Abstract:
A power semiconductor device with improved avalanche capability is disclosed by forming at least one avalanche capability enhancement doped region underneath an ohmic contact doped region. Moreover, a source mask is saved by using three masks process and the avalanche capability is further improved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-In-Part (CIP) of U.S. patent application Ser. No. 13/585,059 of the same inventor, filed on Aug. 14, 2012, entitled “method for manufacturing a power semiconductor device”. 
    
    
     FIELD OF THE INVENTION 
     This invention related generally to the cell structure and device configuration of power semiconductor devices. More particularly, this invention relates to power semiconductor devices with improved avalanche capability. 
     BACKGROUND OF THE INVENTION 
     In U.S. Pat. No. 6,888,196, a conventional structure of power semiconductor device is disclosed, as shown in  FIG. 1 , wherein an N-channel trench MOSFET comprising a plurality of trenched gates  110  surrounded by n+ source regions  112  encompassed in P body regions  114  is formed in an N epitaxial layer  102  over an N+ substrate  100 . To connect the source regions  112  and the body regions  114  to a source metal  122 , a trenched source-body contact structure  118  with vertical sidewalls is employed penetrating through a contact interlayer  120 , the n+ source regions  112  and extending into the P body regions  114 . Furthermore, a p+ body ohmic contact doped region  116  is implanted surrounding bottom of the trenched source-body contact structure  118  to decrease a contact resistance between the P body regions  114  and the trenched source-body contact structure  118 . 
     The conventional structure in  FIG. 1  is accoutering a technical difficulty which is that avalanche always occurs near bottom of the trenched gates  110 , causing a hazardous condition to the power semiconductor device. As we all know that, in the trench MOSFET shown in  FIG. 1 , an avalanche current Iav (illustrated in  FIG. 1 ) flows between the trenched gates  110  and the trenched source-body contact structure  118 , triggering turning-on of a parasitic bipolar transistor (illustrated in  FIG. 1 ) when Iav*Rb&gt;0.7V, wherein Rb is a resistance between the p+ body ohmic contact doped region  116  and channel regions near the trenched gates  110 . As is known to all that, the doping concentration of the p+ body ohmic contact doped region  116  is higher than that of the P body regions  114  (please refer to  FIG. 2  for Y 1 -Y 1 ′ cross section of  FIG. 1 ), which is helpful to decrease resistance Rb, however, as the sidewalls of the trenched source-body contact structure  118  is perpendicular to front surface of the N epitaxial layer  102 , after carrying out implantation through a contact opening and filling with a W (tungsten) plug for formation of the trenched source-body contact structure  118 , the p+ body ohmic contact doped region  116  can be formed only surrounding bottom of the trenched source-body contact structure  118 , resulting in a high resistance Rb underneath the n+ source regions  112 . Therefore, the parasitic bipolar transistor is easily to be triggered turning on due to the high resistance Rb, thus weakening the avalanche capability of the trench MOSFET. 
       FIG. 3  shows another trench MOSFET in prior art disclosed in U.S. Patent Publication No. 20080890357. Comparing to  FIG. 1 , the trench MOSFET in  FIG. 3  comprises a plurality of trenched gates  130  having terrace gate structure for gate resistance reduction, wherein top surface of gate conductive layer filled in gate trenches is higher than the sidewalls of the gate trenches. However, the limitation of poor avalanche capability discussed above is still pronounced in this structure due to the easily turning-on of a parasitic bipolar transistor and the occurring of avalanche near bottom of the trenched gates  130 . 
     For other power semiconductor device, for example trench IGBTs (Insulated Gate Bipolar Transistors), the same disadvantage of poor avalanche capability is also affecting the performance of the power semiconductor device. 
     Accordingly, it would be desirable to provide new and improved power semiconductor devices to avoid the constraint discussed above. 
     SUMMARY OF THE INVENTION 
     The present invention has been conceived to solve the above-described problems with the related art, and it is an object of the invention to provide a technique which makes it possible to avoid the avalanche occurring near bottom of the trenched gates and to prevent the parasitic bipolar transistor from turning-on. Moreover, the present invention provides three masks process to save a source mask and the avalanche capability is further improved. 
     In order to solve the above-described problems, according to a first aspect of the invention, there is provided a power semiconductor device comprising a plurality of trench MOSFETs with each comprising: a substrate of a first conductivity type; an epitaxial layer of the first conductivity type over the substrate, wherein the epitaxial layer has a lower doping concentration than the substrate; a plurality of gate trenches extending into the epitaxial layer, wherein each of the gate trenches has a gate oxide layer lining its inner surface and a doped poly-silicon layer thereon; a body region of a second conductivity type surrounding sidewalls of each of the gate trenches; a source region of the first conductivity type near a top surface of the body region, wherein the source region surrounds top portion of sidewalls of each of the gate trenches, and has a higher doping concentration than the epitaxial layer; a contact insulation layer disposed over the epitaxial layer and covering outer surface of the doped poly-silicon layer; a contact opening locating between every two adjacent of the gate trenches, opened through the contact insulation layer and the source region, and extended into the body region; a body ohmic contact doped region of the second conductivity type formed within the body region, surrounding at least bottom of each the contact opening, wherein the body ohmic contact doped region has a higher doping concentration than the body region; at least one avalanche capability enhancement doped region of the second conductivity type underneath each the body ohmic contact doped region, wherein the avalanche capability enhancement region has a higher doping concentration than the body region but a lower doping concentration than the body ohmic contact doped region; a metal plug filled in the contact opening; a source metal disposed covering top surface of the contact insulation layer; a drain metal disposed on back surface of the substrate; the source region having a doping concentration along sidewalls of the contact opening higher than along an adjacent channel region near the gate trenches at a same distance from the top surface of the epitaxial layer, and the source region having a junction depth along the sidewalls of the contact opening greater than along the adjacent channel region from the top surface of the epitaxial layer. 
     Firstly, the at least one avalanche capability enhancement doped region is formed underneath the body ohmic contact doped region. Second, as Unclamp Inductive Switching (UIS) test is used to evaluate avalanche capability by measuring UIS current at breakdown voltage, in  FIG. 4 , by adding a P* avalanche capability enhancement region  218  underneath a p+ body ohmic contact doped region  216  in an N-channel trench MOSFET (please refer to  FIG. 5  for Y 2 -Y 2 ′ cross section of  FIG. 4 ), the avalanche current lay is shifted from bottom of the gate trenches to underneath the contact opening  212  so that the avalanche current Jay directly flows to the source metal  220  to enhance UIS current (as shown in  FIG. 6 ) at expense of slight degradation of breakdown voltage (as shown in  FIG. 7 ) for depth of the body regions less than 1.0 μm but not affect on breakdown voltage for depth of the body regions greater than 1.0 μm. 
     According to a second aspect of the present invention, there is provided a power semiconductor device comprising a plurality of trench MOSFETs with each comprising only one avalanche capability enhancement doped region underneath the body ohmic contact doped region, wherein the avalanche capability enhancement doped region is formed completely within the body region. 
     According to a third aspect of the present invention, there is provided a power semiconductor device comprising a plurality of trench MOSFETs with each comprising only one avalanche capability enhancement doped region underneath the body ohmic contact doped region, wherein the avalanche capability enhancement doped region is formed partially overlap with the body region and partially extending into the epitaxial layer but shallower than the gate trenches. 
     According to a fourth aspect of the present invention, there is provided a power semiconductor device comprising a plurality of trench MOSFETs with each comprising multiple of avalanche capability enhancement doped regions, and one of which is formed partially overlap with the body region and partially extending into the epitaxial layer but shallower than the gate trenches and others are disposed within the body region, for example in  FIG. 9  having two avalanche capability enhancement doped regions ( 418  and  418 ′). Please refer to  FIG. 10  for the Y 3 -Y 3 ′ cross section of  FIG. 9 . 
     According to a fifth aspect of the present invention, there is provided a power semiconductor device comprising a plurality of trench MOSFETs with each comprising the doped poly-silicon layer protruding out from each of the gate trenches and at least a portion of the doped poly-silicon positioned higher than the sidewalls of each of the gate trenches. 
     According to a sixth aspect of the present invention, there is provided a power semiconductor device comprising a plurality of trench MOSFETs with each comprising the doped poly-silicon layer having a top surface not higher than the sidewalls of each of the gate trenches. 
     According to a seventh aspect of the present invention, there is provided a power semiconductor device comprising a plurality of trench MOSFETs with each comprising the contact opening having sidewalls with taper angle α 1  within the source region, and having sidewalls with taper angle α 2  in the body region with respect to the top surface of the epitaxial layer, wherein the taper angle α 1  is equal to or less than 90 degree and equal to or greater than the taper angle α 2 . By employing this structure with the taper angle α 2  is less than 90 degree, the area of the body ohmic contact doped region is enlarged surrounding not only bottom but also sidewalls of each the contact opening, thus further enhancing UIS performance. 
     According to an eighth aspect of the present invention, there is provided a method of manufacturing a power semiconductor device comprising: forming a trench mask over a top surface of an epitaxial layer of first conductivity type, the trench mask having apertures defining location of a plurality of gate trenches; etching through the apertures in the trench mask to form a plurality of gate trenches in the epitaxial layer; forming a gate oxide layer on inner surface of the gate trenches; depositing a gate conductive layer over the gate oxide layer in the gate trenches and onto the top surface of the epitaxial layer; etching or CMP (Chemical Mechanical Polishing) the gate conductive layer so that the gate conductive layer is removed away from the top surface of the epitaxial layer for formation of a plurality of trenched gates; forming a body region of a second conductivity type extending between every two adjacent of the gate trenches; depositing a contact insulation layer over the top surface of the epitaxial layer; applying a contact mask and following with a dry oxide etching to remove the contact insulation layer from a contact opening defined by the contact mask; implanting the epitaxial layer with a source dopant of the first conductivity type through the contact opening and diffusing the source dopant to form a source region in the contact opening in an active area, thereby a source mask is saved; carrying out a dry silicon etch to make the contact opening further penetrate through the source region and extend into the body region; forming a body ohmic contact doped region of the second conductivity type by ion implantation surrounding at least bottom of the contact opening; and forming at least one avalanche capability enhancement region underneath the body ohmic contact doped region by ion implantation. 
     According to a ninth aspect of the present invention, there is provided a method of manufacturing a power semiconductor device, wherein the ion implantation for formation of at least one avalanche capability enhancement region is carried out with energy ranging from 100 KeV to 300 KeV and with dose from 1E12 to 1E14 cm −2 . 
     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 
       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: 
         FIG. 1  is a side cross-sectional view of a power semiconductor device of prior art. 
         FIG. 2  is a graph showing Y 1 -Y 1 ′ cross section of  FIG. 1 . 
         FIG. 3  is a side cross-sectional view of a power semiconductor device of another prior art. 
         FIG. 4  is a side cross-sectional view of a preferred embodiment according to the present invention. 
         FIG. 5  is a graph showing Y 2 -Y 2 ′ cross section of  FIG. 4   
         FIG. 6  is a graph showing relationship between UIS current and dose of ion implantation through trenched source-body contact for formation of an avalanche capability enhancement doped region. 
         FIG. 7  is a graph showing relationship between breakdown voltage and dose of ion implantation through contact opening for formation of an avalanche capability enhancement doped region. 
         FIG. 8  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 9  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 10  is a graph showing Y 3 -Y 3 ′ cross section of  FIG. 9 . 
         FIG. 11  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 12  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 13  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 14  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 15  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 16  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 17  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 18  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 19  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 20  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 21  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 22  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 23  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 24  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 25  is a side cross-sectional view of another preferred embodiment according to the present invention. 
         FIGS. 26A˜26E  are a serial of side cross-sectional views for showing the processing steps for fabricating the trench MOSFET as shown in  FIG. 21 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the following Detailed Description, reference is made to the accompanying drawings, which forms a part thereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top”, “bottom”, “front”, “back”, etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purpose of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be make without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. It is to be understood that the features of the various exemplary embodiments described herein may be combined with each other, unless specifically noted otherwise. 
     Please refer to  FIG. 4  for a cross sectional-view of a preferred N-channel trench MOSFET which formed on an N+ substrate  200  with back metal  222  of Ti/Ni/Ag on back surface as drain electrode. Onto the N+ substrate  200 , a lighter doped N epitaxial layer  201  is grown, and a plurality of trenched gates  204  are formed therein. The trenched gates  204  in this embodiment is implemented by filling a plurality of gate trenches  202  with a doped poly-silicon layer padded by a gate oxide layer  203 , wherein top surface of the trenched gates  204  is not higher than sidewalls of the gate trenches  202 . The preferred N-channel trench MOSFET further comprises: a P body region  206  formed in upper portion of the N epitaxial layer  201  and extending between every two adjacent of the gate trenches  202 ; an n+ source region  208  near top surface of the P body region  206  and surrounding the sidewalls of the gate trenches  202 ; a contact insulation layer  210  covering top surface of the N epitaxial layer  201  and the trenched gates  204 ; a trenched source-body contact structure  214  implemented by filling a contact opening  212  with a metal plug  215 , for example W (tungsten) plug in this embodiment, padding by a barrier layer  213  of Ti/TiN or Co/TiN or Ta/TiN. As alternative, the metal plug  215  can also be implemented by using a source metal directly filling into the contact opening. Specifically, the trenched source-body contact structure  214  in this embodiment has slope sidewalls penetrating through the contact insulation layer  210  and the n+ source region  208  with a taper angle α 1 , and extending into the P body region  206  with a taper angle α 2 , wherein α 1  is less than 90 degree and greater than α 2 . Therefore, underneath the trenched source-body contact structure  214 , a p+ body ohmic contact doped region  216  is surrounding its bottom and the sidewalls with taper angle α 2  due to the enlargement of implantation area. According to the present invention, in this preferred embodiment, there is only one P* avalanche capability enhancement doped region  218  underneath the p+ body ohmic contact doped region  216  and completely within the P body region  206  to shift avalanche occurrence from bottom of the gate trenches  202  to underneath the trenched source-body contact structure  214 . Onto the contact insulation layer  210  and the trenched source-body contact structure  214 , a front metal of Al alloys or Cu alloys is deposited acting as a source metal  220  to be connected to the n+ source region  208  and the P body region  206  via the trenched source-body contact structure  214 , wherein the source metal  220  is padded by a resistance-reduction layer  220 ′ of Ti or Ti/TiN beneath. 
     Please refer to  FIG. 8  for a cross sectional-view of another preferred N-channel trench MOSFET which is similar to that in  FIG. 4  except that, underneath the p+ body ohmic contact doped region  316 , the P* avalanche capability enhancement doped region  318  is formed partially overlap with the P body region  306  and partially extending into the N epitaxial layer  301  but shallower than the gate trenches  302 . 
     Please refer to  FIG. 9  for a cross sectional-view of another preferred N-channel trench MOSFET which is similar to that in  FIG. 4  except that, underneath the p+ body ohmic contact doped region  416 , there are two avalanche capability enhancement regions: P* 1  and P* 2 , wherein the P* 1  avalanche capability enhancement doped region  418  is formed completely within the P body region  406 , and the P* 2  avalanche capability enhancement doped region  418 ′ is formed partially overlap with the P body region  406  and partially extending into the N epitaxial layer  401  but shallower than the gate trenches  402 . 
     Please refer to  FIG. 11  for a cross sectional-view of another preferred N-channel trench MOSFET which is similar to that in  FIG. 4  except that, the trenched gates  504  protrude out from the gate trenches  502 , which means top surface of the trenched gates  504  is higher than sidewalls of the gate trenches  502  to form terrace trenched gates for gate resistance reduction. 
     Please refer to  FIG. 12  for a cross sectional-view of another preferred N-channel trench MOSFET which is similar to that in  FIG. 8  except that, the trenched gates  604  protrudes out from the gate trenches  602 , which means top surface of the trenched gates  604  is higher than sidewalls of the gate trenches  602  to form terrace trenched gates for gate resistance reduction. 
     Please refer to  FIG. 13  for a cross sectional-view of another preferred N-channel trench MOSFET which is similar to that in  FIG. 9  except that, the trenched gates  704  protrudes out from gate trenches  702 , which means top surface of the trenched gates  704  is higher than sidewalls of the gate trenches  702  to form terrace trenched gates for gate resistance reduction. 
     Please refer to  FIG. 14  for a cross sectional-view of a preferred N-channel trench IGBT which formed on a P+ substrate  800  with back metal  822  of Ti/Ni/Ag on back surface as collector electrode. Onto the P+ substrate  800 , a first N+ epitaxial layer  810 ′ and a second N epitaxial layer  801  is successively grown, and a plurality of trenched gates  804  are formed inside the second N epitaxial layer  801 . The trenched gates  804  in this embodiment is implemented by filling a plurality of gate trenches  802  with doped poly-silicon layer padded by a gate oxide layer  803 , wherein top surface of the trenched gates  804  not higher than sidewalls of the gate trenches  802 . The preferred N-channel trench IGBT further comprises: a P base region  806  formed in upper portion of the second N epitaxial layer  801  and extending between every two adjacent of the gate trenches  802 ; an n+ emitter region  808  near top surface of the P base region  806  and surrounding the sidewalls of the gate trenches  802 ; a contact insulation layer  810  covering top surface of the second N epitaxial layer  801  and the trenched gates  804 ; a trenched emitter-base contact structure  814  implemented by filling a contact opening  812  with a W plug  815  padded by a barrier layer  813  of Ti/TiN or Co/TiN or Ta/TiN. Specifically, the trenched emitter-base contact structure  814  has slope sidewall penetrating through the contact insulation layer  810  and the n+ emitter region  808  with a taper angle α 1 ′, and extending into the P base region with a taper angle α 2 ′, wherein α 1 ′ is less than 90 degree and greater than α 2 ′. Therefore, underneath the trenched emitter-base contact structure  814 , a p+ base ohmic contact doped region  816  is surrounding its bottom and the sidewall with taper angle α 2 ′ due to the enlargement of implantation area. According to the present invention, in this preferred embodiment, there is only one avalanche capability enhancement doped region P*  818  underneath the p+ base ohmic contact doped region  816  and completely within the P base region  806  to shift avalanche occurrence from bottom of the gate trenches  802  to underneath the trenched emitter-base contact structure  814 . Onto the contact insulation layer  810  and the trenched emitter-base contact structure  814 , a front metal of Al alloys or Cu alloys is deposited acting as an emitter metal  820  to be connected to the n+ emitter region  808  and the P base region  806  via the trenched emitter-base contact structure  814 , wherein the emitter metal  820  is padded by a resistance-reduction layer  820 ′ of Ti or Ti/TiN beneath. 
     Please refer to  FIG. 15  for a cross sectional-view of another preferred N-channel trench IGBT which is similar to that in  FIG. 14  except that, underneath the p+ base ohmic contact doped region  916 , the P* avalanche capability enhancement doped region  918  is formed partially overlap with the P base region  906  and partially extending into the second N epitaxial layer  901  but shallower than the gate trenches  902 . 
     Please refer to  FIG. 16  for a cross sectional-view of another preferred N-channel trench IGBT which is similar to that in  FIG. 14  except that, underneath the p+ base ohmic contact doped region  1016 , there are two avalanche capability enhancement doped regions: P* 1  and P* 2 , wherein the P* 1  avalanche capability enhancement doped region  1018  is formed completely within the P base region  1006 , and the P* 2  avalanche capability enhancement doped region  1018 ′ is formed partially overlap with the P base region  1006  and partially extending into the second N epitaxial layer  1001  but shallower than the gate trenches  1002 . 
     Please refer to  FIG. 17  for a cross sectional-view of another preferred N-channel trench IGBT which is similar to that in  FIG. 14  except that, the trenched gates  1104  protrude out from gate trenches  1102 , which means top surface of the trenched gates  1104  is higher than sidewalls of the gate trenches  1102  to form terrace trenched gates for gate resistance reduction. 
     Please refer to  FIG. 18  for a cross sectional-view of another preferred N-channel trench IGBT which is similar to that in  FIG. 15  except that, the trenched gates  1204  protrude out from gate trenches  1202 , which means top surface of the trenched gates  1204  is higher than sidewalls of the gate trenches  1202  to form terrace trenched gates for gate resistance reduction. 
     Please refer to  FIG. 19  for a cross sectional-view of another preferred N-channel trench IGBT which is similar to that in  FIG. 16  except that, the trenched gates  1304  protrude out from gate trenches  1302 , which means top surface of the trenched gates  1304  is higher than sidewalls of the gate trenches  1302  to form terrace trenched gates for gate resistance reduction. 
     Please refer to  FIG. 20  for a cross sectional-view of another preferred N-channel trench MOSFET which is similar to that in  FIG. 4  except that, the n+ source region  2001  has a doping concentration along sidewalls of the trenched source-body contact structure  2002  higher than along an adjacent channel region near the gate trenches  2003  at a same distance from the top surface of the N epitaxial layer  2004 , and the n+ source region  2001  has a junction depth along the sidewalls of the trenched source-body contact structure  2002  greater than along the adjacent channel region at a same distance from the top surface of the N epitaxial layer  2004 . 
     Please refer to  FIG. 21  for a cross sectional-view of another preferred N-channel trench MOSFET which is similar to that in  FIG. 8  except that, the n+ source region  2101  has a doping concentration along sidewalls of the trenched source-body contact structure  2118  higher than along an adjacent channel region near the gate trenches  2103  at a same distance from the top surface of the N epitaxial layer  2104 , and the n+ source region  2101  has a junction depth along the sidewalls of the trenched source-body contact structure  2118  greater than along the adjacent channel region at a same distance from the top surface of the N epitaxial layer  2104 . 
     Please refer to  FIG. 22  for a cross sectional-view of another preferred N-channel trench MOSFET which is similar to that in  FIG. 9  except that, the n+ source region  2201  has a doping concentration along sidewalls of the trenched source-body contact structure  2202  higher than along an adjacent channel region near the gate trenches  2203  at a same distance from the top surface of the N epitaxial layer  2204 , and the n+ source region  2201  has a junction depth along the sidewalls of the trenched source-body contact structure  2202  greater than along the adjacent channel region at a same distance from the top surface of the N epitaxial layer  2204 . 
     Please refer to  FIG. 23  for a cross sectional-view of another preferred N-channel trench MOSFET which is similar to that in  FIG. 11  except that, the n+ source region  2301  has a doping concentration along sidewalls of the trenched source-body contact structure  2302  higher than along an adjacent channel region near the gate trenches  2303  at a same distance from the top surface of the N epitaxial layer  2304 , and the n+ source region  2301  has a junction depth along the sidewalls of the trenched source-body contact structure  2302  greater than along the adjacent channel region at a same distance from the top surface of the N epitaxial layer  2304 . 
     Please refer to  FIG. 24  for a cross sectional-view of another preferred N-channel trench MOSFET which is similar to that in  FIG. 12  except that, the n+ source region  2401  has a doping concentration along sidewalls of the trenched source-body contact structure  2402  higher than along an adjacent channel region near the gate trenches  2403  at a same distance from the top surface of the N epitaxial layer  2404 , and the n+ source region  2401  has a junction depth along the sidewalls of the trenched source-body contact structure  2402  greater than along the adjacent channel region at a same distance from the top surface of the N epitaxial layer  2404 . 
     Please refer to  FIG. 25  for a cross sectional-view of another preferred N-channel trench MOSFET which is similar to that in  FIG. 13  except that, the n+ source region  2501  has a doping concentration along sidewalls of the trenched source-body contact structure  2502  higher than along an adjacent channel region near the gate trenches  2503  at a same distance from the top surface of the N epitaxial layer  2504 , and the n+ source region  2501  has a junction depth along the sidewalls of the trenched source-body contact structure  2502  greater than along the adjacent channel region at a same distance from the top surface of the N epitaxial layer  2504 . 
       FIGS. 26A to 26E  are a serial of exemplary steps that are performed to form the preferred N-channel trench MOSFET in  FIG. 21 . In  FIG. 26A , an N epitaxial layer  2104  is first grown on an N+ substrate  2100 . After applying a trench mask (not shown), a plurality of gate trenches  2103  are trenched to a certain depth into the N epitaxial layer  2104 . Then, a sacrificial oxide layer (not shown) is grown and then removed to eliminate the plasma damage may introduced during etching process. Next, an oxide layer is grown overlying inner surface of the gate trenches  2103  to serve as a gate oxide  2105 , onto which a doped poly-silicon layer is deposited so that the doped poly-silicon layer  2106  overflows onto top surface of the epitaxial layer  2104 . Then, the doped poly-silicon layer is etched by CMP (Chemical Mechanical Polishing) or plasma etching back to be removed away from the top surface of the N epitaxial layer  2104  to form a plurality of trenched gates  2106 . 
     In  FIG. 26B , a P body mask (not shown) is optionally used for the following P type dose implantation, then, the step of P type dose diffusion is performed to form P body regions  2107 . After that, another insulation layer is deposited onto top surface of the N epitaxial layer  2104  and the trenched gates  2106  to serve as a contact insulation layer  2108 . A contact mask (not shown) is applied onto the contact insulation layer  2108 , and a contact opening  2102  is formed by performing a dry oxide etch process above between every two adjacent of the trenched gates  2106 , then a step of n+ type dose is implanted for the formation of n+ source regions  2101  followed by diffusion. Therefore, a mask for forming source regions is saved. 
     In  FIG. 26C , a dry silicon etch is carried out and the contact opening  2102  is further etched through the n+ source region  2101  and etching into the P body region  2107  with slope sidewalls. 
     In  FIG. 26D , after removing the contact mask, a BF2 ion implantation is carried out to form a p+ body ohmic contact doped region  2109  underneath the contact opening  2102  and wrapping its bottom as well as its sidewalls encompassed in the P body region  2107 . Then, a Boron ion implantation is carried out with dose from 1E12 cm −2  to 1E14 cm −2  and with energy ranging from 100 KeV to 300 KeV to form a P* avalanche capability enhancement doped region  2110  underneath the p+ body ohmic contact doped region  2109  and not touching with channel regions near the gate trenches  2103 . 
     In  FIG. 26E , after activating the implanted dopant in  FIG. 26D , a barrier layer  2117  of Ti/TiN or Co/TiN or Ta/TiN is deposited along inner surface of the contact opening  2102 , onto which, tungsten material is deposited and then etched back to be a W plug  2113  for formation of a trenched source-body contact structure  2118 . Next, a front metal layer of Al alloys or Cu alloys is deposited padded by a resistance-reduction layer  2114 ′ Ti or Ti/TiN and over the contact insulation layer  2108  as well as the trenched source-body contact structure  2118  to serve as a source metal  2114 . Last, after backside grinding, drain metal  2115  of Ti/Ni/Ag is deposited onto back surface of the N+ substrate  2116 . 
     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.