# US 5,702,958 A · Method for the fabrication of bipolar transistors

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- Number: US 5,702,958 A
- Document ID: US5702958A
- Status: Granted
- Kind: A
- Application number: 08/287,568
- Filed: 1994-08-09
- Granted: 1997-12-30
- Earliest claimed: 1994-08-09
- Assignees: Texas Instruments Incorporated
- Inventors: William U. Liu; Darrell G. Hill
- Web page: https://patentgrep.com/patent/US5702958A

## CPC

- H10D 10/021: {of heterojunction BJTs [HBT]}
- H10D 10/821: {Vertical heterojunction BJTs}
- H10D 62/85: being Group III-V materials, e.g. GaAs
- Y10S 148/011: Bipolar transistors
- Y10S 148/072: Heterojunctions
- H10D: INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
- Y10S: TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS

## Abstract

The invention described herein includes, in one of its forms, a method for fabricating a semiconductor device having ledge material (148, 150, 152, 162) extending over an undercut region. The method comprises the step of forming a layer of material 164 in tensile stress over the undercut region, or region to be undercut. The layer of material in tensile stress can be a dielectric, such as silicon nitride, and provides support for the ledge material in subsequent processing steps.

## Claims

### Claim 1

A method for fabricating a semiconductor device having semiconductor ledge material extending over an undercut region, said method comprising the step of forming a layer of material in tensile stress and coupled to said semiconductor ledge material over said undercut region or region to be undercut, said layer of material in tensile stress remaining over said undercut region or region to be undercut during steps of said method of fabrication following the formation of said undercut region, whereby said layer of material in tensile stress provides support for said semiconductor ledge material.

### Claim 2 (depends on 1)

The method of claim 1 wherein said layer of material in tensile stress comprises a dielectric.

### Claim 3 (depends on 2)

The method of claim 2 wherein said dielectric is chosen from the group consisting of silicon nitride, silicon oxide, polyimide, and oxynitride.

### Claim 4 (depends on 2)

The method of claim 2 wherein said dielectric is silicon nitride.

### Claim 5

A method for fabricating a bipolar transistor comprising the steps of:
a. providing a material structure including:
a subcollector layer;
a collector layer;
a base layer over said subcollector layer; and
an emitter layer over said base layer;
b. forming a base mesa by removing portions of said emitter and base layer, said base mesa lying over said collector and subcollector layer;
c. forming a layer of dielectric over said base mesa;
d. removing a portion of said collector layer from beneath said base mesa after said layer of dielectric is formed; and
e. with said layer of dielectric still over said base mesa, forming contacts to said subcollector layer, whereby a ledge portion formed by said removal of said collector layer is mechanically supported by said layer of dielectric.

### Claim 6 (depends on 5)

The method of claim 5 wherein said layer of dielectric is in tensile stress.

### Claim 7 (depends on 5)

The method of claim 5 wherein said dielectric is chosen from the group consisting of silicon nitride, silicon oxide, polyimide, and oxynitride.

### Claim 8 (depends on 5)

The method of claim 5 wherein said dielectric is silicon nitride.

### Claim 9

A method for fabricating a bipolar transistor comprising the steps of:
a. providing a material structure including:
a subcollector layer;
a collector layer over said subcollector layer;
a base layer over said subcollector layer; and
an emitter layer over said base layer;
b. forming a base mesa by removing portions of said emitter and base layers, said base mesa lying over said collector and subcollector layers;
c. removing a portion of said collector layer from beneath said base mesa;
d. forming a layer of dielectric over said base mesa after said step of removing a portion of said collector layer, said layer of dielectric remaining over said base mesa during subsequent steps of said method of fabrication, whereby a ledge portion formed by said removal of said collector layer is mechanically supported by said layer of dielectric.

### Claim 10 (depends on 9)

The method of claim 9 wherein said layer of dielectric is in tensile stress.

### Claim 11 (depends on 9)

The method of claim 9 wherein said dielectric is chosen from the group consisting of silicon nitride, silicon oxide, polyimide, and oxynitride.

### Claim 12 (depends on 9)

The method of claim 9 wherein said dielectric is silicon nitride.