# Patent

## Title
Stress release structures for metal electrodes of semiconductor devices

## Abstract

This invention teaches stress release metal electrodes for gate, drain and source in a field effect transistor and stress release metal electrodes for emitter, base and collector in a bipolar transistor. Due to the large difference in the thermal expansion coefficients between semiconductor materials and metal electrodes, significant strain and stresses can be induced in the devices during the fabrication and operation. The present invention provides metal electrode with stress release structures to reduce the strain and stresses in these devices.

## Background
This is a divisional application of U.S. patent application Ser. No. 12/802,152.


FIELD OF INVENTION
This invention relates to metal electrodes for gate, drain and source in a field effect transistor and metal electrodes for emitter, base and collector in a bipolar transistor. Due to the large difference in the thermal expansion coefficients between epitaxial materials and metal electrodes, significant strain and stresses can be induced in the devices during the fabrication and operation. The present invention provides metal electrode with stress release structures to reduce the un-wanted strain or stresses in these devices.
BACKGROUND OF THE INVENTION
In electronic circuits, devices in the forms of bipolar junction transistor (BJT), heterojunction bipolar transistor (HBT), metal semiconductor field effect transistor (MESFET), metal oxide semiconductor field effect transistor (MOSFET), pseudomorphic high electron mobility field effect transistor (pHEMT) and metamorphic high electron mobility field effect transistor (mHEMT) are used for switching and amplification, whereas devices in the forms of p-n junction and Schottky junction are used as rectifiers. The semiconductors for the above devices may be selected from a group of Si, Si/Ge, GaAs, InGaAs, GaN, InGaN, AlInN and etc. For high power applications, the above-mentioned devices are operated at high current densities and the un-wanted joule heating in the semiconductor regions and interfaces between the semiconductor and metal electrodes is high. The un-wanted joule heating leads to an increase in the temperature of the devices and circuits. Take GaN HEMTs as an example, the local channel temperature can be as high as 500-1000 K at a bias voltage of 20 V. In addition to the variation of temperatures during the operation, the devices and circuits may go through heating process during the fabrication and after the metal electrodes have been deposited.
It is noted that for the formation of metal electrodes, the selection of materials is determined by the following considerations: [1] low electrode resistance, [2] good adhesion, [3] low thermal diffusion, and [4] good stability. Due to these requirements, materials for forming metal electrodes in electronic devices are limited. For III-V compounds, various metals such as Ti, Al, Au, Ge, Ni, Pd, Zn, W and their combinations have been developed for both ohmic electrodes and Schottky electrodes. For Si based devices and circuits, materials for metal electrodes are often selected from a group of W, WSi2, Ti, TiN, Cu, Al, TaSi2, TiSi2, etc.
As shown in Table 1, although the above-described materials are suitable for forming low resistance electrodes to the semiconductors listed, their thermal expansion coefficients are substantially larger than that of the semiconductors in the list. Therefore, during the fabrication or operation when the temperatures are raised, there is more severe expansion of the metal electrodes on the semiconductor substrate than that of the semiconductor, causing tensile stresses in the semiconductors and the interfaces. Contrarily, when the temperatures are reduced, the shrinkage of the metal electrodes is more severe than that of the semiconductors, causing a compressive stresses in the semiconductors and the interfaces. Microscopic defects are often formed in semiconductors due to these strain and stresses. The formation of these microscopic defects may lead to performance degradation and lifetime reduction in the devices.







TABLE 1







Thermal expansion coefficients of some materials













Coefficient

Coefficient



Material
(×10−6/K)
Material
(×10−6/K)















GaAs
5.73
Au
14.1



InGaAs
5.05
Cu
17



InP
4.6
Ti
9



Ge
5.6
Ni
13



Si
3.2
Al
23 (12.9)



GaN
4
AlN
4.5



InN
3
Alumina
6-7



Sapphire
4.5
Silicon carbide
3.2



Epoxy Resin
10-100











For power devices operated at high frequencies, the power density can not be easily reduced due to the limitation of phase delay. A typical power density is 1 W per mm length of the metal electrodes for GaAs-based devices. Such high power density will cause elevated temperatures in the channel layers. Take HEMTs for power application as an example, the GaAs-based semiconductor channel temperature can rise to 150° C. whereas the ones based on GaN can be more than 200° C. Formation of microscopic defects in the semiconductor channels caused by strain and stresses is therefore inevitable during fabrication and operation, especially for power devices.
SUMMARY OF THE INVENTION
The un-wanted temperature-change-induced stresses on the semiconductors in a device are substantially reduced using a metal electrode structure have a plurality of stress release sections comprising: A substrate with semiconductor layers and metal electrodes, each of the metal electrodes has a plurality of stress release sections; each of the stress release sections has a section length, separated from semiconductor top surface by a cavity having a cavity height. The section distance between adjacent stress release sections is selected to be substantially greater than emitter electrode width. With the presence of the stress release sections according to this invention, a portion of the stresses to be induced in the semiconductor layers will be distributed to the stress release sections and absorbed by them through slight deformation of the stress release sections. Hence, the un-wanted induction of stresses in the semiconductor layers and the associated defects will be reduced.



DESCRIPTION OF THE DRAWINGS
FIG. 1a shows a schematic cross-sectional view of a prior art (HBT) device, 1b-1d are cross-sectional views along A-A′. B-B′ and C-C′ taken at different portions of the HBT device in 1a. 
FIG. 2a is a schematic cross-sectional view of the HBT device according to this invention, 2b-2d are cross-sectional views along D-D′. E-E′ and F-F′ taken at different portions of the HBT device showing stress release sections in metal electrodes according to this invention.
FIG. 3a is a schematic cross-sectional view of a prior art HEMT device, 3b-3c are cross-sectional views of G-G′ and H-H′ taken at different portions of the HEMT device.
FIG. 4a is a schematic cross-sectional view of the HEMT device according to this invention, 4b-4c are cross-sectional views of I-I′ and J-J′ taken at different portions of the HEMT device showing stress release sections in metal electrodes according to this invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Heterojunction Bipolar Transistors (HBTs)
FIG. 1a shows a schematic side view of a simplified prior art heterojunction bipolar transistor (HBT) structure (100), which includes a substrate (101), a collector layer (102), a base layer (103) and an emitter layer (104). Electrical contacts are obtained from collector electrodes (105), base electrodes (106) and emitter electrodes (107). The materials for base electrodes may be different from those used for the collector electrodes and emitter electrodes, due to the difference in the conduction types. FIG. 1b illustrates a cross-sectional view along line A-A′, FIG. 1c shows a cross-sectional view along B-B′ whereas FIG. 1d is a C-C′ cross-sectional view of the prior art HBT structure (100). When the temperature of the HBT structure (100) is raised during fabrication or operation, the emitter metal expansion (107e) of the emitter electrodes (107) will be greater than the emitter layer expansion (104e), as shown in FIG. 1b. Similarly, the base metal expansion (106e) of the base electrodes (106) will be greater than the base layer expansion (103e, FIG. 1c) whereas the collector metal expansion (105e, FIG. 1d) of the collector electrodes (105) will be greater than the collector layer expansion (102e). Above differences in corresponding expansions due to the difference in the expansion coefficients will be more severe when the emitter electrode length (Lf 108) is large compared to the emitter electrode width (Wf 109 in FIG. 1a). This induces stresses in the sandwiched emitter (104), base (103) and collector (102) layers. Similarly, stresses will be induced in the sandwiched base (103) and collector (102) layers due to larger expansion (106e) in base electrode (106), and in the collector layer (102) due to larger expansion (105e) in the collector electrodes (105). In addition, stresses will also be induced in the emitter layer/emitter electrode interface (104i, FIG. 1b), the base layer/base layer electrode interface (103i, FIG. 1c) and the collector layer/collector electrode interface (102i, FIG. 1d), deteriorating adhesion of the metal electrode layers to the semiconductor layers.
FIG. 2a shows a schematic side view of a simplified HBT structure (200) according to this invention, said structure includes a substrate (201), a collector layer (202), a base layer (203) and an emitter layer (204). Electrical contacts are obtained from collector electrodes (205), base electrodes (206) and emitter electrodes (207). The materials for base electrodes may be different from those used for the collector electrodes and emitter electrodes, due to the difference in the conduction types. FIG. 2b illustrates a cross-sectional view of the simplified HBT structure (200) along line D-D′. An E-E′ cross-sectional view and an F-F′ cross-sectional view of the device (200) is illustrated respectively in FIGS. 2c and 2d. 
According to an embodiment of this invention, a plurality of emitter stress release sections (210) is provided in the emitter electrodes (207) (see FIG. 2b), each of the emitter stress release sections (210) has a section length (210L), separated from emitter front surface (204s) by a emitter cavity (210C) having a emitter cavity height (210H). The section distance (212) between two adjacent emitter stress release sections is selected to be substantially greater than emitter electrode width (207W, FIG. 2a). With the presence of the emitter stress release sections (210) according to this invention, a portion of the stresses to be induced in the sandwiched emitter (204), base (203) and collector (202) layers due to the emitter electrodes (207) will be distributed to the emitter stress release sections (210) and absorbed by them through slight deformation of the emitter stress release sections. Hence, the un-wanted induction of stresses in the semiconductor layers will be reduced.
According to another embodiment of this invention, a plurality of base stress release sections (220, FIG. 2c) in the base electrodes (206) is provided, each of the base stress release sections (220) has a length (220L), separated from base front surface (203s) by a base cavity (220C) having a base cavity height (220H). The section distance (222) between two adjacent base stress release sections is selected to be substantially greater than base electrode width (206W, FIG. 2a). With the presence of the base stress release sections (220) according to this invention, a portion of the stresses to be induced in the sandwiched base (203) and collector (202) layers due to the base electrodes (206) will be distributed to the base stress release sections and absorbed by them through slight deformation of the base stress release sections. Hence, the un-wanted induction of stresses in the semiconductor layers (203, 202) will be reduced.
According to still another embodiment of this invention, a plurality of collector stress release sections (230, FIG. 2d) in the collector electrodes (205) is provided, each of the collector stress release sections (230) has a length (230L), separated from collector front surface (202s) by a collector cavity (230C) having a collector cavity height (230H). The section distance (232) between adjacent collector stress release sections is selected to be substantially greater than collector electrode width (205W, FIG. 2a). With the presence of the collector stress release sections (230) according to this invention, a portion of the stresses to be induced in the collector layer (202) due to the collector electrodes (205) will be distributed to the collector stress release sections and absorbed by them through slight deformation of the collector stress release sections. Hence, the un-wanted induction of stresses in the semiconductor layer (202) will be reduced.
High Electron Mobility Transistors (HEMTs)
FIG. 3a shows a schematic side view of a simplified prior art HEMT structure (300) which includes a substrate (301), a buffer layer (302), a channel layer (303) and semiconductor contact layers (304, 305) with a thickness (390). Electrical contacts are obtained from a source electrode (306) and a drain electrode (307). The source electrode (306) and the drain electrode (307) have a drain/source electrode height (306H). In the HEMT structure (300), a gate electrode (310) is placed substantially in the central portion of the uncovered channel layer (303). The gate electrode (310) has a stem portion (311) with a gate length (311L) and a head portion (312) having a head portion length (312L). The materials for semiconductor contact layers (304, 305) may be different from those used for the gate electrode (310). To enhance the integrity during operation, a layer of passivation material (313) such as silicon nitride may be deposited.
A cross-sectional view along line G-G′ of the simplified prior art HEMT structure (300) is shown in FIG. 3b, while a cross-sectional view along the H-H′ line of the structure is illustrated in FIG. 3c. When the temperature of the HEMT structure (300) is raised during the fabrication or operation, thermal expansion (307e, FIG. 3b) of the drain electrodes (307) and the drain semiconductor contact layer (305) will be greater than the expansion (303e, FIG. 3b) in the semiconductor channel layer (303) due to differences in the expansion coefficients. In a similar fashion, the thermal expansion (not shown) of the source electrodes (306) and the source semiconductor contact layer (304) will be greater than that in the semiconductor channel layer (303) underneath it. The stresses induced by the expansion differences will be more severe when the widths (307W, FIGS. 3b and 306W, not shown) is large compared to the lengths (306L, 307L, FIG. 3a) of the source/drain metal electrodes (306, 307) and will induce stresses in the sandwiched semiconductor contact layers (304, 305) and the channel layer (303).
As illustrated in FIG. 3c (the passivation layer 313 is not shown), when the temperature of the structure (300) is raised, the gate metal expansion (310e) of the gate electrode (310) will be greater than the channel layer expansion (303e) which will induce stresses in the channel layer (303) under the stem portion (311) of the gate electrode (310). This expansion difference will be more severe when the width (301W, FIG. 3c) is large compared to the gate length (311L, FIG. 3a).
FIG. 4a shows a schematic side view of a simplified HEMT structure (400) according to the present invention, said structure includes a substrate (401), a buffer layer (402), a channel layer (403) and semiconductor contact layers (404, 405) with a thickness (490). Electrical contacts are obtained from a source electrode (406) and a drain electrode (407). The drain and source electrodes have a drain/source electrode height (406H). The gate electrode (410) is placed substantially in the central portion of channel layer, (403) where it is not covered by the semiconductor contact layers (404, 405). The gate electrode (410) has a stem portion (411) with a stem central axis, a stem (or gate) length (411L) and a stem height. The gate electrode also has a head portion (412) having a head central axis, a head length (412L) and a head height. The central axis of the head portion may be in alignment with the central axis of the stem portion forming a T-gate. It may also not be aligned with the central axis of the stem portion forming a Γ-gate structure. To enhance the integrity during operation, a layer of passivation material (413) such as silicon nitride may be deposited.
According to one embodiment of this invention, a cross-sectional view along line I-I′ of the HEMT structure (400) is shown in FIG. 4b, where a plurality of drain stress release sections (420) is provided in the drain metal layers (405, 407). Each of said drain stress release sections (420) has a section length (420L), separated from channel layer surface (403s) by a drain electrode cavity (420C) having a drain electrode cavity height (420H). According to another embodiment of the invention, a plurality of source stress release section similar to the drain stress release sections (420) is also provided in the source electrodes (406). Each of said source stress release sections has a section length, separated from the semiconductor electrode layer surface (430s) by a source electrode cavity. Since the source stress release sections can have the same structure and dimensions as the drain stress release sections (420), the source stress release sections are not illustrated and described separately.
The section distance (422) between two adjacent drain/source stress release sections is selected to be substantially greater than the drain/source electrode length (406L, 407L, FIG. 4a). With the presence of the drain/source stress release sections according to this invention, a portion of the stresses to be induced in the sandwiched electrode layers and channel layer (403) will be distributed to the drain/source stress release sections and absorbed by them through slight deformation of the drain/source stress release sections. Hence, the un-wanted induction of stresses in the semiconductor layers and the metal semiconductor interface will be reduced.
According to another embodiment of the present invention, as shown in a cross-sectional view along J-J′ line of the HEMT structure (400) in FIG. 4c (passivation layer 413 is not shown), a plurality of gate stress release sections (430) in the gate electrode (410), each of said gate stress release sections (430) has a length (430L), separated from channel layer surface (403s) by a gate electrode cavity (430C) having a gate electrode cavity height (430H). The section distance (432) between two adjacent stress release sections (430) is selected to be substantially greater than channel length (411L, FIG. 4a). According to this invention, in order to avoid unwanted leakage current in the channel layer (403) introduced by the gate electrode cavities (430C), the charge carriers in the channel layer immediately under the gate electrode cavities (430C) may be removed, dispersed or de-activated, which is done by ion implantation in selected areas using energetic ions of various elements.
With the presence of the gate stress release sections (430) according to this invention, a portion of the stresses to be induced in the channel layer (403) under the stem portion of the gate electrodes (411) due to the expansion differences will be distributed to the gate stress release sections (430) and be absorbed by them through slight deformation of the stress release sections (430). Hence, the un-wanted stresses induced in the channel layers and the gate electrode/channel layer interface will be reduced.
Creation of Corrugated Metal Electrodes
The corrugated drain/source electrodes for a HEMT can be made by creating drain/source sacrificial layer structures to cover selected locations for the stress release sections before the deposition of semiconductor contact layers (404, 405) and the drain/source electrodes (406, 407). After the formation of the drain/source electrodes (406, 407), these drain/source sacrificial layer structures are then removed and leaving suspended sections in the electrodes which are separated from the semiconductor channel layer surface (403s) by a drain/source electrode cavity. The section length of the drain/source stress release sections is determined by the length of the drain/source sacrificial layer structures and the drain/source electrode cavity height is determined by the thickness of the drain/source sacrificial layer structures. The section distance between two adjacent drain/source stress release sections is determined by the distance between two adjacent drain/source sacrificial layer structures.
Similarly, the corrugated gate electrodes for a HEMT can be made by creating gate sacrificial layer structures to cover selected locations for gate stress release sections before the deposition of the gate electrode. After the formation of the gate electrode, these gate sacrificial layer structures are removed and leaving in the gate electrode suspended sections which are separated from the channel layer surface (430s) by a gate electrode cavity. The section length of the gate stress release sections is determined by the length of the gate sacrificial layer structures and the gate electrode cavity height is determined by the thickness of the gate sacrificial layer structures. The section distance between two adjacent gate stress release sections is determined by the distance between two adjacent gate sacrificial layer structures.
The corrugated collector electrodes for a HBT can be created by forming sacrificial layer structures before the deposition of the collector electrodes layer and by removing the sacrificial layer structures after the formation of the collector electrodes. The section length of the stress release sections in the collector electrodes are determined by the length of the scarification layer structures and the collector cavity height is determined by the thickness of the sacrificial layer structures. The section distance between two adjacent collector stress release sections is determined by the distance between two adjacent sacrificial layer structures.
The corrugated base electrodes for a HBT can be created by forming sacrificial layer structures before the deposition of the base electrodes layer and by removing the sacrificial layer structures after the formation of the base electrodes. The section length of the stress release sections in the base electrodes are determined by the length of the scarification layer structures and the base cavity height is determined by the thickness of the sacrificial layer structures. The section distance between two adjacent base stress release sections is determined by the distance between two adjacent sacrificial layer structures.
The corrugated emitter electrodes for a HBT can be created by forming sacrificial layer structures before the deposition of the emitter electrodes layer and by removing the sacrificial layer structures after the formation of the emitter electrodes. The section length of the stress release sections in the emitter electrodes are determined by the length of the scarification layer structures and the emitter cavity height is determined by the thickness of the sacrificial layer structures. The section distance between two adjacent emitter stress release sections is determined by the distance between two adjacent sacrificial layer structures.
Although the description of the present invention is made with reference to a structure of HBT and a structure of HEMT, the structures provided for reducing stresses are equally applicable to MESFET, MOSFET and two terminal devices such as LED, Lasers. Therefore, the spirit of the present invention should not be considered to be limited to the HBT and HEMT.

## Claims

1. A heterojunction bipolar transistors with stress release metal electrodes comprising:
a substrate;
a collector layer with a front surface;
at least one collector electrode having a length, a width, a bottom surface contacting said front surface of said collector layer, wherein said collector electrode has of collector stress release sections along said length of said collector electrode each said collector stress release section has a section length and forms a collector electrode cavity having a collector electrode cavity height, causing a portion of said bottom surface of said collector electrode at location of each said collector stress release section from contacting said front surface of said collector layer;
a base layer with a front surface;
at least one base electrode having a length, a width, and a bottoms surface contacting said front surface of said base layer, wherein said base electrode has a plurality of base stress release sections along aid length of said base electrode, each said base stress release section has a section length and forms a base electrode cavity having a base electrode cavity height, causing a portion of said bottom surface of said base electrode at location of each said base stress release section from contacting said front surface of said base layer;
an emitter layer with a front surface; and
at least one emitter electrode having a length, a width, and a bottom surface contacting said front surface of said emitter layer, wherein said emitter electrode has a plurality of emitter stress release sections along said length of said emitter electrode, each said emitter stress release section has a section length and forms an emitter electrode cavity having an emitter electrode cavity height, causing a portion of said bottom surface of said emitter electrode at location of each said emitter stress release section from contacting said front surface of said emitter layer.

2. A heterojunction bipolar transistors with stress release metal electrodes as defined in claim 1, wherein materials for base electrodes, collector electrodes and for emitter electrodes are selected from a group of metals and metal alloys.

3. A heterojunction bipolar transistors with stress release metal electrodes as defined in claim 1, wherein a section distance between two adjacent collector stress release sections is selected to be substantially greater than said width of said collector electrode.

4. A heterojunction bipolar transistors with stress release metal electrodes as defined in claim 1, wherein a section distance between two adjacent base stress release sections is selected to be substantially greater than said width of said base electrode.

5. A heterojunction bipolar transistors with stress release metal electrodes as defined in claim 1, wherein a section distance between adjacent emitter stress release sections is selected to be substantially greater than said width of said emitter electrode.

6. A heterojunction bipolar transistors with stress release metal electrodes as defined in claim 1, wherein said collector stress release sections are fabricated by creating collector sacrificial layer structures before deposition of said collector electrode and by removing said collector sacrificial layer structures after formation of said collector electrode; said section length of said collector stress release sections is determined by length of said collector sacrificial layer structures and said collector cavity height is determined by thickness of said collector sacrificial layer structures.

7. A heterojunction bipolar transistors with stress release metal electrodes as defined in claim 1, wherein said base stress release sections are fabricated by creating base sacrificial layer structures before deposition of said base electrode and by removing said base sacrificial layer structures after formation of said base electrode; said section length of said base stress release sections is determined by length of said base sacrificial layer structures and said base cavity height is determined by thickness of said base sacrificial layer structures.

8. A heterojunction bipolar transistors with stress release metal electrodes as defined in claim 1, wherein said emitter stress release sections are fabricated by creating emitter sacrificial layer structures before deposition of said emitter electrode and by removing said emitter sacrificial layer structures after formation of said emitter electrode; said section length of said emitter stress release sections is determined by length of said emitter sacrificial layer structures and said emitter cavity height is determined by thickness of said emitter sacrificial layer structures.