Adhesive bonding and miscellaneous chemical manufacture – Delaminating processes adapted for specified product – Delaminating in preparation for post processing recycling step
Patent
1983-08-11
1986-01-21
Lacey, David L.
Adhesive bonding and miscellaneous chemical manufacture
Delaminating processes adapted for specified product
Delaminating in preparation for post processing recycling step
156617SP, 422109, 422249, 364500, C30B 1528, G06F 1546
Patent
active
045655987
DESCRIPTION:
BRIEF SUMMARY
Crystal growth by the Czochralski method has traditionally required the presence of a skilled operator to watch the progress of a growing crystal and to affect diameter changes by making appropriate adjustments to the temperature of the melt. The relationship between melt temperature and crystal diameter depends on a balance between the sum of the heat passing from the melt to the crystal plus the heat of solidification with the heat that can be conducted away from the interface and lost to the environment. The qualitive relationship is that a melt temperature increase will cause a decrease in crystal diameter, but an exact relationship depends strongly on the existing crystal size and shape and will change throughout a growth run. To obtain good crystal quality it is necessary to maintain a constant growth rate as both stoichiometry and impurity concentration are growth rate sensitive. A closed loop servo type control over the crystal diameter is thus desirable to obtain good quality crystals reproducibly. Additional reasons for automatic control are that growth runs may last several hours to days making human supervision particularly tedious and that often visibility of the growing crystal is impaired by, for instance, thermal insulation in the growth chamber.
To achieve automatic control, some method of sensing the crystal diameter must replace the operator's vision. Many methods have been used, with that of weighing either the growing crystal or the melt plus crucible being the most popular and widely applicable. Weighing the crucible is mechanically simpler than weighing the crystal but crucible weighing does suffer the disadvantage that levitation effects from the r.f. heating and evaporation losses from the melt can contribute to the weight signal. Such effects, however, can be taken account of in signal processing.
Methods of using the processed weight signal to produce an appropriate influence on melt temperature such as to achieve servo control over the diameter, are known. The best approach is to first differentiate the weight signal, the result of which should be constant for constant diameter growth, and then compare this with an expected rate of change of weight. The error signal thus obtained can be used in a standard way to derive a control signal to adjust the melt temperature.
The electronic processing can be performed both by analogue and digital means. The analogue methods are considerably less expensive when compared with the cost of a mini-computer, but a digital control system is much more powerful because it allows easy modifications to control algorithms even during a growth run.
The following specifications show details of known prior art in this general field:
British Pat. No. 1,465,191, National Research Development Corporation which uses an r.f. heater supply unit to provide a melt and includes a load cell and draw position indicator to achieve a regulated crystal diameter by automatic means achieved through a system of heat control or draw regulation.
British Pat. No. 1,494,342, National Research Development Corporation which has a load cell associated with the pull rod and uses a differentiator and comparator.
Japanese Patent Specification No. 56-92195 Fujitsu K.K. et al, which uses a load cell connected to a computer through an A.D. converter, which computer is also connected to a power control to regulate the r.f. heating current.
The object of the invention however is to provide an improved method of and means for automatically producing crystals of a required diameter and this is achieved by measuring the temperature of the melt while also measuring the loss of weight of the melt. A microprocessor processes both factors to regulate the crystal growth conditions.
When the system, on which the present invention is based, is used without automatic diameter control, a standard Czochralski crystal puller is used, heating power to the crystal puller being provided by a low voltage industrial r.f. generator of say 50 kW capacity and operating at a nominal frequency of perhaps 45
REFERENCES:
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patent: 4008387 (1977-02-01), Green et al.
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patent: 4058429 (1977-11-01), Duncan et al.
patent: 4073355 (1978-02-01), Schmidt et al.
Valentino et al., Diameter Control of Czochralski Grown Crystals, Journal of Crystal Growth 26, No. 1, Nov. 1974, pp. 1-5.
"Automatic Diameter Control of Czochralski Grown Crystals," Journal of Crystal Growth 19 (1973), 187-192, North-Holland Publishing Co., by A. E. Zinnes, B. E Nevis and C. D. Brandle.
Satoh et al., "Automatic Control System for Czochralski Growth of Large Diameter LiNbO.sub.3 Crystals;, Fujitsu Scientific and Technical Journal, pp. 93-113, (Mar. 1976).
Bardsley et al., "The Weighing Method of Automatic Czochralski Crystal Growth", Journal of Crystal Growth, vol. 40, pp. 21-28, (1977).
Lacey David L.
The Commonwealth of Australia
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