Optical: systems and elements – Polarization without modulation – Depolarization
Reexamination Certificate
1999-12-23
2001-11-27
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Polarization without modulation
Depolarization
C359S490020, C359S506000, C359S352000
Reexamination Certificate
active
06324003
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally related to optical stress plates that are used for delaying optical wavefronts, and methods for making the same.
2. Related Art
Many semiconductor fabrication systems utilizes photolithography techniques in the fabrication of semiconductor wafers. During fabrication, one or more layers of circuit patterns are built up on a semiconductor wafer. This is accomplished by illuminating a reticle with light, where the reticle contains a desired circuit pattern. The resulting reticle image is then projected onto photosensitive resist that covers the semiconductor wafer. After a series of exposures and subsequent processing, a semiconductor wafer containing the desired circuit pattern is manufactured.
It is well known that the smallest feature that can be fabricated on the semiconductor wafer is limited to the optical wavelength of the light used in the illumination system. It is also well known that the upper limit on circuit clock speeds varies inversely with the size of the semiconductor features. Therefore, the demand for higher clock speeds necessitates that semiconductors have smaller circuit features. Circuit features of 0.25 &mgr;m (micrometers) have been achieved with photolithography systems using light wavelengths of 193 m (nanometers). To achieve geometries below 0.25 &mgr;m, even smaller wavelengths (e.g. 157 nm) must be used.
The illumination system used in photolithography includes various optical components that manipulate light to project a reticle image on the semiconductor wafer. One common component in the illumination system is an optical delay plate (also called a stress plate). Stress plates can be used to delay or retard a light wavefront by a specified amount. Stress plates can also be used to convert the polarization of light from one polarization to another. For example, a ¼ (quarter) wave stress plate that is rotated 45 degrees to the incident light converts linearly polarized light to circularly polarized light and visa-versa. In another example, horizontally polarized light is converted to vertically polarized light by using a ¼ wave stress plate and a mirror. This is done by transmitting the horizontally polarized light through the ¼ wave stress plate to generate circularly polarized (CP) light. The CP light is then reflected off the mirror to reverse the CP polarization. Finally, the reflected CP light is sent back though the ¼ wave stress plate to generate vertically polarized light.
In order for a stress plate to function as desired, it must be fabricated from a material that will transmit sufficient light at the wavelength of interest. Conventional stress plates are made of fused silica or man-made quartz. These conventional materials do not sufficiently transmit light at wavelengths that are below 193 nm. As stated above, the smallest feature that can be fabricated on the semiconductor wafer is limited to the optical wavelength of the light used in the system. As such, photolithography systems that utilize conventional stress plates can manufacture features that are no smaller than approximately 0.25 &mgr;m. Therefore, what is needed is a stress plate that is functional at optical wavelengths that are below 193 nm (including 157 nm) to support the fabrication of semiconductor wafers having circuit features that are smaller than 0.25 &mgr;m.
SUMMARY OF THE INVENTION
The present invention is directed at a Calcium Fluoride (CaF
2
) optical stress plate and a method for making the same. The CaF
2
stress plate has surfaces that lie in CaF
2
cubic planes, and delays an optical wavefront that is incident to a set of cubic planes along a transmission axis. To implement the desired delay, the CaF
2
stress plate has a first index of refraction that is seen by a first field component of the optical wavefront, and a second index of refraction that is seen by a second field component of the optical wavefront. The optical delay of the stress plate is proportional to the differences between the two indexes of refraction. In one embodiment, the first index of refraction is the characteristic index of refraction for CaF
2
material, and the second index of refraction is higher or lower than the characteristic index of refraction. In an alternate embodiment, the first index of refraction is higher than the characteristic index of refraction and the second index of refraction is lower than the characteristic index of refraction.
An advantage of fabricating an optical delay from CaF
2
is that CaF
2
is able to transmit light having wavelengths that are below 193 nm (including wavelengths at 157 nm) with relatively little attenuation when compared to traditional optical materials such as fused silica or man-made quartz. Therefore, CaF
2
stress plates enable semiconductor fabrication systems to produce circuit features of 0.25 &mgr;m and smaller using light wavelengths of 157 nm and below.
The present invention also includes a method of fabricating a CaF
2
stress plate that has a specified optical delay from a sample of CaF
2
. The method includes the step of determining the orientation of the cubic planes for the CaF
2
sample, as the sample is typically oriented along the cleave planes. Next, the sample is processed to generate a CaF
2
plate whose surfaces are oriented in CaF
2
cubic planes. This can be done by cutting a CaF
2
plate from the CaF
2
sample along the identified cubic planes and then polishing the sample to a commercial finish. Next, a compressive or tensile force is applied perpendicular to at least one pair of the cubic plane surfaces, and perpendicular to the transmission axis for the incident optical wavefront. The compressive or tensile force has the effect of modifying the characteristic index of refraction for electromagnetic fields that are oriented along the direction of the force vector. After which, the CaF
2
stress plate effectively has two indexes of refraction, where the amount optical delay is proportional to the difference between the indexes of refraction. Next, the amount of optical delay is measured to determine if the measured delay is sufficiently close to the specified delay. If it is not, then more compression or stress can be applied until the desired delay is achieved. In embodiments of the invention, an applied force of 300 psi (pounds per square inch) to 500 psi causes an optical delay of 90 degrees for the incident optical wavefront.
In an alternate embodiment, shear forces are applied to the CaF
2
plate instead of compressive or tensile forces. The shear forces are applied along mechanical surfaces that are rotated 45 degrees to the cubic planes of the CaF
2
plate. As with the compressive or tensile forces, the shear forces operate to change the index of refraction of the CaF
2
plate in the direction of a resultant force vector.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost character(s) and/or digit(s) in the corresponding reference number.
REFERENCES:
patent: 3600611 (1971-08-01), Treharne
patent: 0 942 297 A2 (1999-09-01), None
patent: 0 834 753 A1 (1998-04-01), None
patent: 0 942 300 A2 (1999-09-01), None
patent: 1098897 (1966-07-01), None
Cheng J.C. et al., “Photoelastic modulator for the 0.55-13-&mgr;m range,” Applied Optics, vol. 15, No. 8, Aug. 1976, pp. 1960-1965.
McClay J.A. et al., “157nm optical lithography: The accomplishments and the challenges,” Solid State Technology, Cowan Publishing Corp., vol. 42, No. 6, Jun. 1999, pp. 57, 59-60, 62, 64, 66, 68.
Copy of International Search Report, Application No. PCT/US00/34645, issued Jun. 22, 2001, 10 pages.
Max Born et al.,Principles of Optics,Electromagnetic Theory of Propagation, Interference and Diffraction of Light, Stress Birefringence and Form Birefringence, See 14.5, Pergamon Press Inc., 1980, pp
Curtis Craig
Silicon Valley Group Inc.
Spyrou Cassandra
Sterne Kessler Goldstein & Fox P.L.L.C.
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