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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection exposure apparatus for
forming fine patterns required for manufacture of LSIs, as well as a
polarizer for use in such a projection exposure apparatus.
2. Description of the Related Art
FIG. 8 shows an optical system of a conventional projection exposure
apparatus. The optical system includes a lamp house 1, a mirror 2, a fly's
eye lens 3 disposed in front of the lamp house 1 with the mirror 2 between
the lamp house 1 and the fly's eye lens 3, an aperture member 4 located in
front of the fly's eye lens 3, a relay lens 5A, a stop 6, a mirror 7, a
condenser lens 5B, a photo mask 8 on which a circuit pattern is formed, a
projection lens 9, and a wafer 10 with the projection lens 9 between the
mask 8 and the wafer 10.
Light rays emanating from the lamp house 1 reach the fly's eye lens 3
through the mirror 2, and are split into regions by the individual lenses
3a of the fly's eye lens 3. The rays which have passed through the
individual lenses 3a pass through an aperture portion 4a of the aperture
member 4, the relay lens 5A, the stop 6, the mirror 7 and the condenser
lens 5B, and illuminate the entire surface of the exposure area of the
mask Therefore, on the surface of the mask 8, the rays from the individual
lenses 3a of the fly's eye lens 3 are laid on top of one another, and the
mask 8 is thus uniformly illuminated. The light rays which have passed
through the mask 8 in the manner described above pass through the
projection lens 9 and then reach the wafer 10. A resist film on the
surface of the wafer 10 is exposed to this illumination light, whereby
transfer of the circuit pattern is achieved.
As a result, a resist pattern is formed on the surface of the wafer 10.
When the illumination light illuminates the wafer 10, part of the
illumination light passes through the resist film and is then reflected by
the surface of the wafer 10, generating a standing light wave
corresponding to the thickness of the resist film. Thus, the dimensions of
the resist pattern vary according to the local thickness of the resist,
deteriorating the resist dimension accuracy.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a projection
exposure apparatus preventing deterioration in the resist pattern accuracy
due to a standing light waves.
Another object of the present invention is to provide a polarizer
preventing generation of standing light waves.
In order to achieve the above object, according to one aspect of the
present invention, there is provided a projection exposure apparatus which
comprises a light source, a condenser lens for condensing an illumination
light emanating from the light source on a mask on which a circuit pattern
has been formed, a projection lens for condensing on a surface of a wafer
the illumination light which has passed through the mask, and a polarizer
disposed on the surface of a pupil of the projection lens for converting
the illumination light converged onto the surface of the wafer into
polarized light having radial planes of polarization with respect to an
optical axis of the projection lens.
According to another aspect of the present invention, there is provided a
polarizer which comprises a first conical lens having a concave form, and
a second conical lens having a convex form and made of a material having
an index of refraction different from the index of refraction of the first
conical lens, the second conical lens being fitted into a concave portion
of the first conical lens. The polarizer converts incident light parallel
to axes of the first and second conical lenses into polarized light having
radial planes of polarization with respect to the axes.
According to another aspect the present invention, a polarizer comprises a
plurality of linearly polarizing filters each having a fan-like shape
having a predetermined central angle, the linearly polarizing filters
being disposed in a radial fashion about a common center forming a disk
shape, the linearly polarizing filters passing therethrough polarized
light having planes of polarization along radial directions of the disk.
The polarizer converts incident light parallel to a central axis which
passes through the common center and is perpendicular to the disk into
polarized light having radial planes of polarization with respect to the
central axis.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an optical system of a projection exposure apparatus according
to a first embodiment of the present invention;
FIG. 2 is a plan view of a polarizer employed in the first embodiment;
FIG. 3 shows how a light is converged onto a wafer in the first embodiment;
FIG. 4 shows the relationship between the angle of incidence and the
reflectances of p-polarized light and of an s-polarized light;
FIG. 5 is a perspective view of a polarizer employed in a second embodiment
of the present invention;
FIG. 6 is a plan view of a polarizer employed in a third embodiment of the
present invention;
FIG. 7 shows a change-over device employed in a fourth embodiment of the
present invention; and
FIG. 8 shows an optical system of a conventional projection exposure
apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention will be described below with reference
to the accompanying drawings.
A first embodiment of the present invention will now be described with
reference to FIGS. 1 through 4.
Referring first to FIG. 1, an exposure apparatus includes a lamp house 11
emanating illumination light, a mirror 12, a fly's eye lens 13 disposed
front of the lamp house 11 with the mirror 12 between the lamp house 11
and the fly's eye lens 13, an aperture member 14 located in front of the
fly's eye lens 13, a relay lens 15A, a stop 16, a mirror 17, a condenser
lens 15B, a photo mask 18 on which a circuit pattern is formed, a
projection lens 19, and a wafer with the projection lens 19 between the
mask 18 and the wafer 20. A polarizer 21 is disposed on the surface of the
pupil of the projection lens 19 in order to convert the illumination light
into polarized light having radial polarization with respect to the
optical axis of the projection lens 19.
The operation of this embodiment will be described below. The illumination
light emanating from the lamp house 11 reaches the fly's eye lens 13
through the mirror 12, and is split into regions corresponding to
individual lenses 13a of the fly's eye lens 13. The light rays emanating
from the individual lenses 13a pass through the aperture portion of the
aperture member 14, the relay lens 15A, the stop 16, the mirror 17 and the
condenser lens 15B, and illuminate the entire surface of the exposure area
of the mask 18. Therefore, on the surface of the mask 18, the light rays
from the individual lenses 13a of the fly's eye lens 13 are laid on top of
one another, and the mask surface is thus illuminated uniformly. The light
which has passed through the mask 18 in the manner described above, is
converted to polarized light by the polarizer 21 disposed on the surface
of the pupil of the projection lens, 19, and then reaches the wafer 20. A
resist film on the surface of the wafer 20 is exposed to the illumination
light, whereby transfer of the circuit pattern is achieved.
FIG. 2 is a plan view of the polarizer 21 which converts the illumination
light which passes therethrough into polarized light having the radial
planes of polarization indicated by bidirectional arrows A. Since the
polarizer 21 is located on the surface of the pupil of the projection lens
19, if the circuit pattern on the mask 18 has parallel lines close to the
a resolution limit of the lens 19, a light source image S.sub.0 of the
zeroth order and light source images S.sub.1 and S.sub.2 of the .+-. first
order are formed on the polarizer 21, as shown in FIG. 2.
The rays of illumination light which have been converted into polarized
light by the polarizer 21 are converged onto the wafer 20 with radial
planes of polarization with respect to an optical axis L, as shown in FIG.
3. In other words, the illumination light is incident on a principal plane
20a of the wafer 20 in the form of p-polarized light.
The reflectance Rp of the p-polarized light and the reflectance Rs of the
s-polarized light when the angle of incidence is .theta.i and the angle of
transmission is .theta.t are given by the following equations:
Rp=tan.sup.2 (.theta.i-.theta.t)/ tan.sup.2 (.theta.i+.theta.t)
Rs=sin.sup.2 (.theta.i-.theta.t)/ sin.sup.2 (.theta.i+.theta.t)
FIG. 4 shows the relation between the reflectances of the p-polarized light
and s-polarized light and the angle of incidence on an inclined surface
made of a resist having an index of refraction n of 1.68. The reflectance
of the p-polarized light is smaller than that of the s-polarized light on
the whole. Particularly, the reflectance of the p-polarized light is
reduced to zero when the angle of incidence is the angle of polarization.
In the example shown in FIG. 4, the angle of polarization is about 60
degrees. The broken line in FIG. 4 indicates the reflectance of
non-polarized light.
In the projection exposure apparatus shown in FIG. 1, since the
illumination light is incident on a single spot on the surface of the
wafer 20 at an convergent angle of about 30 degrees, if an inclined
surface having an inclination angle of, for example, 40 degrees is present
on the surface of the wafer 20, the actual angle of incidence at which the
light is incident on the inclined surface ranges between 10 and 70
degrees. Therefore, the area obtained by integrating the curve
representing the reflectance in FIG. 4 in the range of angle of incidence
from 10 degrees and 70 degrees corresponds to the intensity of light which
is reflected by the inclined surface having the inclination angle of 40
degrees. More specifically, when a p-polarized light alone illuminates the
resist for exposure, the intensity of the light which is reflected by the
inclined surface corresponds to the area indicated by a hatched portion in
FIG. 4, and is thus less than half of the intensity of light reflected
when non-polarized light is used.
Thus, when the polarizer 21 is disposed so that the illumination light is
incident on the wafer 20 in the form of p-polarized light with respect to
the principal plane 20a of the wafer 20, the intensity of light reflected
by the surface of the wafer 20 is reduced, thus restricting the generation
of the standing light waves caused by the reflected light and preventing
variations in the dimensions in the resist which would occur according to
the local thickness of the resist film.
As a polarizer for providing radially polarized light, a polarizer 41 shown
in FIG. 5 may also be employed. The polarizer 41 consists of a first
conical lens 42 having a concave form, and a second conical lens 43 having
a convex form. The concave first conical lens 42 has a conical concave
portion 42a, and the second conical lens 43 is fitted into this concave
portion 42a. In addition, the first and second conical lenses 42 and 43
are made of materials having different indexes of refraction. Therefore,
light beam L1 and L2 incident on the polarizer 41 parallel to a central
axis B of the first and second conical lenses 42 and 43 are separated into
p-polarized light and s-polarized light at the contact surface of the two
conical lenses 42 and 43. The p-polarized light propagates straight ahead
while the s-polarized light is reflected in the radial direction of the
cones. In other words, the polarizer 41 acts as a polarized beam splitter.
It is, however, to be noted that the contact surface between the conical
lenses 42 and 43 has a conical form and hence the light which is incident
on the polarizer 41 parallel to the central axis B and then propagates
straight ahead within the polarizer 41 is converted into polarized light
having the radial planes of polarization with respect to the central axis
B.
Also, a polarizer 51 shown in FIG. 6 may be employed. The polarizer 51
consists of a plurality of fan-shaped, i.e., shaped as segments of a
circle, linearly polarizing filters 52 which are disposed in a radial
fashion around a common center C of the filters 52 so that the entire
shape of the polarizer 51 is a circular. The filters 52 are disposed such
that they pass therethrough polarized light having planes of polarization
in the radial direction, as indicated by bidirectional arrows in FIG. 6.
Consequently, light incident on the polarizer parallel to the central axis
which passes through the common center C and is perpendicular to the disk
is converted into polarized light having radial planes of polarization
with respect to the central axis.
A polarizer need not be fixedly disposed on the surface of the pupil of the
projection lens 19 but be detachably provided. FIG. 7 shows a device used
for such an arrangement. A disk-shaped base plate 30 has first and second
open portions 30a and 30b. A polarizer 31 for providing radially polarized
lights is mounted in the first open portion 30a, and no polarizer is
provided in the second open portion 30b. The base plate 30 is provided in
such a manner as to be rotatable around a central portion 32 thereof. It
is therefore possible to selectively locate either of the open portions
30a and 30b on the optical path of an illumination light by rotating the
base plate 30. More specifically, polarized light having radial planes of
polarization with respect to the optical axis are obtained by selecting
the open portion 30a, and a non-polarized illumination light is obtained
by selecting the open portion 30b. Thus, selection between polarized light
and non-polarized light is made possible for exposure according to an
exposure process.
If a reflection-preventing film is formed on the surface of each of the
polarizers 21, 31, 41 and 51, generation of the standing light waves can
be further restricted, thus improving the resist pattern accuracy.
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