# US 7,745,289 B2 · Method of forming a FET having ultra-low on-resistance and low gate charge

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- Number: US 7,745,289 B2
- Document ID: US7745289B2
- Status: Granted
- Kind: B2
- Application number: 10/997,818
- Filed: 2004-11-24
- Published: 2005-07-14
- Granted: 2010-06-29
- Earliest claimed: 2000-08-16
- Assignees: Fairchild Semiconductor Corporation
- Inventors: Izak Bencuya; Brian Sze-Ki Mo; Ashok Challa
- Web page: https://patentgrep.com/patent/US7745289B2

## CPC

- H10D 30/668: {having trench gate electrodes, e.g. UMOS transistors}
- H10D 30/0297: {using recessing of the gate electrodes, e.g. to form trench gate electrodes}
- H10D 62/157: {Impurity concentrations or distributions}
- H10D 62/393: {Body regions of DMOS transistors or IGBTs (cell layout of DMOS H10D62/127)}
- H10D 62/151: {of IGFETs (of IGFETs having LDD or DDD structure H10D30/601; of thin film transistors H10D30/6713)}
- H10D 64/516: {the thicknesses being non-uniform}
- H10D: INORGANIC ELECTRIC SEMICONDUCTOR DEVICES

## Abstract

In accordance with an exemplary embodiment of the invention, a substrate of a first conductivity type silicon is provided. A substrate cap region of the first conductivity type silicon is formed such that a junction is formed between the substrate cap region and the substrate. A body region of a second conductivity type silicon is formed such that a junction is formed between the body region and the substrate cap region. A trench extending through at least the body region is then formed. A source region of the first conductivity type is then formed in an upper portion of the body region. An out-diffusion region of the first conductivity type is formed in a lower portion of the body region as a result of one or more temperature cycles such that a spacing between the source region and the out-diffusion region defines a channel length of the field effect transistor.

## Claims

### Claim 1

A method of forming a field effect transistor, comprising:
epitaxially forming a substrate cap region of a first conductivity type silicon over and in contact with a substrate of the first conductivity type silicon;
epitaxially forming a body region of a second conductivity type silicon over and in contact with the substrate cap region;
forming a plurality of trenches each extending at least through the body region;
lining the sidewalls and bottom of each trench with a dielectric material;
at least partially filling each trench with a conductive material; and
forming a plurality of source regions of the first conductivity type in an upper portion of the body region,
wherein during one or more temperature cycles, dopants of the first conductivity type in the substrate cap region out-diffuse into a lower portion of the body region to form an out-diffusion region of the first conductivity type extending from an interface between the body region and the substrate cap region into the body region such that a spacing between the source regions and the out-diffusion region defines a length of a channel region of the field effect transistor.

### Claim 2 (depends on 1)

The method of claim 1 wherein:
each trench extends through the out-diffusion region, and
the conductive material in each trench extends through a substantial depth of the out-diffusion region.

### Claim 3 (depends on 1)

The method of claim 1 wherein during the one or more temperature cycles the substrate cap region influences said out-diffusion of the dopants of the first conductivity type into the body region such that the length of the channel region varies less and is thus substantially predictable.

### Claim 4 (depends on 1)

The method of claim 1 wherein the substrate cap region has a thickness less than or equal to 2 μm.

### Claim 5 (depends on 1)

The method of claim 1 wherein the out-diffusion region has a graded doping concentration decreasing in a direction away from an interface between the out-diffusion region and the substrate cap region.

### Claim 6

A method of forming a field effect transistor, comprising:
providing a substrate of a first conductivity type silicon;
forming a substrate cap region of the first conductivity type silicon such that a junction is formed between the substrate cap region and the substrate;
forming an epitaxial layer of the first conductivity type silicon over the substrate cap region;
implanting dopants of a second conductivity type into the epitaxial layer and driving the dopants into the epitaxial layer to thereby form a body region having a junction with the substrate cap region;
forming a plurality of trenches each extending at least through the body region;
lining the sidewalls and bottom of each trench with a dielectric material;
at least partially filling each trench with a conductive material; and
forming a plurality of source regions of the first conductivity type in an upper portion of the body region,
wherein during one or more temperature cycles, dopants of the first conductivity type in the substrate cap region out-diffuse into a lower portion of the body region to form an out-diffusion region of the first conductivity type extending from the junction between the body region and the substrate cap region into the body region such that a spacing between the source regions and the out-diffusion region defines a length of a channel region of the field effect transistor.

### Claim 7 (depends on 6)

The method of claim 6 wherein the substrate cap region is epitaxially formed over and in contact with the substrate.

### Claim 8 (depends on 6)

The method of claim 6 wherein the substrate cap region is formed in the substrate by implanting dopants of the first conductivity type into the substrate.

### Claim 9

A field effect transistor, comprising:
a substrate of a first conductivity type silicon;
an epitaxial substrate cap region of the first conductivity type silicon over and in contact with the substrate;
an epitaxial body region of a second conductivity type over and in contact with the substrate cap region;
a plurality of trenches each extending at least through the body region;
a dielectric material lining the sidewalls and bottom of each trench;
a conductive material at least partially filling each trench;
a plurality of source regions of the first conductivity type in an upper portion of the body region; and
an out-diffusion region of the first conductivity type extending from an interface between the body region and the substrate cap region into the body region such that a spacing between each source region and the out-diffusion region defines a length of a channel region of the field effect transistor, the channel region extending vertically along a sidewall of each trench.

### Claim 10 (depends on 9)

The field effect transistor of claim 9 wherein:
each trench extends through the out-diffusion region, and
the conductive material in each trench extends through a substantial depth of the out-diffusion region.

### Claim 11 (depends on 9)

The field effect transistor of claim 9 wherein the dielectric material in each trench is thicker along the bottom of each trench than along the sidewalls of each trench.

### Claim 12 (depends on 9)

The field effect transistor of claim 9 wherein the out-diffusion region has a graded doping concentration decreasing in a direction away from an interface between the out-diffusion region and the substrate cap region.

### Claim 13 (depends on 9)

The field effect transistor of claim 9 wherein the substrate cap region has a thickness of less than or equal to two micrometers.