US3814827A - Method and apparatus for floating zone melting of a semiconductor rod - Google Patents

Abstract

Floating zone melting of a silicon rod is monitored by a television camera which provides electrical pulses for controlling the melting zone to a programmed diameter and tangent angles of the melting zone profile.

Description

I United States Pat nt [m Stut .1 June 4, 1974 54] METHOD AND APPARATUS FOR [56] References Cited FLOgATlN?) ZONE MELTING OF A UNITED STATES PATENTS SEM CON UCTOR ROD 3,243,509 3/l966 Stut 13/1 X [75] lnventorz- Hans Stut, Grohenzell; Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin P E and Mun Germany Attorney, Agent, or Flrm-H1ll, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson [22] Filed: Sept. 25, 1973 [2]] Appl. No.: 400,702 57 ABSTRACT Floating zone melting of a silicon rod is monitored by o e gn Appllcatlon Prl0rlty Data a television camera which provides electrical pulses Sept. 28, l972 Germany 2247651 for controlling the melting zone to a programmed di- June 28. 1973 Germany 2332968 ameter and tangent angles of the melting zone profile.

[52] U.S. Cl 13/1, l'3/DlG. l, 219/1043 r 511 lm. Cl. H05b-5/08, HOSb 9/00 1 I [58] Field of Search l3/l 26, DIG. l;

m E 2 l2 r3n 19 I 2 t 20 3 H i i 2 K 521 L 11. 6

i511 l5b 23 7 10a 10b 1 bu 5 U BACKGROUND or THE INVENTION the recorded pictures to control the melting process.

Description of the Prior Art Techniques such as those generally set forth above are described in the German Letters Pat. No.

1,23 l ,76l corresponding to US. Ser. No. 231,182 now U.S. Pat. No. 3,757,071 and in the German patent application P2l 13720. However, they have only proven advantageous in they-case of zone melting in a q'uasistationary condition, wherein the diameter of the initial rod and that of the rod which hasbeen decrystalized from the melting zone only differslightly from each other, orwher'ein and approximately constant relation to each other exists. if, however, in the decrystalizing rod a conical junction between rod parts having different diameters is provided, the energy which is directed to the melting zone must be adjusted to the volume of the'required melting zone, as well as to the cooling conditions and the electrical coupling relations between the induction coil creating the melting zone and the melting zone itself.IThis is the case, for example, if a small core crystal is joined to a storage rod which is to be remelted by zone melting and has a larger diameter, and if the melting zone is at first createdat the border between the core and the storage rod.

Such a control is impossible by means of the presently published methods for a self-acting control of the melting zone process. Y

SUMMARY OF THE INVENTION In view of the foregoing, it is a primary object of theinvention to provide techniques, including methodand apparatus, for pulling a cone-shaped transit we between a core crystal which is fused to a storage rod, wherein the desired diameterof the semi-conductor rod which is to be melted may differ from' the diameter of the storage rod in accordance with'a prescribed program, and atthe same time avoid in the quasi-' stationary condition, deviations from the desired diameter of the' semiconductor rod which is to be melted,

which deviations might occur due to fluctuations of th diameter of the storage rod.

The foregoing object isachieved in that information concerning the volume of the melting zone,proper angles and the melting and the solidifying front and at the turning point during the course of the melt-liquid part of a volume control system interposed in the actual di-;

ameter control loop for the rod. The angle deviations of the actual angles from the desired values are employed for adjusting the applied energy. With these techniques, the part of the melting zone which determines the growth of the semiconductor rod is scanned in alinear manner in order to determine the volume,

the diameter of the melting zone per line isdetermined,

raised to the second power and added. In addition, at the same time, the diameter at the solidifying front and the angles at the projecting points; solidifying front, melting front and turning point are determined from the pulses of the linearly scanned melting zone. The geometric form of the conical rod part is determined by the respective diameter and the respective angle whereby the diameter and the corresponding angle constitute a function of the distance which was covered by the solidifying front. Accordingly, the diameter and the angles of projection are dependent in the form of a cosine function on the distance which has covered by 1 the solidifying front. Furthermore, the respective desired'angle-is determined for controlling the energy supply, according tothe invention, from the coupling of the diameterand the corresponding angle via the distance covered bythe solidifying front as a function of the diameter. in the methods heretofore known, in the sense of an automatic control, either the volume which is covered by the melting zone or the energy which is directed to the melting zone have been maintained as a contant level. In particular, the first method wherein the volume is maintained at a constant level, is sufficient for a quasi-stationary operation, whereby neither the traveling speedof the melting zone nor the diameters or the diameter relation, respectively, of the two rod parts, nor the radial or axial heat flowconditions change.

The advantages of the method according to the present invention, with respect to the methods which were heretofore known, resides in the fact that, independent of the aforementioned constancy, the control of the entire process can be carried out in such a way that the energy supply as well as the distance of the rod ends which limit the melting zone correspond to the respeca steady course of the diameter of the rod length is to tive optimum stability or growth conditions, respectively. Such a case is'provided, for example, if a monocrystalline silicon rod starting froma thin core crystal is to be melted to a given diameter, whereby basically the geometric dimensions change and therefore also the energy which is required in the corresponding por-' tions of the melting coil changes. t

For the realization of a program for the junction from core crystal to desired diameter of the rod to be melted,

beprovided because of the pulling conditions, the initialconditions and the final conditions. According to the invention, it is therefore suggested to base the program on a cosine function of the diameter and the respective angle of the continuous path which corresponds in a first approximation to the course of the cosine-shaped junction having been determined through tests. The cosine function which comes nearest the values which were determined in experiments amounts to:

is assigned to the respective value, r is the radius of the core crystal, R is the radius of the finished cylindrical cosine-shaped junction, which preferably corresponds approximately to the diameter of the rod to be melted.

In addition, it has become obvious that for the cylindrical growth a certain opening angle of the melt-liquid zone is required at the solidifying front, whereby this angle is'again dependent to a certain extent on the diameter of the solidifying front; a course of the opening angle canalso be determined via the length of the junction zone, which may be mathematically derived, for example, by differentiating the above-captioned diameter function.

Also, the'function of the angle of diameter a f (d) can be derived from the two functions of the diameter and the respective angle depending on the covered distance d f (x The application of this function for controlling the angle can also be advantageous if, due

to the interferences and discontinuities in the growth,

the function d f (Xk), considered ideal, cannot be accurately maintained.

Also, changes of thegeometric and thermal conditions are to be expected if the melting zone approaches the rod end, or if the storage rod exhibits irregular diameters, of if the surrounding atmosphere changes or is changed, for example, the protective gas or vacuum, of if a doping gas flow strikes the melt-liquid zone.

BRIEF DESCRIPTION OF THE DRAWING Other objects, features and advantages of the invention, its organization, construction and'operation will be best understood from the following detailed description taken in conjunction with the accompanying-drawing, on which the single figure schematically illustrates the method and apparatus for practicing the present invention.

" DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, a program is, for example, fed into a data processing arrangement 11. The program which reflects the dependency of the rod diameter beginning at the core crystal and on which the distance covered by the phase border or the induction heating coil 12, respectively, is dependent on the functional course of the method which is to be described.

A television camera'20 scans the melting zone 2 of nism comprises an automatic volume control amplifier 4, which is subjacent to the actual diameter control loop comprising a control amplifier and an amplifier 5 for adjusting the diameter deviation to the desired volume value. The energy supply to the heating coil 12 is controlled by way of a frequency generator 18 .(oscillater) from acontrol amplifier 6 having a parallel angle These two automatic control systems are coupled with each other by way of a third geometry" automatic control system and the program unit ll in such a way that the desired geometric form of the junction is created without rendering the melt-liquid zone unstable, which means without causing the melt-liquid zone to freeze or flow off.

In the above method for crystal-free zone melting of a semiconductor rod, the melting zone is supervised by means of a television camera under constantly maintained recording conditions, whereby from the pictures of the melting zone recorded by the television camera electrical pulses are conducted by way of an optoelectronic image transformation. This method is characterized in that the information is gained from the pulses supplied by the television camera, concerning the volume of the melting zone, projection angles at the melting front and the solidifying front, as well as the turning point during the course of the melt-liquid part of the melting zone and the diameter of the semiconductor rod at the solidifying front, in that this information is used for changing the distance of the rod ends to limit the melting zone and for changing the supplied energy, whereby the distance changing of the rod ends limits the melting zone for achieving a given diameter and is carried out by way of an automatic volume control system connected subjacent to the actual diameter automatic control system and in that the angle deviation of the projection angles from their desired value is employed for the correction of the energy supplied to the melting zone.

The present invention achieves improvement in crucible-free zone melting of a semiconductor rodv disclosed in the aforementioned patent application.

Consequently, the invention concerns a device for floating zone melting of a semiconductor rod, whereby the vertically oriented semiconductor rod, which is supported at its ends by rod mountings which can be shifted in the direction of the rod axis, is surrounded coaxially by an induction heating coil creating the melting zone and which can be shifted axially, whereby furthermore an automatic control of the energy supplied automatic control system by which the angle of a respective diameter is adjusted to the desired. value.

to the induction heating coil, as well as of the distance between the tworod mountings is provided by using an electronic camera as a control guide in such a way that I in case of an unchanged maintained geometry of the lens of the electronic camera in respect of the melting zone traveling through the semiconductor rod, the linear scanning of the imageof the melting zone projected on an image converter in the electronic camera runs perpendicular to the image of the axis of the semiconductor rod.

According to the present invention, it is provided:

that a member for supervising the positions x of the crystalization front requests from a data processing arrangement, on the one hand. the magnitude d (x used as a control value for the control of the distance of the two rod mountings and, on the other hand, the magnitude a (x used as a control value for controlling the energy supplied to the induction heating coil, for the angle a (1%) between the vertical and the melting zone profile at the intersection with the crystalization border;

that the control value d (x supplied by the data processing arrangement to the input stage of a first cascade automatic control system and the control value a (x supplied to the input stage of a second cascade automaticcontrol system experiences a comparison with the respective actual value supplied by the electronic camera by way of an evaluation calculator, while a signal in proportion to the respective control deviation is created as an input signal for the second stage of the respective cascade automatic control system; and

that furthermore in the second control stages of the two cascade automatic control systems-having a Pl characteristic a comparison of the signals from the input stages, changed by integration, takes place with the actual values of a new control magnitude supplied by a control sensing element, which result operates by way of a third control stage provided with a respective recoupling directly on the respective control path at the rod mountings or the directing elements being effective in respect of the respective control path at the rod mountings or'the heating of the melting zone, respectively.

The device according to the invention is described in more detail by referring again to the drawing.

In the drawing, a semiconductor rod 1 whichis to be the semiconductor rod 1, for example the lower mounting la, is designed to be axially shiftable in the direction indicated by the double headed arrow 9' relative to the second mounting so that a melting zone 2 covers the entire cross section of the semiconductor rod and the two rod parts separated by the melting zone can be brought closer to or moved further apart from each other. This will result within the scope of the stability limits in a compression or a stretching, respectively, or the melting zone 2 so that'the mechanism controlling this movement is usually referred to as a compressionstretch mechanism." In this case, various possibilities are known so that an illustration in the drawing anda corresponding discussion in the specification is superfluous. The drive of the stretchmompression mecha-' nism is supplied by-a motor 8 which is controlledby the output of a first cascade automatic control system'.

The induction heating coil 12 issupplemented in the usual manner by a capacitor 12a connected in parallel thereto to form a high frequency resonant circuit with a fixed setting. This high frequency resonant circuit is loosely coupled with a high frequency generator 18 by way of a coupling inductor 19. The frequency of the electrical oscillations supplied by the generator 18 can be continuously changed in a frequency range contain ing the resonant frequency of the resonant circuit formed by the induction heating coil 12 and a capacitor 12a. so that activation of the adjustment of the HF generator 18, the energy which is transferred to the induc tion heating coil 12 can be changed. It should be mentioned that also other known possibilities for adjusting the energy supplied to the induction heating coil 12 can be utilized, for example, by using a resonant circuitcapacitor 12a with a variable capacity and/or by utilizing a varibl'eco'upling inductor 19 to the HF generator- .18.

As was previously described in the aforementioned application P224765L3, an electronic camera 20, for

example a conventional television recording camera which can be coupled with a television playback device in order to make a visual control possible of the television picture of the melting zone 2 is employed as a control sensor to monitor the melting zone 2. A suitable orientation of the electronic camera 20 in respect to the melting zone 2 and the constant maintenance of the recording conditions is provided for in such a way that each pulse supplied by the camera 20 per scanning cycle contains a definite information concerning a certain diameter of the picture recorded by the camera 20. This is the very case if the scanning lines on the target of the camera 20 are oriented vertical to the picture of the axis of the semiconductor rod 1 and if the optical axis of the recording lens which is directed onto the melting zone 2 maintains its length and inclination with respect to the rod axis. This meansthat the camera 20 'must be shifted in case of a non-stationary melting zone 2, whereby its orientation is maintained in respect of itself with the speed of the crystalization front. Since the pulse, corresponding to the crystalization front in the television picture, of the pulse sequence supplied per scanning cycle distinguishes itself as a border pulse having a specially high amplitude, it is possible, by

means of a filter arrangement installed in an evaluation calculator arrangement 3 connected to the camera 20, to filter out without difficulty this pulse p from the individual pulse sequences and to determine the value of d (x which is in proportion to its length. The proportionality factor which is required for this purpose is obtained by means of adjusting the arrangement. Also required for the determination of the marginal angle a (.r,;) is the diameter d (x A) which is assigned to the I scanning line adjacent to the crystaliz'ationborder in the electronic camera 20 which, viewed from the crystalization border is already assigned to the picture of the liquid material. The following equation results:

whereby is the distance corresponding to the distance of the two neighboring scanning linesin the scale of the actual melting zone. It is hereby pointed out that instead of the angle a (x also a trigonometric function of this angle, preferably tg a (x can be used as a control magnitude since a clear relation exists between the angle a (x and its trigonometric functions. In a suitable opto-electronic or mechano-optical supervision device 21, the location x of the crystalization front of the melting zone 2 is supervised and the values corresponding to the location coordinate x preferably distance between crystalization front of the melting value and calculates with this numerical value the respective control value of d (x and at (x that is the values d (x and a (x For example the equation d k) o R (R r )cosx 1r/p. can be employed for determining the value d (x which is also used as a basis for the corresponding value a (x under consideration of the volume change. In the above equation r,, is the radius of the core crystal, R is the radius of the thicker normal rod, and 'n' and p. are constants. The coordination supervision device 21 may also operate, however, in such a way that any time the crystalization front has covered a distances, for example s 0.5 mm, a signal will appear by means of which the values ofd (x and a (x can be determined, according to a program which can be called upon from the data processing arrangement 11, in this case being designed as a memory. It is thereby possible to provide the signals a different character in case of a forward movement of the crystalization front than in the case of a backward movement, so that in case of a backward movement values which were already called upon can be prepared for a renewed call from storage.

An electrical voltage in proportion to the respective value of d (x is emitted from the data processing arrangement ll by-way of the line 10b to a first input-of rectedto the second input of the stage 10 by way ofthe line 10a. The input stage 10 of this cascade automatic control system is provided by a control amplifier l at whose output a voltage appears which'is in proportion to the control deviation of d (x This voltage is then directed to the second control stage of this cascade automatic control system.'At first it is'integrated in an integ'rator 5,-an amplifier element which has the task of forming the time integral of the control deviation of the value d (.r;,-) in such a way that (I) it changes its sign with the control deviation, (2) the time integral re ceives a constant value as soon as the'deviation of d (17,-) disappears, and in addition (3) the time integral is adjusted under the prerequisite of (2) above'in such a way that it becomes equal to the actual value, which is I to be described, as soon main constant;

The signal supplied by the integration stage is directed to the input of acorrection or control amplifier 4 by way of the line 4!). A second input of the amplifier 4 is supplied with a magnitude as an actual value which was also obtained by integration, via the line 4a, which signal is binding for the geometric condition of the melting zone 2. It is advantageous to use the integral via the sum of sq uares of all diameter values determined by the electronic camera 20. This means in practice that all pulses, provided they refer to the picture of the actual melting zone 2 in the camera are transformed by multiplication with the above-captioned proportionality factor into the corresponding diameter values d (3) and that the calculator 3 is allowed to calculatethe sum 2 as the value d r is to rekin (5.)?

and to transform the same into an electrical voltage which is in proportion to the obtained value of S. ,The

summation during the formation of the value S takes scanning lines 2,, form part of the picture of the actual melting zone 2. The previously mentioned value for the correction amplifier 4 which is supplied by the integrator 5 is to be adjusted in such a way that it equals the value S as soon as the program for the value d (x requires a pulling zone with a constant rod diameter.

The output signal which is supplied by the amplifier 4 is directed to a third control stage formed by a recoupled amplifier 7 of the cascade forming the first control loop. For the purpose of recoupling, the shaft of the motor 8 of the stretch-compression mechanism, which is operated by the third control stage, is firmly coupled with a tachometer generator 8a and the voltage supplied therefrom is applied to a second input of the amplifier 7. Therefore, stable rates of revolution of the motor 8 and a definite operation of the stretchcompression mechanism are achieved.

As a control value, the previously mentioned controller-output a (x is applied to the input of the second cascade control loop or system of the arrangement according to the invention by way of the line 6!) and the respective actual value via the line 6a in the form of voltages which are in proportion to the actual values. The first stage of this second cascade control system is provided by a correction or control amplifier 6 which creates a signal in proportion to the deviation of the value a (x This signal is directed to the second stage of the cascade automatic control system. This control cascade is formed by a motor 13, a potentiometer l4 which is connectedto a non-illustrated direct current voltage source, and a correction or control amplifier 15. The P] control character of this stage is provided by the motor 13 which changes the tapping point of the potentiometer l4 and therefore the direct voltage which is supplied as a value via the potentiometer 14. The actual value is supplied by a resonant circuit containing the capacitor 12a and the induction heating coil 12. It should constitute a measure for the energy which is supplied to the resonant circuit, and therefore to the induction heating coil 12. I i

The alternating voltage at the capacitor 12a supplies. after rectification and possible smoothing by the rectifier arrangement 24, a voltage U which can be read at 21 volt meter 22 and which forms a measure for the energy accepted by the resonant circuit. This voltage is applied-to one input of an amplifier 15 by way of the line 15b. The voltage supplied by the potentiometer 14 is applied as a control value to the outer input of the amplifier 15' by way of the line 15b.

The motor 13 is activated as soon as a profile deviation ofa (x occurs. If it is zero, a balance should be given at the amplifier 15 between the value supplied by the potentiometer l4 and the voltage value U,. as the actual value supplied by the arrangement 24, which can be easily achieved by corresponding adjustment of the potentiometer setting. If the energy supply from the generator 18 for the coupling between the melting zone 2 and the induction heating coil 12 should change, the voltage U and also the actual value a (x will likewise change. In such a case, the control stages 6 and 15 will be activated until a balance is again established at the amplifier l5 and the deviation of a (x has disappeared. t

The output of-the second control stage 15 of the second cascade control system is connected to a recoupled third control stage comprising the amplifier 16, whose output operates a motor l7 to incluence the adjustment -9 of the generator 18. The recoupling or feedback is provided by a resistor 23 connected between the output of the amplifier 16 and a second input of the amplifier. Contrary to the feedback conditions of the first cascade control system, this feedback system does not deal with a tachometer generator recoupling, but with an armature voltage recoupling which is sufficient in this case.

In summary, the arrangement illustrated in the figure should be described briefly one more time in its effectiveness.

In the electronic television camera 20, by means of which a picture or image of the melting zone 2 is recorded, an electronic scanning of this image takes place as described, for example, in the German application P21137202. Each scanning cycle a sequence of discrete electrical pulses (square pulses) is obtained which are assigned to the individual picture scanning lines. These pulses are passed onto an electronic evaluation device 3 wherein the calculation operations which are required for the sum for determination of the diameter d (x the actual value of the crystalization front of the melting zone, the marginal angle (.r,,) and the sum S for the charging of the correction 4, are carried out with the aforementioned pulses as output magnitudes. Thereby, the length of the pulses? (duration) is the decisive magnitude', the amplitudes (pulse levels or height) only serve for selecting the various pulses, in particular the pulse relating to the-crystalization front and therefore the pulse P corresponding to the diameter d (x and the above mentioned neighboring pulse corresponding to the diameter d (.r,,- )t).

In addition, and if necessary, the actual values-' which are determined in the evaluation device 3 are also fed into the data processing arrangement for control of the program, which is indicated by the connecting line 311. i

From the data processing arrangement 11, the control values d (x and a (.r,,) are made available, which values are necessary for the input stages of the two cascade control systems, whereby these control values are called upon according to the location x of the crystalization front of the melting zone 2. The control-deviation is generated in the first cascade control system and servesfor controlling the distance of the two mountings 1a and lbof the semiconductor rod 1 which is tobe melted, by means of time integration to a control magnitude which only causes the motor 8'energization to disappear if the diameter of the rod growing from the melting zone 2 is to remain constant and a deviationdoes not exist. The respective actual value is the sum v is ear which is in proportion to the volume of the melting zone 2 (or a part of this volume which can be observed) and which should border the crystalization front.

The influencing of the energy supply to the induction heating coil 12 takes place in a similar manner byway of a second cascade automatic control system of the arrangement according to the invention. The deviation of the marginal angle a(.r,,) which is applied at its input amplifier controls the second control system by way of an actuator or motor 13 which is similar to the integrator in the integration effect, whereby as a control magnitude for the second stage of this cascade the energy supply of the induction heating coil 12 is employed. Whereas the electronic television camera 20 is used as a control sensor for the determination of the sum S, a different control sensor, namely a rectifying and filter circuit 24 is employed. Also, in this automatic control system, a recoupled output stage 16 which influences the control path is employed to control the motor 17 to, in turn, control the frequency of the generator 18. Also, the rate of revolution of the motor 17 becomes, due to the feedback provided by the resistor 23, approximately proportional to the deviation of the energy.

The program for the control value d (x),) and a (x,,) is in any case selected insuch a way that it is'in accordance with the stability of the melting zone 2.

Although I have described my invention by reference to a particular representative system, many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention. I therefore intend to include within the patent warranted hereon all such changes and modifications as may' reasonably and properly be included within the scope of my contribution to the art.

I claim:

l. A method for the floating zone melting of a semiconductor rod, in particular a rod consisting of silicon, comprising the steps of supplying energy to a zone of the rod for melting the zone; monitoring the molten zone with a television camera under constantly maintained recording conditions; generating electrical pulses from the recorded image representing volume of the molten zone, projection angles of the zone, the solidifying front and the turning point over the course of the melt-liquid partof the molten zone and the diameter of the semiconductor rod at the solidifying front; limiting the melting zone in responseto the information obtained from the generated pulses to obtain a desired diameter; and controlling the energy supplied in response to information from the electrical pulses concerning the angle deviations of the projection angles.

2. The method according to claim v1, comprising the steps of determining the volume of the part of the melting zone determining growth of the semiconductor rod by linearly scanning the part with a diameter of the melting zone established per line; squaring and adding the information representative of the diameter and determining from the pulses obtained by linear scanning the diameter at the solidifying front and the angle at the projecting points of the molten zone, the solidifying front, melting front and the turning point.

3. The method according to claim 2 wherein a prescribed geometric fomi of the conical rod part is determined by the respective diameter and the respective angle as a function of the distance covered by the solidifying front.

-4. The method according to claim 3 comprising the step of determining the diameter and the respective projecting angle as a cosine function dependent on the distance covered by the solidifying front.

5. The method according to claim 3, comprising the step of controlling the supplied energy in accordance with the respective desired projection angle as a function of the distance covered by the solidifying front as a function of the diameter.

6. Apparatus for floating-zone melting of a semiconductor rod, particularly a silicon semiconductor rod, comprising: I

a pair of rod mounts holding the rod vertically, said rod mounts being axially shiftable relative one another along the direction of the rod axis;

a rod mount actuator connected to one of said rod mounts for axially moving said one mount;

an induction heating coil surrounding the rod;

variable energizing supply means connected to said coil for energizing said coil to heat the rod and cre ate a melting zone;

a supply actuator connected to said supply means for controlling the energy supplied to the rod;

an electronic television camera operating to linearly scan the melting zone perpendicular to the rod axis and produce actual value information signals corresponding to rod diameter and profile angle of the melting zone and the crystalization front thereof;

a data processor receiving the information signals;

? first and second multi-stage control systems connected to said data processor and'to said rod mount actuator and said supply actuator, respectively;

monitoring means adjacent the rod for monitoring the position of the crystalization front and requesting first and second magnitudes from said data processor corresponding to the diameter at the crystalization front and to,the angle of the melting zone profile at the crystalization front, respectively, said first magnitude applied to said first control system and said second magnitude applied to said second control system;

a calculator connected between said television camera and said control systems for providing respective signals which are proportional to the signals representing actual value information;

first comparison stages in the respective control systems for comparing the respective values received from said calculator with those received from said data processor to provide respective control signals;

means for sensing actual values of rod diameter, profile angle, and heating coil energization;

means for integrating the control signals;

second stages in the respective control systems for comparing the sensed actual value with the integrated control signal; and

third stages in the respective control systems, each having a feedback circuit, coupled directly to said rod mount actuator and said supply actuator, respectively.

7. Apparatus according to claim 6, wherein said data processor is a calculator for calculating respective control signals representing rod diameter and profile angle at a monitored position, and said data processor is connected to said first stages of said control system.

8. Apparatus according to claim 6, wherein said data processor includes a memoryfor storing information corresponding to rod diameter and profile angle and is responsive to said monitoring means to provide signals associated with the monitored position. to said first stages of said control systems.

9. Apparatus according to claim 6, wherein said first control system comprises said first stage, an integrating stage as a part of said integrating means connected to said first stage as a second stage and an amplifier having inputs connected to said calculator and to said integrator and an output connected to said third stage.

10. Apparatus according to claim 9 wherein said calculator comprises means for supplying a sum of the squares of the pulses per scanning cycle generated by the television camera and a diameter value assigned to the image of the actual melting zone, and operates to evaluate'the pulses as to rod diameter.

11. Apparatus according to claim 10, wherein said third stage includes an amplifier connected to said rod mount actuator, 21 tachometer included in the feedback circuit of said-third stage and connected to and operated by said rod mount actuator to create a feedback voltage, and means connecting said feedback voltage to said amplifier of said third stage.

12. Apparatus according to claim 6 wherein said first stage of said second control system is operable to provide a signal which is proportional to the deviation of the profile angle, and comprising a potentiometer connected to an auxiliary voltage source, a potentiometer actuator connected to said first stage and operated thereby to provide a control value, a correction amplifier in said second stage of said second control system connected to receive said control value and connected to said heating coil to receive a signal value representative of heating coil energization.

13. Apparatus according to claim 12, wherein the connection between said correction amplifier and said heating coil includes rectifier means.

14. Apparatus according to claim 12, wherein said correction amplifier of said second control system is connected to said third control stage, said third control stage including an amplifier, and said feedback circuit of said third control stage comprising a resistor con nected from the output of said amplifier to and input thereof, said output of said third stage amplifier connected to and controlling the operation of said supply actuator for varying the energy supplied to said melting zone.

15. Apparatus according to claim 14, wherein said variable energizing supply means includes a high frequency generator variable in frequency for supplying high frequency energy to said induction heating coil, a resonant circuit including said heating coil loosely coupled to said high frequency generator, said high frequency generator being steadily variable over a frequency range which includes the resonant frequency of said resonant circuit.

Claims (15)

1. A method for the floating zone melting of a semiconductor rod, in particular a rod consisting of silicon, comprising the steps of supplying energy to a zone of the rod for melting the zone; monitoring the molten zone with a television camera under constantly maintained recording conditions; generating electrical pulses from the recorded image representing volume of the molten zone, projection angles of the zone, the solidifying front and the turning point over the course of the melt-liquid part of the molten zone and the diameter of the semiconductor rod at the solidifying front; limiting the melting zone in response to the information obtained from the generated pulses to obtain a desired diameter; and controlling the energy supplied in response to information from the electrical pulses concerning the angle deviations of the projection angles.

2. The method according to claim 1, comprising the steps of determining the volume of the part of the melting zone determining growth of the semiconductor rod by linearly scanning the part with a diameter of the melting zone established per line; squaring and adding the information representative of the diameter and determining from the pulses obtained by linear scanning the diameter at the solidifying front and the angle at the projecting points of the molten zone, the solidifying front, melting front and the turning point.

3. The method according to claim 2 wherein a prescribed geometric form of the conical rod part is determined by the respective diameter and the respective angle as a function of the distance covered by the solidifying front.

4. The method according to claim 3 comprising the step of determining the diameter and the respective projecting angle as a cosine function dependent on the distance covered by the solidifying front.

5. The method according to claim 3, comprising the step of controlling the supplied energy in accordance with the respective desired projection angle as a function of the distance covered by the solidifying front as a function of the diameter.

6. Apparatus for floating-zone melting of a semi-conDuctor rod, particularly a silicon semiconductor rod, comprising: a pair of rod mounts holding the rod vertically, said rod mounts being axially shiftable relative one another along the direction of the rod axis; a rod mount actuator connected to one of said rod mounts for axially moving said one mount; an induction heating coil surrounding the rod; variable energizing supply means connected to said coil for energizing said coil to heat the rod and create a melting zone; a supply actuator connected to said supply means for controlling the energy supplied to the rod; an electronic television camera operating to linearly scan the melting zone perpendicular to the rod axis and produce actual value information signals corresponding to rod diameter and profile angle of the melting zone and the crystalization front thereof; a data processor receiving the information signals; first and second multi-stage control systems connected to said data processor and to said rod mount actuator and said supply actuator, respectively; monitoring means adjacent the rod for monitoring the position of the crystalization front and requesting first and second magnitudes from said data processor corresponding to the diameter at the crystalization front and to the angle of the melting zone profile at the crystalization front, respectively, said first magnitude applied to said first control system and said second magnitude applied to said second control system; a calculator connected between said television camera and said control systems for providing respective signals which are proportional to the signals representing actual value information; first comparison stages in the respective control systems for comparing the respective values received from said calculator with those received from said data processor to provide respective control signals; means for sensing actual values of rod diameter, profile angle, and heating coil energization; means for integrating the control signals; second stages in the respective control systems for comparing the sensed actual value with the integrated control signal; and third stages in the respective control systems, each having a feedback circuit, coupled directly to said rod mount actuator and said supply actuator, respectively.

7. Apparatus according to claim 6, wherein said data processor is a calculator for calculating respective control signals representing rod diameter and profile angle at a monitored position, and said data processor is connected to said first stages of said control system.

8. Apparatus according to claim 6, wherein said data processor includes a memory for storing information corresponding to rod diameter and profile angle and is responsive to said monitoring means to provide signals associated with the monitored position to said first stages of said control systems.

9. Apparatus according to claim 6, wherein said first control system comprises said first stage, an integrating stage as a part of said integrating means connected to said first stage as a second stage and an amplifier having inputs connected to said calculator and to said integrator and an output connected to said third stage.

10. Apparatus according to claim 9 wherein said calculator comprises means for supplying a sum of the squares of the pulses per scanning cycle generated by the television camera and a diameter value assigned to the image of the actual melting zone, and operates to evaluate the pulses as to rod diameter.

11. Apparatus according to claim 10, wherein said third stage includes an amplifier connected to said rod mount actuator, a tachometer included in the feedback circuit of said third stage and connected to and operated by said rod mount actuator to create a feedback voltage, and means connecting said feedback voltage to said amplifier of said third stage.

12. Apparatus according to claim 6 wherein said first stage of said second control system is operable to proVide a signal which is proportional to the deviation of the profile angle, and comprising a potentiometer connected to an auxiliary voltage source, a potentiometer actuator connected to said first stage and operated thereby to provide a control value, a correction amplifier in said second stage of said second control system connected to receive said control value and connected to said heating coil to receive a signal value representative of heating coil energization.

13. Apparatus according to claim 12, wherein the connection between said correction amplifier and said heating coil includes rectifier means.

14. Apparatus according to claim 12, wherein said correction amplifier of said second control system is connected to said third control stage, said third control stage including an amplifier, and said feedback circuit of said third control stage comprising a resistor connected from the output of said amplifier to and input thereof, said output of said third stage amplifier connected to and controlling the operation of said supply actuator for varying the energy supplied to said melting zone.

15. Apparatus according to claim 14, wherein said variable energizing supply means includes a high frequency generator variable in frequency for supplying high frequency energy to said induction heating coil, a resonant circuit including said heating coil loosely coupled to said high frequency generator, said high frequency generator being steadily variable over a frequency range which includes the resonant frequency of said resonant circuit.

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