to make a heavily-doped contact region  208  on top surface of each mesa with heavier concentration than said second P+ body region  216 . In  FIG. 6I , after deposition of a barrier layer  222  of Ti/TiN or Co/TiN or Ta/TiN, front metal layer  220  is formed covering the front surface of each mesa to contact said P++ heavily-doped contact region  208 , while extending into the upper portion of each gate trench to contact N+ source regions  212 . And said front metal  220  is isolated from the doped poly  210  by said PSG layer  206 . Next, a back metal  290  is deposited on rear side of said substrate  200  after a grinding process. 
         [0029]    Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention.