Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-07-20
2003-12-30
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S030000, C716S030000
Reexamination Certificate
active
06670080
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pattern creating apparatus and a pattern creating method and, more particularly, to those suitably applicable, for example, to the design of circuit patterns of photomasks or reticles used in an exposure step to expose a photosensitive substrate to exposure light in a microscopic circuit pattern in production of various devices such as semiconductor chips including IC, LSI, and so on, display elements including liquid crystal panels etc., detecting elements including magnetic heads etc., image pickup elements including CCD etc., and so on.
2. Related Background Art
For manufacturing the devices such as the IC, LSI, liquid crystal panels, and so on by the photolithography technology, it was conventional practice to employ projection exposure to project a circuit pattern of a photomask or a reticle or the like (hereinafter referred to as “mask”) onto a photosensitive substrate such as a silicon wafer or a glass plate or the like (hereinafter referred to as “wafer”) coated with a photoresist or the like by a projection optical system, so as to transfer (or expose) the circuit pattern thereonto.
In recent years, in response to the tendency toward higher integration of the above devices, there are demands for much finer patterns to be transferred onto the wafer, i.e., for increase of resolution to higher values. In the above projection exposure technology, which is the core of the micro-fabrication technology for wafers, therefore, the increase of resolution is now labored for forming an image (circuit pattern image) in the size (line width) of 0.3 &mgr;m or less.
A schematic diagram of a conventional projection exposure apparatus is illustrated in FIG.
19
. In
FIG. 19
, reference numeral
191
designates an excimer laser as a light source for deep ultraviolet exposure,
192
an illumination optical system,
193
illumination light emerging from the illumination optical system
192
,
194
a mask,
195
object-side exposure light emerging from the mask
194
and then entering an optical system (projection optical system)
196
,
196
a demagnification type projection optical system,
197
image-side exposure light emerging from the projection optical system
196
and then entering a substrate
198
,
198
a wafer being a photosensitive substrate, and
199
a substrate stage for holding the photosensitive substrate.
The laser light emitted from the excimer laser
191
is guided to the illumination optical system
192
by a routing optical system (
190
a,
190
b
) and is regulated by the illumination optical system
192
so as to be the illumination light
193
having predetermined light intensity distribution, distributed light distribution, spread angle (numerical aperture NA), etc., which illuminates the mask
194
. In the mask
194
a pattern is made of chromium or the like on a quartz substrate and in the size equal to the inverse of a projection magnification of the projection optical system
196
(for example, two, four, or five) times a microscopic pattern to be formed on the wafer
198
. The illumination light
193
is transmitted and diffracted by the microscopic pattern of the mask
194
to become the object-side exposure light
195
.
The projection optical system
196
converts the object-side exposure light
195
to the image-side exposure light
197
, which forms the microscopic pattern of the mask
194
on the wafer
198
at the above projection magnification and with well-suppressed aberration. The image-side exposure light
197
, as illustrated in the enlarged view in the lower part of
FIG. 19
, converges at a predetermined numerical aperture NA (=sin(&thgr;)) on the wafer
198
to form the image of the microscopic pattern on the wafer
198
. When the microscopic pattern is successively transferred onto mutually different shot areas (areas to become one chip or plural chips) in the wafer
198
, the substrate stage
199
is stepped along the image plane of the projection optical system to change the position of the wafer
198
relative to the projection optical system
196
.
The above-stated projection exposure apparatus using the KrF excimer laser as a light source, which is presently becoming mainstream, has high resolving power, but it is technologically difficult to form a pattern image of not over 0.15 &mgr;m thereby, for example.
The projection optical system
196
has the limit of resolution determined from a trade-off between the optical resolution and the depth of focus originating in exposure wavelength (used in exposure). The resolution R and the depth of focus DOF of patterns resolved by the projection exposure apparatus are expressed by the Rayleigh's formula as indicated by Eq. (1) and Eq. (2) below.
R=k
1
(&lgr;/
NA
) (1)
DOF=k
2
(&lgr;/
NA
2
) (2)
In these equations, &lgr; is the exposure wavelength, NA the numerical aperture on the image side to indicate brightness of the projection optical system
196
, and k
1
and k
2
are constants determined by development process characteristics of the wafer
198
etc., which are normally values of about 0.5 to 0.7.
A potential way for improvement in the resolution to decrease the resolution R is “employment of higher NA” to increase the numerical aperture NA, but there is a limit of evolution in the employment of higher NA over a certain point, because the depth of focus DOF of the projection optical system
196
has to be kept at a value over a certain level in practical exposure. It is thus seen from these Eqs. (1), (2) that “shortening of wavelength” to decrease the exposure wavelength &lgr; is eventually necessary for the improvement in the resolution.
There, however, arises a significant issue with progression in the wavelength shortening of the exposure wavelength. The issue is that there exists no glass material for the lenses constituting the projection optical system
196
. Most of glass materials have transmittances close to 0 in the deep ultraviolet region and fused silica is available at present as a glass material produced for the exposure apparatus (the exposure wavelength of about 248 nm) by a special production method. The transmittance of this fused silica is also lowered quickly at the exposure wavelengths of not more than 193 nm.
It is very difficult to develop a practical glass material in the region of exposure wavelengths of not more than 150 nm corresponding to microscopic patterns having the line widths of 0.15 &mgr;m or below. The glass material to be used in the deep ultraviolet region also needs to satisfy plural conditions including durability, index uniformity, optical strain, workability, and so on, as well as the transmittance, which makes it doubtful whether there exists a practical glass material or not.
As described above, the conventional projection exposure methods and projection exposure apparatus need to decrease the exposure wavelength down to about 150 nm or less in order to form the patterns having the line widths of 0.15 &mgr;m or below on the wafer. Against it, there exists no practical glass material in this wavelength region at present, and thus no pattern has been allowed to be formed in the line width of 0.15 &mgr;m or below on the wafer.
U.S. Pat. No. 5,415,835 discloses the technology of forming the microscopic pattern by two-beam interference exposure, and this two-beam interference exposure permits formation of the pattern in the line width of 0.15 &mgr;m or below on the wafer.
The two-beam interference exposure will be explained referring to FIG.
15
. In
FIG. 15
the two-beam interference exposure is effected as follows; laser light L
151
from laser
151
, which has coherence and which is a bundle of parallel rays, is split into two beams, laser beams L
151
a,
L
151
b,
by half mirror
152
and the two split beams are reflected by respective plane mirrors
153
a,
153
b
so as to make the two laser beams (parallel beams with coherence) intersect at a certain angle in the range greater than 0 but less than 90° on t
Kochi Tetsunobu
Sugita Mitsuro
Mohamedulla Saleha R.
Young Christopher G.
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