Abstract:
A trench MOSFET with closed cells having split trenched gates structure in trenched gates intersection area in cell corner is disclosed. The invented split trenched gates structure comprises an insulation layer between said split trenched gates with thick thermal oxide layer in center portion of the trenched gates intersection area, therefore further reducing Qgd of the trench MOSFET without increasing additional Rds.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the device configuration and manufacturing methods for fabricating the semiconductor power devices. More particularly, this invention relates to an improved and novel device configuration and manufacturing process for providing trench Metal-Oxide-Semiconductor-Field-Effect-Transistor (MOSFET) with split trenched gate structures in cell corners for gate charge reduction. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventional trench Metal-Oxide-Semiconductor-Field-Effect-Transistor (MOSFET) comprises either closed cell array or stripe cell array. Compared to that having stripe cell array, trench MOSFET having closed cell array has lower Rds (resistance between drain and source, similarly hereinafter) resulted from a greater channel length. However, it also has disadvantage of higher Qgd (gate charge between gate and drain, similarly hereinafter) contributed from inherent existence of intersection area among the closed cells. 
         [0003]    Please refer to  FIG. 1  for top view of a conventional N-channel trench MOSFET having closed cells with truncated corners of prior art. The Qgd aforementioned of the N-channel trench MOSFET is composed of Qgd 1  and Qgd 2 , wherein Qgd 1  is the gate charge between gate and drain in the intersection area (as illustrated in  FIG. 1 ) among the truncated corners of the closed cells, as shown in  FIG. 2A  of C 1 -D 1  cross section of  FIG. 1 , and Qgd 2  is the gate charge between gate and drain in non-intersection area of trenched gates, as shown in  FIG. 2B  of A 1 -B 1  cross section of  FIG. 1 . When unit cell (as illustrated in  FIG. 1 ) has 1.0 um pitch, the portion of Qgd 1  contributes about 40% of total Qgd due to a large area in the trenched gates intersection area. 
         [0004]    For more detailed, please refer to  FIG. 2B  for A 1 -B 1  cross-sectional view of  FIG. 1 , where a trenched source-body contact  102  having top view of rectangular shape is formed with vertical sidewall and located in center portion between every two trenched gates  108  which induce Qgd 2  in each unit cell. A plurality of n+ source regions  104  encompassed in P body regions  106  are formed having uniform doping profile distributed from the vertical sidewall of the trenched source-body contact  102  to channel region near the trenched gates  108  filled with doped poly-silicon layer. 
         [0005]    Accordingly, it would be desirable to provide a new and improved trench MOSFET configuration and manufacturing method to reduce Qgd in closed cell structure without increasing Rds. 
       SUMMARY OF THE INVENTION 
       [0006]    It is therefore an aspect of the present invention to provide a new and improved trench MOSFET by forming split trenched gates in cell corners in each trenched gates intersection area. Meanwhile, an insulation layer is formed between the split trenched gates with thick bottom thermal oxide layer underneath in center portion of the trenched gates intersection area. Therefore, Qgd 1  is reduced because the center portion of the trenched gates intersection area is the composite oxide layer not poly-silicon layer. 
         [0007]    Another aspect of the present invention is to form a doped area with dopant type opposite to epitaxial layer to further reduce Qgd 1  by surrounding the split trenched gates underneath each the trenched gates intersection area. 
         [0008]    Briefly, in a preferred embodiment, this invention disclosed a trench MOSFET comprising a plurality of closed cells with a substantial square shape for each cell, formed in an epitaxial layer of a first conductivity type onto a substrate of the first conductivity type, further comprising a plurality of trenched gates, wherein each trenched gates intersection area in cell corners comprising: split trenched gates along trench sidewalls of the trenched gates, wherein the trench sidewalls are padded with a gate oxide layer; an insulation layer covering top surface of the trenched gates and the epitaxial layer, and disposed between the split trenched gates; one thermally grown oxide layer formed between the insulation layer and the split trenched gates; another thermally grown oxide layer formed underneath the insulation layer in center portion of each the trenched gates intersection area, having thicker oxide than the gate oxide layer. 
         [0009]    In an exemplary embodiment, the trench MOSFET further comprises: a plurality of source regions of the first conductivity type encompassed in body regions of second conductivity type in upper portion of the epitaxial layer and extending between every two adjacent of the trenched gates; a trenched source-body contact in each the closed cell and penetrating through the insulation layer covering the epitaxial layer, and further extending between through the source region and into the body region to connect the source region and the body region to a source metal covering top surface of the insulation layer; an ohmic body doped region of the second conductivity type encompassed in the body region and wrapping at least bottom of each the trenched source-body contact underneath the source region, wherein the ohmic body doped region has a higher doping concentration than the body region. In an exemplary embodiment, the trench MOSFET further comprises a on-resistance reduction doped region of the first conductivity type surrounding bottom of each the trenched gate and the trenched gates intersection area, wherein the on-resistance reduction doped area has a higher doping concentration than the epitaxial layer. In an exemplary embodiment, the trench MOSFET further comprises a gate-drain charge reduction doped region of the second conductivity type surrounding bottom of each the trenched gates intersection area. In an exemplary embodiment, the trench MOSFET further comprises a void existing between the split trenched gates in each the trenched gates intersection area. In an exemplary embodiment, the source region has a Gaussian distribution profile from sidewalk of the trenched source-body contact to adjacent channel regions near the trenched gates. In an exemplary embodiment, the trenched source-body contact has slope sidewalls and the ohmic body doped region surrounds bottom and sidewall of the trenched source-body contact underneath the source region. In an exemplary embodiment, the trenched gates have rounded trenched gates corners and the trenched source-body contact has circular shape form top view. 
         [0010]    Furthermore, this invention discloses to method to manufacture a trench MOSFET comprising the steps of: opening a plurality of gate trenches in an epitaxial layer of a first conductivity type; carrying out ion implantation of the first conductivity type dopant above the gate trenches to form a on-resistance reduction doped region in the epitaxial layer and surrounding bottom of each the gate trenches as well as each trenched gates intersection area, wherein the doping concentration of the on-resistance reduction doped region is higher than that of the epitaxial layer; forming a gate oxide layer covering top surface of the epitaxial layer, and along inner surface of the gate trenches and the trenched gates intersection area; depositing a doped poly-silicon layer onto the gate oxide layer and etching the doped poly-silicon layer to a pre-determined depth; carrying out a body ion implantation of second conductivity type dopant to form body regions in upper portion of the epitaxial layer; applying a poly mask and performing dry poly-silicon etching to form a poly-silicon hole in center portion of the doped poly-silicon layer in the trenched gates intersection area, wherein the poly hole extends from top surface of the doped poly-silicon layer in the trenched gates intersection area to expose the gate oxide on bottom of the trenched gates intersection area; carrying out an ion implantation of the second conductivity type dopant to form a gate-drain charge reduction doped region of the second conductivity type below the poly-silicon hole; Carrying out a body diffusion to form the body regions of the second conductivity type as well as gate-drain charge reduction doped region underneath the trenched gates intersection area; depositing an insulation layer onto entire top surface and filling into the poly hole; providing a contact mask and carrying out a dry oxide etching to open contact openings through the insulation layer; carrying out ion implantation of the first conductivity type and diffusion step to form source regions in upper portion of the body regions with a Gaussian distribution profile form edge of the contact openings to adjacent channel regions near the gate trenches; carrying out a dry silicon etching to make the contact openings further extending through the source regions and into the body regions to form contact trenches; carrying out ion implantation of the second conductivity type and followed by a step of RTA to form an ohmic body doped region in the body regions and surrounding at least bottom of each the contact trench underneath the source regions, wherein the ohmic body doped region has higher doping concentration than the body region; depositing a barrier layer overlying inner surface of the contact trenches and top surface of the insulation layer; depositing a metal material onto a barrier metal layer and etching back the metal material leaving it within the contact trenches; etching back the barrier layer removing it from top surface of the insulation layer; depositing a front metal layer onto top surface of the insulation layer and covering the metal material to function as a source metal. 
         [0011]    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 
         [0012]    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: 
           [0013]      FIG. 1  is a top view of a trench MOSFET with closed cells of prior art. 
           [0014]      FIG. 2A  is C 1 -D 1  cross section of the trench MOSFET in  FIG. 1 . 
           [0015]      FIG. 2B  is A 1 -B 1  cross section of the trench MOSFET in  FIG. 1 . 
           [0016]      FIG. 3  is a top view of a trench MOSFET according to the present invention. 
           [0017]      FIG. 4A  is a preferred A 2 -B 2  cross section of the trench MOSFET in  FIG. 3  according to the present invention. 
           [0018]      FIG. 4B  is a preferred C 2 -D 2  cross section of the trench MOSFET in  FIG. 3  according to the present invention. 
           [0019]      FIG. 5A  is another preferred A 2 -B 2  cross section of the trench MOSFET in  FIG. 3  according to the present invention. 
           [0020]      FIG. 5B  is another preferred C 2 -D 2  cross section of the trench MOSFET in  FIG. 3  according to the present invention. 
           [0021]      FIG. 6  is another preferred C 2 -D 2  cross section of the trench MOSFET in  FIG. 3  according to the present invention. 
           [0022]      FIG. 7  is another preferred C 2 -D 2  cross section of the trench MOSFET in  FIG. 3  according to the present invention. 
           [0023]      FIG. 8  is another top view of a trench MOSFET according to the present invention. 
           [0024]      FIG. 9A˜9I  are a serial of cross-sectional views for showing the processing steps for fabricating the trench MOSFET with A 2 -B 2  cross section as  FIG. 5A  and C 2 -D 2  cross section as  FIG. 7   
           [0025]      FIG. 10  is a cross-sectional view for showing an alternative step during fabrication process in  FIG. 9F . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0026]    Please refer to  FIG. 4A  and  FIG. 4B  for a preferred A 2 -B 2  cross section and C 2 -D 2  cross section of the trench MOSFET in  FIG. 3  which shows a plurality of substantial square closed cells with split trenched gates. In  FIG. 4A , an N-channel trench MOSFET is formed on an N+ substrate  200  supporting an N epitaxial layer  202 . A plurality of gate trenches  203  are formed within the N epitaxial layer  202  and padded with a gate oxide layer  204  along inner surface. Onto the gate oxide layer  204 , A doped poly-silicon layer  205  is deposited filling within the gate trenches  203  to form a plurality of trenched gates for the N-channel trench MOSFET. A plurality of P body regions  206  in upper portion of the N epitaxial layer  202  surround the trenched gates and encompass n+ source regions  207  near top surface of the N epitaxial layer  202 . A tungsten plug  208  padded with a barrier metal layer of Ti/TiN or Co/TiN is formed filling a contact trench  209  with slope sidewalls to function as a trenched source-body contact which penetrates through an insulation layer  210 , the n+ source region  207  and extending into the P body region  206  to connect the n+ source region  207  and the P body region  206  and connects to a source metal  211  of Al alloys overlying a layer of Ti or Ti/TiN. According to the present invention, each the n+ source region  207  has a Gaussian distribution doping profile from sidewalls of the contact trench  209  to adjacent channel regions near the trenched gates. Within the P body region  206 , a p+ ohmic body doped region  212  is formed surrounding bottom and sidewalls of each the contact trench  209  underneath the n+ source region  207 . 
         [0027]      FIG. 4B  shows the trenched gates intersection area in cell corner which comprises a gate trench  203 ′ padded by the gate oxide layer  204  along inner surface. The trenched gates intersection area further comprises split trenched gates of the doped poly-silicon  205  formed along sidewalls of the gate trench  203 ′. The insulation layer  210  described above also extends between the split trenched gates with a thermal oxide layer  213 ′ underneath in center portion of the trenched gates intersection area. Meanwhile, between the split trenched gates  205  and the insulation layer  210 , there is another thermal oxide layer  213  along sidewall of the split trenched gates, wherein the thermal oxide layer  213 ′ is thicker than the gate oxide  204  because the thermal oxide layer  213 ′ also comprises the gate oxide layer  204  along trench bottom. Because the center portion of the trenched gates intersection area comprises the thermal oxide layer  213 ′ not doped poly-silicon, Qgd 1  of the N-channel trench MOSFET is obviously reduced compared to  FIG. 2A , as illustrated in  FIG. 4B . 
         [0028]    Please refer to  FIG. 5A  and  FIG. 5B  for another preferred A 2 -B 2  cross section and C 2 -D 2  cross section of the trench MOSFET in  FIG. 3 . In  FIG. 5A , the N-channel trench MOSFET has a similar structure to  FIG. 4A  except that, there is an N* on-resistance reduction doped region  314  formed within the N epitaxial layer  302  and surrounding bottom of each the trenched gate to further reduce Qgd of the N-channel trench MOSFET wherein the N* on-resistance reduction doped region  314  has a higher doping concentration than the N epitaxial layer  302 . In  FIG. 5B , there is also an additional N* on-resistance reduction doped region  314  formed within the N epitaxial layer  302  and surrounding bottom of the trenched gates intersection area compared to  FIG. 4B . 
         [0029]    Please refer to  FIG. 6  for another preferred C 2 -D 2  cross section of the trench MOSFET in  FIG. 3 . Compared to  FIG. 5B , the trenched gates intersection area in  FIG. 6  has an additional P* gate-drain charge reduction doped area  414 ′ formed within the N epitaxial layer  402  and surrounding bottom of the trenched gates intersection area to further reduce Qgd 1 . 
         [0030]    Please refer to  FIG. 7  for another preferred C 2 -D 2  cross section of the trench MOSFET in  FIG. 3 . Compared to  FIG. 6 , the insulation layer  510  in  FIG. 7  has a void  515  existing between the split trenched gates of doped poly-silicon layer  505  due to the insulation layer  510  not able to fill up the narrow area between the split trenched gates during fabrication process. 
         [0031]    Please refer to  FIG. 8  for another top view of the trench MOSFET according to this invention. Compared to  FIG. 3 , except for the implementation of the P* gate-drain charge reduction doped region, the trench MOSFET in  FIG. 8  has rounded trenched gate corners and circular trenched source-body contact to further save die area. 
         [0032]    Referring to  FIGS. 9A to 9I  for a series of cross-sectional views to illustrate the processing steps for manufacturing a trench MOSFET with C 2 -D 2  cross sectional as  FIG. 7  and A 2 -B 2  cross section as  FIG. 5A . In  FIG. 9A , a trench mask (not shown) is applied to open a plurality of gate trenches  503  by dry silicon etching process in an N epitaxial layer  502  supported on an N+ substrate  500 , wherein the gate trench located in gate trenches intersection area is illustrated as  503 ′, as shown in C 2 -D 2  cross section. Then, a sacrificial oxide layer (not shown) is grown and removed to repair the sidewall surface of the gate trenches  503  and  503 ′ damaged by the trench etching process. Next, a screen oxide  516  is grown for preventing ion implantation damage. Then an Arsenic ion implantation is carried out to form N* on-resistance reduction region  514  surrounding bottom of each gate trench  503  and  503 ′ with higher doping concentration than the N epitaxial layer  502 . 
         [0033]    In  FIG. 9B , the screen oxide  516  is first removed and a gate oxide layer  504  is deposited or grown overlying inner surface of the gate trenches  503  and  503 ′ and also onto top surface of the N epitaxial layer  502 . After that, the gate trenches  503  and  503 ′ are filled with a doped poly-silicon layer  505  followed by dry etching or CMP (Chemical Mechanical Polishing) of the doped poly-silicon layer  505  to remove it from above the top of the gate trenches and further to a pre-determined depth, forming a plurality of trenched gates for the trench MOSFET. 
         [0034]    In  FIG. 9C , a Boron ion implantation is carried out to form a P type implantation area  517  in upper portion of the N epitaxial layer  502 . Next, after applying a poly mask  518 , a dry poly etching is carried out to form a poly hole  519  defined by the poly mask  518  in center portion of the doped poly-silicon layer  505  in the gate trench  503 ′. The poly hole  519  is extending from top surface of the doped poly-silicon layer  505  in the gate trench  503 ′ to expose center bottom of the gate trench  503 ′, therefore implementing split trenched gates structure in trenched gates intersection area in cell corner as shown in C 2 -D 2  cross section. 
         [0035]    In  FIG. 9D , after removing the poly mask  518 , a P type dopant ion implantation is carried out to form a P* gate-drain charge reduction doped region  520  in upper portion of the P type implantation area  517 , as well as in the trench bottom underneath the poly hole  519 . 
         [0036]    In  FIG. 9E , a step of body diffusion is performed to form a plurality of P body regions  506  extending between the trenched gates, as well as a P* gate-drain charge reduction doped region  514 ′ underneath the trenched gates intersection area and surrounding bottom of the split trenched gates. Then, a step of thermal oxidation is carried out in the body diffusion to form a thermal oxide layer along sidewall of the split trenched gates and covering top surface of the plurality of trenched gates and the N epitaxial layer  502 . Meanwhile, at bottom of the poly hole  519 , the thermal oxidation also increases thickness of the gate oxide  504  on center portion of the trench bottom to form another thermal oxide layer  513 ′. Obviously, the thermal oxide layer  513 ′ is thicker than the gate oxide  504 . 
         [0037]    In  FIG. 9F , an insulation layer  510  comprising BPSG (Boron Phosphorus Silicon Glass) and undoped TEOS (Tetraethyl Orthosilicate) is deposited covering the first thermal oxide layer  513  and extending between the split trenched gates to fill the poly hole and reach the second thermal oxide layer  513 ′. During the insulation layer deposition process, a void  515  is induced due to the insulation layer  510  not able to fill up the narrow poly hole area between the split trenched gates. Then, a contact mask (not shown) is applied onto the insulation layer  510  to define location of contact trench. Next, a dry oxide etching is performed to removing the insulation layer and the thermal oxide layer from where according to the contact mask to form a plurality of contact openings  520  with slope sidewalls. Then, an n+ source ion implantation and diffusion is carried out through the contact openings  520  to form n+ source regions  507  in upper portion of the P body regions  506  with a Gaussian distribution profile from edge of the contact openings  520  to channel regions near the trenched gates. 
         [0038]    In  FIG. 9G , the contact openings  520  are etched to further extending through the n+ source regions  507  and into the P body regions  506  with slope sidewalls by dry Silicon etching to form a plurality of contact trenches  509 . 
         [0039]    In  FIG. 9H , a step of P type dopant BF 2  ion implantation is carried out to form a p+ ohmic body doped region  512  within the P body region  506  and surrounding bottom and sidewalls of each the contact trench  509  underneath the n+ source regions  507 . Then, a RTA (Rapid Thermal Annealing) is performed to activate the P type dopant in the p+ ohmic body doped region  512 . 
         [0040]    In  FIG. 9I , a layer of Ti/TiN  521  is first deposited along inner surface of each the contact trench  509  and top surface of the insulation layer  510  to function as barrier metal layer, then, tungsten metal is deposited onto the barrier layer and filling into the contact trench  509 . After that, the tungsten metal and the barrier metal layer is etched back to be left within the contact trench  509  to act as tungsten plug  508 . Next, a layer of Al alloys overlying a Ti or Ti/TiN layer, or Ti/Ni/Ag is deposited onto the tungsten plug  508  and the insulation layer  510  to act as a source metal  511  which connected to the n+ source regions  507  and the P body regions  506  via the tungsten plug  508 . 
         [0041]      FIG. 10  is a cross-sectional view for showing an alternative step during fabricating the trench MOSFET which is similar with  FIG. 9F  except that, before the N type dopant ion implantation, a thin screen oxide layer  525  is deposited along inner surface of the contact opening  520  to minimize ion implantation damage. The screen oxide layer  525  is then removed before the dry Silicon etching as in  FIG. 9G . 
         [0042]    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.