Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
2000-12-08
2002-03-12
Pelham, Joseph (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C219S422000, C117S217000
Reexamination Certificate
active
06355910
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heating element for heating crucibles as disclosed in claims
1
,
7
and
17
as well as to an arrangement of heating elements for heating crucibles as disclosed in claim
13
. In particular, these heating elements can be used for heating a crucible when growing semiconductor crystals such as silicon, germanium and gallium arsenide.
2. Prior Art
A known device for growing semiconductor crystals using the Liquid Encapsulated Czochralski (LEC) process is schematically shown in FIG.
7
. Such device is known for example from Ishiki, M., Recent Development of Bulk Crystal Growth, Research Signpost 1998, pages 63-64 (ISBN: 81-86481-58-3). A crucible
1
for receiving a molten mass
2
of semiconducting material is arranged on a crucible holder
3
within a cooled recipient
4
. The molten semiconductor
2
is covered with a molten mass of boron oxide
12
to prevent volatile components (e.g. As in GaAs melts) from escaping by evaporation. To that end, the steam pressure in the recipient
4
has to be greater than the steam pressure of the volatile component (e.g. >2 bars in the case of GaAs). For heating the crucible
1
, there is provided an arrangement of heating elements consisting of a lower heating element
100
surrounding the crucible
1
at its bottom and in a wall region adjacent to the bottom, and an upper heating element
200
provided above the lower heating element
100
and surrounding the upper region of the crucible wall. The lower heating element
100
is connected via power leads
110
to electrodes
120
provided at the bottom of the recipient and the upper heating element
200
is connected via power leads
210
to electrodes
220
which are also provided at the bottom of the recipient. A crystal nucleus
5
is connected to a pulling device
6
fed through the cover of the recipient
4
and arranged above the crucible
1
. Also, a growing crystal
7
being pulled from the moltem mass
2
is connected to the pulling device
6
via said crystal nucleus
5
. A drive is provided for rotating the crucible holder
3
and the crystal nucleus
5
in opposite directions during the growing process. The arrangement of heating elements
100
,
200
is thermally insulated by an annular insulating pipe
8
surrounding the heating elements and consisting of a thermally insulating material. Only a small gap is provided between the insulating pipe
8
and the heating elements
100
,
200
in order to prevent excessive gas convection which, due to the high pressure within the recipient, may result in large temperature fluctuations in particular with semiconductor materials having a highly volatile component such as gallium arsenide or indium phosphide.
U.S. Pat. No. 4,533,822 discloses a tulip-shaped heating element used as a bottom heater, which heating element is formed as a jacket-type hollow body having an upper edge and a lower edge, said hollow body comprising a first hollow cylindrical portion having a first cross section and a second portion adjacent thereto and having a tapering cross section, for surrounding a bottom region of a crucible.
DE 44 23 196 A1 discloses a hollow cylindrical heating element
200
for heating crucibles, having an upper edge and a lower edge, and comprising at least one power lead extending from said lower edge as an extension of the cylinder wall of said hollow body.
However, a combination of the known tulip-shaped heating element
100
and the hollow cylindrical heating element
200
in the vertical arrangement shown in
FIG. 7
requires an interruption of the tubular thermal insulation with the insulating pipe
8
at least at two openings
9
,
10
for feeding through the power leads
210
to the upper heating element
200
. Through these openings
9
,
10
, a cold gas may flow in from the space surrounded by the cooled recipient
4
, thus affecting crystal growth.
The vertical arrangement of two heating elements as shown in
FIG. 7
enables variable adjustment of the axial temperature gradient within the inner space. However, an abrupt thermal transition at the gap
11
between the lower heating element
100
and the upper heating element
200
causes a disturbance of the axial temperature distribution in this region as well as unfavorable mechanical stresses in the growing single crystal, reducing the yield of single crystal material.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a heating element and an arrangement of heating elements for heating a crucible, in particular for growing semiconductor single crystals, preventing disturbances in the desired temperature distribution in the region of the molten material and the growing crystal and improving its yield.
This object is achieved by a heating element according to claims
1
,
7
or
17
and by an arrangement of heating elements according to claim
13
. Further improvements of the invention are presented in the dependent claims.
The invention will be better understood from the following description and figures of preferred embodiments which are presented for illustrative purposes only and are not to be construed as limiting the scope of the present invention.
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patent: 5330729 (1994-07-01), Oda et al.
patent: 5660752 (1997-08-01), Vilzmann
patent: 5766347 (1998-06-01), Shimomura et al.
patent: 5968266 (1999-10-01), Iino et al.
patent: 6093913 (2000-07-01), Schrenker et al.
patent: 4204777 (1992-02-01), None
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Eichler Stefan
Köhler Andreas
Seidl Albrecht
Dike, Bronstein, Roberts & Cushman, IP Group of Edward & Angell
Freiberger Compound Materials GmbH
Neuner, Esq. George W.
Pelham Joseph
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