JPH10231194A - Apparatus and method for heat-treatment of fluorite single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a heat-treating apparatus for producing a single crystal having low strain and useful for an optical system of an eximer laser stepper by placing temperature-controllable heaters of plural circuits on the side of the outer part of a gas-tight vessel holding a single crystal and placing plural reflection plates at the upper and the lower parts in the vessel. SOLUTION: A gas-tight vessel 1 for holding a fluorite single crystal 8 is provided with independently temperature-controllable upper heater 3, middle heater 4 and lower heater 5 on the side of the outer part of the vessel and with plural upper and lower reflection plates 9, 10 at the upper and the lower parts in the gas-tight vessel 1 to form the objective heat-treatment apparatus. A carbon vessel 2 holding a fluorite single crystal 8 and a fluorination agent 7 is placed in the gas-tight vessel 1, the vessel 1 is evacuated to <=10<-1> Pa, the fluorite single crystal is maintained at a definite temperature between 800 deg.C and 1300 deg.C while uniformizing the temperature distribution of the fluorite single crystal by the upper and the lower reflection plates 9, 10 and the temperature is slowly lowered.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fluorite single crystal heat treatment apparatus and a heat treatment method. The fluorite single crystal treated by the heat treatment method of the present invention is useful for a lens, a prism, and the like that require high-precision imaging performance and constitute an optical system of an excimer laser stepper.

[0002]

BACKGROUND OF THE INVENTION The present invention relates to a heat treatment of a fluorite single crystal which can be used as an optical system of an excimer laser stepper. In recent years, lithography technology for drawing an integrated circuit pattern on a wafer has been rapidly developing. The demand for higher integration of integrated circuits is only increasing, and to achieve this, it is necessary to increase the resolution of the stepper projection lens. The resolution of the projection lens is governed by the wavelength of the light used and the NA (numerical aperture) of the projection lens. To increase the resolution, the wavelength of the light used is made shorter and the NA of the projection lens is made larger. (Increase the diameter).

[0003] First, the shortening of light wavelength will be described. The wavelength used for the stepper is already g-line (wavelength 436n
m), i-line (wavelength 365 nm), and a shorter wavelength KrF excimer laser light (wavelength 248)
nm), ArF excimer laser light (wavelength 193 nm) and the like, it is no longer possible to use optical glass for the optical system in view of transmittance. For this reason, it is common to use quartz glass or fluorite for the optical system of the excimer laser stepper.

Next, the enlargement of the diameter will be described. This is not limited to a large diameter, and the optical material used in the optical system of the excimer laser stepper needs to be a single crystal in fluorite. With the higher performance of steppers, recently the diameter of φ150 mm ~
Fluorite single crystals having a large diameter of about 250 mm have been required.

Hereinafter, a conventional method for producing a fluorite single crystal will be described. Fluorite single crystals have been produced by the Bridgman method (also called the Stockberger method or the crucible descent method). In the case of a fluorite single crystal used in the ultraviolet or vacuum ultraviolet region, it is common to use a high-purity raw material produced by chemical synthesis without using natural fluorite as a raw material. The raw material can be used as a powder, but in this case, a semi-molten product or a pulverized product thereof is usually used because the volume of the material when melted is drastically reduced. A crucible filled with the above-mentioned raw materials is placed in the growing apparatus, and the inside of the growing apparatus is 10 ⁻³ to 10 ⁻⁴ Pa
Keep in a vacuum atmosphere. Next, the temperature of the growing apparatus is raised to the melting point of fluorite or higher (1370 ° C. to 1450 ° C.), and the raw materials are melted. In order to suppress temporal fluctuations in the temperature of the growing apparatus, control using a constant power output or highly accurate PID control is performed. In the crystal growth stage, the crucible is pulled down at a speed of about 0.1 to 5 mm / H, whereby the crystal is gradually crystallized from the lower part of the crucible. When the crystal is crystallized to the uppermost part of the melt, the crystal growth is completed, and simple slow cooling is performed while avoiding rapid cooling so that the grown crystal (ingot) does not break. When the temperature of the growing apparatus has dropped to about room temperature, the ingot is taken out.

Since the residual stress and strain are very large in this state, a simple heat treatment is performed in the ingot. The fluorite single crystal thus obtained is cut into an appropriate size for each target product. The cut fluorite single crystal is subjected to a simple heat treatment in an annealing apparatus.

[0007]

SUMMARY OF THE INVENTION In the prior art,
The heat treatment simply performed with the ingot could reduce the strong strain generated at the stage of growing the crystal to such an extent that cutting was possible. Therefore, satisfactory distortion could not be obtained to the extent that an optical system could be configured. Further, even in a heat treatment performed on a fluorite single crystal cut to an appropriate size, a fluorite single crystal with a good strain that can be used for a high-precision optical system like an excimer laser stepper could not be obtained. In particular, when the fluorite single crystal has a large diameter and an increased volume, it has been more difficult to remove the strain.

[0008] The present invention has been made in view of the above points. An object of the present invention is to provide a heat treatment apparatus and a heat treatment method for obtaining a fluorite single crystal that can be used for a highly accurate optical system.

[0009]

In order to solve the above problems, the present inventors have taken the following measures. First, as a first means, an airtight container for storing a fluorite single crystal is provided, and a fluorite single crystal equipped with two or more circuits capable of independently controlling the temperature is provided outside the airtight container. A crystal heat treatment apparatus was provided, and a plurality of reflectors were installed on the upper and / or lower part of the apparatus.

As a second means, the fluorite single crystal is maintained at about 800 ° C. to 1300 ° C. in a non-oxygen atmosphere while flattening the temperature distribution in the airtight container by using the above-mentioned apparatus, and thereafter gradually. Then, the temperature was lowered to room temperature.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Strain observed after production of a fluorite single crystal occurs during the crystal growth process. Strain is caused by the temperature distribution in the growing device with a large temperature gradient. In general,
In a method of growing a single crystal for crystallizing a melt, it is essentially necessary to have a temperature gradient in a growing apparatus.
Therefore, strain is inevitably generated in the fluorite single crystal grown by implementing the single crystal growing method for crystallizing the melt.

Therefore, the heat treatment in the present invention is required. By performing the heat treatment in the present invention, the strain of the fluorite single crystal generated during the growth can be eliminated. In the heat treatment of the present invention, the fluorite single crystal was maintained at a temperature as high as possible below the melting point of fluorite. The rearrangement of atoms is performed in the process of this high temperature to eliminate the strain. Furthermore, it is important to equalize the temperature inside the heat treatment apparatus irrespective of the position, that is, to flatten the temperature distribution in order to remove the distortion when the high temperature is maintained and when the temperature is decreased. For this purpose, in the heat treatment according to the present invention, a plurality of reflectors are provided at an upper part and / or a lower part in a heat treatment apparatus using two or more independent heaters to flatten the temperature distribution.

[0013]

FIG. 1 is a sectional view of an apparatus according to the present invention. At the time of heat treatment, the atmosphere inside the heat treatment apparatus is shut off from the atmosphere by the airtight container 1. A carbon container 2 is further placed in the airtight container. The fluorite single crystal 8 to be heat-treated is placed in this carbon container. Teflon and ammonium acid fluoride are also stored as the fluorinating agent 7 in the carbon container. In order to prevent uneven temperature distribution due to heat radiation from the upper and lower parts of the heat treatment apparatus, five carbon reflectors 9 and 10 are provided at regular intervals at the lower part and at regular intervals. Thereafter, the chamber is evacuated to a vacuum, and when it is confirmed that the pressure becomes about 10 ^-1 Pa or less, the evacuation is terminated.

An ordinary heat treatment apparatus often has a structure having an outlet at the top. In that case, heat escapes largely in the upper part of the heat treatment apparatus, and the temperature becomes relatively low. Therefore, by independently controlling the heaters of the two circuits,
Good temperature distribution is obtained in the heat treatment apparatus. In the case of a structure having an installation surface below the heat treatment apparatus, the temperature of the lower part of the heat treatment apparatus also tends to be low. Although the lowering of the temperature in the lower part can be prevented to some extent by strengthening the heat insulating material, it is effective to independently control the heaters of the three circuits to form a better temperature distribution. Specifically, an upper heater 3, a middle heater 4, and a lower heater 5 are provided.

In the process of increasing the temperature, the temperatures of the three-circuit heaters are controlled independently. In this process, the fluorinating agent is vaporized, and the inside of the growing apparatus becomes a fluorine atmosphere. When the entire growing apparatus reaches a steady state of 1200 ° C., it is maintained for 24 hours. The temperature at which this high temperature is maintained is a high temperature close to the melting point of fluorite13
Although about 00 ° C. is desirable, considering the durability of the present heat treatment apparatus, it is better to keep the temperature at the time of maintaining the high temperature at about 1200 ° C. or less. The temperature distribution was measured by inserting a thermocouple into this hermetic container. Was improved from 12 ° C. to 2 ° C.

Thereafter, the temperature is gradually lowered to room temperature, and the pressure is released to the atmospheric pressure to take out the fluorite single crystal.

[0017]

By performing the heat treatment using the apparatus according to the present invention, it was possible to produce a fluorite single crystal having a good strain.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a heat treatment apparatus according to an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Airtight container 2 ... Carbon container 3 ... Upper heater 4 ... Middle heater 5 ... Lower heater 6 ... Thermocouple 7 ... Fluorinating agent 8 ... Single fluorite Crystal 9: Upper reflector 10: Lower reflector A, B: Temperature measurement point

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masaki Shiozawa 2-3-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nikon Corporation (72) Inventor Shuichi Takano 1642-26 Kumakawa, Fussa-shi, Tokyo 26 Applied Optical Laboratory Co., Ltd. (72) Inventor Hidemi Nishikawa 1642 Kumakawa, Fussa-shi, Tokyo 26-applied light laboratory

Claims (7)

[Claims] 1. A heat treatment apparatus having an airtight container for storing a fluorite single crystal, and having two or more heaters capable of independently controlling the temperature disposed outside the airtight container,
A fluorite single crystal heat treatment apparatus in which a plurality of reflectors are arranged at an upper part and / or a lower part in an airtight container. 2. A heat treatment apparatus for a fluorite single crystal, wherein the reflection plate is made of carbon, molybdenum or stainless steel. 3. A fluorite single crystal is housed in an airtight container in a heat treatment apparatus having two or more heaters capable of independently controlling the temperature, and a plurality of upper and / or lower reflectors in the airtight container are provided. While flattening the temperature distribution generated in the fluorite single crystal by the plate, the fluorite single crystal is heated to 800 ° C or more in a non-oxygen atmosphere.
A heat treatment method for a fluorite single crystal, characterized in that the temperature is maintained at a constant temperature of 1300 ° C. or lower and then the temperature is gradually lowered. 4. A method for heat treating a fluorite single crystal according to claim 3, wherein the airtight container KoNoboru heated which is evacuated to a vacuum of about 10 ^{@ 1} Pa, a fluorite single crystal 800 ° C. to 1,300 A heat treatment method for a fluorite single crystal, characterized in that the temperature is maintained at a constant temperature of not more than ℃ and then the temperature is gradually lowered. 5. The method for heat treating a fluorite single crystal according to claim 4, wherein the vacuum is released after the temperature of the fluorite single crystal is lowered to room temperature. 6. The fluorite single crystal heat treatment method according to claim 3, wherein the fluorite single crystal is cut into a desired size from a fluorite single crystal ingot manufactured by the Bridgman method. A method for heat treating a fluorite single crystal, characterized in that: 7. The fluorite single crystal according to claim 3, wherein the fluorite single crystal has a temperature distribution of 5 ° C. or less. Heat treatment method.