US3822111A

US3822111A - Apparatus for pulling up semiconductor crystals

Info

Publication number: US3822111A
Application number: US00327759A
Authority: US
Inventors: T Suzuki; K Hoshi; Y Tamate
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1971-02-25
Filing date: 1973-01-29
Publication date: 1974-07-02
Anticipated expiration: 1991-07-02

Abstract

Apparatus for controlling the diameter of a crystal pulled from a semiconductor melt in a crucible including a float which has a shape substantially similar to the crystal and which is pulled from a liquid filled tub in synchronism with the pulling up of the crystal, and an electronic circuit for comparing the weight of the float with the weight of the crystal during the pull up operation. The rate at which the crystal is pulled up and the temperature of the crucible are controlled as a function of the relative weights of the float and the crystal as they are being pulled up.

Description

United States Patent 1191 Suzuki et a].

[ APPARATUS FOR PULLING UP SEMICONDUCTOR CRYSTALS [75] Inventors: Toshihiko Suzuki; Kinji Hoshi;

- Yasuo Tamate, all of Kanagawa,

Japan [73] Assignee: Sony Corporation, Tokyo, Japan [22] Filed: Jan. 29, 1973 ['21 Appl. No; 327,759

7 Related US. Application Data [63] Continuation of Ser. No. 229,469, Feb; 25, I972,

abandoned.

30 Foreign Application Priority Data Feb. 25, i971 Japan 46-9517 Nov. 8, 1971 Japan 46-88838 [52] US. Cl. 23/273 SP, 23/301 SP, 65/352, 432/13, 432/45, 432/156 [51] Int. Cl. B0ld 9/00, F27b- 14/08 [58] Field of Search 23/273 R, 273 SP, 301 SP; 432/l3,45, I56; 65/86, 352

1451 July 2, 1974 56] 3 References Cited UNITED STATES PATENTS 2,683,676 7/1954 Little et al. 65/352 x 3,244,494 4/1966 Apple et al. 432/13 3,337,303 8/1967 136161121111 23/273 3? 3,4l6,898 12/1968 Shiroki et al. 23/273 SP Primary ExaminerJohn J. Camby Attorney, Agent, or Firm-Lewis H. Eslinger, Esq.; Alvin Sinderbrand, Esq.

57 ABSTRACT Apparatus for controlling the diameter of a crystal pulled from a semiconductor melt in a crucible including a float which has a shape substantially similar to the crystal and which is pulled from a liquid filled tub 'in synchronism with the pulling up of the crystal, and

an electronic circuit for comparing the weight of the float with-the weight of the crystal during the pull up operation. The rate at which the crystal is pulled up and the temperature of the crucible are controlled'as a function of the relative weights of the float and the crystal as they are being pulled up.

40 Claims, 6 Drawing Figures SHEU E [If Q This is a continuation of application Ser. No. 229,469, filed Feb. 25, 1972 now abandoned.

- BACKGROUND OF THE INVENTION The invention relates to apparatus for pulling up semiconductor crystals, and more particularly to an improved apparatus for pulling up rod shaped, semiconductor crystals, having a predetermined cross-sectional area, from a semiconductor melt in a crucible.

There are several methods for automatically controlling the diameters of the pulled up crystals. In general the crystal is formed from a seed crystal which is immersed in a crucible containing the melted semiconductor material. The seed crystal is slowly withdrawn from the melt in the crucible as a crystal of larger diameter is grown upon the seed crystal. The diameter of the grown crystal is controlled by the temperature at which the melt in the crucible is maintained and by the rate at which the crystal is withdrawn from the crucible.

If the crystal is withdrawn at a-relatively fast rate it has a smaller diameter than if it is withdrawn at a relatively slow rate. One existing method for controlling the diameter of crystals to be grown utilizes strain gages which produce an electronic signal representative of the weight of the crystal. This signal is compared with another electronic signalfrom a variable resistor which is indicative of the length of the crystal. The use of strain gages leads to many inaccuracies since such gages are not highly linear and they are temperature dependent. For example when a crystal of 75mm in diameter is pulled up, sensitivity of 0. lg or less is needed even if the diameter of the crystal is measured only with an accuracy of plus or minus 0.5mm. Another problem is that the signal from the variable resistor, typically a slide wire resistor, also contains a large amount of electronic noise which makes. accurate readings difficult.

Another prior art method utilizes an optical scanning device which senses the diameter of the interface between the growing crystal and the surface of the melted material in the crucible. One problem with such a system is that typically the end of the crystal is tapered and to measure the diameter of thetapered outer sprout of the crystal it is necessary tomove the scanning device. This is a cumbersome procedure and it leads to inaccurate readings. It is also necessary to rotate the crystal in order to measure the interface and if the rotation is at all eccentric the reading is inaccurate. Still another problem is that if heating dispersion is non-uniform within-thecrucible the surface of the material in the crucible is vibrated, thereby causing a detection error.

SUMMARY OF THE INVENTION The above and other disadvantages are overcome by the apparatus of the presentinvention which comprises a crucible containing a melted material from which the I signal in'response to the difference of the weights, and means responsive to the signal for controlling the rate at which the crystal is pulled.

The float has a shape substantially similar' to the shape of the crystal which is to be pulled from the crucible. The ratio of the cross-sectional area of the float tothe finally finished crystal is selected to be the same as the ratio of the liquid within the container to the crystal in specific gravity. Means are also provided for controlling the temperature of a heater surrounding the crucible so that the temperature of the melt may be varied as a function of the difference in weight of the crystal and the float.

It is therefore an object of the invention to provide an apparatus for automatically controlling the pull up rate of a semiconductor crystal from a melt in a crucible.

It is still another object of the invention to provide an improved apparatus for controlling a cross-sectional area of the growing crystalat a desired constant value 'as represented by a predetermined physical standard.

tion of the invention taken in conjunction with the acr companying drawings.

BRIEF-DESCRIPTION or THE DRAWINGS FIG. I is a vertical elevational view, of one embodiment of the invention;

FIG. 2 is a more-detailed diagram of the electronic controls of the embodiment of FIG. I;

FIG. 3a is a diagrammatic illustration of the float of the embodiment of FIG. I;

FIGS. 3b and 3c are illustrative graphs for use in explaining the mathematical operation of the control circuits of the embodiment of FIG. 1;

FIG. 4 is a vertical elevational view, partly in-section, 0f the furnace of another embodiment of the invention.

DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS Referring now more particularly to FIG. I a hermetically sealed furnace I is comprised of a thick part la which generally constitutes an L shaped frame which supports a box like structure containing the melting apparatus and a thin part II) which projects upwardly from the top of part'lla. A crucible 2, made of a quartz material, is carried in a support 3 made of graphite or the like. The crucible 2 and the support 3 are rotatably driven on a shaft 4 which is bearing mounted in the support Ia and which has a wormgear 6 affixed to one end. The worm gear 6 is turned by a motor 5.

The motor 5 and the lower end of the shaft 4 are sup-. ported by a vertically displaceable table 7. A shaft 10 is threaded in the table 7 and has a worm gear 9 fitted to its lower end which is turned by a motor 8. The cru- .Cible lift motor 8 and the threaded shaft 10 are supported by a stationery table ll which constitutes a horizontal leg of the portion 1a of the furnace.

partly in section,

A melted semiconductor material 12 (hereinafter referred to as the melt) suchas a silicon material is contained in the crucible 2. A substantially cylindrical heater 13 made of graphite is disposed about the periphery of the crucible support 3 and spaced from it such that the crucible support 3 and the crucible 2 are rotated by the shaft 4 within the heater 13. The crucible 2 is heated to a temperature which is typically 1,420 degrees centigrade'by the heater 13.

A substantially cylindrical insulator l4 surrounds the heater 13 and has a predetermined spaced relationship with it. The insulator l4 prevents the radiation of heat outwardly and reflects the heat. radiated from the heater 13 inwardly upon the crucible 2. t

r A hollow, outer shaft 16 passes through a closed bearing in the upper leg 1b of the hermetically sealed furnace l. A crystal pulling shaft 17 passes through the outer shaft 16'. A seed chuck 18, in which a seed 19 is positioned, is attached to a lower portion of the crystal pulling shaft 17. When the seed 19 is immersed into the melt 12 of the crucible 2 and is then slowlypulled up from the melt, the seed 19 grows into a single crystal 20. A pair of closed metal seals 21a and 21b close the ends of the outer shaft 16 about a crystal pulling-shaft 17 to maintain the hermetic seal of the furnace, l.

The upper endof the outer shaft 16 is rotatably fitted within the lower portion 22a of a movable rack 22. The movable rack 22 is vertically displaceable by means of a rotating shaft 27 threaded in the lower portion 22a of the rack 22. Thus the outer shaft 16 maybe axially shifted vertically by vertical displacement of the moving rack.

The outer shaft 16is also rotated in a predetermined direction which is opposite to the direction of rotation of the crucible 2. A worm gear 24 is attached to the upper end of the outer shaft 16 and engages a worm drivemotor 23 affixed to the lower portion 22a of the rack 22. The crystal pulling shaft 17 is rotated with the outer shaft 16.

The upper end of the crystal pulling shaft 17 is connected to a first weight'detector 25 which is suspended from the upper end portion 22b of the moving rack 22 through a first suspending installation 26. The first suspending installation 26 comprises a vertically mounted rod 261) having its upper end fastened in the upper portion 221; of the rack 22 and having its lower end conically flared, with the apex of the conical portion pointing upwardly. A bearing holder 26a has an upper outer rim portion 26d which rides on the conical portion of the rod 26b through bearings 26c. The upper portion 26d is threadably engaged in the bearing holder 26a and the weight detector 25 is fastened to the lower surface of the bearing holder 26a.

The lower end of the rod 27 is seated in a shoulder portion 10 of the hermetically sealed furnace 1. A worm gear 29 is affixed to the lower end of the threaded shaft 27 and engages a motor 28. As the motor 28 rotates the threaded shaft 27 the rack 22 is slowly moved vertically with respect to the furnace l.

.A float 35 is immersed in the liquid 31. The tub 32 is provided with a inlet port 33 at its lower end through which liquid is supplied and has an outlet port 34 at its upper end. Liquid is supplied to the tub 32 at a rate sufficient to maintain the liquid at a constant level 31a as the float 35 is removed vertically from the tub 32.

The upper end of the float 35 is threaded at 39 to engage the lower end of a pulling rod 38. The upper end of the rod 38 is connected to a second weight detector 37 which, in turn, is suspended from the upper portion 22b of the rack 22 by a second suspending installation 36.

The second suspending installation 36 and the second weight detector 37 are of substantially similar construction to the first suspending installation 26 and the first weight detector 25. The second suspending installation comprises a bearing holder 36a and a rod 36b having its upper end fastened to the portion 22b of the rack 22 and having its lower end conically flared. The bearing holder 360 has an upper portion 36d threadably engaged with it. The upper portion 36d rides on the flared portion of the rod 36b by means of a bearing assembly 36c and the weight detector 37 is attached to the lower surface of the bearing holder 36a.

With reference now to FIGS. 1 and 3a the float 35 comprises a main body portion 35b which has a substantially uniform. diameter corresponding to the desired diameter of the final single crystal. At the upper portion of the float the diameter is reduced in a shoulder portion 35a'. The diameter is also gradually reduced at the lower end of the float in a sprout portion 360. The length of the float is selected to be substantially the same as the length of the final single crystal. The crosssectional area ratio of the float 35 to the finally finished single crystal is selected to be the' same as the ratio of the liquid 31 to the single crystal 20 in specific gravity. The float 35 may be made of a variety of materials such as aluminum, polytetrafluoroethylene, available under the tradename of Teflon and manufactured by DuPont Company, or zircon. In some cases it may be hollow and in other cases it may be solid, depending on the material and the desired weight.

The first and

second weight detectors

25 and 37 are constructed of semiconductive materials and are adapted to convert the weights of the crystal 20 and the float 35 into electrical signals. The weight detectors are comprised of strain gages which may be constructed, for example, of adjacent portions of an identical silicon wafer. In this way they will be guaranteed to have substantially similar characteristics and will be equally affected by temperature. Using identical gages cancels out any inherent inaccuracies in the gages.

The output signals of the first and

second weight detectors

25 and 37 are compared by a comparator circuit 40 and the compared output from the circuit 40 is supplied to a processing circuit 41. The processing circuit 41 has three output signals which are fed respectively to a power control 42, a lift speed control 43 and a pull up speed control 44. The power control 42 determines how much electrical power is supplied to the heater 13. The lift speed control 43 determines the speed at which the motor 8 lifts the crucible 2. The pull up speed control 44 determines the speed at which the motor 28 raises the rack 22. Thus the processing circuit .41, through the three output controlsignals, determines the rate and diameter at which the crystal 20 is grown. The reason that the position of the melt level in the crucible 2 is controlled by the motor 8 is that when the relative position of the melt level to the heater l3 is changed, the cross-sectional area of the single crystal is also changed.

In operation, the growth of the single crystal 20 is i started by immersing the seed 19 in the melt 12. At the start of the growth of the crystal the float 35 is immersed inthe liquid 31 with the upper end of the float 35a coinciding with the liquid level 31a in the tub 32. The crucible support 3 and the crucible 2 are rotated relative to the heater 13 by means of the rotating shaft 4 and the crystal pulling shaft 17 is rotated in the opposite direction at a predetermined speed by means of the motor 23 and the worm gear 24. By rotating the crucible 2 within the heater 13 the heat emission of the heater 13 is uniformly averaged. The crystal pulling motor 28 is driven under'the control of the controller 44 to raise the rack 22 at a predetermined speed. As the single crystal 20 grows it increases its weight and thereby produces an output signal from the first weight detector 25.

Simultaneously with the pulling up of the crystal 20 the float 35 is lifted from the liquid 31a by the upward movement of the rack 22. As the float 35 is lifted from the liquid 31 the buoyant forces acting on the float 35 decrease and therefore the weight of the float as sensed by the second weightdetector 37 increases. Liquid is supplied through the inlet 33 in an amount sufficient to make up for the falling of the liquid level due to the removal of the float 35. I

'At the start of the pulling up of the single crystal 20 and the float 35 the outputs-of the first and

second weight detectors

25 and 37 are adjusted to be identical to each other and the motor 28 is set to raise therack 22 at a predetermined speed. When the weight detected by the first weight detector 25 is greater than the weight detected by the second weight detector 37, the speed of the crystal pulling motor 28 is increased under the control of a signal from the output of the comparator circuit 40 which is fed through the processing circuit 41 to the crystal pull up speed controller 44. The result is that the single crystal 20 is thereby made smaller in diameter. When the weight detected by the first weight detector 25 is less than the weight detected by the second detector 37, the pull up speed of the single crystal 20 is reduced in the same manner thereby making the single crystal 20 larger in diameter.

The grown single crystal 20 has a shoulder corresponding to the shoulder a of the float and a sprout corresponding to the sprout 35c of the float. Most importantly the crystal 20 has a uniform main body portion corresponding in diameter to the main body portion 35b of the float. Thus the length and diameter of the float 35 may be selected to give the desired shape of the finished single crystal. In order to obtain any particularly shaped single crystal in the same apparatus only the float 35 need be changed.

Although the crystal pulling shaft 17 is described above as being rotatable in another embodiment the shaft 17 is stationery and the heater 13 is rotated. In such an embodiment no mechanical rotational noise is transmitted by the shaft 17 to the strain gage in the weight detector 25. In other embodiments the second weight detector 37 is suspended from a movable rack which is synchronized with the rack 22. In still other embodiments the level 31a in the tub 32 is maintained constant by lifting the tub 32 rather than by continuously supplying liquid through the input port 33. In still further embodiments the float 35 is held stationery and the tub 32 is lowered. In all embodiments when the tub A 32 and the crucible 2 have the same shape, it is not necessary to correct for the depression of the liquid level 31a due to the raising of the float 35 because the level 31a and the melt level are then depressed synchronously.

Referring now to FIG. 2, the details of the control system represented by the

circuits

40, 41, 42, 43 and 44 will be explained in greater detail. A description of the main structure will not'be given again since it was described in reference to FIG. l. The weight of the pulled up single crystal 20 is detected by the first weight detector 25 and an output signal is supplied by it to one input terminal of a polarization error amplifier 47. The weight of the float 35 is detected by the second weight detector 37 and an output signal is supplied by it to one input of a zero adjuster 45. An output signal corresponding to a predetermined diameter of the pulled up single crystal 20 is supplied from a diameter adjuster 46 to the other input of the-zero adjuster 45, so that the "output of the diameter adjuster is superposed upon the output of the zero adjuster 45. The output of the zero adjuster 45 issupplied to the other inpu terminal of the polarization error amplifier 47.

At the start of the pulling up of the single crystal 20, the output of the zeroadjuster 45 is adjusted such that the output of the polarization error amplifier 47 is zero. The output of the polarization error amplifier 47 is fed to a PID controller 48 where it is'differentiated with respect to the length of the crystal by a differentiation circuit. The controller 48 produces an output signal representative of the difference between the set crosssectional area of the float and the cross-sectional area of the growing crystal as will be explained in further detail below with regard to FIGS. 3b and 3c. The output from thecontroller 48 is supplied to the crystal pulling motor 28 through an amplifier 49 and is also supplied to the crucible lift motor 8 through a crucible lift controller 50.

A programed control signal source 51 is provided for producing a variable control signalin response to the length of the pulled up single crystal 20. This control signal is supplied to a constant-power controller 53 through an adder circuit 52. Power is supplied to the heater 13 from the constant-power controller53 under.-

Constant power may be applied to the heater 13 when the control signal is zero. v Variation in the pull up speed of the single crystal 20 can be made smaller by controlling heating power supplied to the heater 13 as a functionof the difference output from the PID controller 48 of the set crosssectional area of the float 35 and the physical crosssectional area of the crystal 20. The output signal derived from the PID controller 48 is compared in a comparator 54 with a reference signal from a programed reference source 55. When the difference of the set cross-sectional area of the float 35 and the physical cross-sectional area of the crystal exceedsa predeter- 7 v mined value, an output signal is derived from the comparator 54. v

This output signal is differentiated with respect to the length of the crystal by adifferentiation circuit 56 which supplies its differentiated output signal to the adder circuit 52. The differentiated signal is thereby added to the control signal from the source 51 or to a signal from a manual power regulator 57 coupled to the adder circuit 52. The heating power supplied to the heater l3'from the constant-power controller 53 may thus be controlled by the-added output signal.

An error output signal derivedfrom the polarization error amplifier 47 is also differentiated with respect to the length of the single crystal by a differentiation circuit 58 to detect the difference of the set crosssectional area of the float 35 and the physical crosssectional-area of the crystal 20. The detected output is supplied to a graphic meter 60 through a DC amplifier 59 and is then recorded. It will of-course be understood that the recorded signal is very useful for a subsequent crystalline analysis.

In order to obtain the desired shape of the single crystal-20, the ratio of the pull up speed of the single crystal 20 to the lift speed of the crucible 2 must bealways equal to the ratio of the cross-sectionalarea of the'cru- 'cible 2 to the cross-sectional area of the single crystal 20. When the diameter of the single crystal 20 is constant, the ratio of the pull up speed of the single crystal 20 to the lift speed of the crucible 2 is constant. When the diameter is gradually increased or decreased, the

lift speed of the crucible 2 mustbe changed in proportion to variationof the cross-sectional area of the single crystal 20, assuming the pull up speed of the single crystal 20 is held constant. a

If the incrementof the pull up speed of the single crystal 20 is expressed by AV, the increment of the cross-sectional area of the single crystal 20 is expressed by AA and the equation AA KAV is established, then the coefficient K is variable, that is, the larger the diameter of the single crystal 20, i.e., the cross-sectional area thereof, the larger is the coefficient K. The larger the cross-sectional area of the single crystal 20 in a particular pull up speed variation, the larger'is AA in the same condition. The transfer functions for the motors 8 and 28 must always be kept constant in order to operate the control system for the motors 8 and 28 in a stable condition. To

portional sensitivity of the PID controller 48 must be always constant. When the value of thecoefflcient K is changed in one direction in response to the variation of the cross-sectional area of the single crystal 20 as described above, the proportional sensitivity of the PID controller 48 must be changed in the reverse direction. To accomplish this the crucible lift controller 50 and the proportional sensitivity of the PID controller 48 for the above-described control system are controlled by a control signal representative of the variation of the weight of the single crystal 20 due to the increase in its length. The derivation of this control signal is explained below. The proportional sensitivity of the crucible lift motor 8 and the crystal pulling motor 28 are also controlled by the control signal.

- A detected output E1 is derived from the firstweight detector 25 and is differentiated with respect tothe length l of the single crystal 20 by a differential circuit 61. The conditions of the detected output El with respect to l' are shown in curves a, b and c of FIG. 3B. The curve a indicates the condition of the output El with respect to the length l at the shoulder 35a of the float 35. The curve b indicates the condition of the output El with respect to the length l at the middle part of the float 35. The curve c indicates the condition of the output El with respect to the length l at the sprout 350 of the float 35.

The conditions of a differentiated output signal (dEl/dl) derived from the differential circuit 61 with respect to the length l are shown in the curves a, h and c of FIG. 3C. The curve a indicates the'condition of (dEl/dl) with respect to the length l at the shoulder 35a of the float 35.- The curve b indicates the condition of (dE/dl) with respectto the length l at the middle part 35b of the float 35. The curve c indicates the condition of the differentiated output (dEl /dl) with respect to the length l at the sprout 35c of the float 35 The differentiated output is supplied to a comparator 63 through an amplifier 62 to compare itwith a reference signal derived from a programed reference source 64.

The output signal from the comparator 63 is supplied to the crucible lift controller 50 to' control the magnitude of its output and thereby to control the crucible lift motor 8 and the lift speed of the crucible 2, as shown in curves 0, b and c of FIG. 3C at the shoulder 35a, the middle part 35b and the sprout 350 of the float 35, respectively. The output of the comparator 63 is also supplied to the PID controller 48 so that-the proportional sensitivity of the PID controller is controlled in the reverse relation to that shown in curves a, b and c of FIG. 3C at the shoulder, at the middle part and at the sprout of the crystal 20, respectively.

Thus, the lift speed of the crucible 2 is changed in response to the variation of the cross-sectional area of the single crystal 20 at the shoulder and sprout of the single crystal, with result that the meltlevel of the melt 12 within the crucible 2 is kept constant in height at the shoulder and sprout of the single crystal. Furthermore the transfer functions of the control system for the motors 8 and 28 are always kept constant to operate them in a stable condition.

When thelength ofthe single crystal 20 to be pulled is over a predetermined value and the quantity of the melt 12 is below apredetermined value, the melt 12 is loweredin temperature so that the cross-section area of the single crystal to be pulled up is thereby increased. This results in a faster pull up speed of the single crystal 20. When the pull up speed of the single crystal 20 is too sudden, a good quality single crystal can not be obtained. The crystal in such cases is nonuniform in specific resistance.

A sudden increase in the cross-sectional area of the single crystal 20 is prevented by controlling the heating temperature of the crucible 2 as a function of the length of the single crystal 20. This is accomplished by controlling the heating power supplied to th heater 13 by means of the output of the programed controlsignal source 51 in respone to the length of the single crystal 20. Here, an input signal of the differential circuit 61 may be derived from the output of the second weight detector 37. r

Thus, when the melt 12 tends to be too low in temperature power supplied to the heater 13 from the constant-power controller 53 is gradually increased by the control signal derived from the control signal source 51, and the heating temperature of the crucible 2 be- 9 comes gradually higher. As a result, the cross-sectional area of the single crystal is not greatly increased and the'pull up speed of the single crystal 20 is not suddenly increased. I

Referring now to FIG. 4 the furnace of another embodiment of the invention is shown. As will be explained in further detail the furnace prevents temperature and pressure drift of the liquid coolant in the furnace cooling system from influencing the temperature control system for the crystal melt. I

Since a single crystal pulling up furnace is operated at a high temperature,-typically in excess of 1,400C, the single crystal pulling up furnace has hollow, double structure furnace walls through which cooling water circulates. This ensures that the outer surfaces of the walls are temperature controlled and prevented from becoming so hot that they constitute a danger to the operator or an overload on the air conditioning system of the room which contains the furnace.

When the temperature or circulation rate of the cooling water is rapidly changed, its cooling effect is thereby changed, and the heat supplied to the crucible is also changed. This adversely affects the otherwise highly accurate and indespensable temperature control of the crucible. Therefore prevention of drift of the cooling effect caused by temperature drift and pressure drift of the cooling water is indespensable' to the realization .of highly accurate temperature control of the crucible. I

In the furnace depicted in FIG. 4, an inside cooler 65 is disposedaround and spaced from a heater 14' to prevent the leakage of heating power from the furnace. A fluid intake line 66 which is passed from the furnace exterior through the furnace walls, is connected to the lower end of the inside cooler 65. A motorized control valve 67 is provided in the line 66 to control the quantity of cooling liquid flowing through the cooler 65. An

outlet line 68 is connected to the upper portion of the cooler 65 and passes from the interior to of the furnace through the walls.

A temperature detector, such as a thermoelectric couple, is provided at a suitable place within the furnace to sense the temperature of the cooler 65. In the embodiment depicted in FIG. 4 a temperature detector 69 is placed opposite the center of the crucible 2 and on the inside of the cooler 65. The output of the temperature detector 69 is connected to the input of a controller 70 which measures the difference between the temperature sensed by the-detector 69 and a preset temperature and controls the valve 67 as a function of that difference. I

The furnace wall 711 is hollow and also contains a the exterior cooling liquid. An intake line 72 for supplying cooling I water is connected to the hollow interior of the wall 711 at the lower end'of the furnace. A motorized control valve 76 regulates the amount of cooling water which enters the hollow furnace wall 71. An outlet line 73 is connected to the upper end of the hollow interior of the furnace wall 71. A temperature detector 74, such as thermoelectric couple, is attached to the inside of the furnace wall 7ll The output of the temperature detector 74 is connected to the input of a controller 75 which is similar in construction to the controller 70. The controller 75 operates the control valve 76 as a function of the temperature sensed by the detector 74.

Thus the furnace of the embodiment of FIG. 4 allows the temperature of the heating element 14' and the furnace coolant to be held substantially constant and free from variations caused by drift of the pressure or temperature of the cooling apparatus which surrounds the heating element. 9

The terms and expressions which have been employed here are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions, of excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.

We claim as our invention:

1. Apparatus for producing a semiconductor crystal comprising:

a. a crucible containing a melt of the'semiconductor;

b. a tub filled with a liquid; I c. a float immersed in the liquid of the tub;

(1. means for simultaneously pulling up a growing crystal from the crucible and for removing the float from the liquid; e. comparator means for comparing the weights of the crystal and the float and for producing an output signal representative of the weight difference between the crystal and the float; and I f. control means responsive to the output signal of the comparator means for controlling the speed at which the crystal and float are pulled up 2. Apparatus for producing a semiconductor crystal as recitedin claim 1 wherein the float has the desired shape of the crystal to be pulled up from the crucible.

3. Apparatus for producing a semiconductor crystal as recited in claim ll wherein the control means includes means for vertically moving the crucible.

4. Apparatus for producing a semiconductor crystal as recited in claim ll wherein the control means further controls the heating temperature of the crucible.

' 5. Apparatus for producing a semiconductor crystal as recited in claim 1 wherein the means for pulling up the crystal and the float includes a vertically movable member, and wherein thecomparator means includes a first weight detector connected between'the vertically movable member and the crystal and a second weight detector connected between the vertically movable member and the float. I I

6. Apparatus for producing a semiconductor crystal as recited in claim 11 further comprising means forrotating the crucible and the crystal in opposite directions relative to each other.

7. Apparatus for producing a semiconductor crystal as recited in claim 1 wherein the control means includes means for-sensing the. length of the crystal and for producing a first output signal representative of the length of the crystal, means for sensing the weight of I the crystal and for producing a second output signal representative of the crystal weight, and means responsive to the first and second output signals for controlling the rate of response of the control means as a function of the rate of change of the second signal with respect to the rate of change of the first signal. I

8. Apparatus for producing a semiconductor crystal as recited in claim I wherein the tub is fed with a continuous supply of liquid for maintaining the liquid level constant in height.

9. Apparatus for producing a semiconductor crystal as recited in claim 1 wherein the tub and the crucible have interior hollow shapes which are substantially identical.

10. Apparatus for producing a semiconductor crystal as recited in claim 1 wherein the tub is lifted up for maintaining the height of the liquid level constant.

11. Apparatus for producinga semiconductor crystal as recited in claim 1 further comprising means for heating the crucible and means for rotating the crucible relative to the heating means.

12. Apparatus for producing a semiconductor crystal as recited in claim 1 further comprising:

a. means for heating the crucible;

b. a furnace for enclosing the crucible and the heating means; and -c. means for cooling the furnace.

13. Apparatus for producing a semiconductor crystal as recited in claim 12 further comprising:

a. means for sensing the temperature of the furnace and for producing a control signal representative of the furnace temperature and l b. means for controlling the cooling means in accordance with the control signal. 14. Apparatus for producing a semiconductor crystal comprising:

a. a crucible containing a melt of the semiconductor;

b. a tub filled with a liquid; I c. a float immersed in the liquid of the tub;

d. means for simultaneously pulling up a growing the crystal and the float and for producing an output signal representative of the weight difference between the crystal and the float; and

f. control means responsive to the output signal of the comparator means for controlling the shape of the growing crystal.

15 Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the float has the desired shape of the crystal to be pulled up from the crucible.

16. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the control means includes means for vertically moving the crucible.

17. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the means for controlling the shape of the growing crystal includes means vfor controlling the speed at which the growing crystal and ence signal, and 'means responsive to the comparator output signal forvertlcally moving the crucible at-a controlled speed such that the ratio of the pull-up speed of the growing crystal to the lift speed of the crucible equals the ratio of the cross-sectional area of the crucible to the cross-sectional area of the crystal, taken at the surface of the melt in the crucible.

19. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the control means further controls the heating temperature of the crucible.

adjacent portions of the same silicon wafer.

22. Apparatus for producing a semiconductor crystal as recited in claim 14, further comprising means for rotating the crucible.

23. Apparatus for producing a semiconductor crystal as recited in claim 14, further comprising means for rotating the growing crystal.

24. Apparatus for producing a semiconductor crystal as recited in claim 14 further comprising means for rotating the crucible and the crystal in opposite directions relative to eachother.

25. Apparatus for producing a semiconductor crystal as recited in claim 14, further comprising a hermetically sealed furnace surrounding the crucible and wherein the means for pulling up the growing crystal includes a rotatable, hollow outer shaft, a rotatable, crystal pulling, inner shaft, the outer shaft having a portion which extends into the furnace and" means for effecting a hermetic seal between the outer shaft and the inner shaft, the inner shaft being freely movable in a vertical direction within the outer shaft.

26. Apparatus for producing a semiconductor crystal as recited in claim 14 further comprising a heater disposed about the periphery of the crucible and an insulator surrounding the heater and having a predetermined spaced relationship with it for preventing the radiation of heat outwardly and for reflecting the radi ated heat inwardly upon the crucible.

27. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the growing crystal pulling up means further comprises a vertically movable frame, a support for the growing crystal and suspension means connected between the frame and the crystal support for reducing rotational friction of the crystal support, the suspension means including a vertically mounted rod having a flared end, a bearing holder encompassing the flared end of the rod, and bearings within the holder which ride on the flared portion of the rod.

28. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the ratio of crosssectional area of the float, at any given longitudinal position, to the cross-sectional area of the crystal at a corresponding longitudinal position is substantially equal to the ratio of the specific gravities of the liquid in the tub and the crystal.

for comparing the output Signals and generating a representative output. signal, processing means responsive to the comparator output signal for generating first, second and third control signals, means responsive to the first control signal for controlling the temperature of the melt within the crucible, means responsive to the second control signal for controlling the speed at which the growing crystal is pulled up and means responsive to the third signal for lifting the crucible at a controlled speed.

30. Apparatus for producing a semiconductor crystal as recited in claim 29 further comprising diameter adjuster means for generating a signal corresponding to a predetermined diameter of the growing crystal, a zero adjuster means having two inputs and an output, one of the inputs being supplied with the signal representative of the weight of the float and the other input being supplied with the signal from the diameter adjuster means so that the output of the diameter adjuster means is superposed upon the output of the zero adjuster means, and wherein the comparator means includes a polarization error amplifier having one input supplied with the signal representative of the weight of the growing crystal and another input which is supplied with the output from the zero adjuster means, the zero adjuster means including means for initially adjusting the output of the polarization error amplifier to be substantially zero.

31. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the control means includes programmed source means for producing a variable control signal in response to the length of the growing crystal, the programmed relationship between the length of the growing crystal and the magnitude of the variable control signal being such that the magnitude is zero until the length of the growing crystal is a predetermined fraction of its desired finished length, the magnitude thereafter increasing proportionally with the length of the growing crystal, and menas responsive to the magnitude of the variable control signal for controlling the temperature of the melt.

32. Apparatus for producing a semiconductor crystal as recited in claim 14, wherein the control means further comprises means for generating a control signal representative of the difference in the set crosssectional area of the float and the physical crosssectional area of the growing crystal, means for comparing the control signal with a programmed reference signal and for generating an output signal when the control signal exceeds a predetermined value, means for differentiating the output signal with respect to the length of the growing crystal, and means responsive to the differentiated output signal for controlling the temperature of the melt.

33. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the control means includes means for sensing thelength of the crystal and for producing a first output signal representative of the length of the crystal, means for sensing the weight of the crystal and for producing a second output signal representative of the crystal weight, and means responsive to the first and second output signals for controlling the rate of response of the control means as a function of the rate of change of the second signal with respect to the rate of change of the first signal 34. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the tub is fed with a continuous supply of liquid for maintaining the liquid level constant in height.

35. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein'the tub'and the crucible have interior hollow shapes which are substantially identical. e

36. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the tub is lifted up for maintaining the height of the liquid level constant.

37. Apparatus for producing a semiconductor crystal as recited in claim 14 further comprising means for lowering the tub to remove the float from the liquid in the tub.

38. Apparatus for producing a semiconductor crystal as recited in claim 14 further comprising means for heating the crucible and means for rotating the crucible relative to the heating means.

39. Apparatus for producing a semiconductor crystal as recited in claim 14 further comprising:

a. means for heating the crucible;

b. a furnace for enclosing the crucible and the heating means; and i c. means for cooling the furnace.

40. Apparatus for producing a semiconductor crystal as recited in claim 39 further comprising:

a. means for sensing the temperature of the furnace and for producing a control signal representative of the furnace temperature and b. means for controlling the cooling means in accordance with the control signal.

Claims

1. Apparatus for producing a semiconductor crystal comprising: a. a crucible containing a melt of the semiconductor; b. a tub filled with a liquid; c. a float immersed in the liquid of the tub; d. means for simultaneously pulling up a growing crystal from the crucible and for removing the float from the liquid; e. comparator means for comparing the weights of the crystal and the float and for producing an output signal representative of the weight difference between the crystal and the float; and f. control means responsive to the output signal of the comparator means for controlling the speed at which the crystal and float are pulled up.

2. Apparatus for producing a semiconductor crystal as recited in claim 1 wherein the float has the desired shape of the crystal to be pulled up from the crucible.

3. Apparatus for producing a semiconductor crystal as recited in claim 1 wherein the control means includes means for vertically moving the crucible.

4. Apparatus for producing a semiconductor crystal as recited in claim 1 wherein the control means further controls the heating temperature of the crucible.

5. Apparatus for producing a semiconductor crystal as recited in claim 1 wherein the means for pulling up the crystal and the float includes a vertically movable member, and wherein the comparator means includes a first weight detector connected between the vertically movable member and the crystal and a second weight detector connected between the vertically movable member and the float.

6. Apparatus for producing a semiconductor crystal as recited in claim 1 further comprising means for rotating the crucible and the crystal in opposite directions relative to each other.

7. Apparatus for producing a semiconductor crystal as recited in claim 1 wherein the control means includes means for sensing the length of the crystal and for producing a first output signal representative of the length of the crystal, means for sensing the weight of the crystal and for producing a second output signal representative of the crystal weight, and means responsive to the first and second output signals for controlling the rate of response of the control means as a function of the rate of change of the second Signal with respect to the rate of change of the first signal.

8. Apparatus for producing a semiconductor crystal as recited in claim 1 wherein the tub is fed with a continuous supply of liquid for maintaining the liquid level constant in height.

11. Apparatus for producing a semiconductor crystal as recited in claim 1 further comprising means for heating the crucible and means for rotating the crucible relative to the heating means.

12. Apparatus for producing a semiconductor crystal as recited in claim 1 further comprising: a. means for heating the crucible; b. a furnace for enclosing the crucible and the heating means; and c. means for cooling the furnace.

13. Apparatus for producing a semiconductor crystal as recited in claim 12 further comprising: a. means for sensing the temperature of the furnace and for producing a control signal representative of the furnace temperature and b. means for controlling the cooling means in accordance with the control signal.

14. Apparatus for producing a semiconductor crystal comprising: a. a crucible containing a melt of the semiconductor; b. a tub filled with a liquid; c. a float immersed in the liquid of the tub; d. means for simultaneously pulling up a growing crystal from the crucible and for removing the float relative to the liquid in the tub; e. comparator means for comparing the weights of the crystal and the float and for producing an output signal representative of the weight difference between the crystal and the float; and f. control means responsive to the output signal of the comparator means for controlling the shape of the growing crystal.

15. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the float has the desired shape of the crystal to be pulled up from the crucible.

17. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the means for controlling the shape of the growing crystal includes means for controlling the speed at which the growing crystal and the float are pulled up.

18. Apparatus for producing a semiconductor crystal as recited in claim 17 wherein the control means includes means for generating a control signal representative of the variation of the weight of the growing crystal due to the increase in its length, a reference source for supplying a reference signal, comparator means for comparing the control signal with the reference signal and for generating an output signal representative of the difference between the control signal and the reference signal, and means responsive to the comparator output signal for vertically moving the crucible at a controlled speed such that the ratio of the pull-up speed of the growing crystal to the lift speed of the crucible equals the ratio of the cross-sectional area of the crucible to the cross-sectional area of the crystal, taken at the surface of the melt in the crucible.

20. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the means for pulling up the crystal and the float includes a vertically movable member, and wherein the comparator means includes a first weight detector connected between the vertically movable member and the crystal and a second weight detector connected between the vertically movable member and the float.

21. Apparatus for producing a semiconductor crystal as recited in claim 20, wherein the first and second weight detectors comprise strain gages constructed of adjacent portions of the same silicon wafer.

24. Apparatus for producing a semiconductor crystal as recited in claim 14 further comprising means for rotating the crucible and the crystal in opposite directions relative to each other.

25. Apparatus for producing a semiconductor crystal as recited in claim 14, further comprising a hermetically sealed furnace surrounding the crucible and wherein the means for pulling up the growing crystal includes a rotatable, hollow outer shaft, a rotatable, crystal pulling, inner shaft, the outer shaft having a portion which extends into the furnace and means for effecting a hermetic seal between the outer shaft and the inner shaft, the inner shaft being freely movable in a vertical direction within the outer shaft.

26. Apparatus for producing a semiconductor crystal as recited in claim 14 further comprising a heater disposed about the periphery of the crucible and an insulator surrounding the heater and having a predetermined spaced relationship with it for preventing the radiation of heat outwardly and for reflecting the radiated heat inwardly upon the crucible.

28. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the ratio of cross-sectional area of the float, at any given longitudinal position, to the cross-sectional area of the crystal at a corresponding longitudinal position is substantially equal to the ratio of the specific gravities of the liquid in the tub and the crystal.

29. Apparatus for producing a semiconductor crystal as recited in claim 14 and further comprising means for generating separate output signals representative of the weights of the growing crystal and of the float, means for comparing the output signals and generating a representative output signal, processing means responsive to the comparator output signal for generating first, second and third control signals, means responsive to the first control signal for controlling the temperature of the melt within the crucible, means responsive to the second control signal for controlling the speed at which the growing crystal is pulled up and means responsive to the third signal for lifting the crucible at a controlled speed.

32. Apparatus for producing a semiconductor crystal as recited in claim 14, wherein the control means further comprises means for generating a control signal representative of the difference in the set cross-sectional area of the float and the physical cross-sectional area of the growing crystal, means for comparing the control signal with a programmed reference signal and for generating an output signal when the control signal exceeds a predetermined value, means for differentiating the output signal with respect to the length of the growing crystal, and means responsive to the differentiated output signal for controlling the temperature of the melt.

33. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the control means includes means for sensing the length of the crystal and for producing a first output signal representative of the length of the crystal, means for sensing the weight of the crystal and for producing a second output signal representative of the crystal weight, and means responsive to the first and second output signals for controlling the rate of response of the control means as a function of the rate of change of the second signal with respect to the rate of change of the first signal.

34. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the tub is fed with a continuous supply of liquid for maintaining the liquid level constant in height.

35. Apparatus for producing a semiconductor crystal as recited in claim 14 wherein the tub and the crucible have interior hollow shapes which are substantially identical.

39. Apparatus for producing a semiconductor crystal as recited in claim 14 further comprising: a. means for heating the crucible; b. a furnace for enclosing the crucible and the heating means; and c. means for cooling the furnace.

40. Apparatus for producing a semiconductor crystal as recited in claim 39 further comprising: a. means for sensing the temperature of the furnace and for producing a control signal representative of the furnace temperature and b. means for controlling the cooling means in accordance with the control signal.