Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
2001-11-06
2002-11-19
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S216000, C438S287000, C438S275000, C438S585000, C438S785000
Reexamination Certificate
active
06482690
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device and a method for fabricating the same.
In recent years, semiconductor devices have been drastically downsized and increasingly required to perform an even wider variety of functions concurrently. Considering such a state of the art, it has become more and more necessary to change the thickness of a gate insulating film, which is provided for multiple MOS transistors on the same semiconductor substrate, from place to place. Specifically, to make all of these transistors operate reliably, the gate insulating film should be partially thickened for some transistors operating at relatively high voltages, but be partially thinned for other transistors operating at relatively low voltages. Another situation where the thickness of the gate insulating film has to be changed arises when several kinds of threshold voltages need to be prepared for these transistors.
In addition, as semiconductor devices have been miniaturized, the necessity of reducing the thickness of a gate insulating film has also been increasing day after day. For that purpose, according to a proposed technique, a thinner gate insulating film was provided by making the insulating film of tantalum oxide, which has a relative dielectric constant higher than that of a silicon dioxide film used conventionally. Specifically, the relative dielectric constant of the tantalum oxide film is about 25, whereas that of the conventional silicon dioxide film is about 3.9. Accordingly, the dielectric strength of the tantalum oxide film is about 6 times as high as that of the silicon dioxide film.
If a voltage as high as 1.5 V or more is applied to the gate electrode, however, then the tantalum oxide film is hard to use as the gate insulating film anymore in view of the leakage current characteristic thereof. Thus, in such a situation, two types of transistors, i.e., transistors including a tantalum oxide film as gate insulating film and transistors including a silicon dioxide film as gate insulating film, have to be formed on the same semiconductor substrate.
Hereinafter, a method for fabricating a semiconductor device, which includes a transistor with a silicon dioxide gate insulating film and a transistor with a tantalum oxide gate insulating film on the same semiconductor substrate, will be described as first prior art example with reference to FIGS.
11
(
a
) through
11
(
e
). In FIGS.
11
(
a
) through
11
(
e
), the tantalum oxide gate insulating film will be formed in a first region on the left-hand side, and the silicon dioxide gate insulating film will be formed in a second region on the right-hand side.
First, as shown in FIG.
11
(
a
), isolation regions
11
are defined on the surface of a semiconductor substrate
10
of silicon, and then a tantalum oxide film
12
is deposited to a thickness of 10 nm, for example, in the first and second regions of the semiconductor substrate
10
.
Next, as shown in FIG.
11
(
b
), a mask
13
is made out of a silicon dioxide or resist film over the first region of the semiconductor substrate
10
, and the tantalum oxide film
12
in the first region is etched away using the mask
13
.
Then, as shown in FIG.
11
(
c
), a silicon dioxide film
14
is formed to a thickness of 5 nm, for example, in the second region of the semiconductor substrate
10
.
Subsequently, a conductor film is deposited over the tantalum oxide and silicon dioxide films
12
and
14
and then patterned into gate electrode shapes. In this manner, first and second gate electrodes
15
and
16
are formed on the tantalum oxide and silicon dioxide films
12
and
14
, respectively, as shown in FIG.
11
(
d
). Thereafter, the tantalum oxide and silicon dioxide films
12
and
14
are etched using the first and second gate electrodes
15
and
16
as respective masks, thereby forming first and second gate insulating films
17
and
18
.
If the tantalum oxide film
12
is used as the gate insulating film, however, then a silicon dioxide film with a relative dielectric constant lower than that of the tantalum oxide film
12
is formed in the interface between the tantalum oxide film
12
and the semiconductor substrate
10
of silicon during a subsequent heat treatment process. As a result, the total relative dielectric constant of the gate insulating film adversely decreases.
An alternative method for fabricating a semiconductor device according to a second prior art example, which was proposed to avoid this problem, will be described with reference to FIG.
12
. As shown in
FIG. 12
, before the tantalum oxide film
12
is deposited, the surface of the underlying semiconductor substrate
10
is nitrided at 900° C. for about 60 seconds within a rapid thermal annealing furnace, thereby forming a nitrogen-containing silicon layer
19
on the surface of the semiconductor substrate
10
.
Also, just after the tantalum oxide film
12
has been deposited, the tantalum oxide film
12
is amorphous and contains a lot of carbon. For these and other reasons, the tantalum oxide film
12
as deposited should be subjected to oxidation or crystallization heat treatment. Furthermore, when the tantalum oxide film
12
is used as gate insulating film, the gate electrode is made of a metal such as Ti, W or TiSi
x
. Thus, it is effective to deposit a TiN, WN or TaN film as barrier metal layer in the interface between the gate insulating film and the gate electrode.
The material of the silicon dioxide film has also been modified recently to cope with the demand for further reduction in thickness of the gate insulating film. Specifically, a silicon oxynitride film, which is obtained by introducing nitrogen into a silicon dioxide film, has been used more and more often instead of the silicon dioxide film essentially consisting of silicon and oxygen only. Examples of known methods for forming the silicon oxynitride film include: (1) forming a silicon dioxide film and then heat-treating it within an ammonium or nitrogen oxide ambient; (2) forming a silicon nitride film and then heat-treating it within an oxygen ambient; (3) conducting a heat treatment within an ambient in which nitrogen or ammonium and oxygen are mixed; and (4) directly heat-treating the surface of a silicon substrate, on which no film has been deposited yet, within a nitrogen oxide ambient.
In a storage capacitor portion of a semiconductor storage device such as DRAM, an ONO film, which is a stack of silicon dioxide and nitride films, has been used.
However, to cope with the downsizing of capacitors, a tantalum oxide film has recently attracted a lot of attention as a new material for a capacitive insulating film. If the tantalum oxide film is used as an insulating film for a storage capacitor portion, the tantalum oxide film has to be subjected to a nitriding or oxidation/crystallization process before or after the deposition as in the gate insulating film.
Also, a semiconductor storage device has been more and more required lately to process several information items at a time by using a plurality of regions with different quantities of charge.
If an MOS transistor with the silicon dioxide gate insulating film and another MOS transistor with the tantalum oxide gate insulating film are formed on the same semiconductor substrate in the above-described manner, then the number of masking process steps increases. In addition, since the heat treatment should be conducted separately to form the tantalum oxide and silicon dioxide films on the semiconductor substrate, the dopant introduced into the semiconductor substrate might diffuse excessively to affect the basic electrical characteristics of the transistors. As a result, it is difficult to ensure desired performance because MOS transistors of a very small size cannot be formed in such a case.
This problem happens frequently when multiple MOS transistors with different types of gate insulating films are formed on the same semiconductor substrate. e.g., when an MOS transistor with a gate insulating film of silicon dioxide or tantalum oxide and an M
Nixon & Peabody LLP
Pompey Ron
Studebaker Donald R.
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