The contamination o.f a melt c aused by the material of the container is
completely avoided by the float ing zoine method. For this reason the\u27 method, firstly realized by P. H. Keck and M. J. Golay1, is nowadays u sed very oft~n for obtaining various materials of extremely high purity. The method is especially valuable in the case of chemically active melts and certainly it is the best way for obtaining single c rystals when extreme purity is primarily required. Zone melting can be achieved by different heating techniques, high frequency being used mostly. Electron beam heating wais introduced by A. Calverley et al.2 Ln the case o f ma•terials with very high melting point it has the advantage over high frequency heating. In the present article an improvement
of this method as well as details of the equipment and floating zone refining of silicon are described

M. Sikirica

Z. Ban

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CR 0 AT IC A CHEMIC A ACT A 34 (1962) 175 BILJESKE NOTES CCA-259 546.28 .05 :542.14 The Floating-Zone Refining of Silicon by Electron Beam Heating Z. Ban and M. Sikirica Department of Structural and Ino-rganic Chemistry, Institute »Ri:aer Bo!ikovic«, Zagreb, Croatia, Yugoslavia Received April 6, 1962 The contamination o.f a melt caused by the material of the container is completely avoided by the floating zoine method. For this reason the' method, firstly realized by P. H. Keck and M. J. Golay1, is nowadays used very oft~n for obtaining various materials of extremely high purity. The method is espe-cially valuable in the case of chemically active melts and certainly it is the best way for obtaining single crystals when extreme purity is primarily requi-red. Zone melting can be achieved by different heating techniques, high fre-quency being used mostly. Electron beam heating wais introduced by A. Cal-verley et al.2 Ln the case of ma•terials with very high melting point it has the advantage over high frequency heating. In the present article an improve-ment of this method as well as details of the equipment and floating zone refining of silicon are described. The apparatus is shown ·in Fig. 1. It consists of a cylindrical water cooled metal chamber in which the electron source and the head of the assembly, for mounting a rod shaped sample, are enclosed. The chamber has three openings. Two of them serve for vacuum measurements, the third one for the observation of the melting. It is made of a concave glatss window provided with a segment shutter in a fixed position. By turning the window one always exposes the new and transparent part of it, thus enabling the conhnuous observahon o.f the process. On the assembly head there is a movable frame with two specimen holders coupled thrrn1gh a metal ball soc~et. The lower holder rests on an elastic spring. Allowance is thus made for any dilatation or contraction of the sample during the heating and cooling periods. Cracking of the sample is thus avoided. The apparatus is equipped with an automatic cycling system requiring only sporadic control. The regulation of the zone height and the final diameter of the sample is achieved simply by rotating the main shaft as it is seen from Fig. 1. The mechanism for moving the whole frame inside the chamber consists of the mortor dniven gear box. The speed of the zone is therefore adjusted at will from 6 to 16 cm/h. The required vacuum of 10·' torr. is achieved as usually by coupling the rotary ioil pump with a three stage oil difusi•on pump. The whole chamber is mounted on a modified Pollard conduction trapi cooled with liquid air. 176 Z . BAN AND M. SIKIRICA 9 1- METAL CHAMBER • 2- FOCUSING SYSTEM 3- CONCAVE GLASS WINDOW ANO SEGMENT SHUTTER 4- FRAME 5- SPRING 2 6-MAIN SHAFT 7- H. T. ELECTRODE 8 - TUNGSTEN CATHODE 8 9- BALL JOINT 4 5 F ig. 1. Bombarding chamber. The emission cuNent is subject to large fluctuations in the course o.f the process and must therefore be controlled. The controller similar to one descri--bed by Calverley and coworkers fullfiled the requirements. The response time of the controller is 1/ 20 sec. The mains voitage is separately stabilized to 1 p. c .. The high tension rectifier operates from 0 to 7000 V, with a max. emmision. THE FLOATING-ZONE REFINING OF SILICON 177: current of 1000 mA. The equ1pment is safeguarded by 30 lamps of 100 W con-nected in series in the high tension line. They serve both as protective resistance and an additional stabilization in the case of sudden large changes of the -emmission current that cannot be followed by the controller. A stable melting zone is achieved by adjusting the heating v·oltage of the cathode and the high tension for the electr'on acceleration, after which one switches on the automatic control. The circuitry of the latter ·is g:iven in Fig. 2. After the melted zone· reaches the upper end of the sample rod, a relay switches off the cathode· heating (or reduces it to a desired level), and reverses the motor. The frame is now traveling at the maximum speed in reverse. When the second micro-switch is touched, the cathode heating is switched Q11 again. The same micro-switch reverses the motor, and the frame inside the chamber is traveling-backwards at the minimum speed, thus repeating the zone melting process. 7000 v 1 A DC CONTROLLER BOMBARDING CHAMBER 20 V 50 A I· AC MS 1 RELAYS MS 2 TO AC MOTOR Fig. 2. Electrical circuitry of the apparatus The described arpparratus wa's used for zone melting of silicon. The samples were prepared from technical grade microcrystaU-ine siHcon (Merck). Silicon powder 00: different granulatiion was pressed into rods of 100 mm length with a cross section of one cm2 , at 2000 kg/cm?. One hour sintering followed at a temperature of 1380-1400° C, -in a vacuum of 5 · 10- s torrs. Spectwgraphical examination showed_ markedly a; purification effect, which is to be explained by partial evapmation oif the admixtures due to the reduction reactions of the type: Si <sl + Si02 <sl -----?- 2Si0 (gJ Si (s) + MeO Cs) Me (g, s) + SiO (g) The metal and silicon oxide vapors deposit onto the cooler part of the tube. The subsequent floating ·zorne r.efining removes the remaining nonvolatile 1.78 Z. BAN AND M. SIKIRICA impurities owing to. their different distribution coefficients between solid and liquid silicon. By using this prncedure we obtained siliico1n rods (length: 140 mm, mean diameter 10 mm) usually composed of two crystals. On the other hand, when much purer siiricon was used (Sylvania, 6-15 ohm. cm) the primavily obtained bicryistalline rod was ground on its end and remounted with thi:s end at the bottom which thus served as a seed for the single crystal obtained during the zone melting process. One such single crystal is shown in Fig: 3. The resistiv.ities of the obtained single crystails were measured by means of the compensation method, and showed some peculiarities. The minimum resistivity at the bottom pa.rt is varying frnm aibout 200 ohm. cm. in the center with a sharp increase toward the surface where it is greater then 1000 ohm. cm. Dpper sections are showing subsequently lower resistivities, i. e. down to 2_:_3 Fig. 3. Silicon single crystal ohm.cm, with the previously mentioned difference between the core and the surface region. Lower part of the rod w as :wned many times more than the upper one because the maiin pu11pose of the process was to grow the single crystal. Each time when the growth of the single crystal was interrupted at a certain length from the bo'&t,om the process was started again from the begi-ning. Thus the lower part of the rod was zoned at least 20 times and the top part only once. The radial efect of the purification is probaibly due to the preferential evaporation of the impurities owing to the very high local superheating of the molten zone with a very limited stirring. The method therefore seems to be quite useful for simultaneous produc-tion of single crystals and zone purification up to the very high purities, but providing for rotation o.f the one or both parts o.f the silicon rod. THE FLOATING-ZONE REFINING OF SILICON 179 The thermoelectric probe showed that the material is of the p-type as it was •before the zone melting. If the radial effect of the purification is really ,due to the above mentioned possible reason !it must be expected that the silicon of the n-type could be converted into p-type, because boron is probably the ,only impurity that cannot be removed neither by zone melting nor by the preferential evaporation. Acknowledgment: The authors are greatly indebted to Prof. Dr. D. Grdenic for his stimulating interest and revision of the manuscript. Thanks are due to Dr. S . Scav-nicar, who proposed this work, and to Dr. B. Kamenar for helpful discussions and ·encouragement as the work proceeded. We express our gratitude to Mr. R. Mutabzija for the construction of the controller. REFERENCES 1. P . H . Keck and M. J. G o lay, Phys. Rev. 89 (1953) 1298. ·2. A . C a 1ver1 e y et al., J . Sci. Instr. 34 ~1957) 142. 3. J . E . Po 11 a rd, Rev. Sci. Instr. 24 (li953) 996. 4. F. E. B i r beck and A. C a 1ver1 e y, J. Sci. Instr. 36 (1959) 460. IZVOD Ciscenje silicija u lebdeeoj zoni taljenjem pomocu snopa elektrona Z. Ban i M. Sikirica Prikazana je konstrukcija i nacin rada automatskog uredaja za zonalnu rafi-naciju i rast monokristala metodom ledbece zone. Ras.taljena zona se dobiva pomocu fokusiranog snopa elektrona. Opisana je metoda priredivanja sintrovanih stapova od silicija u obliku praska, sto se opcenito smatra ozbiljnim problemom u tehnologiji poluvodica. Uredaj je opremljen automatskim sistemom za kontrolu elektronske emisije i ponavljanja procesa taljenja tako da zahtijeva samo povremenu kontrolu. Eksperi-menti su izvedeni taljenjem silicija i dobiven je monokristal dilZine 110 mm, promjera oko 8 mm. •ODJEL STRUKTURNE I ANORGANSKE KEMIJE, INSTITUT »RUDER BOSKOVIC« ZAGREB Primljeno 4. travnja 1962. 

The Floating-Zone Refining of Silicon by Electron Beam Heating

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CR 0 AT IC A CHEMIC A ACT A 34 (1962) 175 BILJESKE NOTES CCA-259 546.28 .05 :542.14 The Floating-Zone Refining of Silicon by Electron Beam Heating Z. Ban and M. Sikirica Department of Structural and Ino-rganic Chemistry, Institute »Ri:aer Bo!ikovic«, Zagreb, Croatia, Yugoslavia Received April 6, 1962 The contamination o.f a melt caused by the material of the container is completely avoided by the floating zoine method. For this reason the' method, firstly realized by P. H. Keck and M. J. Golay1, is nowadays used very oft~n for obtaining various materials of extremely high purity. The method is espe-cially valuable in the case of chemically active melts and certainly it is the best way for obtaining single crystals when extreme purity is primarily requi-red. Zone melting can be achieved by different heating techniques, high fre-quency being used mostly. Electron beam heating wais introduced by A. Cal-verley et al.2 Ln the case of ma•terials with very high melting point it has the advantage over high frequency heating. In the present article an improve-ment of this method as well as details of the equipment and floating zone refining of silicon are described. The apparatus is shown ·in Fig. 1. It consists of a cylindrical water cooled metal chamber in which the electron source and the head of the assembly, for mounting a rod shaped sample, are enclosed. The chamber has three openings. Two of them serve for vacuum measurements, the third one for the observation of the melting. It is made of a concave glatss window provided with a segment shutter in a fixed position. By turning the window one always exposes the new and transparent part of it, thus enabling the conhnuous observahon o.f the process. On the assembly head there is a movable frame with two specimen holders coupled thrrn1gh a metal ball soc~et. The lower holder rests on an elastic spring. Allowance is thus made for any dilatation or contraction of the sample during the heating and cooling periods. Cracking of the sample is thus avoided. The apparatus is equipped with an automatic cycling system requiring only sporadic control. The regulation of the zone height and the final diameter of the sample is achieved simply by rotating the main shaft as it is seen from Fig. 1. The mechanism for moving the whole frame inside the chamber consists of the mortor dniven gear box. The speed of the zone is therefore adjusted at will from 6 to 16 cm/h. The required vacuum of 10·' torr. is achieved as usually by coupling the rotary ioil pump with a three stage oil difusi•on pump. The whole chamber is mounted on a modified Pollard conduction trapi cooled with liquid air. 176 Z . BAN AND M. SIKIRICA 9 1- METAL CHAMBER • 2- FOCUSING SYSTEM 3- CONCAVE GLASS WINDOW ANO SEGMENT SHUTTER 4- FRAME 5- SPRING 2 6- MAIN SHAFT 7- H. T. ELECTRODE 8 - TUNGSTEN CATHODE 8 9- BALL JOINT 4 5 F ig. 1. Bombarding chamber. The emission cuNent is subject to large fluctuations in the course o.f the process and must therefore be controlled. The controller similar to one descri--bed by Calverley and coworkers fullfiled the requirements. The response time of the controller is 1/ 20 sec. The mains voitage is separately stabilized to 1 p. c .. The high tension rectifier operates from 0 to 7000 V, with a max. emmision. THE FLOATING-ZONE REFINING OF SILICON 177: current of 1000 mA. The equ1pment is safeguarded by 30 lamps of 100 W con-nected in series in the high tension line. They serve both as protective resistance and an additional stabilization in the case of sudden large changes of the -emmission current that cannot be followed by the controller. A stable melting zone is achieved by adjusting the heating v·oltage of the cathode and the high tension for the electr'on acceleration, after which one switches on the automatic control. The circuitry of the latter ·is g:iven in Fig. 2. After the melted zone· reaches the upper end of the sample rod, a relay switches off the cathode· heating (or reduces it to a desired level), and reverses the motor. The frame is now traveling at the maximum speed in reverse. When the second micro-switch is touched, the cathode heating is switched Q11 again. The same micro-switch reverses the motor, and the frame inside the chamber is traveling-backwards at the minimum speed, thus repeating the zone melting process. 7000 v 1 A DC CONTROLLER BOMBARDING CHAMBER 20 V 50 A I· AC MS 1 RELAYS MS 2 TO AC MOTOR Fig. 2. Electrical circuitry of the apparatus The described arpparratus wa's used for zone melting of silicon. The samples were prepared from technical grade microcrystaU-ine siHcon (Merck). Silicon powder 00: different granulatiion was pressed into rods of 100 mm length with a cross section of one cm2 , at 2000 kg/cm?. One hour sintering followed at a temperature of 1380-1400° C, -in a vacuum of 5 · 10- s torrs. Spectwgraphical examination showed_ markedly a; purification effect, which is to be explained by partial evapmation oif the admixtures due to the reduction reactions of the type: Si <sl + Si02 <sl -----?- 2Si0 (gJ Si (s) + MeO Cs) Me (g, s) + SiO (g) The metal and silicon oxide vapors deposit onto the cooler part of the tube. The subsequent floating ·zorne r.efining removes the remaining nonvolatile 1.78 Z. BAN AND M. SIKIRICA impurities owing to. their different distribution coefficients between solid and liquid silicon. By using this prncedure we obtained siliico1n rods (length: 140 mm, mean diameter 10 mm) usually composed of two crystals. On the other hand, when much purer siiricon was used (Sylvania, 6-15 ohm. cm) the primavily obtained bicryistalline rod was ground on its end and remounted with thi:s end at the bottom which thus served as a seed for the single crystal obtained during the zone melting process. One such single crystal is shown in Fig: 3. The resistiv.ities of the obtained single crystails were measured by means of the compensation method, and showed some peculiarities. The minimum resistivity at the bottom pa.rt is varying frnm aibout 200 ohm. cm. in the center with a sharp increase toward the surface where it is greater then 1000 ohm. cm. Dpper sections are showing subsequently lower resistivities, i. e. down to 2_:_3 Fig. 3. Silicon single crystal ohm.cm, with the previously mentioned difference between the core and the surface region. Lower part of the rod w as :wned many times more than the upper one because the maiin pu11pose of the process was to grow the single crystal. Each time when the growth of the single crystal was interrupted at a certain length from the bo'&t,om the process was started again from the begi-ning. Thus the lower part of the rod was zoned at least 20 times and the top part only once. The radial efect of the purification is probaibly due to the preferential evaporation of the impurities owing to the very high local superheating of the molten zone with a very limited stirring. The method therefore seems to be quite useful for simultaneous produc-tion of single crystals and zone purification up to the very high purities, but providing for rotation o.f the one or both parts o.f the silicon rod. THE FLOATING-ZONE REFINING OF SILICON 179 The thermoelectric probe showed that the material is of the p-type as it was •before the zone melting. If the radial effect of the purification is really ,due to the above mentioned possible reason !it must be expected that the silicon of the n-type could be converted into p-type, because boron is probably the ,only impurity that cannot be removed neither by zone melting nor by the preferential evaporation. Acknowledgment: The authors are greatly indebted to Prof. Dr. D. Grdenic for his stimulating interest and revision of the manuscript. Thanks are due to Dr. S . Scav-nicar, who proposed this work, and to Dr. B. Kamenar for helpful discussions and ·encouragement as the work proceeded. We express our gratitude to Mr. R. Mutabzija for the construction of the controller. REFERENCES 1. P . H . Keck and M. J. G o lay, Phys. Rev. 89 (1953) 1298. ·2. A . C a 1ver1 e y et al., J . Sci. Instr. 34 ~1957) 142. 3. J . E . Po 11 a rd, Rev. Sci. Instr. 24 (li953) 996. 4. F. E. B i r beck and A. C a 1ver1 e y, J. Sci. Instr. 36 (1959) 460. IZVOD Ciscenje silicija u lebdeeoj zoni taljenjem pomocu snopa elektrona Z. Ban i M. Sikirica Prikazana je konstrukcija i nacin rada automatskog uredaja za zonalnu rafi-naciju i rast monokristala metodom ledbece zone. Ras.taljena zona se dobiva pomocu fokusiranog snopa elektrona. Opisana je metoda priredivanja sintrovanih stapova od silicija u obliku praska, sto se opcenito smatra ozbiljnim problemom u tehnologiji poluvodica. Uredaj je opremljen automatskim sistemom za kontrolu elektronske emisije i ponavljanja procesa taljenja tako da zahtijeva samo povremenu kontrolu. Eksperi-menti su izvedeni taljenjem silicija i dobiven je monokristal dilZine 110 mm, promjera oko 8 mm. •ODJEL STRUKTURNE I ANORGANSKE KEMIJE, INSTITUT »RUDER BOSKOVIC« ZAGREB Primljeno 4. travnja 1962. 

The Floating-Zone Refining of Silicon by Electron Beam Heating

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