DD270728A1 - ARRANGEMENT FOR SUPPLYING CRYSTALS OF ELECTRICALLY CONDUCTIVE MELTS - Google Patents

Abstract

Arrangement for growing single crystals of electrically conductive melts, in particular silicon or A (III) B (V) single crystals. The invention was based on the object by the application of a suitable magnetic rotating field with adjusted high Feldstaerke to ensure the formation of a uniformly thin diffusion boundary layer at the growth phase boundary and thereby achieve a homogeneous foreign substance incorporation into the crystal. According to the invention, the object is achieved by providing a two-pole electromagnet with a symmetrical pole position in conjunction with a common iron return.

Description

Characteristic of the known state of the art

When a static magnetic field is applied to an electrically conductive melt, electrical currents are induced when the magnetic flux density in a convective volume element changes with time. The vector of the induced current density is

whereby a volume force is produced by the interaction of the magnetic field with these currents:

(y-electrical conductivity, c-speed of light, ο - speed of convection, H-magnetic field strength, ~ j * - current density, ~ £ - volume force).

The consequence is an attenuation of the convection in the melt, ie an increase in the kinematic viscosity.

This damping effect is manifested in crystal growth in stabilization of the crystal radius, in a reduction in temperature fluctuations, and in homogenization of the radial and axial doping distribution (See Fiegl, G., Solid State Technology, August 1983, p.121, Sekinobu, M. et al , JEE, Syst., 1983 p. 32; Suzuki, T. et al., Semiconductor Silicon 1981, vol. 81-5, p.90; Hirata, H., et al., J. Crystal Growth 70 [1984] 330-334).

In principle, two magnet arrangements are distinguished: With axial magnetic fields, the field lines run parallel to the growth direction, while the transverse variant is characterized by a field line course perpendicular to the growth direction.

In both cases, these are static magnetic fields. The axial magnet arrangement causes a symmetrization of the temperature field and thereby largely suppresses the formation of rotational striations (Hoshikawa, K., Jap.

J.Appl.Phys., Vol.21, No.9, Sept. 1982, pL 545). The more homogeneous incorporation of foreign matter is described by (Hurle, D.TJ., and Series, RW, J. Crystal Growth 73 [1985] 1-). 9) is attributed to an increase in the effective distribution coefficient K, _n as a result of the greater kinematic viscosity.

The effect of a transverse magnetic field is more complex, since the melt between the magnetic poles is more or less fixed and turn crystal or crucible past the melt. In particular, in the region of the phase boundary there is an increase in the relative flow velocity of the melt and, as a result, to a thinner diffusion boundary layer.

The more homogeneous doping distribution in the transverse magnetic field must therefore be attributed to a Verrühreffekt at the phase boundary, in addition, the mass transfer in the melt is hindered by the increased viscosity.

In a work by Hirata, H. and Inoue, N., Jap. J. Appl. Phys., Vol. 23, no. 8, August 1984, pp. L527-L530 was found to be under the given special conditions no linear course of the attenuation with the magnetic field strength.

Rather, the measured temperature fluctuations are initially greater with the magnetic field strength and reach a maximum at 0.05T, in order then to fall off in the usual way.

This behavior could not be observed by Suzuki, T. et al., Semiconductor Silicon 1981 under other conditions (crucible geometry, melt volume, etc.).

Essential for the CZ-Si Verfatiren is the ability to control the oxygen removal from the crucible and to adjust the oxygen incorporation in the crystal at a certain level.

As a result of the particular magnet arrangement used, certain flow components are not damped. It is not attenuated convection vortices whose axis coincides with a field axis of symmetry. The higher temperature gradients due to the impaired mass transfer effect even increased flow velocities in the non-damped directions mentioned.

In Japanese Patent Publication 60-33293 (A), it has therefore been proposed to angle the magnetic field at an angle of 45 ° Breeding direction, at the same time flow components of forced convection (due to rotation)

and to influence free convection (a consequence of temperature gradients).

A complete solution to the problem of undamped convection components is not possible with this arrangement

he, ürten, since also here directions perpendicular to the field course remain free to move. In addition, as with joder statically transversal Magnetvaricnte, the thermal symmetry is disturbed and so the demand to breed striation-free material can not be met.

In axial magnetic fields, although the thermal symmetry is not disturbed, the undamped flow components

however, parallel to the growth phase boundary, it is difficult to control the diffusion boundary layer. Therefore, in particular in the edge regions of the grown crystals, it is necessary to calculate with an inhomogeneous resistance, since crucible material is emphasized in these areas.

In addition, the generation of required fields is up to 0.5T (Fiegl, G., Solid State Technology, August 1983, p. 121). Expenses that are on the order of magnitude of the crystal growing plants. Especially big is the effort for the

ironless magnetic systems of the axial fields. A complete mixing of the melt would be advantageous

Stirring, for example by applying a magnetic rotating field, known from metallurgy. A fast Movement of the melt over the growth phase boundary would significantly contribute to a uniformly thin formation of the Diffusion boundary layer act. However, it is very expensive, with the expected pole distances and the consequently large wastage, a rotatable To generate magnetic field of great field strength. From an investigation by H. Fuchs: Minimization of the scattering losses of a three-coil rotating field, ZWG1987, can be deduced

that for the magnetic induction in focus of the field of 0.1 T with 90% scattering loss is expected, this value grows faster than the used electr. Power for a larger magnetic field. Since the scattering is in the pole piece and the relative closeness of the three pole pieces is physical, the high leakage is found to be system moderate.

Present work by Fiegl, G., Solid State Technology, August 1983, p. 121 and Hoshikawa, K. et al., Semiconductor Silicon

1981, p. 101-112 are therefore limited to magnetic rotary fields, which are generated by appropriate design of the crucible heater by the heater current at mains frequency.

For technical reasons, the maximum field strength is limited to 0.08T (Fiegl, G., Solid State Technology, August 1983. Since three-phase mains power was used, the speed with 3000min " ^{1 is also} poorly adapted Depending on the melt volume a resulting rotational speed of the melt of 10-2OmJn ^- 'a. The limitations of experimental degrees of freedom, rotational speed and field strength are optimal Design of the arrangement each obviously prevented. Object of the invention

The aim of the invention is to achieve an increase in the usable crystal yield and the use value by substantially improving the material properties of semiconductor crystals, in particular of silicon.

Explanation of the essence of the invention

The invention is based on the object by the application of a suitable magnetic rotating field with adapted high field strength to ensure the formation of a uniformly thin diffusion boundary layer at the growth phase boundary, thereby achieving a homogeneous foreign substance incorporation into the crystal. By decisively improving the efficiency in the generation of a magnetic rotating field, the use of a magnetic rotating field, even to existing breeding plants with limited space to be made possible. According to the invention the object is achieved in that a two-pole electromagnet is provided with symmetrical pole position in conjunction with a common iron yoke.

Ausfflhrungsbelsplel

An alternating magnetic field, as z. B. consists in a single-phase asynchronous motor can be represented as a superposition of two equal opposite rotating rotating fields.

By mechanically starting such a single-phase asynchronous motor in any direction, the Mitfeld is amplified and weakened the opposing field, so that a torque is generated. From a certain speed, the Mitdrehmoment enough to make the engine full speed up. He can then be charged. Instead of the cage-type squirrel-cage rotor widely used in the art, any solid-state conductive body could be used in a single-phase asynchronous motor.

If the electrically conductive melt of a crystal growing system is rotated rotationally symmetrically to the growth phase boundary of a crystal by an arrangement of the type described, a desired relative velocity between melt and growth phase boundary can be set depending on the rotational speed of the crystal and the magnetic field. Besides the synchronous run, the strong agitation of the melt at the growth phase boundary is significant. By this stirring, a thin and homogeneous formation of the diffusion boundary layer over the growth phase boundary takes place, whereby a uniform incorporation of foreign substances in the crystal is ensured.

Due to the mirror symmetry of the pole arrangement is compared to the three-pole arrangement of ordinary three-phase motors one

Substantial reduction of scattering losses in the pole plane is achieved because the small spatial pole distances avoided

become.

such a mirror-symmetrical arrangement is in principle the best option, with minimal effort

to produce maximum magnetic induction values with good homogeneity.

a) The start of the rotational movement of the melt is mechanically ensured by the forced convection due to crucible and / or crystal rotation.

b) Analogous to the shaded pole motor, a part of the Hauptpoiflächen is separated by a gap and provided with a copper ring as a short-circuit ring.

The direction of rotation of this arrangement is structurally determined. The result is an alternating field in the shaded pole, which acts spatially spiked and temporally displaced, creating a torque in cooperation with the! Nduktionsströmen the melt.

c) The torque required for starting is generated by kurzzeitigos Einschalton an auxiliary coil which is arranged offset by 90 ° to the actual magnetic field is. The necessary phase shift of the auxiliary current can be achieved by a Wiäerstandsschahung (here the ohmic component of Hiifswickung with the reactive component of the main winding a rotating field, since generally the flooding components are not equal, there is a fluctuating starting torque, which has an elliptical rotating field result), by connecting a Choke to the auxiliary winding (the inductive component of the auxiliary winding is amplified, from the ohmic component of the main winding and the reactive component of the auxiliary winding, the rotating field is created) and by switching on a capacitor in front of the auxiliary coil (if correctly dimensioned, a circular rotating field with high starting torque can be achieved) be achieved.

Claims (1)

Arrangement for growing single crystals of electrically conductive melts under the influence of a rotating magnetic field in the melting zone, characterized in that a two-pole electromagnet with symmetrical pole position is provided in conjunction with a common iron yoke. Field of application of the invention The invention can be used in the growth of single crystals of electrically conductive melts, in particular of silicon or AIII single crystals, according to the Czochralski, FZ, Pedestal, Bridgman and Stockbarger methods, preferably to obtain a homogeneous, radial and axial Impurity distribution can be applied.