US2996595A

US2996595A - Apparatus and process for regulating current flow through material

Info

Publication number: US2996595A
Application number: US803256A
Authority: US
Inventors: Kniazuk Michael; Fred R Prediger
Original assignee: Merck and Co Inc
Current assignee: Merck and Co Inc
Priority date: 1959-03-31
Filing date: 1959-03-31
Publication date: 1961-08-15
Anticipated expiration: 1978-08-15

Description

Aug. 15, 1961 M. KNIAZUK ET AL APPARATUS AND PROCESS FOR REGULATING CURRENT FLOW THROUGH MATERIAL Filed March 31, 1959 FRED R PKED/GE? BY zzn I 8 M C. m fi lz, ATTORNEYS United States Patent Ofiicc Patented Aug. 15, 1961 2,996,595 APPARATUS AND PROCESS FOR REGULATING CURRENT FLOW THROUGH MATERIAL Michael Kniazuk, Mountainside, and Fred R. Prediger,

Westfield, N.J., assignors to Merck & Co., Inc., Rahway, N.J., a corporation of New Jersey Filed Mar. 31, 1959, Ser. No. 803,256 '5 Claims. (Cl. 219-20) This invention relates to a process and apparatus for regulating current flow through material characterized by a negative resistance-temperature coefficient, and in particular, relates to a process and an improved power supply for growing a semi-conductor ingot, such as silicon, by a gas phase process wherein an electric current flow through a specimen of the semiconductor heats same for carrying out the growing process.

In accordance with the present state of the art, an ingot of silicon may be grown out of a gas phase process by supporting a filament-like specimen of the silicon in a gas reaction chamber containing a silicon gas. The gas is decomposed by heat which deposits additional silicon on the hot specimen to cause it to grow in diameter. In accordance with the improvements claimed herein, the heat required for growing the semiconductor is generated by a controlled electric current flowing through the silicon filament. This technique provides a convenient and effective method of heating and controlling the temperature of the hot silicon. Silicon has a negative resistancetemperature coeificient. The resistivity of silicon is relatively very high at room temperature; however, its resistance undergoes a large change as the temperature of the hot silicon increases. This phenomenon permits a large temperature generating current to flow through the hot silicon filament for the purpose of growing same.

It is a principal object of this invention to provide an improved process and apparatus for regulating current flow through a specimen of material characterized by a negative resistance-temperature coefficient, and in particular, for heating and controlling the temperature of the material by such current flow.

It is a further object of the invention to provide an improved process and apparatus for growing semiconductor material, for example silicon, in a gas phase process wherein the temperature required for carrying out such process is generated and controlled by regulating the current flow through a specimen of such semiconductor.

In accordance with the foregoing objects, it is a further object of the invention to provide a power supply for regulating current flow through a specimen of material such as silicon or other like material characterized by a negative resistance-temperature coefficient. r The power supply circuit includes a step-down transformer wherein the specimen is coupled alternatively to a high voltage side of the transformer and then to its low voltage side whereby during the first phase of operation, a relatively low but increasing current is caused to flow through the specimen. During a later phase of operation, a relatively high current is caused to flow through the specimen. The power supply circuit also includes an inductive impedance for varying the transformer terminal voltages to :egulate current flow through the specimen during either phase of operation.

In accordance with the preceding object, it is a further object of the invention to employ a saturable reactor raving a variable D.-C. bias as the aforesaid impedance neans, whereby variation of the D.-C. bias effects reguation of current flow through the specimen.

As noted hereinbefore, the cold silicon has a rela- :ively very high resistivity at room temperature. Ac- :ordingly, it is a further object of the invention to heat the specimen of silicon filament by external means to facilitate initial current flow therethrough at the start of the gas phase process. External heating may be discontinued after current flow through the specimen increases sufficiently whereby the filament is able to heat itself.

Further objects and advantages will become apparent from the following description of the invention taken in conjunction with the figures, in which:

FIG. 1 depicts schematically the power supply in accordance with the practice of the instant invention; and

FIG. 2 illustrates the B-H characteristics of the saturable reactor, which curve is used herein to describe the operation of the reactor.

Reference is now made to the figures wherein FIG. 1 illustrates a rod-like silicon filament 10 supported by conventional means in a vacuum-sealed reaction chamber 12. In accordance with the present state of the art, a gas mixture 13 including a carrier gas and a silicon gas, such as silicon tetrahalide is introduced into chamber 12 via an inlet 14. The silicon gas mixture 13 when suitably heated will decompose to deposit additional silicon on the hot filament 10. This action causes filament 10 to grow in diameter. The exhaust gases in chamber 12 are removed through an outlet :15.

The present invention is primarily concerned with the means for heating the silicon mixture 13 and filament 10 by regulating current flow through the latter. Current flow is regulated by the power supply depicted in FIG. 1, which power supply is energized by an A.-C. source 16, for example 440 volts. The power supply includes a stepdown transformer 17 and a saturable reactor 18, which reactor is connected in series with the high voltage side of transformer 17. Reactor 18 is essentially a variable inductive impedance, the inductance of which may be varied considerably by a variable direct current bias in the secondary winding circuit of reactor 18. Specifically, transformer 17 and reactor 18 each have respective primary and secondary windings -19, 20 and 21, 22. The series connected primary windings 19, 21 are connected across source 16. The D.-C. bias for reactor 18 involves a regulated variable D.-C. source depicted at 23 and connected to reactor winding 22.

The path for current flow through filament 10 includes a line 25 connecting filament 10 to one side of source 16. For the purpose of identification, this side of source 16 often will be referred to as the upper side, whereas the other side of source 16 will be referred to as the lower side. The current path continues through filament 10 and line 27, which line is connected to a single pole double throw switch 28 having

terminals

29, 30. Terminal 29 connects to the lower side of source 16 through reactor winding 21. It will be understood that the conductive connections of

lines

25 and 27 to filament 10 may be accomplished by known conventional means.

When switch 28 closes to contact terminal 29, filament 10 is essentially connected across a relatively high voltage and low current source as constituted by transformer primary 19. For this switching condition, the secondary winding circuit of transformer 17 is open, and primary winding 19 draws a negligible magnetizing current for the no-load condition. The terminal voltage applied to primary 19 will be maximum for a minimum voltage drop across reactor 18 and minimum for a maximum voltage drop across reactor 18. When switch 28 closes contact with terminal 30, filament 10 is essentially across a low voltage and high current source as constituted by the terminal voltage of transformer secondary 20. The high current path is depicted in FIG. 1 by heavy bus lines. For the reasons explained hereinafter, it will be seen that filament 10 is connected to the high voltage 3 side of transformer 17 during an early phase of current flow, and is connected to the low voltage and high current side of transformer 17 during a later phase of current flow. Meanwhile, reactor 18 will serve to vary the primary and secondary transformer terminal voltages to control continuously current flow in filament for either position of switch 28.

Reference is now made to FIG. 2, which curve illustrates the 8-H characteristics of the magnetic core of saturable reactor 18, where B denotes fiux density and H denotes the magnetizing force. The reactor inductance may be defined as follows:

where is core flux, N is the number of turns, and I denotes cur-rent. Since 1: and I are directly proportional to B and H, respectively Alfi/AI is essentially the slope of the B-H curve. Consequently, reactor inductance will be a maximum for operating conditions in the region of the origin of the curve coordinates wherein average core flux is zero. On the other hand, reactor inductance decreases for reactor operation in the region where the curve flattens out by reason of an increase in average core flux. The D.-C. bias in the secondary of reactor 18 determines average core flux, which in turn determines the reactor inductance L, whereby a decrease in D.-C. bias results in an increase of reactorinductance, and conversely an increase of D.-C. bias results in a decrease of inductance.

At the start of the gas phase process of growing the silicon, filament 10 is at room temperature wherein the silicon exhibits a very high resistance. Consequently, it is desirable to apply the highest available voltage across filament 10 for the initial phase of current flow there through. This is accomplished by throwing switch 28 to close contact 29 and by biasing reactor 18 with a high DC. bias. The latter action minimizes the voltage drop across reactor 18. Nevertheless, the extremely high resistance of cold silicon will prevent a sufficient rate of current flow build-up through filament 10 to permit the silicon to heat itself. Consequently, external heating means 31 such as a bank of heating elements are disposed in close spaced relationship about chamber 12. Heaters 31 may be connected across source 16 by a switch 32 and serve to heat filament 10 by radiation.

Consequently, it will be understood that at the start of the foregoing process, the voltage applied to filament 10 is the highest available, and that the initial phase of current flow through the silicon filament is facilitated by heating same by external means. The resistance of the heated filament drops which results in increasing current flow therethrough. This action leads to a higher filament temperature. When the current is sufficiently large-to self-heat filament 10, heater 31 may be disconnected from the circuit by opening switch 32. The continuation of current flow through filament 10 is characterized by an increase of its temperature accompanied by further increases in current. To prevent a runaway condition which will melt the silicon, the process may be controlled by decreasing the DC. reactor bias in order to increase reactor inductance. This adjustment will reduce the terminal voltage applied to transformer 17 and in turn the voltage across filament 10 to regulate the current flow therethrough. In actual practice, the D.-C. bias may be varied manually to increase the reactor inductance by suitable increments at predetermined time intervals. It would be keeping within the spirit of this invention to obtain such bias adjustments automatically by a control regulated by a thermostat responsive to the temperature of chamber 12.

When the voltage across filament 10 drops to a predetermined amount, for example 80 volts, switch 28 may be thrown to close terminal 30 in order to cause a large heat generating current to flow through the silicon filament. This switch action couples filament 10 to the low voltage side of transformer 17. During the phase of large current flow, reactor 18 is again employed to control the voltage applied across filament 10 and thus the current flow therethrough in order to obtain and, in addition, to maintain a suitable silicon temperature in chamber 12 for growing the silicon ingot. At the completion of the growing process, the power supply circuit may be opened, for example by a switch 33, whereby the completed ingot is replaced by another silicon specimen and the foregoing process is repeated.

It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim:

1. A power supply for regulating the current flow through a conductor work specimen characterized by a negative resistance-temperature coefi'icient comprising, a source of A.-C. voltage, a step-down transformer having a primary winding and also a normally open circuit secondary winding, said primary winding being coupled to said-source, variable inductor reactance means in electrical series relationship with said primary winding for varying the terminal voltage applied to said conductor work specimen, switching means for conductively connecting the conductor work specimen alternatively across said primary or second winding wherein said conductor work specimen is connected to a relatively high voltage and low current source or a relatively low voltage and high current source, respectively, said conductor being connected to the high voltage and low current source during an early phase of current flow therethrough and; being connected to the low voltage and high current, source during a later phase of current flow therethrough, 5 and means for regulating the inductance of said inductor; means wherein the voltage applied across said transformer primary is varied to regulate current flow through said conductor work specimen for either phase of opera-- tion.

2. A power supply for regulating the heating currentflow through a semiconductor specimen characterized: by a negative resistance-temperature coeflicient comprising, a source of A.-C. voltage, a step-down transformer having a primary winding and also an open circuit secondary winding, said primary winding being conductively coupled to said source, a saturable reactor in electrical series relationship with said transformer primary winding for varying the terminal voltage applied to said semiconductor specimen, means for connecting said specimen, of semi-conductor alternatively across said primary or secondary winding of said transformer wherein said, specimen is connected to a relatively high voltage and low current source or a relatively low voltage and high current source, respectively, said specimen being 6011-; nected to the high voltage source during an early phase, of current flow therethrough and being connected to the low voltage source during a later phase of current flow. therethrough, and means for regulating the inductance of said reactor wherein the voltage across said specimen is controlled to regulate current flow therethrough for either phase of operation.

3. Apparatus as defined in claim 2 further including, heat generating means in close spaced relationship with respect to said specimen and for heating said specimen by radiation until the temperature of said specimen rises to facilitate a buildup of initial current flow therethrough.

4. A power supply for regulating the heating current flow through silicon or like semiconductor specimen material which is characterized by a negative resistance-temperature coefficient comprising, a source of A.-C. voltage a step-down transformer having a primary winding am also an open circuit secondary winding, a saturable reac tor having a primary winding in electrical series relationship with said transformer primary winding and alsc having a secondary winding for varying the terminal voltage applied to said specimen, said source being conductively coupled across the aforesaid series combination of primary windings, means for connecting said specimen of semiconductor alternatively to said primary or secondary winding of said transformer wherein said specimen is connected to a relatively high voltage and low current source or a relatively low voltage and high current source, respectively, said specimen being connected to the high voltage source during an early phase of current flow therethrough and being connected to the low voltage source during a later phase of current flow therethrough, and variable direct current bias means coupled to said reactor secondary winding for regulating the inductance of said reactor wherein the voltage applied to said transformer primary is varied to regulate current fiow through said specimen for either phase of operation.

5. In apparatus for applying an alternating current from a substantially constant voltage source to a relatively cold negative-resistance work specimen to heat same and for regulating the current flow through said work specimen as its temperature rises wherein said apparatus includes a transformer in which the secondary winding has a smaller number of turns than the primary winding, and said apparatus also including an adjustable reactor, the improvement in said apparatus comprising, means connecting said adjustable reactor and the primary winding of said transformer in series across the source of alternating current, means connecting one terminal of said work specimen to one terminal of said source of alternating current, switch means connecting the other terminal of said work specimen to either the series connection between the adjustable reactor and transformer primary winding or one terminal of the secondary winding of said transformer, and means connecting the other terminal of said transformer secondary winding to said one terminal of said work specimen, said work specimen is energized from said alternating current source through said adjustable reactor when said switch means connects said other terminal of said work specimen to the series connection between the adjustable reactor and said primary transformer winding, whereby said adjustable reactor serves to control the amount of current flow through said work specimen, said work specimen is energized from said alternating current source through said transformer secondary winding when said switch means connects said other terminal of said work specimen to said one terminal of said secondary transformer winding, and whereby said adjust-able reactor in series with the primary winding of said transformer also serves to control the current flow through said work specimen.

- References Cited in the file of this patent UNITED STATES PATENTS 1,913,580 Altshuler et al June 13, 1933 2,314,956 Slayter et a1. Mar. 30, 1943 2,338,518 Koch Jan. 4, 1944 2,680,771 Kistler June 8, 1954 2,769,076 Bogdan Oct. 30, 1956