Abstract:
Semiconductor device fabrication method and devices are disclosed. The semiconductor power device is formed on a semiconductor substrate having a plurality of trench transistor cells each having a trench gate. Each of the trench gates having a thicker bottom oxide (TBO) formed by a REOX process on a polysilicon layer deposited on a bottom surface of the trenches.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Priority Claim 
       [0001]    This application is a Continuation-in-Part (CIP) application and claims the priority benefit of a co-pending application Ser. No. 13/560,247 filed on Jul. 27, 2012. Application Ser. No. 13/560,247 is a Divisional application of Ser. No. 12/551,417 filed on Aug. 31, 2009 and now issued as U.S. Pat. No. 8,252,647. The disclosures made in application Ser. Nos. 12/551,417 and 13/560,247 are hereby incorporated by reference in the present patent application. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention generally relates to the methods and configuration for fabricating a trench semiconductor power device, e.g., a DMOS device, and more particularly to the device configurations and methods for fabricating a trench semiconductor power device with variable-thickness gate oxides. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    A DMOS (Double diffused MOS) transistor is a type of MOSFET (Metal Oxide Semiconductor Field Effect Transistor) that uses two sequential diffusion steps aligned to a common edge to form a channel region of the transistor. DMOS transistors are often implemented as a high voltage, high current device as discrete transistors or as components in power integrated circuits. The advantage of such applications is because the DMOS transistors can provide high current per unit area with a low forward voltage drop. 
         [0004]    One particular type of DMOS transistor is a trench DMOS transistor. In this type of DMOS transistor, the gate is formed in a trench and the channel is formed around the sidewalls of the trench gate and the channel extends from the source towards the drain. The trench gate is lined with a thin oxide layer and filled with polysilicon. Compared with a planar gate DMOS device, the trench DMOS allows less constricted current to flow and thereby provides lower values of specific on-resistance. 
         [0005]    In order to improve the device performance, it is often necessary to allow flexibility in the manufacturing processes to more conveniently fabricate a trench DMOS transistor to adjust the thickness of the trench oxide. The device performance is improved by strategically adjusting the thickness of the gate oxide at different portions inside the trench. Specifically, a thinner gate oxide is preferred at the upper portion of the trench to maximize channel current. By contrast, a thicker gate oxide is desired at the bottom portion of trench to support higher gate-to-drain breakdown voltage. 
         [0006]    U.S. Pat. No. 4,941,026 discloses a vertical channel semiconductor device including an insulated gate electrode having a variable thickness oxide, but does not illustrate how to make such a device. 
         [0007]    U.S. Pat. No. 4,914,058 discloses a process for making a DMOS, including lining a groove with a nitride to etch an inner groove having sidewalls extending through the bottom of the first groove, and lining the inner groove with a dielectric material by oxidation growth to obtain increased thickness of the gate trench dielectric on the sidewalls of the inner groove. 
         [0008]    US publication No. 2008/0310065 discloses a transient voltage suppressing (TVS) circuit with uni-directional blocking and symmetric bi-directional blocking capabilities integrated with an electromagnetic interference (EMI) filter supported on a semiconductor substrate of a first conductivity type. The TVS circuit integrated with the EMI filter further includes a ground terminal disposed on the surface for the symmetric bi-directional blocking structure and at the bottom of the semiconductor substrate for the uni-directional blocking structure and an input and an output terminal disposed on a top surface with at least a Zener diode and a plurality of capacitors disposed in the semiconductor substrate to couple the ground terminal to the input and output terminals with a direct capacitive coupling without an intermediate floating body region. The capacitors are disposed in trenches having an oxide and nitride lining. 
         [0009]    A difficulty arises during polysilicon gate backfill in the trench if a thick oxide is uniformly formed in the trench, producing a higher trench aspect ratio (ratio of depth A to width B) as shown in the prior art. By way of example,  FIGS. 1A-1D  are cross-sectional views illustrating a prior art method of forming a single gate of the prior art. As shown in  FIG. 1A , a trench  106  is formed in a semiconductor layer  102 . A thick oxide  104  is formed on the bottom and sidewalls of the trench  106  which increases its aspect ratio A/B. Polysilicon  108  is in-situ deposited into the trench  106 . Due to the high aspect ratio of the polysilicon deposition, a keyhole  110  tends to form as shown in  FIG. 1B . As shown in  FIG. 1C , the poly  108  is etched back followed with an isotropic high temperature oxidation (HTO) oxide etch as shown in  FIG. 1D , throughout which a portion of the keyhole  110  remains. 
         [0010]      FIG. 2  is a cross-sectional view of a current shield gate trench (SGT) device  200  having a shield poly gate with an Inter-Poly Oxide (IPO)  202  between a first polysilicon structure that forms a gate  204  and a second polysilicon structure  206  that acts as a conductive shield. According to one prior art process, such a structure is formed by a process that involves two etch-back steps (of the polysilicon layer  206  and of the IPO oxide layer  202 ) in forming the IPO  202  between the two polysilicon structures  204 ,  206 . Specifically, the polysilicon that forms the shield  206  is deposited in the trench and etched back and HDP oxide is formed on the shield  206  and etched back to make room for deposition of the polysilicon that forms the gate structure  204 . This approach has the drawback of poor IPO thickness controllability across wafer. The IPO thickness depends on two independent and unrelated etch-back steps, which could cause non-uniform and local thinning of IPO thickness due to either under etch-back of Poly or over etch-back of Oxide or a combination of both. 
         [0011]    Also, in the methods discussed above the thickness of the gate trench dielectric on the thick portion of the side wall versus the thickness at the bottom of the trench are linked together. One thickness cannot be altered without affecting the other thickness. 
         [0012]    For the above reasons, there is a need to provide new device configurations and new manufacturing methods for the semiconductor power devices to provide more convenient manufacturing processes to more flexibly adjust the gate oxide thickness along different parts of the trench gates such that the above discussed technical difficulties and limitations can be resolved. 
       SUMMARY OF THE PRESENT INVENTION 
       [0013]    It is an aspect of the present invention to provide a new and improved device configuration and manufacturing method for providing a semiconductor power device with reduced gate to drain capacitance by adjusting the gate oxide thickness, especially, the thickness of the trench bottoms for trenches with a high aspect ratio. 
         [0014]    Another aspect of the present invention is to provide a new and improved device configuration and manufacturing method for providing a semiconductor power device with reduced gate to drain capacitance for high density transistor cells manufactured with trench gates having high aspect ratios. The improved processes provide simplified and low cost processing steps to fabricate thicker bottom oxide (TBO) trenches for high density transistor cells such that the difficulties and imitations encounter by the conventional manufacturing processes can be resolved to produce improved device performance. 
         [0015]    Briefly in a preferred embodiment this invention discloses a semiconductor power device formed on a semiconductor substrate having a plurality of trench transistor cells each having a trench gate. Each of the trench gates having a thicker bottom oxide (TBO) formed by a Poly REOX process on a polysilicon layer deposited on a bottom surface of the trenches. 
         [0016]    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 
         [0017]      FIGS. 1A-1D  are cross-sectional schematic diagrams illustrating trench gate fabrication according to the prior art. 
           [0018]      FIG. 2  is a cross-sectional schematic diagram of a trench gate including an inter-poly oxide (IPO) between Poly1 and Poly2 of the prior art. 
           [0019]      FIGS. 3A-30  are cross-sectional views illustrating a process of fabricating a trench DMOS with variable-thickness gate trench oxides for single poly gate case according to an embodiment of the present invention. 
           [0020]      FIGS. 4A-4M  are cross-sectional views illustrating a process of fabricating a trench DMOS with variable-thickness gate trench oxides for shield poly gate case according to an embodiment of the present invention. 
           [0021]      FIGS. 5A-5F  are cross-sectional views illustrating an alternative process of fabricating a trench DMOS with variable-thickness gate trench oxides for shield poly gate case according to an embodiment of the present invention. 
           [0022]      FIGS. 6A to 6F  are cross-sectional views illustrating an alternative process of fabricating a trench DMOS with a thicker bottom oxide (TBO) for shield poly gate according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    In embodiments of the present invention as illustrated below, separated processing steps are applied to make the bottom dielectric layer to have a greater thickness than the dielectric layer on the trench sidewalls A thicker bottom dielectric layer reduces the capacitance between the trench gate and the drain of the DMOS transistors. 
         [0024]      FIGS. 3A to 3O  are cross-sectional views illustrating the fabrication process steps for manufacturing a trench DMOS with variable-thickness trench gate oxides for a single polysilicon (poly) gate of the type depicted in  FIG. 1D  according to an embodiment of the present invention. 
         [0025]    As shown in  FIG. 3A , a trench  306  of width A is formed in a semiconductor substrate  302 . By way of example and not by way of limitation, the trench  306  is formed by applying a hard mask (not specifically shown), e.g., oxide or nitride, which may then be removed or left in place. Alternatively, the trench  306  may also be formed by applying using a photoresist (PR) mask (not shown). An oxide  304  (or other insulator) is deposited to fill the trench  306 . A chemical mechanical planarization (CMP) is carried out on the oxide  304  followed by an etching back to recess the oxide  304  in the trench  306  as shown in  FIG. 3B , leaving an thick block of the oxide  304  filling a substantially portion of the lower part of the trench and exposing the silicon sidewall of upper portion of the trench. In  FIG. 3C , a thin oxide  308  is then grown on the exposed sidewall of the trench  306  and on the top surface of the semiconductor substrate  302 . By way of example, and not by way of limitation, the thickness of the thin oxide  308  has a range between about 50 Angstroms to 100 Angstroms. 
         [0026]      FIG. 3D  shows a step of depositing a layer of oxide etch resistant material, such as nitride  310 , on top of the oxide  308  and the oxide  304 . In an exemplary embodiment, the nitride  310  may composed of a silicon nitride. Alternatively, the etch resistant layer  310  may compose of a polysilicon layer since the polysilicon layer also has high etch resistance during subsequent oxide etch. The thickness of the nitride  310  determines the bottom oxide sidewall thickness T 1 , which may be between about 500 angstroms and about 5000 angstroms. The nitride  310  is then anisotropically etched back leaving one or more oxide etch resistant spacers  311  on the sidewall of the trench  306  as shown in  FIG. 3E . The thick oxide block  304  may then be anisotropically etched to a predetermined thickness T 2  at the bottom of the trench  306  as shown in  FIG. 3F . The thickness T 2  may be between about 500 angstroms and about 5000 angstroms. The material such as a nitride material that forms the spacer(s)  311  is preferably resistant to the process used to etch the oxide  304 . The spacer(s)  311  therefore act as an etch mask to define a width A′ of a trench etched into the oxide  304 . In this method, the thicknesses T 1  and T 2  are decoupled, i.e., the thickness T 1  does not depend on the thickness T 2 . In general, it is desirable for T 2  to be greater than T 1 . This may be accomplished more easily if the thicknesses T 1  and T 2  are decoupled. After etching, the spacers  311  and thin oxide  308  may be removed leaving behind a trench with a top portion of width A and a narrower bottom portion of width A′ lined by the remaining portion of oxide  304  as shown in  FIG. 3G . 
         [0027]    Gate oxide (or dielectric)  314  may then be grown on top of the semiconductor substrate  302  and on portions of the sidewall of the trench that are not covered by the remaining oxide  304  leaving the top portion with a width A″ that is greater than the width A′ of the bottom portion as shown in  FIG. 3H . The trench “aspect ratio” is effectively reduced for easier filling due to the wide trench top portion having width A″. Conductive material, such as doped polysilicon may then be deposited to fill the trench.  FIG. 3I  shows the polysilicon gap fill  316  in a narrow trench case, e.g., where the width A″ at the top of the trench is about 1.2 microns, where the doped polysilicon can easily fill up the trench completely. The polysilicon  316  is then etched back to form a single gate poly as shown in  FIG. 3J . The polysilicon  316  acts with the gate dielectric  314  as the gate electrode for the device. 
         [0028]    Alternatively,  FIG. 3K  shows the poly gap fill  318  in the wider trench case, e.g., the diameter A″ at the top of the trench is about 3 microns, where poly cannot easily fill up completely, which leaves a gap  319 . A filler material, such as an HDP oxide  320 , may then be deposited to fill the gap  319  and on top of the poly  318  as shown in  FIG. 3L . The filler material  320  may then be etched back as shown in  FIG. 3M  followed by an etching back of the poly  318  and filler material  320  to form a single gate poly  318  as shown in  FIG. 3N . The device may be completed by a standard process e.g., involving ion implant into selected portions of the semiconductor substrate  302  to form a body region  330  and source regions  332 , followed by the formation of a thick dielectric layer  360  on top of the surface and open contact holes through dielectric layer  360  for depositing a source metal  370  to electrically connect to the source and body regions as shown in  FIG. 3O . 
         [0029]    There are a number of variations on the process described above that are within the scope of embodiments of the present invention. For example,  FIGS. 4A-4M  illustrate a process to fabricate a trench DMOS with variable-thickness gate trench oxides for a shield poly gate of the type depicted in  FIG. 2  according to an embodiment of the present invention. In this embodiment, a composite insulator in the form of an oxide-nitride-oxide (ONO) structure is formed on the sidewall and the bottom of the trench. 
         [0030]    As shown in  FIG. 4A , a trench  401  is first formed in a semiconductor substrate  402 . A thin oxide layer  404  is formed on the sidewall of the trench  401 . The thickness of the oxide layer  404  may be between about 50 Angstroms and 200 Angstroms. Nitride  406  is then deposited on top of the oxide layer  404 . Thickness of the nitride layer  406  may be between about 50 Angstroms and 500 Angstroms. The trench  401  may then be filled with oxide  408 , e.g., using LPCVD and high density plasma. The oxide  408  may then be etched back leaving a trench of width A with thick oxide block substantially filling the tower portion of the trench as shown in  FIG. 4B . 
         [0031]    A thin oxide layer  410  (e.g., a high temperature oxide (HTO)) may optionally be deposited on top of the oxide  408 , on the sidewall of the trench  401  and on top of the nitride  406  as shown in  FIG. 4C . The thickness of the oxide  410  may be between about 50 Angstroms and 500 Angstroms. Conductive material, such as doped polysilicon  412  may then be deposited on top of the oxide  410  (or on the nitride  406  if the oxide  410  is not used). The thickness of the poly  412  depends on the desired bottom oxide sidewall thickness T 1 , which may be between about 500 angstroms and about 5000 angstroms. The poly  412  may then be anisotropically etched back to form the poly spacers  413  as shown in  FIG. 4D . 
         [0032]    The oxide  408  is then anisotropically etched to a desired thickness T 2  at the bottom as shown in  FIG. 4E . The thickness of T 2  may be between about 500 angstroms and about 5000 angstroms. The polysilicon that forms the spacers  413  is preferably resistant to the etch process used to anisotropically etch the oxide  408 . The thickness of the poly spacer  413  on the sidewalls of the trench determines the thickness T 1  therefore determines the width A″ of a trench etched into the oxide  408  by the anisotropic etch process. After etching, the spacer  413  may be removed as shown in  FIG. 4F . The “aspect ratio” is effectively enlarged over the top portion of trench for easier gap fill than if a thick oxide were uniformly formed on the bottom and sidewalls of the trench. It is further noted that the bottom thickness T 2  may be determined independently of the sidewall thickness T 1  by simply varying the duration of the anisotropic etch. In general, it is desirable to form T 2 &gt;T 1 . 
         [0033]    Conductive material, such as polysilicon  414  may be deposited to fill the trench in the oxide  408  as shown in  FIG. 4G . The polysilicon  414  may then be etched back to below the top surface of the thick oxide  408 , e.g., by about 1000 Angstroms to 2000 Angstroms to form a gap  416  as shown in  FIG. 4H . The remaining polysilicon  414  may act as a shield electrode for the finished device. An insulator, such as poly reoxidation (reox)  418  may be formed to fill the gap  416  as shown in  FIG. 4I . The thickness of the poly reoxidation  418  may be about 2000 Angstroms to 3000 Angstroms. As the upper portion and the top surface are covered by nitride layer  406 , no oxidation occurs in this area. 
         [0034]    The optional thin oxide  410  may be etched following by etching off the exposed portions of nitride  406  and oxide  404  as shown in  FIG. 4J . 
         [0035]    Gate oxide  420  may then be grown on the sidewall of the trench and on top of the semiconductor substrate  402  as shown in  FIG. 4K . Finally, conductive material, such as doped polysilicon  423  may be deposited to fill the top portion of the trench  401  and then etched back to form an active gate as shown in  FIG. 4L . The thickness of the gate oxide  420  on the sidewalls of the top portion of the trench  401  determines a width A′ of a top portion of the active gate that is formed by the polysilicon  423 . In general gate oxide  420  is much thinner than T 1  and T 2 , in the range of tens to hundreds of Angstroms. Further the top surface of poly  423  may be recessed below oxide layer  420 . 
         [0036]    The fabrication of the device may continue with standard processes to implant body regions  430  and source regions  432 , followed by the formation of a thick dielectric layer  460  on top of the surface and open contact holes through dielectric layer  460  for depositing a source metal  470  to electrically connect to the source and body regions. The device  400  resulting from this process as shown in  FIG. 4M  is constructed on a substrate  402  which comprising a lightly doped Epitaxial layer  402 -E overlaying a heavily doped substrate layer  402 -S. In the embodiment shown in  FIG. 4M , gate trench  401  extends from the top surface of Epitaxial layer  402 -E through the entire  402 -E layer reach into substrate layer  402 -S. Alternatively the bottom of trench  401  may stop within Epitaxial layer  402 -E without reaching substrate layer  402 -S (not shown). The trench  401  has a poly gate electrode  423  disposed in the upper portion of the trench and a poly shielding electrode  414  disposed in the lower portion of the trench with an inter poly dielectric layer  418  in between insulating the two. To optimize the shielding effect, the bottom shielding electrode may electrically connect through layout arrangement to the source metal layer  470  where a ground potential is usually applied in applications. A thin gate oxide layer  420  insulates the gate electrode from the source and body regions in the upper portion of trench. To minimize the gate to drain capacitance of the device therefore to improve the device switching speed and efficiency, body regions  430  is carefully controlled to diffuse to substantially the bottom of gate electrode  423  to effectively reduce the coupling between gate  423  and drain region disposed below the body regions. The bottom shielding (or source) electrode  414  is surrounded by a thick dielectric layer  424  along the lower sidewalls and the bottom of trench to insulate from the drain region. Preferably the dielectric layer  424  is much thicker than the thin gate oxide layer  420  and has a variable thickness that is T 2  on the trench bottom and T 1  on trench sidewalls, whereas T 1 &lt;T 2 . As shown in  FIG. 4M , dielectric layer  424  may further comprise a nitride layer  406  sandwiched between oxide layers  404  and  408 . 
         [0037]      FIGS. 5A to 5F  illustrate another alternative process of fabricating a trench DMOS with variable-thickness gate oxides for a shield poly gate of the type depicted in  FIG. 2  according to an embodiment of the present invention. 
         [0038]    As shown in  FIG. 5A , a trench  501  of width A is formed in a semiconductor substrate  502 . A thin insulator layer such as an oxide layer  504  is grown or deposited on the surfaces of the trench  501  and on the top surface of the semiconductor substrate  502 . A thickness of the oxide  504  may be about 450 Angstroms. A layer of material such as a nitride  506  is then deposited, e.g., to a thickness between about 50 Angstroms and about 500 angstroms, on top of the oxide  504  followed by deposition of another oxide, e.g., HTO (high temperature oxide) oxide  508 , on top of the nitride  506 . The thickness of the nitride  506  may be about 100 Angstroms and the thickness of the HTO oxide  508  may be about 800 Angstroms. In this example, the combined thickness of the oxide  504 , nitride  506  and HTO oxide  508  determines a width A′ of a narrowed trench  501 . In-situ doped polysilicon  510  may then be deposited into the narrowed trench  501  and then etched back to a predetermined thickness of, e.g., between about 500 angstroms and about 2 microns to form a shield electrode. Arsenic may be optionally implanted into at least an upper portion of the polysilicon  510  remaining in the trench to enhance a re-oxidation rate of the polysilicon in a subsequent oxidation step. 
         [0039]    Specifically, as shown in  FIG. 5B , an insulator such as a poly reox layer  512  may be formed by the oxidation of a top portion of the polysilicon  510 . The thickness of the poly reox  512  may be about 3000 Angstroms. The nitride layer  506  ensures that oxide layer  512  is only formed on top of the polysilicon  510 . The HTO oxide  508  may then be removed by an etch process that stops on the nitride layer  506  as shown in  FIG. 5C . This protects the underlying oxide  504  from the etch process that removes the thicker HTO oxide  508 . The nitride  506  may then be removed leaving an upper portion of the trench with a width A″ that is wider than A′ as shown in  FIG. 5D . In this example, the width A″ of the upper portion is determined by the thickness of the thin oxide  504  on the sidewalls of the trench. The thickness uniformity of the inter-poly oxide  512  across the wafer may be improved by use of a thermal oxide. This is because a thermal oxide process oxidizes the top portion of the poly in the trench as opposed to depositing and etching back the oxide on the poly in the trench. 
         [0040]    The oxide can be preserved during the nitride removal process due to high nitride to oxide wet etch selectivity. 
         [0041]    Gate oxide  514  may then be formed (e.g., by growth or deposition) on the thin oxide  504  as shown in  FIG. 5E . The thickness of the gate oxide  514  may be about 450 Angstroms. Alternatively, the thin oxide  504  may first be removed before growing the gate oxide  514 . Finally, a second conductive material, such as doped polysilicon  516 , may be deposited into the remaining portions of the trench over the gate oxide  514 . The polysilicon  516  may be etched back to form a shield gate structure, in which the polysilicon  516  is the gate electrode and the polysilicon  510  is the shield electrode. 
         [0042]    It should be clear to those skilled in the art that in the embodiments described above, only a single mask—an initial mask used to define the gate trenches is required in the formation of the gate trench, gate trench oxides, gate poly, and shield poly. 
         [0043]      FIGS. 6A-6F  are cross-sectional views illustrating the fabrication process steps for manufacturing a trench DMOS with variable-thickness trench gate oxides according to an embodiment of the present invention. 
         [0044]    As shown in  FIG. 6A , an ONO (oxide-nitride-oxide) hard mask  601  is formed on top of a semiconductor substrate  602 , which includes a bottom oxide layer  601 - 1 , a middle nitride layer  601 - 2  and a top oxide layer  601 - 3 . By way of example and not by way of limitation, the bottom oxide layer  601 - 1  may be approximately 200 angstroms, the nitride layer  601 - 2  may be 3500 angstroms, and the top upper oxide layer  601 - 3  may be 1400 angstroms. In  FIG. 6B , a trench mask (not shown) is applied to carry out a hard mask etch and silicon etch to form a trench  606  in the semiconductor substrate  602 . In an exemplary embodiment, the trench etching process is carried out with a ratio of depth B, including the thickness of the hard mask  601 , to width A, i.e., aspect ratio, B/A&gt;3. A trench etching process may first comprise an etchant to remove the ONO hard mask  601 , in order to expose the top surface of the semiconductor substrate  602  and a second etching process to form the trench  606 . Then a thin gate oxide layer (or other insulator)  608  is grown along the sidewalls and on the bottom surface of the trench  606 . In an exemplary embodiment, the thickness of the thin oxide  608  has a range between about 100 Angstroms to 600 Angstroms. 
         [0045]      FIG. 6C  shows a step of depositing a thin layer of polysilicon layer  610  over the gate oxide layer  608  that may have a thickness ranging between 100 to 800 Angstroms on the sidewalls and the bottom surface of the trench  606 . Then a nitride layer  612  is deposited over the polysilicon layer  610 . In an exemplary embodiment, the nitride layer  612  has a thickness ranging between 50 to 300 Angstroms. The nitride layer  612  on the bottom surface of the trench is removed with an etching process, for example a nitride dry etch process, to form a nitride spacer  612  along the sidewalls of the trench  606 . In  FIG. 6D , the manufacturing process proceeds with a polysilicon re-oxidation process, i.e., poly REOX, to oxidize the exposed bottom polysilicon layer  610  to form a bottom poly-REOX oxide layer that combines with the gate oxide layer  608  forming a thick bottom oxide layer  611  on the bottom surface of the trench  606 . 
         [0046]    In  FIG. 6E , the nitride spacer  612  on the sidewalls of the trench  602  is removed by a wet dip and then the trench  606  is filled with a conductive material such as a polysilicon layer  616  for example through chemical vapor deposition (CVD). Excess polysilicon layer  616  is removed and planarized with the surface of the hard mask  601  by a chemical-mechanical planarization (CMP) process. In  FIG. 6F , an poly etch back process is carried out to etch back the polysilicon layer  612  to the surface of the semiconductor substrate  602 , for example with a dry etching process, to generate a poly-recess that is then filled with an oxide layer  618 . Excess oxide layer  618  on top of the polysilicon layer  616  and the top oxide layer  601 - 3  of the hard mask  601  is then planarized by a CMP process to the surface of the nitride layer  601 - 2  of the hard mask  601 . The device may be completed by a standard process to form a trench MOSFET that has a thick bottom oxide (TBO). 
         [0047]    Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. For these embodiments, it is possible to use various alternatives, modifications and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. Any feature, whether preferred or not, may be combined with any other feature, whether preferred or not. In the claims that follow, the indefinite article “A”, or “An” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for.”