FIELD OF THE INVENTION 
     This invention relates generally to the cell structure, device configuration and fabrication process of semiconductor power devices. More particularly, this invention relates to a novel and improved cell structure, device configuration and improved fabrication process of a trench MOSFET (Metal Oxide Semiconductor Field Effect Transistor). 
     BACKGROUND OF THE INVENTION 
     Please refer to  FIG. 1A  for a trench MOSFET  100  disclosed in a prior art of U.S. Pat. No. 7,829,944 and U.S. Pat. No. 7,791,132 having split gate electrodes ( 101 - 1  and  101 - 2 ), wherein each of the split gate electrodes ( 101 - 1  and  101 - 2 ) is asymmetrically disposed in a thick insulation layer  102  between a field plate  103  and trench sidewalls of a deep trench  104 , and isolated from the trench sidewall of the deep trench  104  by a gate oxide  105 .  FIG. 1B  shows manufacturing process for forming the split gate electrodes ( 101 - 1  and  101 - 2 ) of the trench MOSFET  100  of  FIG. 1A  according to the same prior art, wherein a patterned mask layer (not shown) is applied and followed by dry or wet oxide etching steps to form gate trenches  106  into the thick insulation layer  102  to achieve the asymmetrical disposition of the split gate electrodes ( 101 - 1  and  101 - 2  in  FIG. 1A ). That is to say, an additional mask for definition of the location of the split gate electrodes is required in the prior art, which is not cost effective for mass production. 
       FIG. 1C  is a plot showing normalized drift region doping profile versus normalized distance from a P-body region in a N-channel MOSFET, wherein the curve  123  illustrates a dual-gradient doping in drift region according to the prior art U.S. Pat. No. 7,791,132; that is, the doping concentration in the drift region nearest the P-body region (i.e., nearest the source) has a first gradient, and the doping concentration in the drift region farther from the P-body region (i.e., nearest the drain) has a second gradient, with the latter gradient being larger than the former. Plots  120  and  121  depict single-gradient doping concentration profiles optimized for the off state and on-state Vbd (breakdown voltage) respectively. As shown in  FIG. 1C , multiple epitaxial layers with different gradient doping profiles are required, which is also not cost effective for mass production. Moreover, it is difficult to control the various gradient doping profiles in the manufacturing process. 
     Therefore, there is still a need in the art of the semiconductor power device, particularly for trench MOSFET design and fabrication, to provide a novel cell structure, device configuration and fabrication process that would resolve these difficulties and reduce the manufacturing cost. 
     SUMMARY OF THE INVENTION 
     The present invention provides a trench MOSFET with resurf stepped oxide (RSO) which only requires a single epitaxial layer and no additional mask for split gate electrodes to achieve a better cost effective than the prior arts. Moreover, the present invention also provides a diffused drift region having a higher doping concentration than the epitaxial layer, whose doping profile is much easier to control than the multiple epitaxial layers in the prior arts. 
     In one aspect, the present invention features a trench MOSFET having resurf stepped oxide comprising: a substrate of a first conductivity type; an epitaxial layer of the first conductivity type onto the substrate, wherein the epitaxial layer has a lower doping concentration than the substrate; a plurality of gate trenches formed from a top surface of the epitaxial layer and extending downward into the epitaxial layer in an active area; a first gate insulation layer formed along trench sidewalls of a lower portion of each of the gate trenches; a source electrode formed within each of the gate trenches and surrounded by the first gate insulation layer in the lower portion of each of the gate trenches; a second gate insulation layer formed at least along trench sidewalls of an upper portion of each of the gate trenches and upper sidewalls of the source electrode above the first gate insulation layer, the second gate insulation layer having a thinner thickness than the first gate insulation layer; and a pair of split gate electrodes disposed above the first gate insulation layer and adjacent to the second gate insulation layer in the upper portion of each of the gate trenches, wherein each of the split gate electrodes is disposed in the middle between the source electrode and adjacent trench sidewall in each of the gate trenches. Therefore, comparing with the prior art, no additional mask is required for definition of the split gate electrodes in the present invention, and one mask is saved. Meanwhile, each of the split gate electrodes is symmetrically disposed in the middle of the source electrode (field plate in the prior arts) and adjacent trench sidewall of each of the gate trenches. 
     In another aspect, the present invention features a diffused drift region in a mesa between two adjacent gate trenches by performing an angle ion implantation of the first conductivity type, which is the same conductivity type as the epitaxial layer, followed by a diffusion step, wherein the diffused region has a higher doping concentration than the epitaxial layer for on-resistance reduction. Cost of formation of the diffused drift region is much more cost effective than the multiple epitaxial layers in the prior arts. Moreover, the doping concentration of the diffused drift region is much easier to control than the multiple epitaxial layers in the prior arts. Meanwhile, because of the diffusion method, the diffused drift has a higher doping concentration near the trench sidewalls of the gate trenches than in the center of the mesa. 
     In another aspect, the present invention features a trench MOSFET having a resurf stepped oxide and a diffused drift region comprising: a substrate of a first conductivity type; an epitaxial layer of the first conductivity type onto the substrate, wherein the epitaxial layer has a lower doping concentration than the substrate; a plurality of gate trenches formed from a top surface of the epitaxial layer and extending downward into the epitaxial layer in an active area; a first gate insulation layer formed along trench sidewalls of a lower portion of each of the gate trenches; a source electrode formed within each of the gate trenches and surrounded by the first gate insulation layer in the lower portion of each of the gate trenches; a second gate insulation layer formed at least along trench sidewalls of an upper portion of each of the gate trenches and upper sidewalls of the source electrode above the first gate insulation layer, the second gate insulation layer having a thinner thickness than the first gate insulation layer; a pair of split gate electrodes of a second conductivity type disposed above the first gate insulation layer and adjacent to the second gate insulation layer in the upper portion of each of the gate trenches, wherein each of the split gate electrodes disposed in the middle between the source electrode and adjacent trench sidewall in each of the gate trenches; a diffused drift region of the first conductivity having a higher doping concentration than the epitaxial layer, disposed in a mesa between every two adjacent gate trenches; the diffused drift region having a higher doping concentration near the trench sidewalls of the gate trenches than in center of the mesa; a body region of the second conductivity type formed in the mesa, above a top surface of the diffused drift region; and a source region of the first conductivity type formed near a top surface of the body region and adjacent to the split gate electrodes. 
     Preferred embodiments include one or more of the following features: the source electrode and the split gate electrode comprise a doped poly-silicon of the first conductivity type; the source electrode comprises a doped poly-silicon of a second conductivity type or the first conductivity type, and the split gate electrodes comprise a doped poly-silicon of the first conductivity type; trench bottoms of the gate trenches are above a common interface between the substrate and the epitaxial layer; the gate trenches further extend into the substrate; the trench MOSFET further comprises a trenched source-body contact filled with a contact metal plug, penetrating through the source region and extending into the body region; the trench MOSFET further comprises a body contact doped region of the second conductivity type within the body region and surrounding at least bottom of the trenched source-body contact underneath the source region, wherein the body contact doped region has a higher doping concentration than the body region; the trench MOSFET further comprises a termination area which comprises a guard ring connected with the source region, and multiple floating guard rings having floating voltage, wherein the guard ring and the multiple floating guard rings of the second conductivity type have junction depths greater than the body region; the trench MOSFET further comprises a termination area which comprises multiple floating trenched gates having floating voltage and being spaced apart by mesas comprising the body region, wherein the floating trenched gates each comprises a source electrode and a pair of split gate electrodes same as those in the gate trenches in the active area; the trench MOSFET further comprises a termination area which comprises multiple floating trenched gates having floating voltage and being spaced apart by mesas without having the body region, wherein the floating trenched gates each comprises a source electrode and a pair of split gate electrodes same as those in the gate trenches in the active area; the contact metal plug is a tungsten metal layer padded by a barrier metal layer of Ti/TiN or Co/TiN; the contact metal plug is Al alloys or Cu padded by a barrier metal layer of Ti/TiN or Co/TiN or Ta/TiN, wherein the contact metal plug also extends onto a contact interlayer to be respectively patterned as a source metal or a gate metal; the trench MOSFET further comprises at least a trenched source electrode contact filled with a contact metal plug connecting the source electrode to a source metal; the trench MOSFET further comprises at least a gate contact trench filled with a source electrode and a pair of split gate electrodes for gate connection, wherein the split gate electrodes in the gate contact trench are connected to a gate metal through at least a trenched gate contact filled with a contact metal plug. 
     The invention also features a method for manufacturing a trench MOSFET comprising the steps of: (a) growing an epitaxial layer of a first conductivity type upon a substrate of the first conductivity type, wherein the epitaxial layer having a lower doping concentration than the substrate; (b) forming a hard mask such as an oxide onto a top surface of the epitaxial layer for definition of a plurality of gate trenches; (c) forming the plurality of gate trenches, and mesas between two adjacent gate trenches in the epitaxial layer by etching through open regions in the hard mask; (d) keeping the hard mask substantially covering the mesas after formation of the gate trenches to block sequential angle ion implantation into top surfaces of the mesas; (e) carrying out an angle Ion Implantation of the first conductivity type dopant into the mesas through trench sidewalls of the gate trenches followed by a diffusion step to form a plurality of diffused drift regions in the mesas; (f) removing the hard mask after formation of the diffused drift regions; (g) forming a thick oxide layer along inner surfaces of the gate trenches by thermal oxide growth or oxide deposition; (h) depositing a first doped poly-silicon layer filling the gate trenches to serve as source electrodes; (i) etching back the source electrodes from the top surface of the epitaxial layer; (j) etching back the thick oxide layer from the top surface of the epitaxial layer and an upper portion of the gate trenches; (k) forming a thin oxide layer covering a top surface of the thick oxide layer, along upper inner surfaces of the gate trenches and along sidewalls of the source electrodes above the top surface of the thick oxide layer; (l) depositing a second doped poly-silicon layer filling the upper portion of the gate trenches to serve as split gate electrodes; (m) etching back the split gate electrodes by CMP (Chemical Mechanical Polishing) or plasma etch; (n) carrying out a body implantation of the second conductivity type dopant and a step of body diffusion to form body regions; (o) applying a source mask onto the top surface of the epitaxial layer; and (p) carrying out a source implantation of the first conductivity type dopant and a source diffusion to form source regions. 
     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. 1A  is a cross-sectional view of a trench MOSFET with resurf stepped oxide of a prior art. 
         FIG. 1B  is a cross-sectional view for showing manufacturing process for forming the split gate electrodes of the trench MOSFET of  FIG. 1A  of the prior art. 
         FIG. 1C  is a plot showing normalized drift region doping profile versus normalized distance from the P-body region for a two gradient device according to another prior art. 
         FIG. 2A  is a cross-sectional view of a preferred embodiment according to the present invention. 
         FIG. 2B  is another cross-sectional view of the preferred embodiment according to the present invention. 
         FIG. 2C  is a plot showing mesa doping profile along the cross section A-B in  FIG. 2B  according to the present invention. 
         FIG. 3  is a cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 4  is a cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 5A  is a cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 5B  is a cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 5C  is a cross-sectional view of another preferred embodiment according to the present invention. 
         FIG. 6  is a cross-sectional view of another preferred embodiment according to the present invention. 
         FIGS. 7A-7G  are a serial of side cross-sectional views for showing the processing steps for fabricating the trench MOSFET of  FIG. 4 . 
     
    
    
     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. 2A  for a preferred embodiment of this invention wherein an N-channel trench MOSFET  217  is formed in an N− epitaxial layer  202  onto an N+ substrate  200  coated with a back metal  201  of Ti/Ni/Ag on rear side as a drain metal. A plurality of gate trenches  203  are formed starting from a top surface of the N− epitaxial layer  202  and extending downward into the N− epitaxial layer  202  in an active area, wherein trench bottoms of the gate trenches  203  are above a common interface between the substrate  200  and the N− epitaxial layer  202 . Each of the gate trenches  203  is filled with: a first gate insulation layer  204  formed along trench sidewalls of a lower portion of each of the gate trenches  203 ; a source electrode (S, as illustrated)  207  formed within each of the gate trenches  203  and surrounded by the first gate insulation layer  204  in the lower portion of each of the gate trenches  203 ; a second gate insulation layer  205  formed at least along trench sidewalls of an upper portion of each of the gate trenches  203  and upper sidewalls of the source electrode  207  above the first gate insulation layer  204 , wherein the second gate insulation layer  205  has a thinner thickness than the first gate insulation layer  204 ; and a pair of split gate electrodes  206  disposed above the first gate insulation layer  204  and close to the second gate insulation layer  205  in the upper portion of each of the gate trenches  203 , wherein each of the split gate electrodes  206  is disposed in the middle between the source electrode  207  and adjacent trench sidewall in each of the gate trenches  203 . The source electrode  207  and the split gate electrodes  206  comprise a doped poly-silicon of N conductivity type. As an alternative, the source electrode  207  can be implemented comprising a doped poly-silicon of P conductivity type and the split gate electrodes  206  can be implemented comprising a doped poly-silicon of N conductivity type. For further reducing the on-resistance, the N-channel trench MOSFET  217  can be implemented by further comprising an N diffused drift region  208  which has a higher doping concentration than the epitaxial layer  202  and is disposed in a mesa between every two adjacent gate trenches  203 , wherein the N diffused drift region  208  has a higher doping concentration near the trench sidewalls of the gate trenches  203  than in the center of the mesa. According to the present invention, cost of formation of the N diffused drift region  208  is much more cost effective than the multiple epitaxial layers in the prior arts. Moreover, the doping profile of the N diffused drift region  208  is much easier to control than the multiple epitaxial layers in the prior arts. Above a top surface of the N diffused drift region  208 , a p body region  210  is formed in the mesa with an n+ source region  211  near a top surface of the p body region  210  and adjacent to the split gate electrodes  206 . A trenched source-body contact  212  filled with a contact metal plug  213  is penetrating through a contact interlayer  214 , the n+ source region  211  and extending into the p body region  210 , wherein the contact metal plug  213  is Al alloys or Cu padded by a barrier metal layer of Ti/TiN or Co/TiN or Ta/TiN, the contact metal plug  213  is also extended onto the contact interlayer  214  to act as a source metal  215  connected to the n+ source region  211  and the p body region  210 . As an alternative, the contact metal plug  213  can be implemented by using a tungsten metal layer padded by a barrier metal layer of Ti/TiN or Co/TiN, and the source metal can be implemented by using the Al alloys or Cu which is overlying onto the contact interlayer  214  and the contact metal plug. The n+ source region  211  has a uniform doping concentration and junction depth between sidewalls of the trenched source-body contact  212  to adjacent channel regions near the trench sidewalls of the gate trenches  203 . A p+ body contact doped region  216  is formed within the p body region  210 , surrounding at least bottom of the trenched source-body contact  212  underneath the n+ source region  211  to further reduce the contact resistance between the contact metal plug  213  and the p body region  210 . 
       FIG. 2B  shows another cross-sectional view of the trench MOSFET  217  of  FIG. 2A  for showing how the source electrode and the split gate electrodes are connected to the source metal and a gate metal respectively. As illustrated, a plurality of trenched source electrode contacts ( 222 - 1  and  222 - 2 ) filled with the contact metal plugs ( 223 - 1  and  223 - 2 , which are the same as the contact metal plug  213  in  FIG. 2A ) are penetrating through the contact interlayer  214  and extending into the source electrodes  207  and connecting the source electrodes  207  to the source metal  215 . And the trench MOSFET  217  further comprises a gate contact trench  218  filled with the source electrode  207 ′ and a pair of the split gate electrodes  206 ′, which are the same as those in the gate trenches  203  in the active area, for gate connection. A plurality of trenched gate contacts ( 220 - 1  and  220 - 2 ) filled with the contact metal plugs ( 221 - 1  and  221 - 2 , which are the same as the contact metal plug  213  in  FIG. 2A ) are extending through the contact interlayer  214  and extending into the split gate electrodes  206 ′ and connecting the split gate electrodes  206 ′ to a gate metal  219  which is overlying onto the contact interlayer  214 . 
       FIG. 2C  is a plot showing mesa doping profile along the cross section A-B in  FIG. 2B  according to the present invention. As shown in  FIG. 2C , the N diffused drift region  208  in  FIG. 2B  has a higher doping concentration near the trench sidewalls of the gate trenches  203  than in center of the mesa. The doping profile of the N diffused drift region  208  of  FIG. 2C  is much easier to control than the doping profile of the multiple epitaxial layers of  FIG. 1C . 
       FIG. 3  is a cross-sectional view of another preferred embodiment according to the present invention for showing an N-channel trench MOSFET  317  which is similar to the trench MOSFET  217  of  FIG. 2B  except that, in  FIG. 3 , the gate trenches  303  in the active area and the gate contact trench  318  are starting from the top surface of the epitaxial layer  302  and further extending into the N+ substrate  300 . Besides, the N diffused drift region  308  is reaching the interface of the epitaxial layer  302  and the N+ substrate  300 . 
       FIG. 4  is a cross-sectional view of another preferred embodiment according to the present invention for showing an N-channel trench MOSFET  417  which is similar to the trench MOSFET  317  of  FIG. 3  except that, in  FIG. 4 , the contact metal plugs ( 423 - 1  and  423 - 2 ) filled in the trenched source electrode contacts ( 422 - 1  and  422 - 2 ), the contact metal plug  413  filled in the trenched source-body contact  412 , and the contact metal plugs ( 421 - 1  and  421 - 2 ) filled in the trenched gate contacts ( 420 - 1  and  420 - 2 ) are each implemented by using a tungsten metal layer padded by a barrier metal layer of Ti/TiN or Co/TiN. Meanwhile, the source metal  415  made of Al alloys or Cu is overlying onto the contact interlayer  414  and contacting with the contact metal plugs ( 423 - 1 ,  423 - 2  and  13 ) in the trenched source electrode contacts ( 422 - 1  and  422 - 2 ) and in the trenched source-body contact  412  respectively to be connected to the source electrodes, the gate metal  419  also made of Al alloys or Cu is overlying onto the contact interlayer  414  and contacting with the contact metal plugs ( 421 - 1  and  421 - 1 ) in the trenched gate electrode contacts ( 420 - 1  and  420 - 2 ) to be connected to the gate electrodes. 
       FIG. 5A  is a cross-sectional view of another preferred embodiment according to the present invention. Compared with the trench MOSFET  317  of  FIG. 3 , the trench MOSFET  517  of  FIG. 5A  further comprises a termination area  520  comprising multiple floating trenched gates trenches  521  having floating voltage and being spaced apart by the mesas comprising the N diffused drift region  508  without having the p body region, wherein the floating trenched gates  521  each comprises a source electrode  507  and a pair of split gate electrodes  506  which are same as those in the gate trenches  503  in the active area. 
       FIG. 5B  is a cross-sectional view of another preferred embodiment according to the present invention. Compared with the trench MOSFET  317  of  FIG. 3 , the trench MOSFET  527  of  FIG. 5B  further comprises a termination area  530  comprising multiple floating trenched gates  531  each has having floating voltage and being spaced apart by the mesas comprising the p body region  510  and the N diffused drift region  518 , wherein the floating trenched gates  531  each comprises a source electrode  511  and a pair of split gate electrodes  516  which are the same as those in the gate trenches  513  in the active area. 
       FIG. 5C  is a cross-sectional view of another preferred embodiment according to the present invention. Compared with the trench MOSFET  317  of  FIG. 3 , the trench MOSFET  537  of  FIG. 5C  further comprises a termination area  540  comprising a p type guard ring  539  (GR, as illustrated in  FIG. 5C ) connected with the n+ source region  541 , and multiple p type floating guard rings  549  having floating voltage, wherein the p type guard ring  539  and the multiple p type floating guard rings  549  all have junction depths greater than the p body region  550 . 
       FIG. 6  is a cross-sectional view of another preferred embodiment according to the present invention for showing a trench MOSFET  6  which is similar to the trench MOSFET  417  of  FIG. 4  except that, in  FIG. 6 , the n+ source region  611  have a higher doping concentration and a greater junction depth along the sidewalls of the trenched source-body contact  612  than along adjacent channel regions near the trench sidewalls of the gate trenches  603 , and the n+ source region  611  has a Gaussian-distribution doping profile from the sidewalls of the trenched source-body contact  612  to the adjacent channel regions near the trench sidewalls of the gate trenches  603 . 
       FIGS. 7A˜7G  are a serial of exemplary steps that are performed to form the inventive trench MOSFET  417  of  FIG. 4 . In  FIG. 7A , an N− epitaxial layer  701  is grown on an N+ substrate  702 . Next, a hard mask  723  such as an oxide layer is formed onto a top surface of the N− epitaxial layer  701  for definition of areas for a plurality of gate trenches. Then, after dry oxide etch and dry silicon etch, a plurality of gate trenches  703  are etched penetrating through open regions in the hard mask  723 , the N− epitaxial layer  701  and extending into the N+ substrate  702 . Meanwhile, at least a gate contact trench  703 ′ is formed in the same steps, which is also starting from the top surface of the N− epitaxial layer  701  and extending into the N+ substrate  702 . As an alternative, the gate trenches can be formed having trench bottoms disposed above the N+ substrate  702 . Mesas are formed between every two adjacent gate trenches  703  and the gate contact trench  703 ′ in the N− epitaxial layer  701 . 
     In  FIG. 7B , a sacrificial oxide layer (not shown) is first grown and then removed to eliminate the plasma damage after forming the gate trenches  703  and the gate contact trench  703 ′. Keeping the hard mask  723  substantially covering the mesas, a screen oxide  704  is grown along an inner surface of the gate trenches  703  and the gate contact trench  703 ′. Then, a step of angle Ion Implantation of Arsenic or Phosphorus dopant into the mesas is carried out through the open regions in the hard mask  703  and sidewalk of the gate trenches, and followed by a diffusion step as shown in  FIG. 7C  to form a plurality of N diffused drift regions  705  in the mesas. Therefore, the N diffused drift region  705  has a higher doping concentration along trench sidewalls of the gate trenches  703  and the gate contact trench  703 ′ than in center of the mesas. And the doping profile of the N diffused drift regions  705  is much easier to control than the multiple epitaxial layers in the prior arts. 
     In  FIG. 7D , the hard mask  723  and the screen oxide  704  (as illustrated in  FIG. 7C ) are firstly removed. Then, a first gate insulation layer  706  comprising a thick oxide layer is formed lining the inner surface of the gate trenches by thermal oxide growth or thick oxide deposition. Then, a first doped poly-silicon layer is deposited onto the first gate insulation layer  706  to fill the gate trenches  703  and the gate contact trench  703 ′, and is then etched back from the top surface of the epitaxial layer  701  to serve as a source electrode  710 . Next, the first gate insulation layer  706  is etched back from top surface of the epitaxial layer and an upper portion of the gate trenches  703  and the gate contact trench  703 ′. 
     In  FIG. 7E , a second gate insulation layer  711  comprising a thin oxide layer is grown along upper inner surfaces of the gate trenches  703  and the gate contact trench  703 ′, covering a top surface of the first gate insulation layer  706  and along sidewalls of the source electrode  710  above the top surface of the first gate insulation layer  706 . After that, a second doped poly-silicon layer is deposited filling the upper portion of the gate trenches  703  and the gate contact trench  703 ′, and is then etched back by CMP (Chemical Mechanical Polishing) or Plasma Etch to serve as split gate electrodes  712 . Therefore, no mask is required for definition of the location of the split gate electrodes  712 , and one mask is saved for cost reduction. Each of the split gate electrodes  712  is symmetrically disposed in the middle between the source electrode  710  and adjacent trench sidewall in the gate trenches  703  and the gate contact trench  703 ′. Then, a body implantation of p conductivity type dopant is carried out over entire top surface to form p body regions  713  between every two adjacent gate trenches  703  and the gate contact trench  703 ′. After applying a source mask (not shown) onto the top surface of the epitaxial layer  701 , a source implantation of n conductivity type dopant and a diffusion step are successively carried out to form an n+ source region  714  near a top surface of the p body regions  713  between two adjacent gate trenches  703  in an active area. 
     In  FIG. 7F , another oxide layer is deposited onto the top surface of the epitaxial layer  701  to serve as a contact interlayer  715 . Then, after applying a contact mask (not shown) onto the contact interlayer  715 , a plurality of trenched contacts  716  are formed by successively dry oxide etch and dry silicon etch penetrating through the contact interlayer  715 , and extending into the p body regions  713  for trenched source-body contacts, into the source electrodes  710  for trenched source electrode contacts, and into the split gate electrodes  712  for trenched gate contacts, respectively. Next, a BF2 Ion Implantation is performed to form a p+ body contact doped region  717  within the p body regions  713  and surrounding at least bottom of the trenched source body-contacts penetrating through the n+ source region  714  and extending into the p body region  713 . 
     In  FIG. 7G , a barrier metal layer of Ti/TiN or Co/TiN or Ta/TiN is deposited on sidewalls and bottoms of all the trenched contacts  716  followed by a step of RTA process for silicide formation. Then, a tungsten material layer is deposited onto the barrier layer, wherein the tungsten material layer and the barrier layer are then etched back to form: contact metal plugs ( 423 - 1  and  423 - 2 ) for the trenched source electrode contacts ( 422 - 1  and  422 - 2 ); contact metal plug  413  for the trenched source-body contacts  412 ; and contact metal plugs ( 421 - 1  and  421 - 2 ) for the trenched gate contacts ( 420 - 1  and  420 - 2 ). Then, a metal layer of Al alloys or Cu padded by a resistance-reduction layer Ti or Ti/TiN underneath is deposited onto the contact interlayer  715  and followed by a metal etching process by employing a metal mask (not shown) to be patterned as a source metal  718  and a gate metal  719 . 
     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 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.
A trench MOSFET with split gates and diffused drift region for on-resistance reduction is disclosed. Each of the split gates is symmetrically disposed in the middle of the source electrode and adjacent trench sidewall of a deep trench. The inventive structure can save a mask for definition of the location of the split gate electrodes. Furthermore, the fabrication method can be implemented more reliably with lower cost.