Method of rounding a topcorner of trench

Details Method of rounding a topcorner of trench Method of rounding a topcorner of trench

2002-05-29

2003-12-30

Ho, Hoai (Department: 2818)

Semiconductor device manufacturing: process

Chemical etching

Combined with coating step

Reexamination Certificate

active

06670275

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method for increasing rounding effect in a trench top corner, and more particularly to a method for pulling back SiN to increase rounding effect in a shallow trench isolation process (STI) and to avoid Wrap Round in the trench top corner.
BACKGROUND OF THE INVENTION
Trench isolation is a widely used method in the modern VLSI process to isolate elements. The primary principle of the technique is to use the anisotropic dry etch to define a trench and then a filling is added to the trench. As the trend of the modern semiconductor, the dimension of the elements used in the semiconductor is smaller and smaller, which causes the dimension of the isolation layer to decrease. Accordingly, semiconductors having the structure of STI become the mainstream.
With reference to
FIGS. 1A
to
1
H, the conventional STI technique includes the following steps. First, a SiO
2
layer
102
is formed on a substrate
101
of Si. A Si
3
N
4
layer
103
is deposited on the SiO
2
layer
102
to form a shallow trench area
104
. Then, high density plasma chemical vapor deposition is used to precipitate an oxide
105
in the shallow trench area
104
. After the precipitation step, it is necessary to planarize the oxide
105
. A step of rounding the shallow trench corner
104
a
is then processed. After the rounding process, the Si
3
N
4
layer
103
is removed. Finally, cleaning steps including wet cleaning, cell cleaning and tunnel cleaning are performed.
In the conventional method, the oxide
105
is planarized in an etcher and then cleaned. After the post cleaning process, an oxide recessed portion
106
is often formed on the edge of the oxide
105
in the shallow trench area
104
. The recessed portion
106
causes a wafer to have abnormal conductivity, such as double humps in the I
d
-V
G
curve.
Therefore, it is necessary to provide an improved method to form a shallow trench isolation structure to eliminate the wrap around in the trench top corner.
To overcome the aforementioned problem, some introduces a method, as shown in the prior art and in
FIGS. 2A
to
2
I, to protect the edges of the wafer, which includes the following steps. The first step is to prepare a SiO
2
layer
202
and a Si
3
N
4
layer
203
on a substrate
201
of Si. Then, a photoresist layer
204
that defines an opening
205
is formed above the Si
3
N
4
layer
203
. An anisotropic etching is applied to the SiO
2
layer
202
and the Si
3
N
4
layer
203
to form an encasing wall
206
a
around the photoresist layer
204
, the SiO
2
layer
202
, the Si
3
N
4
layer
203
and the opening
205
. Thereafter, a dry etching is introduced to the encasing wall
206
b
that encloses the opening
205
and the substrate
201
to form a shallow trench area
207
. Then, the photoresist layer
204
and the encasing wall
206
b
are removed to expose the unetched area on the substrate
201
and a sharp edge
208
a.
After the removal of the photoresist layer, it is then required to form an oxide of Si
209
on the unetched area of the substrate
201
and the sharp edge
208
a
to change the sharp edge
208
a
to round edge
208
b.
Then it is necessary to precipitate an insulation layer
210
on the oxide of Si
209
and fill the shallow trench area
207
. Last, the SiO
2
layer
202
and the Si
3
N
4
layer
203
are removed from the insulation layer.
This method uses the encasing wall
206
b
of a polymer to fill in the opening
205
. When the polymer is removed and the insulation layer
210
is filled in the space left by the removal of the polymer, the insulation layer
210
is able to protect the corner
211
.
This method does meet the necessary requirements. However, it needs the step of forming the encasing wall and the removal of the encasing wall, it is too complicated and the cost of manufacturing is increased.
According to the foregoing technique, the methods such as wet etch or oxidation to pull back the SiN complicates the process and increases the cost. Furthermore, after the Si
3
N
4
layer is removed, the post cleaning process easily forms wrap round on the trench top corner and thus causes high electric field and pre-breakdown.
To overcome the shortcomings, the present invention intends to provide an improved method for pulling back S
i
N to increase rounding effect in a shallow trench isolation process and to avoid Wrap Round in the trench top corner.
SUMMARY OF THE INVENTION
The primary objective of the present invention is to provide a method for pulling back SiN to increase rounding effect in a shallow trench isolation process and to avoid Wrap Round in the trench top corner.
To order to accomplish the foregoing objective, the method adds an isotropic etching process to pull back the Si
3
N
4
and to increase the trench top corner rounding. After the SiN layer is etched to a predetermined depth, a gas of SF
6
/HB
r
is applied to fully etch the remaining SiN. With the gradient variation of the depth by the SF
6
/HB
r
, and the etch rate selectivity of SiO
2
to Si being less than 1, a top rounding etch step is adopted to continue etching the oxide layer and the substrate to have a reinforced top corner rounding.
Other objects, advantage and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.


REFERENCES:
patent: 6527968 (2003-03-01), Wang et al.

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