GB2024649A - Apparatus for pulling single crystal from melt on a seed - Google Patents

Abstract

An apparatus for pulling a single crystal from a melt on a seed consists of a chamber 1,2 housing a crucible 3, a heater 4 for heating the crucible, and thermal insulation 5a, 5b, 5c shielding the crucible, a rotatable and reciprocable vertical rod 6 for carrying the seed 8, and means for rotating the crucible including a support ring 9 on which the crucible 3 is carried by a holder 10. The ring 9 which is rotatably supported on a mount 21 has a toothed periphery to be driven by a shaft 15 through a gear train 14, 17, 18, 16. In an alternative embodiment the crucible is suspended from the inner periphery of a support ring (23) connected to its outer periphery with a cylindrical stand (32) having a toothed rim (31) carried by a bearing (30) and meshing with a driving gear (27). <IMAGE>

Description

SPECIFICATION Apparatus for pulling single crystal from melt on a seed The invention relates to the art of growing single crystals from a melt, and, more particularly, to pulling single crystal from melt on a seed.

The invention can be effectively used in growing from melt single crystals of large diameter, such as optical and scintillation single crystals (sodium chloride, potassium chloride, potassium bromide, lithium fluoride, sodium iodide, etc.), as well as semiconductor single crystals (germanium, silicon, solid solutions of tin and lead tellurides).

According to the present invention there is provided an apparatus for pulling single crystal from a melt on a seed, comprising a sealed chamber accommodating therein a crucible with the meit, operatively connected with a drive for effecting rotation thereof, a heater with thermal insulation means enclosing the crucible, and a rod extending coaxially with the crucible and carrying the seed holder, the rod being operatively connected with a drive for effecting rotation and vertical displacement of this rod, the operative connection between the crucible and its rotation drive including a support ring accommodated within the chamber and supporting thereon the crucible for their joint rotation during the process of pulling the single crystal.

Such an apparatus can be simple in construction, very reliable in operation, and operable over a wide range of process parameters.

With the apparatus of the invention obstructionfree supply of heat from the heater to the entire crucible surface can be achieved, without overheating the crucible walls and without any undesirable temperature gradient from the periphery of the crucible toward the centre of its bottom.

Furthermore use of the apparatus can allow parasitic crystallization at the bottom ofthe crucible to be precluded.

In a preferred embodiment of the invention the operative connection between the crucible and its rotation drive includes a double-stage reducing gear accommodated within the chamber, the driving pinion of the first stage of this reducing gear being fast with the output shaft of another reducing gear arranged together with the drive outside the sealed chamber, the driving pinion of the second stage of the double-reducing gear cooperating with the support ring for rotating the latter It is expedient that the support ring should be made in the form of a toothed rim. In this way the construction of the apparatus is simplified and the reliability of its performance is enhanced.

In an alternative embodiment the operative connection between the crucible and its rotation drive includes a bearing accommodated in the sealed chamber and encompassing the thermal insulation means, the stationary race of the bearing being secured at the bottom of the chamber coaxially with the seed holder rod, and a reducing gear of which the driving pinion is fast with the output shaft of the rotation drive arranged outside the chamber, and of which the driven member is in the form of a toothed rim mounted on the rotatable race of the bearing and connected with the support ring for rotating the latter.

The support ring may be mounted on a cylindrical stand having its lower end secured to the toothed rim coaxially therewith, so that the stand encloses therein the heater with the thermal insulation means.

It is preferable that the support ring should be mounted on the stand with adjustment-wise displacement in a horizontal plane relative to the axis of the seed holder rod, and the support ring may take the form of two concentric thin-wall rims interconnected with spokes which are uniformly circum ferentially spaced and extend tangentially to the internal rim.

The present invention will be further described in connection with the embodiments thereof, with reference being had to the accompanying drawings, wherein: Figure 1 is a sectional schematic view of an apparatusfor pulling single crystal from melt on a seed in accordance with the invention; Figure 2 is a sectional schematic view of an alternative embodiment of the invention, with a different structure of the crucible rotation drive arrangement; Figure 3 is a plan view ofthe support ring in accordance with one of the features of the present invention.

The invention resides in an apparatus for pulling a single crystal from a melt on a seed. In the presently described embodiment the melt from which the single crystal is to be pulled is either a semiconductive silicon, our a solid solution of tin and lead tellurides. The apparatus schematically illustrated in Figure 1 of the appended drawings comprises a sealed chamber 1 with a lid 2. The chamber 1 accommodates therein a crucible 3 operatively connected with a drive for effecting its rotation, and a heater 4 with thermal insulation means 5 enclosing the crucible 3.

In this embodimentthethermal insulation means is in the form of a graphite block made up of five cylindrical screens 5a, one ceiling screen 5b and one bottom screen Sc arranged at the bottom of the chamber 1. The two of the cylinder-shaped screens 5a, closest to the heater 4, are slotted to accommodate the elements of the operative connection, i.e. of the crucible-driving chain. The entire thermal insulation block is mounted on the bottom screen 5c.

Extending into the chamber 1 coaxially with the crucible 3 is a rod 6 sealingly passing through the lid 2 and carrying the holder7 of a seed 8. The rod 6 is associated with any suitable known per se drive (not shown) for effecting its rotation and vertical reciprocation.

In the presently described embodiment the cruci ble material can be quartz, boron nitride, silicon nitride, etc.

The holder 7 of the seed 8 is made of graphite.

In accordance with the invention, the operative connection between the crucible 3 and its rotation drive includes a support ring 9 having the crucible 3 suspended therefrom for their joint rotation during the crystal-pulling operation.

The crucible 3 in the presently described embodiment is suspended from the support ring 9 on a holder 10 enveloping the crucible 3 and having at the top thereof the shoulder means 11 by which the holder 10 bears upon the support ring 9.

The heater 4 is supported by current conductors 12 which sealingly extend into the chamber 1 through its bottom 13 and are secured thereto.

The operative connection or drive chain between the crucible 3 and the drive effecting its rotation includes in the presently described embodiment, in accordance with the invention, a double-stage reducing gear accommodated within the sealed chamber 1, of which the driving pinion 14 of the first stage is fast with the output shaft 15 of another known per se reducer (not shown) arranged together with its drive motor (not shown, either) outside the sealed chamber 1.

The driving pinion 14 cooperates with the driving pinion 16 of the second stage of the double-stage reducing gear through an idle pinion 17 and a driven pinion 18. In its turn, the driving pinion 16 ofthe second stage cooperates with the support ring 9 for rotating the latter. In the embodiment being described the support ring is a toothed rim with annular grooves 19 and 20, respectively, in its top and bottom planes.

The groove 19 receives therein the shoulder means 11 of the holder 10 of the crucible 3.

The chamber 1 accommodates therein a cylindrical mount 21 adapted to carry the support ring 9. The mount 21 has its bottom end face rigidly secured in an annular groove 22 cut in the bottom screen 5c, while its top end face 9 is received in the annular groove 20 of the support ring 9 so that the support ring is able to rotate by sliding about the top end face of the mount 21.

Figure 2 of the appended drawings illustrates another embodiment of the apparatus for pulling alkali halide single crystals, e.g. sodium chloride, on the seed 8 from a melt. Similarly with the previously described embodiment, the apparatus comprises a sealed chamber 1 with a lid 2. The sealed chamber 1 accommodates a crucible 3 operatively connected with the drive for effecting its rotation, a heater 4 with heat insulation means 5 enclosing the crucible 3 and a rod 6 with the holder 7 of a seed 8, extending coaxiallywith the crucible 3.

In accordance with the invention, the operative connection or drive chain between the crucible 3 and the drive effecting its rotation includes a support ring 23 accommodated in the chamber 1 and adapted to support thereon the crucible 3 for their joint rotation during the crystal-pulling operation.

The crucible 3 is supported by the support ring 23 with aid of hooks 24 provided at the top of the crucible 3. At the horizontal level of the support ring 23, extending sealingly into the chamber 1 through its sidewall 25 is a pusher 26 provided for adjusting the position of the crucible 3 in the chamber 1.

In this embodiment the operative connection between the crucible 3 and the drive for its rotation includes, in addition to the support ring 23, accommodated in the chamber 1, a bearing and a reducing gear of which the driving pinion 27 is fast with the output shaft 28 of the rotation drive (not shown) arranged outside the chamber 1, the shaft 28 sealingly extending into the chamber 1.

The bearing is made up by a rotatable race 29 and a stationary race 30 secured at the bottom 13 of the chamber 1 coaxially with the rod 6 of the holder 7 of the seed 8. The driven member of the reducing gear is in the form of a toothed rim 31 fastened about the rotatable ring 29 of the bearing and connected with the support ring 23 for rotating the latter jointly with the crucible 3.

The last-mentioned connection includes a cylindrical stand 32 carrying the support ring 23 and having its bottom end face secured to the toothed rim 31 coaxially theire-with, so that the mount 32 encompasses the heater 4 with the thermal insulation means 5.

The support ring 23 is carried by the stand 32 with adjustment-wise displacement in a horizontal plane relative to the axis of the rod 6 of the holder 7 of the seed 8 and includes two concentric thin-wall rims, viz, the inner one 33 (Figure 3) and an outer one 34 interconnected with spokes 35 which are uniformly circumferentially spaced and extend tangentially to the internal ring 33.

The embodiment of the herein disclosed apparatus for pulling a single crystal, illustrated in Figure 1, operates, as follows. Before commencing the operation, the holder 10 of the crucible 3 is mounted on the support ring 9, and the crucible 3 with the initial material is placed into the holder, whereafter the crucible rotation drive is energized. The rotation is transmitted to the crucible 3 through the driving pinion 16 of the second stage of the reducing gear, through the driven pinion 18, intermediate pinion 17, and the driving pinion 14 of the first stage from the output shaft 15 of the external reducer. The seed 8 is secured in the holder 7, and the drive rotating the rod 6 is energized. Then the appropriate conditions are set up in the chamber 1 - either vacuum or counter-pressure of an inert gas - and the heater 4 is energized to melt the initial material.The seed 8 is, first, lowered into the melt 36, and after the seed 8 partially melts over, it is gradually pulled by energizing the drive of the vertical displacement of the rod 6 of the holder 7 of the seed 8.

Then the temperature of the melt 36 and the pulling rate are controlled in the known per se appropriate manner to pull a single crystal 37 of the preset diameter.

The operational reliability at temperatures associated with the process can be attained by making the movable parts of the apparatus within the chamber 1 of a mechanically strong, heat-resistant material, e.g. of graphite of semiconductor purity. With both the thermal insulation and the double-stage reducer made of graphite, the high sterility of the crystallization process is provided for.

The embodiment of the presently disclosed apparatus, illustrated in Figures 2 and 3, operates, as follows. Before commencing the operation, the crucible 3 is mounted on the support ring 23, and the initial material (potassium chloride, for instance) is charged into the crucible. Then the rotation drive of the crucible 3 is energized to transmit the driving torque from the output shaft 28 to the crucible 3 via the driving pinion 27, the toothed rim 31, the cylindrical stand 32 and the support ring 23. Then the set screw or pusher 26 is operated to center the crucible to be aligned with the axis of its rotation and with the axis of the rod 6 of the holder 7 of the seed 8. The seed 8 is then secured in the holder 7, and the drive rotating the rod 6 is energized. Then the heater 4 is energized to melt the initial material.The seed 8 is slowly lowered until it comes into contact with the melt 36. Following the partial melting-over of the seed 8 and establishment of a balanced state between the seed 8 and the melt 36, i.e. the state where neither melting of the seed 8 nor crystallisation of the melt thereon takes place, the drive effecting the vertical displacement (in this case, the raising) of the rod 6 of the holder 7 of the seed 8 is energized. The subsequent control of the pulling rate and of the heater temperature provides for the single crystal 37 of the required diameter growing on the seed 8.

As it can be seen from the above detailed description of the embodiments of the herein disclosed apparatus, the present invention provides for precluding the overheating of the melt and the eventually resulting pollution of the melt with the crucible material (e.g. when either silicon or tin and lead tellurides single crystalls are pulled from the melt in a quartz crucible). Furthermore, the present invention provides for having any preset distribution of the specific heating intensity over the entire crucible surface and thus for obtaining any desired temperature field in the melt. In combination with the provision for rotating the crucible at any required speed in the direction opposite to that of the rotation of the growing single crystal, this enables to control effectively the shape of the solid-liquid interface process and the degree of the agitation of the melt.

All in all, the present invention substantially enhances the quality of single crystals being grown.

Claims (8)

1. An apparatus for pulling single crystal from a melt on a seed, comprising a sealed chamber accommodating therein a crucible with the melt, operatively connected with a drive for effecting rotation thereof, a heater with thermal insulation means enclosing the crucible, and a rod extending coaxially with the crucible and carrying the seed holder, the rod being operatively connected with a drive for effecting rotation and vertical displacement of this rod, the operative connection between the crucible and its rotation drive including a support ring accommodated within the chamber and supporting thereon the crucible for their joint rotation during the process of pulling the single crystal.

2. An apparatus as claimed in Claim 1, wherein the operative connection between the crucible and its rotation drive includes a double-stage reducing gear accommodated within the sealed chamber, the driving pinion of the first stage of the reducing gear arranged together with the drive outside the sealed chamber, the driving pinion of the second stage of the double-stage reducing gear cooperating with the support ring for rotating the latter.

3. An apparatus as claimed in Claim 2, wherein the support ring is a toothed rim meshing with the driving pinion of the second stage of the doublestage reducing gear.

4. An apparatus as claimed in Claim 1, wherein the operative connection between the crucible and its rotation drive includes a bearing accommodated within the sealed chamber and having its stationary race fixed to the bottom of the chamber coaxially with the rod of the seed holder, and a reducing gear likewise accommodated within the sealed chamber and having its driving pinion fast with an output shaft of a drive arranged outside the chamber, the driven member of the reducing gear being in the form of a toothed rim mounted on the rotatable race of the said bearing and operatively connected with the support ring for rotating the latter.

5. An apparatus as claimed in Claim 4, wherein the support ring is mounted on a cylindrical stand having its bottom end fixed to the toothed rim coaxially therewith, so that this stand encompasses the heater with the thermal insulation means.

6. An apparatus as claimed in Claim 4, wherein the support ring is mounted on the cylindrical stand with adjustment-wise displacement in a horizontal plane with respect of the axis of the seed holder.

7. An apparatus as claimed in claims 4 to 6, wherein the support ring is made up of two concentric thin-wall rims interconnected with spokes which are uniformly spaced circumferentially and extend tangentially of the internal rim.

8. An apparatus for pulling single crystal on a seed from a melt, substantially as hereintofore described and illustrated in the appended drawings.