Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2002-04-24
2004-12-14
Pham, Long (Department: 2814)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S710000, C438S714000, C438S725000
Reexamination Certificate
active
06831018
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for fabricating a semiconductor device. More particularly, it relates to a method for fabricating a semiconductor device including a step of subjecting a polymer film, which has been deposited on an insulating film through plasma etching using an etching gas including carbon and fluorine, to ashing using an oxygen gas or a gas including oxygen as a principal constituent.
In accordance with recently improved refinement of semiconductor integrated circuit devices, it has become necessary to form a contact hole with a smaller diameter. On the contrary, since the depth of a contact hole has not been largely changed, a technique to form a contact hole with a high aspect ratio (the depth of the contact hole/the diameter of the contact hole) has become necessary.
Also, since a resist film used for forming a hole pattern has been reduced in its thickness, it is significant how the value of (the depth of the contact hole)/(the thickness of the resist film to be etched) is increased, namely, how the value of resist selectivity (=(the etching rate of an insulating film used for forming the contact hole)/(the etching rate of the resist film) is increased.
For example, when the resist selectivity is not sufficiently high, most of the resist film is etched before forming the contact hole, and hence, the contact hole cannot be formed in a good shape. Specifically, the contact hole may have a trumpet-shaped upper opening or adjacent contact holes may be connected to each other because the resist film is removed.
As one method for attaining sufficiently high resist selectivity for forming a contact hole in a good shape, a rigid deposited film is formed by using, as an etching gas, a PFC (perfluorocarbon) gas with a high C/F ratio, such as a C
2
F
6
gas (with a C/F ratio of 2/6), a C
4
F
8
gas (with a C/F ratio of 4/8) or a C
5
F
8
gas (with a C/F ratio of 5/8), or by employing carbon-rich etching conditions. Thus, high resist selectivity can be attained.
Recently, however, oxide film etching process with higher resist selectivity is employed, and hence, there is a problem that a sufficient etching rate of a polymer film formed on the resist film cannot be attained by conventional ashing even if the power is increased.
Further, ashing using an oxygen gas including a fluorine gas for attaining the etching rate invites surface roughness of the wafer or shaving of the underlying substrate.
Now, a conventional method for forming a contact hole will be described with reference to
FIGS. 7A through 7C
and
8
A through
8
C.
First, as shown in
FIG. 7A
, a resist pattern
12
having a contact hole opening is formed on a silicon oxide film
11
formed on an underlying layer
10
made from an etching stopper film of a silicon nitride film or the like, a plug of polysilicon, tungsten or the like, or a lower interconnect.
Next, as shown in
FIG. 7B
, an etching gas
13
including a fluorocarbon gas as a principal constituent is introduced into an etching chamber (not shown), so as to etch the silicon oxide film
11
by using the resist pattern
12
as a mask. In this manner, a contact hole
14
is formed in the silicon oxide film
11
. Thus, a reaction product gas
15
of SiF
4
, CO
2
, H
2
O and others is generated and vaporized. At this point, a rigid polymer film
16
of (C
x
H
y
F
z
)
n
including, as a principal constituent, carbon or fluorine supplied from the plasma of the etching gas
13
is deposited on the top surface of the resist pattern
12
, the bottom and the wall of the contact hole
14
and the inside wall of the etching chamber.
Then, as shown in
FIG. 7C
, an ashing gas
17
of an oxygen gas including a fluorocarbon gas is introduced into an ashing chamber (not shown), so as to ash the polymer film
16
. Thus, oxygen activated by plasma generation power is bonded to carbon, that is, one principal constituent of the polymer film
16
so as to generate carbon dioxide, and fluorine is also vaporized. These gases are removed as a reaction product gas
18
.
At this point, a residual polymer
19
is formed on the silicon oxide film
11
as shown in
FIG. 8A. A
large amount of activated oxygen with high energy is generated by the plasma generation power and the thus generated activated oxygen with high energy reaches the surface of the silicon oxide film
11
. Therefore, fluorine included in the residual polymer
19
is concentrated and pushed into a surface portion of the silicon oxide film
11
by the activated oxygen reaching the surface of the silicon oxide film
11
. As a result, a first fluorine implant layer
21
is formed in the surface portion of the silicon oxide film
11
. Also, gas-phase fluorine included in the reaction product gas
18
is activated by the plasma generation power and reaches again the surface of the silicon oxide film
11
, and then is implanted into a surface portion of the silicon oxide film
11
. Therefore, a second fluorine implant layer
22
is formed in the surface portion of the silicon oxide film
11
.
Furthermore, at this point, fluorine included in the polymer film
16
adhered onto the resist pattern
12
or included in the polymer film adhered onto the inside wall of the chamber and fluorine included in fluorocarbon added to the ashing gas also enters to reach the bottom of the contact hole
14
. Therefore, a portion of the underlying layer
10
exposed within the contact hole
14
is etched to form a recess
23
.
Next, as shown in
FIG. 8B
, the surface of the silicon oxide film
11
and the bottom of the contact hole
14
are wet cleaned with a cleaning agent
24
, so as to remove the residual polymer
19
.
Thus, the residual polymer
19
present on the surface of the silicon oxide film
11
and on the bottom of the contact bole
14
is completely removed in the wet cleaning. However, there is a difference in the etching rate in the wet cleaning between the surface portions of the silicon oxide film
11
where the first fluorine implant layer
21
and the second fluorine implant layer
22
are formed and a surface portion thereof where none of these implant layers are formed. Accordingly, irregularities are caused in the surface portions of the silicon oxide film
11
, which results in surface roughness
25
as shown in FIG.
8
C.
Furthermore, in removing the residual polymer
19
present on the surface of the silicon oxide film
11
and on the bottom of the contact hole
14
by the ashing, if large plasma generation power is applied for the ashing in order to attain a sufficient ashing rate or to definitely remove the residual polymer
19
, fluorine included in the residual polymer
19
or fluorine included in the polymer film deposited on the inside wall of the chamber strikes the surface of the silicon oxide film
11
. As a result, the surface roughness
25
caused in the wet cleaning is further increased.
Moreover, when a large amount of fluorine enters to reach the bottom of the contact hole
14
during the ashing as described above so as to form the recess
23
in the underlying layer
10
exposed within the contact hole
14
(as shown in FIG.
8
A), there arises a problem of increase of contact resistance if the underlying layer
10
is an impurity diffusion layer. Alternatively, if the underlying layer
10
is an etching stopper film, a metal interconnect formed below the etching stopper film is exposed, and hence, the metal interconnect is oxidized by oxygen plasma or absorbs moisture. As a result, there arises a problem of degradation in the device characteristic.
In addition, the fluorine included in the polymer film
16
(shown in
FIG. 7C
) deposited on the surface of the silicon oxide film
11
and the fluorine generated from the fluorocarbon gas added to the ashing gas is activated by the plasma during the ashing, so as to damage parts of the chamber. As a result, there arises a problem of a short life of the parts.
Furthermore, the fluorine having struck the surface of the silicon oxide film
11
during the ashing may not be completely removed but remai
Matsushita Electric - Industrial Co., Ltd.
McDermott Will & Emery LLP
Peralta Ginette
Pham Long
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