Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate
Reexamination Certificate
1999-04-22
2001-08-28
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
30
Reexamination Certificate
active
06281097
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a semiconductor device including a bipolar transistor having a SiGe base epitaxial layer, and further to a method of fabricating the same.
2. Description of the Related Art
In order to improve a high-frequency characteristic in a transistor, there has been suggested a self-aligned selectively grown SiGe base (SSSB) bipolar transistor including an epitaxial layer made of SiGe alloy as a base layer.
FIGS. 1A
to
1
C are cross-sectional views of a base region of such a SSSB bipolar transistor, illustrating respective steps of a method of fabricating the same. As illustrated in
FIG. 1A
, a silicon dioxide film
2
and a polysilicon film
3
are successively formed on a monocrystal silicon substrate
1
acting as a collector. Then, the polysilicon layer
3
acting as a base region is etched to thereby form a second opening
3
a
therethrough. Thereafter, a silicon nitride film
4
is formed over an exposed surface of the polysilicon layer
3
, namely, an upper surface of the polysilicon layer
3
and an inner surface of the second opening
3
a.
Then, the silicon dioxide film
2
is wet-etched through the second opening
3
a
to thereby remove a portion of the silicon dioxide film
2
at a base region. Thus, the silicon dioxide film is formed with a first opening
2
a
which is in connection with the second opening
3
a.
By wet-etching the silicon dioxide film
2
, the silicon dioxide film
2
is side-etched, resulting in that tunnel portions
2
b are formed around the first opening
2
a.
Then, as illustrated in
FIG. 1B
, a SiGe epitaxial film
5
acting as a base layer is selectively grown on a surface of the monocrystal silicon substrate
1
, exposed to the second and first openings
3
a
and
2
a,
and the tunnel portions
2
b.
The growth of the SiGe epitaxial film
5
is discussed in F. Sato et al., “A Super Self-Aligned Selectively Grown SiGe Base (SSSB) Bipolar Transistor Fabricated by Cold-Wall Type UHV/CVD Technology”, IEEE Transactions on Electron Devices, Vol. 41, No. 8, pp. 1373-1378, August 1994, for instance. In the suggested method, disilane and germanium are used as a process gas for growth, and chlorine is used as an etching gas for selective growth.
In addition, an influence of chlorine gas on SiGe epitaxial layer growth is discussed in T. Aoyama et al., “Cl
2
influence on Si
1-x
Ge
x
base epitaxial layer growth for high speed bipolar transistor”, Extended Abstracts of the 1997 International Conference on Solid State Devices and Materials, pp. 528-529.
Referring back to
FIG. 1B
, the SiGe epitaxial film
5
is formed by means of a cold-wall type UHV-CVD apparatus wherein a base pressure is equal to or smaller than 1.5×10
−9
Torr, disilane at 3 sccm and germanium at 2 sccm are used as a growth gas, chlorine at 0.03 sccm is used as an etching gas, and a growth temperature is set at 605 degrees centigrade.
Thus, the SiGe epitaxial layer
5
is formed on a surface of the monocrystal silicon substrate
1
in both the first opening
2
a
and the tunnel portions
2
b.
At the same time, a SiGe polysilicon film
6
is formed on a lower surface of the polysilicon film
3
exposed to the tunnel portions
2
b.
Thus, the SiGe epitaxial film
5
grows upwardly, and the SiGe polysilicon film
6
grows downwardly both in the tunnel portions
2
b
until the SiGe films
5
and
6
come to contact with each other, as illustrated in FIG.
1
C. Thus, a base layer consisting of the SiGe epitaxial film
5
makes electrical contact with the SiGe polysilicon film
6
making electrical contact with the polysilicon film
3
acting as a base electrode layer. Though not illustrated, an emitter layer and an emitter electrode are formed on a surface of the thus formed base layer
5
through the first opening
2
a
of the silicon dioxide film
2
.
A bipolar transistor having such a structure as mentioned above can have a maximum cut-off frequency of 15 GHz.
In a conventional transistor as having been explained with reference to
FIGS. 1A
to
1
C, since the SiGe epitaxial film
5
has a much greater growth rate than that of the SiGe polysilicon film
6
, the SiGe epitaxial film
5
has a greater thickness than that of the SiGe polysilicon film
6
when they come to contact with each other, as illustrated in FIG.
1
C. Hence, it is quite difficult or almost impossible to form the SiGe epitaxial film
5
to have a small thickness.
In particular, in accordance with the above-mentioned conventional method of epitaxial growth, the SiGe epitaxial film
5
occupies about 80% of a height of the tunnel portions
2
b.
A time for electrons to run across a base layer can be made shorter, if the base layer has a smaller thickness. Namely, a base layer having a smaller thickness ensures a higher operation rate of a bipolar transistor. However, as mentioned so far, since it is quite difficult or almost impossible to form the SiGe epitaxial layer
5
thinner, there is paused a problem of difficulty in increasing an operation rate of a bipolar transistor by virtue of a thinner base layer.
In order to form the SiGe epitaxial layer or base layer
5
thinner, the silicon dioxide film
2
may be formed thinner to thereby decrease a height of the tunnel portions
2
b.
However, if the silicon dioxide film
2
is formed thinner, a parasitic capacitor formed between the monocrystal silicon substrate
1
acting as a collector and the polysilicon film or base electrode layer
3
would be increased. Thus, it would be necessary for the silicon dioxide film
2
to have a thickness of about 1000 angstroms at smallest. This means that the base layer
5
illustrated in
FIGS. 1A
to
1
C would have a thickness of 800 angstroms at smallest, which prevents a bipolar transistor from operating at a higher rate.
SUMMARY OF THE INVENTION
In view of the foregoing problems of the conventional bipolar transistor, it is an object of the present invention to provide a semiconductor device including a bipolar transistor which has a thinner base layer, and a smaller parasitic capacitor, and is capable of operating at a higher rate without decreasing a thickness of an insulating film formed between a collector or a substrate and a base electrode layer. It is also an object of the present invention to provide a method of fabricating the same.
In one aspect of the present invention, there is provided a semiconductor device including (a) a silicon substrate, (b) a Si
1-x
Ge
x
base epitaxial layer formed on the silicon substrate, (c) an insulating film formed on the silicon substrate, the Si
1-x
Ge
x
base epitaxial layer making contact at opposite ends thereof with the insulating film, (d) a base electrode layer formed on the insulating film, the Si
1-x
Ge
x
base epitaxial layer making electrical contact at the opposite ends and through an upper surface thereof with the base electrode layer, the Si
1-x
Ge
x
base epitaxial layer having a greater thickness at the opposite ends thereof than a thickness at the central portion thereof.
It is preferable that the insulating film has a thickness equal to or greater than 1000 angstroms.
The semiconductor device may further include a heater for heating the silicon substrate.
For instance, X in the Si
1-x
Ge
x
base epitaxial layer may be in the range of 0 to 15% both inclusive.
There is further provided a semiconductor device including (a) a silicon substrate, (b) an insulating film formed on the silicon substrate, and formed with a first opening, (c) a base electrode layer formed on the insulating film, and formed above the first opening with a second opening having a smaller length than the first opening so that tunnel portions are formed in the insulating film below the base electrode layer, and (d) a Si
1-x
Ge
x
base epitaxial layer formed on the silicon substrate in the first opening of the insulating film, and having a greater thickness in the tunnel portions than a thickness below the second opening.
The semiconductor device may further include a second Si
1-x
Ge
x
layer formed on a lower
McGinn & Gibb PLLC
Mulpuri Savitri
NEC Corporation
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