Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having pulling during growth
Patent
1991-06-12
1995-12-05
Kunemund, Robert
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having pulling during growth
117 28, 117 31, 117213, 117218, 117932, C30B 1512
Patent
active
054719437
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention refers to a process for growing crystals by the Czochralski method using a melt within a crucible, a holden for the crystal to be pulled from the melt, and coaxial to the rotating and pulled crystal holder a separation wall in the form of a hollow cylinder dipping from the melt surface into the melt, and to an equipment for performing the process.
Crystals are grown by the Czochralski method whereby the free-hanging crystal is pulled out of a melt contained in a crucible. Thereby the temperature conditions are precisely monitored. The greatest impact of the Czochralski method is in growth of high-purity crystals for semiconductor technology.
The two-chamber technique allows to reduce segregation effects, to approach a constant composition of melt and thus of the crystals, by supplying fresh melt from a reservoir to that region of the melt from which the crystal is pulled. This is achieved by a hollow cylinder coaxial to the crystal and extending from the melt surface to the crucible bottom to which it is fixed. The supply of melt is achieved through perforations in the separation wall. Such a two-chamber system, or an internal crucible serving a similar purpose, may possibly reduce the axial segregation problem, however, without sufficient reduction of the local segregation problem, the striations problem.
Other processes have become known for the reduction of striations, for instance the reduction of convection by a strong magnetic field. However, these measures require large efforts and, among others, have the disavantages of detrimental effects on electrical properties of the crystals, and of "overheating" of the melt. Furthermore, in case of gallium arsenide the distribution of dislocations is more inhomogeneus than in crystals fabricated without magnetic field.
SUMMARY OF THE INVENTION
The purpose of the present invention is to establish a process and an equipment which does not only reduce the axial segregation problem, but which additionally reduces the local and radial segregation problem. Furthermore, improvements of the structural perfection of the pulled crystals as well as an improved solid-liquid interface are to be achieved.
According to this invention these tasks are achieved by a separation wall which is not extending to the crucible bottom and which is rotated in the same direction with the crystal holder. The rotation rates of crystal holder and separation wall may differ, but the rotation rates may also be identical.
In addition, the crucible can be rotated in a known manner whereby its sense of rotation may be equal or contrary to that of crystal holder and separation wall. In a further elaboration it is forseen to achieve a constant melt level by adding material to the region between separation wall and crucible wall whereby the quantity of feed material corresponds to the material pulled as crystal from the melt.
The effect of this invention can be understood by an annular space within the rotating separation wall (beneath the crystal) which is filled with melt, and which is quasi "separated" from the remainder fraction of the melt. In this internal melt volume the natural and the forced convection will be reduced, and this effect is enforced with increasing rotation rate of crystal and separation wall. Especially the formation of Taylor-Proudman cells will be suppressed in this region, and the disturbing effect of Marangoni convection is significantly reduced.
The axial segregation problem in present state-of-the-art is significantly reduced in this invention by a comparably fast approach of a steady-state concentration in the crystal. This concentration remains constant during a certain growth period, until towards the end of the process (at a distribution coefficient less than one) it is increasing. The quality and thus the yield of the pulled crystal is thus increased. This advantage is even more significant in a continous process in which continuously the material quantity pulled as crystal from the melt is replenished, and where a st
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patent: 4874458 (1989-10-01), Nishizawa
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patent: 5290395 (1994-03-01), Matsumoto et al.
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