US3259467A

US3259467A - Apparatus for pulling rod-shaped crystals of semiconductor material from a melt in acrucible

Info

Publication number: US3259467A
Application number: US327659A
Authority: US
Inventors: Rummel Theodor
Original assignee: Siemens AG
Current assignee: Siemens and Halske AG; Siemens AG
Priority date: 1962-12-07
Filing date: 1963-12-03
Publication date: 1966-07-05
Anticipated expiration: 1983-07-05
Also published as: CH426739A; DE1245317B; GB1008300A; NL300759A

Description

July 5 1966 'r. RUMMEL 2 APPARATUS FOR PULLING ROD-SHAPED CRYSTALS OF SEMICOIIDUR MATERIAL FROM A MELT IN A CRUCIBLE Filed Dec. 5, 1963 Fig.4

F|g.3 Fl 9 6 31 Dr l Dr Fig.7 u 33 T was in I=f(l) ml v ggv lm United States Patent 3 Claims. (31. 23-473 My invention relates to the method of pulling rodshaped crystals of semiconductor material from a molten mass of the material contained in a crucible, and has for its main object to reliably provide for a constant cross section of the crystalline rod being produced.

When producing crystals by pulling them out of a melt in a crucible, it is difficult to accurately determine the diameter of the resulting rod-shaped crystals and, above all, to maintain the diameter at a desired constant value during the entire course of the pulling operation. While it is known to regulate the diameter of the recrystallizing rod during crucible-free (floating) zone melting by inductively sensing the diameter with the aid of the inductionheater coil employed for producing the molten zone or by means of an additional measuring coil, or also with the aid of optical scanning devices, the application of such diameter-responsive sensing means to the pulling of crystals from a melt in a crucible is not readily possible because it is extremely difficult, if at all feasible, to place the sensing or scanning devices sufiiciently close to the liquid-solid boundry of the solidifying material, This difiiculty is to a great extent due to the fact that the crucible, prior to melting, supplied with chunks of solid material so that the sensing devices could be placed close to the measuring location only atter the solid pieces have been melted, but this would require an extremely accurate and careful observation and control of the sensor adjusting motion. This, however, is greatly impeded by the crucible rim and often also by heat shielding and other auxiliary means that prevent of obscure direct observation.

Another difiiculty of regulating the diameter of the growing rod-shaped crystal for constancy is due .to the fact that the level of the melt in the crucible is not constant and that level fluctuations cannot be sufficiently avoided even with the aid of .a regulating device.

It is an object of my invention to devise a method of pulling rod-shaped crystals of semiconductor material from a melt in a crucible that avoids the above-mentioned short-comings and difficulties and affords by relatively simple means an accurate control of the diameter or cross section of the rod being pulled from the melt so as to maintain the rod diameter at a desired constant value.

According to my invention, the adjustment and maintenance of a given rod diameter during the pulling operation is effected by continuously determining the weight (W) and the length (L) of the rod being pulled and hence the weight-to-length differential quotient (dW/dL) that corresponds to the datum value of the rod diam- -tected with the aid of transducers that furnish respective measuring voltages, and these are compared with each other in an electric network that furnishes an error voltage for controlling the crystal pulling operation.

The continuous determination and regulation of the rod diameter by ascertaining the rod weight (W) in dependence upon the pulling length (L) affords performing all necessary measuring operations With the aid of sensing means that need not be located in the immediate vicinity of the liquid-solid phase boundary, thus eliminating the fundamental difficulties of the above-mentioned other methods. When operating, according to another feature of my invention, at a constant crystal pulling speed, the Weight of the rod being pulled increases in proportion to time; consequently it is then only necessary to control the temperature of the melt in dependence upon the continuously measured weight of the rod being pulled. However, the control of the pulling operation according .to the invent-ion can be effected also by varying the pulling speed.

Particularly in cases where the resulting semiconductor crystals are to have uniform doping, it is preferable to perform the method at constant crystal pulling speed and to vary the temperature of the melt. In this case, the preferably electric measuring magnitude or signal voltage resulting from the change in weight of the rod is employed by means of suitable transmitting devices for controlling the temperature of the melt in the crucible. For maintaining a constant pulling speed, a synchronous electric motor may be employed, or also a motor equipped with a centrifugal-type speed regulator so as to operate the pulling device at a constant driving speed.

When performing the method of the invention while manitaining the temperature of the melt at a constant value, the crystal pulling speed is to be varied in an analogous manner under control by the measuring magnitude or signal voltage resulting from the changes in weight of the crystal.

According to another feature of my invention it is preferable to sense the crystal weight with the aid of resistance strain gauges. It is of advantage to employ several strain-gauge strips in order to eliminate errors due to bending and deformation perpendicularly to the direction of the strain to be sensed. By connecting several strain-gauge strips in series, a multiple of the slight resistance changes that occur as a result of changes in crystal weight, can be obtained. The accuracy found attainable with such a device is particularly satisfactory. For example, with a crystal diameter of 28.0 mm, a change in diameter of 0.25 mm. can still be ascertained in this manner.

Applicable in lieu of strain gauges are any other suitable deformationor force-responsive sensors and transducers, for example piezoelectric transducers. The in crease in crystal weight can also be determined indirectly by continuously measuring the reduction in total weight of the melt-containing crucible.

When operating with resistance strain gauges or other resistive transducers in the above-described manner, the change in electric resistance resulting from the change in crystal Weight is preferably utilized by connecting the weight-sensing resistor in a bridge network or other comparator network together with a continuously varying reference resistor so that the corresponding voltage drops are opposed to each other and jointly furnish a difference voltage which, since only very small values of this voltage are effective, is indicative of the above-mentioned differential quotient: The occurring differential enror voltage is then employed, preferably upon amplification in an electronic amplifier, for controlling the temperature of the melt in the crucible or for controlling the crystal pulling speed.

Analogously, when operating with a constant tempera- Patented July 5, 1966 '21 for the crucible 1.

ture of the melt, the control of the crystal pulling operation according to the invention can be effected by varying the pulling speed under control by the differential error voltage resulting from the comparison of the weightresponsive current or voltage with a reference current or voltage varying in dependence upon the length of the crystal being pulled and hence in accordance with the pulling travel.

The foregoing and other objects, advantages and features of my invention, said features being set forth with particularly in the claims annexed hereto, will be apparent from the following description of embodiments illustrated by way of example in the accompanying drawing, in which:

FIG. 1 shows schematically and in section a crystal pulling apparatus operating in accordance with the invention.

FIG. 2 is a horizontal cross section through a tubular 1 pulling structure and strain gauges that form part of the apparatus according to FIG. 1.

FIG. 3 is a schematic diagram of a temperature regulating circuit applicable in conjunction with the apparatus according to FIGS. 1 and 2.

FIG. 4 shows schematically an electric drive for the pulling structure of apparatus otherwise as shown in FIG. 1, but in con-junction with an electric control system for operation at constant temperature of the melt and variable crystal pulling speed.

FIGS. 5, 6 and 7 show three different modifications of regulating circuits applicable in lieu of the regulating circuit according to FIG. 3, in conjunction with apparatus as shown in FIG. 1.

As shown in FIG. 1, an evacuated processing vessel 10 'contains a crucible 1 with molten semiconductor material 2 such as silicon, germanium, indium antimonide or any other semiconductor substance suitable for the production of mono-crystals in the above-described manner. The top portion of the processing vessel 10 has a central neck 5 with a cylindrical seal 6 traversed by a pulling structure 9 in form of a cylindrical and tubular rod. The rod-shaped crystal is fastened by means of a clamping device 4 to the bottom portion 8 of the tubular structure 9. As a rule, and as shown, only a thin see-d crystal is clamped in the holder 4 and, when commencing the pulling process, is partly immersed in the melt 2, to be thereafter withdrawn from the melt with the result that semiconductor material will crystallize to the seed and thereby produce a rod whose diameter depends upon the temperature of the melt as well as upon the pulling speed. During pulling operation, the increase in weight of the crystal 3 is continuously sensed by strain-gauge strips 7 which are cemented to the wall of the thin-walled end portion 8 of the pulling structure 9. The electric leads 11 and 12 of the strain gauges 7 extend upwardly through the interior of the tubular structure 9 to the outside of the processing vessel, the bottom of the tubular structure 9 being hermetically sealed for preserving the vacuum in the vessel during pulling operation, it being understood that it is preferable to have the vessel continuously connected to a vacuum pump and also to provide for opening the vessel before commencing and after terminating the crystal pulling operation. The gauge strips are protected in tube portion 8 from being heated by radiation from the melt 2. For this purpose, cooled heat-shielding members (not shown) may be added if desired, or a coolant may be supplied to the gauge strips throtigh the interior of the tubular pulling structure 9.

schematically shown in FIG. 1 is a resistance heater The heater 21 is connected in series with an adjustable rhe-ostat 22 and through an amplifier 23 to the

output terminals

33, 34 of the bridge network shown in FIG. 3. This circuit diagram relates to operation at constant pulling speed and hence uniform upward travel of the tubular pulling structure 9 during steady-state operation. In FIG. 3 the variable resistance of the strain gauges 7 is denoted by R This resistance, varying in dependence upon the increasing Weight of the crystal, is compared in the bridge network with a resistance R1 which varies continuously in proportion to the pulling speed and consequently at a constant rate of change.

provide the signal voltage which, upon amplification in amplifier 23 (FIG. 1), serves for controlling the temperature of the melt. The regulation tends to vary the temperature of the melt 2 in the sense required for reducing the differential error voltage. As a result, the diameter of the crystal rod is maintained at the desired constant value.

When operating with a constant temperatur of the melt 2, the differential signalvoltage issuing from the above-described bridge circuit is used for correspondingly controlling the pulling speed. One way of doing this is schematically shown in FIG. 4. During operation, the tubular pulling structure 9 is displaced upwardly by pinch rollers 39 and 40 driven from the armature 41 of a motor which has a main field winding 42 and a speed-controlling field winding 43. Winding 43 is excited through a control rheostat 44 whose tap is displaced by a solenoid 45 in the output diagonal of a direct-current bridge network essentially corresponding to the one described above with reference to FIG. 3.

Another way of practicing the invention, relating to operation at constant crystal pulling speed, is to use the weight-responsive measuring current for charging a capacitor, periodically discharging the capacitor, and comparing the resulting capacitor discharge voltage with a controlled reference voltage having a constant rate of change in accordance with the constant pulling speed. The resulting differential error voltage is then available for controlling the temperature of the melt. The modified regulating network shown in FIG. 5, operates in this manner. a

It will be understood that

terminals

11 and 12 according to FIG. 5 are to be connected to the

leads

11 and 12 according to FIG. 1, the

output terminals

33 and 34 according to FIG. 5 being identical with those designated by 33 and 34 in FIG. ll. In the network of FIG. 5, a capacitor C is continuously charged by current whose magnitude is controlled by the strain gauges 7 and consequently is determined by the varying weight of the crystal r-od being pulled at constant speed. The capacitor C is periodically discharged by actuation of a switch S, preferably an electronic switch operating in given intervals of time. The capacitor discharge produces a voltage drop U along a resistor R4. This voltage drop is compared with a controllaible voltage U which, aside from the internal resistance of the voltage source, is equal to the voltage drop U which it produces across a resistor R6. The voltage difference AU: U U U U appears across a resistor R5 and between the

output terminals

33, 34 and thus is employed for regulating the crystal pulling operation for constancy of the rod diameter.

According to the embodiment of the regulating circuit shown in FIG. 6, the measuring current, varying in dependence upon the crystal weight, is passed through an inductance coil D and the reactive voltage U occurring along the coil is impressed between the

output terminals

33 and 34 for controlling the crystal pulling operation.

The regulating network according ,to FIG. 7 corre- This variation may be effected, for example, by providing the resistor R1 with a slide contact as shown sponds in principle to that described above with reference to FIG. 5, except that the reference voltage U is not produced 'with the aid of voltage from an auxiliary generator but is derived from the current 1: (L) that is a function of the rod length and hencecontrolled by the abovementioned resistor R1 (FIG. 4). voltage U is compared with the voltage drop U; of a resistor R7 connected between the

terminals

11 and 12 and consequently dependent upon the weight-responsive current 1: (G). The voltage diiference appears at a resistor R9 and is impressed across the

terminals

33 and 34. This difference voltage is employed for controlling the pulling operation in the above-described manner.

To those skilled in the art it will be obvious upon a study of this disclosure that my invention permits of various other modifications with respect to processing equipment and circuitry and consequently may be given embodiments other than particularly illustrated and described herein, without departing from the essential features of my invention and within the scope of the claims annexed hereto.

I claim:

1. Apparatus for pulling rod-shaped crystals of semiconductor material from a melt in a crucible, comprising a processing vessel;

a crucible containing the melt positioned in said processing vessel;

heater means in operative proximity with said crucible for heating the melt at a substantially constant temperature;

crystal pulling means mounted in said vessel above said crucible for substantially vertical movement, said crystal pulling means comprising a substantially vertically displaceable member, driving means in operative proximity with said member for moving said member in a substantially vertical direction and crystal holder means on said member for seeding and pulling a crystal rod upwardly out of the melt when said member is moved upwardly;

strain gage means amxed to the member of said crystal pulling means for providing an electrical signal having a magnitude indicative of the weight of said crystal rod;

circuit means including variable resistance means having a resistance-varying arm mechanically coupled to the driving means of said crystal pulling means for providing an electrical signal having a magnitude indicative of the length of said crystal rod;

electrical comparator means including said strain gage means and said circuit means for comparing the electrical signals corresponding to the weight and the length of the crystal rod to produce an electric control signal proportional to the differential quotient of the rate of change of weight to the rate of change of length of said crystal rod; and

control means connected to said electrical comparator means and coupled to the driving means of said crystal pulling means for controlling the pulling speed by the electric control signal to maintain the difierential quotient constant thereby to maintain the diameter of the rod-shaped crystal constant.

2. Apparatus for pulling rod-shaped crystals of semiconductor material from a melt in a cruicible, comprising a processing vessel;

a crucible containing the melt positioned in said processing vessel;

heater means in operative proximity with said crucible for heating the melt at a substantially constant temperature; crystal pulling means mounted in said vessel above said This length-responsive crucible for substantially vertical movement, said crystal pulling means comprising a substantially vertically displaceable member, driving means in operative proximity with said member for moving said member in a substantially vertical direction and crystal holder means on said member for seeding and pulling a crystal rod upwardly out of the melt when said member is moved upwardly;

piezoelectric force gage means aflixed to the member of said crystal pulling means for providing an electrical signal having a magnitude indicative of the weight of said crystal rod;

circuit means including variable resistance means havin g a resistance-varying arm mechanically coupled to the driving means of said crystal pulling means for providing an electrical signal having a magnitude indicative of the length of said crystal rod;

electrical comparator means including said piezoelectric force gage means and said circuit means for comparing the electrical signals corresponding to the weight and the length of the crystal rod to produce an electric control signal proportional to the diiferential quotient of the rate of change of weight to the rate of change of length of said crystal rod; and

control means connected to said electrical comparator means and coupled to 'the driving means of said crystal pulling means 'for controlling the pulling speed by the electric control signal to maintain the differential quotient constant thereby to maintain the diameter of the rod-shaped crystal constant.

3. Apparatus for pulling rod-shaped crystals of semiconductor material from a melt in a crucible, comprising crystal pulling means mounted in said vessel above said crucible for substantially vertical movement, said crystal pulling means comprising a substantially vertically displaceable elongated tube extending from a sealed lower end in said processing vessel through said vessel to an open upper end outside said vessel, driving means in operative proximity with said tube for moving said tube in a substantially vertical direction and crystal holder means on said tube for seeding and pulling :a crystal rod upwardly out of the melt when said tube is moved upwardly;

resistance strain gage means afiixed to the inside of the tube of said crystal pulling means near the lower end thereof for providing an electrical signal having a magnitude indicative of the weight of said crystal rod;

electrical leads connected to said resistance strain gage means and extending upwardly through the tube of said crystal pulling means and through the upper end of said tube to outside said vessel;

electrical comparator means including said resistance strain gage means and said circuit means for comparing the electrical signals corresponding to the weight and the length of the crystal rod to produce an electric control signal proportional to the diiferentional quotient of the rate of change of weight to the rate of change of length of said crystal rod; and

control means connected to said electrical comparator means and coupled to the driving means of said crystal pulling means for controlling the pulling speed by the electric control signal to maintain the 7 differential quotient constant thereby to maintain 2,992,311 7/ 1961 Keller 23 -301 the diameter of the rod-shaped crystal constant. 3,046,379 7/ 1962 Keller et a1 23-301 References Cited by the Examiner DAVID L. RECK, Primary Examiner.

UNITED STATES PATENTS 5 HYLAND BIZOT, Examiner.

2,908,004 10/ 1959 Levinson 1 N. F, MARKVA, Assistant Examiner.

2,913,561 111/1959 Rummel et a1. 1481.6

Claims

1. APPARATUS FOR PULLING ROD-SHAPED CRYSTALS OF SEMICONDUCTOR MATERIAL FROM A MELT IN A CRUCIBLE, COMPRISING A PROCESSING VESSEL; A CRUCIBLE CONTAINING THE MELT POSITIONED IN SAID PROCESSING VESSEL; HEATER MEANS IN OPERATIVE PROXIMITY WITH SAID CRUCIBLE FOR HEATING THE MELT AT A SUBSTANTIALLY CONSTANT TEMPERATURE; CRYSTAL PULLING MEANS MOUNTED IN SAID VESSEL ABOUT SAID CRUCIBLE FOR SUBSTANTIALLY VERTIICAL MOVEMENT, SAID CRYSTAL PULLING MEANS COMPRISING A SUBSTANTIALLY VERTICALLY DISPLACABLE MEMBER, DRIVING MEANS IN OPERATIVE PROXIMITY WITH SAID MEMBER FOR MOVING SAID MEMBER IN A SUBSTANTIALLY VERTICAL DIRECTION AND CRYSTAL HOLDER MEANS ON SAID MEMBER FOR SEEDING AND PULLING A CRYSTAL ROD UPWARDLY OUT OF THE MELT WHEN SAID MEMBER IS MOVED UPWARDLY; STRAIN GAGE MEANS AFFIXED TO THE MEMBER OF SAID CRYSTAL PULLING MEANS FOR PROVIDING AN ELECTRIICAL SIGNAL HAVING A MAGNITUDE INDICATIVE OF THE WEIGHT OF SAID CRYSTAL ROD; CIRCUIT MEANS INCLUDING VARIABLE RESISTANCE MEANS HAVING A RESISTANCE-VARYING ARM MECHANICALLY COUPLED TO THE DRIVING MEANS OF SAID CRYSTAL PULLING MEANS FOR PROVIDING AN ELECTRICAL SIGNAL HAVING A MAGNITUDE INDICATIVE OF THE LENGTH OF SAID CRYSTAL ROD; ELECTRICAL COMPARATOR MEANS INCLUDING SAID STRAIN GAGE MEANS AND SAID CIRCUIT MEANS FOR COMPARING THE ELECTRICAL SIGNALS CORRESPONDING TO THE WEIGHT AND THE LENGTH OF THE CRYSTAL ROD TO PRODUCE AN ELECTRIC CONTROL SIGNAL PROPORTIONAL TO THE DIFFERENTIAL QUOTIENT OF THE RATE OF CHANGE OF WEIGHT TO THE RATE OF CHANGE OF LENGTH OF SAID CRYSTAL ROD; AND CONTROL MEANS CONNECTED TO SAID ELECTRICAL COMPARATOR MEANS AND COUPLED TO THE DRIVING MEANS OF SAID CRYSTAL PULLING MEANS FOR CONTROLLING THE PULLING SPEED BY THE ELECTRIC CONTROL SIGNAL TO MAINTAIN THE DIFFERENTIAL QUOTIENT CONSTANT THEREBY TO MAINTAIN THE DIAMETER OF THE ROD-SHAPED CRYSTAL CONSTANT.