Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – Insulative material deposited upon semiconductive substrate
Reexamination Certificate
2000-05-22
2001-06-26
Dang, Trung (Department: 2823)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
Insulative material deposited upon semiconductive substrate
C438S004000, C438S585000, C438S948000, C438S974000
Reexamination Certificate
active
06251804
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for enhancing adhesion of photo-resist to silicon nitride surfaces. More particularly, the present invention relates to a method for enhancing adhesion of a photo-resist layer to a silicon nitride layer on a semiconductor wafer, and therefore improves the critical dimension performance of a silicon nitride layer during a lithographic process.
2. Description of the Prior Art
Lithography technology is in wide use at various steps during an integrated circuit fabrication process, and plays a central role in microcircuit technology. Typically, five to twenty complete lithographic operations are required on each wafer. To transfer the desired pattern from a patterned photo-mask to a wafer, the wafer is coated with a light-sensitive photo emulsion, known as photo-resist. The photo-resist coating process consists of spinning the wafer at high speed after a small quantity of pre-filtered photo-resist has been placed on it. The photo-resist film thickness is inversely proportional to the square root of the spin rate; typically, spinning speeds range from 1000 to 6000 rpm and result in photo-resist films that are about 0.5 to 3 m thick. The photo-resist becomes degraded upon exposure to ultraviolet light. The photo-resist film must adhere sufficiently well to the underlying film so that it does not lift off during subsequent processing. This is usually not a problem, provided that appropriate procedures are used for surface preparation.
Extreme care must be taken to use clean, dry wafers to obtain good adhesion of the photo-resist. A dehydration bake process is thus routinely performed for a couple minutes to remove residual water. A dip in a coupling agent, well known as a priming process, just prior to photo-resist application, is then used to enhance adhesion of the photo-resist to the surfaces, especially silicon dioxide surfaces, of the wafer. Hexamethyldisilizane, commonly abbreviated as HMDS, is often used for this purpose. Other adhesion promotors which can be used include trichlorophenylsilane, trichlorobenzene, and xylene. The coupling agent can also be applied by vapor-plating, in a bath process. By virtue of HMDS, the surface energy of a silicon dioxide layer on the wafer can be adjusted to a level that is approximately equivalent to that of the photo-resist so as to enhance the adhesion.
However, adhesion of the photo-resist to some surfaces, such as silicon nitride surfaces, often presents a serious problem, especially when the line width is below 0.15 m. Please refer to FIG.
1
.
FIG.1
is a cross-sectional diagram of defined gate electrodes
14
having a cap nitride layer
16
on each gate electrode
14
by virtue of a photo-resist layer
16
and HMDS
22
on a semiconductor wafer
10
after performing a dry etching process according to the prior art method. As shown in
FIG.1
, the semiconductor wafer
10
comprises a silicon substrate
12
and a plurality of gate electrodes
14
with a 0.15 line width defined on the silicon substrate
12
. Each gate electrode
14
comprises a gate oxide layer
19
on the silicon substrate
12
, a polysilicon layer
18
on the gate oxide layer
19
, and a cap nitride layer
16
on the polysilicon layer. On each cap nitride layer
16
a photo-resist layer
20
, acting as a hard mask during the dry etching process, is formed and HMDS
22
between the photo-resist layer
20
and the cap nitride layer
16
is applied according to the prior art.
Despite the application of the HMDS
22
to the surfaces of the cap nitride layer
16
, the photo-resist layers
20
are still lifted off during a routine wet cleaning process that is performed before the dry etching process that is used to define the gate electrodes
14
. This causes a pattern transfer failure. In addition, variations of the critical dimension (CD) performance are usually observed when using typical optical printing methods on the cap nitride layer
16
. This occurs because, gradually, native oxidation of the surface of the cap nitride layer changes the optical properties of the surface, such as the reflection index, on the cap nitride layer
16
. The problems of photo-resist patterning on silicon nitride layer are especially severe. Consequently, an economic and effective solution to the above mentioned issues must be found.
SUMMARY OF THE INVENTION
It is therefore a primary objective of this invention to provide a method for enhancing adhesion of photo-resist to a silicon nitride surface.
Another objective of the invention is to provide a method for enhancing adhesion of the photo-resist to silicon nitride surfaces and, at the same time, improving the critical dimension performance of the silicon nitride surfaces during a lithographic process.
In accordance with the present invention, the silicon nitride layer is formed on a semiconductor wafer. The semiconductor wafer comprises a silicon substrate, a gate oxide formed on the silicon substrate, and a polysilicon layer formed on the gate oxide layer. Specifically, the silicon nitride layer is formed on the surface of the polysilicon layer. An oxidation process is first performed on the surface of the semiconductor wafer using an oxidant to transform most of the dangle bonds and Si-N bonds on the surface of the silicon nitride layer into Si-O bonds or Si-ON bonds. An HMDS (hexamethyl disilazane) layer is then formed on the surface of the silicon nitride layer. A photo-resist layer is next formed on the surface of the HMDS layer. Finally, a soft bake process is performed to remove solvents from the photo-resist layer and an exposure process is performed on the photo-resist layer to define a predetermined pattern in the photo-resist layer.
In the preferred embodiment according to this invention, the oxidation process is performed by using ozone-dissolved deionized water. The ozone acts as the oxidant.
In another embodiment according to this invention, the oxidant is an oxygen plasma. In still another embodiment according the present invention, the oxidant is a solution comprising hydrogen peroxide (H
2
O
2
) and sulfuric acid (H
2
SO
4
).
The most important feature of the present invention is that the surfaces of the silicon nitride layer are first subjected to an oxidation process before coating HMDS in order to transform most of the dangle bonds and Si-N bonds on the surface of the silicon nitride layer into Si-O bonds or Si-ON bonds so that the HMDS later applied to the surfaces of the silicon nitride layer will adhere more tightly to the silicon nitride layer. Thus, the adhesion of the photo-resist to the silicon nitride layer is significantly improved.
Furthermore, by controlling the time period of the oxidation process, all of the dangle bonds or Si-N bonds on the surface of the silicon nitride layer can be transformed into Si-O bonds or Si-ON bonds so that the optical properties across the surfaces of the silicon nitride layer are equalized. Thus, the critical dimension performance can be improved.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.
REFERENCES:
patent: 4330569 (1982-05-01), Gulett et al.
patent: 5714037 (1998-02-01), Puntambekar et al.
patent: 5783365 (1998-07-01), Tsujita
Dang Trung
Hsu Winston
United Microelectronics Corp.
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