Optical: systems and elements – Holographic system or element – For reconstructing image
Reexamination Certificate
2002-02-06
2003-12-02
Juba, John (Department: 2872)
Optical: systems and elements
Holographic system or element
For reconstructing image
C359S012000, C359S900000, C359S197100, C430S002000, C430S311000, C430S397000, C356S505000
Reexamination Certificate
active
06657756
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for reconstructing images from total internal reflection holograms and particularly for increasing the effective reconstruction efficiency of total internal reflection holograms.
BACKGROUND ART
The principles of total internal reflection (TIR) holography are described in U.S. Pat. No. 4,857,425 since which time many advances have been made to maximise the advantages of TIR holography when applied to microelectronics manufacturing. Other prior art references are U.S. Pats. Nos. 4,917,497, 4,966,428, 5,187,372, 5,640,257, 5,695,894 and European Patent Appl. No. 98303677.
U.S. Pat. No. 4,966,428 discloses an apparatus based on TIR holography for manufacturing integrated circuits wherein a holographic image formed in a first recording medium on a glass plate is replayed by scanning it with a narrow collimated beam of light so as to print the image into a second recording medium on a silicon wafer arranged in proximity and parallel to the first recording medium. A “zigzag” or raster scanning scheme is disclosed wherein the beam scans in lines of alternate direction with a partial overlap between successive scan lines whereby the complete holographic image recorded in the first recording medium is printed into the second recording medium. This scanning technique provides a uniform time-integrated illumination of the hologram and also maximises the power of the replay beam arriving at the hologram in order to minimise the replay time.
U.S. Pat. No. 5,695,894 discloses a TIR holographic system for changing the scale of a pattern reconstructed from a TIR hologram wherein a first computer controlled stage produces a raster scan of the illumination beam across the hologram while a second computer controlled stage simultaneously displaces a photoresist coated wafer relative to the hologram in orthogonal directions in the plane of the wafer. Changing the scale of the pattern printed from the hologram onto a wafer is important for a multi-level process in which a pattern has to be printed onto a wafer with a pattern already printed on it such that the two patterns are accurately registered, or aligned, with respect to each other. In such cases there may exist small differences in the corresponding dimensions of the respective patterns arising from, for example, an irreversible expansion of the wafer caused by a high-temperature, post-exposure treatment. To counteract this so that accurate overlay is achieved over the complete pattern, the difference in scale between the patterns on the wafer and in the hologram is first determined by measuring the relative positions of reference alignment marks included alongside the respective patterns and then the difference is compensated by providing a relative motion between the wafer and hologram as the illumination beam scans during the printing. Since this procedure produces some loss in resolution of printed image, U.S. Pat. No. 5,695,894 additionally teaches that an amount of convergence or divergence be introduced into the illumination beam so that the image instantaneously reconstructed from the hologram by the scanning beam is itself magnified or demagnified by the measured amount. For this purpose an apparatus is disclosed comprising a beam decollimator and a prism through which the beam passes before it illuminates the hologram. Adjustment of the separation of lenses in the beam decollimator changes the degree of convergence or divergence of the beam whereas the prism compresses the beam in one plane so that the magnification or demagnification of the image reconstructed from the hologram is rendered isotropic.
EP-A-0421645 discloses the combination of a raster scanning of the illumination beam in a TIR holographic system with a continuous measurement of the local separation of the hologram and wafer surfaces at the location of the beam as it scans and a continuous adjustment of that separation to the focal distance of the image reconstructed from the hologram in order that the image is accurately printed in focus on the wafer surface. This technique overcomes the problem caused by the unevenness of wafer surface and the limited depth-of-focus of high-resolution images.
European Patent Application No. 98303677 discloses an enhancement of the previous method to further improve the accuracy with which the image is printed in focus on the wafer surface wherein the dimension of the collimated illumination beam in the scanning direction is additionally compressed relative to its dimension in the stepping direction.
A further important consideration for the application of TIR holography to microelectronic manufacturing is the time it takes to print a pattern from a hologram onto a wafer or other substrate. Using the raster scan techniques described in the prior art, the time, t, it takes to print a pattern of length, l, and width, w, from a hologram onto a substrate can be estimated as:
t
=
l
w
v
s
equ
.
(
1
)
where v is the scanning speed of the beam and s is the stepping distance between successive scan lines (for this estimation the time it takes for the beam to decelerate, step and then accelerate between scan lines has been neglected and the beam dimensions have been assumed to be negligible compared to the pattern size).
However, the scan speed and step size of the beam during the scanning also have to satisfy the condition:
E
=
η
P
v
s
equ
.
(
2
)
where E is the exposure dose required by the photoresist, P is the power of the laser beam at the output of the laser and &eegr; is the efficiency, or transmission, of the complete optical system between the laser and the wafer surface.
From equs. (1) and (2) it therefore follows that:
t
=
l
w
E
η
P
equ
.
(
3
)
Evaluating this for a pattern of dimensions 30 cm×40 cm, a photoresist of sensitivity 100 mJ/cm
2
, an argon laser of output power 2 W and an optical system efficiency of 50% produces:
t
=
30
×
40
×
0.1
0.5
×
2
=
120
s
=
2
m
i
n
u
t
e
s
Although this is acceptable for industrial application it would be advantageous if it could be reduced.
It is clear from equ. (3) that the time it takes to print a pattern of certain dimensions may be reduced by any of i) increasing the sensitivity of the resist, ii) increasing the transmission of the optical system, and iii) increasing the laser power. However, i) is usually difficult because photoresists are generally mature and optimised products, ii) is difficult because of the properties of TIR holograms, and iii) although feasible, in that an additional or more powerful laser source can be added to the system, may be unattractive because of operating costs.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method and apparatus to reduce the time required to print an image from a total internal reflection hologram, in particular it is an object to increase the effective reconstruction efficiency of a total internal reflection hologram.
According to the invention there is provided a method for reconstructing an image from a total internal reflection hologram that includes the steps of
arranging the hologram in relation to a first face of a coupling body;
generating a substantially collimated illumination beam;
directing the illumination beam through a second face of the coupling body so that it reconstructs the image recorded in the hologram;
recycling at least once the light in the illumination beam that does not reconstruct the imagine wherein the light reflected from the hologram is directed out of the coupling body through the second or a third face and is subsequently redirected as a recycled beam through the second face of the coupling body such that it also reconstructs the image recorded in the hologram;
scann
Assaf Fayez
Holtronic Technologies Plc
Juba John
LandOfFree
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