Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
1999-04-14
2001-03-20
Utech, Benjamin L. (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S724000, C438S738000
Reexamination Certificate
active
06204193
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application No. P10-113566 filed Apr. 23, 1998 which application is incorporated herein by reference to the extent permitted by law.
FIELD OF THE INVENTION
The present invention relates to a method for etching applied to a fine processing represented by a semiconductor and an electronic device part.
BACKGROUND OF THE INVENTION
In a semiconductor device used in a VLSI of recent years, a severe demand in fine processing rises according to development of its high integration and high performance. Taking the structure of a DRAM as an example, the width of wiring is reduced with the distance of wiring being reduced, and the hole diameter of a contact hole also becomes small. As a result, the distance between the wiring and the contact hole becomes small, and there arises a fear of electric short circuit. In order to prevent the same, a layer of silicon nitride is inserted in addition to an interlayer insulating film formed with an oxide film.
FIG. 1
is a schematic cross sectional view of a DRAM of a COB structure for describing the problems of the conventional dry etching method.
A bit line
106
is formed on a silicon substrate
107
, and an oxide film
103
is formed on the bit line
106
. A silicon nitride film
104
is formed on the oxide film
103
, and an oxide film
103
is formed on the silicon nitride film
104
. A word line
105
is formed inside the oxide film
103
. A silicon nitride film
104
is formed on the oxide film
103
, and an oxide film
103
is formed on the silicon nitride film
104
. A silicon nitride film
102
is formed on the oxide film
103
, and a capacitor part
101
is formed on the silicon nitride film
102
. A contact hole
108
is opened from the capacitor part
101
to a transistor at the lower part of the figure.
In order to produce the contact hole
108
, a laminated film composed of the oxide film
103
and the silicon nitride films
102
and
104
should be etched as shown in FIG.
1
.
As an etching gas that can etch both the oxide film and the silicon nitride film, a CHF
3
series gas can be exemplified. As an example of fine processing technique in recent years, processing of a contact hole using a polymask instead of a resist mask is being employed. Submicron processing, which has not been accomplished by the resist mask, can be realized by using the polymask.
However, when a contact hole is produced with the polymask by using the CHF
3
series gas singly, there arises a phenomenon in that the selective ratio of the mask and the oxide film to shift the mask.
Furthermore, the conventional etching method involves the following problems.
FIGS. 2
to
4
are schematic cross sectional views showing a part of a production process of a semiconductor device using the conventional etching method, and also describing the problems associated with the conventional etching method.
As shown in
FIG. 2
, a silicon nitride film
204
is formed on a silicon substrate
205
, and an oxide film
203
is formed on the silicon nitride film
204
. An etching mask (poly-Si)
201
is formed on the oxide film
203
. When a contact hole
202
is formed in the oxide film
203
and the silicon nitride film
204
by etching with the etching mask
201
as a mask by using a CHF
3
series gas singly, the shape of the contact hole becomes a bowing shape.
Thereafter, a hole filler
207
, such as poly-Si, is accumulated on the poly-Si (etching mask)
201
to bury the contact hole
202
as shown in
FIG. 3. A
hollow space
206
is formed inside the contact hole
202
since the contact hole
202
has the bowing shape.
The hole filler
207
is then subjected to etch back. The hollow part
206
is etched at a faster rate than the other part as shown in
FIG. 4
, and there arises a problem in that the silicon substrate
205
at the bottom of the contact hole
202
is etched, which is not planned to be etched.
As a method for preventing such a problem, a method is considered in that after etching the oxide film
203
with a C
4
F
8
series gas, the silicon nitride film
204
is etched with a CHF
3
series gas. The oxide film is easily etched with the C
4
F
8
series gas, but the silicon nitride film is not easily etched by that gas. In order to practice such a method, after etching the oxide film
203
with the C
4
F
8
series gas, a fluorocarbon series reaction product deposited inside the contact hole must be removed with an O
2
plasma (ashing), and then further cleaned with sulfuric acid and aqueous hydrogen peroxide, followed by etching the silicon nitride film
204
by using the CHF
3
series gas. In the case where the multi-layer film comprising plural oxide films and silicon nitride films is produced as shown in
FIG. 1
, such a method requires the removing step of the reaction product and cleaning step for each films, to increase the cost.
In order to suppress the cost, on the other hand, a method is considered in that the removing step of the reaction product and the cleaning step are omitted, and after etching the oxide film
203
with a C
4
F
8
series gas, the etching gas is switched from the C
4
F
8
series gas to a CHF
3
gas, to continuously etch the silicon nitride film
204
. However, as shown in
FIG. 5
, the etching rate (etching amount) of the silicon nitride film under the oxide film is decreased in proportion to the over-etching amount of the oxide film with the C
4
F
8
series gas, and when the over-etching amount reaches a specific value, an etching stop phenomenon occurs. Therefore, the removing step of the reaction product and the cleaning step cannot be omitted.
FIGS. 6A
,
6
B, and
6
C are schematic cross sectional view in
FIG. 5
showing the phenomenon in that etching stop occurs when the over-etching amount of the oxide film is increased.
As shown in
FIG. 6A
, a silicon nitride film
304
is formed on a silicon substrate
305
, and an oxide film
303
is formed on the silicon nitride film
304
. An etching mask
301
is formed on the oxide film
303
. The oxide film
303
is then etched with the etching mask
301
as a mask by using a C
4
F
8
series gas
306
to immediately before exposing the surface of the silicon nitride film
304
. In this case, no reaction product is formed inside a contact hole
302
.
As shown in
FIG. 6B
, a silicon nitride film
304
is formed on a silicon substrate
305
, and an oxide film
303
is formed on the silicon nitride film
304
. An etching mask
301
is formed on the oxide film
303
. The oxide film
303
is then etched with the etching mask
301
as a mask by using a C
4
F
8
series gas
306
to immediately before exposing the surface of the silicon nitride film
304
. In this case, a fluorocarbon series reaction product
307
is formed inside a contact hole
302
.
As shown in
FIG. 6C
, a silicon nitride film
304
is formed on a silicon substrate
305
, and an oxide film
303
is formed on the silicon nitride film
304
. An etching mask
301
is formed on the oxide film
303
. The oxide film
303
is then over-etched with the etching mask
301
as a mask by using a C
4
F
8
series gas
306
. In this case, the amount of a fluorocarbon series reaction product
307
formed inside a contact hole
302
is larger than the case of FIG.
6
B.
It is understood from these figures that when the oxide film
303
is etched with a C
4
F
8
series gas, the fluorocarbon series reaction product
307
starts to be accumulated inside the contact hole
302
on exposing the silicon nitride film
304
as an underlayer. The amount of the reaction product depends on the over-etching amount of the oxide film
303
with a C
4
F
8
series gas as expected from FIG.
5
. When the over-etching time is further prolonged, the etching effect of the ion is cancelled by the reaction product accumulated inside the contact hole, and the etching is stopped. Therefore, after etching the oxide film
303
, the fluorocarbon series reaction product
307
is evaporated by ashing with oxygen in the form of COF as a reaction product of O
2
and CF, and then the silicon nitri
Chen Kin-Chan
Sonneschein Nath & Rosenthal
Sony Corporation
Utech Benjamin L.
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