Abstract:
A trench Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) structure with guard ling, includes: a substrate including an epi layer region on the top thereof a plurality of source and body regions formed in the epi layer; a metal layer including a plurality of metal layer regions which are connected to respective source and body regions forming metal connections of the MOSFET; a plurality of metal contact plugs connected to respective metal layer regions; an insulating layer deposited on the epi layer formed underneath the metal layer with a plurality of metal contact holes therein for contacting respective source and body regions; and a guard ring wrapping around the trench gates with contact metal plug underneath the gate metal layer.

Description:
CROSS REFERENCE 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 12/111,797 filed on Apr. 29, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a trench MOSFET structure with a guard ring and the method for manufacturing the same, and more particularly to a structure of a trench MOSFET which solves low breakdown voltage in contacted trench gate area and the method for manufacturing the same. 
         [0004]    2. The Prior Arts 
         [0005]    In the structure of a trench Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) or vertical transistor, the gate of the transistor is formed in a trench on top of a substrate and the source/drain regions are formed on both sides of the gate. This type of vertical transistor allows high current to pass through and channel to be turned on/off at a low voltage. 
         [0006]    Referring to  FIG. 1 , a cross-sectional diagram of the structure of a trench MOSFET is shown. An N-type doping epitaxial region  105  is provided on a N+ substrate  100 . A plurality of trenches  106  are formed on the N-type doping epitaxial region  105  that has lower doping concentration than the substrate  100 . The surface of trenches  106  which is covered a gate oxide layer  110  thereon are filled with a doped polysilicon layer to form a plurality of trenched gates  115 . A plurality of P-type doping regions  120  are formed on both sides of the trenched gates  115 . A plurality of N+ source regions  125  are formed in the P-type body regions  120 . A metal layer  160  is deposited on the top of the MOSFET structure for formation of source metal  160   b,  and gate metal field plate metal  160   a  (gate metal  160   a  also services as field plate metal here) of the MOSFET. An insulating layer  130  is formed on top surface of the epitaxial layer and the trenched gates  115  for insulating. Metal contact plugs  137  are formed in the P-type body regions  120  for connecting source metal  160   b,  and in a wide trench of the said trenched contact gates  116  for connecting gate metal  160   a  (The wide trenched contact gate  116  allows metal contact having more tolerance into the doped polysilicon in the wide trench without shorting to the P-type body regions  120  resulted from misalignment). A plurality of P+ heavily-doped regions  121  are formed at the bottom of the metal contact plug  137 . The MOSFET structure of the prior arts also has a P-type guard ring  170  which is formed aside the P-type body regions  120  underneath the field plate metal  160   a  of the MOSFET to increase breakdown voltage in termination. However, the structure in  FIG. 1  has low breakdown voltage occurring on trench bottom of the trenched contact gate  116  as result of wider trench which has deeper trench depth than the trench depth in active area. The trench depth is deeper when the trench width is wider because more open area allows more etching gas goes into trench during dry etching silicon process. When reverse bias between drain and gate/source increases, avalanche will first occur on the trench bottom of the trenched contact gate  116  because it has deeper trench gate. 
         [0007]    The present invention provides a new structure of trench MOSFET structure with a guard ring wrapped around the contacted trench gate which improves the lack of the prior art. 
       SUMMARY OF THE INVENTION 
       [0008]    This invention provides a trench Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) with a guard ring. The MOSFET structure with guard ring, comprising: a substrate comprising an epitaxial layer region on the top thereof; a plurality of source and body regions formed between two adjacent trenched gates in active area; a metal layer comprising source metal layer and gate metal layer regions which are respectively connected to source and body regions, and at least a trenched contact gate forming metal connections of the MOSFET; a plurality of metal contact plugs connected to the respective metal layer regions; an insulating layer deposited on the epi layer formed underneath the metal layer with a plurality of metal contact holes filled with the metal contact plugs therein for contacting the source and body regions; and a guard ring wrapping around the trenched contact gates in such a way that a drift region formed between the guard ring and the substrate underneath the trenched contact gate with the largest height located below center of the trenched contact gate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    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: 
           [0010]      FIG. 1  is a cross-sectional diagram depicting a trench MOSFET structure with a guard ring; 
           [0011]      FIGS. 2A-2F  are cross-sectional diagrams illustrating forming a trench MOSFET structure with guard ring on a substrate in accordance with an embodiment of the present invention; and 
           [0012]      FIG. 3  is a cross-sectional diagram illustrating the trench MOSFET structure with guard ring in accordance with another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    The present invention is described by the following specific embodiments. Those with ordinary skills in the arts can readily understand the other advantages and functions of the present invention after reading the disclosure of this specification. The present invention can also be implemented with different embodiments. Various details described in this specification can be modified based on different viewpoints and applications without departing from the scope of the present invention. 
         [0014]    The present invention as shown in  FIG. 2F  provides a trench MOSFET structure with a guard ring comprising: a substrate of a first conductivity doping type including an epitaxial layer of the first conductivity doping type formed on the top thereof with doping concentration less than the substrate; a plurality of source regions of the first conductivity doping type formed in the epitaxial layer and a plurality of first type body regions of a second conductivity doping type formed beneath the sources in active area; an insulating layer formed on the epitaxial layer; a plurality of first type trenches in the active area vertically penetrating through the insulating layer and the source regions and extending into the body regions, and having an gate oxide layer formed thereon, the first type trenches filled with a doped polysilicon layer as trenched gates for current conduction; at least one second type trench having the gate oxide layer formed thereon, the second type trench being deeper and wider than the first type trench extending from surface of the body regions and into a guard ring region, the second type trench filled with the doped polysilicon as trenched contact gate between the active and termination areas; a first metal contact plug penetrating through the insulating layer and extending into the second type trench, the first metal contact plug being connected to a first metal layer formed on the insulating layer, the first metal contact serving as gate metal in a termination area; a plurality of second metal contact plugs, penetrating through the insulating layer and the source regions, and extending into the body regions, the second metal contact plugs being connected to a second metal layer formed on the insulating layer, the second metal layer serving as source metal; the guard ring formed in the trenched contact gate area, and termination area extending from the top surface of the epitaxial layer having the second conductivity doping type, the guard ring having a junction depth deeper than that of the body region, and the guard ring wraps around sidewall and bottom of the trenched contact gate; the guard ring having doping concentration near the trenched contact gate bottom gradually decreases from sidewall of the trenched contact gate toward center of the trenched contact gate bottom; a drift region of the first conductivity doping formed between the substrate and the guard ring underneath the trenched contact gate with a height of the drift region defined by a distance from the substrate to the guard ring which is greater under center of the trenched contact gate bottom than that under sidewalls of the trenched contact gate; and the gate metal formed above the body region, guard ring and overlapping top surface of the epitaxial layer in the termination area servicing as a metal field plate for breakdown voltage enhancement. a second type body region with said second conductivity doping type formed between said active area and said trenched contact gate; and a third type body region with said second conductivity doping type formed in said termination area. 
         [0015]    In another embodiment as shown in  FIG. 3 , the trench MOSFET structure further comprising a plurality of third metal contact plugs penetrating through the insulating layer and extending into the body regions, the third metal contact plugs being connected to a third metal layer formed on the insulating layer, the third metal layer overlapping the body region, the guard ring region and the epitaxial layer serving as a metal field plate for breakdown voltage enhancement. 
         [0016]    Referring to  FIG. 2A , an N+ doped substrate  200  having a N-type doping epitaxial layer region  205  thereon is provided. Lithography and dry etching processes are performed to form a plurality of trenches  206  in the N-type epi layer  205 . The trenches  206  comprise a first type trenches  206   b  and  206   c,  a second type trench  206   a,  and the second type trench  206   a  is deeper and wider than both of the first type trenches  206   b  and  206   c.  Then, a deposition or thermally grown process is performed to form a silicon oxide layer on the surface of the N-type source region  205  and the trenches  206 , which acts as a gate oxide layer  210  of a trench MOSFET. Prior to the gate oxide layer  210  is formed, a sacrificial oxide is grown and wet etched for removal silicon damage along the trench  206  surface induced by the dry trench etch. At last, a doped polysilicon layer is formed on the gate oxide layer  210  and filled in the trenches  206  by a deposition process. Thereafter, the doped polysilicon layer on the gate oxide layer  210  is removed by a dry etching process or a CMP (chemical-mechanical polishing process) and the doped polysilicon layer on the each trench  206  is removed by a polysilicon etching back process, and a plurality of gate structures  215  of the trench MOSFET in the trench are formed. The gate structure  215  comprises a first type trenched gates  215   b  and  215   c,  and a second trenched gate  215   a  which are respectively formed in the first type trenches  206   b  and  206   c,  and the second trench  206   a.  The second type trenched gate  215   a  is used as trenched contact gate for connection of gate metal layer. 
         [0017]    Referring to  FIG. 2B , a second mask  240  is formed over the gate oxide layer  210  and the gate structure  215  by lithography to define a doping zone. Then, a guard ring  270  are formed in the N-type doping region  205  by an ion implantation and diffusion processes. After processes of forming the guard ring  270 , the second mask  240  is removed. The guard ring  270  surrounds the trenched contact gate  215   a  while the doping zone of the guard ring  270  covers the trenched contact gate  215   a  with a doping depth of the guard ring  270  deeper than the trenched contact gate  215   a.  Moreover, a N type drift region  290  ( FIGS. 2B and 2C ) is formed under the trench contact gate between the substrate  200  and the guard ring  270  with a non-uniform height defined by a distance from the substrate  200  to the guard ring  270 . The height of the N type drift region  292  ( FIG. 2D ) under the center of the trench contact gate  215   a  is greater than those heights  291  and  293  ( FIG. 2D ) under sidewall of the trenched contact gate  215   a.    
         [0018]    Referring to  FIG. 2C  and  FIG. 2D , a third mask  250  is formed to define a plurality of P-body regions  220  formed in the N-type doping epitaxial region  205  by an ion implantation, the third mask  250  removal and diffusion process. The P-body regions  220  include a first type P-body region  220   c  in active area, a second type P-body region  220   b  between the active area and the trenched contact gate, and a third type P-body region  220   a  in termination area. After that, a forth mask  251  (see  FIG. 2D ) is formed so as to facilitate formation of N+ source regions  225  in the first type P-body region  220   c  and the second type P-body region  220   b  by ion implantation and thermal diffusion processes after the forth mask  251  is removed. 
         [0019]    Referring to  FIG. 2E , an insulating layer  230  is formed on the gate oxide layer  210  and the gate structure  215 . This insulating layer  230  is a silicon dioxide layer formed by a deposition process. After the deposition of the insulating layer  230 , a fifth mask  252  is formed on the surface of the insulating layer  230  by lithography. This fifth mask  252  defines the locations of metal contacts of the trench MOSFET. Thereafter, a dry etching process is performed by using the fifth mask  252  as the etching mask, such that metal contact holes  241   a,    241   b,  and  241   c  are formed in the insulating layer  230 , the N+ sources regions  225 , the P-body regions  220 , and the trenched contact gate  215   a.  The first metal contact holes  241   b  and  241   c  are respectively corresponding to the P-body regions  220   b  and  220   c  while the second metal contact hole  241   a  is corresponding to the trenched contact gate  215   a.  Then, an ion implantation process is carried out to form P+ heavily-doped regions  221  at bottom of contact  241   b  and  241   c.    
         [0020]    Referring to  FIG. 2F , the metal contact holes  241   a,    241   b,  and  241   c  can be filled with tungsten metal  237  padded with a barrier metal layer Ti/TiN or Co/TiN to form the metal contact plugs  237   a,    237   b,  and  2371   c  respectively. Besides tungsten metal, aluminum metal or copper metal is used as the contact plug or the front metal layer of the trench MOSFET. After etch back of the contact metal  237 , a metal layer Ti/Aluminum alloys  260  is deposited on the insulating layer  230 , the first metal contact plugs  237   b  and  237   c,  the second metal contact plug  237   a,  and the metal layer  260  comprises a first metal layer region  260   b  and a second metal layer region  260   a  which are separated and are metal connections of the trench MOSFET. The first metal layer region  260   b  is corresponding to connection of both the N+ source region  225  and the P-body region  220 , and the second metal layer region  260   a  is corresponding to connection of the trenched contact gate  215   a.    
         [0021]    Referring to  FIG. 2F , the MOSFET structure with guard ring of the present invention has a MOSFET structure comprises the N+ doped substrate  200 , the N-type doping epi layer region  205 , the plurality of trenches  206 , the plurality of gate structure  215 , the gate oxide layer  210 , the plurality of P-body regions  220 , the plurality of P+ heavily-doped regions  221 , the plurality of N+ source regions  225 , the insulating layer  230 , the plurality of metal contact plugs ( 237   a,    237   b,  and  237   c ), the metal layer  260 , and the guard ring  270 . The metal layer  260  comprising the first metal layer region  260   b  and the second metal layer region  260   a  are formed on the top of the MOSFET structure, and the first metal layer region  260   b  and the second metal layer region  260   a  are formed as the source metal, and the gate and field plate metal of the MOSFET, respectively. The gate structure  215  comprising the first type trenched gates  215   b  and  215   c,  and the second trenched gate as trenched contact gate  215   a  which are covered the gate oxide layer  210  and are filled in the trenches  206  to be used as the gate of the MOSFET. The insulating layer  230  is formed between the metal layer  260  and the gate structure  215  for insulating, and the metal contact plugs  237   a,    237   b,  and  237   c  are penetrated through the insulating layer  230  and contacted with the metal layer  260 . Although the MOSFET structure of the present invention has the partial structure which is similar to prior arts, the guard ring  270  is particularly different from the prior arts. The guard ring  270  wraps around the trenched contact gate  215   a  while the second metal layer region  260   a  of the metal layer  260  covers the second contact plug  237   a  and the trenched contact gate  215   a.  The P+ heavily-doped regions  221  are formed at the bottom of the first type trenched gates  215   b  and  215   c.    
         [0022]    Referring to  FIG. 2F  again, according to the embodiment said above, the guard ring  270  can wrap around the trenched contact gate  215   a,  a part of the second type P-body region  220   b  between the active area and termination area and the third type P-body region  220   a  in termination area while the first metal layer region  260   b  and the second metal layer region  260   a  of the metal layer  260  covers the first metal contact plugs  237   b  and  237   c,  and the second contact plug  237   a,  respectively. 
         [0023]    Referring to  FIG. 3 , a second embodiment of the present invention, the MOSFET structure with guard ring of the present invention is similar to the first embodiment of the present invention and has a MOSFET structure comprises a N+ doped substrate  300 , a N-type doping epi layer region  305 , a plurality of trenches  306 , a plurality of gate structure  315 , a gate oxide layer  310 , a plurality of P-body regions  320 , a plurality of P+ heavily-doped regions  321 , a plurality of N+ source regions  325 , a insulating layer  330 , a plurality of contact metal plugs ( 337   a,    337   b,    337   c,  and  337   d ), multiple metal layer  360 , and a guard ring  370 . The metal layer  360  comprising a first metal layer region  360   c,  a second metal layer region  360   b,  and a third metal layer  360   a  is formed on the top of the MOSFET structure, and the first metal layer region  360   c,  the second metal layer region  360   b,  and the third metal layer  360   a  are formed as the source metal, the gate metal, and the field plate metal of the MOSFET respectively. The gate structure  315  comprises the first type trenched gate  315   b,  and the second gate  315   a  which are covered the gate oxide layer  310  and are filled in the trenches  360  to be used as a gate of the MOSFET. The insulating layer  330  is formed between the metal layer  360  and the gate structure  315  for insulating, and the metal contact plugs  337   a,    337   b,    337   c,  and  337   d  penetrate through the insulating layer  330  and contacted with the metal layer  360  respectively. The guard ring  370  is particularly different from the prior arts. the guard ring  370  can wrap around the trenched contact gate  315   a,  a part of the second type P-body region  320   b  between the active area and termination area and the third type P-body region  320   a  in termination area while the first metal layer region  360   c  and the second metal layer region  360   b  of the metal layer  360  covers the first metal contact plugs  337   b  and  337   c,  and the second metal contact plug  337   a,  respectively. Moreover, the guard ring having doping concentration near the trenched contact gate bottom gradually decreases from sidewall of the trenched contact gate toward center of the trenched contact gate bottom. The doping concentration at point A in  FIG. 3  is higher than that at point B. 
         [0024]    Referring to  FIG. 3  again, according to the embodiment said above, the guard ring  370  can wrap around the trenched contact gate  315   a,  a part of the second type P-body region  320   b  between the active area and termination area and the third type P-body region  320   a  in termination area while the first metal layer region  360   c  and the second metal layer region  360   b  and the third metal layer region  360   a  of the metal layer  360  covers the first metal contact plugs  337   b  and  337   c,  the second metal contact plug  337   a  and third metal contact plug  337   d,  respectively. 
         [0025]    Although various embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and are within the purview of the appended claims without departing from the spirit and intended scope of the invention.