Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Charge transfer device
Reexamination Certificate
2003-05-05
2004-03-23
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Charge transfer device
C257S016000, C257S019000, C257S037000, C257S055000, C257S063000, C257S192000, C257S249000
Reexamination Certificate
active
06710382
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device including a gate electrode and a method for fabricating the device.
Recently, as semiconductor devices have been downsized drastically and as the number of devices integrated on a chip has been increasing steeply, dual-gate CMOSFETs (complementary metal-oxide semiconductor field-effect transistors) have been used more and more widely.
Hereinafter, a p-channel MOSFET included in a known dual-gate CMOS device will be described as a typical known semiconductor device with reference to FIG.
11
.
As shown in
FIG. 11
, a gate electrode
3
of polysilicon is formed over a semiconductor substrate
1
of silicon with a gate insulating film
2
interposed between them. Normally, the gate electrode
3
is doped with a dopant, e.g., boron (B), by an ion implantation technique. The boron ions are implanted into a polysilicon film, of which the gate electrode
3
will be made, at an energy low enough to form a boron concentration profile in the gate electrode
3
with one of its peaks located near the upper surface thereof and to prevent the boron atoms from penetrating through the gate insulating film
2
into the semiconductor substrate
1
.
In this case, if the polysilicon film to be the gate electrode
3
is annealed after having been doped with boron, the boron atoms in the polysilicon film diffuse toward the semiconductor substrate
1
. Any inappropriate condition for the annealing process causes the boron atoms in the polysilicon film to permeate through the gate insulating film
2
in the semiconductor substrate
1
. Then, the dopant concentration in the semiconductor substrate
1
changes to degrade the device characteristics. Also, where a metal layer is deposited on the polysilicon film to form a poly-metal gate electrode and then a silicon nitride film to be a hard mask is deposited on the metal layer and annealed or where a silicon nitride film to be a sidewall is deposited on the gate electrode
3
and annealed, the permeation of the boron atoms into the semiconductor substrate
1
is observed noticeably.
To suppress the boron atoms from permeating the semiconductor substrate
1
, various measures have been taken; a silicon oxynitride film that can suppress the boron atom permeation to a certain degree is adopted as the gate insulating film
2
.
However, even if the silicon oxynitride film is used as the gate insulating film
2
, the boron atom permeation is not completely suppressible. Particularly, where the silicon oxynitride film is extremely thin (less than 3 nm, for example) to catch up with performance enhancement of devices, the silicon oxynitride film can suppress the boron atom permeation just slightly to say the least.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to prevent a dopant introduced into a gate electrode from permeating a semiconductor substrate.
To achieve this object, a first inventive semiconductor device includes a gate electrode that has been formed over a semiconductor substrate with a gate insulating film interposed between the gate electrode and the substrate. The gate electrode includes: a silicon germanium layer; and an upper silicon layer that has been formed on the silicon germanium layer.
In the first inventive device, a gate electrode includes: a silicon germanium layer; and an upper silicon layer that has been formed on the silicon germanium layer. Thus, in doping the gate electrode with a dopant such as boron, the dopant can be introduced by an ion implantation process into the silicon germanium layer through the upper silicon layer. So, the dopant can be implanted sufficiently shallow while the penetration of the dopant into a semiconductor substrate, which is usually caused by a channeling phenomenon, is suppressible. Accordingly, it is possible to prevent the dopant, with which the gate electrode has been doped, from permeating the semiconductor substrate even if the gate electrode is subsequently subjected to an annealing process, for example. As a result, any variation in device characteristics, which might result from a change in dopant concentration in the semiconductor substrate, is suppressible.
Also, in the first inventive device, the bandgap of the silicon germanium layer for the gate electrode may be changed by controlling a germanium concentration in the silicon germanium layer. And the threshold voltage controllability of the gate electrode can be improved by changing the bandgap. In that case, the gate electrode does not have to be doped with any dopant. As a result, any variation in device characteristics, which might result from the permeation of a dopant from the gate electrode into the semiconductor substrate, is suppressible with more certainty.
Further, in the first inventive device, since the silicon germanium layer is covered with the upper silicon layer, cross contamination (contamination of the semiconductor substrate or a reactor), caused by germanium atoms released from the silicon germanium layer, is also suppressible. Accordingly, a process for forming a gate electrode out of silicon layers can be utilized.
In one embodiment of the present invention, the gate electrode may further include a lower silicon layer under, the silicon germanium layer.
In such an embodiment, the lower silicon layer with a surface morphology better than that of the silicon germanium layer exists under the silicon germanium layer for the gate electrode. Thus, the breakdown strength of the gate insulating film can be improved compared to a situation where the silicon germanium layer and the gate insulating film are in direct contact with each other.
In another embodiment, the gate electrode may further include a metal layer on the upper silicon layer, and a silicon nitride film may have been formed over the gate electrode.
Then, the gate electrode is implementable as a poly-metal gate electrode. And even though the silicon nitride film has been formed over the gate electrode, any dopant existing in the gate electrode hardly permeates the semiconductor substrate.
In this particular embodiment, an insulating layer preferably exists between the gate electrode and the silicon nitride film.
In that case, the dopant, existing in the gate electrode, even less likely permeates the semiconductor substrate when the insulating layer is made of silicon dioxide, for example.
A second inventive semiconductor device includes a gate electrode that has been formed over a semiconductor substrate with a gate insulating film interposed between the gate electrode and the substrate. The gate electrode includes a silicon germanium layer that has been deposited in an amorphous state.
In the second inventive device, a gate electrode includes a silicon germanium layer that has been deposited in an amorphous state. Thus, in doping the gate electrode with a dopant such as boron, the dopant can be introduced by an ion implantation process into the silicon germanium layer in the amorphous state. So, the dopant can be implanted sufficiently shallow while the penetration of the dopant into a semiconductor substrate, which is usually caused by a channeling phenomenon, is suppressible. Accordingly, it is possible to prevent the dopant, with which the gate electrode has been doped, from permeating the semiconductor substrate even if the gate electrode is subsequently subjected to an annealing process, for example. As a result, any variation in device characteristics, which might result from a change in dopant concentration in the semiconductor substrate, is suppressible.
Also, in the second inventive device, the bandgap of the silicon germanium layer for the gate electrode may be changed by controlling a germanium concentration in the silicon germanium layer. And the threshold voltage controllability of the gate electrode can be improved by changing the bandgap. In that case, the gate electrode does not have to be doped with any dopant. As a result, any variation in device characteristics, which might result from the permeation of a dopant from the gate el
Kubo Hiroko
Yoneda Kenji
Matsushita Electric - Industrial Co., Ltd.
Nelms David
Pham Ly Duy
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