Abstract:
A semiconductor device includes a field shield region that is doped opposite to the conductivity of the substrate and is bounded laterally by dielectric sidewall spacers and from below by a PN junction. For example, in a trench-gated MOSFET the field shield region may be located beneath the trench and may be electrically connected to the source region. When the MOSFET is reverse-biased, depletion regions extend from the dielectric sidewall spacers into the “drift” region, shielding the gate oxide from high electric fields and increasing the avalanche breakdown voltage of the device. This permits the drift region to be more heavily doped and reduces the on-resistance of the device. It also allows the use of a thin, 20 Å gate oxide for a power MOSFET that is to be switched with a 1V signal applied to its gate while being able to block over 30V applied across its drain and source electrodes, for example.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to semiconductor device structures with improved packing/cell density and breakdown, and in particular MOSFETs having a gate electrode located in a trench, more specifically a low-voltage trench-gated power MOSFET having an improved breakdown characteristic, a thin gate oxide to reduce the gate drive voltage, and a high cell density to lower the on-resistance of the MOSFET.  
       BACKGROUND OF THE INVENTION  
       [0002]     MOSFETs have become the preferred devices for switching currents in numerous fields, including the computer and automotive industries. Three of the principal characteristics of MOSFETs are their gate drive voltage, their on-resistance (R ds -on) and their avalanche breakdown voltage (V B ). The gate drive voltage is determined primarily by the gate oxide thickness; the thinner the gate oxide, the lower the gate drive voltage. However, a thinner gate oxide leads to a lower breakdown voltage, especially for trench power MOSFETs. The breakdown voltage is normally provided largely by a lightly-doped “drift” region that is located between the drain and body regions of the MOSFET. For example, in MOSFET  10  shown in  FIG. 1 , a lightly-doped N-epitaxial (epi) layer  104  is grown on a heavily-doped N+ substrate  102 , which serves as the drain of the device. (Note that  FIG. 1  is not drawn to scale; for example, substrate  102  would typically be much thicker than epi layer  104 .) A trench is formed in the top surface of epi layer  104 , frequently using a reactive ion etch (RIE) process. The walls of the trench are lined with a gate oxide layer  112 , and the trench is filled with a conductive material, often doped polycrystalline silicon (polysilicon), which serves as a gate electrode  110 . The top portion of the epi layer  104  is implanted with a P-type impurity such as boron to form a P-body region  108 , and using appropriate photoresist masks, N and P type dopants are implanted and diffused to form N+ source regions  110  and P+ body contact regions  118  at the surface of epi layer  104 . The implantations used to form P-body region  108 , N+ source regions  110  and P+ body contact region  118  are frequently performed before the trench is etched.  
         [0003]     A borophosphosilicate layer  116  is deposited and patterned so that it covers and isolates the gate electrode  110 , and a metal layer  114  is deposited over the top surface of the device. Metal layer  114 , which can be an aluminum or copper alloy, makes an ohmic electrical contact with N+ source regions  110  and P+ body contact regions  118 .  
         [0004]     Current flows vertically through MOSFT  10  from the N+ drain  102  and through an N− drift region  106  and a channel region (denoted by the dashed lines) in P-body region  108  to the N+ source regions  110 .  
         [0005]     The trench is typically made in the form of a lattice that creates a number of MOSFET cells. In a “closed cell” arrangement, the MOSFET cells may be hexagonal, square or circular. In an “open cell” arrangement, the cells are in the form of parallel longintudinal stripes.  
         [0006]     When MOSFET  10  is reverse-biased, the N+ drain region  102  is biased positively with respect to the N+ source regions  110 . In this situation, the reverse bias voltage appears mainly across the PN junction  120  that separates N-drift region  106  and P-body region  108 . N-drift region  106  becomes more and more depleted as the reverse bias voltage increases. When the depletion spreading reaches the boundary between N+ substrate  102  and N-drift region  106 , any further increases in the reverse bias are seen at PN junction  120 . Thus making N-drift region  106  thicker generally provides greater protection against breakdown. Furthermore, there is a generally inverse relationship between the avalanche breakdown voltage of PN junction  120  and the doping concentration of N-drift region  106 , i.e, the lower the doping concentration of N-drift region  106 , the higher the breakdown voltage V B  of PN junction  120 . See Sze,  Physics of Semiconductor Devices,  2 nd  Ed., page 101,  FIG. 26 , which provides a graph showing the relationship between the doping concentration and V B  for several semiconductor materials.  
         [0007]     Thus, to increase the breakdown voltage of junction  120 , one would like to reduce the doping concentration of N-drift region  106 . This in turn, however, reduces the quantity of charge in N-drift region  106  and accelerates the effect of depletion spreading. One solution would be to increase the thickness of N-drift region  106 , but this tends to increase the on-resistance of MOSFET  10 .  
         [0008]     U.S. Pat. No. 5,216,275 describes a high voltage drift structure useful for trench power MOSFETs, diodes, and bipolar transistors. The drift structure includes a “composite buffer layer” that contains alternately arranged areas of opposite conductivity.  
         [0009]     In low voltage and high density trench MOSFETs there is another limitation. A high field at the bottom of the gate oxide, which limits the breakdown voltage and the oxide thickness. U.S. Pat. No. 5,168,331 proposes a floating, a lightly doped P− region just below the trench gate oxide to reduce the field which it does. However, P-shield region (e.g., boron atoms) out diffuse towards the P-body, which increases Rds on and/or requires the packing density to be reduced.  
         [0010]     The present invention overcomes these problems.  
       SUMMARY OF THE INVENTION  
       [0011]     A trench-gated semiconductor device according to this invention includes a semiconductor substrate of a first conductivity type. An epitaxial layer is formed on the substrate. First and second trenches are formed in the epitaxial layer, the first and second trenches being separated by a mesa. Each of the trenches comprises a gate dielectric layer, the gate dielectric layer lining the walls and floor of the trench, and a gate electrode bounded by the gate dielectric layer. A body region of a second conductivity type is located in the mesa. A source region of the first conductivity type is located adjacent a wall of the trench and the top surface of the epitaxial layer. A drift region of the epitaxial layer is located below the body region and doped with material of the first conductivity type. A field shield region of the second conductivity type is located below each of the trenches, the sides of the field shield region being bounded by dielectric sidewall spacers. The dielectric sidewall spacers separate the field shield region from the drift region of the epitaxial layer. A metal layer lies on top of the epitaxial layer and is in electrical contact with the source region and the body region. The field shield region is electrically connected to the source region and the body region.  
         [0012]     With this structure, depletion regions form on both sides of dielectric sidewall spacers when the MOSFET is in an off condition and blocking a voltage. This increases the avalanche breakdown voltage of the device and allows the drift region to be doped more heavily, reducing the on-resistance of the MOSFET. The dielectric spacers bordering the field shield region confine the field shield region to the area directly beneath the trench floor. Use of the field shield region decouples the gate oxide thickness from the breakdown voltage of the device.  
         [0013]     As a result, the cell packing density can be increased, and the gate oxide thickness can be reduced to achieve a threshold voltage as low as 1V Vgs while maintaining a high breakdown voltage.  
         [0014]     This invention also includes a process for fabricating a trench-gated semiconductor device. The process includes providing a semiconductor substrate of a first conductivity type; forming an epitaxial layer of the first conductivity type on the substrate; forming first and second trenches in the epitaxial layer, the first and second trenches being separated by a mesa; forming dielectric sidewall spacers on the walls of the trenches; forming a “field shield region” on the bottom of the trench by partially filling the trench with a semiconductor material of a second conductivity type; removing portions of the dielectric sidewall spacers above the field shield region; forming a dielectric layer on the walls of the trenches above the field shield region and on the top surface of the field shield region; and filling an upper portion of the trenches with a conductive gate material.  
         [0015]     In one variation of the process, source regions are formed in the mesa by forming a first dielectric layer above the conductive gate material, depositing a layer of polysilicon containing a dopant of the first conductivity type on the entire top surface of the structure and directionally etching the layer of polysilicon to leave a polysilicon spacer adjacent a vertical surface of the first dielectric layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a cross-sectional view of a conventional trench-gated MOSFET.  
         [0017]      FIG. 2A  is a cross-sectional view of a MOSFET/IGBT which includes a field shield region in accordance with this invention.  
         [0018]      FIG. 2B  is a cross-sectional view of a semiconductor device containing a generalized field shield region in accordance with this invention.  
         [0019]      FIG. 2C  is a cross-sectional view of a Schottky barrier diode containing a field shield region in accordance with this invention.  
         [0020]      FIG. 2D  is a cross-sectional view of a vertical JFET device containing a field shield region in accordance with this invention.  
         [0021]      FIGS. 3A and 3B  illustrate techniques for establishing electrical contact between the field shield regions and the source regions in the MOSFET shown in  FIG. 2 .  
         [0022]      FIGS. 4A-4H  illustrate a process sequence for fabricating the MOSFET/IGBT shown in  FIG. 2A   
         [0023]      FIGS. 5A-5G  illustrate a process sequence for fabricating an alternative embodiment of the invention.  
         [0024]      FIG. 6  illustrates a variation of the MOSFET shown in  FIG. 5G .  
         [0025]      FIG. 7  shows another alternative embodiment of a MOSFET in accordance with this invention.  
         [0026]      FIGS. 8A-8C  illustrate a process for forming the field shield contact shown in  FIG. 3B  when the process shown in  FIGS. 4A-4H  is used to manufacture the MOSFET.  
         [0027]      FIG. 9  illustrates an alternative process for forming the field shield contact shown in  FIG. 3B  when the process shown in  FIGS. 5A-5G  is used to manufacture the MOSFET.  
         [0028]      FIG. 10  illustrates a preferred structure of the termination region when the structure of  FIG. 3A  is used to contact the field shield region.  
         [0029]      FIG. 1  illustrates a high-voltage termination structure that can be fabricated with three-mask process shown in  FIGS. 5A-5G .  
         [0030]      FIGS. 12A and 12B  illustrate a preferred structure for contacting the gate of the MOSFET when the MOSFET is manufactured using the process shown in  FIGS. 4A-4H .  
         [0031]      FIGS. 13A and 13B  illustrate a preferred structure for contacting the gate of the MOSFET when the MOSFET is manufactured using the process shown in  FIGS. 5A-5G .  
         [0032]      FIGS. 14A-14C  illustrate portions of stripe (open cell), square and hexagonal patterns in which the trenches and mesas can be formed in devices according to this invention. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0033]      FIG. 2A  shows a MOSFET  30  in accordance with this invention. MOSFET  30  is formed on an N+ substrate  302  and an overlying epi layer  304 . Trenches  306  are formed in epi layer  304 , and trenches  306  are lined with a gate oxide (SiO 2 ) layer  310  and filled with agate  308 . Alternatively, layer  310  could be formed of silicon nitride (Si 3 N 4 ). Gate  308  is typically formed of heavily-doped polysilicon and can include a silicide.  
         [0034]     A mesa between trenches  306  includes a P-body region  316 . Within P-body region  316  are N+ source regions  312  and a P+ body contact region  314 . The top surface of gate  308  is covered with a BPSG layer  324 . A source metal layer  326 S overlies BPSG layer  324  and makes electrical contact with N+ source regions  312  and P+ body contact regions  314 . Similarly, a metal layer  325  contacts N+ substrate  302 , which functions as the drain. The electrical contact between metal layer  325  and N+ substrate  302  could be ohmic or could include a Schottky barrier.  
         [0035]     The remaining portion of epi layer  304  is divided into N drift region  318  and P field shield regions  320 . Each of P field shield regions  320  is located below one of trenches  306  and is separated laterally from N drift region  318  by oxide sidewall spacers  322 . In some embodiments, field shield regions  320  could extend downward to N+ substrate  302 .  
         [0036]      FIG. 2A  illustrates the present innovation in a “U” shaped trench gate device. However the basic “field shield region” bounded by dielectric sidewalls only or by dielectric sidewalls and a dielectric top wall is applicable to devices of many shapes, including devices having gates in U-shaped or V-shaped grooves and planar structures.  
         [0037]     A key innovation in  FIG. 2A  is the structure below the trench gate; the P− field shield region  320  is laterally bounded by dielectric sidewalls  322  and bounded on the bottom by the PN junction with by N-region  318 . This more general structure is illustrated in  FIG. 2B . As shown in  FIG. 2B , P-field shield region  320  may be electrically biased either by shorting P-field shield region  320  to the top surface electrode of the N-region, or P-field shield region  320  may be biased independently with a separate voltage source. The contact with the top surface of the N-region can be either a Schottky barrier or an ohmic contact.  
         [0038]     P-shield regions  320  can be formed by a selective epitaxial deposition after the RIE etch of the silicon and after the formation of a sidewall oxide. The basic structure shown in  FIG. 2B  is applied to a trench MOSFET (N+ substrate) and also IGBT (P+ substrate) structure in  FIG. 2A  to improve the blocking capability with thin gate oxide. The structure shown in  FIG. 2B  can be applied to make a low barrier height diode such as the Schottky barrier diode, as shown in  FIG. 2C , or the vertical JFET structure, as shown in  FIG. 2D . The devices shown in  FIGS. 2A-2D  share the novel field shield structure, which is a P-region bounded by dielectricwalls on the sides and a PN junction below. The dielectric sidewalls prevent the spread of the P region expansion laterally by blocking the lateral diffusion of acceptors (e.g., boron) during device processing at high temperatures (e.g., above 800° C.). Of course, the polarities may be reversed in which case the field shield region would be formed of N-type material.  
         [0039]     Each of field shield regions  320  is connected to P-body regions  316  and N+ source regions  312  in the third dimension, outside the plane of the drawing.  FIGS. 3A and 3B  illustrate how this can be done.  FIG. 3A  is a cross-sectional view taken at the end of one of trenches  306  showing how field shield regions  320  can be connected to P-body regions  316  and N+ source regions  312 . A P-well  328  is formed by ion implantation through a mask and diffusing a P-type dopant such as boron at the ends of trenches  306 . As the P-type dopant diffuses, the P-well expands laterally under the sidewall spacers  322  and merges with the field shield regions  320 . A P+ contact region  330  is formed beneath an opening in BPSG layer  324  at the surface of epi layer  304  to form an ohmic contact with metal layer  326 S. P+ contact region  330  can be formed during the same process step as P+ body contact region  314 , shown in  FIG. 2 . Since metal layer  326 S is in electrical contact with N+ source regions  312  and P+ body contact regions  314  (see  FIG. 2 ), field shield regions  320  are likewise in electrical contact with N+ source regions  312  and P+ body contact regions  314 .  
         [0040]     Field shield regions  320  can also be connected to N+ source regions  312  and P+ body contact regions  314  by means of a wide trench, as shown in  FIG. 3B . Wide trench  602  is an extension of trench  306  and may be located at the end of each rectangular trench cell, for example. At the bottom of trench  602  is a P shield region  604 , which is an extension of field shield region  320 . Also included in trench  602  are polysilicon spacers  606 , BPSG spacers  610 , and a metal plug  612 . Metal plug  612  extends downward from metal layer  326 S. A P+ region  608  within P shield region  604  provides an ohmic contact with metal slug  612 . Therefore, since P shield region  604  is an extension of field shield region  320 , and since metal layer  326 S is in electrical contact with N+ source regions  312  and P+ body contact regions  314 , this structure forms an electrical link between field shield region  320  and both N+ source regions  312  and P+ body contact regions  314 .  
         [0041]     Referring again to  FIG. 2 , with this structure depletion regions form on both sides of dielectric sidewalls or sidewall spacers  322  when MOSFET  30  is turned off, with N+ substrate  302  biased positive with respect to source regions N+. This increases the avalanche breakdown voltage of the device and allows N drift region  318  to be doped more heavily, reducing the R ds -on of MOSFET  30 .  
         [0042]      FIGS. 4A-4H  illustrate a process sequence that can be used to fabricate MOSFET  30 . The process begins with the formation of epi layer  304  on top of N+ substrate  302 . Because of the additional voltage blocking capability described above, epi layer  304  can be doped with an N-type dopant such as phosphorus to a concentration of 4×10 16  cm −3  to 8×10 16  cm −3 , for example, as compared with the normal doping concentration of 1×10 16  cm −3  to 2.5×10 16  cm −3  for a trench MOSFET with 30V breakdown. Prior to the process step illustrated in  FIG. 4A , the structure is masked, and boron is implanted at a dose in the range of 1×10 13  cm −3  to 5×10 13  cm −3  to form P− wells, such as the P well  328  shown in  FIG. 3A  that is used to contact the field shield regions.  
         [0043]     A second photoresist mask is then formed over what is to be the active area of the device, and a thick field oxide layer (e.g., 0.2-1.0 μm thick) is thermally grown in what are to be the voltage termination regions (die edges) of the MOSFET. Then, as shown in  FIG. 4A , a pad oxide layer  404  is thermally grown on the surface of epi layer  304  and a silicon nitride layer  402  is deposited over pad oxide layer  404 . A third photoresist mask (trench mask) is formed atop nitride layer  402 , and nitride layer  402  and oxide layer  404  are etched through an opening in the trench mask to form openings  406 .  
         [0044]     As shown in  FIG. 4B , trenches  408  are etched through openings  406 . Trenches  408  can be relatively deep (e.g., 3 μm deep). An oxide layer which will form sidewall spacers  322 , which can be 0.05 to 0.1 μm thick, is grown thermally on the walls and bottoms of trenches  408 , and a directional reactive ion etch (RIE) process is used to remove the oxide layer from the bottoms of trenches  408 , leaving sidewall spacers  322 . Oxide layer  404  and nitride layer  402  are removed.  
         [0045]     As shown in  FIG. 4C , a P-type epitaxial layer is selectively deposited in the trenches  408  and then etched back to a thickness of 1.0-1.5 μm, for example. This forms field shield regions  320 .  
         [0046]     Referring to  FIG. 4D , the exposed portions of sidewall spacers  322  are removed by isotropic oxide etch, typically diluted HF(hydrofluoric acid), leaving the field shield regions  320  and the portions of sidewall spacers that are embedded between field shield regions  320  and N epi layer  304 .  
         [0047]     As shown in  FIG. 4E , gate oxide layer  310  is thermally grown on the exposed portions of the walls and the floor of trenches  408  and the upper portion of trenches  408  are then filled with polysilicon gate  308 , which is preferably heavily doped with an N-type dopant by ion implantation, POCl3 or in situ. The polysilicon typically fills the top surface of epi layer  304  and is etched back by using a fourth, polysilicon mask so that it is coplanar with the top surface of epi layer  304  (although typically the polysilicon is etched back slightly into the trenches).  
         [0048]     As shown in  FIG. 4F , a P-type dopant is implanted and diffused to form P-body regions  316 . This can be done without a mask. A fifth photoresist mask (source mask) is then formed on the top surface of the structure, and the source mask is patterned photolithographically to create openings where the N+ source regions  312  are to be located. Next, an N-type dopant is implanted to form N+ source regions  312 . The mask is then removed.  
         [0049]     BPSG layer  324  is deposited. A sixth photoresist mask (contact mask) is formed on BPSG layer  324 , with openings over the mesas, and BPSG layer  324  is etched, as shown in  FIG. 4G . Using the contact mask, a second P-type dopant is implanted to form P+ body contact regions  314 . A thermal diffusion typically follows each of these implants to activate the dopant.  
         [0050]     As shown in  FIG. 4H , metal layer  326  is deposited over the top surface of the structure to make an ohmic contact with N+ source regions  312  and P+ body contact regions  314 . Metal layer  326  can be formed of Al:Si and can be from 1.3 to 5.0 μm thick. Typically a thin Ti/TiN barrier layer (not shown) is deposited under metal layer  326 . The result is MOSFET  30 , shown in  FIG. 2 . A seventh photoresist mask (metal mask) is formed over metal layer  326 , and metal layer  326  is etched through the metal mask to separate metal layer  326 S that contacts N+ source regions  312  from the portion (not shown) that contacts the gate  308 .  
         [0051]      FIGS. 5A-5G  illustrate a process that can be used to form an alternative embodiment of the invention. This process can use as few as three masks and as many as seven masks. However,  FIGS. 5A-5G  illustrate a three-mask version of the process. The process described above in  FIGS. 4A-4C  is carried out, except that a blanket implant and diffusion to form P body region  316  is performed before pad oxide layer  404  and nitride layer  402  are deposited. As described above, trench mask is used to define the location of the trench. After field shield region  320  has been formed, as shown in  FIG. 4C , pad oxide layer  404  and nitride layer  402  are left in place, as shown in  FIG. 5A . The doping concentration of field shield region  320  may be in the range of 5×10 16  to 5×10 17  cm −3 , for example.  
         [0052]     The exposed portions of oxide layers  322  are then removed. Gate oxide layer  310  is thermally grown on the exposed sidewalls of the trench and on the exposed upper surface of field shield region  320 . The upper portion of trench  408  is then filled with polysilicon gate  308 , which is preferably doped with an N-type dopant by ion implantation, POCl3, or preferably in situ. The polysilicon is etched back so that its top surface adjoins nitride layer  402 . As described above a BPSG layer  324  is deposited on he top surface of the structure and etched back, using an RIE process, or planarized, using a chemical-mechanical polishing technique, until the top surface of BPSG layer  324  is coplanar with the top surface of nitride layer  402 , thereby forming a BPSG plug  470 . The resulting structure is shown in  FIG. 5B .  
         [0053]     Nitride layer  402  is then removed, preferably without a mask, to yield the structure shown in  FIG. 5C .  
         [0054]     As shown in  FIG. 5D , the structure is then heated in a dry-oxidation furnace (e.g., at 900-1000° C. for 10-30 minutes) to oxidize the exposed sidewalls of polysilicon gate  308 , forming oxide layers  472 .  
         [0055]     As shown in  FIG. 5E , pad oxide layer  404  is removed, and a P-type dopant is implanted and diffused to adjust the threshold voltage of the MOSFET to be formed. The areas in which this dopant is located is labeled  474 . An N-type dopant is implanted and diffused to form N+ source layer  476 .  
         [0056]     As shown in  FIG. 5F , a second, N+ doped polysilicon layer is deposited over the top surface of the structure, and is then removed using a directional RIE process to leave N+ polysilicon spacers  478  adjacent the sidewalls of BPSG layer  470 . Polysilicon spacers  478  also abut the exposed surfaces of oxide layers  472 . A second BPSG layer is deposited over the top surface of the structure and is then removed using a directional RIE process to leave BPSG spacers  480  adjacent polysilicon spacers  478 . As a result, at this point of the process both polysilicon spacers  478  and BPSG spacers  480  are attached to the sides of BPSG layer  470 . Alternatively, a silicon nitride layer could be deposited instead of the second BPSG layer in which case spacers  480  would be made of nitride.  
         [0057]     Using BPSG layer  470  and spacers  478  and  480  as a mask, the top surface of epi layer  304  is etched using an RIE process to remove the exposed portions of N+ source layer  476 . Using the same mask, a P-type dopant is implanted at a relatively low energy to form P+ body contact regions  482 . This produces the structure illustrated in  FIG. 5F .  
         [0058]     BPSG layer  470  and BPSG (or nitride) spacers  480  are etched (e.g., about 500 Å) to expose more of N+ polysilicon spacers  478  and N+ source layer (now region)  476 . In this process all of BPSG spacers may be removed.  
         [0059]     As shown in  FIG. 5G , a barrier metal layer  481  formed of Ti/TiN is deposited by sputtering or CVD. Barrier metal layer  481  could be 1000 Å thick. This is followed by the deposition of metal layer  326 , which could be from 2 to 8 μm thick. Metal layer  326  could be made of Al and could include up to 1% Si and 0.4% Cu. A photoresist metal mask is then typically formed atop metal layer  326 , and metal layer  326  is etched to separate the metal layer  324 S that contacts the N+ source regions  476  (shown in  FIG. 5G ) from the portion (not shown) that contacts the gate  308 .  
         [0060]     The result of this process is MOSFET  40 , shown in  FIG. 5G .  
         [0061]     In an alternative embodiment, nitride spacers  486  are substituted for polysilicon spacers  478  and BPSG spacers  480 , producing MOSFET  42  shown in  FIG. 6   
         [0062]      FIG. 7  shows an alternative embodiment according to the invention. Again, MOSFET  50  is formed in epi layer  304  that is grown on N+ substrate  302 . Trenches  306  are formed in epi layer  304 , and trenches  306  are lined with gate oxide layer  310  and filled with polysilicon gate  308 . Deep trenches  450  are formed in the mesas between trenches  306 . The walls of each trench  450  are lined with oxide sidewall spacers  458 , and each trench  450  contains a P shield region  452  and a P+ contact region  456 . Within the mesa between trenches  306  are a P-body region  454 , N+ source regions  312  and P+ body contact regions  460 . The top surface of each gate  308  is covered with a BPSG layer  324 . Source metal layer  326 S overlies BPSG layer  324  and makes electrical contact with N+ source regions  312 , P+ body contact regions  460  and P+contact region  456 . Similarly, metal layer  325  contacts N+ substrate  302 , which functions as the drain. The remainder of epi layer  304 , outside the mesa between trenches  306 , includes N drift region  318 , which is more lightly doped than N+ substrate  302 .  
         [0063]     Thus, P+ body contact regions  460 , P-body regions  454 , N+ source regions  312 , P+contact region  456  and P shield region  452  are all biased to the source potential through metal layer  326 S. When MOSFET is blocking voltage in an off condition, depletion regions spread outward from sidewall spacers  458  into N drift region  318 . Thus, a vertical junction field-effect transistor (JFET) forms between adjacent deep trenches  450 , underneath trenches  306 . The N drift region  318  is largely depleted by the adjacent deep trenches  450  when MOSFET is blocking a voltage. This increases the breakdown potential of MOSFET  50  and protects the corners of trenches  306  and gate oxide layers in trenches  306  from the high electric field that would otherwise result from a high source-to-drain voltage and high gate-to-drain voltage. N drift region  318  can be doped to a higher concentration than would otherwise be possible, reducing the on-resistance of MOSFET  50 .  
         [0064]     MOSFET  50  can be fabricated with a conventional process, except that an additional mask and etch for the deep trenches  450  is required. An oxide layer is grown on the sidewalls and floor of the deep trenches  450 , and the oxide layer is removed from the floor of the deep trenches  450  by an RIE process to leave oxide spacers  458 . A selective epi growth process is used to form P shield regions  452 . After the formation of the P shield regions  452 , a normal trench MOSFET process can be used to fabricate trenches  306  and the remainder of MOSFET  50 .  
         [0065]     Referring again to  FIG. 3B , a manufacturing process for making electrical contact with the field shield regions by means of a wide trench is illustrated in  FIGS. 8A-8C . This is part of the process flow illustrated in  FIGS. 4A-4H . Pad oxide layer  402  and nitride layer  404  are patterned ( FIG. 4A ) so as to form wide trenches  602  in the locations on the chip where the field shield region is to be contacted. The process steps described in  FIGS. 4B-4D  are then undertaken to form P shield region  604 . When N+ polysilicon layer  308  is deposited ( FIG. 4E ), it conforms to the contours of wide trench  602 , as shown in  FIG. 8A . When BPSG layer  324  is deposited ( FIG. 4G ), it likewise conforms to the contours of wide trench  602 , as shown in  FIG. 8B . Referring further to  FIG. 8B , when BPSG layer  324  is masked, an opening is formed in the central region of wide trench  602 , and BPSG layer  324 , polysilicon layer  308  and the thin oxide layer over P shield region  604  are etched through this opening to form the structure shown in  FIG. 8B . This produces polysilicon spacers  606  and BPSG spacers  610  on the walls of wide trench  602 . P shield region  604  contains a P+ region  608 , which can be formed at the same time as P+ body contact region  314 . When metal layer  326  is deposited ( FIG. 4H ), it flows into wide trench  602  and forms an electrical contact with P shield region  604 , as shown in  FIG. 8C  The use of this process in the basic process sequence shown in  FIGS. 5A-5G  produces a similar result, except that, as shown in  FIG. 9  there is no polysilicon layer  308  on the die surface, only inside the trenches. Therefore, in the three-mask process, N+ polysilicon and BPSG sidewall spacers are formed on the vertical surfaces of BPSG layer  324 . As mentioned above, a portion of metal layer  326  (not shown) is used to contact the polysilicon gate  308 .  
         [0066]      FIG. 10  shows a termination edge region  650  that may be used with the field shield contact structure shown in  FIG. 3A , which contains a P well. A section  404 A of oxide layer  404  is left remaining on top of epi layer  304 , with an opening  654  adjacent the end of trench  306 . This can be done in the seven-mask process illustrated in  FIGS. 4A-4H . A heavily-doped N+ polysilicon layer  308 A is formed over oxide layer  404 A. Polysilicon layer  308 A can be a portion of the polysilicon layer that is deposited to form gate  308  (see  FIG. 4E ) and a mask can be applied before the polysilicon is etched back into the trench to form layer  308 A. Using the contact mask, a portion  324 A of BPSG layer  324  is left remaining on top of polysilicon layer  308 A, with an opening  658  over polysilicon layer  308 A. Finally, after metal layer  326  has been patterned, using the metal mask, the portion  326 S that contacts the source regions also contacts P+ region  330  and polysilicon layer  308 A.  
         [0067]     If the field shield is contacted in the manner shown in  FIG. 9 , using a wide trench, a termination structure of the kind shown in  FIG. 11  may be employed. In the edge termination region  700 , oxide layer  310 A, N+ polysilicon layer  308 A and three trenches  702 A,  702 B and  702 C are formed by using the trench and contact mask levels. There are no active field plates on the surface of the voltage termination structure shown in  FIG. 11 , where the process is reduced to three mask levels. The three trenches  702 A,  702 B and  702 C are typically longitudinal trenches that are parallel to each other and are parallel to and adjacent to an edge of the semiconductor die. Trenches  702 A- 702 C may be formed in the same manner and at the same time as trenches  306  in the active region of the MOSFET (see  FIG. 5B ). The internal structure of trenches  702 A- 702 C is identical to that of trenches  306 . Each P-shield region  320  and each polysilicon region  308 A “floats” with respect to both source and the drain potentials, because there is no direct electrical contact. Therefore, the three trenches filled with polysilicon  308 A, isolated by silicon dioxide layer  310 A, act like “floating” p-n junctions (floating rings) with a field plate to reduce the electric field by dividing the voltage among three trenches  702 A- 702 C. Either the P field shield region  320  below each of trenches  702 A- 702 C is in electrical contact with the poly silicon  308 A or floating. The contact mask is designed such that a portion  324 B of BPSG layer  324  is left over trenches  702 A- 702 C. BPSG layer  324 B is removed from the active region of the device side to allow metal layer  326 S, which is in contact with the N+ source regions  476 , to make contact with P+ region  482 . BPSG layer  324  is also removed from the saw street area of the chip (right side of  FIG. 11 ). Polysilicon spacers  478  and BPSG spacers  480  are also shown on the sidewalls of BPSG layer  324 B in  FIG. 11 .  
         [0068]      FIGS. 12A and 12B  illustrate a structure for contacting the gate  308  when the process shown in  FIGS. 4A-4H  is used to manufacture the MOSFET. As shown in  FIG. 12A , oxide layer  404  are nitride layer  402  are masked so that they are not removed at the point described above (see  FIG. 4B ). Similarly, when the polysilicon layer which will form gate  308  is deposited, and before it is etched back into the trench, the polysilicon layer is masked in the area where the gate contact is to be made, forming polysilicon layer  308 B, which is essentially an extension of gate  308  outside the trench. Polysilicon layer  308 B is thus in electrical contact with gate  308 . Nitride layer  324  D is an extension of nitride layer  324 . An opening is formed in the contact mask (see  FIG. 4G ) so that when BPSG layer  324 D is etched, an opening  710  is formed. When metal layer  326  is deposited, it fills the opening  710  and makes contact with polysilicon layer  308 B. The metal mask is configured such that the section of metal layer  326  that contacts polysilicon layer  308 B becomes the gate metal portion  326 G.  
         [0069]      FIGS. 13A and 13B  illustrate a way of contacting the gate if the process described in  FIGS. 5A-5G  is used to manufacture the MOSFET. This process is similar to the one described in  FIGS. 5F-5G , except that polysilicon is inside a wider trench region,  306 W.  
         [0070]      FIGS. 14A-14C  illustrate three patterns in which the gate trenches and mesas may be formed: stripe, square and hexagonal geometries. Devices of the present invention may be formed in any of these or other trench lay out patterns.  
         [0071]     While specific embodiments of this invention have been described, it should be understood that these embodiment are illustrative, and not limiting. Many other embodiments according to this invention will be apparent to persons of skill in the art. For example, while the embodiments described above involved MOSFETs, this invention is also applicable to other semiconductor devices, such as trench insulated gate bipolar transistors (IGBTs), vertical power junction field-effect transistors (JFETs) and power bipolar devices. Moreover, while N-channel devices have been described, these principles of this invention can be used with P-channel devices by reversing the polarities.