Radiation imagery chemistry: process – composition – or product th – Registration or layout process other than color proofing
Reexamination Certificate
2002-01-25
2002-10-08
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Registration or layout process other than color proofing
C430S311000, C430S313000, C430S314000, C430S318000, C430S327000, C430S329000, C430S510000, C430S512000, C430S950000
Reexamination Certificate
active
06461776
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of forming a resist pattern by lithography and an anti-reflective layer used in forming the resist pattern.
An anti-reflection technology, to suppress reflection of exposure light from a substrate, has been known as a peripheral technology of lithography that meets the necessary conditions of dimensional precision and resolution required of ULSI manufacture. When exposure light is reflected from the substrate, a thin-film interference occurs in a resist film, producing exposure variations in the direction of resist film thickness, called standing waves, and pattern dimension variations called multiple interferences caused by resist film thickness variations. The former degrades the resolution, while the latter deteriorates the dimensional precision. Halation, which is caused by exposure light being reflected on uneven surfaces of the substrate in diagonal directions and in random directions, poses a problem that areas that are originally intended to be shielded from exposure are exposed, making it impossible to form a desired pattern. These problems depend on the intensity of the reflected light from the substrate. The more the reflected light is reduced, the more these problems are mitigated. For this reason, growing efforts are being focused on the reduction of the reflected light from the substrate.
The anti-reflection methods may be classified largely into two groups by their working principle. One group of methods uses as an anti-reflective film a so-called photoabsorptive film with a strong capability to absorb exposure light, and the second group utilizes light interference to prevent reflection. As a representative for the former, an ARC (Anti-Reflective Coating) method is available, which applies a photoabsorptive organic film over the substrate before applying a resist. Light that has passed through the resist toward the substrate is absorbed by this photoabsorptive organic film before being reflected by the substrate surface, so that the light reflected from the substrate and returning to the resist is mitigated.
Examples of anti-reflective films of the second group include Si and TiN. The anti-reflective film of Si, SiO
x
N
y
:H,TiN, etc. is deposited over a metal such as W and Al to such a thickness that the reflected light from the resist/anti-reflective film interface and the reflected light from the anti-reflective film/substrate interface are in opposite phase with each other in order to reduce the reflection. Conventionally, these methods have been employed in reducing the reflection of light.
The ARC method is described in the Proceedings of SPIE, 1991, vol. 1463, pp. 16-29 and in Japan Patent Laid-Open No. 93448/1984. The anti-reflective film using light interference is described in Japan Patent Laid-Open No. 6540/1984 and 130481/1982, in the Proceedings of SPIE, 1994, vol.2197, pp. 722-732 and in the Technical Digests of International Electron Device Meeting, 1982, pp.399-402.
SUMMARY OF THE INVENTION
Regarding the problems of the conventional anti-reflection technology, explanations are given separately to the anti-reflection technology using light interference and to the ARC technology using light absorption.
The anti-reflection method using light interference requires the reflectivity of the resist/anti-reflective film interface and the reflectivity of the anti-reflective film/substrate interface to be equal in order to cancel the reflected light from these interfaces. Because the reflected light from-the resist/anti-reflective film interface and the reflected light from the anti-reflective film/substrate interface need to be in opposite phase, the thickness of the anti-reflective film must be made constant at any location. This is close to impossible to realize on uneven surfaces of the substrate because, as shown in
FIG. 2
, a thickness
21
of a stepped portion of the anti-reflective film is greater than that of a thickness
22
of a planar portion. When the substrate surface layer is a transparent film such as a silicon oxide film, the reflected light from the reflective interface under the silicon oxide film and the reflected light from the resist/anti-reflective film interface must be set in opposite phase. This requires a precise film thickness control including the silicon oxide film. If the silicon oxide film is used as an inter-layer film on the uneven surface of the substrate, the film thickness control is impossible because the thickness of the silicon oxide film varies greatly depending on locations. In these cases, therefore, satisfactory anti-reflection cannot be obtained with the light interference anti-reflection film. There is another problem that, to make the reflectivities equal, it is necessary to optimize the complex index of refraction of the anti-reflective film material according to the substrate material (more precisely, the complex-index of refraction of the substrate material). That is, the anti-reflection method using light interference makes it necessary to change the anti-reflective film material each time the substrate material is changed. The anti-reflection method therefore lacks versatility.
Because it deposits a film, the ARC method has the advantages of being simple and versatile, that is, it does not depend on the substrate material. On the other hand, the ARC method has a problem that the anti-reflective film has a large thickness and therefore this method is not suited for forming a microfine pattern. When there is a step in the substrate surface, the thickness
31
(see
FIG. 3
) of the anti-reflective film
30
over a stepped portion is smaller than the thickness
32
at the side of the stepped portion and the thickness
33
of the planar portion. This makes it necessary to set the thickness over the stepped portion greater than is required. Further, when performing lithography on a planar substrate, the anti-reflective film should be formed thick. To increase photoabsorbance in the anti-reflective film and at the same time reduce the film thickness requires increasing the photoabsorbance level of the anti-reflective film. As the photoabsorbance level increases the reflectivity of the interface between the anti-reflective film and the resist increases, making it impossible to produce a satisfactory reflection prevention effect. For example, when the extinction coefficient of the anti-reflective film, which indicates photoabsorption, exceeds 0.5, the interface reflection between the resist and anti-reflective film much increases. Hence, to obtain a sufficient reflection prevention effect, the thickness of the anti-reflective film needs to be increased. When a thick anti-reflective film is used, the ratio of film thickness to pattern width, i.e., an aspect ratio, becomes extremely large in microfine patterns, making the anti-reflective film very difficult to process. At the same time, the formed pattern will easily collapse, resulting in a faulty product. For example, if a 0.2 &mgr;m pattern is to be formed at ±5% precision, the reflectivity of the substrate needs to be kept within 0.23% (energy reflectivity). To achieve this reflectivity requires the thickness of the anti-reflective film to be 0.15 &mgr;m or greater because of the relationship between the above mentioned photoabsorbance level and the interface reflection. The aspect ratio for the pattern is 0.75. Finer patterns require higher dimensional precision and accordingly the reflectivity must be reduced further, forcing the thickness of the anti-reflective film and the aspect ratio to become still larger.
The present invention has been accomplished with a view to overcoming the above-mentioned problems experienced with conventional technologies.
That is, it is an object of this invention to provide a resist pattern forming method and an anti-reflective film used in the method, which can produce a satisfactory reflection prevention effect even when there are large steps in a substrate surface; which can be used widely irrespective of the substrate material without being affecte
Asai Naoko
Tanaka Toshihiko
Uchino Shoichi
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Young Christopher G.
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