US3284172A

US3284172A - Apparatus and process for preparing semiconductor rods

Info

Publication number: US3284172A
Application number: US403489A
Authority: US
Inventors: Richard J Binder
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1964-10-13
Filing date: 1964-10-13
Publication date: 1966-11-08
Anticipated expiration: 1983-11-08
Also published as: GB1128033A

Description

Nov. 8, 1966 R. J. BINDER 3,

APPARATUS AND PROCESS FOR PREPARING SEMICONDUCTOR RODS Filed Oct. 15, 1964 HIGH FREQUENCY 55 susCEPTOR GENERATOR T as 64 56 e|- j w HOD RECTIFIER gfiI DETECTOR 2.] j g 0 0 II 0 x r L g c 0 o A II 0 SATURABLE CURRENT SELF- CORE MAGNET'C ADJUSTING BALANCING REACTOR AMPL'F'ER TRANSMITTER POTENTIOMETER POWER sOuRC 5+ FIG-l INVENTOR RICHARD J. BINDER G3 GI PJJvZML-MT FIG.2 ATTORNEY United States Patent 3,284,172 APPARATUS AND PROCESS FOR PREPARING SEMICUNDUCTOR RODS Richard J. Binder, St. Louis, Mo., assignor to Monsanto Company, St. Louis, Mo., a corporation of Delaware Filed Oct. 13, 1964, Ser. No. 403,489 6 Claims. (Cl. 23301) This invention relates to the preparation of semiconductor rods and is particularly concerned with an apparatus and process for temperature regulation incident to the melt drawing of semiconductor materials into monocrystalline semiconductor rods.

In conventional methods for the production of monocrystalline semiconductor rods 'by the melt drawing of semiconductor materials, the semiconductor material is melted within a susceptor by use of inductive heating through the medium of a high frequency work coil and through manipulative arrangements a semiconductor rod is produced by the drawing or pulling of the melt into rod form. In melt drawing methods such as the foregoing, it is of prime importance that the melt of semiconductor material 'be maintained at substantially optimum growing temperature. It is well-known that in order to grow acceptable mono-crystalline semiconductor rods, the delicate balance at the interface between the melt and the growing crystal must be maintained. For example, if the temperature varies less than about 1 C. above or below the optimum growing temperature, such deleterious effects as crystal melting or melt freezing will occur and such will affect the quality of the formed semiconductor rod.

Various methods for controlling the temperature of the melt of semiconductor material during the melt drawing process are known and such methods rely on controlling the temperature by optical means, temperature-sensitive means, electrical means and various combinations of these means. For example, in the automatic control by thermal-electrical means the temperature of the apparatus used in crystal pulling is controlled by a thermal-electrical system through the power input to the high frequency source thereby controlling the temperature of the melt of the semiconductor material. In this example, a secondary loop inductively coupled to the high frequency power circuit is used as a feed back loop to the primary electrical control system. However, such feedback means has many limitations among which are its tendency to be influenced by stray or spurious electromagnetic radiation frequencies and its dependency on the space relationship of the feedback loop to the work coil.

As will be more fully discussed hereinafter, this invention is directed to an improved electrical control means which minimizes or obviates the limitations of the prior art methods.

According to the present invention, there is provided an improved process and apparatus for the temperature regulation of the melt of semiconductor material as a monocrystalline semiconductor rod is pulled or drawn therefrom by means of a mechanical arrangement while the temperature of the melt is controlled by primary control means which senses temperature fluctuations in the melt, converts the temperature fluctuations to an error voltage representing the difference between the fluctuated temperature and a desired melt temperature and utilizes this error voltage to control the power of the work coil used as the heating means to form the melt, and a secondary control circuit means capacitively coupled to the work coil which monitors the voltage of the work coil, is responsive to voltage changes in this coil and sums any voltage changes with the error voltage to stabilize the power in the work coil and thereby stabilize the temperature of the melt, all of which will be more fully discussed hereinafter.

The various objects and features of the invention will be brought out in the description and in order to facilitate the description and understanding of the invention, reference is made to the appended drawings in which:

FIGURE 1 shows the principles underlying the operation of one embodiment of the present invention; and

FIGURE 2 is a schematic diagram of a means for summing the voltage change fro-m the cathode follower and the error voltage from the self-balancing potentiometer.

Reference is now made to FIGURE 1 which illustrates schematically an embodiment of the present invention. A semiconductor material 51, such as silicon or germanium, is placed in a susceptor 52, such as a quartz lined graphite crucible, within an enclosure (not shown) which is maintained at a temperature below the melting point of the semiconductor material in order to maintain the rod in a solid state as it is drawn from the melt. Suitable means are provided for supporting the susceptor within the enclosure. A high frequency work coil 53 connected to a suitable source of alternating voltage (high frequency generator 54) is mounted concentrically about a portion of the susceptor 52. Within the circuit of the work coil 53 and the high frequency generator 54 a resonating capacitor 55 is provided in parallel to permit a large circulating current in the resonant circuit of which the work coil is a part, while the high frequency source has to supply only a relatively small current. When the work coil is energized the semiconductor material within the susceptor is melted. The semiconductor material is pulled or drawn into rod form from the melt or puddle usually by means of a seed crystal through a suitable holder and mechanical arrangement.

The primary control circuit for controlling the temperature of the melt of semiconductor material is provided by a thermal-electrical control system. This system is provided with a detector 56, that is, a thermopile or bolometer, which receives infra-red radiation from the susceptor at the temperature at which such is operated. The intensity of this radiation is proportional to the temperature of the susceptor and thus provides a means through which changes in the temperature of the susceptor can be detected which reflect temperature changes in the melt portion of the semiconductor material. The detector generates a small potential as a function of the incident radiation (infra-red radiation). This potential is impressed across a self-balancing potentiometer 57 which is provided with a set point in order to provide a predetermined potential value and which measures the difference, if any, between the set point potential and the input potential and transmits this difference or error voltage to a current adjusting transmitter 58. The self-balancing potentiometer can be provided, if desired, with a recording unit or visual indication unit which can record or indicate the difference in potential. The current adjusting transmitter 58 is essentially a high gain and high impedance output amplifier and serves to generate a milliamp current from the millivolt potential difference or error voltage input. The current output from the current adjusting transmitter is applied to the magnetic amplifier 59 and amplified thereby. The output of the magnetic amplifier is applied to windings formed on the core of a saturable core reactor 60. Thus, by varying the current on the control winding, the reactor 60 can be employed to control the power from a power source to the high frequency generator 54 thereby controlling the power in the work coil and thus the temperature of the susceptor and melt.

Provided, along with the primary control circuit, is a further electrical control circuit (secondary control circuit) which improves the operation of the primary control circuit. The secondary control circuit is capacitively coupled with the high frequency power circuit and is responsive to changes in the voltage of the power circuit without materially influencing the work coil voltage and, when the voltage of such circuit starts to change from a predetermined value, the change is immediately detected :by the secondary control circuit which along with the primary control circuit lends toward restoring the voltage to the predetermined value thereby maintaining the melt with a more uniform temperature and thus aiding in producing a semiconductor rod of acceptable quality.

The secondary control circuit is provided with measuring capacitors, 61 and 62, which prevent any appreciable current from being drawn from the high frequency power circuit, a rectifier 63 which rectifies the alternating current voltage to a pulsed direct current voltage, and a cathode follower 64 which permits the pulsed direct current voltage from the circuit to be used and/ or measured without materially influencing the voltage of the work coil circuit. The output from the cathode follower (DC potential) is fed to the current adjusting transmitter 58 where this potential i summed with the potential output from the self-balancing potentiometer 57, that is, the error voltage.

Referring now to FIGURE 2 which is a schematic diagram of a portion of the current adjusting transmitter which sums the voltage change from the cathode follower and the error voltage from the self-balancing potentiometer. The current adjusting transmitter is provided with a high voltage source B+(D.C. battery), a plate load resistor 70, an amplifying triode 71 and a cathode resistor 72 which are serially connected. Between the cathode resistor 72 and ground G there is provided input connections, 73 and 74, for impressing across the connections the voltage change from the cathode follower (V f). The error voltage from the self-balancing potentiometer (V,,) is impressed across the grid 75 of the amplifying triode 71 and ground G Between the plate load resistor 70 and the amplifying triode 71 there is provided a connection 76 between which and ground G the summation voltage (V,,) is taken to be used in the portion of the current adjusting transmitter which serves to generate a millia-mp current from the summation voltage. As can be observed from this figure, the summation voltage will at all times be a function of the sum of the eflective grid voltage, that is, V +V In operation, the self-balancing potentiometer is set at a predetermined potential value so that no potential difference is generated therefrom when the temperature of the susceptor and thus the melt is at its predetermined temperature value. If, for any reason, the temperature of the susceptor starts to vary or fluctuate, this change is immediately detected via the detector and the selfabal-ancing potentiometer, which as hereinbefore explained, generates a potential or error voltage used to control, via the current adjusting transmitter, magnetic amplifier and saturable core reactor, the power source to restore the power in the work coil and thus the temperature of the susceptor to its predetermined value. The error voltage from the self-balancing potentiometer, therefore, represents the difference between the fluctuated temperature and a desired temperature. Such a control means, however, has many limitations with perhaps the major limitation being the slowness of response of the control circuit due to, among other things, the inherent time lag between the change in power and the change in temperature of the melt which is dependent on the change in power, as well as the time delay of the detector in generating the potential from the infra-red radiation received.

The secondary control circuit is provided in order to anticipate the primary control circuit and improve its operating characteristics. Since the secondary control circuit monitors the voltage of the work coil which volttage is proportional to the temperature of the melt of the semiconductor material, such circuit can detect changes in work coil voltage from many sources, among which are variations in voltage supplied to the power source, changes in load impedance of the work coil circuit, and changes in the voltage due to changes in the power by the primary control circuit in controlling the temperature of the melt. Additionally, the secondary control circuit is much more rapid in reacting to such changes in the voltage of the work coil circuit as compared to the primary control circuit responding to temperature changes in the melt. Therefore, the secondary control circuit will tend toward stabilizing the voltage of the work coil and thus the temperature of the melt.

The secondary control circuit, being capacitively coupled with said work coil, is provided wit-h adjusting means so that at a predetermined voltage in the work coil which corresponds to the desired temperature of the melt there is a fixed voltage output from the cathode follower. In operation, when the work coil voltage changes from its predetermined value, the change is immediately detected "by the secondary control circuit, the potential output of which is fed to the current adjusting transmitter and summed with the potential difierence or error voltage, if any, from the self-balancing potentiometer. Since, however, the change, detected by the secondary control circuit, is transmitted to the current adjusting transmitter more rapidly than any temperature change in the melt as reflected by the error voltage generated in the self-balancing potentiometer, the secondary control circuit tends to anticipate or override the primary control circuit and thus stabilize the voltage of the work coil thereby tending to stabilize the temperature of the melt of semiconductor material.

As can be appreciated from the foregoing, the electrical secondary control circuit improves the operation of the melt drawing method and because it is capacitively coupled withinthe work coil circuit an accurate and rapidly responsive control means is provided which results in the production of a mono-crystalline semiconductor rod of acceptable quality.

What'is claimed is:

1. In a method of melt drawing a monocrystalline semiconductor rod wherein a semiconductor material is melted within a susceptor by use of inductive heating through the medium of a work coil energized by a high frequency electrical source, and thereafter drawing said semiconductor material into rod form from said melt by means of a mechanical arrangement while controlling the temperature of said melt by sensing temperature fluctuations in said melt, converting said temperature fluctuations to an error voltage representing the difference between the fluctuated temperature and a desired temperature, and utilizing said error voltage to control the power source to said high frequency energy source and thereby control the power in said work coil, the improvement which comprises utilizing changes in the voltage of said work coil without materially influencing said voltage 'by summing with said error voltage to stabilize the power in said work coil and thereby stabilize the temperature of said melt.

2. In a method of melt drawing a monocrystalline semiconductor rod wherein a semiconductor material is melted within a susceptor by use of inductive heating through the medium of a work coil energized by a high frequency electrical source, and thereafter drawing said semiconductor material into rod form from said melt by means of a mechanical arrangement while controlling the temperature of said 'melt by sensing temperature fluctuations in said melt, converting said temperature fluctuations to an error voltage representing the difference between the fluctuated temperature and a desired temperature, and utilizing said error voltage to control the power source to said high frequency energy source and thereby control the power in said work coil, the improvement which comprises monitoring the voltage of said work coil by means of a capacitively coupled electrical circuit, and summing changes in said voltage with said error voltage to stabilize the power in said work coil and thereby stabilize the temperature of said melt.

3. The method of claim 2, wherein said electrical circuit is comprised of, in combination, measuring capacitors for preventing any appreciable current from being drawn from said work coil, a rectifier for rectifying the alternating current votlage to a pulsed direct current voltage and a cathode follower for permitting the pulsed direct current voltage to be used without materially influencing said work coil voltage.

4. In an apparatus for melt drawing a monocrystalline semiconductor rod comprising a susceptor for containing a melt of semiconductor material, a work coil surrounding said susceptor for melting said semiconductor material within said susceptor by use of inductive heating when said work coil is energized by a high frequency electrical source, drawing means for drawing said semiconductor material into rod form from said melt, and temperature control means for controlling the temperature of said melt comprising sensing means for'sensing temperature fluctuations in said melt, means for converting said temperature fluctuations to an error voltage representing the difference between the fluctuated temperature and a desired temperature, and means for utilizing said error voltage to control the power source to said high frequency energy source thereby controlling the power in said work coil, the improvement which comprises circuit means capacitively coupled to said work coil and responsive to changes in the voltage of said work coil and means for summing said voltage changes with said error voltage to stabilize the power in said work coil and thereby stabilize the temperature of said melt.

5. Apparatus according to claim 4, wherein said circuit means is comprised of, in combination, measuring capacitors for preventing any appreciable current from being drawn from said work coil, a rectifier for rectifying the alternating current voltage to a pulsed direct current voltage and a cathode follower for permitting the pulsed direct current voltage to be used without materially influencing said work coil voltage.

6. Apparatus according to claim 5, wherein said means for summing said voltage changes with said error voltage is comprised of, in combination, a high voltage direct current voltage source, a plate load resistor, an amplifying triode and a cathode resistor for providing a summation voltage which is a function of the effective grid voltage of said triode.

References Cited by the Examiner UNITED STATES PATENTS 2,752,473 6/ 1956 Hoge 219-497 2,916,593 12/1959 Herrick 23-273 3,177,336 4/1965 Fischer 219-497 3,206,286 9/1965 Bennett et a1. 23-301 NORMAN YUDKOFF, Primary Examiner. G. P. HINES, Assistant Examiner.

Claims

1. IN A METHOD OF MELT DRAWING A MONO-CRYSTALLINE SEMICONDUCTOR ROD WHEREIN A SEMICONDUCTOR MATERIAL IS MELTED WITHIN A SUSCEPTOR BY USE OF INDUCTIVE HEATING THROUGH THE MEDIUM OF A WORK COIL ENERGIZED BY A HIGH FREQUENCY ELECTRICAL SOURCE, AND THEREAFTER DRAWING SAID SEMICONDUCTOR MATERIAL INTO ROD FORM FROM SAID MELT BY MEANS OF A MECHANICAL ARRANGEMENT WHILE CONTROLLING THE TEMPERATURE OF SAID MELT BY SENSING TEMPERATURE FLUCTUATIONS IN SAID MELT, CONVERTING SAID TEMPERATURE FLUCTUATIONS TO AN ERROR VOLTAGE REPRESENTING THE DFFERENCE BETWEEN THE FLUCTUATED TEMPERATURE AND A DESIRED TEMPERATURE, AND UTILIZING SAID ERROR VOLTAGE TO CONTROL THE POWER SOURCE TO SAID HIGH FREQUENCY ENERGY SOURCE AND THEREBY CONTROL THE POWER IN SAID WORK COIL, THE IMPROVEMENT WHICH COMPRISES UTILIZING CHANGES IN THE VOLTAGES OF SAID WORK COIL WITHOUT MATERIALLY INFLUENCINNG SAID VOLTAGE BY SUMMING WITH SAID ERROR VOLTAGE TO STABILIZE THE POWER IN SAID WORK COIL AND THEREBY STABILIZE THE TEMPERATURE OF SAID MELT.