Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
1999-01-29
2002-02-19
Fourson, George (Department: 2823)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S669000, C438S705000, C438S734000, C438S974000
Reexamination Certificate
active
06348418
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photoresist pattern forming method, and more particularly to a method of forming a photoresist pattern on a silicide film (of an interconnect metal compound of silicon and a metal element).
2. Description of the Related Art
In the manufacture of transistors and other semiconductor devices, it has been customary to use tungsten silicide (WSi) as conductive material for gate electrodes and wirings. The conventional method for formation of this tungsten silicide film is exemplified by chemical vapor deposition (CVD) and sputtering; with recent progress in miniturization of semiconductor integrated circuits, chemical vapor deposition is increasingly becoming more popular than sputtering as a tungsten silicide film whose resistivity is lower by approximately 20%-40% can be obtained.
Conventionally, in the photolithographic process for the tungsten silicide film formed by CVD, using grays (436 nm in wavelength) and i rays (365 nm in wavelength) of a high-voltage mercury lamp as a light source, exposure takes place over a novolak-based photoresist coated and solidified on the tungsten silicide film to form a photoresist pattern. Recently, however, the design requirement for semiconductor integrated circuits has come down below 35 nm. In an effort to obtain a preciser photoresist pattern to meet such design criterion, exposure using a krypton fluoride (KrF) excimer laser as a light source (248 nm in wavelength) is realized. In this exposure using KrF excimer laser, it has become practical to use a chemical amplification photoresist instead of the novolak-based photoresist, which is too large in light absorption to obtain a good resist pattern.
A chemical amplification photoresist is a positive photoresist to which an acid catalyst reaction is applied. The positive photoresist is generally composed of a base resin, such as polyhydroxystyrene, which is to be insoluble with an alkali developing liquid when the protection radical is in a bonded state at a predetermined portion and to be soluble with the alkali developing liquid when the radical is in a free state, a photoionizer, which emits hydrogen ions upon exposure to light, a small mass of additive for controlling the performance, and an organic solvent for spinner-coating.
When this chemical amplification photoresist coated on the tungsten silicide film and dried to become solid is exposed to irradiation of far ultraviolet rays as of a KrF excimer layer light source, hydrogen ions are generated from the photoionizer to start chemical amplification. When the hydrogen ions are bonded with the base resin in place of the protection radical of the base resin during post exposure baking (PEB), the base resin will be soluble with the alkali developing liquid. In the meantime, the freed protection radical will also reacts with water to generate hydrogen ions again so that the above-mentioned dissolving with the alkali developing liquid will be expedited in chain reaction.
Therefore, if the positive chemical amplification photoresist is developed by the alkali developing liquid, a desired photoresist pattern can be obtained even with insufficient exposure.
The manner in which the chemical amplification photoresist is patterned according to the conventional phtolithographic process will now be described in further detail with reference to FIGS.
3
(
a
) through
3
(
c
) of the accompanying drawings.
Firstly, a silicon oxide film
32
is formed on the surface of a silicon substrate (wafer)
31
as by thermal oxidation. Then a polysilicon film
33
is grown on the silicon oxide film
32
as by CVD. Subsequently, a tungsten silicide (WSi) film
34
is formed by CVD using dichlorosilane (SiH
2
Cl
2
) and tungsten hexafluoride (WF
6
) as raw material (FIG.
3
(
a
)). However, since this tungsten silicide film
34
is very large in internal stress, it is necessary to terminate supplying tungsten hexafluoride, namely, to supply only dichlorosilane at the final stage of such film forming process to intend a reduced stress.
Then, in order to increase adhesion of the photoresist, the semiconductor substrate is exposed to hexamethyldisilazane (HMDS) atmosphere to make the substrate surface hydrophobic. Subsequently, an Si-rich tungsten silicide film
35
is coated with a chemical amplification photoresist
37
(FIG.
3
(
b
)). Then, pattern exposure takes place using KrF excimer laser, for example, as a light source, and finally, only unnecessary portions of the photoresist are removed to form a photoresist pattern
38
(FIG.
3
(
c
)).
However, this conventional photoresist pattern forming method has the following problems:
As is mentioned above, in order to relax internal stresses of the tungsten silicide film
34
, at the final stage of formation of the film, supply of tungsten hexafluoride is terminated, namely, only dichlorosilane is continued to be supplied. Even during this final stage, the tungsten silicide film
34
is slightly formed as an Si-rich tungsten silicide film
35
containing silicon excessively (FIG.
3
(
a
)).
This Si-rich tungsten silicide film
35
contains also a high-concentration of chlorine originating from dichlorosilane. Therefore, as is mentioned above, during the surface treatment of the Si-rich tungsten silicide film
35
with hexamethyldisilazane (HMDS) prior to the coating of the chemical amplification resist
37
, a chemical reaction occurs between ammonia generated from hexamethyldisilazane and chlorine coming from the uppermost layer of the Si-rich tungsten silicide film
35
to form a solid
36
of ammonium chloride (NH
4
Cl) as shown in FIG.
4
(
a
).
If the chemical amplification resist
37
is coated to the resulting surface of the Si-rich tungsten silicide film
35
and solidified there (FIG.
4
(
b
)) and then exposure by far ultraviolet rays from a light source as of KrF excimer laser takes place, hydrogen ions serving as a seed to start chemical amplification are generated from the photoionizer in the chemical amplification photoresist and are captured by the solid
36
of ammonium chloride so that chain reaction of release of the protection radical will be obstructed. As a result, the chemical amplification photoresist around the solid
36
of ammonium chloride will be free from so decomposing as to be soluble with alkali. Therefore, during the developing, the photoresist to be removed remains unremoved as pattern defects
39
(FIG.
4
(
c
)); even if the tungsten silicide film
34
is etched in the presence of these pattern defects
39
, precise gate electrodes and wirings as designed cannot be achieved, which might result in products defective yet in electrical property.
SUMMARY OF THE INVENTION
With the foregoing prior art problems in view, it is an object of the present invention to provide a method of forming a photoresist pattern without causing pattern defects while a metal silicide film formed on a substrate, especially a tungsten silicide film formed by CVD is patterned.
According to a first aspect of the present invention, the above object is attained by a method of forming a photoresist pattern, comprising the steps of: forming a metal silicide film on a substrate; etching a surface layer of the metal silicide; coating the metal silicide film with a photoresist so as to form a photoresist film on the etched surface layer of the metal silicide film; and patterning the photoresist film by photolithography.
Preferably, the etching of the surface layer of the metal silicide film is carried out with a liquid containing hydrogen peroxide in a predetermined concentration before the coating of the photoresist. The substrate is preferably a silicon substrate.
According to a second aspect of the invention, the method further includes the step of making the surface layer of the metal silicide film hydrophobic before the coating of the photoresist and subsequent to the etching of the surface layer of the metal silicide film.
Preferably, the step of making the surface layer of the metal silicide film hydrophobic is carri
Arata Hiromi
Inoue Shuichi
Okamura Kenji
Estrada Michelle
Fourson George
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