Nikon Corporation

Shuuichi Takano

Shigeru Sakuma

Tsutomu Mizugaki

Kazuo Kimura

A manufacturing method for a single crystal of calcium fluoride by which it is possible to obtain a single crystal of calcium fluoride with adequately small double refraction, which can be used in optical systems for photolithography, and in particular, a single crystal of calcium fluoride with a large diameter (&oslash;200 mm or larger) having superior optical properties, which can be used for photolithography with a wavelength of 250 nm or less. A manufacturing method for a single crystal of calcium fluoride, having its optical properties improved through an annealing process in which a single crystal of calcium fluoride is contained in a sealable container, and said container is sealed and vacuumed, followed by, a process of heating with a heater arranged external to said container so that the temperature inside said container is raised to a first temperature, which is lower than the melting point of said single crystal of calcium fluoride, a process by which the temperature inside said container is maintained at said first temperature for a designated period of time, and a process by which the temperature inside said container is lowered to room temperature, wherein, the maximum temperature of the annealing process is set to be a first temperature within the range of 1020 to 1150&deg; C. Also provided is a manufacturing method for a single crystal of calcium fluoride having its optical properties improved through an annealing process such that, its maximum temperature during the thermal process is set to a first temperature which is within the range of 1020 to 1150&deg; C., and which is maintained for a designated period of time, and its cooling speed for reaching a second temperature, which is in the range of (or around) 600 to 800&deg; C., from said first temperature is set to be 1.2&deg; C./hour or less, or its cooling speed for reaching a second temperature, which is in the range of (or around) 700 to 900&deg; C., from said first temperature is set to be 1.2&deg; C./hour or less.

Manufacturing method for calcium fluoride and calcium fluoride for photolithography

The present invention provides a method for calculating the birefringence of an optical element and selecting the direction of minimum birefringence in the optical element. The birefringence of the optical element is calculated by converting known piezo-optical constants in a specified three-dimensional orthogonal coordinate system for the optical material into piezo-optical constants in an arbitrary three-dimensional orthogonal coordinate system. The amount of change in the refractive index &Dgr;n of the optical material in a first direction and the amount of change in the refractive index &Dgr;n of the optical material in a second direction which is perpendicular to the first direction are calculated using a uniaxial stress that is applied to the optical material along the first direction. The amount of birefringence as seen from a third direction perpendicular to the first direction and the second direction is determined in the arbitrary three-dimensional orthogonal coordinate system by determining the difference between the amount of change in the refractive index &Dgr;n and the amount of change in the refractive index &Dgr;n

OPTICAL ELEMENT MADE FROM FLUORIDE SINGLE CRYSTAL, METHOD FOR MANUFACTURING OPTICAL ELEMENT, METHOD FOR CALCULATING BIREFRINGENCE OF OPTICAL ELEMENT AND METHOD FOR DETERMINING DIRECTION OF MINIMUM BIREFRINGENCE OF OPTICAL ELEMENT

A manufacturing method for a single crystal of calcium fluoride by which it is possible to obtain a single crystal of calcium fluoride with adequately small double refraction, which can be used in optical systems for photolithography, and in particular, a single crystal of calcium fluoride with a large diameter (&oslash; 200 mm or larger) having superior optical properties, which can be used for photolithography with a wavelength of 250 nm or less. A manufacturing method for a single crystal of calcium fluoride, having its optical properties improved through an annealing process in which a single crystal of calcium fluoride is contained in a sealable container, and said container is sealed and vacuumed, followed by, a process of heating with a heater arranged external to said container so that the temperature inside said container is raised to a first temperature, which is lower than the melting point of said single crystal of calcium fluoride, a process by which the temperature inside said container is maintained at said first temperature for a designated period of time, and a process by which the temperature inside said container is lowered to room temperature, wherein, the maximum temperature of the annealing process is set to be a first temperature within the range of 1020 to 1150&deg; C. Also provided is a manufacturing method for a single crystal of calcium fluoride having its optical properties improved through an annealing process such that, its maximum temperature during the thermal process is set to a first temperature which is within the range of 1020 to 1150&deg; C., and which is maintained for a designated period of time, and its cooling speed for reaching a second temperature, which is in the range of (or around) 600 to 800&deg; C., from said first temperature is set to be 1.2&deg; C./hour or less, or its cooling speed for reaching a second temperature, which is in the range of (or around) 700 to 900&deg; C., from said first temperature is set to be 1.2&deg; C./hour or less.

The present invention provides a method for calculating the birefringence of an optical element and selecting the direction of minimum birefringence in the optical element. A material for manufacturing the optical element is a fluoride single crystal with a birefringence value that is reduced compared to conventional materials. The fluoride single crystal is cut from a fluoride single crystal ingot obtained by crystal growth so that the &lcub;111&rcub; crystal planes are two parallel planes, after which the optical performance is improved by subjecting this material to a heat treatment. The birefringence of the optical element is calculated by converting known piezo-optical constants in a specified three-dimensional orthogonal coordinate system for the optical material into piezo-optical constants in an arbitrary three-dimensional orthogonal coordinate system. The amount of change in the refractive index &Dgr;n,of the optical material in a first direction along the direction of one coordinate axis of the arbitrary three-dimensional orthogonal coordinate system, and the amount of change in the refractive index &Dgr;n,of the optical material in a second direction which is perpendicular to the first direction are calculated using a uniaxial stress that is applied to the optical material along the first direction and the piezo-optical constants in the arbitrary three-dimensional coordinate system. The amount of birefringence as seen from a third direction perpendicular to the first direction and the second direction is determined in the arbitrary three-dimensional orthogonal coordinate system by determining the difference between the amount of change in the refractive index &Dgr;n,and the amount of change in the refractive index &Dgr;n

Optical element made from fluoride single crystal, method for manufacturing optical element, method for calculating birefringence of optical element and method for determining direction of minimum birefringence of optical element

An annealing method for a single crystal of fluoride is provided. The method includes the steps of removing at least one of attached objects and absorbed objects from the surface of the single crystal of fluoride to clean the surface, thereafter annealing the single crystal of fluoride, including heating the single crystal of fluoride and gradually cooling the heated single crystal of fluoride, and removing a deteriorated layer which is formed on the surface of the single crystal of fluoride during the annealing step.

Method for annealing single crystal fluoride and method for

A manufacturing method for a single crystal of calcium fluoride includes the steps of degassing calcium fluoride powder particles to desorb impurities from surfaces of the calcium fluoride powder particles, preprocessing the degassed calcium fluoride powder particles by fusing the degassed calcium fluoride powder particles in a crucible to obtain a preprocessed product, and re-fusing the preprocessed product in a crucible to grow a single crystal of calcium fluoride.

Growing community of inventors

Inagi, Japan

Shuuichi Takano