Semiconductor device manufacturing: process – Formation of electrically isolated lateral semiconductive... – Grooved and refilled with deposited dielectric material
Reexamination Certificate
2000-05-15
2002-04-16
Smith, Matthew (Department: 2825)
Semiconductor device manufacturing: process
Formation of electrically isolated lateral semiconductive...
Grooved and refilled with deposited dielectric material
C438S296000, C438S426000, C438S431000, C438S437000, C438S756000, C438S425000
Reexamination Certificate
active
06372602
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of forming a semiconductor device, and more particularly to a method of forming a shallow trench isolation with an isolation oxide film free of any divot and any void as well as a method of preventing formation of any divot and any void in the isolation oxide film of the shallow trench isolation.
The shallow trench isolation structure is used as an isolation structure in an advanced ultra large scale integrated circuit device for further improvements in scaling down and isolation property. This shallow trench isolation structure is superior in isolation characteristics as compared to a local oxidation of silicon structure. This shallow trench isolation structure is also smaller in occupied area than the local oxidation of silicon structure. Accordingly, the shallow trench isolation structure is more suitable for further increase in high integration of the integrated circuit. The number of the semiconductor device having the shallow trench isolation structure has been on the increase.
The conventional method of forming the shallow trench isolation structure will be described.
FIGS. 1A through 1E
are fragmentary cross sectional elevation views illustrative of semiconductor devices with shallow trench isolation structures in sequential steps involved in the conventional method of forming the same.
With reference to
FIG. 1A
, a thermal oxide film
4
as a pad oxide film is formed on a silicon substrate
2
. A silicon nitride film
6
as a chemical mechanical polishing stopper is deposited on the thermal oxide film
4
. A resist film is applied on the silicon nitride film
6
. The resist film is then patterned by a lithograph technique to form a resist pattern
8
.
With reference to
FIG. 1B
, the resist pattern
8
is used to carry out an isotropic etching to the silicon nitride film
6
, the thermal oxide film
4
and the silicon substrate
2
, whereby a trench is formed which penetrates the silicon nitride film
6
and the thermal oxide film
4
and reaches a predetermined depth from the surface of the silicon substrate
2
.
With reference to
FIG. 1C
, the resist pattern
8
is removed. A plasma oxide film
10
is entirely deposited by a plasma enhanced chemical vapor deposition method over the silicon nitride film
6
and within the trench, whereby the trench is filled with the plasma oxide film
10
. The plasma oxide film
10
is polished by use of the silicon nitride film
6
as a polishing stopper, whereby the plasma oxide film
10
remains only within the trench.
With reference to
FIG. 1D
, the silicon nitride film
6
is removed by a wet etching.
With reference to
FIG. 1E
, the pad oxide film
4
is further removed by a wet etching, whereby the isolation oxide film
10
projects upwardly from the surface of the silicon substrate
2
.
The shallow trench isolation is superior in device characteristic but causes a problem in the fabrication processes. Divots are formed in the isolation oxide film.
FIG. 2
is a fragmentary cross sectional elevation view illustrative of an isolation oxide film with divots. Namely, the silicon nitride film
6
as the chemical mechanical polishing stopper is removed, whereby the isolation oxide film
10
projects upwardly from the surface of the pad oxide film
4
. This pad oxide film
4
is then removed by a wet etching. This wet etching is an isotropic etching. The pad oxide film
4
and the isolation oxide film
10
are also silicon oxide films. The wet etching is effective to the silicon oxide films, or not only the pad oxide film
4
but also the isolation oxide film
10
. Namely, as the pad oxide film
4
is removed by he wet etching, the isolation oxide film
10
is also isotropically etched by the wet etching in isotropic directions as shown in arrow marks “a”, “b”, and “c” thereby forming divots
22
in FIG.
2
. After the removal of the pad oxide film
4
, further wet etching processes for removing the silicon oxide films are carried out, whereby the divots are further enlarged.
A mechanism of forming the divots
22
will be described in detail. As described above, the wet etching is carried out to the pad oxide film
4
. Since the wet etching is an isotropic etching. The etching direction is isotropic and includes the three directions “a”, “b” and “c”. The direction “a” is a downward direction to etch the isolation oxide film
10
. The direction “b” is a lateral direction to etch the isolation oxide film
10
. The isolation oxide film
10
is etched in the two directions “a” and “b” before the pad oxide film
4
is completely etched. After the pad oxide film
4
has been completely etched, the isolation oxide film
10
is etched not only in the directions “a” and “b” but also in the direction “c”. The etching in the direction “c” forms the divots
22
. Since the pad oxide film
4
is very thin, for example, has a thickness in the range of 5-30 nanometers, the pad oxide film
4
is removed in a very short time, whereby the surface of the silicon substrate
2
is shown and the etching to the isolation oxide film
10
in the direction “c” is started to form the divots
22
.
After the wet etching process has been carried out and the divots
22
have been formed, a photo-lithography process is carried out. The divots
22
form a difference in level or steps which provides an influence of halation to the photo-lithography process. A dry etching is carried out to a film over the divots
22
by use of the resist mask prepared by the photo-lithography process, provided that the film has a variation in thickness due to the divots
22
. This makes it difficult to ensure an accurate selectively and control to the shape.
The above conventional method causes a further problem as follows. As described above, the trench groove is filled by the CVD oxide film deposited by the plasma enhanced chemical vapor deposition method to form the isolation oxide fi within the trench.
FIG. 3
is a fragmentary cross sectional elevation view illustrative of a trench isolation with a void formed in a trench in a semiconductor substrate in accordance with the conventional method. The isolation oxide film within the trench may have a void or a cavity
20
. If the trench has a high aspect ratio of a depth to a width of the trench, a lateral direction deposition rate could not be ignored in relation to a vertical direction deposition rate, whereby the vertical direction deposition of the isolation oxide film by the plasma enhance chemical vapor deposition method is limited by the lateral direction deposition thereof. Namely, the deposition in the lateral direction could not be ignored, whereby the deposition rate in the shallow portion of the trench is made faster than the deposition rate in the deep portion of the trench. This makes it possible that the trench is made blockage in the shallow portion thereof, whereby the silicon oxide is no longer supplied to the deeper portion of the trench, whereby no longer deposition of the silicon oxide appears in the deeper portion of the trench. As a result, the void or cavity
20
is formed in the isolation oxide film in the trench. If the void or cavity
20
is formed in the isolation oxide film within the trench, a deep groove or depressed portion is formed in the isolation oxide film during the chemical and mechanical polishing method. This deep groove or depressed portion causes the same problem as described above. Namely, the deep groove or depressed portion form a difference in level or steps which provides an influence of halation to the photo-lithography process. A dry etching is carried out to a film over the deep groove or depressed portion by use of the resist mask prepared by the photo-lithography process, provided that the film has a variation in thickness due to the deep groove or depressed portion. This makes it difficult to ensure an accurate selectively and control to the shape.
In the above circumstances, it had been required to develop a novel method of forming a shallow trench isolation free from the above problem.
SUMMARY OF THE IN
Anya Igwe W.
NEC Corporation
Smith Matthew
Young & Thompson
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