Abstract:
A structure of trench MOS rectifier and a method of forming the same are disclosed including a plurality of trenches formed in the n− drift epitaxial layer, a plurality of MOS structure formed on the substrate either in discrete islands or in rows. Asides the MOS gates there are source regions formed under the mesas. A top metal served as an anode is then formed on the resulted front surface connecting the MOS gates and the adjacent source regions.

Description:
RELATED APPLICATION 
       [0001]    This application is a divisional application of prior application Ser. No. 14/215,224, which was filed on Mar. 17, 2014 of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention pertains to a semiconductor device, in particularly, to a trench MOS (metal/oxide/semiconductor) device and a method of making the same. 
       DESCRIPTION OF THE PRIOR ART 
       [0003]    The Schottky diode is an important power device and is widely applied for power supply switch, motor control, telecom switch, industrial automation, electrical automation, etc., and several high speed power-switches. What spotlights on the Schottky diode may be due to good performance thereof. For instance, the forward bias voltage drop is low; the reverse recovery time (t RR ) is very short; and the breakdown voltage may withstand as high as about 250 voltages at a reverse bias. However, due to the image charge potential barrier lower, the leakage current of Schottky diode is higher than that of PN type diodes and increasing with the reverse bias voltage increases. Another drawback of the Schottky diode is that the reliability of the metal-semiconductor junction is decreasing when an operating temperature is soaring to a critical level thereby lowering the bearing surge voltage capability during forward and reverse biased. 
         [0004]    There are several of conventional trench rectifier devices are developed. Of the one, please refer to another Taiwan patent Application with a series no. 101,140,637 by the present inventor. 
         [0005]    Recently, to solve the foregoing problem, a novel MOS rectifier diode is developed. As shown in the  FIG. 1 , a top metal layer formed on the MOS gate, a metal layer or poly-Si layer  15 /gate oxide layer  10  is connected to the source electrode  5 . A heavily doped source electrode  5  is formed in the p-well. While the region beneath the MOS gate, the current does not flow from the left toward the right but downward to the n+ substrate  5  via a vertical channel  30  due to equal potential between the drain electrode and the source electrode. At a reverse bias, the vertical channel is cutoff by a depletion region generated by the p-wells. The MOS ensure the performance of trench rectifier device is similar to a Schottky diode while it is forward biased whereas the performance of it has much improvement while it is under reverse biased. It is because image charge potential barrier lowering does not occur so that the leakage current is a constant rather than increase with increasing the reverse biased voltage. 
         [0006]    The present invention discloses a novel trench MOS rectifier device to use any planar area of the substrate that can be utilized so as to reach the purpose of lower forward biased voltage V F  and least leakage current. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to disclose a novel trench MOS rectifier device having a plurality of planar MOS gates to lower a switched on forward voltage and least leakage while under reversal biased. 
         [0008]    In accordance with a first preferred embodiment of the present invention, a trench rectifier device is formed in an n type lightly doped epitaxial layer (n− epi-layer) supported by a heavily doped n type semiconductor substrate having a top metal layer as an anode electrode over planar MOS gates formed on the mesas and on the implanted regions aside the MOS gates. 
         [0009]    The trench rectifier device comprises a plurality of trenches in parallel and spaced each other with a mesa formed in the n− epi-layer. A trench oxide layer conformally is formed on bottoms and sidewalls of the trenches. A first poly-Si layer having conductive impurities in-situ doped is formed to fill the trenches and to form a trench MOS structure. A plurality of planar MOS gates having a second poly-Si layer/a thinner gate oxide layer are then formed on the mesas. The second poly-Si layer has conductive impurities in-situ doped. The asides the MOS structure are p-type impurity doped regions formed into the mesas. After exposed planar gate oxide removal, a top-metal layer served as anode of the rectifier device is then formed on the MOS structures and on the p-implanted regions. A bottom metal layer formed on the backside of the heavily doped n type semiconductor substrate served as cathode. 
         [0010]    The modified structure of the first preferred embodiment further comprises n+ regions formed in the p implanted regions to further lower the forward voltage. 
         [0011]    In accordance with a second preferred embodiment of the present invention, a trench rectifier device is formed in an n− epi-layer supported by a heavily doped n type semiconductor substrate having a top metal layer served as n anode over a plurality of rows of planar MOS gates formed on the mesas and first poly-Si layer and on the implanted regions aside the MOS gates. 
         [0012]    The trench rectifier device comprises a plurality of trenches in parallel and spaced each other with a mesa formed in the n− epi-layer. A thermal trench oxide layer is formed on bottoms and sidewalls of the trenches. A first poly-Si layer having conductive impurities in-situ doped is formed to fill the trenches and to form a trench MOS structure. A plurality of rows of planar MOS gates having a second poly-Si layer/a thinner gate oxide layer are then formed on the mesas and a first poly-Si layer. The two sides of the MOS structure are p-type impurity doped regions formed into the mesas. After exposed planar gate oxide removal, a top-metal layer served as anode electrode of the device is then formed on the MOS structures and on the p-implanted regions. A bottom metal layer formed on the backside of the heavily doped n type semiconductor substrate served as cathode. 
         [0013]    The modified structure of the second preferred embodiment further comprises n+ regions formed in the p implanted regions to further lower the forward voltage. 
         [0014]    In accordance with a third preferred embodiment of the present invention, a trench rectifier device is formed in an n type lightly doped epitaxial layer supported by a heavily doped n type semiconductor substrate having a top metal layer served as an anode over a plurality of rows of first poly-Si layer included a plurality of planar MOS gates on the mesas and on the implanted regions. 
         [0015]    The trench rectifier device comprises a plurality of trenches in parallel and spaced each other with a mesa formed in the n− epi-layer. A thermal trench oxide layer is formed on bottoms and sidewalls of the trenches. A planar gate oxide layer is formed on the mesas. A first poly-Si layer having conductive impurities in-situ doped is then formed to over-fill the trenches and the planar gate oxide. The first poly-Si layer is then pattern as a plurality rows including planar MOS gates having a first poly-Si layer/a thinner gate oxide layer formed on the mesas. The two sides of the MOS structure are p-type impurity doped regions formed into the mesas. After exposed planar gate oxide removal, a top-metal layer served as anode electrode of the device is then formed on the MOS structures and on the p-implanted regions. A bottom metal layer formed on the backside of the heavily doped n type semiconductor substrate served as cathode. 
         [0016]    The modified structure of the second preferred embodiment further comprises n+ regions formed in the p implanted regions to further lower the forward voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a cross-sectional view illustrating a rectifier having anode electrode connected a planar MOS gate with implanted regions in accordance with a prior art; 
           [0018]      FIG. 2 a    is a top view illustrating a trench MOS rectifier (the top metal layer is not shown) in accordance with a first preferred embodiment of the present invention; 
           [0019]      FIG. 2 b    is a top view illustrating the modified design of the trench MOS rectifier (the top metal layer is not shown) in accordance with the first preferred embodiment of the present invention; 
           [0020]      FIG. 2 c    is a top view illustrating a trench MOS rectifier (the top metal layer is not shown) in accordance with a second preferred embodiment of the present invention; 
           [0021]      FIG. 2 d    is a top view illustrating the modified design of the trench MOS rectifier (the top metal layer is not shown) in accordance with the second preferred embodiment of the present invention; 
           [0022]      FIG. 2 e    is a top view illustrating a trench MOS rectifier (the top metal layer is not shown) in accordance with a third preferred embodiment of the present invention; 
           [0023]      FIG. 2 f    is a top view illustrating the modified design of the trench MOS rectifier (the top metal layer is not shown) in accordance with the third preferred embodiment of the present invention; 
           [0024]      FIG. 3  is a cross-sectional view illustrating a plurality of trenches formed in the n− epitaxial layer and a trench oxide layer successively formed thereon in accordance with the first preferred embodiment of the present invention; 
           [0025]      FIG. 4  is a cross-sectional view illustrating a first poly-Si layer refilled the trenches and then an etch back performed to remove the first poly-Si layer and the trench oxide layer over the mesas in accordance with the first preferred embodiment of the present invention; 
           [0026]      FIG. 5  is a cross-sectional view illustrating a planar oxide layer formed on the mesas; in accordance with the first preferred embodiment of the present invention; 
           [0027]      FIG. 6  is a cross-sectional view illustrating a second poly-Si layer formed and then a photoresist patterned formed on the second poly-Si layer in accordance with the first preferred embodiment of the present invention; 
           [0028]      FIG. 7A ,  FIG. 7B  and  FIG. 7C  are the cross-sectional views respectively, along the A-A′ line, the B-B′ line and the C-C′ line shown in  FIG. 2 a    illustrating the second poly-Si layer patterned and p type impurity implanted regions formed under the mesas in accordance with the first preferred embodiment of the present invention; 
           [0029]      FIG. 8A ,  FIG. 8B  and  FIG. 8C  are the cross-sectional views respectively, along the A-A′ line, the B-B′ line and the C-C′ line shown in  FIG. 2 a    illustrating the final structure of a trench MOS rectifier in accordance with the first preferred embodiment of the present invention 
           [0030]      FIG. 9A ,  FIG. 9B  and  FIG. 9C  are the cross-sectional views respectively, along the A-A′ line, the B-B′ line and the C-C′ line shown in  FIG. 2 b    illustrating n+ impurities implanted region formed in the p regions using a photoresist pattern as a mask in accordance with a modified structure of the first preferred embodiment of the present invention; 
           [0031]      FIG. 10A ,  FIG. 10B  and  FIG. 100  are the cross-sectional views respectively, along the A-A′ line, the B-B′ line and the C-C′ line shown in  FIG. 2 b    illustrating the modified structure of a trench MOS rectifier in accordance with the first preferred embodiment of the present invention; 
           [0032]      FIG. 11A ,  FIG. 11B  and  FIG. 11C  are the cross-sectional views respectively, along the A-A′ line, the B-B′ line and the C-C′ line shown in  FIG. 2 c    illustrating the second poly-Si layer patterned and p type impurity implanted regions formed under the mesas in accordance with the second preferred embodiment of the present invention; 
           [0033]      FIG. 12A ,  FIG. 12B  and  FIG. 12C  are the cross-sectional views respectively, along the A-A′ line, the B-B′ line and the C-C′ line shown in  FIG. 2 c    illustrating the exposed planar gate oxide layer removed in accordance with the second preferred embodiment of the present invention; 
           [0034]      FIG. 13A ,  FIG. 13B  and  FIG. 13C  are the cross-sectional views respectively, along the A-A′ line, the B-B′ line and the C-C′ line shown in  FIG. 2 c    illustrating the final structure of a trench MOS rectifier in accordance with the second preferred embodiment of the present invention; 
           [0035]      FIG. 14A ,  FIG. 14B  and  FIG. 14C  are the cross-sectional views respectively, along the A-A′ line, the B-B′ line and the C-C′ line shown in  FIG. 2 d    illustrating the modified structure of a trench MOS rectifier in accordance with the second preferred embodiment of the present invention; 
           [0036]      FIG. 15  is a cross-sectional view illustrating a plurality of trenches formed in the n− epitaxial layer and a trench oxide layer successively formed thereon and then a CMP (chemical mechanical polishing) performed to remove the trench oxide layer over the mesa in accordance with the third preferred embodiment of the present invention; 
           [0037]      FIG. 16  is a cross-sectional view illustrating a planar gate oxide layer formed on the mesa in accordance with the third preferred embodiment of the present invention; 
           [0038]      FIG. 17A ,  FIG. 17B  and  FIG. 17C  are the cross-sectional views respectively, along the A-A′ line, the B-B′ line and the C-C′ line shown in  FIG. 2 e    illustrating the first poly-Si layer patterned and p type impurity implanted regions formed under the mesas in accordance with the third preferred embodiment of the present invention; 
           [0039]      FIG. 18A ,  FIG. 18B  and  FIG. 18C  are the cross-sectional views respectively, along the A-A′ line, the B-B′ line and the C-C′ line shown in  FIG. 2 e    illustrating the final structure of a trench MOS rectifier in accordance with the third preferred embodiment of the present invention; 
           [0040]      FIG. 19A ,  FIG. 19B  and  FIG. 19C  are the cross-sectional views respectively, along the A-A′ line, the B-B′ line and the C-C′ line shown in  FIG. 2 f    illustrating the modified structure of a trench MOS rectifier in accordance with the third preferred embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0041]    The present invention discloses a trench MOS device structure, as described in the following figures hereinafter, the uppercase A, B, C in FIG. # A, FIG. # B, FIG. # 0  represent, respectively, along the cutting lines AA,′BB′ and CC′ of the top views  FIG. 2 a   - FIG. 2 e   . The label “+” and “−” following n or p represent, respectively, heavily doped (implanted) and lightly doped (implanted). To facilitate illustrating the detailed structure, a top metal layer  180  is skipped in the top plan views. As to the detailed connection relationship between elements in the semiconductor device, please refer to the cross-sectional views in the following figures. 
         [0042]    In accordance with a first preferred embodiment of the present invention, a trench rectifier device is illustrated in the plan-view  FIG. 2 a    and cross-sectional views,  FIG. 8A ,  FIG. 8B  and  FIG. 8C . The figures show an n− epi-layer  105  on a n+ semiconductor substrate  100  having a plurality of trenches  115  in parallel and spaced each other with a mesa  118  formed therein. A trench oxide layer  120  is conformally formed on bottoms and sidewalls of the trenches. A first poly-Si layer  130  having conductive impurities in-situ doped is formed on the trench oxide layer  120  and filled the trenches  115  to form a trench MOS structure. A plurality of MOS structures formed on the mesas  118 . Each MOS structure has a second poly-Si layer  140  formed on the planar gate oxide layer  127 , which is formed on the mesa  118 . The second poly-Si layer  140  has conductive impurities in-situ doped. Asides of the MOS structure are p-type impurity doped region  135  formed into the mesas as sources regions. A top-metal layer  180  served as anode electrode of the device is then formed on the MOS structures and on the sources regions  135  to connect them. A bottom metal layer  190  as a cathode is formed on the backside of the n+ semiconductor substrate  100 . 
         [0043]    Alternatively, each p-type impurity implanted region  135  further comprises two separate n+ impurity implanted regions  145 . The n+ impurity implanted regions  145  adjacent the MOS structure. Please refer to plan-view  FIG. 2B  and cross-sectional views  FIG. 10A ,  FIG. 10B  and  FIG. 100 . 
         [0044]    In accordance with a second preferred embodiment of the present invention, a trench rectifier device is illustrated in the plan-view  FIG. 2 c    and cross-sectional views,  FIG. 13A ,  FIG. 13B  and  FIG. 13C . The figures shows a n− epi-layer  105  on a n+ semiconductor substrate  100  semiconductor substrate  100  having a plurality of trenches  115  in parallel and spaced each other with a mesa  118  formed therein. A trench oxide layer  120  conformally formed on bottoms and sidewalls of the trenches  115 . A first poly-Si layer  130  having conductive impurities in-situ doped formed on the trench oxide layer  120  and filled the trenches  115  to form a trench MOS structure. A plurality of row of planar MOS structures formed on and across the mesas  118  and trenches. Each planar MOS structure has a second poly-Si layer  140  formed on the planar gate oxide layer  127 , which is formed on the mesa  118  and on the first poly-Si layer  130 . Asides the MOS structure are p-type impurity doped regions  135  formed into the mesas as sources regions. A top-metal layer served as anode electrode of the rectifier device is then formed on the planar MOS structures and on the sources regions  135 . A bottom metal layer as a cathode is formed on the backside of the n+ semiconductor substrate  100 . 
         [0045]    The differences between two preferred embodiments are at the morphology of the planar MOS structure. In the first preferred embodiment, the MOS structures are formed on the mesas  118  only but in the second preferred embodiment, the MOS structures are in a form of rows formed across the mesas  118  and first poly-Si layer  130 . 
         [0046]    In an alternative embodiment of the second preferred embodiment, each p-type impurity implanted region  135  further comprises two separate n+ impurity implanted regions  145 . The n+ impurity implanted region  145  adjacent the MOS structure. Please refer to plan-view  FIG. 2 d    and cross-sectional views  FIG. 14A ,  FIG. 14B  and  FIG. 14C . 
         [0047]    In a third preferred embodiment of the present invention, a trench rectifier device is illustrated in the plan-view  FIG. 2 e    and cross-sectional views,  FIG. 18A ,  FIG. 18B  and  FIG. 18C . The figures shows a n− epi-layer  105  on a n+ semiconductor substrate  100  having a plurality of trenches  115  in parallel and spaced each other with a mesa  118  formed therein. A trench oxide layer  120  is conformally formed on bottoms and sidewalls of the trenches  115 . A planar gate oxide layer  127  is formed on the mesas  118 . A first poly-Si layer  130  having conductive impurities in-situ doped is then formed on the trench oxide layer  120  and filled the trenches  115  until over the mesas  118  by a predetermined thickness. A plurality of row of defined first poly-Si layer  130  including planar MOS structures formed on the mesas  118 . Asides the MOS structure are p-type impurity doped regions  135  formed into the n− epi-layer  105 . A top metal layer is as an anode electrode of the device is then formed on the first poly-Si layer and on the sources regions  135 . A bottom metal layer  190  served as a cathode is formed on the backside of n+ semiconductor substrate  100 . 
         [0048]    In an alternative embodiment of the third preferred embodiment, each p-type impurity implanted region  135  further comprises two separate n+ impurity implanted regions  145 . The n+ impurity implanted region  145  adjacent the MOS structure. Please refer to plan-view  FIG. 2 e    and cross-sectional views  FIG. 19A ,  FIG. 19B  and  FIG. 19C . 
         [0049]    The detailed processes for forming trench rectifier are as follows. 
         [0050]    Please refer to  FIG. 3 . The cross-sectional view depicts an n− epi-layer  105  on a n+ semiconductor substrate  100  having a plurality of trenches  115  in parallel and spaced each other with a mesa  118  formed therein. The trenches  115  may be formed by a dry etch using a photoresist pattern layer or a hard mask layer with a patterned nitride layer/pad oxide as an etching mask (not shown). 
         [0051]    Subsequently, a thermal oxidation process is carried out to form a trench oxide layer  120  conformally formed on the sidewalls and bottoms of the trenches and the mesas  118 . The processes can repair the damage during etching. 
         [0052]    Referring to  FIG. 4  a first poly-Si layer  130  with in-situ doped conductive impurities is deposited within the trenches  115  until overfilled. Thereafter, an etching back or a chemical mechanical polishing technology is performed to remove the first poly-Si layer  130  overflowed and the trench oxide layer  120  on the mesa using the surface of the n− epi-layer  105  as an etching stop layer. 
         [0053]    Next, please refer to  FIG. 5 ; a thermal oxidation is carried out to form a planar gate oxide layer  127  on the first poly-Si layer  130  and the mesa  118 . The planar gate oxide layer  127  is a thinning oxide layer about 1-50 nm in thickness. The trench oxide layer  120  has about two folds to 100 folds in thickness than the planar gate oxide layer  127 . 
         [0054]    Thereafter, a second poly-Si is deposited on the planar gate oxide layer  127 . A photoresist pattern  142  is formed on the second poly-Si layer  140  to define the positions of the planar MOS gate structure. 
         [0055]    An anisotropic etch is performed to pattern the second poly-Si layer  140  using the photoresist pattern  142  as an etching mask. The cross-sectional view  FIG. 7A  along the cutting line AA′ of the  FIG. 2 a    depicts the planar MOS gate, and the cross-sectional view  FIG. 7B  along the cutting line BB′ of the  FIG. 2 a    depicts source regions asides the trench MOS. In the regions, the second poly-Si layer is removed and a first ion implantation is carried out by implanting p type impurities to form p regions  135 . After patterning, the photoresist pattern is stripped off. Hereinafter, unless otherwise note, the ion implantations will be performed by blanketing so that all the exposed areas will be implanted with the impurity ions. However, the regions with the impurity ions in the first poly-Si layer  130  and the second poly-Si layer  140  are skipped without shown in the figures for simplicity. 
         [0056]    The doses used for implanting the p implanted region  135  are to make the concentration of the p impurity ions is higher than that of the n− epi-layer  105  by 1-3 order of magnitude. For example, the implant doses are between about 1E12-1E14/cm 2  and the implant energy is between about 10 keV-1000 keV. 
         [0057]    The cross-sectional view  FIG. 7C  along the cutting line CC′ of the  FIG. 2 a   , shows the planar MOS gate and the p implant region  135 . The cutting line CC′ is along the longitudinal direction of the trenches  115 . An anneal process is then carried out to activate the implanted ions. 
         [0058]    Thereafter, a dilute HF or NH 4 F buffer solution is used to remove the exposed planar gate oxide layer  127 . Finally, a top-metal layer  180  is deposited on the exposed area to connect the planar gate and the source region  135 . Alternatively, before forming the top metal layer  180 , a self-aligned silicide process is performed. The silicide process includes sequentially sputtering Ti and TiN on the exposed area and then performing rapid thermal anneal (RTA) process to make the metal layer reactive with the second poly-Si layer  140  and the n− epi-layer  105  to form metal silicide (not shown). The un-reactive metal layers are then removed by wet etching. The top metal layer  180  may be one layer or two or three stack layers such as TiNi/Ag, TiW/AI or Al etc.  FIG. 8A - FIG. 8C  shows cross-sectional views of the structure. 
         [0059]    The modified embodiment of the first preferred embodiment is to form two separated n+ regions  145  in the p implanted region  135 , as shown in  FIGS. 9A-9C .  FIGS. 10A-10C  show cross-sectional views of the final structure of the trench MOS rectifier.  FIG. 2 b    is the top-view without the top-metal layer  180 . The doses for n+ regions  145  implantation are between about 1E13-1E15/cm 2 . 
         [0060]    In accordance the second preferred embodiment, the processes before forming the photoresist layer on the second poly-Si layer  140  shown in  FIGS. 6A-6C  are the same as the first preferred embodiment. 
         [0061]    Please refer to  FIGS. 11A-11C . A photoresist pattern layer  142  for planar MOS structure definition is formed on the second poly-Si layer  140 . The photoresist pattern layer  142  masked the second-poly Si layer  140  is to define a plurality of rows of MOS gates along the AA′ cutting line of  FIG. 2 c   . The photoresist pattern layer  142  exposed the second-poly SI  140  is to define the implanted regions along the BB′ cutting line of  FIG. 2 c   . An anisotropic etching is then carried out to pattern the second poly-Si layer  140  using the photoresist pattern layer  142  as etching mask. After that a first ion implantation is performed to implant p-type conductive ions to form the p-region  135 . The doses and the energy for the first implantation are the same as that of the first preferred embodiment. 
         [0062]    After stripped off the photoresist layer  142 , an anneal process is then carried out to activate the implanted ions. A dilute HF or NH 4 F buffer solution is then used to remove the exposed planar gate oxide layer  127 . 
         [0063]    Finally, a top-metal layer  180  is deposited on the exposed area to connect the planar gate and the source region. Alternatively, before forming the top metal layer  180 , a self-aligned silicide process is performed. 
         [0064]    The top metal layer  180  may be one layer or two or three layers such as TiNi/Ag, TiW/AI or Al etc.  FIG. 13A - FIG. 13C  shows cross-sectional views of the structure.  FIG. 2 c    shows the top view of the trench MOS rectifier in accordance with the second preferred embodiment. 
         [0065]    The modified embodiment of the second preferred embodiment is to form two separated n+ regions  145  in the p implanted region  135 , using the implantation mask shown in  FIGS. 9A-9C .  FIGS. 14A ˜ 14 C show cross-sectional views of the final structure of the trench MOS rectifier.  FIG. 2 b    is the top-view without the top-metal layer  180 .  FIG. 2 d    shows the top view of the trench MOS rectifier in accordance with the modified second preferred embodiment . . . . 
         [0066]    The aforementioned first and second preferred embodiments, the poly-Si layers include the first poly-Si layer  130  and the second poly-Si layer  140 . According to a third preferred embodiment, only the first poly-Si layer  130  is deposited. 
         [0067]    Turning back to  FIG. 3 , the trench oxide layer  120  is formed on the sidewalls and bottom of the trenches  115  and the mesas  118 . Next, a CMP method is carried out to remove the trench oxide layer  120 . A planar gate oxide layer  127  is formed by thermal oxidation. The planar gate oxide layer  127  is much thinner than the original trench oxide layer  120 , as shown in  FIG. 16 . 
         [0068]    Thereafter, a first poly-Si layer  130  with in-situ doped conductive impurities is deposited to fill the trenches  115  and over the mesa  118  by a predetermined thickness. A photoresist pattern layer  132  is deposited on the first poly-SI layer  130  to define a plurality of MOS gates. An anisotropic etching is then performed using the photoresist pattern layer  132  as etch mask. Please refer to  FIG. 17A  to  FIG. 17C . The photoresist pattern layer  132  masked a plurality of rows of MOS gates, which is along the AA′ cutting line of  FIG. 2 e   . Nevertheless, the regions along the BB′ cutting line of  FIG. 2 e    is exposed. A first ion implantation to with p-type conductive impurities is then implanted into the exposed region to form p regions  135 . The doses and energy of the first ion implantation are between about 1E12-1E14/cm 2  and between about 10 keV-1000 keV, respectively. 
         [0069]    The photoresist pattern layer  132  is then removed. After that an anneal process is then performed to activate the ions. Thereafter, a dilute HF or NH 4 F buffer solution is used to remove the exposed planar gate oxide layer  127 . Finally, a top-metal layer  180  is deposited on the exposed area to connect the planar gate and the source region  135 . Alternatively, before forming the top metal layer  180 , a self-aligned silicide process is performed. The top metal layer  180  may be one layer or two or three stack layers such as TiNi/Ag, TiW/AI or Al etc.  FIG. 18A - FIG. 18C  shows cross-sectional views of the structure. 
         [0070]    The modified embodiment of the third preferred embodiment is to form two separated n+ regions  145  in the p implanted region  135 .  FIGS. 19A-190  show cross-sectional views of the final structure of the trench MOS rectifier.  FIG. 2 f    is the top-view without the top-metal layer  180  shows the top view of the trench MOS rectifier in accordance with the modified second preferred embodiment. 
         [0071]    The differences among the three preferred embodiments include follows: in the first preferred embodiment, the MOS gates are formed on the mesas only but in the second preferred embodiment, a plurality of rows of the MOS gates are formed across the trenches and mesas. Furthermore, in the first and second preferred embodiments, the poly-Si in the trenches and the poly-Si as MOS gates are formed separately, while in the third preferred embodiment the poly-Si in the trenches and the poly-Si as MOS gates are deposited at the same step. Also in the third preferred embodiment a plurality of rows of the MOS gates are formed across the trenches and mesas, but the poly-Si inside the trenches and MOS gates over it are integrated as a whole part. 
         [0072]    The benefits of the present invention are: 
         [0073]    (1). In comparison with the conventional planar MOS rectifier, the trench MOS rectifier has better capability to decrease the leakage current while the device is under reverse bias. 
         [0074]    (2). The trench MOS rectifier according to the present invention includes also a planar MOS structure which has a thinner planar gate oxide layer than the trench gate oxide so that the device has a lower switch on voltage and least the leakage current. 
         [0075]    (3). The p bodies at two sides of a planar MOS gate further comprise an n+ region each furthermore to decrease the forward voltage V F . 
         [0076]    As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrated of the present invention rather than limiting of the present invention. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures.