Die for shaped crystal growth from a molten bath

Details Die for shaped crystal growth from a molten bath Die for shaped crystal growth from a molten bath

2000-06-08

2001-12-04

Kunemund, Robert (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Apparatus

For crystallization from liquid or supercritical state

C117S210000, C117S211000, C117S212000

Reexamination Certificate

active

06325852

ABSTRACT:

TECHNICAL FIELD
The subject of this invention is a die for the drawing of crystals from a molten bath, that may be used in particular, in solidification methods referred to as capillary growth methods.
These methods consist of fabricating an item made of a polycrystalline or monocrystalline material by drawing it through a die from a bath of the molten material and crystallizing the molten material coming out of the die on a seed.
The invention is particularly applicable to the production of items with a complex shape, made of various materials, for example, oxides, silicon, aluminum, or other metals, in a mono- or poly-crystalline form.
STATE OF THE PRIOR TECHNOLOGY
Methods of manufacturing items made of a monocrystalline or polycrystalline material by growth from a molten bath have been known for a long time and have been described, for example, in document 1: H. E. La Belle, Jr., Journal of Crystal Growth, 50, 1980, pages 8 to 17, and in document 2: Y. A. Tatarchenko, Shaped Crystal Growth, Kluwer Academic Publishers, 1993, 6, pages 1 to 18.
In document 1, the drawing method referred to as EFG (Edge defined, Film-fed Growth) is especially described, a method wherein a die is used that comprises a capillary channel produced in a material that is wetted by the molten bath.
In document 2, various crystal growth methods are described, and in particular, preforming methods that use dies of suitable shape.
Document 3: EP-A-653 504 describes a method of manufacturing items in a polycrystalline or monocrystalline material by crystal growth from a liquid bath of the molten material. In this method, a die is used through which the molten material that one wishes to crystallize on the seed is drawn, and the die and/or the seed is subjected to at least one translational movement in a plane perpendicular to the direction of drawing. In particular, this allows the production of items of complex shape.
Document 4: U.S. Pat. No. 4,430,305 describes a die with capillary tubes suitable for drawing items made of silicon having the shape of a plate. This die has a particular shape in order to encourage the appearance of the impurity SiC only on one side of the plate being produced.
Recent developments in preforming methods make use of drawing devices which permit the development of particular shapes by the use of vertical drawing, combined with relative horizontal movement of the seed on which solidification is occurring, with respect to the die which is supplying the molten material.
Certain installations are derived from a Czochralski furnace, where a seed and crucible are driven in a rotational movement &ohgr; about a vertical axis Z, and a translational movement along this same axis.
FIG. 1
appended shows the simple case of traditional preforming of a bar, where only vertical translation of the seed is used.
In such a method, supply takes place in general from a crucible
1
containing the liquid
2
to be solidified, through a die
3
which comprises at least one internal bore, either a capillary channel
5
, wherein the liquid coming from the crucible can circulate, or by capillary rise in the case where the liquid wets the die material, or under the effect of a pressure difference between the surface of the liquid
2
in the crucible and the outlet from the die. At the outlet from the die
3
, a liquid meniscus
7
is obtained and the solidification of the liquid is initiated on a monocrystalline seed
9
and is continued by the drawing of this seed and of the formed solid
10
, upwards in the direction of the arrow, at speed Vz. The precise shape of the solid obtained depends on the distance between the solidification interface
11
and the outlet from the die
3
, but always stays close to the latter. The position of this interface can be adjusted by controlling the supply of heat (calories) to the crucible
1
.
For local preforming, there is an extra movement: the rotation of the seed about itself.
In document 3, the method described enables one to draw curved shapes, using traditional preforming (i.e. without rotating the seed), or round items using local preforming (i.e. with rotation of the seed).
FIG. 2
appended shows how to obtain a curved rod
19
of circular cross-section, by using the combined movements of vertical drawing and translation in the horizontal direction, of both the seed
9
and the item undergoing solidification.
In this figure, one can see the die
3
bored with a central channel
5
, through which the liquid moves to form a meniscus
7
at the outlet of the die with a solid-liquid interface
11
of the item undergoing solidification
19
. In this case, the seed
9
is displaced in the vertical direction at speed Vz causing a horizontal translational movement to be described at the die at a suitable speed Ux. The diameter of the die is matched to provide the desired diameter d of the curved rod
19
, this diameter also depending on the position of the interface
11
above the die
3
. In order to obtain a rod having the shape of an elliptical arm with half-axes a and b, the horizontal translational movement of the die is carried out between a first position wherein the seed and the die are aligned and a second position wherein the seed is the distance b from the die, and the speed of vertical drawing is regulated and the horizontal translation function Ux of the die in relation to time t, is such that one has:
Vz=(&pgr;a/2&tgr;)·sin(&pgr;t/2&tgr;) and Ux=(&pgr;b/2&tgr;)·cos(&pgr;t/2&tgr;)
with &tgr; representing the duration of drawing and a and b representing respectively the half-axes of the ellipse.
In the methods described above one uses the same dies as those developed for traditional preforming, that is to say dies whose upper face is horizontal, or perpendicular to the axis of the bore and to the direction of drawing. With such dies, the control of the shape given to the preforming material occurs only at the fluid meniscus
7
. Its geometry depends on the capillary constant of the molten material and the contact limit conditions on the die. The shape of the meniscus and its contact on the die are therefore the only parameters which locally define the shape of the crystal.
Whatever the height Hm of the meniscus, linked to the nature of the material and to the thermal profile in the furnace, the equilibrium of the fluid drop on the horizontal die always leads, for large drawing angles, to too small a crystal thickness for the method of pulling away horizontally to be viable.
It has also been observed that the use of dies wherein the upper face is horizontal is also restricted by the problem of attachment of the meniscus over the entire section of this face. The control of the shape then becomes deficient and makes the method of pulling away horizontally non-viable.
In
FIG. 3
, which represents diagrammatically a preforming method analogous to that in
FIG. 2
, and which uses the same reference numbers, it can be seen that the thickness Ec of the crystal
19
drawn with the die
3
with a horizontal upper face, depends on the characteristic dimension Ef of the die
3
, the height of the meniscus Hm and the drawing angle &agr;. This drawing angle is uniquely a function of the speeds imposed by the growth. For example, in
FIG. 2
, where the speed of horizontal translation is designated Ux and the speed of vertical drawing is Vz, the drawing angle is given by:
tg
⁢
 
⁢
 
⁢
α
=
Ux
Vz
(R1)
When the height of the meniscus Hm is negligible compared with Ef (this is the case for example for sapphire), the thickness Ec of the crystal is given by:
Ec=Ef·cos &agr;  (R2)
The equation (R2) reveals a major handicap of preforming using traditional dies: the geometric limitation of the drawing angle required to keep the crystal from having zero thickness. Quite obviously, a drawing angle of 90° to the vertical is impossible with a traditional die with a horizontal top.
Hence, these traditional dies are not suitable for large drawing angles and, in particular to create horizontal pulling away of the crystal grow

Affiliated with

Also associated with

Rating

Die for shaped crystal growth from a molten bath does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Die for shaped crystal growth from a molten bath, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Die for shaped crystal growth from a molten bath will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2576298