US3033660A

US3033660A - Method and apparatus for drawing crystals from a melt

Info

Publication number: US3033660A
Application number: US20360A
Authority: US
Inventors: Okkerse Boudewijn
Original assignee: US Philips Corp
Current assignee: US Philips Corp; North American Philips Co Inc
Priority date: 1959-05-05
Filing date: 1960-04-06
Publication date: 1962-05-08
Anticipated expiration: 1979-05-08
Also published as: CH407051A; GB953538A; NL238924A; DE1136670B

Description

May 8, 1962 B. OKKERSE METHOD AND APPARATUS FOR DRAWING CRYSTALS FROM A MELT Filed April 6, 1960 INVENTOR BOUDEWIJN OKKERSE AGENT United States Patent 3,033,660 METHOD AND APPARATUS FOR DRAWING CRYSTALS FROM A MELT Boudewijn Okkerse, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York,

N.Y., a corporation of Delaware Filed Apr. 6, 1960, Ser. No. 20,360 Claims priority, application Netherlands May 5, 1959 11 Claims. (Cl. 23301) This invention relates to a method of drawing crystals from a melt. Such a method is commonly employed for manufacturing monocrystalline bodies from semi-conductor materials, such as germanium, silicon and intermetallic compounds. These bodies have to satisfy the requirement of homogeneity, that is to say, that their crystal lattice exhibits few dislocations and that the physical properties, for example the specific resistance, are the same at any point of the crystal.

In a known method, the melt is produced in a cylindrical crucible having a flat or concave bottom; then a socalled seed crystal is brought into contact with the melt and subsequently slowly drawn upwards so that material crystallizes out and grows to the crystal lattice of the seed.

The homogeneity of the resulting body may show deviations; it may deviate in the direction of drawing, that is to say, in the vertical direction, and at right angles thereto. The homogeneity in the former direction is influenced inter alia by the composition of the melt and by variations in the drawing rate. The factors which can influence the transverse homogeneity, however, were not exactly known.

It has already been suggested to rotate the seed during the drawing process in order to avoid dislocations and other defects in the crystal. It was found that this measure materially reduces the occurrence of such defects but does not completely eliminate them, and it is an object of the present invention to create better conditions for homogeneity of the crystal.

The present invention is based on the recognition that defects in the crystal are due to temperature fluctuations in the melt, in particular in the part of the melt immediately under the crystal.

Uncontrollable convection currents and/or eddies must be considered as an important cause of the occurrence of fluctuations in the melt.

It is a particular object of the invention to create more favourable conditions in the melt under the crystal. According to the invention, the crucible has a part situated above the bottom, which part hereinafter will be referred to as a projection and the diameter of which is at least equal to that of the drawn crystal.

Due to the provision of the projection the convection currents encounter a greater flow resistance in the shallow part of the melt over this projection than in the deeper part of the melt, which hereinafter will be referred to as an annular channel, whereas the currents in this annular channel enhance the uniformity of the melt both with respect to the temperature and to the composition thereof. In addition, the crystal is rotated about its axis in the usual manner, the comparatively thin layer of the melt under the crystal thus being thoroughly stirred.

. Preferably the projection is cylindrical; its upper surface is preferably flat or substantially flat.

The inner diameter of the crucible is preferably at least twice the diameter of the projection. Thus, the annular channel has a width such that the flow resistance is low enough both for horizotnal and for vertical convection currents. Hence a disturbing influence of the heat exchange between the lateral wall of the crucible and the melt will be hardly perceptible near the growing crystal.

3,033,660 Patented May 8, 1962 The upper surface of the projection preferably lies at a level with respect to the bottom of the crucible of at least one third of the diameter of the projection. With such a level a possible horizontal temperature gradient in the bottom of the crucible will be scarcely perceptible at the upper surface of the projection. Hence, the temperature at the upper surface will depend to a higher degree upon the heat exchange between the projection and the surrounding melt, in which there are no large temperature variations.

During the drawing process, the spacing between the crystal and the projection is preferably maintained con stant.

This may be effected by making the projection vertically movable with respect to the crucible and gradually lowering it during the drawing of the crystal according as the level of the melt falls. The projection may be shaped as a disc arranged on a shank which is passed through an aperture in the bottom and by means of which the position of the projection can be controlled from with out. The clearance between the wall of the aperture and the shank must be so slight that the upward pressure due to the surface tension exceeds the hydrostatic pressure of the melt. The upward pressure can be calculated in grams per sq. cm. by means of the following formula:

see

where 7 is the surface tension in dynes/cm., g the acceleration of gravity in cm./sec. and R and R are radii of curvature of the surface of the liquid in two directions at right angles to one another in cm. For a clearance of 0.2 mm., with molten germanium contained in a graphite crucible, the surface tension is to be assumed to be 500 dynes/cm., R A cm. and R =infinity. If it is further assumed that g=l000 cm./sec. the upward pres-, sure is found to be 50 g./sq. cm.

The hydrostatic pressure of the germanium melt must not exceed this upward pressure and consequently the level of the melt must lie at most slightly more than 8 cm. above the bottom, and this is generally sufficient in practice.

The constant spacing between the crystal and the projection may alternatively be maintained by supplying melt to the crucible during the drawing of the crystal so that the level of the melt in the crucible is constant. For this purpose, the crucible preferably, by means of a duct, communicates with a container the volume of which can be varied. Preferably this container comprises a cylinder and piston arranged under the crucible. Between the piston and the cylinder a small clearance is again admissible provided that the upward pressure due to the surface tension again exceeds the hydrostatic pressure of the melt filling the container.

According to a further preferred embodiment, the level of the melt is maintained at a level of at most 6 mm. above the upper surface of the projection.

The invention will now be described more fully with reference to some embodiments illustrated by the accom= panying drawing. In the figures of this drawing, apparatus in accordance with the invention is shown partly in cross section and partly in perspective.

FIGURE 1 shows an apparatus in which the projection is vertically movable.

FIGURE 2 shows an apparatus in which the pro'jec-l tion has a fixed position relative to a crucible communicating with a container. I

Example I a mm. An aperture 3" is centrally of the bottom 2 of the crucible; The projection" comprises a graphite disc 4' having a diameter of 18 mm. and a height of 6 mm. This disc is arranged on a vertical shank 5, which passes through the aperture 3 with a clearance of 0.2 mm. and is capable of free vertical movement through this aperhim, so that the position ofthe disc 4 can readily be adjusted from without.

The crucible is arranged on a hollow cylindrical support 6 and is surrounded by a high-frequency coil 7. The crucible contains molten germanium 9 which cannot flow away through the aperture 3 since the upward pressure produced by the surface tension exceeds the hydrostatic pressure. The high-frequency coil 7 is energized so that heat is produced in the lateral wall 8 of the crucible. This heat is transferred to the melt 9 so that convection currents are produced therein.

The disc 4 is adjustedso that its upper side It) lies 4 mm. below the level of the melt. Thus, the convection currents will be kept away from the melt over the disc 4 and will travel along an annular channel 11.

A rod-shaped germanium crystal12 having a diameter of 15 mm. is in contact with the melt over the disc 4' arid is drawn upwards at a rate of 1 mm. per minute so as to grow. While being drawn up, the crystal may be rotated about its vertical axis. During the drawing process, the level of the melt in the crucible falls. Now the disc 4 is gradually lowered so that the ditference in height between the level of the melt and the upper surface of the disc is maintained at 4 mm. and the ldiquilcll layer under the crystal is maintained at a constant ept g The current strength in the coil is controlled so that the diameter of the growing crystal is maintained at mm. so'that the crystal does not project beyond the disc 4 horizontally.

Example II In this example use is made of an apparatus shown in FIGURE 2. In this figure, reference number 29 denotes a cylindrical graphite crucible having an inner diameter of 50 mm., the bottom 21 of this crucible being provided with a central cylindrical projection 22 having a diameter of mm; and a height of 10 mm. This projection has a flat upper surface 23.

The crucible 20 is supported by a graphite disc 24 having .a rim 25 projecting on either side of the disc. The'rimmed disc is supported by a hollow graphite cylinder 26'closed at the .upper end. Owing to the provision of the broad-rimmed disc two

chambers

27 and 28 having horizontal boundary surfaces are produced between the bottom 21 and the upper surface of the cylinder 26.

A vertically movable piston 29 provided with a connecting rod 30 fits within the cylinder 26 with a clear anceof 0.1 mm. The cylinder 26 and the piston 29 form a container 31 of variable volume. The container 31' is connected to the crucible 20 by a duct 32 which extends in part through a rod-shaped connecting member 33 interposed between the upper surface of the cylinder 26 and the bottom 21 of the crucible and passing through an aperture 34 in the disc 24.

The assembly comprising the crucible 20, the disc 24 and the cylinder 26 containing the piston is arranged on a hollow support 36 and surrounded by a high-frequency coil 35. m

In operation, .the container 31 and the duct 32 are completely filled and the crucible 20' is partly filled with molten germanium 38 The connecting rod of the piston 29 is adjusted so that the level of the melt 38 in the crucible is at a height of 3 mm. above the upper surface 23 of the projection 22.

The high-frequency coil is' energized so that heat is produced in the walls of the crucible 20 and the container 31' and also in the rim 25. Due to the presence of disc 24' provided with the rim 25 and the chambers 4- 27 and 28, additional heat is supplied to the bottom 21 of the crucible and to'the upper end of the container 31.

The convection currents produced in the melt 38 are restricted to an annular channel 39 surrounding the projection 22 so that the layer of the melt over the projection is not influenced by these currents. 7

By means of a drawing mechanism (not shown) a germanium crystal 40 is drawn from the melt at a point directly over the projection 22 at a rate of 1 mm. per minute so as to grow gradually. By controlling the highfrequency current in the coil the diameter of the growing crystal is maintained at 19 mm. so that it does not project horizontally beyond the projection 22. By rotation of the crystal, the part of the melt over the projection can be thoroughly stirred.

By the gradual growth of the crystal, melt is continuously being withdraum from the crucible. By simultaneously raising the piston 29, however, melt is supplied to the crucible from the container 31 through the duct 32. The: speed. at which the piston travels is controlled so that the level of the melt 38 in the crucible is maintained at a height of 3 mm; above the upper surface 23 of the projection 22.

What is claimed is:

l. A method of drawing crystals from a melt contained in a crucible having bottom and side wall portions and a projection extending upwardly from the bottom wall within and spaced from the said side walls and whose upper extremity possesses a given diameter, comprising the steps of maintaining the melt within the crucible at a level above the upper extremity of the projection, and drawing a crystal upwards from a region of the melt lying over the said extremity.

2. A method of drawing crystals from a semicondu tive melt contained in a crucible having bottom and side Wall portions and a projection extending upwardly from the bottom wall within and spaced from the said side walls and whose upper extremity possesses a given diameter, comprising the steps of maintaining the melt within the crucible at a level just above the upper extremity of the projection, and drawing a growing crystal having a diameter not greater than said given diameter upwards from a region of the melt lying only over the said extremity. 4

3. A method as set forth in claim 2 including the step of maintaining substantially constant the meltlevel above the said upper extremity by lowering the projection while the crystal is drawn upwards.

4. A method as set forth in claim 2 including the step of maintaining substantially constant the melt level above the said upper extremity by introducing melt into the crucible while the crystal is drawn upwards.

5. A method of drawing crystals from a semiconductive melt contained in a crucible having bottom and side wall portions and a projection extending upwardly from the bottom wall within and spaced from the said side walls and whose upper extremity is cylindrical possessing a given diameter and has a flat upper surface, comprising the steps of maintaining the melt within the crucible at a level not more than 6 mm. above the upper surface of the extremity of the projection, and drawing a growing crystal having a diameter not greater than said given diameter upwards from a region of the melt lying only over the said extremity.

6. Apparatus for drawing crystals comprising a crucible having bottom and side wall portions and a projection extending upwardly from the bottom wall within and spaced from the said side walls and whose upper eX- tremity possesses a given diameter, a melt of semiconductive material within said crucible, means for maintaining the melt at a level just above the upper extremity of the projection, and means for drawing a crystal upwards from a region of the melt lying only over the said extremity.

7. Apparatus as set forth in claim 6, wherein the projection is vertically movable, and means are provided for lowering the projection while the crystal is drawn upwards to maintatin the melt level just above the said extremity.

8. Apparatus as set forth in claim 6, wherein means are provided for feeding melt into the crucible to maintain the melt level.

9. Apparatus for drawing crystals comprising a crucible having bottom and side Wall portions and a projection extending upwardly from the bottom wall Within and spaced from the said side walls and Whose upper extremity possesses a substantially flat upper surface and is cylindrical with a given diameter, a melt of semiconductive material within said crucible, means for maintaining the melt at a level just above the upper extremity of the projection, and means for drawing a growing crystal having a diameter not greater than the said given diameter upwards from a region of the melt lying only over the said extremity.

10. Apparatus as set forth in claim 9, wherein the spacing between the crucible side walls is at least twice the said given diameter, and the flat upper surface of the said extremity is spaced from the bottom wall at a distance at least equal to one-third the said given diameter.

11. Apparatus as set forth in claim 9, wherein the melt level is maintained at a distance not more than 6 mm. above the upper surface of the said extremity, and said distance is maintained substantially constant.

References Cited in the file of this patent UNITED STATES PATENTS 1,552,884 Schnurpfeil Apr. 8, 1925 2,793,103 Emeis May 21, 1957 FOREIGN PATENTS 754,767 Great Britain Aug. 15, 1956

Claims

1. A METHOD OF DRAWING CRYSTALS FROM A MELT CONTAINED IN A CRUCIBLE HAVING BOTTOM AND SIDE WALL PORTIONS AND A PROJECTION EXTENDING UPWARDLY FROM THE BOTTOM WALL WITHIN AND SPACED FROM THE SAID SIDE WALLS AND WHOSE UPPER EXTREMITY POSSESSES A GIVEN DIAMETER, COMPRISING THE STEPS OF MAINTAINING THE MELT WITHIN THE CRUCIBLE AT A LEVEL ABOVE THE UPPER EXTREMITY OF THE PROJECTION, AND DRAWING A CRYSTAL UPWARDS FROM A REGION OF THE MELT LYING OVER THE SAID EXTREMITY.