Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2001-03-30
2003-10-21
Rutledge, D (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S071000, C359S494010
Reexamination Certificate
active
06636295
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to an exposure apparatus and, more particularly, to an exposure apparatus for use in the manufacture of devices such as semiconductor devices (e.g., IC's or LSI's), image pickup devices (e.g., CCD's), display devices (e.g., liquid crystal panels), or magnetic heads, for example.
In projection exposure apparatus for the manufacture of semiconductor devices such as IC's or LSI's, increases in density (integration) of a semiconductor device have required an extraordinarily high optical performance of an exposure apparatus to assure a smallest linewidth of 0.2 micron on a wafer surface.
Generally, when a circuit pattern of a reticle is projected by a projection optical system onto a wafer surface (projection plane being equivalent to an image plane). the resolvable linewidth of the circuit pattern is largely influenced by the wavelength (&lgr;) of exposure light and the numerical aperture (NA) of the projection optical system as well as the quality of uniformness of an illuminance distribution upon the projection plane.
Particularly, when the smallest linewidth is on an order of 0.2 micron or less, a suitable light source for exposure will be a KrF excimer laser (&lgr;=193 nm). As regards the uniformness of an illuminance distribution on a projection plane in an exposure apparatus using such a laser light source, it should desirably be about 1% or less.
FIG. 8
is a schematic view of a main portion of an optical system of a projection exposure apparatus.
In the drawing, a laser light beam (pulse light) from an excimer laser
1
is transformed by a beam shaping optical system
3
into a desired shape, and it is then incident on a light entrance surface of an optical integrator
4
having small lenses arrayed two-dimensionally. The light is divided and collected by the optical integrator, whereby a plurality of secondary light sources are produced adjacent to a light exit surface of the optical integrator
4
. Divergent light beams from these secondary light sources, defined adjacent to the light exit surface of the integrator, are collected by a condenser lens
6
, such that they are superposed one upon another to illuminate an aperture of a field stop
9
which is disposed at a position optically conjugate with the circuit pattern surface of the reticle
12
placed on the surface to be illuminated.
Light beams from the field stop
9
are directed by way of a lens
10
a
, a deflecting mirror
11
, and a lens
10
b
, to illuminate the same region (circuit pattern) of the reticle
12
. The lenses
10
a
and
10
b
cooperate to define an imaging lens system, and they function to project an image of the aperture of the stop
9
, having a uniform light intensity distribution, onto the circuit pattern of the reticle
12
.
The thus illuminated circuit pattern of the reticle
12
is projected, in a reduced scale, by a projection optical system
13
onto a wafer
15
which is held on an X-Y-Z stage
16
. With this procedure. a resist applied to the wafer
15
surface is exposed with a reduced image of the circuit pattern. The X-Y-Z stage
16
is movable in an optical axis direction (Z) and in two directions (x and y) orthogonal thereto. The optical system
13
comprises lens systems
13
a
and
13
b
, and an aperture stop
14
between the lens systems
13
a
and
13
b
. This aperture stop
14
functions to determine the position, shape and size (diameter) of the pupil of the optical system
13
.
Denoted at
5
is an aperture stop of an illumination optical system, and it has an aperture having a function for determining the size and shape of an effective light source which comprises the above-described secondary light sources The aperture of this aperture stop
5
is imaged by an imaging optical system, comprising lenses
6
,
10
a
,
10
b
and
13
a
, upon the aperture of the aperture stop
14
, namely, upon the pupil plane.
Denoted at
7
is a beam splitter which comprises a glass plate with or without a dielectric film formed thereon. The glass plate
7
is disposed obliquely with respect to the optical axis, between the lens
6
and the field stop, and it functions to reflect a portion of each of the plural light beams from the integrator
4
, toward a light quantity detecting means
8
. On the basis of the light quantities of each of the pulse lights as detected by the light quantity detecting means
8
, the light quantities of pulse lights sequentially impinging on the wafer
15
surface are detected. By using the thus obtained light quantity data, the exposure amount of the resist on the wafer
15
is controlled.
Denoted at
101
is a quarter waveplate which is a birefringent plate having its crystal axis tilted by 45 deg. with respect to the polarization direction (y direction) of linearly polarized light from the excimer laser
1
. It functions to convert the linear polarization of this laser light into circular polarization.
It has been considered that, once an optical system such as shown in
FIG. 8
is assembled, even if the polarization direction of the laser light from the excimer laser
1
shifts slightly, owing to the polarization conversion function of the quarter waveplate
101
, the glass plate
7
can be continuously irradiated with approximately circularly polarized light in which a polarization component of laser light (P-polarized light) parallel to the sheet of the drawing and a polarization component (S-polarized light) perpendicular to the sheet of the drawing have approximately the same intensities, such that the light quantity ratio between the light (exposure light for wafer
15
) passed through the glass plate
7
and the light (to be detected by the light quantity detecting means
8
) reflected by the glass plate
7
can be maintained substantially constant.
FIGS. 10A and 10B
illustrate the practical assembling procedure for the integrator
4
.
FIG. 10A
is a sectional view of the integrator
4
along a section containing the optical axis.
FIG. 10B
is a sectional view of the integrator, along a section perpendicular to the optical axis. As shown in
FIG. 10A
, the integrator
4
is an aggregation of bar lenses each having convex spherical surfaces at opposite ends thereof. Also, as shown in
FIG. 10B
, the bar lenses having spherical surfaces at their opposite ends are accumulated and then, by pressing them in two directions, they are bonded and secured, whereby the integrator
4
is assembled.
In this assembling procedure, each bar lens of the integrator
4
may be distorted as forces are applied thereto in radial directions. Thus, these bar lenses may have different birefringent properties. Additionally, even inside each bar lens, the birefringence may differ in dependence upon the position. For these reasons, in the structure of
FIG. 8
, there is a possibility that, even if the laser light as it enters the integrator
4
is circularly polarized light, the light as it emerges from the integrator
4
is not circularly polarized light. Also, light beams emitted from the bar lenses may be in different polarization states.
This raises a problem that, in the structure of
FIG. 8
, due to a change in polarization direction of the linearly polarized laser light from the excimer laser
1
and to the polarization dependency of the reflection factor (transmission factor) of the glass plate
7
(beam splitter), the light quantity ratio between the light (exposure light for wafer
15
) passed through the glass plate
7
and the light (to be received by detecting means
8
) reflected by the glass plate
7
cannot be maintained constant in a required range, such that precise exposure control cannot be done stably.
The light entrance surface of each bar lens is optically conjugate with the wafer
15
surface. Because these bar lenses may have different birefringent properties, due to the polarization light transmission characteristic of the glass plate
7
or the mirror
11
, there may occur illuminance non-uniformess. Further, if the polarization state of the laser light or
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