Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
2000-03-15
2003-12-16
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
C438S745000
Reexamination Certificate
active
06664196
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of cleaning an electronic device having a component containing a refractory metal such as tungsten (W) and to a method of fabricating the same. More particularly, it relates to a method of cleaning an electronic device having a polymetal gate electrode structure (such as W/WN
x
/Poly-Si) or a metal gate electrode structure (such as W/WN
x
) for reducing the resistance of the gate electrode of a MOS transistor and to a method of fabricating the same.
In recent years, increasing miniaturization, higher integration density, higher-speed operation, and lower power consumption has been required of a VLSI device.
In the process of fabricating a VLSI device, the operating speed of the VLSI device is further increased by reducing the resistance of the gate electrode of a MOS transistor. To reduce the resistance of the gate electrode of the MOS transistor, there has been developed a polymetal gate electrode or metal gate electrode using a metal not containing silicon, specifically a refractory metal, as a replacement of a conventional polysilicon gate electrode or silicide gate electrode.
As an exemplary structure of the polymetal gate electrode, a structure in which W/WN
x
/Poly-Si is deposited on a gate oxide film or a structure in which W/TiN/Poly-Si is deposited on a gate oxide film has been used.
As an exemplary structure of the metal gate electrode, a structure in which W/WN
x
is deposited on a gate oxide film or a structure in which W/TiN is deposited on a gate oxide film has been used.
As the refractory metal composing the polymetal gate electrode or metal gate electrode, tungsten has been used predominantly (Monthly Semiconductor World, pp.76-81, 1998, 9).
In cleaning an electronic device having a conventional polysilicon gate electrode or silicide gate electrode, a cleaning solution which is a mixture of an acidic or alkaline solution and aqueous hydrogen peroxide (H
2
O
2
) has been used predominantly to remove a particle (having a diameter of about 10 &mgr;m or less), resist residue, a polymer, or a metal. Such a method of cleaning an electronic device is generally known as an RCA cleaning method.
However, if an electronic device having a polymetal gate electrode, metal gate electrode, or the like containing a refractory metal such as tungsten is cleaned by such a conventional cleaning method as the RCA cleaning method, tungsten or the like composing the gate electrode is dissolved disadvantageous, so that the reliability of the electronic device is reduced.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to ensure cleaning of an electronic device, while preventing the dissolution of a refractory metal such as tungsten contained in a component of the electronic device.
The present inventors have examined the cause of the dissolution of tungsten contained in the component of the electronic device when a conventional cleaning method is used.
Referring to FIGS.
4
(
a
) to
4
(
c
), a description will be given to the case of forming a polymetal gate electrode containing tungsten on a substrate and then cleaning the substrate in accordance with the conventional cleaning method. In the present specification, a substrate formed with a component such as a gate electrode will be termed an electronic device.
First, as shown in FIG.
4
(
a
), a gate oxide film
2
is formed by thermal oxidation on a silicon substrate
1
. Then, a polysilicon (Poly-Si) film
3
A, a tungsten nitride (WN
x
) film
3
B, and a tungsten film
3
C as the materials of a polymetal gate electrode are deposited successively on the gate oxide film
2
, followed by a silicon nitride film
4
deposited on the tungsten film
3
C. Subsequently, dry etching is performed successively with respect to the silicon nitride film
4
, the tungsten film
3
C, the tungsten nitride film
3
B, and the polysilicon film
3
A by using a resist pattern (not shown) covering a region in which a gate electrode is to be formed as a mask, thereby forming a polymetal gate electrode
3
composed of the polysilicon film
3
A, the tungsten nitride film
3
B, and the tungsten film
3
C on the silicon substrate
1
with the gate oxide film
2
interposed therebetween. Thereafter, the resist pattern is removed by ashing.
Next, the silicon substrate
1
is cleaned by sequentially using a first cleaning solution composed of a SPM solution (a solution mixture of sulfuric acid (H
2
SO
4
) and aqueous hydrogen peroxide) and a second cleaning solution composed of an APM solution (a solution mixture of aqueous ammonium (NH
4
OH), aqueous hydrogen peroxide, and water).
During the cleaning process, as shown in FIG.
4
(
b
), the tungsten nitride film
3
B and the tungsten film
3
C, each composing the polymetal gate electrode
3
, are etched in a direction parallel to the silicon substrate
1
.
The present inventors tried cleaning the silicon substrate
1
by increasing the respective concentrations of aqueous hydrogen peroxide contained in the SPM solution and in the APM solution.
As a result, as shown in FIG.
4
(
c
), the tungsten nitride film
3
B and the tungsten film
3
C, each composing the polymetal gate electrode
3
, were entirely dissolved to disappear.
This proves that aqueous hydrogen peroxide contained in the conventional cleaning solution is the cause of the dissolution of tungsten contained in the component of the electronic device.
The present inventors also assumed the formula representing the chemical reaction between tungsten and aqueous hydrogen peroxide to be
W+2H
2
O
2
→WO
2
+2H
2
O (Oxidation of Tungsten)
2WO
2
+6H
2
O
2
→H
2
W
2
O
11
+5H
2
O (Dissolution of Tungsten)
3H
2
W
2
O
11
+7H
2
O →2H
2
W
3
O
16
30 8H
2
O (Dissolution of Tungsten).
It may be considered that, if the electronic device having the component containing tungsten is cleaned with the cleaning solution containing aqueous hydrogen peroxide, the chemical reaction proceeds catalytically between tungsten and aqueous hydrogen peroxide in accordance with the foregoing chemical reaction formula, so that tungsten contained in the component of the electronic device is dissolved.
The present invention has been achieved based on the foregoing findings. Specifically, a first method of cleaning an electronic device according to the present invention comprises the step of: cleaning an electronic device having a component containing tungsten by using a cleaning solution composed of an acidic solution which does not substantially contain aqueous hydrogen peroxide or an alkaline solution which does not substantially contain aqueous hydrogen peroxide.
In accordance with the first cleaning method, the electronic device is cleaned by using the cleaning solution composed of the acidic solution which does not substantially contain aqueous hydrogen peroxide or the alkaline solution which does not substantially contain aqueous hydrogen peroxide. Compared with the case where a cleaning solution containing aqueous hydrogen peroxide is used, the etching rate for tungsten can be reduced significantly and particles, resist residue, a polymer, or the like can be removed thoroughly. This ensures cleaning of the electronic device, while preventing the dissolution of tungsten contained in a component of the electronic device.
In the present specification, the cleaning solution which does not substantially contain aqueous hydrogen peroxide is defined as a cleaning solution which contains aqueous hydrogen peroxide (at a concentration of 30wt %) at a volume ratio of less than about 0.2%.
In the first cleaning method, the cleaning solution is preferably a solution mixture of tetramethyl ammonium hydroxide and water.
In the arrangement, the etching rate for tungsten can be reduced significantly compared with the case where a cleaning solution containing aqueous hydrogen peroxide is used and particles, resist residue, a polymer, or the like can be removed thoroughly.
In this case, if a concentration of tetramethyl ammonium hydroxide in the s
Matsumoto Michikazu
Wada Yukihisa
Lattin Christopher
Matsushita Electric - Industrial Co., Ltd.
Niebling John F.
Nixon & Peabody LLP
Studebaker Donald R.
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