Optical waveguides – Optical imaging tunnel
Reexamination Certificate
2000-06-15
2002-02-12
Healy, Brian (Department: 2874)
Optical waveguides
Optical imaging tunnel
C385S005000, C385S007000, C385S146000
Reexamination Certificate
active
06347176
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of illumination, and in particular to light tunnels used in optical systems such as illuminators to achieve uniform illumination.
BACKGROUND OF THE INVENTION
Achieving uniform illumination is necessary in numerous optical applications, and is particularly important in the fields of microscopy, and the relatively new field of photolithography. Many illumination uniformization techniques have evolved over the years to meet the increasing demands on illumination uniformity. With the advent of the laser in the 1960's, new techniques have been developed to deal with illumination non-uniformities arising from interference effects due to the coherent nature of laser light.
In certain applications, such as photolithography, materials processing and the like, it is desirable to illuminate an object with light having an intensity distribution that is both macroscopically and microscopically uniform. Here, macroscopic means dimensions comparable to the size of the object being illuminated and microscopic means dimensions comparable to the size of the wavelength of the illumination. In many of these applications, it is further desirable to use a pulsed laser source and to have a spatially uniform intensity distribution. However, the light output of a pulsed laser source is spatially non-uniform. Macroscopically, the light beam often has a gaussian-like cross-section (“profile”). A great deal of effort has gone into fabricating lasers that emit a beam having a more uniform profile, but even these are only uniform to +/−10% over limited areas. As a result, it is often necessary to use auxiliary optics with a pulsed laser light source to make the illumination more uniform.
The challenge in producing a spatially uniform intensity distribution from a laser source arises from its inherent temporal and spatial coherence. When two incoherent light beams overlap, the intensities of the two beams add. However, when two coherent beams overlap, the electric fields of the two beams add, which can produce an intensity having an interference pattern comprising fringes not present in an incoherent illumination system. As a result, the traditional methods of producing uniform illumination with incoherent sources are typically unsuitable for coherent sources like lasers.
With reference to
FIGS. 1A and 1B
, there are shown schematic cross-sectional diagrams of conventional illumination uniformizer apparatus
10
and
70
for achieving uniform macroscopic illumination. The conventional uniformizer apparatus works well for incoherent (i.e., “non-laser”) sources, but is inadequate for coherent (i.e., “laser”) sources. For many applications, apparatus
10
of
FIG. 1A
comprises, along an optical axis A, a laser light source
16
emitting short pulses of coherent light L (e.g., 10 ns/pulse) comprising light rays R
1
and R
2
, a condenser optical system
24
, and a hollow light tunnel
30
with an interior region
32
, upper and lower walls
36
and
40
, respectively, and corresponding highly reflective inner surfaces
36
i
and
40
i
and outer surfaces
36
o
and
40
o
respectively. Light tunnel
30
further includes an input end
50
adjacent optical system
24
, and an output end
56
at the distal end of tunnel
30
from optical system
24
. A material often used for walls
36
and
40
of hollow light tunnel
30
is quartz, which is often coated with a high-reflectivity material such as a metal or a dielectric.
With reference to
FIG. 1B
, apparatus
70
includes the same elements, except that instead of hollow light tunnel
30
, apparatus
70
includes a solid light tunnel
80
having an index of refraction n
1
, upper and lower surfaces
86
and
90
, an input end
94
and an output end
98
. A material often used for solid light tunnel
70
is fused quartz, which has a refractive index of about 1.5 in the visible wavelengths. Apparatus
10
and
70
are commonly used with incoherent sources to achieve better than +/−1% uniformity at their respective output ends
56
and
98
.
Because of the coherent nature of light source
16
, intersecting light rays R
1
and R
2
passing through the light tunnel produce a light intensity distribution in the form of a standing sinusoidal wave pattern P
s
at the output ends
56
and
98
of light tunnels
30
and
80
, respectively. Here, two rays R
1
and R
2
and a central ray RS are shown for the sake of illustration. The period of standing wave pattern P
s
is determined by the wavelength of the laser light and the angle between intersecting light rays R
1
and R
2
, between rays R
1
and RS, and between rays R
2
and RS. In practice, there are many pairs of intersecting light rays (depending on the number of reflections), with each pair producing a standing wave pattern. The length and width of light tunnels
30
and
80
define the angle between intersecting rays R
1
, R
2
, and RS and the path length difference (i.e., the phase) between the intersecting rays determines the relative position of the irradiance maxima in standing wave pattern P
s
.
A prior art technique for eliminating interference effects (e.g., standing wave pattern P
s
) to achieve uniform illumination using a light tunnel is the breaking of the coherent light into packets and adding the packets incoherently, or by rotating a random diffuser between the light source and the light pipe entrance.
There are several U.S. patents directed to such techniques for eliminating interference effects that are relevant to light tunnel illumination systems. For example, U.S. Pat. No. 4,744,615, entitled “LASER RAY HOMOGENIZER,” describes a coherent laser ray having a possibly non-uniform spatial intensity distribution that is transformed into an incoherent light ray having a substantially uniform spatial intensity distribution by homogenizing the laser ray with a light tunnel. When the cross-section of the light tunnel is a polygon (as preferred) and the sides of the tunnel are all parallel to the axis of the tunnel (as preferred), the laser light at the exit of the light tunnel (or alternatively at any image plane with respect thereto) has a substantially uniform intensity distribution and is incoherent only when the aspect ratio of the tunnel (length divided by width) equals or exceeds the co-tangent of the input ray divergence angle theta and when W
min
=>2RL
coh
, where W
min
is the minimum required width for the light tunnel, L
coh
is the effective coherence length of the laser light being homogenized and R is the chosen aspect ratio for the light tunnel. This approach restricts the ratio of the tunnel's length to width and consequently, the number of bounces for the light rays. However, the number of bounces affects the “macro-uniformity” of the output of the tunnel. As a result, this approach can impact the macro-uniformity at the output of the homogenizer tunnel.
U.S. Pat. No. 5,224,200, entitled “COHERENCE DELAY AUGMENTED LASER RAY HOMOGENIZER,” describes a system in which the geometrical restrictions on a laser ray homogenizer are relaxed by using a coherence delay line to separate a coherent input ray into several components each having a path length difference equal to a multiple of the coherence length with respect to the other components. The components recombine incoherently at the output of the homogenizer, and the resultant ray has a more uniform spatial intensity suitable for microlithography and laser pantogography.
U.S. Pat. No. 4,511,220, entitled “LASER TARGET SPECKLE ELIMINATOR,” describes an apparatus for eliminating the phenomenon of speckle with regard to laser light reflected from a distant target whose roughness exceeds the wavelength of the laser light. The apparatus includes a half plate wave member, a first polarizing ray splitter member, a totally reflecting right angle prism, and a second polarizing ray splitter member, all of which are in serial optical alignment, that are used in combination to convert a linearly (i.e., vertically) polarized light ray, which is
Grek Boris
Hawryluk Andrew M.
Stites David G.
Healy Brian
Jones Allston L.
Ultratech Stepper, Inc.
Wood Kevin S
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