Weighing scales – With weigher loading or unloading means – Hoist
Patent
1996-11-23
1998-06-09
Gellner, Michael L.
Weighing scales
With weigher loading or unloading means
Hoist
177245, 177253, 177DIG9, 117201, G01G 1900, C30B 2300
Patent
active
057638381
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to an apparatus for weighing a grown crystal, and in particular, to an apparatus for weighing the weight of a crystal which is growing by being pulled from a melt thereof.
BACKGROUND ART
As a method for producing monocrystal of silicon semiconductor and the like, Czochralski Process is used in which a seed crystal is dipped in a melt of a semiconducting material and the like and then pulled up while being rotated so as to grow the monocrystal. In an apparatus for producing crystal based on the Czochralski Process, in order to control the size of the monocrystal being grown (to be referred to as a "grown crystal") through the pulling, the cross-sectional area or the diameter of the grown crystal at the boundary surface between the melt and the grown crystal (to be referred to as "boundary surface") is measured. Based on the measured value of the cross-sectional area or the diameter, the speed of pulling the seed crystal and the temperature of the melt are controlled so as to produce the desired diameter of the grown crystal. Conventional method to measure the diameter of the grown crystal at the boundary surface is, for instance, an image measurement method in which the diameter of a grown crystal is determined based on its image obtained by a television (TV) camera, or a method in which weight increment of the grown crystal is measured and its diameter is calculated on the basis of the measured weight increment.
The FIG. 9(A) shows an example of conventional wire-type apparatus 41 for producing monocrystal. With the apparatus 41 of FIG. 9(A), a heater 42 heats melt 1 in a crucible 44, and a seed crystal secured to the lower end of a wire rope 46 having its upper end connected to a drum 48 depends by way of a pulley 47 located above the crucible 44 and is dipped in the melt 1. Being slowly wound up onto the drum 48, the wire rope 46 ascends gradually, and its lower end is slowly pulled up from the melt 1 so as to form and develop a grown crystal 3. In the illustrated example, a temperature sensor 43 measures the temperature of the heater 42. The apparatus 41 of FIG. 9(A) has a TV camera 45 for taking picture of the inside of the crucible 44, the boundary between the melt 1 and the grown crystal 3 in particular, for facilitating the determination of the diameter of the grown crystal 3 based on the picture or image obtained by the TV camera 45.
The conventional image method of determining the diameter of the grown crystal 3 has a shortcoming in that, with the development of the grown crystal 3, the top surface level of the melt 1 in the crucible 44 is gradually lowered as shown in FIGS. 9(B) and 9(C). With the TV camera 45 at the fixed position, as the surface level is lowered, it becomes increasingly difficult to take picture of the melt surface or the above-mentioned boundary surface. Without the picture of the boundary surface, it is impossible in the image method to determine the diameter of the grown crystal 3 at the crystallizing point.
To avoid the shortcoming of the image method, it has been proposed to measure the weight of the grown crystal 3 acting on the pulley 47 of FIG. 9 by using a load cell 49 as shown in FIG. 10(A). If it is assumed that the weight of the pulley 47 is negligible and the pulley 47 acts as a lever with two arm lengths L.sub.1 and L.sub.2, the output of the load cell 49 can be given by the following equation (1) as a function of the weight Mg of the grown crystal 3. When the arm lengths ratio (L.sub.1 /L.sub.2) is known, one can get the weight Mg of the grown crystal 3 from the output of the load cell 49 by using the relationship of the equation (1). In the equation (1), F represents winding force of the drum 48, and L.sub.1 and L.sub.2 represent horizontal radii of the pulley 7 on the sides of the grown crystal 3 and the drum 48, respectively.
The method of FIG. 10(A), however, has a shortcoming in that comparatively large measurement error is inevitable. To analyze such error, a test was made by connecting a weight member of 60 kg, in
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"Overload Protection in Electronic Weighing Systems: Part 5", Measurment and Control, vol. 12, By Al Brendel, p. 471, Nov. 1979.
Morimura Toshiaki
Noguchi Yoshitaka
Oka Satoshi
Gellner Michael L.
Gibson Randy W.
Ohkura Electric Co. Ltd.
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