Abstract:
A trench power MOSFET device is disclosed wherein the method of manufacturing produces a high density MOSFET cell with good breakdown characteristics.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method for producing power MOSFET transistors with high cell density. 
     BACKGROUND OF THE INVENTION 
     A power MOSFET normally requires considerable &#34;silicon real estate&#34; to perform power control and/or switching with a transistor. At the same time, low &#34;on-resistance&#34; is required for efficient device operation. Low specific &#34;on-resistance&#34; is also desirable because device cost is proportional to device area on the chip and lower on-resistance is usually achieved only by increasing the size of the device. Such transistors often require extensive isolation between adjacent devices and a large gate width to prevent source-drain shorting through various breakdown processes. The subject invention allows fabrication of a power MOSFET with reduced gate width, in part through use of a relatively deep gate oxide positioned adjacent to the source and drain regions. This achieves low specific &#34;on-resistance&#34;, can be fabricated in general shape die areas, and has low associated production costs. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to provide a method for producing power MOSFET transistors. 
     Another object is to provide a method for producing power transistors of high cell density. 
     Another object is to provide power transistors using a buried polysilicon component. 
     Other objects of the invention, and advantages thereof, will become clear by reference to the detailed description and the accompanying drawings. 
     The foregoing objects may be achieved by a method comprising the steps of: 
     providing an n+ substrate with a relatively thick layer of n- material contiguous thereto and thinner layers of p, n+, oxide and silicon nitride material stacked on top of the n- material in that order; providing a substantially rectangular or trapezoidal groove in the structure that reaches through the nitride layer, oxide layer, thin n+ layer, p layer and through a portion of the thick n- layer to expose a bottom wall and side walls of the groove; providing a thick oxide layer on the groove bottom wall and a thin oxide layer on the groove side walls; providing polysilicon that fills the groove and forms a polysilicon layer covering the top of the groove; planarizing the top surface of the structure to provide a substantially planar top surface; providing a covering layer of substantially undoped oxide that selectively covers the polysilicon at the top of the groove and immediately adjacent to the groove; removing substantially all of the polysilicon, thin nitride layer, thin oxide layer and thin n+ layer at the top surface of the structure not covered by the undoped oxide covering layer, to expose the p layer; and providing a metallized layer of material covering and contiguous to the p layer, the top of the groove and all regions of material adjacent to the boundary of the groove. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of the initial semiconductor substrate/active layers combination to which the method may be applied. 
     FIG. 2 is a sectional view of the material combination of FIG. 1 with a rectangular or trapezoidal groove etched therein. 
     FIGS. 3a, 3b and 3c are sectional views of the structure of FIG. 2, with an oxide applied to the exposed walls of the groove by (a) oxide ion implantation, (b) heavy ion implantation and enhanced etching and (c) anisotropic etching, respectively. 
     FIG. 4 is a sectional view of the structure of FIGS. 3 with bulk polysilicon grown in and on top of the groove and top surface of the structure. 
     FIG. 5 is a sectional view of the structure of FIG. 4, after, substantially all of the polysilicon has been removed from the top surface of the structure. 
     FIG. 6 is a sectional view of the structure of FIG. 5, with an electrically insulating layer of oxide introducted at the top of the groove. 
     FIG. 7 is a sectional view of the structure of FIG. 6, with the nitride layer and adjacent oxide layer removed to expose the p layer adjacent to the groove, and with a layer of metallized material applied to the top surface of the structure. 
     FIG. 8 is a sectional perspective view of a three-dimensional gate region that may be produced by the invention. 
     FIG. 9 is a plan view of a gate/source configuration that may be produced by the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention provides a high cell density structure that is suitable for power MOSFETs. With reference to the sectional view shown in FIG. 1, one begins with a layered combination structure 21 of semiconductor materials, the structure including: an n+ substrate layer 23 of Si or other suitable semiconductor material; an n- layer 25 of thickness substantially 2-5 μm contiguous to the top surface of substrate 23; a p layer 27 of thickness substantially 250-750 nm contiguous to the top surface of layer 25; an n+ layer 29 of thickness substantially 150-500 nm contiguous to the top surface of layer 27; a layer 31 of oxide material such as SiO 2  of thickness substantially 25-75 nm contiguous to the top surface of layer 29; and an initial covering layer 33 of protective nitride material such as Si 3  N 4  of thickness substantially 100-200 nm contiguous to the top surface of layer 31. The nitride material that comprises layer 33 has substantially different thermal expansion and lattice parameters than does the crystalline silicon in layer 29, and the thin layer 31 of SiO 2  material can take up most of the resulting stress of the layer 31/layer 33 interface that would otherwise appear in the silicon material that comprises layer 29. Provision of the SiO 2  layer 31 is optional but is preferred here. 
     A portion of predetermined shape of the top surface of protective layer 33 is masked and etched, preferably using an anisotropic dry etching process (&#34;etchant&#34;) that will produce a substantially rectangular or trapezoidal groove 35 in the transverse cross section shown in FIG. 2, where this groove reaches through all the thickness of each of layers 33, 31, 29 and 27 and reaches through a portion, but not all, of the thickness of n- layer 25. The bottom of the groove 35 may be adjacent to the top surface of the n+ layer 23 in FIG. 2; but this will reduce the breakdown voltage of the gate oxide and require use of smaller gate voltages. The n- layer 25 is 6-60 times as thick as each of the layers 27, 29, 31 and 33 so that terminating the action of this etchant in the interior of the n- layer 25 should not be a problem. 
     The groove 35 is then processed to form a thick oxide layer 41 on the exposed surface of the bottom wall 37 of groove 35 and a relatively thin oxide layer 43 on the exposed surfaces of the side walls 39 of the groove 35, as indicated in FIGS. 3a, 3b and 3c. The thin oxide layer 43 will serve as gate oxide. These oxide layers may be produced by any one of at least three techniques. In a first approach, one may use ion implantation of oxygen at the bottom wall 37, followed by growth of a thin oxide layer (50-200 nm) on the bottom wall as 41 and on the side walls as 43, as indicated in FIG. 3a. 
     In a second approach: (1) an oxide layer is grown thermally on the bottom wall 37 and side walls 39; (2) a second layer (not shown), of thickness substantially 100 nm, of nitride such as silicon nitride Si 3  N 4  is substantially uniformly deposited in the groove and across the top surface of the structure; (3) an ion implant, using As, P, Sb, Bi or a similar ion, is performed on the bottom wall 37 of the groove 35 and on the remaining top surface 45 of the structure; (4) the second layer of nitride is removed by etching, taking advantage of the enhancement of the etching rate at the bottom wall 37 and top surface 45 by the heavy ion implant; (5) a thick oxide layer 41&#39; (0.5-1) is grown thermally at the bottom wall 37 of the groove 35; (6) nitride (and some oxide) that adheres to the side walls 39 of the groove 35 is removed; and (7) a thin oxide layer, of thickness substantially 100 nm, is grown on the groove side walls (43&#39;) and on the groove bottom wall (41&#39;). This is partly illustrated in FIG. 3b. 
     Third, a procedure similar to the second technique above may be employed, with an anisotropic dry etch (to selectively remove nitride from the top surface and groove bottom wall) replacing the heavy ion implant step (3) and the first etch step (4), as indicated in FIG. 3c. After one of these three techniques is applied, the oxide layer 41&#39; at the bottom wall 37 of the groove 35 should be 100-500 nm thick and the oxide layer 43&#39; at the sidewalls 39 of the groove 35 should be 30-150 nm thick. 
     Polysilicon 47, in a bulk filler layer, is now deposited in the groove and at the top of the groove, as illustrated in FIG. 4. The polysilicon is doped either in situ (preferably) or post-doping, with phosphorous or other suitable n type dopant. 
     A dry etchant is now applied at the top surface of the polysilicon to remove the polysilicon at the top surface and produce a planarization of the top surface of the structure, as indicated in FIG. 5. The remaining first nitride layer 33 and underlying thin oxide layer 31 are removed from the top surface. A local oxidization step is then employed to grow a thick layer 49 of oxide on top of the polysilicon 47 in the groove 35, with a small amount of this oxide extending beyond the side wall boundaries 39 of the groove 35 at the top surface, as indicated in FIG. 6, so that no polysilicon is exposed. 
     Using &#34;bird beaks&#34; (shown in FIG. 6) for etch control around the top of the groove 35, the top surface of the structure, except the region overlying and immediately adjacent to the groove 35, is etched to remove most of the n+ layer 29 and expose the underlying p layer 27, as indicated in FIG. 7. A layer of metallization 51 is then applied to the entire top surface of the structure to produce a source contact for the device, as shown in FIG. 7. 
     FIG. 8 is a sectional perspective view of a three-dimensional gate region that may be produced by the invention, showing the gate 53, gate contact 55, source 57 and drain 59. The presence of the large &#34;bulk&#34; of polysilicon 47 adjacent to the channel 61 will help insure that gate oxide breakdown does not occur for the high electrical fields needed to drive a power MOSFET. 
     FIG. 9 shows, in plan view, a gate-source configuration that may be produced by the invention. 
     Although the preferred embodiments of the invention have been shown and described herein, variation and modification may be made without departing from the scope of the invention.