GB1561112A

GB1561112A - Zone melting

Info

Publication number: GB1561112A
Application number: GB2748276A
Authority: GB
Original assignee: Wacker Siltronic AG
Current assignee: Siltronic AG
Priority date: 1975-07-01
Filing date: 1976-07-01
Publication date: 1980-02-13
Also published as: DE2529329C3; DK147438B; NL7606167A; DK294076A; FR2315993A1; DE2529329B2; BE843542A; JPS526309A; JPS5617313B2; FR2315993B1; DK147438C; IT1066271B; DE2529329A1

Description

(54) ZONE MELTING (71) We, WACKER-CHEMITRONIC GESELLSCHAFT FUR ELEKTRONIK GRUNDSTOFFE MBH., a Body Corporate organised according to the laws of the Federal Republic of Germany, of 8263 Burghausen, Federal Republic of Germany, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a method of monitoring the zone melting of a rod of semi-conductor material and controlling the zone melting process in accordance with monitored data.

Rods of semiconductor material which are polycrystalline, that is, contain large numbers of small crystals, may be converted into rods substantially composed of a single crystal by melting and recrystallising them.

This may be brought about without the use of a crucible by the so-called zone-melting process, in which a vertically mounted rod, held at both ends, is gradually and progressively heated along its length by a heater which moves in relation to the rod, in a direction parallel to the rod axis, and is generally displaceably mounted so as to move along a stationary rod.

As the heater moves along the rod, the molten zone shifts with it and hence the previously heated region of the rod gradually cools and re-crystallises, thereby progressively increasing the size of the monocrystalline portion of the rod.

It has been proposed to film the form of the molten zone with a television camera with a multiple photo-cell system, for example, with the system known as "Videcon".

The form of the zone is obtained by subjecting it to linear scanning with an electron beam, and the image obtained via the television camera supplies pulses proportional to the brightness of the image, from which data on the dimensions characterising the form of the molten zone can be deduced, according to which the zone-melting process can be regulated.

The problem underlying the present invention was to provide a process for monitoring the zone melting of a rod of semiconductor material, and controlling the zonemelting process in accordance with the data resulting from the monitoring, which is cheaper, simpler and less susceptible to disturbance than previously proposed processes, preferably by providing data in the form of analogue or digital data which can be fed into a process regulator so that fully automatic zone pulling can be provided, if desired.

The present invention provides a process for the zone melting of a vertically extending rod of semi-conductor material, in particular of silicon, which comprises applying radially symmetric heating to the rod progressively along its length so that successive regions of the rod are heated in turn, monitoring the heated molten zone with one or more arrays of electromagnetic radiationsensitive elements, the or each array being arranged to receive radiation emitted from the molten zone, and controlling the molten zone in accordance with the data registered by the radiation-sensitive array elements.

The process according to the invention may be used to monitor at least one linear dimension of the molten zone, and the energy supply and/or pulling speed may be controlled in accordance with this information.

The radiation-sensitive elements are preferably photosensitive elements and, especially, diodes, for example in the form of a multi-diode array. The spatial dimensions of the molten zone may then be determined by the number of illuminated diodes between two opposite bright/dark transitions of the free molten surface and/or between two opposite solid/liquid phase transitions between the molten zone and the solid semiconductor rod.

The heat may be applied by means of heating means surrounding the rod coaxially and arranged to move along the rod.

The invention also provides zone-melting apparatus for a vertically extending rod of semi-conductor material, which comprises heating means arranged to provide radially symmetric heating to a portion of a rod along its length, means for causing relative movement between the heating means and the rod in a direction along or parallel to the axis of the rod, monitoring means comprising one or more arrays of electro-magnetic radiation-sensitive elements, the or each array being arranged to receive radiation emitted grom the molten zone of the rod in the region of the heating means, and control means for controlling the molten zone in accordance with the data registered by the radiation-sensitive array elements.

It is possible using the process according to the invention to measure important dimensions of the molten zone, in particular its height and width, and to ascertain accurately the position of the solidification front, this being the interface between liquid and recrystallised semiconductor material. The data may be used in the form of analogue or digital electrical signals and may be used either directly or indirectly in the control of the zone-melting progress and consequent frowth of the monocrystalline semiconductor rod.

It may be possible, using the data obtained, to regulate the energy supply or pulling speed directly and, if desired, continuously in such a way that deviations from the desired diameter of the monocrystalline rod as well as changes in the zone height can constantly be compensated for. For the indirect use of the data obtained, a computer can be interposed for comparing the data obtained with nominal values of a programme which has been previously fed in, the computer, if desired, being programed to control the whole growing process by means of a regulating system.In this manner, not only deviations from a strictly predetermined diameter and/or a strictly predetermined zone height can be corrected and virtually eliminated or minimised, but also continuous, desired alternations in meltingzone height or diameter while the monocrystalline rod portion is growing, such as the conical-shaped transition between the seed crystal and the substantially constant diameter semi-conductor rod, can be controlled fully automatically.

The molten zone which is to be determined is produced by melting the polycrystalline rod, generally by means of high frequency coils and, as a rule, is contained by the surface tension of the liquid semiconductor material and by the solid rod portions bounding the molten zone above and below. Whereas the monocrystalline rod portion formed from the molten zone as a result of re-solidification generally forms a sharply defined transition to the molten zone, an irregular layered melting of the polycrystalline rod portion is in most cases observed, resulting in an ill-defined solid/ liquid interface between the said polycrystalline rod portion and the molten zone.

While the measurement of the diameter of the molten zone can be obtained by simple associated with the number of illuminated diodes or other radiation-sensitive elements, since there is a sharp light/dark interface between the sides of the radiationemitting molten zone and the comparatively dark background, the exact determination of the zone height is far more difficult owing to the multiplicity of solid/liquid phase transitions in the region of the end portion of the melting zone proximate the polycrystalline rod portion. Using the process according to the invention, however, it is possible to determine accurately the first solid-liquid phase transition between the polycrystalline rod and the molten zone and thereby to obtain accurate reproduceable values for the zone height.The criterion for the clear determination of a solid/liquid phase transition is the sharp reduction in the photo-current occurring from one photodiode of the array, onto which an image of the molten zone is projected, to an adjacent diode, which reduction is brought about by the different spectral composition of the light which is radiated across the transition from white-red glowing silicon, which is still just solid, to liquid silicon.

Moreover, the invention also allows for the simultaneous determination of several dimensional values of the molten zone by using several multi-diode arrays or arrays of other radiation-sensitive elements, preferably also employing semi-translucent mirrors. Thus, in a particularly advantageous process according to the invention, the vertical and horizontal dimensions of the molten zone are determined simultaneously by two multi-diode arrays which are arranged parallel to the dimensional values which are to be measured in the direction of passage and reflection, respectively, of a semi-translucent mirror set up at an angle of 45" in front of the molten zone of the semi-conductor rod.

The multi-diode arrays can be arranged in combination with a lens system, in the form of commercially obtainable photodiode cameras.

The accuracy of measurement of a linear dimension of the molten zone depends on the chosen scale of reproduction and the number of diodes composing the multidiode array used in determining that particular dimension. For example, if the scale of reproduction is 1:1, and multi-diode arrays are used which contain, per 100 mm, 512, 1024, or 1872 diodes, then, the dimensional values of the melting zone can be determined correspondingly with accuracies to within 0.2 mm, 0.1 mm, and 0.05 mm respectively.

The monitoring arrangement can be made to follow, preferably in a self-adjusting manner, the molten zone passing through the rod, by way of comparing a particular monitoring signal at a solid/liquid phase transition between the molten zone and the rod with a predetermined fixed nominal value and, thereby controlling the position of the monitoring arrangement with regard to the molten zone by means of a position control circuit, which takes account of the difference between the value of the monitoring signal and the predetermined nominal value.

The diameter of the molten zone is normally measured at a position between the heating induction coil and the region up to and including the solidification front between the heating induction coil and the region up to and including the solidification front between the molten zone and the monocrystalline rod portion, and preferably in the region of 1 to 2 mm above this solidification front. With a self-adjusting monitoring system it is possible to ensure that, during the entire zone-melting process, the diameter of exactly the same place in the molten zone is monitored. When measuring the zone height, it is more or less irrelevant at what place in the molten zone the measurement is monitored, but, once chosen, the particular position in the zone at which the measurement is taken must of course be kept the same for the whole zone-melting process to enable valid comparison.

The data on the form of the molten zone, obtained by way of the multi-diode or other radiation-sensitive arrays, can be fed as standard dimensions to a molten-zone regulating system with which there is associated, as a nominal value transmitter, a computer programmed according to the respective, desired crystal diameter.

Compared to previously proposed and known measuring processes where the molten zone is determined, for example, with a "Videcon" system, the process according to the invention permits zone-melting processes to be carried out with fundamental improvements which reside, in particular, in greater local dissociation, practically unlimited life of the multi-diode arrays used in the process, considerably smaller cost of electronic circuitry, easy positioning of the entire monitoring arrangement without unduly tedious readjustment, and simple maintenance with relatively low costs.

A process according to the invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1 shows the measuring arrangement used in the process; Figure 2 shows schematically the principle of the diameter determination; Figure 3 shows schematically the principle of the measurement of the zone height; and Figure 4 shows a control scheme for the exact positioning of the measuring system with regard to the molten zone.

With reference to the drawings, and initially to Figure 1, two diode cameras, one for monitoring the height x of the molten zone 6 of a rod 9 and the other for monitoring the diameter y of the melting zone, comprise multi-diode arrays 1 and 2, respectively. The two diode cameras are each mounted on a sliding carriage 3, which can be adjusted in height, the multi-diode array 2 being arranged horizontally and facing the molten zone 6 which is to be monitored, and the multi-diode array 1 being arranged to face in a direction parallel to the axis of the molten zone.A lens 4 is arranged to receive radiation emitted from the region of the molten zone 6, and to project it onto a semi-translucent mirror 5 lying at 45" to the axis of the molten zone, from where it is reflected and transmitted to the multi-diode arrays 1 and 2, respectively, the two multi-diode arrays thereby receiving simultaneously an image of the molten zone.

The rod 9 is of polycrystalline structure and the molten zone is formed by the heating action of an induction coil 10 arranged coaxially around the rod. The induction coil 10 is movable along the length of the rod 9, and the molten zone 6 can hence progress along the rod, the polycrystalline structure of the rod thereby being changed to a monocrystalline structure.

The diameter y of the molten zone 6 is measured at an accurately adjustable distance Ax above the solidification front 7 of the rod portion 8 growing in monocrystalline form, by means of the horizontally arranged multi-diode array 2. The vertically arranged multi-diode array 1 serves, firstly, to measure the distance between the semiconductor rod portions 8 and 9 bounding the molten zone 6, that is, the zone height x, and, secondly, to position the entire monitoring system with regard to the solidification front 7 in such a manner that the position of the solidification front 7 is always projected at the same place with respect to the array 1, so that the monitoring system automatically follows the molten zone as it passes along the rod of semi-conductor.The value for Ax registered by the multi-diode array 1 is transmitted as a control value RAx to a nominal value regulator 11, and there compared with a predetermined nominal value forAx. In accordance with the difference between these two values, the monitoring system is kept at a constant alignment with respect to the solidification front 7 of the molten zone 6, by the action of the motor 13, which raises and lowers the platform 3, controlled by the speed regulator 12, which permits both forward and reverse movement. The distance Ax, between the place in the molten zone 6 at which the diameter y is measured and the solidification front 7 is thereby kept constant.

A second control value, Rx, obtained by the multi-diode array 1 by determining the zone height x, and a third control dimension, Ry, obtained by the multi-diode array 2 by determining the diameter y of the melt, may be optionally used. in regulating a stretching and compressing device shown), or in controlling the energy supply to the heater 10. In this case, the control Rx and Ry are fed into a computer which stores predetermined nominal values for Rx and Ry, the computer being connected to the stretching and compressing device and/or to a device regulating the. energy supply to the heating coil. 10, and being programmed to control these devices so as to tend to produce a resultant monocrystalline rod of a desired diameter, the value of which has previously been fed into the computer.It is therefore possible, in this way, to control fully automatically the entire zone-melting process from the addition or development of the seed crystal via the conical transition to the larger substantially constant diameter of the growing rod.

Figure 2 illustrates the principle for monitoring the diameter y of the molten zone 6, by means of the horizontally arranged multidiode array 2. The value of the diameter y is equal to the distance between the points 14 and 15 on the bright/dark transitions between the molten zone 6 and the background, and is proportional in a simple manner to the number of the diodes of the multi-diode array 2 which are illuminated.

The cut-off point between non-illuminated or only slightly illuminated diodes 19 and the illuminated diodes 17 is accurately determined by means of a predetermined trigger threshold 18. The number of illuminated diodes 17, and hence a measure of the value of the diameter y of the molten zone 6, is emitted in an output signal as the controlled dimension Ry. The output signal 22 may be in series, in parallel or in analogue form, as desired.

The zone height measurement procedure carried out by means of the multi-diode array 1, is illustrated schematically in Figure 3. The diode array 1 is scanned in a manner similar to that in which the diode array 2 is scanned as described above. In contrast to the determination of the diameter y, where the image of the molten zone 6 stands out sharply against a dark background, there is no sharp bright/dark transition at either end of the moltebn zone 6 since the solid portions of the rod adjacent the molten zone also emit light, and the intensity spectrum falling on the array 1 is a continuous curve, thereby initiating a photocurrent at each diode of this array.However, at the transition 7 between the glowing white-red, just solidified, silicon (or other semi-conductor material) of the growing monocrystalline rod portion 8 and the just melted, liquid silicon of the molten zone 6, a pronounced change in the quality of emitted light occurs, and a corresponding pronounced difference occurs in the photocurrents produced at the two diodes either side of the image of the transition interface on the array 1, at the threshold point 25, where the steeply sloping photocurrent curve falls below a trigger threshold value 27. The reason for this is the change in the spectrum of the emitted light and the associated change in the photocurrent yield of the individual silicon photodiodes.However, difficulties arise on the other side of the molten zone 6, since the polycrystalline rod portion 9 does not melt across a sharply defined interface, but rather individual lumps 23 which are still solid come away from the rod portion 9 and float in the molten zone 6, thus giving rise to several solid-liquid transitions at this end portion of the molten zone 6. To take account of this situation, the zone height is measured in two stages. In the first stage, only the number of solid-liquid phase transitions or the thresholds in the photocurrent are counted, that is, the number of inclined edges in the output signal 30. In the second stage, the clock pulses 29 are counted from the edge 31 of the first-stored peak in the output signal 30, corresponding to the threshold point 25 of the photocurrent 24, to the edge 32 of the last-stored peak in the output signal 30, corresponding to the threshold value 26 of the photocurrent 24, and, if in the interval between two starting pulses 28 this number of solid-liquid transitions coincides with the preceding, stored measurement value, they are emitted as part of the output signal determining Rx.

If the number of edges has changed, the two values obtained during the first and second measuring stages are erased by clearing the two appropriate counters and the process is begun again. This prevents the possibility of a layer of polycrystalline rod material, which has still not melted and is floating in the molten zone, causing an incorrect measurement of the zone height.

The self-adjusting positioning process of the monitoring system with respect to the changing location along the rod of the molten zone 6 is illustrated in Figure 3, insofar as it is a constituent of the zone height measurement, and in Figure 4. The individual photodiodes of the multi-diode array 1 are scanned by means of a shift register and their states are established. At the same time the position of the solid/liquid phase transition at the solidification front 7 can be established very accuratelsy by taking into account where the intensity curve shown in Figure 3 cuts the trigger threshold value-line 27.To position the monitoring system, the number of clock pulses 29 from the starting pulse 28 to the first inclined edge 31 of the output signal 30 is counted, which number corresponds to the lower solid/liquid phase transition at the solidification front 7 and to the threshold point 25 on the photocurrent 24, is transmitted, via a transformer 33 to a calculating unit 35 and compared with a fixed nominal value 34 which has been previously fed into the unit 35.An output corresponding to the difference between these values is fed into an intermediate store 36 and a digital-analogue converter 37, amplified in an operational amplifier 38, and fed from there into a position control circuit 39 which, by means of the motor 13, keeps the entire measuring system in one position with respect to the molten zone 6 so that, as described earlier, the image of the solid/liquid phase transition between the solidification front 7 and the molten zone 6 is always projected at the same place on the multi-diode array 1 monitoring the vertical dimensions of the molten zone 6.

WHAT WE CLAIM IS: 1. A process for the zone melting of a vertically extending rod of semi-conductor material, which comprises applying radially symmetric heating to the rod progressively along its length so that successive regions of the rod are heated in turn, monitoring the heated molten zone with one or more arrays of electro-magnetic radiation-sensitivie elements, the or each array being arranged to receive radiation emitted from the molten zone, and controlling the molten zone in accordance with the data registered by the radiation-sensitive array elements.

2. A process as claimed in Claim 1, wherein the radiation-sensitive elements are photosensitive elements.

3. A process as claimed in Claim 1 or Claim 2, wherein the radiation-sensitive elements are diodes.

4. A process as claimed in any one of Claims 1 to 3, wherein the control of the rod heating is determined by the number of radiation-sensitive array elements which receive radiation of a predetermined quality.

5. A process as claimed in any one of Claims 1 to 4, wherein the molten zone of the rod is heated by heating means which surround the rod coaxially and are movable along the length of the rod.

6. A process as claimed in any one of Claims 1 to 5, wherein at least one linear dimension of the molten zone is monitored.

7. A process as claimed in Claims 4 and 6, wherein the radiation-sensitive elements are photosensitive elements and the value of the molten zone linear dimension being monitored is directly propoertional to the number of photo-sensitive array elements which is illuminated.

8. A process as claimed in Claim 6 or Claim 7, wherein the heating of the rod is controlled to standardise the or each monitored linear dimension of the heating zone by regulating the energy supplied during heating and/or by regulating the rate at which the heated molten zone passes along the rod and/or by regulating external stretching and compressing forces acting on the rod.

9. A process as claimed in any one of Claims 6 to 8, wherein the or one of the arrays is arranged to monitor the diameter of the molten zone by detection of two bright/dark vertically orientated interfaces between molten zone and background.

10. A process as claimed in any one of Claims 6 to 9, wherein the or one of the arrays is arranged to monitor the height of the melting zone between the two horizontally orientated solid/liquid phase transition interfaces between the solid rod material and the melt of the molten zone by detection of the change in the quality of radiation occurring at these two interfaces.

11. A process as claimed in any one of Claims 6 to 10, wherein there is provided a plurality of arrays of radiation-sensitive elements, each array being arranged to monitor a different linear dimension of the molten zone.

12. A process as claimed in Claim 11, wherein radiation-receiving means is provided which is arranged to project the radiation pattern of the molten zone onto each array.

13. A process as claimed in Claim 12, wherein the radiation-receiving means comprises at least one semi-translucent mirror.

14. A process as claimed in any one of Claims 10 to 13, wherein there are provided two arrays of radiation-sensitive elements arranged to monitor simultaneously the diameter of the molten zone and the height of the molten zone, respectively.

15. A process as claimed in Claim 13 and Claim 14, wherein the arrays for measuring the diameter and height of the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. floating in the molten zone, causing an incorrect measurement of the zone height. The self-adjusting positioning process of the monitoring system with respect to the changing location along the rod of the molten zone 6 is illustrated in Figure 3, insofar as it is a constituent of the zone height measurement, and in Figure 4. The individual photodiodes of the multi-diode array 1 are scanned by means of a shift register and their states are established. At the same time the position of the solid/liquid phase transition at the solidification front 7 can be established very accuratelsy by taking into account where the intensity curve shown in Figure 3 cuts the trigger threshold value-line 27.To position the monitoring system, the number of clock pulses 29 from the starting pulse 28 to the first inclined edge 31 of the output signal 30 is counted, which number corresponds to the lower solid/liquid phase transition at the solidification front 7 and to the threshold point 25 on the photocurrent 24, is transmitted, via a transformer 33 to a calculating unit 35 and compared with a fixed nominal value 34 which has been previously fed into the unit 35.An output corresponding to the difference between these values is fed into an intermediate store 36 and a digital-analogue converter 37, amplified in an operational amplifier 38, and fed from there into a position control circuit 39 which, by means of the motor 13, keeps the entire measuring system in one position with respect to the molten zone 6 so that, as described earlier, the image of the solid/liquid phase transition between the solidification front 7 and the molten zone 6 is always projected at the same place on the multi-diode array 1 monitoring the vertical dimensions of the molten zone 6. WHAT WE CLAIM IS:

1. A process for the zone melting of a vertically extending rod of semi-conductor material, which comprises applying radially symmetric heating to the rod progressively along its length so that successive regions of the rod are heated in turn, monitoring the heated molten zone with one or more arrays of electro-magnetic radiation-sensitivie elements, the or each array being arranged to receive radiation emitted from the molten zone, and controlling the molten zone in accordance with the data registered by the radiation-sensitive array elements.

molten zone are linear arrays arranged each at right angles to the axis of the rod and at right angles to each other, and arranged to receive light by transmission and reflection, respectively, from a semi-translucent mirror of the radiation collecting means arranged to receive radiation from the molten zone and at an angle of 45" to the axis of the rod.

16. A process as claimed in Claim 10 or in any one of Claims 11 to 15 when dependent on Claim 10, wherein relative movement is caused between the means monitoring the molten zone and the rod, so that the projection of the solid/liquid phase transition interface between the melt and the previously heated rod portion always falls in substantially the place on the array arranged to monitor the height of the molten zone.

17. A process as claimed in Claim 16, wherein the position of the monitoring means with respect to the molten zone is automatically controlled in accordance with the radiation pattern received on the array monitoring the height of the molten zone by means of a position control circuit.

18. A process as claimed in Claim 16 or Claim 17, and in any one of Claims 11 to 15, wherein the plurality of arrays of radiation-sensitive elements are arranged so as to be fixed in position in relation to each other, and the entire monitoring means, comprising the arrays, is caused to move along the rod.

19. A process as claimed in Claim 8 orin any one of Claims 9 to 18 when dependent on Claim 8, wherein the values of the monitored linear dimension or dimensions are transmitted to a regulating system which controls the heating process of the rod so as to tend to produce a monocrystalline rod of a predetermined diameter.

20. A process for the zone-melting of a vertically extending rod of semi-conductor material substantially as hereinbefore described with reference to the accompanying drawings.

21. Zone-melting apparatus for a vertically extending rod of semi-conductor material, which comprises heating means arranged to provide radially symmetric heating to a portion of a rod along its length, means for causing relative movement between the heating means and the rod in a direction along or parallel to the axis of the rod, monitoring means comprising one or more arrays of electro-magnetic radiationsensitive elements, the or each array being arranged to receive radiation emitted from the molten zone of the rod in the region of the heating means, and control means for controlling the molten zone in accordance with the data registered by the radiationsensitive array elements.

22. Apparatus as claimed in Claim 21, wherein the heating means is arranged to move along the rod.

23. Apparatus as claimed in Claim 21 or Claim 22, wherein the heating means surrounds the rod coaxially.

24. Apparatus as claimed in any one of Claims 21 to 23, wherein the radiationsensitive elements are photosensitive elements.

25. Apparatus as claimed in Claim 24, wherein the radiation-sensitive elements are diodes.

26. Apparatus as claimed in any one of Claims 21 to 25, wherein the monitoring means is arranged to monitor at least one linear dimension of the molten zone.

27. Apparatus as claimed in Claim 26, wherein the control means is arranged to standardise the or each monitored linear dimension of the molten zone by regulating the energy supplied during heating and/or by regulating the rate at which the heated molten zone passes along the rod and/or by regulating external stretching and compressing forces acting on the rod.

28. Apparatus as claimed in Claim 26 or Claim 27, wherein there is provided a plurality of arrays of radiation-sensitive elements, each array being arranged to monitor a different linear dimension of the molten zone.

29. Apparatus as claimed in Claim 28, wherein radiation receiving means is provided which is arranged to project the radiation pattern of the molten zone onto each array.

30. Apparatus as claimed in Claim 29, wherein the radiation-receiving means comprises at least one semi-translucent mirror.

31. Apparatus as claimed in any one of Claims 28 to 30, wherein there are provided two arrays of radiation-sensitive elements arranged to monitor simultaneously the diameter of the molten zone and the height of the molten zone, respectively.

32. Apparatus as claimed in Claim 29 and Claim 31, wherein the arrays for measuring the diameter and height of the molten zone are linear arrays arranged each at right angles to the axis of the rod and at right angles to each other, and arranged to receive light by transmission and reflection, respectively, from a semi-translucent mirror of the radiation-receiving means arranged to receive radiation from the molten zone and at an angle of 45" to the axis of the rod.

33. Apparatus as claimed in any one of claims 21 to 32, wherein the monitoring means can move vertically to follow the molten zone along the rod.

34. Apparatus as claimed in Claim 33, wherein the monitoring means is arranged to monitor the height of the molten zone and there is provided a position control circuit which is arranged to control automa tically the vertical position of the monitoring means in accordance with the radiation pattern received by the array arranged to monitor the molten zone height.

35. Apparatus as claimed in Claim 33 or claim 34, wherein the plurality of arrays of radiation-sensitive elements are arranged so as to be fixed in position in relation to each other, and the entire monitoring means, comprising the arrays, is caused to move along the rod.

36. Zone-melting apparatus for a vertically extending rod of semi-conductor material substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.