Polarization beam splitter for photolithography

Details Polarization beam splitter for photolithography Polarization beam splitter for photolithography

2002-10-04

2004-01-20

Nguyen, Thong (Department: 2872)

Optical: systems and elements

Having significant infrared or ultraviolet property

Multilayer filter or multilayer reflector

C359S490020, C359S490020, C359S355000

Reexamination Certificate

active

06680794

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to optics, and in particular, to optics in microlithography.
2. Related Art
Photolithography (also called microlithography) is a semiconductor fabrication technology. Photolithography uses ultraviolet or visible light to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as, diodes, transistors, and integrated circuits, can be fabricated using photolithographic techniques. Exposure systems or tools are used to carry out photolithographic techniques, such as etching, in semiconductor fabrication. An exposure system can include a light source, reticle, optical reduction system, and a wafer alignment stage. An image of a semiconductor pattern is printed or fabricated on the reticle (also called a mask). A light source illuminates the reticle to generate an image of the particular reticle pattern. An optical reduction system is used to pass a high-quality image of the reticle pattern to a wafer. See, Nonogaki et al.,
Microlithography Fundamentals in Semiconductor Devices and Fabrication Technology
(Marcel Dekker, Inc.: New York, N.Y. 1998), incorporated in its entirety herein by reference.
Integrated circuit designs are becoming increasingly complex. The number of components and integration density of components in layouts is increasing. Demand for an ever-decreasing minimum feature size is high. The minimum feature size (also called line width) refers to the smallest dimension of a semiconductor feature that can be fabricated within acceptable tolerances. As result, it is increasingly important that photolithographic systems and techniques provide a higher resolution.
One approach to improve resolution is to shorten the wavelength of light used in fabrication. Increasing the numerical aperture (NA) of the optical reduction system also improves resolution. Indeed, commercial exposure systems have been developed with decreasing wavelengths of light and increasing NA. For example, Silicon Valley Group Lithography (SVG), formerly Perkins-Elmer, has introduced a number of ultra-violet exposure systems with catadioptric types of optical reduction systems and step-and-scan reticle/wafer stage systems. See, e.g., Nonogaki, at section 2.5.5, pp. 20-24. These UV exposure systems available from SVG have light sources operating at a wavelength of 248 nanometer (nm) with an associated NA of 0.5 or 0.6, and at a wavelength of 193 nm with an associated NA of 0.5 or 0.6. However, light at wavelengths equal to or less than 170 nm, and for example at 157 nm, has not been made available in photolithographic applications for semiconductor fabrication. A numeric aperture greater than 0.6, and for example at 0.75, is also not available.
Catadioptric optical reduction systems include a mirror that reflects the imaging light after it passes through the reticle onto a wafer. A beam splitter cube is used in the optical path of the system. A conventional beam splitter cube, however, transmits 50% of input light and reflects 50% of the input light. Thus, depending upon the particular configuration of optical paths, significant light loss can occur at the beam splitter.
In UV photolithography, however, it is important to maintain a high light transmissivity through an optical reduction system with little or no loss. Exposure time and the overall semiconductor fabrication time depends upon the intensity or magnitude of light output onto the wafer. To reduce light loss at the beam splitter, a polarizing beam splitter and quarter-wave plates are used.
FIGS. 1A and 1B
illustrate an example conventional polarizing beam splitter cube
100
used in a conventional catadioptric optical reduction system. Polarizing beam splitter cuber
100
includes two prisms
110
,
150
, and a coating interface
120
. Prisms
120
,
150
are made of fused silica and are transmissive at wavelengths of 248 nm and 193 nm. Coating interface
120
is a multi-layer stack. The multi-layer stack includes alternating thin film layers. The alternating thin film layers are made of thin films having relatively high and low indices of refraction (n
1
and n
2
). The alternating thin film layers and their respective indices of refraction are selected such that the MacNeille condition (also called Brewster condition) is satisfied. In one example, the high index of refraction thin film material is an aluminum oxide. The low index of refraction material is aluminum fluoride. A protective layer may be added during the fabrication of the stack. Cement or glue is included to attach one of the alternating layers to a prism
150
at face
152
or to attach the protective layer to prism
110
at face
112
.
As shown in
FIG. 2A
, the MacNeille condition (as described in U.S. Pat. No. 2,403,731) is a condition at which light
200
incident upon the multi-layer stack is separated into two beams
260
,
280
having different polarization states. For example, output beam
260
is an S-polarized beam, and output beam
280
is a P-polarized beam.
FIG. 2B
shows the advantage of using a polarizing beam splitter in a catadioptric optical reduction system to minimize light loss. Incident light
200
(usually having S and P polarization states) passes through a quarter-wave plate
210
. Quarter wave plate
210
converts all of incident light
200
to a linearly polarized beam in an S polarization state. Beam splitter cube
100
reflects all or nearly all of the S polarization to quarter wave plate
220
and mirror
225
. Quarter wave plate
220
when doubled passed acts like a half waveplate. See, e.g., “Waveplates,” <http://www.casix.com
ew/waveplate.htm, two pages. Quarter wave plate
220
converts the S polarization light to circular polarization, and after reflection from mirror
225
, converts light into P-polarized light. The P-polarized light is transmitted by beam splitter cube
100
and output as a P-polarized beam
290
toward the wafer. In this way, the polarizing beam splitter
100
and quarter wave plates
210
,
220
avoid light loss in a catadioptric optical reduction system that includes a mirror
225
. Note, as an alternative, mirror
225
and quarter wave plate
220
can be positioned at face B of cube
100
rather than at face A and still achieve the same complete or nearly complete light transmission over a compact optical path length.
Polarizing beam splitter cube
100
, however, is not transmissive at wavelengths less than 170 nm. Prisms
120
,
150
are made of fused silica which is opaque at wavelengths less than 170 nm. Similarly, coating interface
120
is also based on the MacNeille condition which is only explicitly described for infra-red wavelengths. Such coatings
120
are not effective at ultraviolet wavelengths less than 170 nm. Cement or glue used in bonding coating interface
120
to fused silica prisms
110
,
150
can degrade when exposed to light at 170 nm or less.
What is needed is a polarizing beam splitter that supports an even higher resolution. A polarizing beam splitter is needed that is transmissive to light at ultraviolet wavelengths equal to or less than 170 nm, and for example at 157 nm. A polarizing beam splitter is needed that can image at high quality light incident over a wide range of reflectance and transmittance angles. A polarizing beam splitter is needed that can accommodate divergent light in an optical reduction system having a numeric aperture at a wafer plane greater than 0.6, and for example at 0.75.
SUMMARY OF THE INVENTION
The present invention provides a ultraviolet (UV) polarizing beam splitter. The UV polarizing beam splitter is transmissive to light at wavelengths equal to or less than 170 nm, and for example at 157 nm. The UV polarizing beam splitter can image at high quality light incident over a wide range of reflectance and transmittance angles. The UV polarizing beam splitter can accommodate divergent light in an optical reduction system having a numeric aperture at a wafer plane greater than 0.6, and for example at 0.75. In different embodiments, the U

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