Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2002-01-16
2004-04-13
Kim, Peter B. (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S077000
Reexamination Certificate
active
06721040
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to exposure apparatuses, and more particularly to an exposure method and apparatus used to expose an object to be exposed, such as a single crystal substrate for a semiconductor wafer, and a glass substrate for a liquid crystal display (LCD), a device fabricating method using the exposed object, and a device fabricated from the exposed object. The exposure method and apparatus of the instant invention are applicable to the fabrication of various types of devices, for example, semiconductor chips such as ICs and LSIs, display devices such as liquid crystal panels, detecting devices such as magnetic heads, and imaging devices like CCDs.
The conventional photolithography for fabricating devices, such as ICs, LSIs, and liquid crystal panels, has utilized projection exposure methods and apparatuses. Such methods and apparatuses use a projection optical system to project or transfer a circuit pattern on a photo-mask or reticle (called “a mask” hereinafter) onto a photoresist-applied, photosensitive substrate, such as a silicon wafer and a glass plate (called a “wafer” hereinafter), thereby exposing the substrate with the circuit pattern.
The higher integration of these devices accordingly requires a smaller pattern to be transferred to a chip area on a wafer, that is, the higher resolution, as well as a larger area for each chip area on the wafer. Therefore, the projection exposure method and apparatus for taking a lead in the fine wafer processing technology are also required to improve the resolution and exposure area so that an image with a size (or line width) of 0.5 &mgr;m or less can be formed in a wider area.
A typical schematic of a conventional projection exposure apparatus is shown in FIG.
13
. In
FIG. 13
,
191
denotes an excimer laser as a light source for far ultraviolet exposure,
192
an illumination optical system,
193
an illumination beam,
194
a mask,
195
an object-side exposure beam emitting from the mask
194
and incident upon the optical system
196
,
196
a demagnification projection optical system,
197
an image-side exposure beam emitting from the optical system
196
and incident upon the wafer
198
which is a photosensitive substrate, and
199
a substrate stage that holds the photosensitive substrate.
The laser beam emitted from the excimer laser
191
is directed to the illumination optical system
192
by a directing optical system, and then turned by the illumination optical system
192
into the illumination beam
193
with a specified light intensity distribution, a luminous intensity distribution, and an open angle (number of apertures NA). The illumination beam
193
, in turn, illuminates the mask
194
. The mask
194
forms on its quartz substrate a chromium pattern, which is a reciprocal times of projection optical system
196
's projection power (for example, twice, four times or five times) as large as the minute pattern formed on the wafer
198
. The illumination beam
193
is diffracted after transmitting through the minute pattern on the mask
194
, and turned into the object-side exposure beam
195
. The projection optical system
196
converts the object-side exposure beam
195
into the image-side exposure beam
197
for forming an image representative of mask
194
's minute pattern on the wafer
198
with the above projection power and sufficiently small aberration. The image-side exposure beam
197
converges, as shown in the enlarged part at the bottom of
FIG. 13
, onto the wafer
198
with a specified NA (=sin &thgr;), creating an image of the minute pattern on the wafer
198
. In order to form a minute pattern sequentially on multiple different areas (or shot areas each of which will becomes one or more chips) on the wafer
198
, the substrate stage
199
stepwise moves along the image plane of the projection optical system and shifts a position of the wafer
198
relative to the projection optical system
196
.
However, it is difficult for the projection exposure apparatuses, which use currently widespread excimer lasers as a light source, to form a pattern of 0.10 &mgr;m or less.
The projection optical system
196
has its limits in resolution based on the trade-off between the optical resolution R dependent on the wavelength of an exposure beam (called an “exposure wavelength” hereinafter) and a depth of focus (“DOF”). R and DOF in the projection exposure apparatus are given as following Rayleigh's formulas (1) and (2):
R=k
1
(&lgr;/
NA
) (1)
DOF=k
2
(&lgr;/
NA
) (2)
&lgr; is an exposure wavelength, NA is the number of apertures at the image side of the projection optical system
196
, and values of k
1
and k
2
usually fall between about 0.5-0.7, at most about 0.4 even for a resolution enhancement like a phase shift. These formulas indicates that the superior resolution with a smaller value of R is obtainable from the higher NA or the increased number of apertures NA, but the projection optical system
196
needs relatively large DOF in the actual exposure and the NA can be increased to only some extent. As a result, it is understood that a shorter wavelength or reduced exposure wavelength &lgr; is needed for the higher resolution.
Nevertheless, the shortened wavelength would possibly raise a critical problem in that there would be no glass materials available for lenses in the projection optical system
196
. Transmittances of most glass materials are close to 0 in the far ultraviolet range. Even synthetic quartz, which is fabricated by a special fabricating method, as a glass material for an exposure apparatus (with the exposure wavelength of 248 nm), drops its transmittance drastically for the exposure wavelength of 193 nm or shorter. It seems very difficult to develop practical glass materials as have sufficiently high transmittance for an exposure wavelength of 150 nm or less, which allows for a minute pattern transfer of 0.10 &mgr;m or less. Further, glass materials for use in the far ultraviolet range need to satisfy such certain conditions in view of durability, uniform refractive index, optical distortion, and manufacturability, in addition to the transmittance, that practical glass materials for the exposure wavelength of 150 nm or shorter become harder to be available.
The conventional projection exposure method and apparatus thus need to use the shortened exposure wavelength of about 150 nm or below to form a pattern of 0.10 &mgr;m or less on the wafer
198
, but cannot do that since no practical glass material is available in this wavelength range.
In the meantime, a fine processing apparatus configured as a scanning near field optical microscope (referred to as an “SNOM” hereinafter) has recently been proposed to provide optical fine processing of a size of 0.1 &mgr;m or less. This apparatus utilizes evanescent or near-field light oozed out of a fine aperture of, for example, 100 nm or less to locally expose the resist beyond the limit of a light wavelength. However, such a SNOM lithographic apparatus disadvantageously has a poor throughput since it is adapted to use one or more processing probes for the fine process as if drawing a picture with a single stroke of brush.
As in Japanese Laid-Open Patent Application No. 8-179493, one proposed solution for this problem provides a photo mask with a prism, leads light to the prism at an angle of incidence producing the total reflection, and utilizes the near field light oozed out of the total-reflection surface to transfer the entire photo mask pattern onto the resist at one time.
It is requisite for the batch exposure apparatus using the prism and near field light, as disclosed in the above reference, to set a distance between the prism/mask and the resist surface to 100 nm or less. However, in reality, it is difficult to set the distance between them to 100 nm or less throughout the entire prism/mask surface, due to the limitative surface precision and flatness of the prism/mask and the substrate. Any slight tilt in positioning the pris
Inao Yasuhisa
Kuroda Ryo
Saito Kenji
Canon Kabushiki Kaisha
Kim Peter B.
Morgan & Finnegan , LLP
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