Semiconductor device manufacturing: process – Forming bipolar transistor by formation or alteration of... – Using epitaxial lateral overgrowth
Reexamination Certificate
1999-04-21
2002-11-19
Fourson, George (Department: 2823)
Semiconductor device manufacturing: process
Forming bipolar transistor by formation or alteration of...
Using epitaxial lateral overgrowth
C438S343000, C438S349000, C438S481000
Reexamination Certificate
active
06482712
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device and a method for fabricating the same, and more particularly, to a bipolar transistor and a method for fabricating the same.
FIG. 1
shows a cross-sectional view of a hetero-junction bipolar transistor formed using a compound semiconductor. Its fabrication process will be explained below.
First of all, an n+GaAs subcollector layer
12
, n−GaAs collector layer
13
, p+GaAs base layer
14
, n−GaAs emitter layer
15
, and n+GaAs emitter cap layer
16
are sequentially grown on a GaAs substrate
11
through epitaxy. Then, a metal is deposited thereon and patterned to form an emitter electrode
17
. The emitter cap layer
16
and the emitter layer
15
are patterned to form an emitter portion.
Thereafter, a metal is deposited on the exposed base layer
14
to form a base electrode
18
and the base layer
14
is patterned to form a base portion. At the same time, the collector layer
13
is etched. Then a metal is deposited on the exposed subcollector layer
12
and patterned to form a collector electrode
19
. The subcollector layer
12
is etched to form a collector portion.
For increasing the operation speed of the semiconductor device, it is required that the base layer be thin and the emitter area be small. However, in the aforementioned conventional method, a narrow space is formed between the emitter electrode
17
and base electrode
18
and a thin base layer is formed. Thus, the base layer may be easily damaged during etching of the emitter layer. This makes the fabrication process difficult.
In order to solve the above problem, a technique using selective epitaxy has been proposed. This technique will be explained below with reference to FIG.
2
.
As shown in
FIG. 2
, a subcollector layer
12
, a collector layer
13
and a base layer
14
are sequentially formed on a substrate
11
through epitaxy. An epitaxy mask is formed of Si
3
N
4
on the portion where a base is to be formed, and then the epitaxy is carried out. Accordingly, a selective epitaxy is accomplished to thereby grow an emitter layer
15
and an emitter cap layer
16
on the portion where the epitaxy mask (Si
3
N
4
) is not formed. That is, the emitter layer
15
and emitter cap layer
16
are not grown on the epitaxy mask. Here, the emitter layer
15
and emitter cap layer
16
are formed in the shape of an overhang around the epitaxy mask as shown in FIG.
2
.
Thereafter, the epitaxy mask is etched, and a metal layer is deposited thereon. By doing so, an emitter electrode
17
and a base electrode
18
are simultaneously formed using the overhang. Then, the portion of the metal layer formed where the base electrode
18
is not formed is etched through photolithography. The base layer
14
and collector layer
13
are also selectively etched through photolithography. Then a collector electrode
19
is formed on the exposed subcollector layer
12
.
According to this technique, the emitter electrode
17
and base electrode
18
are simultaneously formed, and the collector electrode
19
is formed on the exposed subcollector layer
12
. If the emitter electrode
17
and base electrode
18
cannot be formed using the same metal material, the emitter electrode
17
is formed first before the epitaxy mask is etched. Then the epitaxy mask is etched and the base electrode
18
is formed by depositing a metal on portions of the base layer
14
.
In the aforementioned method for fabricating the bipolar transistor as shown in
FIG. 2
, which attempts to solve the problems of the device shown in
FIG. 1
, the base layer
14
is grown by a first selective epitaxy, and the emitter layer
15
and emitter cap layer
16
are grown by a second selective epitaxy using a Si
3
N
4
layer as an epitaxy mask. This technique has the following advantages. Only the Si
3
N
4
mask layer is removed in order to form the base electrode
18
, and the emitter electrode
17
and base electrode
18
can be simultaneously formed using the overhang of the emitter layer
15
.
In the aforementioned technique, however, the emitter-base junction, which is an important portion of the bipolar transistor, is exposed during the fabrication process, which results in a deterioration of junction characteristics. Also, two-time epitaxy must be carried out complicating the fabrication process. Furthermore, although the surface area of the emitter layer
16
can be decreased, the surface area of the emitter electrode
17
and emitter layer
16
becomes identical. Thus, if the surface area of the emitter layer
16
is small, the surface area of the emitter electrode
17
becomes small, which makes it difficult to connect the emitter electrode
17
to a pad, or to another electrode.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a bipolar transistor and a method for fabricating the same to overcome the above and other problems of the conventional bipolar device and method, whereby a hetero-junction bipolar transistor having a small emitter area and a thin base layer is fabricated through selective epitaxy, for increasing operation speed of the transistor.
To accomplish the above and other objects of the present invention, there is provided a bipolar device including an insulating layer formed on a substrate; a collector formed on the substrate, the collector being adjacent the insulating layer, the collector having an inclined portion where the collector contacts the insulating layer; a base formed on the inclined portion of the collector and a portion of the collector; and an emitter formed on the base corresponding to the inclined portion of the collector.
Here, the collector is thicker than the insulating layer, whereas the base is thinly formed on the inclined portion of the collector and thickly formed on a portion of the collector. The bipolar device further includes an emitter electrode formed on the emitter and the insulating layer, a base electrode formed on the thick portion of the base, and a collector electrode formed on a portion of the collector where the base is not formed.
It is preferable that the substrate is formed of one of GaAs, InP, GaP and Si.
For achieving the objects of the present invention, there is further provided a method for fabricating a bipolar device, including the steps of forming an epitaxial growth retarding layer on a substrate, the layer being angled to the orientation of the substrate by a predetermined angle; forming a collector layer on the substrate, the collector layer being adjacent to the retarding layer, the collector layer having an inclined portion and its top extending from the inclined portion; forming a base layer on the collector layer, the base layer being formed thinner on the inclined portion of the collector layer than on the top of the collector layer; and forming an emitter layer on the base layer.
The present method further includes the steps of depositing and patterning a conductive material on the retarding layer and base layer to form an emitter electrode; selectively etching the emitter layer to form an emitter and, at the same time, exposing the base layer; depositing and patterning a conductive material on the base layer to form a base electrode; and selectively etching the base layer to form a base, selectively etching a portion of the collector, and depositing and patterning a conductive material on the conductive layer to form a collector electrode.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Briefly described, the present invention is directed to a bipolar device including an insulating layer formed on a substrate, a collector f
Birch & Stewart Kolasch & Birch, LLP
Fourson George
LG Semicon Co. Ltd.
Pham Thanh
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