Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Havin growth from molten state
Reexamination Certificate
2002-08-22
2004-09-28
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Havin growth from molten state
C117S002000, C117S083000
Reexamination Certificate
active
06797058
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of French Patent Application, Serial Number 00 03771, filed Mar. 24, 2000 entitled Process And Device For Growing Single Crystals, Especially Of Caf
2
, by Patrick Herve.
The present invention relates to a process for growing single crystals and to an associated device suitable for carrying out said process. Said process and device are very particularly suitable for growing single crystals of calcium fluoride (CaF
2
).
Ultra-high-performance optical systems are required to increase the level of integration of electronic components on a semiconductor wafer insofar as a radiating light of very low UV wavelength (below 248 nm) is necessary to improve the resolution.
To obtain such optical systems, the most widespread technique to date uses fused silica. Another technique already being exploited, uses single crystals of calcium fluoride (CaF
2
).
Such single crystals or single crystals of the same type (single crystals of alkaline earth metal fluoride, in general, or even single crystals of silica) are obtained by the so-called Stockbarger or Bridgman method. In said method the crystals are generated in a furnace, inside which a crucible containing the molten material is moved from top to bottom, along a vertical axis, from a hot zone into a cold zone. The temperature of said hot zone is maintained above the melting point of the material in question (in the case of CaF
2
it is above 1525° C.). The crucible advances at a speed of about 0.3 to 5 mm/h.
As it passes from the hot zone to the cold zone, the material goes through a zone of high thermal gradient. Crystallization takes place inside the crucible when the material reaches the zone in which the temperature is below its melting point. The fixed crystallization front propagates inside the crucible, within the material, from bottom to top, insofar as said crucible is caused to move downwards.
To prevent any oxidation of the material and the components of the furnace, said furnace is generally maintained under vacuum. The crucible is made of a material resistant to chemical attack and is generally a graphite crucible.
In fact, said method is mainly carried out according to two variants.
According to the first variant, which is carried out batchwise, a stack of crucibles (each crucible in said stack containing the material in question) is loaded cold into the upper zone of a tower furnace, which is to become the hot zone of said furnace. The furnace, loaded in its upper zone, is heated. In a first stage, the stack of crucibles is kept hot, at a temperature above the melting point of the material in question, in said upper zone, or hot zone, of said furnace. In a second stage, said stack is lowered into and held in the lower zone, or cold zone, of said furnace, which is maintained at a temperature below that of the hot zone and below the melting point of said material in question.
The main disadvantages of this operational variant are its low productivity and mediocre yield. The productivity is hampered by dead times before and after the growth of the crystals (dead times for loading the furnace, evacuating said furnace, heating the two zones of said furnace, cooling said two zones and unloading said furnace). The yield is affected by the fact that each crucible in the stack is not exposed to the same thermal conditions over a complete cycle of said stack. In particular, the crucible at the bottom of said stack is brought into contact with the cold zone more rapidly and remains in said cold zone for longer. In addition, such discontinuous thermal conditions (hot zone/cold zone) inside one and the same furnace are difficult to control.
According to the second variant, which is described in Russian author's certificate no. 2161891 filed on Aug. 8, 1975, the aim is to carry out a continuous heat treatment. A stack of crucibles occupies the whole height of a tower furnace, which has two superposed thermal treatment chambers. When the stack undergoes a translational movement from top to bottom, each crucible in said stack passes successively through the upper chamber and then the lower chamber. For the emptying of each crucible at the bottom of the stack (the crucible which has therefore passed successively through said two thermal treatment chambers), the vertical translational movement of said stack is stopped (the crucible surmounting said crucible at the bottom of the stack being clamped between jaws for supporting and stabilizing said stack). This involves a degree of discontinuity in the thermal treatment and in any case entails decelerations and accelerations of the movement of the stack, and jerks, which are responsible for vibrations within the treated material. This is highly detrimental to optimized growth of the desired single crystals.
In such a context, the inventors designed and developed an optimized process for growing single crystals. Said process is also based on the so-called Stockbarger or Bridgman method explained above. It is optimized in that it is a truly continuous thermal treatment process which ensures that each crucible in the stack moving translationally through the furnace has the same history (very particularly as regards the thermal conditions), without jerks and with no vibration.
The prior art mentioned above and the invention mentioned below are explained only in terms of the mechanical aspect of the process (and its associated device) in question; the chemical aspect, and especially the advantageous presence of a fluorinating agent, has been generally described in the literature and is not repeated here.
The process and device of the invention are referred to in the present text as a process and device for growing single crystals. This description cannot imply a limitation. They can be described more generally as a process and device for growing crystals (polycrystals and monocrystals) insofar as they are obviously suitable for generating polycrystals. Their use for generating such polycrystals only is not excluded from the framework of the present invention (although it hardly seems pragmatic insofar as polycrystals can be obtained much more simply). The process and device of the invention can be described more precisely as a process and device for growing crystals which have been optimized to give a valuable yield of single crystals.
The process of the invention is therefore a process for growing single crystals (of the CaF
2
, BaF
2
, Magnesium Fluoride, optical fluoride crystals), carried out in the absence of impurities (under vacuum, in general, and/or in a controlled atmosphere) in a tower furnace inside which two superposed thermal treatment chambers are arranged, namely a first chamber, or melting chamber, and a second chamber, or annealing chamber, a sizeable thermal gradient being created between said first and second chambers.
Said process of the invention comprises:
the support and the translational movement along a vertical axis, inside said furnace, of a stack of crucibles containing the starting material (said starting material, inside each crucible, generally being introduced in the form of a powder or a previously melted disk), the height of said stack of crucibles, in operation, being greater than the sum of the heights of said superposed first and second chambers and each stack being supported and moved translationally in a direction such that each of the crucibles constituting said stack passes successively through said first chamber and then said second chamber under the action of means acting on at least the crucible at the bottom of said stack and arranged in a third chamber, or translation chamber, positioned underneath said pair of first and second chambers; and
the loading of a new crucible, upstream of said first chamber, at one end of said stack, and the unloading of the crucible which has successively passed through said first and second chambers, at the other end of said stack, the loading and unloading operations being performed at the same frequency so that the height of said stack, in
Corning Incorporated
Douglas Walter M.
Hiteshew Felisa
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