# Patent

## Title
Process for manufacture of low voltage power MOSFET device

## Abstract

A trench type power MOSFET has a thin vertical gate oxide along its side walls and a thickened oxide with a rounded bottom at the bottom of the trench to provide a low RDSON and increased VDSMAX and VGSMAX and a reduced Miller capacitance. The walls of the trench are first lined with nitride to permit the growth of the thick bottom oxide to, for example 1000 Å to 1400 Å and the nitride is subsequently removed and a thin oxide, for example 320 Å is regrown on the side walls. In another embodiment, the trench bottom in amorphized and the trench walls are left as single crystal silicon so that oxide can be grown much faster and thicker on the trench bottom than on the trench walls during an oxide growth step. A reduced channel length of about 0.7 microns is used. The source diffusion is made deeper than the implant damage depth so that the full 0.7 micron channel is along undamaged silicon. A very lightly doped diffusion of 1000 Å to 2000 Å in depth could also be formed around the bottom of the trench and is depleted at all times by the inherent junction voltage to further reduce Miller capacitance and switching loss.

## Background
RELATED APPLICATIONS
This application is a divisional of U.S. application Ser. No. 09/814,081, filed Mar. 21, 2001, now U.S. Pat. No. 6,434,966 entitled Low Voltage Power MOSFET Device and Process for its Manufacture which claims priority to Provisional Application Ser. No. 60/194,386 filed Apr. 4, 2000 in the name of Naresh Thapar.


FIELD OF THE INVENTION
This invention relates to power MOSFET devices and their methods of manufacture, and more specifically relates to such devices with reduced RDSON, reduced gate capacitance and increased gate breakdown voltage.
BACKGROUND OF THE INVENTION
In present trench type power MOSFET devices, a vertical gate oxide is formed simultaneously within the vertical walls of the trench and at the trench bottom. In order to provide a low RDSON, the vertical oxide should be relatively thin. However, the gate to drain capacitance is determined by the thickness of the gate oxide at the trench bottom, and the gate breakdown voltage VGSMAX is limited by the curvatures of the oxide at the trench bottom corner. Thus, the desire for a thin vertical gate oxide for low RDSON is contradictory to the need of a thick oxide at the bottom of the trench for improved VGSMAX and a low gate to drain capacitance. It would be desirable to harmonize these trade-offs.
A further problem exists in present trench type power MOSFETS due to the conventional formation of the channel and source diffusions. Thus, these regions are usually formed by an implant followed by the diffusion. The implants are known to cause surface damage which extends to a particular depth, depending on the process variables. Therefore, the vertical channel, which extends from the point at which the source intersects the trench wall to the bottom of the channel diffusion, will include damaged silicon caused by the earlier implants. This then increases threshold voltage and increases the channel resistance. It would be desirable to avoid the influence of implant damage on the conduction channel of the device.
A still further problem exists in current power MOSFETs in that the inherent Miller capacitance of the structure increases the gate charge QG of the device and thus increases switching loss. It would be desirable to reduce Miller capacitance to reduce switching loss.
BRIEF SUMMARY OF THE INVENTION
In accordance with a first aspect of this invention, a novel structure and process are provided which result in the production of a thick gate oxide at the bottom of the trench and a significantly thinner gate oxide along its vertical wall. Thus, a trench is first etched in conventional fashion, through a source diffusion layer and channel diffusion layer and the trench walls and bottom have a silicon nitride coating deposited thereon. A thermally grown pad oxide may be formed before depositing the silicon nitride. The silicon nitride at the bottom surface of the trench is then reactively etched away, and a silicon dioxide layer is then grown on the exposed silicon at the bottom of the trench. The bottom oxide layer is grown to any desired thickness, for example, 1000 Å to 1400 Å in comparison to the conventional oxide thickness of about 320 Å used for the side wall gate oxide in the conventional device. During its growth, the oxide and the silicon at the corners of the trench round out to smooth or round the otherwise sharp bottom corners of the trench.
In another embodiment of the invention, the thicker bottom oxide is formed by amorphizing the trench bottom by using a heavy dose ion implantation (for example 1E16 atoms/cm2) of a neutral species ion, for example, Argon, after the trench etch. This process makes use of the fact that amorphized silicon oxidizes 3 or 4 times faster than the single crystal silicon surface along the vertical trench walls.
Thereafter, the silicon nitride layer remaining on the trench walls is removed by a wet etch which leaves the thick bottom oxide layer intact. A thin gate oxide (320 Å for example) is then grown on the exposed side walls.
The resulting structure has the desired thick bottom oxide and rounded silicon bottom for increased VDSMAX and VGSMAX and reduced Miller capacitance; and a thin gate oxide on the trench walls for reduced RDSON.
The channel length of the new device is also reduced to about 0.7 microns (from 1.2 microns for the same voltage in the prior art).
In forming the source and channel diffusions, the impurity atoms are implanted into the silicon and diffused before forming the trench. When the diffusions are shallow, as for a low voltage device with a reduced channel length (for example, 0.5 microns) part of the blocking voltage is held off only by the channel. However, the channel may include silicon along the trench wall which was damaged during the implant process. In accordance with a further feature of the invention, the source diffusion in a short channel low voltage MOSFET is made intentionally deeper than the implant damage depth. In this way, the full channel length will be along undamaged silicon so that threshold voltage and RDSON characteristics are unaffected by silicon crystal implant damage.
As a still further feature of the invention, and to further reduce Miller capacitance, a very lightly doped P− or N− diffusion of about 1000 Å to about 2000 Å in depth is formed around the bottom of the trench. The P− diffusion will be depleted at all times by the inherent junction voltage, thus reducing Miller capacitance and switching loss. This concept is applicable to planer devices as well as trench devices.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a cross-section of a trench with a P−/N− implant to reduce Miller capacitance.
FIG. 1B shows an alternative embodiment to the structure of FIG. 1A.
FIG. 2 is a cross-section of one cell element of a planar geometry chip with the novel P−/N− implant.
FIG. 3 shows a silicon chip with a trench at a stage of processing in which the trench has a thick silicon dioxide layer on its bottom and a silicon nitride layer on its walls.
FIG. 4 shows the structure of FIG. 3 after the nitride layer is removed from the walls and is replaced by a thin oxide.
FIG. 5 shows the trench structure of FIG. 4 in which a reduced channel length is used with the source diffusion being deeper than the depth of the implant damage.



DETAILED DESCRIPTION OF THE DRAWINGS
Referring first to FIG. 1A, there is shown in cross-section, a very small portion of a trench MOSFET at an early stage of its manufacture. Thus, an N+ chip 10 of monocrystalline silicon is first prepared and an N− layer 11 of epitaxially grown silicon is formed thereon. A P− dopant is then implanted and diffused into the top of layer 11 to form P− channel diffusion 12 to a given depth. Thereafter, an N+ source impurity is implanted through a suitable mask to form spaced N+ source regions such as source region 13 which are also diffused to a given depth. Source region 13 may have any desired topology and can be a stripe or a cell or the like.
Thereafter, trenches such as trench 20 are etched in the silicon, and a silicon dioxide gate layer 21 having a thickness of about 320 Å is grown along the walls and bottom of the trench 20. A conductive polysilicon gate 22 is formed within trench 20 and the top of the polysilicon 22 is capped with an oxide cap 23.
A source electrode 24 is then formed over the top surface of the device and in contact with source regions 13 and channel region 12. A drain metal 25 is also applied to the bottom of body 10.
It is to be noted that the same elements in FIGS. 1A, 1B and 2 to 5 carry the same identifying numerals.
The structure described to this point is the well known trench MOSFET structure. In accordance with a first feature of the invention, and to reduce Miller capacitance (the capacitance between gate and drain) and thus QG for increased speed, a very lightly doped P−/N− diffusion 30 is formed in the trench wall before or after the gate oxide step but before the formation of polysilicon 22. The diffusion depth is preferably 1000 Å to 2000 Å deep. This P− diffusion 30 will have a concentration sufficiently low to be at all times depleted by the built-in junction voltage to substrate 11, thereby reducing Miller capacitance.
FIG. 2 shows that the same diffusion 30 can be used in an otherwise conventional planar MOSFET structure having gate oxide 40, polysilicon 31, LTO layer 42 and the source metal 24.
FIG. 1B shows a further embodiment of the invention of FIGS. 1A and 2 in which the P− diffusion of FIG. 1A is replaced by a 3 micron deep N− region 50 which receives trench 20 and acts like a very thin N type depletion layer. N− region 50 has a concentration of about 1×1014 atoms/cm3 and will be fully depleted by the built-in junction charge.
The doping concentration for P− region 30 of FIGS. 1A and 2 can be produced by an implant dose of about 1×1012 atoms/cm2. By comparison, N− region 50 is 50 Ω cm and N+ region 11 is 0.003 ohm cm material.
FIGS. 3 and 4 show a novel structure and process for reducing the gate-to-drain capacitance of the device by using a thick bottom oxide but thin side wall oxide in the trench, thus preserving low on-resistance. More specifically, in prior art trench devices, the bottom oxide thickness was that of the side wall thickness which is about 320 Å. In accordance with the present invention, the bottom oxide layer is increased in thickness to 1000 Å to 1400 Å and the trench is rounded at the trench corners to increase the drain/source breakdown voltage and gate-drain breakdown voltage at those corners.
Thus, after the trench 20 is formed in FIG. 3, its walls and bottom are lined with silicon nitride as by a conventional Si3N4 deposition process. The nitride at the bottom of the trench is then removed, using a suitable reactive etch process, leaving intact the nitride on the trench walls, shown as nitride layers 60 in FIG. 3. Thereafter, oxide is grown on the exposed silicon bottom of trench 20, forming the thick oxide layer 61 in FIG. 3. Layer 61 may have a thickness if from 1000Å to 1400 Å (by way of example) and, fortuitously, will create rounded edges 62, 63 (or a rounded trench bottom periphery if the trench is polygonal in cross-section). Thereafter, the nitride 60 of FIG. 3 is removed by a suitable nitride etch, for example, H3PO4 which leaves intact the oxide 61. A thin gate oxide 62 (FIG. 4) may then be grown on the exposed side walls of trench 20 to a thickness of 300 Å to 320 Å.
In another embodiment of the invention, after the trench 20 is formed, a heavy dose implant (for example, 1E16 atoms/cm2) of a neutral species, (for example, Argon) is applied to the bottom of the trench only, converting the singe crystal silicon at the trench bottom to an amorphized silicon. An oxide growth step then takes place of growing oxide on the single crystal trench walls and on the amorphized bottom trench surface. The oxide on the amorphized silicon grows about 3 to 4 times faster than on the trench walls, thus providing the trench with the desired thickened bottom.
The device is then completed in the well known manner by the formation of the polysilicon gate and source and drain electrodes.
The resulting device will have reduced gate-to-drain capacitance and will have improved voltage withstand ability because of the thick oxide 61 and the rounded trench bottom edges, rather than the sharper trench edges of the prior art.
It is desirable for devices such as that of FIG. 4 to have a shorter channel length while still holding off reverse voltage in the channel only. Thus, it was conventional for low voltage trench MOSFETs to use a channel length along the trench wall of about 1.3 microns. It has been found, as shown in FIG. 5, that a reduced channel length can be used, for example, 0.7 microns. To make such devices, it becomes necessary to consider that the channel may include a significant length of implant induced damage caused during the implant of P− region 12 and N+ source region 13. In accordance with another aspect of the present invention, the source region 13 is intentionally diffused to a depth greater than the depth of the implant damage. In this way, the full reduced length channel is in undamaged silicon so that it can better hold off full source to drain voltage.
Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

## Claims

1. The process for forming a trench type MOSgated device in a semiconductor body comprising a highly doped substrate of a first conductivity and a lightly doped semiconductor layer of said first conductivity formed over said substrate; said process comprising the steps of etching a trench having spaced side walls and a flat bottom surface in said semiconductor layer, said trench terminating in said semiconductor layer and said flat bottom surface and said sidewalls being generally normal to one another thereby forming a sharp edge at the point of their cross-section; amorphizing the bottom surface of the trench but not its side walls; and thereafter growing a silicon dioxide layer on the side walls and bottom of the trench at the same time, wherein the silicon dioxide layer is substantially thicker at the bottom of said trench than on the side walls and said edges are round.

2. The process of claim 1 wherein the said silicon dioxide coating on said walls have a thickness of greater than 1000 Å.

3. The process of claim 1, wherein the step of amorphizing the silicon at the bottom of said trench is carried out by an ion implant of a neutral species into the bottom of said trench.

4. The process of claim 3, wherein said ion implant is at a heavy dose of greater than about 1E16 atoms/cm2 .

5. The process of claim 4, wherein the silicon oxide coating on said walls have a thickness of greater than about 1000 Å.