Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-09-28
2003-06-10
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06576375
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-274722, filed Sep. 28, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a photomask having a anti-reflection structure.
Pattern transfer is carried out in the following manner in the process that forms a resist pattern by applying a resist on a film to be processed (hereinafter called “to-be-processed film”) on a substrate and performing exposure and development using an exposure apparatus, such as a stepper. The light that has emitted from a light source and condensed by a lens is focused again on the resist by the lens, thereby forming the latent image of the same pattern as the mask (the latent image of a reduced pattern in the case of reduction projection) on the resist. As a result, a mask pattern is transferred on the resist. The mask that is used in this pattern transfer normally has a thin film of a chromium pattern formed on thick glass.
There is a significant difference between refractive indexes of the glass and chromium or refractive indexes of chromium and air. At the time light actually passes the mask, therefore, reflection at the interface between glass and chromium or the interface between chromium and air becomes very large in this structure.
As a solution to this shortcoming, a three-layer structure of chromium oxide/chromium/chromium oxide has been proposed in place of the single layer of chromium as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 23277/1978. The mask with this three-layer structure is designed on the premise of the use of g rays having an exposure wavelength of 435 nm or i rays of 365 nm. With such an exposure wavelength, the use of the three-layer structure drops the reflectance to 10% or less, which shows a sufficient anti-reflection effect.
Recently, however, the exposure wavelength tends to become shorter as patterns become finer and the exposure light is being shifted to KrF excimer laser light with an exposure wavelength of 248 nm or ArF excimer laser light with an exposure wavelength of 193 nm. The aforementioned refractive indexes also vary in accordance with the shift from the longer wavelength to the shorter one. With this difference in refractive index, therefore, the three-layer structure disclosed in the Japanese publication cannot suppress the reflectance to less than 10% and does not demonstrate a sufficient anti-reflection effect.
Specifically, the reflectance at, for example, the interface between glass and chromium oxide or the interface between chromium and air significantly exceeds 10% in the case of KrF excimer laser light of 248 nm or ArF excimer laser light of 193 nm. Therefore, more than 10% of the incident light is reflected at those two interfaces and repeats complex reflection inside the exposure apparatus.
When such light, which is called stray light, reaches the resist through multiple reflection, the stray light, unlike the normal exposure light, has not undergone normal diffraction and thus works as noise. This noise reduces the resolution of the resist, reducing the exposure latitude and focus depth, and further causes adverse influences, such as a change in the pattern shape according to the numerical aperture of the mask.
One way to solve this problem is to eliminate stray light by lowering the reflectance at the aforementioned interfaces and the structure which has a lamination of a anti-reflection film/glass/chromium/chromium oxide/anti-reflection film is actually proposed in Jpn. Pat. Appln. KOKAI Publication No. 51240/1992. This structure does not however reduce reflection at the interface between glass and chromium and demonstrates an inadequate performance as a anti-reflection structure.
The following show the results of an experiment conducted using a conventional photomask.
FIG. 1
is a cross-sectional view exemplifying the structure of the conventional photomask. This conventional photomask is produced by the following method.
First, a chromium oxide film
3
having a thickness of 30 nm, a chromium film
4
having a thickness of 70 nm and a chromium oxide film
5
having a thickness of 30 nm were laminated in the named order on a quartz substrate
1
having a thickness of 0.9 inch by sputtering. Next, a photosensitive resin with a thickness of 0.5 &mgr;m was applied on the chromium oxide film
5
. Then, this photosensitive resin was exposed with an electron beam and developed, yielding a line and space (L/S) pattern of 0.7 &mgr;m/0.7 &mgr;m comprised of the photosensitive resin.
Next, with this pattern of the photosensitive resin used as a mask, the chromium oxide film
3
, the chromium film
4
and the chromium oxide film
5
were dry-etched using gas which essentially consists of chlorine. Then, the photosensitive resin was removed, thus completing a photomask as shown in FIG.
1
.
This photomask was observed with a laser microscope using a He—Cd laser with a wavelength of 325 nm. It was confirmed that the L/S pattern of the chromium oxide film
3
/chromium film
4
/chromium oxide film
5
accurately had a dimension of 0.7 &mgr;m/0.7 &mgr;m.
Next, the light reflectance of this mask was measured with KrF excimer laser light of 248 nm. The reflectance at the time of irradiating light from the bottom side of the quartz substrate
1
was 15.3% and the reflectance at the time of irradiating light from the major surface side of the quartz substrate
1
or from the chromium film
4
side was 13.2%, both being very high.
Further, a pattern was actually formed with the photomask prepared in the above-described manner and the performance of the photomask was inspected under the following conditions. A silicon oxide film
22
with a thickness of 200 nm as a to-be-processed film was formed on a silicon substrate
21
by CVD. Then, a solution of DUV 30 (a product by Brewer Science Limited) was provided with spin-coating method on this silicon oxide film
22
and baked for 90 seconds at 225° C., providing a anti-reflection film
23
with a thickness of 60 nm. A resist UV6, produced by Shipley Corporation, was applied on this anti-reflection film
23
and baked for 60 seconds at 130° C., yielding a resist
24
having a thickness of 300 nm.
Then, the photomask was exposed at 27 mJ/cm
2
using KrF excimer scanner S202A (NA=0.6 and mask magnification of ×4), manufactured by Nikon Corporation. Thereafter, the to-be-processed substrate was baked for 90 seconds at 130° C. and developed with 2.38% of tetramethylammonium hydroxide (TMAH) for 45 seconds.
FIG. 2
shows an exemplary cross-sectional view of a to-be-processed substrate including the resist pattern that is obtained by the above-described process. While an L/S pattern of 0.175 &mgr;m/0.175 &mgr;m which was the ¼ pattern of the photomask pattern was formed as designed, the resist had round corners. Then, exposure was conducted while the focus position is varied. The results showed that the focus depth which made the dimension fall within ±10% became very shallow, about 0.4 &mgr;m.
As apparent from the results of the experiment, the resolution was deteriorated due to an increased amount of stray light originated from the photomask, and the resist and the exposure apparatus did not demonstrate the intended performances, making the focus depth shallower. In addition, the pattern did not have a rectangular shape.
While the exposure wavelength tends to become shorter as patterns become finer, the above-described conventional method of forming a resist pattern causes the amount of stray light to increase as the reflectance increases in accordance with the shortened exposure wavelength. This results in shortcomings, such as the deterioration of the resolution of the resist.
BRIEF SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a photomask which has a low reflectance even when the exposure wavelength becomes shorter.
To achieve the above object, according to
Azuma Tsukasa
Kanemitsu Hideyuki
Miyoshi Seiro
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Huff Mark F.
Kabushiki Kaisha Toshiba
Mohamedulla Saleha R.
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