Control of rod diameter responsive to a plurality of corrected parameters
United States Patent 1191 Keller Apr. 29, 1975 [75] Inventor: Wolfgang Keller, Munich, Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin and Munich, Germany [22] Filed: Mar. 19, 1973 [21] Appl. No.: 342,457  
 3,321,299 5/1967 Binder 23/273 SP 3,617,392 l/l971 Locke 23/273 SP 3,644,151 2/1972 Keller 23/301 SP Primary ExaminerNorman Yudkoff Assistant E.\&#39;aminerFrank Sever Attorney, Agent, or Firm-Hill, Gross, Simpson, Van Santen, Steadman, Chiara 8L Simpson [57] ABSTRACT A method for controlling the course of a floating melting zone process, in which a melting zone is electromagnetically induced in a semiconductor rod includes the steps of monitoring the r.f. frequency of the signal applied to induce heating in the rod, monitoring the voltage drop across the induction heating coil, and monitoring the current drawn by the r.f. generator, and controlling, in response to these three variables, the frequency of the signal and the velocity at which the silicon rod is pulled, in order to bring all three of the independent variables into agreement with desired values, to produce a rod of a constant desired diameter.  
 13 Claims, 2 Drawing Figures CONTROL OF ROD DIAMETER RESPONSIVE TO A PLURALITY OF CORRECTED PARAMETERS BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates to a method for the control of a floating zone melting process. and more particularly to such a method in which a constant diameter semiconductor rod is produced.  
 2. The Prior Art Processes for the control of a floating zone melting process have been described in the literature, and are well known. They are described, for example, in German Pat. No. 1,277,813 and German Pat. No. 1,153,908. In principle, the method employs an induc tion heating coil surrounding a semiconductor rod, and a drive for moving the rod through the coil in its longitudinal direction. The zone of the rod within the induction coil is melted by induction heating and this Zone is caused to traverse the length of the rod as the rod is moved relative to the coil. As a result, the semiconductor rod is transformed from polycrystaline form into a single crystaline form, the semiconductor material is purified, and the diameter of the rod may be modified if desired.  
  The diameter of the rod is influenced partly by the size and shape of the volume of liquid material in the melting zone, by the velocity at which the rod is moved in its longitudinal direction, and also by the rate at which energy is applied to the induction coil. Normally, only one or two variables have been sensed, and one or two parameters adjusted in performing the process, and optimum results have not been achieved.  
 SUMMARY OF THE INVENTION It is a principal object of the present invention to provide a means for controlling a floating zone melting process which makes use of three different variables for determining the control signals.  
  Another object of the present invention is to provide a method for drawing semiconductor rods using a floating melting zone process, in which the diameter of the rods produced by such process have predetermined diameters.  
  These and other objects and advantages of the present invention will become manifest by an examination of the following description and the accompanying drawings:  
  In one embodiment of the present invention there is provided a method for controlling a floating melting zone process including the steps of monitoring the current drawn by an r.f. generator, monitoring the voltage drop across the induction heating coil, monitoring the frequency of the r.f. generator, and selectively and independently controlling the frequency of the r.f. generator and the velocity at which the semiconductor rod is pulled in response to pairs of the monitored variables.  
 BRIEF DESCRIPTION OF THE DRAWINGS Reference will now be made to the accompanying drawings in which:  
  FIG. 1 is a schematic circuit diagram, partly in functional block diagram form, of control apparatus incorporating an illustrative embodiment of the present invention; and  
  FIG. 2 is a schematic diagram of a switch network employed in the apparatus of FIG. 1.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, the semiconductor rod I is illustrated as passing through an induction coil 2. and the zone of the rod 1 within the vicinity of the coil 2 is somewhat distorted as it is the melting zone. A capacitor 3 is connected in parallel with the coil 2, and the r.f. generator 4 is connected to the coil 2 in series with a coil 5. One end of the coil 2 and the corresponding terminals of the capacitor 3 and the generator 4 are also connected to ground. The circuit including the coils 2 and 5 and the capacitor 3 is tuned to the frequency of the generator 4.  
  A meter 6 is connected to the ungrounded terminal of the coil 2, and serves to indicate the r.m.s. voltage across the coil 2. Similarly, a meter 7 is connected to the ungrounded terminal of the generator 4 and serves to indicate the frequency of the generator 4. A meter 8 is connected with the r.f. generator 4, and serves to indicate the current drawn by the generator.  
  The meter 6 is a volt meter, the meter 7 is a frequency meter, and the meter 8 is an ammeter. Their details are not illustrated, since all of them are well known to those skilled in the art. It will also be understood by those skilled in the art that all of the connections to the meters are not illustrated, in the interest of simplifying the drawings.  
  Control units 9, 10 and 11 are connected in circuit with the meters 6, 7 and 8, respectively, and operate to derive three separate output signals in response, respectively, to the voltage indicated by the meter 6, the frequency indicated by the meter 7, and the current indicated by the meter 8. Each of the control devices 9, l0 and 11 is provided with an additional input signal which are respectively V f0, and I and these additional inputs represent the desired values for the voltage, frequency, and current, respectively. The three control devices 9, l0 and 11 each generate a dc. signal at the output thereof which is responsive to the difference, or error between the desired value of its respective parameter and the instantaneous value thereof, as indicated by its respective meter.  
  The details of the control units 9, l0 and 11 are not illustrated, because they also are well known to those skilled in the art. In one arrangement, each of the control devices may take the form of a known apparatus for developing a dc. voltage having a level proportional to the instantaneous value of current, voltage or frequency, and the input signals representative of the desired values are do. signals produced by potentiometers connected across a dc. source. The two do. voltages for each parameter may be connected to the inverting and noninverting inputs of a differential amplifier, the output of which is responsive to the difference in level between the two inputs. Other known arrangements may be used alternatively.  
  The outputs of the three control devices, 9, l0 and 11 are connected to three inputs of a switch unit 12, which functions to select two of the inputs and connect them to two output terminals. The two output terminals are connected by way of lines 14 and 15, respectively, to the r.f. generator 4, and to the drive unit 13 which controls the movement of the semiconductor rod 1. The effect of a change in the voltage on the line 14 is to change the frequency of the r.f. generator 4 by varying the effective capacitance internally of the oscillator circuit. This can be accomplished readily by use of a varactor diode or the like, in a manner well known to those skilled in the art. The drive 13 is controlled in response to the voltage on the line 15 by means of conventional servo techniques.  
  The switch 12 selects two of the three input signals and makes an appropriate adjustment in the frequency of the oscillator 4 and the speed of the drive, as controlled by the servo unit 13, until these two parameters are brought into agreement with their desired values. Then another pair of the three input signals is selected. and the generator frequency and drive velocity are again varied, as necessary, in order to bring their measured values into line with the desired values therefor. This operation continues repetitively, with a different pair of the three input signals being selected successively, and employed to control continuously the frequency of the oscillator 4 and the velocity of the drive.  
  The details of the switch unit 12 are illustrated in FIG. 2. The unit 12 incorporates two separate switches, one of which is a triple pole-double throw switch 16, and the other is a double pole-triple throw switch 17. The two stator terminals of each of the poles of the switch 16 are connected to a separate pair of the lines connected to the outputs of the control units 9, l and 11 over the lines 18, 19 and 20, to the terminals 21, 22 and 23 respectively. The three common terminals of the switch 16 are connected to the three stator terminals of a first pole of the switch 17. The first pole selects one of the terminals 21-23 for connection to an output terminal 24, and the second pole of the switch 17 selects one of the poles of the switch 16 for connection to a terminal 25. The terminal 24 is connected by the line 14 to control the frequency of the r.f. generator 4, and the terminal 25 is connected by the line to control the velocity of the drive, under the control of the servo unit 13.  
  In operation, the switch 17 is positioned to one of its three positions, and then the switch 16 is switched repetitively back and forth between its two positions in order to select first one of the difference or error signals applied to the terminals 21-23 and then another. The third signal (viz., the one not selected by the switch 16) is the one which is connected directly to the output terminal 24 by means of the switch 17. Any one of the three input signals is selectable for direct connection to the generator 4 via the terminal 24, while the other two are successively and repeatedly chosen for connection to the drive unit 13. The switch 16 is periodically changed in position whenever the difference signals selected by the switch 16 vanish, indicating that the parameter represented by the difference signal has been adjusted to its desired value. Thus, each of two parameters is adjusted by controlling the velocity of the drive via the drive unit 13, and any effect such adjustment has on the third parameter, represented by the signal connected directly to the terminal 24, causes a corresponding adjustment in the frequency of the generator 4. In this manner all three parameters are readily brought into agreement with their desired values, and maintained at such desired values throughout the process. As the diameter of the treated semiconductor rod 1 is a function of the three parameters which are monitored in the present invention, the desired diameter may be produced merely by applying signals representative of the frequency, voltage and current parameters which produce a rod having the desired diameter. The correct values for these three parameters are readily determined by experiment, for any semiconductor material which is being used.  
  A double pole-double throw switch 26 is provided for interchanging the signals applied to the terminals 24 and 25 when desired.  
  It will be apparent that the apparatus of the present invention may be employed in order to practice other processes besides the floating zone melting process which has been described herein. The process of the present invention is applicable wherever it is desired to control a complex process by means of two separate control units in reponse to three independent parameters.  
 What is claimed is:  
  1. A method for the control of a floating zone melting process in which a zone in a semiconductor rod is heated inductively by means of r.f. energy supplied to a coil surrounding the rod, and including drive means for moving the heated Zone longitudinally through the rod, comprising the steps of sensing the frequency of said r.f. generator, sensing the current drawn by said r.f. generator, sensing the voltage drop across said coil, comparing said sensed frequency, current and voltage with a reference frequency, current and voltage respectively to yield resulting parameters representing the respective errors between said frequencies, currents and voltages, and controlling with said resulting parameters the frequency of said generator and the relative velocity between upper and lower rod holder of said drive means for producing a rod having a predetermined diameter.  
  2. The method according to claim 1, including the step of periodically operating switch means to utilize a first pair of said resulting parameters, controlling said frequency and relative velocity in response to said first pair of parameters, thereafter choosing a second pair of parameters, and controlling said frequency and velocity in response to said second pair of parameters.  
  3. The method according to claim 2, including the step of repeatedly operating said switch means to utilize said first pair of parameters and then said second pair of parameters for the control of said frequency and relative velocity.  
  4. The method according to claim 3, including the step of periodically operating said switch means to utilize said second pair of parameters after said first pair of parameters have been brought into coincidence with predetermined values therefor.  
  5. The method according to claim 4, including the step of periodically operating said switch means to utilize said first pair of parameters after said second pair of parameters has been brought into coincidence with predetermined values therefor.  
  6. The method according to claim 1, including the steps of controlling the frequency of said generator in response to one of said three parameters, and controlling the relative velocity of said drive means alternately in response to the other two of said three parameters.  
  7. The method according to claim 1, including the steps of controlling the relative velocity of said drive means in response to one of said three parameters, and controlling the frequency of said generator alternately in response to the other two of said three parameters.  
  8. The method according to claim 1. including the step of periodically operating switch means to utilize first and second ones of said three parameters. controlling the frequency of said generator in response to said first parameter and simultaneously controlling the velocity of said drive means in response to said second parameter, and cyclically utilizing different pairs of said three parameters, whereby either said frequency or said relative velocity or both is controlled in response to a different parameter during each cycle than in the next previous cycle.  
  9. The method according to claim 8, including the step of initiating a new cycle by utilizing a different pair of said parameters when the parameters chosen during the next previous cycle have been brought into coincidence with described values therefor.  
  10. A method for controlling a complex process having first and second control means for controlling two parameters in said process and first, second and third sensing means for sensing three parameters which are interrelated with each other. including the steps of comparing said three sensed parameters with three reference parameters representing the respective errors between said respective sensed and reference parameters, to yield resulting parameters, cyclically utilizing different pairs of said resulting parameters. and operating said first and second control means in response to the pair of resulting parameters utilized at any given time. and operating said first and second control means in response to the pair of parameters selected at any given time.  
  11. The method according to claim 10, wherein at least one of said controlled parameters is one of said sensed parameters.  
  12. The method according to claim 10, wherein at least one of said controlled parameters is not a sensed parameter.  
  13. Apparatus for controlling a complex process comprising, in combination; first and second control means for controlling two parameters in said process. first second and third sensing means for producing signals responsive to three different parameters in said process. means for producing three signals representative of desired values for said three sensed parameters, means for producing three signals each representative of the difference between one of said three sensed parameters and its desired value, a double pole-triple throw switch means having the three stator terminals of one pole connected with said three difference signals to select one of said difference signals for connection to said first control means, said second control means being connected through the second pole of said double pole-triple throw switch to one of three lines, and a triple pole-double throw switch having its common terminals connected with said three lines and having its stator terminals connected with said three difference signals to select either two of said three difference signals other than said one difference signal for connection to said second control means. l=