Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
1999-04-27
2002-08-20
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S069000
Reexamination Certificate
active
06437852
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure system for exposing photosensitive substrates, such as silicon plates and glass, to light through patterns designed for devices, such as semiconductors including an IC, an LSI, etc., a liquid crystal panel, a magnetic head, a CCD (image sensor), and so on.
2. Description of Related Art
In manufacturing an IC, an LSI, a liquid crystal element, etc., by photolithography, a projection aligner (projection exposure apparatus) is employed. The projection aligner is arranged to perform an exposure by projecting through a projection optical system a pattern of a photomask or a reticle (hereinafter referred to as a “mask”) onto a substrate, such as a silicon plate or a glass plate, which is coated with a photoresist or the like (hereinafter referred to as a “wafer” in general).
FIG. 1
schematically illustrates the arrangement of a conventional projection aligner. In
FIG. 1
, there are illustrated a KrF excimer laser
251
used as a light source, an illumination optical system
252
, illumination light
253
, a mask
254
, exposure light
255
on the object side, a projection optical system
256
, exposure light
257
on the image side, a photosensitive substrate (wafer)
258
, and a substrate stage
259
which holds the photosensitive substrate
258
.
In the conventional projection aligner, a laser beam emitted from the excimer laser
251
is led to the illumination optical system
252
. At the illumination optical system
252
, the laser beam is converted into the illumination light
253
having a light intensity distribution, a luminous distribution, etc., which are predetermined. The illumination light
253
falls on the mask
254
. A circuit pattern which is to be eventually formed on the photosensitive substrate
258
is beforehand formed on the mask
254
with chromium or the like. The incident illumination light
253
passes through the mask
254
and is diffracted by the circuit pattern to become the object-side exposure light
255
. The projection optical system
256
converts the exposure light
255
into the image-side exposure light
257
to image the circuit pattern on the photosensitive substrate
258
at a predetermined magnification with sufficiently small aberrations. As shown in an enlarged view at the lower part of
FIG. 1
, the image-side exposure light
257
converges on the photosensitive substrate
258
at a predetermined NA (numerical aperture=sin&thgr;) to form the image there. To have the circuit pattern formed in a plurality of shot areas on the photosensitive substrate
258
, the substrate stage
259
is arranged to be movable stepwise to vary the relative positions of the photosensitive substrate
258
and the projection optical system
256
.
However, with the conventional projection aligner using the KrF excimer laser arranged as described above, 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 as follows. The resolution of the projection optical system is limited by a trade-off between an optical resolution and the depth of focus due to the wavelength of the exposure light. The resolution R of the resolving pattern of the projection aligner and the depth of focus DOF can be expressed by the following Rayleigh's formulas (1) and (2):
R
=
k
1
λ
NA
(
1
)
DOF
=
k
2
λ
NA
2
(
2
)
In the above formulas, &lgr; represents the wavelength of the exposure light, NA represents a numerical aperture indicative of the brightness of the optical system on the light exit side, and k1 and k2 represent constants which are normally between 0.5 and 0.7.
According to the formulas (1) and (2), in order to make 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 value 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 brightness value NA.
Meanwhile, known exposure apparatuses capable of giving also a high degree of resolution include probe-type exposure apparatuses.
FIG. 16
schematically shows the arrangement of a proximity-field probe exposure apparatus as one example of such probe-type exposure apparatuses.
Referring to
FIG. 16
, there are illustrated an exposure part
151
, a laser light (beam) source
152
, light source control means
153
, an optical fiber transmission part
154
, an optical fiber probe
155
, an alignment part
156
, a wafer
157
, a wafer stage
158
, and a wafer stage control part
159
. In this example, the exposure part
151
which is arranged to generate exposure light is fixed. The wafer
157
, which is a photosensitive substrate, is arranged to have its position controlled by moving the wafer stage
158
relative to the probe
155
according to information on the measured position of an alignment mark obtained with the alignment part
156
in its position (
1
). At the same time, the generation of exposure light from the probe
155
is controlled. Under such control, the wafer
157
is exposed to light of a circuit pattern in the neighborhood of its position (
2
).
The proximity-field probe exposure is carried out by introducing the exposure light into the optical fiber probe
155
which has its tip sharply formed. A circuit (exposure) pattern is formed on the wafer
157
, i.e., the photosensitive substrate, by exposing the wafer
157
to a non-propagating component of the exposure light, i.e., proximity-field light, which seeps out from a microaperture part of the tip of the optical fiber probe
155
, depends on the shape or size of the microaperture part and has a tiny spread less than a wavelength.
FIG. 17
schematically shows an example of the optical fiber probe
155
. In the case of this example, the probe
155
is completely covered with a metal coating
155
b
except an aperture part
155
a
of the tip for the purpose of efficiently converging the light propagating inside of the optical fiber toward the tip of the probe
155
and also for the purpose of preventing the S/N ratio of the exposure light from being degraded at the proximity field by scattering or transmission of non-proximity-field light taking place in the neighborhood of the aperture part
155
a.
The resolution of such an exposure system is determined by the aperture diameter and the sharpness of the tip of the optical fiber probe
155
.
According to the current level of technology, an optical fiber probe can be prepared with the aperture diameter of the tip measuring less than 50 nm. The resolution of such a probe is much finer than that of the above-stated conventional projection aligner which has the aperture diameter of the tip of the probe at 200 nm or thereabout.
There are other known probe exposure methods, besides the above-stated method of using the proximity-field light. Other known probe exposure methods include a method called STM (scanning tunneling microscopy) which uses a tunneling current and a method called AFM (atomic force microscopy) which uses an interatomic force.
However, these probe exposure methods have a shortcoming in that the rate of throughput attainable by these methods is low. This is because an exposure area which can be covered by one shot of exposure in the probe exposure method is substantially the same as the minute size of the tip of the probe. In order to depict a circuit pattern over a wide exposure area, the wide area, therefore, must be exposed by spending much time.
FIG. 18
shows a probe part of the probe exposure apparatus, in which the probe part is made into a multiple probe so as to improve the rate of throughput by performing the simultaneous exposure using the multiple probe. For example, the rate of throughput can be increased by an N number of times by arranging th
Adams Russell
Nguyen Hung Henry
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