Substantially-uniform-temperature annealing

Details Substantially-uniform-temperature annealing Substantially-uniform-temperature annealing

2001-07-20

2003-09-23

Pelham, Joseph (Department: 3742)

Electric heating

Heating devices

Combined with container, enclosure, or support for material...

C219S390000, C219S394000, C219S399000, C117S204000, C373S119000, C373S137000

Reexamination Certificate

active

06624390

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to annealing and more particularly to annealing of single crystals to yield single crystals with low stress birefringence such as for use as optical lenses.
BACKGROUND OF THE INVENTION
The increase in the processing speed, functionality, and integration in integrated circuits (ICs) has been achieved through continuous reduction in the feature sizes of the ICs. A portion of the manufacturing of the ICs affecting attainable feature sizes is photolithography. During photolithography, a pattern of the IC is transferred from a mask to a wafer, e.g., a semiconducting wafer. Imaging characteristics of modern projection optical photolithography equipment are dominated by diffraction effects. The resolution (i.e. the smallest feature size that can be printed on the wafer) is k
1
&lgr;/NA, where &lgr; is the wavelength of the light source, k
1
is a constant approximately equal to 0.5, and NA is the numerical aperture of the projection optics. The depth of focus of the projection printer over which the image quality is not degraded is limited and is equal to k
2
&lgr;/(NA)
2
, where k
2
is a constant that depends on k
1
. Thus, to decrease the feature size either the wavelength of exposure must be reduced or the NA of the optics must be increased.
Increasing the optics NA to reduce feature size results in a substantial reduction in the depth of focus (~(NA)
−2
), which is undesirable, particularly because the depth of focus must be larger than any variations in the flatness of the photoresist surface. Therefore, the semiconductor industry is pursuing the use of short wavelength exposure sources for achieving smaller and smaller feature sizes. KrF, ArF, and F
2
excimer lasers are presently available as light sources for, respectively, 248, 193, and 157 nm photolithography. The synthetic fused silica, however, that has been the optical material of choice for higher wavelength exposure sources, exhibits significant loss of transmittance at wavelengths below 200 nm.
Single crystals of Calcium Fluoride (CaF
2
) exhibit the desirable optical properties for sub 200-nm-photolithography. Furthermore, for historical reasons the production knowledgebase for CaF
2
is relatively extensive. Other single crystals of fluoride such as BaF
2
and LiF are also possible material candidates, but are significantly behind CaF
2
in production technology, and may be less desirable, e.g., due to toxicity and corrosiveness (BaF
2
) and/or expense (LiF). Therefore, single crystal CaF
2
are desirable and suitable optical material for 193 and 157 nm optical steppers. Presently, CaF
2
crystals as large as 30 cm in diameter and 10 cm in height are used in photolithography equipment.
Single crystals of CaF
2
are grown by directional solidification from the melt phase. In this process layers of the melt are continuously solidified, by changing the temperature of the crystal, to form a single crystal boule. The crystal boule is subsequently cooled to room temperature. The transfer of heat from and through the crystal sets up temperature gradients (i.e. temperature non-uniformities) and associated thermal stresses in the single crystal. CaF
2
is a relatively weak material, especially at elevated temperatures, and therefore experiences plastic deformation under thermal stresses during the crystal growth process. The accumulation of plastic strain during the crystal growth process results in generation of residual stresses in the crystal at room temperature. Residual stresses, in turn, cause stress birefringence through spatial variations in the material's index of refraction, and an associated degradation of optical characteristics of components made from this material.
Annealing is used to reduce residual stresses in crystals that have experienced plastic deformation during the crystals' growth process. To anneal a crystal, the crystal is maintained at an elevated temperature close to its melting point temperature for a period of time. This constant temperature is intended to allow existing residual stresses to relax. The crystal is cooled to room temperature. During cooling, temperature gradients associated with the cooling of the crystal generate thermal stresses in the crystal that may cause the crystal to undergo plastic deformation.
Due to the nature of the material, temperature variations to which a single crystal is exposed to during growth and annealing result in large thermal stresses leading to plastic deformation of the crystal and, hence, large residual birefringence.
SUMMARY OF THE INVENTION
In general, in an aspect, the invention provides a system for heating optical members. The system includes a thermally-conductive inner housing defining an interior volume for receiving an optical member to be heated, a thermally-insulative outer housing at least partially containing the thermally-conductive inner housing, and a heating structure disposed outside the inner housing and configured to provide heat through the thermally-conductive inner housing and into the interior volume defined by the inner housing.
Implementations of the invention may include one or more of the following features. The inner housing is configured such that an inner surface defining the interior volume has a substantially uniform temperature in response to the inner housing receiving the heat provided by the heating structure. The inner housing is configured to define the interior volume to be axi-symmetric.
Further implementations of the invention may include one or more of the following features. The system further comprises a controller coupled to the heating structure and configured to control the heating structure such that the member disposed in the interior volume is heated substantially without being plastically deformed. The controller is configured to control the heating structure such that a resolved shear stress of a CaF
2
optical member disposed in the interior volume does not exceed about 0.5 e
(990/T)
MPa where T is average temperature of the member in Kelvin.
Further implementations of the invention may include one or more of the following features. A portion of the outer housing in contact with and supporting the inner housing has a thermal conductivity different than at least one other portion of the outer housing. An inner boundary of the outer housing is disposed in contact with substantially an entire outer boundary of the inner housing. The inner housing and at least a portion of the outer housing are an integral structure, with the inner housing and the at least a portion of the outer housing being layers of the integral structure with different thermal conductivity.
Further implementations of the invention may include one or more of the following features. The inner housing comprises at least one of high-thermal-conductivity graphite and high-thermal-conductivity carbon. The interior volume is cylindrical and directions of highest thermal conductivity of the inner housing are parallel with inner surfaces of the inner housing. The interior volume is cylindrical and directions of lowest thermal conductivity of the inner housing are perpendicular with inner surfaces of the inner housing. Directions of lowest thermal conductivity of the outer housing are perpendicular with outer surfaces of the inner housing.
Further implementations of the invention may include one or more of the following features. The inner housing has substantially orthotropic thermal conductivity. The outer housing comprises at least one of low-thermal-conductivity graphite, low-thermal-conductivity carbon, low-thermal-conductivity porous graphite, low-thermal-conductivity porous carbon, low-thermal-conductivity fibrous graphite, low-thermal-conductivity fibrous carbon. The outer housing has substantially orthotropic thermal conductivity. The system further comprises another thermally-conductive housing, the another thermally-conductive housing substantially contains the thermally-insulative outer housing. The another thermally-conductive housing is displaced from the outer hou

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