Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
1999-07-01
2001-01-16
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S290000, C257S437000, C428S446000, C428S448000
Reexamination Certificate
active
06174644
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to an anti-reflective coating formed on the bottom of a photoresist layer and more particularly to the deposition of a silicon nitride film having a low index of refraction, 1.9, over a silicon nitride film having a higher index of refraction, 2.1, to form the anti-reflective coating.
(2) Description of the Related Art
U.S. Pat. No. 5,378,659 to Roman et al. describes the use of a silicon-rich silicon nitride layer, having an absorptive index greater than 0.25, as an anti-reflective coating for use in photolithographic processing.
U.S. Pat. No. 5,441,914 to Taft et al. describes using a thin silicon layer between a patterned tungsten silicide layer and an overlying patterned silicon nitride anti-reflective layer to prevent delamination of the anti-reflective layer.
U.S. Pat. No. 5,216,542 to Szczyrbowski et al. describes a five layer system to provide an effective anti-reflective effect.
U.S. Pat. No. 5,126,289 to Ziger describes the use of an organic material which is highly absorptive of deep ultra violet actinic light to provide anti-reflection effects as well as surface planarization.
U.S. Pat. No. 5,449,639 to Wei et al. describes a method of metal etching using a disposable metal anti-reflective coating.
The anti-reflective coating described in this Patent Application uses a second silicon nitride layer, having a low silicon to nitrogen ratio and a relatively low index of refraction, formed over a first silicon nitride layer, having a high silicon to nitrogen ratio and a relatively high index of refraction, to provide an effective anti-reflective coating.
SUMMARY OF THE INVENTION
Patterns are typically formed in a layer of material, such as silicon nitride, on a semiconductor substrate by forming a photoresist layer over the layer of material, exposing the pattern in the photoresist layer, developing the exposed photoresist layer, and using the developed photoresist layer as a mask to form the pattern in the layer of material. The photoresist layer is exposed using light which has passed through a mask and is focussed on the photoresist layer. During the exposure of the photoresist layer light can enter the photoresist layer and set up multiple reflections within the photoresist layer. The multiple reflections within the photoresist layer will cause constructive and destructive interference at various points within the photoresist layer which will degrade the pattern formed photoresist layer and in the layer of material where the desired image is to be formed.
Anti-reflective coatings are often used to solve the problems caused by the effect of standing waves in a layer of photoresist. Two types of conventional anti-reflective coatings are shown in
FIGS. 1A and 1B
.
FIG. 1A
shows a semiconductor substrate
10
with a layer of pad oxide
12
formed thereon. A layer of silicon nitride
14
is formed on the layer of pad oxide. In order to form a pattern in the layer of silicon nitride
14
a layer of photoresist
16
is formed on the layer of silicon nitride
14
. The photoresist is exposed using a light
30
which has passed through a mask and focussed on the layer of photoresist
16
. An layer of anti-reflective material
18
is formed over the layer of photoresist to prevent multiple reflections within the layer of photoresist
16
from setting up standing waves and distorting the pattern formed in the photoresist.
A second type of conventional anti-reflective coating is shown in FIG.
1
B. In this case the anti-reflective coating
18
is formed under the photoresist layer
16
and over the layer of silicon nitride
14
to be patterned. The anti-reflective coating shown in
FIG. 1B
with the anti-reflective coating under the photoresist layer is more effective in preventing problems due to standing waves but this type of anti-reflective coating increases the complexity of the process flow.
It is an objective of this Invention to provide a method of forming an anti-reflective coating over a silicon nitride layer and under a photoresist layer which does not increase the complexity of the process flow and provides good anti-reflective characteristics.
It is another objective of this Invention to provide a method of forming a pattern in a layer of silicon nitride using an anti-reflective coating under the photoresist layer which does not increase the complexity of the process flow and provides good anti-reflective characteristics.
It is another objective of this Invention to provide an anti-reflective coating under a photoresist layer for use in forming a pattern in a silicon nitride layer.
These objectives are achieved by causing variations in the silicon nitride layer as it is being formed in order to use part of the silicon nitride layer as an in-situ anti-reflective coating.
FIG. 2
shows a diagram of a light beam
30
illuminating a layer of first material
40
having an index of refraction n
1
, a layer of second material
42
having an index of refraction n
2
, and a layer of photoresist
44
having an index of refraction n
3
. The light has a wavelength &lgr;
0
. The layer of second material
42
will act as an effective anti-reflective coating preventing standing waves in the photoresist layer
44
if the index of refraction of the second material, n
2
, is equal to the square root of n
1
multiplied by n
3
, and if the thickness, t, of the layer of second material
43
is equal to the wavelength, &lgr;
0
, divided by the quantity of the index of refraction of the second material, n
2
, multiplied by four, &lgr;
0
/(4n
2
)
The index of refraction of the photoresist, n
3
, is about 1.68 and the index of refraction of silicon nitride can be varied from 1.9 to 2.1 depending on the ration of silicon to nitrogen in the silicon nitride layer. In this invention the layer of first material
40
is silicon nitride with a high silicon to nitrogen ratio and having an index of refraction of about 2.1. The layer of second material is silicon nitride with a low silicon to nitrogen ratio having an index of refraction of about 1.9. The square root of 1.68 multiplied by 2.1 is 1.878 which is very nearly equal to 1.9.
For an i line source having a wavelength of 3650 Angstroms the thickness of the layer of second material, silicon nitride with an index of refraction of 1.9, is about 480 Angstroms which is 3650 Angstroms divided by the quantity of 1.9 multiplied by 4. For an application using a silicon nitride layer having a thickness of 1500 Angstroms the thickness of the layer of first material, silicon nitride with an index of refraction of 2.1, is about 1020 Angstroms.
The objectives of the invention are achieved by forming a first silicon nitride layer with a high silicon to nitrogen ratio and an index of refraction of 2.1. A second silicon nitride layer with a low silicon to nitrogen ratio is formed on the first silicon nitride layer wherein the second silicon nitride layer has an index of refraction of 1.9 and a thickness of 480 Angstroms. When a layer of photoresist is formed over the second silicon nitride layer the second silicon nitride layer forms an effective anti-reflection layer.
REFERENCES:
patent: 5126289 (1992-06-01), Ziger
patent: 5216542 (1993-06-01), Szczyrbowski et al.
patent: 5378659 (1995-01-01), Roman et al.
patent: 5441914 (1995-08-01), Taft et al.
patent: 5449639 (1995-09-01), Wei et al.
patent: 5639687 (1997-06-01), Roman et al.
Cheng Po-Chieh
Shieh Meng-Shiun
Ackerman Stephen B.
Ashton Rosemary
Baxter Janet
Prescott Larry J.
Saile George O.
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