Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – Insulated gate formation
Reexamination Certificate
1999-12-02
2002-08-13
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
Insulated gate formation
Reexamination Certificate
active
06432803
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device and a fabrication method therefor and, more particularly, to a MOS transistor having a thin gate insulating film and a low-resistance gate electrode formed on the gate insulating film and a fabrication method therefor.
To implement a semiconductor device composed of a MOS transistor which is smaller in size, higher in integration, and operable with a lower voltage, it is necessary to reduce the resistances of the materials of a wire, an electrode, and the like each composing the semiconductor device and thereby reduce a delay time resulting from wiring resistance.
Accordingly, a multilayer film composed of a polysilicon film and a metal silicide film has been used for the gate electrode of the MOS transistor.
In a MOS transistor with an extremely small design rule of 0.10 &mgr;m or less, however, a sufficiently reduced resistance can not be obtained with the gate electrode formed of the multilayer film composed of the polysilicon film and the metal silicide film. As a substitute, therefore, a metal gate process has been considered in which the gate electrode is formed of a refractory metal film such as a tungsten film.
Referring to FIGS.
15
(
a
), (
b
), (
c
), and (
d
), a method of fabricating a semiconductor device according to a first conventional embodiment will be described, in which the gate electrode is formed by the metal gate process.
First, as shown in FIG.
15
(
a
), an insulating film
11
for isolation and a p-type semiconductor region
12
are formed successively in a surface portion of a semiconductor substrate
10
. A silicon oxide film
13
, serving as a gate insulating film, is then formed on a region of the semiconductor substrate
10
surrounded by the insulating film
11
for isolation. Thereafter, a target made of tungsten is sputtered in an argon ambient, whereby a tungsten film
14
serving as a gate electrode is deposited over the entire surface of the semiconductor substrate
10
.
Next, as shown in FIG.
15
(
b
), a resist pattern
15
is formed on a region of the tungsten film
14
in which a gate electrode is to be formed. Then, as shown in FIG.
15
(
c
), the tungsten film
14
and the silicon oxide film
13
masked with the resist pattern
15
is etched to form a gate electrode
14
A and a gate oxide film
13
A.
Next, as shown in FIG.
15
(
d
), an n-type lightly doped region
16
is formed by implanting an n-type dopant by using the gate electrode
14
A as a mask, followed by sidewalls
17
formed on the side surfaces of the gate electrode
14
A. Thereafter, an n-type heavily doped region
18
is formed by implanting an n-type dopant by using the gate electrode
14
A and sidewalls
17
as a mask. A heat treatment is then performed to activate the n-type lightly doped region
16
and the n-type heavily doped region
18
.
Next, contacts, metal wires, and the like are formed, though they are not shown in the drawing. As a result, the semiconductor device having the gate electrode
13
A made of tungsten is obtained.
It has been reported that, if the heat treatment is performed in a nitrogen ambient at a temperature of 900 to 1100° C. for about 30 minutes after the formation of the gate electrode
14
A composed of the tungsten film
13
, the internal stress of the gate electrode
14
A can be reduced and the reliability of the MOS transistor is improved thereby (N. Yamamoto, S. Iwata, and H. Kume, “The influence of Internal Stresses in Tungsten-Gate Electrodes on the Degradation of MOSFET Characteristics Caused by Hot Carriers: IEEE Trans, Electron Device, vol. ED-34, pp. 607-614 1987).
Hereinafter, a second conventional embodiment of the method of fabricating a semiconductor device disclosed in Japanese Unexamined Patent Publication No. 10-233505 will be described, in which the gate electrode is formed by the metal gate process.
In the second conventional embodiment, a nitrogen-containing tungsten film composed of a composite of tungsten and tungsten nitride is formed on a gate insulating film by sputtering a target made of tungsten in a nitrogen ambient. A heat treatment is then performed with respect to the nitrogen-containing tungsten film to diffuse nitrogen contained in the nitrogen-containing tungsten film, thereby preventing the reliability of the gate insulating film from deteriorating.
Since further scaling down of a semiconductor integrated circuit requires a thinner gate insulating film, the achievement of higher reliability is becoming increasingly important in a MOS transistor having an extremely thin gate insulating film. It is also known that the material of the gate electrode greatly affects the reliability of the gate insulating film.
In the case where the tungsten film is used for the gate electrode, as in the first conventional embodiment, the heat treatment for reducing the internal stress of the gate electrode should be performed, so that the internal stress of the gate electrode changes as a crystal grows within the tungsten film composing the gate electrode. As a result, a mechanical stress is exerted on the gate insulating film adjacent the gate electrode, which causes a new problem of reduced reliability of the gate insulating film. When the gate insulating film is extremely thin, in particular, the deterioration of the gate insulating film caused by the heat treatment is remarkable.
FIGS.
16
(
a
) and (
b
) show the results of a TDDB (Time Dependent Dielectric Breakdown) evaluation performed with respect to the reliability of a gate insulating film in a MOS transistor having a gate electrode made of tungsten and a silicon oxynitride film with a thickness of 3.5 nm, of which FIG.
16
(
a
) shows the result of Weibull-plotting the relationship between the charge-to-breakdown value (Q
bd
) and the cumulative fault probability when a negative bias was applied to the gate electrode and FIG.
16
(
b
) shows the result of Weibull-plotting the relationship between the value Q
bd
and the cumulative fault probability when a positive bias was applied to the gate electrode. For comparison, there is also shown the case where a gate electrode made of polysilicon is used.
As can be seen from FIG.
16
(
a
), the value Q
bd
when the negative bias was applied to the gate electrode is higher with the use of the gate electrode made of tungsten than with the use of the gate electrode made of polysilicon. As can be seen from FIG.
16
(
b
), the value Q
bd
when the positive bias was applied to the gate electrode is lower with the use of the gate electrode made of tungsten than with the use of the gate electrode made of polysilicon.
Thus, in the MOS transistor having the gate electrode made of tungsten and the extremely thin gate insulating film with a thickness of about 3.5 nm, the reliability of the gate insulating film deteriorates when the positive bias is applied to the gate electrode.
To deposit the nitrogen-containing tungsten film composed of the composite of tungsten and tungsten nitride on the gate insulating film by sputtering the target made of tungsten in the nitrogen ambient and diffuse nitrogen contained in the nitrogen-containing tungsten film, as in the second conventional embodiment, a high-temperature, long-term heat treatment should be performed at a temperature of, e.g., 900° C. for about 1 minute. In the process of high-temperature heat treatment, nitrogen leaves the gate electrode and the internal stress of the gate electrode changes, while the mechanical stress is exerted on the gate insulating film adjacent the gate electrode. Therefore, the process is not satisfactory in terms of preventing the deterioration of the reliability of the gate insulating film.
In addition, the high-temperature heat treatment for diffusing nitrogen causes another problem of adversely affecting an element formed on the semiconductor substrate, such as a transistor. In particular, a MOS transistor with an extremely small design rule of 0.10 m or less may have its characteristics significantly changed by the high-temperature heat treatment.
SUMMARY OF THE INVE
Moriwaki Masaru
Yamada Takayuki
Hoang Quoc
Matsushita Electric Industrial Co., Inc.
Nelms David
Nixon & Peabody LLP
Studebaker Donald R.
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