Illumination – Light fiber – rod – or pipe – Specific material
Reexamination Certificate
2000-02-16
2003-04-29
Tso, Laura K. (Department: 2875)
Illumination
Light fiber, rod, or pipe
Specific material
C362S551000, C362S259000
Reexamination Certificate
active
06554464
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to systems to uniformize illumination, and in particular to such systems employing a light tunnel as an optical integrator.
BACKGROUND OF THE INVENTION
Achieving uniform illumination is necessary in numerous optical applications, including microscopy, and various other forms of imaging, such as photolithography. Many illumination uniformity techniques have evolved over the years for the variety of imaging applications. With the advent of the laser in the 1960's, new techniques have to be developed to deal with non-uniformities arising from interference effects due to the coherent nature of laser light.
In many applications, such as microlithography, or materials processing, it is desirable to illuminate an object with a light beam having an intensity distribution that is both macroscopically and microscopically spatially 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 light used. In many of these applications, it is further desirable to use a pulsed laser source.
However, the output of most lasers is spatially non-uniform. Macroscopically, the laser output often has a gaussian-like profile. A great deal of effort has gone into fabricating lasers with more “square” profiles, but even these are only uniform to +/−5-10% over limited areas. As a result, it is often necessary to use auxiliary optics in conjunction with the laser source in an attempt to make the illumination more uniform.
The greatest challenge in producing uniform illumination from a laser source arises from the inherent temporal and spatial coherence of the laser source. When two incoherent beams overlap, the intensities of the two beams are added. However, when two coherent beams overlap, the electric fields of the two beams are added and can produce interference patterns. (fringes) that are absent in an incoherent illumination system. As a result, the traditional methods used to produce uniform illumination with incoherent sources are not suitable for laser sources. This is particularly true where the application utilizes only one or a few pulses so that time-averaging to achieve uniformization is not a practical option.
FIGS. 1
a
and
1
b
show schematic cross-sectional diagrams of conventional illumination uniformizer apparatus
4
and
8
, respectively, for achieving macroscopic illumination through the use of a light tunnel. Apparatus
4
and
8
both include a pulsed light source
10
emitting pulses of coherent light
12
, a condenser optical system
16
, and a light tunnel
24
.
In apparatus
4
(
FIG. 1
a
), light tunnel
24
comprises a hollow light tunnel body
30
with a central axis A
1
, an upper wall
34
and a lower wall
38
, each with a highly reflective inner surface
42
and
44
, respectively, an input end
50
and an output end
54
. The latter includes upper and lower edges
60
and
62
, respectively. An exemplary material for walls
34
and
38
of hollow light tunnel body
30
is any material that is coated with a highly reflective surface such as metallic coatings or dielectric coatings.
In apparatus
8
(
FIG. 1
b
), light tunnel
24
comprises a solid light tunnel body
80
having a central axis A
2
, an index of refraction n
1
, upper and lower surfaces
84
and
86
, respectively, which reflect light via total internal reflection (TIR) (as such, these surfaces can be considered reflective surfaces), and input and output ends
90
and
94
, respectively. Output end
94
includes upper and lower edges
106
and
108
, respectively. Solid light tunnel body
80
works best when it is made from an optically transparent material with a high index of refraction, such as glass, fused quartz or Al
2
O
3
.
Apparatus
4
and
8
are commonly used to achieve macro-uniformities of approximately +/−1% uniformity. However, because of the coherent nature of lasers, these illumination methods produce significant micro-non-uniformities.
With continuing reference to
FIGS. 1
a
,
1
b
, above, coherent light
12
from the light source
10
is condensed by condenser optical system
16
and enters light tunnel
24
at entrance end
50
or
90
over a range of angles. Two light rays
100
and
102
are shown, with light ray
100
representing a central, straight-through ray, and ray
102
representing a ray having a single reflection (bounce) off inner surface
44
or
86
. Other rays having more bounces are typically present, but are not shown. Light rays
100
and
102
then exit the light tunnel at output end
54
or
94
at various angles and output end positions. “Edge rays” are the light rays that exit the light tunnel at or near edges
60
and
62
, or
106
and
108
, of the output end.
A phenomenon called “edge-ringing” occurs when a coherent edge ray “folds” or “reflects” and interferes with itself. In other words, edge-ringing occurs where a reflected edge ray (e.g., ray
102
) overlaps (interferes) with a non-reflected edge ray. This edge-ringing is related to the spatial coherence of light source
10
. The greater the spatial coherence of light source
10
, the greater the edge-ringing. Here, “ringing” refers to the damped sinusoidal variation in the irradiance distribution of light I(x) as a function of the distance x across output end
54
or
94
of light tunnel
24
, such as shown in
FIG. 2
, where “x” is the distance from the edge of the light tunnel towards the center. The vertical dashed line corresponds to the edge of the light tunnel edge (e.g., edge
106
) or a knife-edge placed at the output end
54
or
94
. Larger values of “x” extend away from the edge and towards the center of the light tunnel.
Two types of edge-ringing can occur in light tunnels. The first type, described above, is caused by coherent light rays (edge rays) interacting with other rays near edges
60
and
62
or
106
and
108
at the output end of a light tunnel. The second type is coherent light rays interacting with a “knife-edge” placed near the center of output end
54
or
94
of light tunnel
24
, as mentioned above. For example, a knife-edge might be placed at output end
54
or
94
to reduce the size of the downstream illumination field (not shown).
Traditionally, use of light tunnels in combination with spatially coherent light sources does not work well because the coherence of the laser beam leads to non-uniformities at the output of the light tunnel. The coherence of the laser produces both interference fringes in the light tunnel (from overlapping orders) and ringing at the edges of the light tunnel, which results in illumination non-uniformity.
There are several prior art designs for reducing interference effects in light tunnels. Unfortunately, each has significant shortcomings.
U.S. Pat. No. 4,744,615, entitled “Laser beam homogenizer,” describes an apparatus wherein a coherent laser beam having a possibly non-uniform spatial intensity distribution is transformed into an incoherent light beam having a substantially uniform spatial intensity distribution by homogenizing the laser beam with a light tunnel (a transparent light passageway having flat internally reflective side surfaces). It has been determined that when the cross-section of the 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) will have a substantially uniform intensity distribution and will be incoherent only when the aspect ratio of the tunnel (length divided by width) equals or exceeds the co-tangent of the input beam divergence angle theta and when W
min
=L
coh
(R+sqrt(1+R
2
))>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. A shortcoming of this technique, howe
Fong Yu Chue
Hawryluk Andrew M.
Stites David G.
Wang Weijian
Jones Allston L.
Tso Laura K.
Ultratech Stepper, Inc.
LandOfFree
Apparatus for and method of reducing or eliminating... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus for and method of reducing or eliminating..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus for and method of reducing or eliminating... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3017353