Abstract:
A trench DMOS transistor structure having a low resistance path to a drain contact located on an upper surface and methods of making the same. The transistor structure comprises: (1) a first region of semiconductor material of a first conductivity type; (2) a gate trench formed within the first region; (3) a layer of gate dielectric within the gate trench; (4) a gate electrode within the gate trench adjacent the layer of gate dielectric material; (5) a drain access trench formed within the first region; (6) a drain access region of conductive material located within the drain access trench; (7) a source region of the first conductivity type within the first region, the source region being at or near a top surface of the first region and adjacent to the gate trench; (8) a body region within the first region below the source region and adjacent to the gate trench, the body region having a second conductivity type opposite the first conductivity type; and (9) a second region of semiconductor material within the first region below the body region. The second region is of the first conductivity type and has a higher dopant concentration than the first semiconductor region. Moreover, the second region extends from the gate trench to the drain access trench and is self-aligned to both the gate trench and the drain access trench.

Description:
STATEMENT OF RELATED APPLICATION  
       [0001]     This application is a divisional of co-pending U.S. patent application Ser. No. 10/144,214, filed May 13, 2002, entitled “Trench DMOS Transistor Structure Having A Low Resistance Path To A Drain Contact Located On An Upper Surface,” which is a continuation-in-part of U.S. Ser. No. 09/516,285, filed Mar. 1, 2000, also entitled “Trench DMOS Transistor Structure Having A Low Resistance Path To A Drain Contact Located On An Upper Surface.” Both of the related applications are incorporated herein by reference in their entireties. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to MOSFET transistors and more generally to DMOS transistors having a trench structure.  
       BACKGROUND OF THE INVENTION  
       [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 the same edge to form the channel region of the transistor. DMOS transistors are often high voltage, high current devices, used either as discrete transistors or as components in power integrated circuits. DMOS transistors can provide high current per unit area with a low forward voltage drop.  
         [0004]     A typical discrete DMOS transistor structure includes two or more individual DMOS transistor cells which are fabricated in parallel. The individual DMOS transistor cells share a common drain contact (the substrate), while their sources are all shorted together with metal and their gates are shorted together by polysilicon. Thus, even though the discrete DMOS circuit is constructed from a matrix of smaller transistors, it behaves as if it were a single large transistor. For a discrete DMOS circuit it is desirable to maximize the conductivity per unit area when the transistor matrix is turned on by the gate.  
         [0005]     One particular type of DMOS transistor is a so-called trench DMOS transistor in which the channel is present on the sidewall of a trench, with the gate formed in the trench, which extends from the source towards the drain. The trench, which is lined with a thin oxide layer and filled with polysilicon, allows less constricted current flow than the vertical DMOS transistor structure and thereby provides lower values of specific on-resistance. Examples of trench DMOS transistors are disclosed in U.S. Pat. Nos. 5,072,266, 5,541,425, and 5,866,931.  
         [0006]     One example is the low voltage prior art trench DMOS transistor shown in the cross-sectional view of  FIG. 1 . As shown in  FIG. 1 , trench DMOS transistor  10  includes heavily doped substrate  11 , upon which is formed an epitaxial layer  12 , which is more lightly doped than substrate  11 . Metallic layer  13  is formed on the bottom of substrate  11 , allowing an electrical contact  14  to be made to substrate  11 . As is known to those of ordinary skill in the art, DMOS transistors also include source regions  16   a ,  16   b ,  16   c , and  16   d , and body regions  15   a  and  15   b . Epitaxial region  12  serves as the drain. In the example shown in  FIG. 1 , substrate  11  is relatively highly doped with N-type dopants, epitaxial layer  12  is relatively lightly doped with N type dopants, source regions  16   a ,  16   b ,  16   c , and  16   d  are relatively highly-doped with N type dopants, and body regions  15   a  and  15   b  are relatively highly doped with P type dopants. A doped polycrystalline silicon gate electrode  18  is formed within a trench, and is electrically insulated from other regions by gate dielectric layer  17  formed on the bottom and sides of the trench containing gate electrode  18 . The trench may extend into the heavily doped substrate  11  to reduce any resistance caused by the flow of carriers through the lightly doped epitaxial layer  12 , but this structure also limits the drain-to-source breakdown voltage of the transistor. A drain electrode  14  is connected to the back surface of the substrate  11 , a source electrode  22  is connected to the source regions  16  and the body regions  15  by source/body metal layer  23 , and a gate electrode  19  is connected to the polysilicon  18  that fills the trench forming the gate.  
         [0007]     Another example of a trench DMOS device is disclosed in U.S. Pat. No. 4,893,160 and shown in the cross-sectional view of  FIG. 2 . As shown in  FIG. 2 , partially completed trench DMOS device  30  includes substrate  11 , epitaxial region  12 , body regions  15   a  and  15   b , and source regions  16   a ,  16   b ,  16   c , and  16   d . However, in comparison to the device shown in  FIG. 1 , N+ region  39  is added along the lower sides and bottom of trench  36 , or alternatively just along the bottom of trench  36 . At this step in the fabrication process, a layer of oxide  35  is present on the silicon surface. This structure improves the device performance by allowing carriers to flow through a heavily doped region at the bottom of the trench, thereby reducing the local resistance.  
         [0008]     It would be desirable to provide further improvements to trench DMOS devices. For example, there is a need for a trench DMOS device that provides a low on-resistance and which is relatively simple and inexpensive to fabricate.  
       SUMMARY OF THE INVENTION  
       [0009]     According to a first aspect of the invention, a trench MOSFET device is provided. The device comprises: (1) a first region of semiconductor material of a first conductivity type; (2) a gate trench formed within the first region; (3) a layer of gate dielectric within the gate trench; (4) a gate electrode within the gate trench adjacent the layer of gate dielectric material; (5) a drain access trench formed within the first region; (6) a drain access region of conductive material located within the drain access trench; (7) a source region of the first conductivity type within the first region, the source region being at or near a top surface of the first region and adjacent to the gate trench; (8) a body region within the first region below the source region and adjacent to the gate trench, the body region having a second conductivity type opposite the first conductivity type; and (9) a second region of semiconductor material within the first region below the body region. The second region is of the first conductivity type and has a higher dopant concentration than the first semiconductor region. Moreover, the second region extends from the gate trench to the drain access trench, and it is self-aligned to both the gate trench and the drain access trench.  
         [0010]     The gate electrode can be formed of various conductive materials, for example, aluminum, alloys of aluminum, refractory metals, doped polycrystalline silicon, suicides, and combinations of polycrystalline silicon and refractory metals.  
         [0011]     While the first region can be an epitaxial layer deposited on the semiconductor substrate (which is beneficially doped to the first conductivity type), an epitaxial layer is not necessary with the present invention. Hence, the first region can correspond to a semiconductor substrate, if desired.  
         [0012]     The gate trench can take on a number of shapes. In some preferred embodiments, the gate trench has the shape of an octagonal, hexagonal, circular, square or rectangular mesh or lattice when viewed from above.  
         [0013]     In some embodiments, the drain access trench is greater in width than the gate trench. In others, the drain access trench is of equal or lesser width than the gate trench.  
         [0014]     The conductive material of the drain access region can comprise, for example, doped polycrystalline silicon, silicides and/or metal (for example, aluminum, refractory metals, and alloys thereof).  
         [0015]     In certain embodiments, an oxide layer is provided adjacent the sidewalls of the drain access trench.  
         [0016]     According to another aspect of the invention, a method of making a semiconductor device is provided. The method comprises: (a) providing a first region of semiconductor material of a first conductivity type; (b) etching a gate trench and a drain access trench within the first region; (c) forming a second semiconductor region within the first region, the second region: (i) extending from the gate trench to the drain access trench, (ii) being self-aligned to both the gate trench and the drain access trench, (iii) being of the first conductivity type, and (iv) having a higher dopant concentration than the first region; (d) forming a layer of gate dielectric material within the gate trench; (e) depositing a gate electrode within the gate trench adjacent the layer of gate dielectric material; (f) depositing a drain access region of conductive material within the drain access trench; (g) forming a body region within the first region above the second region and adjacent the gate trench, the body region having a second conductivity type opposite the first conductivity type; and (h) forming a source region of the first conductivity type above the body region and adjacent the gate trench.  
         [0017]     In some embodiments the gate trench and the drain access trench are formed simultaneously. In this case, the second semiconductor region is preferably formed using a single implantation step.  
         [0018]     In other embodiments, the gate trench is formed in a different etching step from the drain access trench. In this case, the gate trench can be formed prior to the drain access trench, or vice versa. Moreover, a first implantation step can be performed after formation of the gate trench, and a second implantation steps can be performed after formation of the drain access trench. The drain access region can comprise a metal region and/or a polysilicon region.  
         [0019]     In some embodiments, the gate and drain access trenches are formed prior to the formation of the body and source regions. In others, the gate and drain access trenches are formed subsequent to the formation of the body and source regions.  
         [0020]     In some embodiments, a dielectric material layer is formed adjacent sidewalls of the drain access trench, in which case the dielectric material layer can be formed, for example, in the same process step as the gate dielectric material.  
         [0021]     In some embodiments the gate electrode is a doped polysilicon or silicide electrode, and the drain access region is a metal region.  
         [0022]     In other embodiments, the gate electrode is a doped polysilicon or silicide electrode, and the drain access region at least partially comprises a doped polysilicon or silicide region. In these embodiments, the drain access region can be entirely formed of doped polysilicon or silicide, and the gate electrode and the drain access region can be formed in different polysilicon or silicide formation steps. Alternatively, the drain access region can partially comprise a doped polysilicon or silicide region that is introduced in the same polysilicon or silicide formation step as the gate electrode, in which case (a) the drain access region can further comprise an additional doped polysilicon or silicide region, which is introduced in a subsequent polysilicon or silicide formation step or (b) the drain access region can further comprise a metal region, which is introduced in a metal deposition step. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIGS. 1 and 2  each show cross-sectional views of a conventional trench DMOS transistor.  
         [0024]      FIG. 3  shows a cross-sectional view of a trench DMOS transistor constructed in accordance with the prior art.  
         [0025]      FIG. 4  shows an embodiment of the trench DMOS transistor constructed in accordance with the present invention.  
         [0026]      FIGS. 5   a - 5   d  illustrate a sequence of process steps forming the trench DMOS transistor shown in  FIG. 4 .  
         [0027]      FIGS. 6-8  show top views of various geometries in which a plurality of trench DMOS transistors constructed in accordance with the present invention may be arranged.  
         [0028]      FIGS. 9   a - 9   d  illustrate a sequence of process steps for forming a trench DMOS transistor in accordance with an embodiment of the present invention.  
         [0029]      FIGS. 10   a - 10   b  illustrate a sequence of process steps for forming a trench DMOS transistor in accordance with another embodiment of the present invention.  
         [0030]      FIGS. 11   a - 11   f  illustrate a sequence of process steps for forming a trench DMOS transistor in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0031]      FIG. 3  shows a trench DMOS transistor  100  constructed in accordance with the prior art. One notable advantage of this structure is that because it is self-isolated it can be used not only in discrete components but also in integrated circuits. However, it requires the formation of a buried layer and the deposition of an epitaxial layer. As shown in  FIG. 3 , trench DMOS transistor  100  includes a substrate  25 , heavily doped buried region  11 , and an epitaxial region  12 , which is more lightly doped than buried region  11 . While the substrate  25  may be N-type or P-type, a P-type substrate will typically be preferred when the structure is to be incorporated into an integrated circuit, since junction isolated devices may be readily fabricated. The DMOS transistor also includes source regions  16   a  and  16   b  and body regions  15   a  and  15   b . As is well known to those of ordinary skill in the art, the body regions  15   a ,  15   b  may include a deeper more heavily doped region and a shallower, more lightly doped region. In the example shown in  FIG. 3 , buried region  11  is relatively highly doped with N type dopants, epitaxial region  12  is relatively lightly doped with N type dopants, source regions  16   a  and  16   b  relatively highly doped with N type dopants, and body regions  15   a  and  15   b  include portions that are relatively highly doped and relatively lightly doped with P type dopants.  
         [0032]     A polycrystalline silicon gate electrode  18 , which is formed within a trench, is electrically insulated from other regions by a gate dielectric layer  17  formed on the bottom and sides of the trench containing gate electrode  18 . The trench extends into the heavily doped buried region  11 . In contrast to the structures shown in  FIGS. 1 and 2 , in this device the drain electrode is located on the top surface rather than the back surface of the structure. More specifically, a drain access region  26  extends from the top surface of the device to the heavily doped buried region  11 . The drain access region  26  is heavily doped and of the same conductivity type as the buried region  11 . The drain access region provides a low resistance path from the heavily doped buried region  11  to a drain electrode  14 .  
         [0033]     Finally, similar to the devices shown in  FIGS. 1 and 2 , a source and body electrode  22  is connected to the source regions  16  and the body regions  15  through source and body metal layer  23 , and a gate electrode  19  is connected to the polysilicon  18  that fills the trench.  
         [0034]     One problem with the device structure shown in  FIG. 3  is that it can be relatively expensive to manufacture because it requires the deposition of an epitaxial layer, i.e., region  12 , which is inherently expensive to produce.  
         [0035]     According to an embodiment of the present invention, depicted in  FIG. 4 , the epitaxial region  12  is eliminated so that fabrication of the device is considerably simplified. As shown in  FIG. 4 , trench DMOS transistor  100  includes a substrate  25  in which the device is formed. Similar to the previously depicted structures, the DMOS transistor shown in  FIG. 4  includes source regions  16   a ,  16   b ,  16   c  and  16   d  and body regions  15   a  and  15   b . As is commonly the case, in the example shown in  FIG. 4 , substrate  25  is doped with N-type dopants (although alternatively, P-type dopants may be used), source regions  16   a ,  16   b ,  16   c , and  16   d  are relatively highly doped with N type dopants, and body regions  15   a  and  15   b  are both relatively highly doped and relatively lightly doped with P type dopants. Polycrystalline silicon gate electrodes  18   a ,  18   b ,  18   c  and  18   d  are each formed within a gate trench. The gate electrodes  18   a ,  18   b ,  18   c  and  18   d  are electrically insulated from other regions by gate dielectric layers  17   a ,  17   b ,  17   c  and  17   d  formed on the bottom and sides of each respective gate trench. Additional trenches defining drain access regions  26   a ,  26   b , and  26   c  also extend from the top surface of the device.  
         [0036]     A low resistance path for the drain is provided by adding heavily doped regions along the lower sides and bottom of the gate trenches and the drain access trenches, or alternatively, only along the bottom of the gate trenches and drain access trenches. The heavily doped regions merge laterally, forming continuous, heavily doped regions  39   a ,  39   b  and  39   c  that extend from the bottom of each gate trench to its associated drain access trench. The drain access regions  26   a ,  26   b  and  26   c  are preferably heavily doped with the same conductivity type dopant as heavily doped regions  39   a ,  39   b  and  39   c . The drain access regions  26   a ,  26   b  and  26   c  provide low resistance paths from the heavily doped regions  39   a ,  39   b  and  39   c  to the drain electrode, which is preferably located on the top surface of the device.  
         [0037]     As will be discussed in more detail in connection with  FIGS. 5   a - 5   d , the heavily doped regions  39   a ,  39   b  and  39   c  are preferably formed by diffusing a species such as phosphorous and/or arsenic through the gate and access trenches before they are filled with polysilicon. The gate and drain access trenches should be sufficiently close to one another to ensure that the dopants diffusing therethrough merge together to form the continuous, low resistance path between the trenches. These heavily doped regions are self-aligned to the bottoms of the gate and the drain access trenches.  
         [0038]     As previously mentioned, the structure shown in  FIG. 4  advantageously eliminates the need for an epitaxial layer  12  as well as the need for a layer formed below the epitaxial layer, such as the region  11  shown in  FIG. 3 .  
         [0039]     The inventive DMOS devices shown in  FIG. 4  may be fabricated in accordance with conventional trench DMOS processing techniques with the appropriate modification of the deposition and etching steps. For example, the  FIG. 4  device begins by forming the bodies  15   a  and  15   b  and the source regions  16   a - 16   d  in diffusion steps and the gate and drain access trenches in etching steps. Additional details concerning such steps may be found, for example, in previously mentioned U.S. Pat. No. 4,893,160. Next, a dielectric layer  17  such as a silicon dioxide layer is grown in the trenches, followed by the introduction of a diffusing species, e.g., an n-type species such as phosphorous or arsenic, to the bottom of the trenches by a technique such as ion implantation. The diffusing species is then diffused to form the continuous, heavily doped regions  39 .  FIG. 5   a  shows the structure at the end of this stage of fabrication with the heavily doped regions  39  self-aligned to the bottoms of the trenches.  
         [0040]     Next, as shown in  FIG. 5   b , the gate trenches are filled and the drain access trenches are partially filled with doped polysilicon  18 . As is well known to those of ordinary skill in the art, polysilicon will more quickly fill a narrow trench of a given depth than a wider trench of the same depth, since it deposits in an essentially uniform layer. Accordingly, in some embodiments of the invention such as those shown in the figures, it may be desirable to make the width of the drain access trench greater than the width of the gate trench. In this way, as shown in  FIG. 5   b , when the gate trench is filled with polysilcon (polycrystalline silicon) the drain access trench will be only partially full.  
         [0041]     In either case, after the gate trench is filled with polysilicon, an isotropic etch is used, which removes the polysilicon in the drain access trenches while leaving it in the gate trenches. A subsequent etch process is employed to remove the silicon oxide layer lining the drain access trench producing the device of  FIG. 5   c . Next, as shown in  FIG. 5   d , the drain access trench is filled with N type doped polysilicon using CVD, which also covers the surface of the wafer. An isotropic etch is preformed to form the drain access region  26 . A conductor other than doped polysilicon, for example, a metal conductor, can also be used to fill the trench.  
         [0042]      FIGS. 6-8  show top views of various surface geometries in which a plurality of the inventive DMOS transistors may be arranged. The arrangements include drain access cells  40  and transistor cells  50 . The drain access cells  40  denote the structure defined by the drain access trench and the adjacent gate trenches, which are interconnected by the low resistance path at the bottom of the drain access trench and the surrounding transistor cells. The transistor cells  50  denote the structure defined by the conventional DMOS transistor structure, which includes the gate trenches, the source regions and the body region. While these or any other geometries may be employed, the octagonal arrangement shown in  FIG. 6  is particularly advantageous because it allows the relative areas occupied by the transistor cells and the drain access cells to be adjusted independently of one another so that a minimum device on-resistance can be achieved.  
         [0043]     Various processing schemes have been developed, in addition to the processing scheme set forth above in connection with  FIGS. 5   a - 5   d , for the production of various devices in accordance with the present invention.  
         [0044]     For example, referring now to  FIGS. 9   a - 9   d , a layer of silicon oxide, preferably silicon dioxide, can be deposited over a structure like that illustrated in  FIG. 5   b , covering the structure and filling the trenches that are only partially filled with polycrystalline silicon. The silicon dioxide layer is then etched using techniques known in the art, for example, plasma etching, to produce silicon dioxide regions  24 . The trenches are preferably filled with silicon dioxide regions  24  at this point to provide a planarized structure, which, in turn, improves the quality of subsequent masking steps.  
         [0045]     This structure is then subjected to a plasma silicon etching step to remove the exposed polycrystalline silicon at the top surface of the structure, producing polysilicon regions  18 . Then, the exposed polycrystalline silicon that remains is oxidized, for example, using a wet or dry oxidation step, to form a thin oxide layer  27  on the polycrystalline silicon regions  18 , as illustrated in  FIG. 9   a.    
         [0046]     A masking layer, such as a silicon nitride layer, is then deposited over the structure of  FIG. 9   a . This layer is then, in turn, masked and etched as is known in the art, producing a patterned masking layer  28 . The silicon dioxide regions  24  of  FIG. 9   a  are then etched through apertures in the patterned masking layer  28  using an anisotropic plasma silicon dioxide etching step. (Alternately, thin oxide layer  27  is not formed, and nitride layer  28  is masked and etched, eliminating the need for an anisotropic oxide etch.) After this, the polysilicon at the trench bottom is likewise anisotropically etched. Finally, the silicon dioxide layer at the trench bottom is anisotropically etched, completing the formation of trenches  21 , illustrated in  FIG. 9   b.    
         [0047]     A layer of doped polycrystalline silicon is then deposited, covering the structure and filling the trenches  21 . This polycrystalline silicon layer is etched in a plasma etching step, planarizing the overall structure and producing polysilicon regions  18 ′. Finally, the exposed polycrystalline silicon is oxidized, for example, using a wet or dry oxidation step, to form a thin oxide layer  27 ′ on the newly exposed polycrystalline silicon regions  18 ′ as illustrated in  FIG. 9   c . As discussed below in connection with  FIG. 9   d , the thin oxide layer  27 ′ is removed in a subsequent contact etching step. Hence, the above step of forming the thin oxide layer  27 ′ is clearly an optional one. However, by forming the thin oxide layer  27 ′ over the polycrystalline silicon regions  18 ′, the issue of photoresist adhesion to polysilicon, a problem well known in the art, is effectively addressed.  
         [0048]     Although the structure of  FIG. 9   c  is similar to that illustrated in  FIG. 5   d , substantially different processing steps were used in their production. The process leading to the structure of  FIG. 9   c  is advantageous relative to that leading to the structure of  FIG. 5   d , because the polysilicon along the drain access trench sidewall is retained, reducing the likelihood of processing problems that reduce process yield.  
         [0049]     Referring now to  FIG. 9   d , a masking layer (not shown) is preferably applied and patterned using techniques known in the art. Silicon dioxide regions, and in some areas, silicon nitride regions as well, are then etched through apertures in the patterned masking layer, for example, using plasma etch techniques, or wet etches such as buffered oxide and phosphoric acid, forming contact openings. Finally, a conductive layer, for example, a metal layer such as aluminum, aluminum-copper, or aluminum-copper-silicon, is deposited over the structure, masked and etched using techniques known in the art to produce drain contact regions  29   a  and source/body contact regions  29   b  as illustrated in  FIG. 9   d , as well as gate contacts (not shown), completing the structure.  
         [0050]     A further device design and processing scheme will now be discussed in connection with  FIGS. 10   a  and  10   b . Beginning with a structure like that of  FIG. 9   a  above, a masking layer, such as a silicon nitride layer, is deposited, masked and etched as is known in the art, producing a patterned masking layer  28 . The silicon dioxide regions  24  (see  FIG. 9   a ), which have a significantly higher etch rate than thermally grown oxide, are then etched through apertures in the patterned masking layer  28  using an anisotropic silicon dioxide etching step. After this, the polysilicon at the trench bottom is likewise anisotropically etched. Finally, the silicon dioxide layer at the trench bottom is etched, completing the formation of trenches  21 , to produce the structure of  FIG. 10   a . (As with the process sequence in  FIG. 9 , the growth of thin oxide layer may be eliminated, removing the need for an anisotropic etch.)  
         [0051]     Silicon dioxide regions over the source/body regions are etched without the need for an additional mask, for example, using a buffered oxide etching step. Finally, a conductive layer, for example, a metal layer such as aluminum, aluminum-copper, aluminum-copper-silicon or tungsten, is deposited over the structure, covering the structure and filling the trenches  21 . The metal layer is then masked and etched, using techniques known in the art, to produce drain contact regions  29   a  and source/body contact regions  29   b  as illustrated in  FIG. 10   b . The structure of  FIG. 10   b  is advantageous relative to that of  FIG. 9   d , for example, in that lower resistance drain contacts are produced. (As an alternative example, one metal such as tungsten may be used, with suitable liner layers such as Ti/TiN to fill the trench, and a second metal or set of metals may be used as the metal on the surface.)  
         [0052]     Yet another device design and processing scheme will now be discussed in connection with  FIGS. 11   a - 11   f . As in  FIG. 5   a , bodies  15  and source regions  16  are first formed in implantation/diffusion steps and gate trenches  21   g  are formed in etching steps. Next, a dielectric layer  17 , such as a silicon dioxide layer, is grown in the trenches and on the upper surface, followed by the introduction of a diffusing species, e.g., an n-type species such as phosphorous, to the bottom of the trenches by a technique such as ion implantation. The diffusing species is then diffused to form heavily doped regions  39   a .  FIG. 11   a  shows the structure at the end of this stage of fabrication. This structure differs from the structure of  FIG. 5   a  in that the wide drain access trenches of  FIG. 5   a  are not formed at this stage of device fabrication.  
         [0053]     Next, doped polysilicon is provided over the structure, filling the gate trenches  21   g . The doped polysilicon layer is subsequently etched in a plasma etch process, creating doped polysilicon regions  18 . Then, the remaining exposed polycrystalline silicon is oxidized, for example, using a wet or dry oxidation step, to form a thin oxide layer  27  on the polycrystalline silicon regions  18 , as illustrated in  FIG. 11   b.    
         [0054]     A first masking layer, such as a first silicon nitride layer, is then deposited over the structure of  FIG. 11   b , and a second masking layer such as silicon dioxide is deposited over the silicon nitride. This second layer is then masked and etched as is known in the art, producing a patterned masking layer  28   b . Photomask and etch processes are then repeated to produce patterned masking layer  28   a . The exposed silicon dioxide regions  17  are then etched through mutual apertures in the patterned masking layers  28   a  and  28   b , while the masking layer of photoresist is still present, using a silicon dioxide etching step. The resulting structure is illustrated in  FIG. 11   c.    
         [0055]     After this etching step drain access trenches  21   d  are then etched in the exposed silicon through the mutual apertures in the patterned masking layers  28   a ,  28   b  and silicon dioxide  17  using an anisotropic silicon etching step. Note that the drain access trenches  21   d  need not be of the same depth as the earlier provided gate trenches, because they are formed in separate process steps. An n-type species such as phosphorous, is then provided at the bottom of the trenches  21   d  by a technique such as ion implantation and diffusion, forming heavily doped regions  39   b . The resulting structure is illustrated in  FIG. 11   d . Regions  39   b  overlap regions  39   a . Together, regions  39   a  and  39   b  form heavily doped regions that extend from the bottom of each gate trench to an associated drain access trench.  
         [0056]     A partial silicon nitride etch is then performed, removing those portions of patterned masking layer  28   a  that are not covered by the patterned masking layer  28   b . The remaining portions of patterned masking layers  28   b  and  28   a  are then used as a mask for a subsequent contact etching step, in which exposed portions of silicon dioxide layer  17  and  28   b  are removed. The resulting structure is illustrated in  FIG. 11   e.    
         [0057]     Finally, a conductive layer, for example, a metal layer, or combination of metal layers such as discussed above, is deposited over the structure, covering the surface and filling the drain access trenches  21   d , using techniques known in the art to produce drain contact regions  29   a  and source/body contact regions  29   b  as illustrated in  FIG. 11   f , as well as a gate contact (not shown), completing the structure.  
         [0058]     Although various embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and are within the purview of the appended claims without departing from the spirit and intended scope of the invention.