Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-07-01
2003-10-14
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S311000, C430S396000
Reexamination Certificate
active
06632574
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mask for use in exposing a photosensitive substrate with a pattern designed for devices, such as a semiconductor including an IC, an LSI, etc., a liquid crystal panel, a magnetic head and a CCD (image sensor), and an exposure method and an exposure apparatus which use the mask, and more particularly, relates to a mask, an exposure method and an exposure apparatus which are adapted for manufacturing such devices to a high degree of integration.
2. Description of Related Art
Heretofore, in manufacturing an IC, an LSI, a liquid crystal element, etc., by photolithography techniques, a projection exposure apparatus is used which performs an exposure by projecting through a projection optical system a pattern of a photomask or a reticle (hereinafter referred to generally as “mask”) onto a photosensitive substrate, such as a wafer or a glass plate, which is coated with a photoresist or the like.
In recent years, the degree of integration of devices, such as an IC, an LSI and a liquid crystal element, is increasing more and more. As one of demands for minute and fine working of semiconductor wafers according to such an increase of the degree of integration, a device pattern is required to be more finely and minutely formed, i.e., to have a higher resolution.
For such a requirement, the projection exposure technique, which plays a main role in the art of accomplishing minute work, is being developed these days so as to form a pattern image of a line width not greater than 0.5 &mgr;m over a wider range.
FIG. 21
is a schematic diagram showing the arrangement of a conventional exposure apparatus.
In
FIG. 21
, there are illustrated an excimer laser light source
171
, an illumination optical system
172
, illumination light
173
, a mask
174
, object-side exposure light
175
, a projection optical system
176
, image-side exposure light
177
, a photosensitive substrate (wafer)
178
, and a substrate stage
179
arranged to hold the photosensitive substrate
178
.
In the conventional exposure apparatus shown in
FIG. 21
, a laser beam emitted from the excimer laser light source
171
is led to the illumination optical system
172
. At the illumination optical system
172
, the laser beam is converted into the illumination light
173
having a predetermined light intensity distribution, a predetermined luminous distribution, etc., and the illumination light
173
is then made incident on the mask
174
. On the mask
174
, a circuit pattern which is to be formed on the photosensitive substrate
178
is beforehand formed with chromium or the like in a predetermined magnified size. The illumination light
173
, passing through the mask
174
, is diffracted by the circuit pattern to be converted into the object-side exposure light
175
. The projection optical system
176
converts the object-side exposure light
175
into the image-side exposure light
177
to image the circuit pattern on the photosensitive substrate
178
at a predetermined magnification with sufficiently small aberrations. As shown in an enlarged view at the lower portion of
FIG. 21
, the image-side exposure light
177
converges on the photosensitive substrate
178
at a predetermined NA (numerical aperture=sin &thgr;) to be imaged there. To have the circuit pattern formed in a plurality of shot areas on the photosensitive substrate
178
, the substrate stage
179
is arranged to be movable stepwise to vary the relative positions of the photosensitive substrate
178
and the projection optical system
176
.
In the above projection exposure apparatus using an excimer laser, which is currently widely used, however, it is difficult to form a pattern image of a line width not greater than 0.15 &mgr;m.
The reason for this difficulty is explained below. The resolution of the projection optical system is limited by a trade-off between the optical resolution and the depth of focus due to the wavelength of the exposure light. The resolution R and the depth of focus DOF in the resolving pattern by the projection exposure apparatus can be expressed by the following Rayleigh's formulas (a1) and (a2):
R
=
k
1
λ
NA
(a1)
DOF
=
k
2
λ
NA
2
(a2)
where &lgr; is the wavelength of the exposure light, NA is a numerical aperture indicative of the brightness of the optical system, k
1
and k
2
are constants which are determined by the developing process characteristic, etc., of the photosensitive substrate and are normally between 0.5 and 0.7.
According to the formulas (a1) and (a2), in order to make the value of the resolution R smaller for a higher degree of resolution, it is necessary either to make the wavelength &lgr; smaller for a shorter wavelength or to make the NA larger for a higher degree of brightness. At the same time, however, the depth of focus DOF required for a necessary performance of the projection optical system must be kept at least at a certain value. This requirement imposes some limitation on the increase of the NA. As a result, the shortening of the wavelength is considered a sole solving method.
The attempt to shorten the wavelength, however, encounters several serious problems other than the problem related to the above formulas. The most serious problem lies in that it becomes hardly impossible to find any optical material usable for the projection optical system. An optical system which is actually mountable on the exposure apparatus as the current projection optical system in view of the amount of aberration, the precision of working, the controllability, etc., is one including a refractive system, i.e., a lens. Almost all optical materials used for lenses have transmission factors near “0” in the short wavelength region, i.e., in the far ultraviolet region. Although there are a fused quartz material, etc., as an optical material which is manufactured by a special manufacturing method for an exposure apparatus, the transmission factor of the fused quartz also abruptly drops for the wavelength not greater than 193 nm. It is thus extremely difficult to develop any optical material practically usable for an exposure wavelength not greater than 150 nm required for a pattern of a line width not greater than 0.15 &mgr;m, because the optical material is required to satisfy a plurality of conditions relative to durability, uniform refractive index, optical strain, workability, etc., in addition to the transmission factor.
The above conventional projection exposure method necessitates the shortening of the wavelength for performing a pattern exposure depending on the formulas (a1) and (a2), and, therefore, causes a problem in that there exists no usable optical material, so that it is impossible to realize an exposure for a pattern image of a line width not greater than 0.15 &mgr;m.
There is another exposure method called a two-light-flux interference exposure method.
FIG. 22
is a schematic diagram for explaining the two-light-flux interference exposure method. According to the two-light-flux interference exposure method, coherent light emitted from a laser light source
71
is divided by a half-mirror
72
into two light fluxes. Mirrors
73
a
and
73
b
are arranged to deflect the two light fluxes respectively at some angles to cause the two light fluxes to join together on a photosensitive substrate
74
in such a way as to form interference fringes there. Then, the photosensitive substrate
74
is exposed according to a distribution of light intensity made by the interference fringes, so that a periodic pattern is formed according to the distribution of light intensity.
The resolution R obtained by the two-light-flux interference exposure method is expressed by the following formula (a3), where the resolution R is assumed to be the width of each of lines and spaces (L&S), i.e., the width of each of bright and dark bands of the interference fringes, e represents the angle of incidence on the photosensitive substrate
74
of each of the two light fluxes
71
a
and
71
b
, and NA sin &thgr;.
R
Huff Mark F.
Mohamedolla Saleha R.
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