US3271551A - Method for crucible free zone melting - Google Patents

Description

Sept. 6, 1966 W. KELLER METHOD FOR CRUCIBLE FREE ZONE MELTING Filed Sept. 8, 1964 United States Patent 3,271,551 METHQD FOR CRUCIBLE FREE ZONE MELTING Wolfgang Keller, Pretzfeld, Germany, assignor to Siemens-Schuckertwerke Aktiengesellschaft, lierliniemensstadt, Germany, a corporation of Germany Filed Sept. 8, 1964, Ser. No. 394,834 Claims priority, application Germany, Sept. 6, 1963, S 87,138 1 Claim. (Cl. 219-1041) My invention relates to a method of crucible free zone melting of a rod-shaped body held at its ends, in which a melting zone is produce-d by means of an induction heater surrounding the body.

In crucible free zone melting of semiconductor material or the like, a melting zone is usually produced in a rod-shaped semiconductor body that is supported at its ends. The melting zone is passed through the body due to relative motion between the semiconductor rod and the heating device. It is known to use as heating device an induction coil surrounding the rod and connected parallel with a capacitor in a heating circuit which is coupled to an output circuit of a high-frequency generator having a frequency of 3 to 5 rnI-Iz., for example.

It is customary to have the high frequency generator operate at such a frequency that, when the rod zone is molten, the operating point lies on the inductive portion of the voltage-current characteristic, this being the curve portion of the current-voltage characteristic of the heating circuit which rises with an increase in conductivity (German Patent 962,006).

In crucible free zone melting with inductive heating of the melting zone, it is also known to use the current supplied to the heating circuit as a measure of the coupling degree and hence as indicative of the thickness of the rodshaped body being processed, thus employing the variation in heating current for regulating the thickness of the body. Preferably, the distance between the two rod holders is controlled in dependence upon these current variations so as to move the rod holders toward one another when the inductive coupling decreases, thus compressing and thickening the melting zone between the two rod portions and thereby increasing the degree of coupling. This affords automatic regulation of the rod diameter, for example by comparing the current drawn by the heating circuit with a datum value, and moving one of the rod holders upon deviation from this datum value in the direction and by the amount required to eliminate the deviation.

Such a diameter regulation of the rod-shaped body has also been performed by tuning the high-frequency generator and the heating circuit so that the high-frequency generator operates on the inductive side of the resonance curve of the heating circuit. However, this involves difficulties with relatively thick rods, for example silicon rods of over 20 mm. diameter, causing the regulating mechanism to get out of step occasionally. With such large rod diameters the regulation may pass beyond the resonance point of the resonance curve; and the regulating device will then operate in the wrong direction, which results in failure of the regulating performance.

It is therefore an object of my invention to overcome the difiiculties of the known methods and particularly to prevent the regulating mechanism from getting out of step when working with relatively thick semiconductor rods.

According to my invention, I provide a method for crucible free zone melting in which the semiconductor rod is inductively heated by a coil energized from a high-frequency generator, which always operates on the capacitive side of the resonance curve of the heating circuit when melting the rod at the melting zone.

Other features which are considered as characteristic for my invention are set forth in the appended claim.

Although the invention has been illustrated and described as a method for crucible free zone melting, it is nevertheless not intended to be limited to the details shown since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claim.

The method of this invention, however, together with additional objects and advantages thereof will be best understood from the following description when read in connection with the accompanying drawings, in which:

FIG. 1 is a partly schematic view of an apparatus employed for carrying out the method of my invention;

FIG. 2 is a graph showing the resonance curve of the heating circuit; and

FIG. 3 is an enlarged view of the melting zone in the rod-shaped body of the apparatus shown in FIG. 1.

The zone melting is carried out preferably within a hell or other receptacle under high vacuum, and the semiconductor rod and its holders as well as the inductance heating coil and, if desired, also the devices for displacing the holder, are mounted within the receptacle. Such design features which are not essential to an understanding of the invention proper herein and which are known per se for apparatus of this general type are not illustrated in the drawing.

Referring now to the drawings and particularly to FIG. 1, there is shown a rod-shaped body 2 of semiconductor material such as silicon, germanium or the like held between two holders 3 and 4. A heating coil 5 surrounds the rod and produces a melting zone 6 by means of inductive heating. The heating coil 5 is so arranged that it can be moved along the rod 2 at a given speed, and is connected in parallel with capacitor 7 to form an inductive heating or tank circuit therewith. The heating coil 5 and capacitor 7 are connected to the output terminals 8 of a high-frequency generator 9 which is energized for example, by a direct current source 10. The high-frequency generator 9 is of the type shown and described in Patent No. 3,046,379 of which I am a coinventor. For the purpose of control or regulation, the plate current I (see FIG. 2) of the high-frequency generator 9 which is proportional to the current fed into the heating circuit is conducted through a variable resistance 11 serially connected in the plate circuit of the generator 9. The voltage drop developed across the resistance 11 is measured with a suitably connected voltmeter 12a. The voltage drop across the resistor 11 acts on the coil of a relay 13 in a differential relation to an adjustable datum voltage from a source 12.

The relay 13 is polarized, and its contacts 14 are in an inactive position when the voltage drop across resistor 11 is substantially equal to the datum voltage of source 12. However, the contacts 14 close either one or the other of the two stationary contacts respectively, which are reversely connected to the poles of a current source 16, when the voltage drop across resistor 11 and hence the magnitude of the generator current deviates from the datum value i.e. increases or decreases respectively.

The relay 13 thus operates the reversing contacts 14, to energize a motor 15 connected to the direct-current source 16. Depending upon the increase or decrease of the heating circuit current and the consequent corresponding deviation in the voltage drop across the resistor 11 as compared to the datum voltage of the source 12, the contacts 14 will engage one or the other of the stationary contacts respectively to effect rotation of the motor 15 in one or the other rotary direction respectively. The motor 15, for example, drives a gear 18 through a reduction gear system (not shown) in a desired rotary direction so that the toothed rack 17 with which it is in meshing engagement and which is connected with the displaceable holder 3 is displaced respectively toward or away from the melting zone 6. The holder 4 is fixed against displacement in the direction of the rod axis but is however, rotatable about the rod axis by means such as is disclosed in the aforementioned Patent No. 3,046,379, which also discloses suitable means for displacing the heating coil along the rod.

The high-frequency generator 9 generally operates on a side of the resonance curve of the induction heating or tank circuit formed by the coil 5 and capacitor 7. A typical resonance curve is shown in FIG. 2 with voltage values U applied to the heating circuit or plate current I of the high-frequency generator as ordinate and the frequency values j as abscissa, both at a linear rate. If the high-frequency generator 9 operates on the inductive or ascending side, for example at point I, of the resonance curve of the heating circuit 5, 7, as shown in FIG. 2, the following regulation takes place upon deviation of the rod diameter from a desired value. A change takes place in the coupling of the melting zone 6 to the heating coil 5 when there is a deviation of the rod diameter from the desired value, and the current supplied to the heating circuit 5, 7 and thereby also the plate current of the high-frequency generator varies accordingly. This causes variation in the voltage drop across the resistance 11, so that the relay 13, suitably polarized, actuates the relay plunger to move in the necessary direction to switch the reversible motor 15 with suitable polar connections to the current source 16 so that the motor rotates the gear 18 for displacing the rack 17 and the holder 3 in the appropriate direction. The deviation from the desired rod diameter value is thus again equalized or adjusted 5 and the semiconductor material crystallizing from the melting zone shows a reduced deviation from the desired value, or relatively no deviation at all.

It has been found that when using a so-called fiat coil as for example, the one depicted in FIG. 3, particularly, the melting of the rod does not take place continuously but rather, in an intermittent manner. When assuming for example, that the flat coil 5a in FIG. 3 is travelling upwardly, then, the fixed rod parts 2a and 2b, the melting zone 6a and the heating coil 511 will have substantially the relative spatial arrangement shown in FIG. 3. The rod material thus melts at the upper part of the melting zone 6a and crystallizes again at the lower part thereof. At times, an isolated portion or island of the rod part 2a which has slowly begun to glow suddenly becomes molten and flows downward into the bulge of the melting zone 6a. This abruptly loosens the coupling between the heating coil 5a and the melting zone 6a, since the heating coil is also located in the upper region of the melting zone, i.e. at its narrow neck. This neck is suddenly constricted, due to the sudden melting of a portion of the solid semiconductor material and, hence, loosens heating circuit, which as previously mentioned, can cause the operating point to pass over and beyond the resonance point or peak of the resonance curve. Consequently, the regulation falls out of step and, as a rule, causes the melting zone to harden and interrupts the zone melting operation.

If, in accordance with my invention, the high-frequency generator and the heating circuit are so tuned or matched that the high-frequency generator operates on the capacitive side of the resonance curve of the heating circuit, for example at point II in FIG. 2, this dangerous swing or aberration of the control devicecan then no longer occur. Naturally in this instance, the control means have to be adjusted to the opposite polarity than if the generator were operated on the inductive side of the curve. Preferably, the control means should be so constructed that regulation or control therewith can take place along the inductive as well as the capacitive side, i.e. at the operating points I and II. In such a case the melting zone can pass upwardly along the semiconductor rod when the control means is operating on the capacitative side of the resonance curve whereas, during the downward passage of an incandescent zone through the semiconductor rod, the control means operates on the inductive side.

By employing auxiliary devices for varying the field of the inductive heating coil, such as levitating coils or short circuit rings, the normal form of the melting zone is disturbed. It is, therefore advantageous to carry out my inventive method Without using such auxiliary devices and only by using a flat coil such as 'is illustrated in FIG. 3.

I claim:

In a method of zone melting a semiconductor rod in which the rod is vertically supported at both ends and a molten zone is formed in the rod by a surrounding inductive heating coil connected in parallel with a capacitor and forming therewith a heating circuit having specific resonance curve characteristics, the heating circuit is energized by current from a high-frequency generator varying in accordance with a change in the diameter of the rod for actuating a regulating device to vary the axial spacing between the rod ends until the current assumes a datum value, and the zone is caused to move lengthwise of the rod, the improvement which comprises operating the high-frequency generator at a frequency in the capacitive leg of the resonant curve of the heating circuit for melting the rod at the zone while moving the zone upwardly along the rod, and operating the high-frequency generator at a frequency in the inductive leg of the resonant curve of the heating circuit for maintaining the zone as an incandescent solid while moving the zone downwardly along the rod.

References Cited by the Examiner UNITED STATES PATENTS 2,913,561 11/1959 Rummel et a1 219--10.43

FOREIGN 'PATENTS 962,006 4/ 1957 Germany.

RICHARD M. WOOD, Primary Examiner.

ANTHONY BARTIS, Examiner.

L. H. BENDER, Assistant Examiner.

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