Method for fabricating LOCOS isolation

Details Method for fabricating LOCOS isolation Method for fabricating LOCOS isolation

1999-01-05

2001-05-01

Nelms, David (Department: 2818)

Semiconductor device manufacturing: process

Formation of electrically isolated lateral semiconductive...

C438S439000

Reexamination Certificate

active

06225186

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for fabricating an isolation for a semiconductor device, and more particularly to a method for fabricating a local oxidation of silicon (LOCOS) isolation.
2. Description of the prior art
To improve the bird's beak (encroachment effect) at the edge of a LOCOS isolation, a polysilicon spacer serving as a buffer layer is widely used in the LOCOS isolation process.
Referring to
FIGS. 1A through 1E
, the cross-sectional side views of a conventional method for fabricating a LOCOS isolation are depicted in sequence.
Referring now to
FIG. 1A
, a cross-sectional view of the starting step is schematically shown. In
FIG. 1A
, a first pad oxide layer
12
and a silicon nitride layer
14
are sequentially formed on the active region of a silicon substrate
10
. Then, using silicon nitride layer
14
as an etching mask, the silicon substrate
10
is etched to form a recess
15
by anisotropic etching.
Next, as shown in
FIG. 1B
, a second pad oxide
16
is formed on the surface of the recess
15
by thermal oxidation, which preferably uses a gas containing O
2
as the reactive gas.
Now as shown in
FIG. 1C
, a polysilicon spacer
18
, which extends the recess
15
, is formed on the side walls of the silicon nitride layer
14
. The polysilicon spacer
18
can be formed by the steps of depositing, and etching back a polysilicon layer. A thin silicon nitride layer
20
is deposited on the surface of the polysilicon spacer
18
by selective chemical vapor deposition.
Referring now to
FIGS. 1C and 1D
, a local oxide
22
(isolation), which has a deeper concave portion
24
, is grown above the recess
15
by thermal oxidation.
Next, referring to
FIG. 1E
, the silicon nitride layer
14
is removed via wet etching to leave local oxide
22
a.
However, the LOCOS isolation process suffers from problems, for example the deeper concave portion
24
can cause an uneven surface. Moreover, the thin silicon nitride layer
20
formed on the surface of the polysilicon spacer
18
limits the oxidation of polysilicon spacer
18
. Therefore, a portion of polysilicon spacer
18
does not react into local oxide
22
during the thermal oxidation step. Also, the conventional method described above may cause the encroachment to be too large.
SUMMARY OF THE INVENTION
In view of the above disadvantages, an object of the invention is to provide a method for fabricating a LOCOS isolation by changing the thin silicon nitride layer and the polysilicon layer steps.
The above object is attained by providing a method for fabricating a LOCOS isolation, which comprises the steps of (a)forming a masking layer on the active region of a silicon substrate; (b)anisotropically etching said silicon substrate by using said masking layer as an etching mask, thereby forming a recess; (c)depositing a thin nitride layer; (d)depositing a polysilicon layer on said thin nitride layer; (e)etching back said polysilicon layer and said thin nitride layer to expose the surface of said masking layer, thereby forming a polysilicon spacer extending said recess and said masking layer; (f)growing a local oxidation of silicon (LOCOS) isolation above said recess, said isolation forming on the side wall of said masking layer.
An aspect of the invention is to provide a method for fabricating a LOCOS isolation, wherein said masking layer is a silicon nitride layer.


REFERENCES:
patent: 4835115 (1989-05-01), Eklund
patent: 4897364 (1990-01-01), Nguyen et al.
patent: 5298451 (1994-03-01), Rao
patent: 5399515 (1995-03-01), Davis et al.
patent: 5459083 (1995-10-01), Subrahmanyan et al.

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