JP2005089204A - Apparatus and method for producing crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus for producing a crystal with which a good single crystal can be grown while introduction of crystal defects is suppressed by providing a means for stirring impurities segregated in the vicinity of an interface in the apparatus for producing the crystal used for growing a fluoride single crystal. <P>SOLUTION: Stable growth of the crystal is realized by suppressing compositional supercooling by providing a mechanism capable of stirring a melt in a crucible with ultrasonic waves and diffusing the impurities segregated in the vicinity of the interface. The ultrasonic waves are introduced into the melt by inserting a rod-like member made of carbon in the crucible. Further, the ultrasonic waves are introduced into the melt through a rod for supporting the crucible accommodating a raw material. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a crystalline material manufacturing apparatus manufactured by melting and solidifying at a high temperature and a crystal manufacturing method using the same.

The present invention also relates to a fluoride crystal production apparatus suitable for various optical elements, lenses, window materials, prisms and the like used in a wide wavelength range from the vacuum ultraviolet region to the far infrared region, and a crystal production method using the same. In particular, the present invention relates to a method for producing a calcium fluoride (CaF ₂ ) crystal that is a material of an optical element used in an exposure apparatus for photolithography using an excimer laser.

  In the production of a single crystal material, a method in which a polycrystalline material as a raw material is melted at a high temperature and then a crystal is grown on the seed crystal by a method such as providing a temperature gradient inside the melt is often used. As a typical manufacturing method, there is a vertical Bridgman method (VB method, see US Pat. Nos. 2,149,076 and 2,214,976) in which a crucible moves in a furnace having a temperature distribution. is there.

  In the crystal manufacturing apparatus used in the VB method, a crucible charged with raw materials is installed in the furnace body, and an annular heating element is arranged in the vertical direction around the crucible. The temperature inside the furnace is controlled to be lower than the melting point. In the VB method, the temperature distribution is used to melt the raw material in the crucible at the upper part of the furnace body, and then when the crucible is pulled down to the lower part of the furnace body at a predetermined speed, when passing through the temperature range near the melting point. Single crystals are grown by successively depositing from the melt while preserving the crystal orientation on the surface of the seed crystal or the grown single crystal.

  At this time, precipitation of impurities occurs on the surface of the already grown single crystal (a phenomenon in which impurities are eliminated on the liquid side of the solid-liquid interface is generally called segregation). In practice, the so-called compositional supercooling effect due to segregation makes it easy for the melt to solidify at a fixed distance from the surface of the single crystal. It becomes polycrystalline.

  In order to suppress the formation of polycrystals due to such compositional supercooling, it is generally known that it is effective to increase the temperature gradient in the vicinity of the growth surface of a single crystal where precipitation occurs. Yes. For example, in the invention disclosed in Patent Document 1, by providing a heat shield plate that blocks the movement of heat in the vertical direction around the crucible, a clear difference is provided in the temperature above and below the crystal growth position, thereby The temperature gradient at the growth position is increased to suppress polycrystallization due to compositional supercooling.

Japanese Patent Laid-Open No. 10-251097

  However, the operation of increasing the temperature gradient at the crystal growth position in order to suppress the formation of a polycrystal due to the compositional supercooling as described above may deteriorate the quality of the grown crystal.

  Hereinafter, the problem caused by the high temperature gradient will be described by taking calcium fluoride as an example.

Calcium fluoride has a high transmittance for short-wavelength light, and is indispensable for the formation of ultrafine patterns associated with high integration of semiconductor integrated circuits (KrF excimer laser light (wavelength 248 nm), ArF excimer laser light (wavelength 193 nm), F _2. It is considered to be an effective material for an optical system of an exposure apparatus using excimer laser light such as excimer laser (wavelength 157 nm). At this time, when a polycrystal is used, it is difficult to ensure the surface accuracy required for each lens constituting the optical system of the exposure apparatus. In addition, impurities are easily segregated at the crystal interface, and the uniformity of the refractive index is impaired, and the durability of the lens is problematic. Therefore, a large-diameter single crystal calcium fluoride is used as a projection lens for an excimer laser exposure apparatus. It is desired and grown by the above-described VB method or the like.

  When growing a single crystal of calcium fluoride, there is a problem that the crystal grown by compositional supercooling becomes polycrystalline as described above, and therefore a constant temperature gradient is added in the vicinity of the crystal growth surface. It is necessary.

  However, a calcium fluoride crystal that has undergone a rapid temperature change has an increased birefringence due to crystal defects that occur at that time, and the quality deteriorates from an optical viewpoint. It is also disclosed in Japanese Patent Application Publication No. 11-240787. This phenomenon is the same in other fluoride crystals, and it is known that birefringence easily increases even with a relatively small temperature change, particularly in a high temperature range just below the melting point. Furthermore, the crystal that has undergone a rapid temperature change becomes brittle and hinders subsequent handling. In other words, adding a large temperature gradient to the crystal growth surface reduces the quality of the grown single crystal. Therefore, a method for growing a single crystal without increasing the temperature gradient and regardless of the influence of compositional supercooling is desired.

  In order to suppress compositional supercooling without changing the temperature gradient, it is necessary to reduce the concentration of impurities segregated at the growth interface which is the root cause.

  Therefore, the present invention provides a means for stirring impurities segregated in the vicinity of the interface in a crystal manufacturing apparatus used for growing a fluoride single crystal, so that a single crystal can be satisfactorily produced while suppressing the introduction of crystal defects. It is an object of the present invention to provide a crystal production apparatus that can be grown and a crystal production method using the same.

  In order to achieve the above object, the present invention provides a mechanism capable of agitating the melt in the crucible with ultrasonic waves and diffuses impurities in the vicinity of the interface due to segregation, thereby suppressing compositional supercooling. And stable crystal growth can be performed.

  As a means described above, the invention according to claim 2 is an invention relating to a crystal manufacturing apparatus characterized in that an ultrasonic wave is introduced into a melt by inserting a carbon rod-like member into a crucible.

  The invention according to claim 3 is an invention relating to a crystal manufacturing apparatus, characterized in that ultrasonic waves are introduced into the melt through a rod that supports a crucible containing raw materials.

  The invention according to claim 7 is an invention according to a crystal manufacturing method, wherein a single crystal is manufactured by using the crystal manufacturing apparatus according to claims 1 to 6.

  According to the present invention, it becomes possible to suppress the formation of polycrystals due to compositional supercooling, and it is possible to grow a high-quality single crystal even if it is a fluoride single crystal or the like.

  According to the present invention, a crystal capable of growing a single crystal satisfactorily while suppressing the introduction of crystal defects by providing means for stirring the melt in a crystal manufacturing apparatus used for growing a single crystal of fluoride. A manufacturing apparatus and a crystal manufacturing method using the same can be provided.

  In addition, according to the present invention, a high-quality single crystal that suppresses the birefringence to a minimum can be obtained by providing means for adjusting the crucible moving speed in a crystal manufacturing apparatus used for growing a single crystal of calcium fluoride crystal. It is possible to provide a crystal manufacturing apparatus that can be stably grown and a crystal manufacturing method using the crystal manufacturing apparatus.

  In the case where the crystal is grown by the crucible pulling method according to the present invention, the influence of the stirring of the melt on the crystal growth in the crucible will be described below.

  FIG. 3 shows the concentration distribution in the liquid phase of impurities excluded from the interface.

  The segregation coefficient was k. The impurity concentration near the interface is high due to the effect of segregation. FIG. 3 also shows the equilibrium temperature corresponding to the concentration distribution in the liquid phase. The equilibrium temperature in the liquid phase changes greatly near the interface where the concentration change is large. The supercooling is caused by a change in the equilibrium temperature due to the concentration of impurities excluded on the liquid phase side.

  Therefore, in order to manufacture a high-quality and large-diameter crystal, a mechanism for diffusing impurities that cause supercooling to obtain a uniform concentration is required. If the impurities in the melt can be stirred and the impurity concentration in the liquid phase can be made uniform, the equilibrium temperature in the melt can be made uniform, and as a result, compositional supercooling can be suppressed.

  The melt stirring mechanism according to the present invention will be described with reference to FIG.

  In order to introduce ultrasonic waves into the melt 5a in the crucible as shown in FIG. 4, a bar member 7 made of graphite is inserted into the melt. The diameter of the rod-like member 7 is set to 10 mm or less so as to suppress the influence on the temperature distribution in the melt as much as possible.

  Further, the position of the rod-shaped member 7 in the melt is adjusted and held at a certain distance from the interface while the crucible moves to the low temperature region and the crystal grows. A piezo element 8 for generating ultrasonic waves is attached to the end of the rod-like member 7. The ultrasonic wave 16 introduced into the melt via the rod-shaped member 7 agitates and diffuses the impurities due to segregation by vibrating the melt 5a as a medium. Ultrasonic stirring is suitable for diffusing impurities near the interface because a stronger force acts near the interface as compared with other methods. Impurities are diffused by ultrasonic stirring and the compositional supercooling region disappears, so that stable crystal growth can be realized over the entire growth process.

  The melt stirring method by the second mechanism according to the present invention is exactly the same as the above stirring function except that ultrasonic waves are introduced into the melt through the crucible support rod.

  By stirring the melt with ultrasonic waves as described above, a high-quality large-diameter crystal that has been stably grown over the entire growth process can be produced.

<Embodiment 1>
An embodiment of the present invention will be described with reference to FIG.

  FIG. 1 is a schematic diagram of a crystal manufacturing apparatus according to Embodiment 1 of the present invention, and shows a configuration for growing a single crystal of calcium fluoride.

  In this apparatus, a graphite crucible 4 for growing a crystal is installed inside a furnace chamber 10 which can be kept in vacuum by a vacuum exhausting means 12, and a side surface made of graphite which is a heating means arranged around the crucible. A heat insulating material 13 is disposed around the heater 3 and the periphery thereof. The crucible 4 is held by a crucible support rod 6 inserted from the outside of the furnace chamber, and can rotate and move up and down 11.

  Further, in order to control the crystal growth surface to any of {111}, {100}, and {110}, a seed crystal 15 is provided at the lower part of the crucible. In order to stir the melt in the crucible, a graphite rod 7 having a diameter of 10 mm is inserted, and the position of the piezoelectric element 8 which is an ultrasonic vibrator and the graphite rod 7 is adjusted at the end thereof. An elevating mechanism 9 is attached. The position of the graphite rod 7 is adjusted and held at an appropriate position so that the distance from the interface is kept constant during crystal growth.

  The vibration frequency of the ultrasonic wave was set to 40 kHz because it may cause aggregation if it is too high. In the present embodiment, the side heater 3 is composed of two upper and lower stages, the upper stage is a temperature higher than the melting point of the crystal to be grown to maintain the temperature of the melt 5a, and the lower stage is to hold the grown crystal 5b. The temperature is set to an appropriate temperature, and is adjusted so as to be the melting point temperature of crystals grown in the crucible in the vicinity of the middle of the upper and lower heaters. The temperature control of the heater is performed on behalf of the temperature of each heater by a thermocouple 2 installed outside the middle of each heater. The temperature distribution in each heater is thermally designed to have an almost uniform temperature distribution by controlling the current distribution inside the heater, but it changes within a certain range due to the flow of heat to and from the outside. To do.

  Single crystal growth is achieved by maintaining the temperature distribution and pulling down the crucible sequentially while stirring the melt with ultrasound to further melt the surface of the grown single crystal at a location near the melting temperature. The liquid is deposited so that the whole becomes a single crystal.

In growing a calcium fluoride single crystal, the descending speed of the crucible depends on the furnace structure, but is preferably 0.1 mm to 5 mm per hour. Further, it is desirable to rotate the crucible at 1 rph or more so that the temperature unevenness of the heater does not affect the crystal growth. The degree of vacuum during the growth is preferably maintained in the furnace chamber 8 at about 10 ⁻³ to 10 ⁻⁴ Pa.

The length of the fluoride crystal to be grown is usually about 100 to 400 mm, and a single crystal is grown in a growth time of about 300 to 700 hours. Subsequently, the fluoride crystal that has grown is heat-treated in an annealing furnace to relieve thermal stress remaining in the crystal during growth. Thereafter, it is molded into the required optical article shape. By combining various lenses thus obtained, a projection optical system and illumination optical system suitable for excimer lasers, particularly ArF excimer lasers and F ₂ excimer lasers can be constructed. An exposure apparatus for photolithography can be configured by combining an excimer laser light source, an optical system having a lens made of calcium fluoride crystal obtained by the manufacturing method of the present invention, and a stage capable of moving the substrate.

<Embodiment 2>
FIG. 2 is a schematic diagram of a crystal manufacturing apparatus according to Embodiment 2 of the present invention.

  In FIG. 2, the raw material melt can be stirred by providing the ultrasonic vibrator 8 in the crucible support rod. The configuration other than the ultrasonic wave generation portion is the same as that in FIG.

  By stirring the melt with the crystal production apparatus used in the present embodiment, the production yield of the single crystal was improved by about 1.5 times.

  The present invention can be applied to an apparatus for manufacturing a crystalline material manufactured by melting and solidifying at a high temperature and a crystal manufacturing method using the same.

It is a schematic diagram of the crystal manufacturing apparatus which concerns on Embodiment 1 of this invention. It is a schematic diagram of the crystal manufacturing apparatus which concerns on Embodiment 2 of this invention. It is a figure which shows the relationship between the impurity distribution which concerns on this invention, and the equilibrium temperature in a liquid phase. It is a schematic diagram which shows the case where a melt is stirred with the ultrasonic wave which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Case of crystal growth furnace 2 Thermocouple for heater control 3 Heater 4 Crucible for crystal growth 5a Molten part in crucible 5b Single crystal grown in crucible 6 Support rod of crucible 7 Graphite rod 8 Ultrasonic vibration Child 9 Elevating mechanism of graphite rod 10 Furnace chamber 11 Rotation and vertical movement of crucible 12 Vacuum exhaust means 13 Heat insulating material 14 Cooling water 15 Seed crystal 16 Ultrasonic wave

Claims (7)

A crystal comprising a heating means arranged so as to substantially surround the crucible and a heat insulating means arranged so as to substantially surround the heating means, and solidifying a molten raw material of fluoride by moving the crucible from a high temperature region to a low temperature region In manufacturing equipment,
A crystal manufacturing apparatus, wherein the raw material melt is agitated by ultrasonic waves.   2. The crystal manufacturing apparatus according to claim 1, wherein an ultrasonic wave is introduced into the melt using a carbon rod-shaped member separated from the crucible containing the raw material.   2. The crystal manufacturing apparatus according to claim 1, wherein ultrasonic waves are introduced into the melt through a rod that supports a crucible containing the raw material.   The crystal manufacturing apparatus according to any one of claims 1 to 3, wherein an ultrasonic frequency is 20 kHz or more and 400 kHz or less.   The crystal production apparatus according to claim 1, wherein the crystal to be grown is calcium fluoride.   The crystal manufacturing apparatus according to claim 1, wherein the crystal to be grown has a diameter of 250 mm or more.   A crystal manufacturing method comprising manufacturing a single crystal using the crystal manufacturing apparatus according to claim 1.

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