US3501406A - Method for producing rod-shaped silicon monocrystals with homogeneous antimony doping over the entire rod length - Google Patents

Description

. March 17, 1.970 R.'KAPPELMEYER ETAL 3,501,405

METHOD FOR PRODUCING ROD-SHAPED SILICON MONOCRYSTALS WITH HOMOGENEOUS ANTIMONY DOPING OVER .THE ENTIRE ROD LENGTH Filed June 8, 1967 United States Patent US. Cl. 25262.3 9 Claims ABSTRACT OF THE DISCLOSURE Described is a method of producing rod-shaped silicon monocrystals having homogeneous antimony doping over the entire rod length, by pulling from a melt. The monocrystal is pulled by a crystal seed from a melt contained in a crucible, with an appropriately selected antimony content, whereby a portion of the antimony present in the melt is vaporized during the growth of the crystal. The pulling process is carried out in an evacuable reaction vessel, in a gaseous atmosphere and at a reduced pressure.

The present invention relates to a method of producing rod-shaped silicon monocrystals with homogeneous antimony doping over the entire rod length by pulling from a melt.

Several fields of the semiconductor art require silicon rods having a homogeneous antimony doping across their entire length. These rods may be divided by means of an appropriate dividing process, such as sawing or breaking, into disc-shaped bodies and used as carrier crystals (substrate discs) for epitactic growth layers. The latter may then be processed into semiconductor components, without the need for additional or dividing processes. It is preferred, particularly for series production, that all discs have the same doping concentration, as this is a prerequisite for the reproducible manufacture of a plurality of semiconductor components having equal or almost equal characteristics. However, the production of rod-shaped silicon monocrystals having homogeneous antimony doping over their entire length, entails considerable difliculties in the methods heretofore used. Above all, the specific resistance of a rod length, corresponding approximately to 90% of the crystallized melt, diminishes to about 40% of the value, at the beginning of the rod, in the methods which employ normal conditions (about 760 mm. Hg). Monocrystalline rods having such steep doping gradients are largely unsuitable for direct subsequent processing into carrier crystals.

The present invention utilizes a method whereby the monocrystal is pulled by a crystal seed from a melt held in a crucible containing an appropriate amount of antimony. During the growth of the crystal, a portion of the antimony contained in the melt is vapor-deposited at such amounts that the increase in antimony concentration caused by the distribution coefficient is compensated for, by means of vaporization of the antimony.

The present method is characterized by the fact that the pulling process is carried out in an evacuable reaction vessel, under a protective atmosphere and at a decreased pressure, whereby the atmosphere is preferably comprised of argon. To this end, an argon pressure of about torr (10 mm. Hg) is adjusted in the reaction vessel, after the crystal seed has been dipped into the melt. This pressure, during the further course of the 3,501,406 Patented Mar. 17, 1970 pulling process, may either be maintained at the same value, or may be lowered to a pressure of about 3 torr.

In the present method, silicon is preferably first melted in a high vacuum, at pressures of less than 10 torr, and is thereby freed from volatile contaminations.

The silicon monocrystals, produced in accordance with the present invention, are particularly suitable for the production of carrier crystals for epitactic growth layers. Directly following the coating process, they may be processed into semiconductor components such as transistors, rectifiers and the like, or they may be processed directly into such semiconductor components as transistors, diodes or solid state integrated circuits.

Additional details of the present invention will be derived from the embodiment examples described with respect to the figure.

In a reaction vessel 10 a crystal seed 1 is inserted into a holder 2 which is connected to a driving device, not shown. The connection between the holder 2 and the driving device is through an intermediary member 3. By means of this driving device, the crystal seed 1 together with the monocrystalline silicon rod 4, growing thereon, may be rotated around its longitudinal axis and may be pulled, in accordance with the crystal growth, upward from the melt 5 which is contained in quartz vessel 6. The quartz vessel 6 is arranged within graphite vessel 7, which is heated by high-frequency coil 8, positioned outside the reaction vessel 10. The heating action of the highfrequency coil 8 is amplified by the energy concentrator 9. The quartz vessel is heated by a heat transfer from the graphite vessel 7. The temperature of the melt is determined by means of a Pt/Pt-Rh thermoelement 11, which is contained in a protective tube 12 of aluminum oxide or quartz. The thermoelement 11 may be connected to a regulating control circuit, not shown in the figure, to control the energy supply and thereby the melting temperature. The lower seal of the reaction vessel 10 is the bottom plate 13 through which the tubular vessel holder 14 and the rod-shaped holder 15 for the energy concentrator 9 are hermetically led. Furthermore, an inlet nozzle 16 has been provided through which the protective gas, e.g. argon, from a storage vessel 17 is introduced to the reaction vessel 10, via dosing valve 18. A head portion 20 equipped with a cooling jacket is provided as the upper seal for the reaction vessel 10. The inlet and outlet for the cooling water are through openings 21 and 22. The rod holder 2 which is coupled with the connecting piece 3 is hermetically led through the head 20. Seals 23 and 24 are further provided for sealing the reaction vessel. The under-pressure in the reaction vessel is produced by the pumping aggregate consisting of the diffusion pump 25 and the circulating pump 26. The block valve 27 is also installed into the pump line. The pressure is measured by manometer 28 and Penning measuring tube 29.

Two modes of operation are particularly suitable for carrying out the method of the present invention.

In the first mode of operation, the silicon is first melted at a reduced pressure, for example at 10- torr. The melting temperature is about l400-l450 C. The temperature of the melt is then reduced to the point that the melt remains just about liquid. Thereafter, argon i introduced from the storage vessel into the reaction vessel and the gas pressure is adjusted in the vessel to approximately 500760 torr. After the crystal seed is dipped in and melted on, the pulling of the crystal is initiated. The antimony, serving as the doping material, is thrown in small pieces, e.g. balls of equal weight, into the silicon melt, prior to or following the dipping in of the crystal seed. The crystal which is rotated around its longitudinal axis at a speed of about 10-100 r.p.m., preferably about 50 r.p.m., is now pulled from the antimony doped melt. The pulling speed is approximately 13 mm./min. Thereafter, the gas pressure in the reaction vessel is adjusted to a value of about torr. This value is either maintained or reduced to approximately 3 torr. The pumping capacity is preferably so adjusted that the flow rate of the gas is at least 3 l./ min.

The amount of material to be used depends on the required doping concentration and may be calculated without difficulty from the known distribution coefficients of the employed materials as well as from the vaporization rate of the doping material.

In the second mode of operation, a pre-alloying of antimony and silicon is effected. This is then melted at a pressure of about 760 torr. This may be done by putting pure antimony, together with the silicon, into the crucible and melting it. Subsequently, the crystal seed is dipped into said melt and melted on. The crystal pulling is effected in the manner described in the previous example. Temperature and pressure as well as the travelling speed of the gas are adjusted analogously.

We have found it to be particularly advantageous to adjust an argon pressure of about 10 to approximately 3 torr, since this pressure not only effects the desired vaporization rate of the antimony but also corresponds to the vapor pressure of the silicon monoxide, at a melting temperature of the silicon.

Experience has shown that at a pressure of about 10 torr and less, the silicon monoxide vaporizes at a high rate of vaporization and precipitates at the more distant walls of the reaction vessel or at another location, while at'a normal pressure (even in an argon atmosphere) the silicon monoxide condenses at the upper rim of the quartz crucible. This rapidly growing precipitation is spongy and crumbles off the crucibles edge. The silicon monoxide which, therefore, falls into the melt leads to considerable disturbances. These may be avoided by following the present invention and by effecting the pulling process at a pressure of about 10 torr which may be reduced to 3 torr.

The reduction of the specific resistance across the rod length is virtually eliminated due to the present invention, as compared to a drop of approximately 60%, occurring in the known methods.

We claim:

1. A method of producing rod-shaped silicon monocrystals having homogeneous antimony doping over the entire rod length, by pulling from a melt, which comprises pulling a monocrystal by means of a crystal seed from a melt with an appropriately selected antimony content, contained in a crucible, whereby a portion of the antimony present in the melt is vaporized during the growth of the crystal, said pulling process being carried out in an evacuable reaction vessel in which is a gaseous atmosphere at a pressure under about 10 torr.

2. The method of claim 1, wherein a pressure of approximately 10 torr is maintained constant, during the entire pulling process.

3. The method of claim 1, wherein the pressure during the pulling process is reduced from about 10 torr to about 3 torr.

4. The method of claim 1, wherein the silicon is melted under a high vacuum at pressures of less than 10* torr.

5. The method of claim 4, wherein the pulling process is carried out in an argon atmosphere.v

6. The method of claim 5, wherein the vessel has a volume of about liter and the argon flow rate is about 3-5 l./min.

7. The method of claim 6, wherein the pulling process is carried out at a temperature corresponding approximately to the melting temperature of the silicon.

8. The method of claim 7, wherein the silicon monocrystal is pulled from the melt at a velocity of about 1 to 3 mm./ min.

9. The method of claim 8, wherein the speed of rotation of the silicon monocrystal, rotating around its longitudinal axis, is adjusted to approximately 10 to 100 r.p.m.

References Cited UNITED STATES PATENTS 2,981,687 4/1961 Parmee 25262.3 3,167,512 l/l965 Ziegler 25262.3 3,296,036 1/1967 Keller 25262.3 X

HELEN M. MCCARTHY, Primary Examiner J. COOPER, Assistant Examiner US. Cl. X.R.

2330l; 148l7l,

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