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Diameter control of lec grown GaP crystals
Journal of Crystal
Growth 21 (1974) 310-312
DIAMETER
H. J. A. VAN Philips Research Received
0 North-Holland
CONTROL
DIJK,
Co.
OF LEC GROWN
C. M. G. JOCHEM,
Laboratories,
21 September
Publishing
Eindhoven,
GaP CRYSTALS
G. J. SCHOLL
and P. VAN
DER
WERF
The Netherlands
1973
Gallium phosphide single-crystals have been pulled by the liquid encapsulated Czochralski technique. The process is remotely-controlled and observed by a closed circuit TV system. However, the visibility needed to permit the growth of crystals with a constant diameter is limited by several process conditions. Moreover an automatic diameter control was felt to be desirable. The X-ray shadowgraph method has been selected as the most appropriate one for this particular system. For that purpose the puller chamber was redesigned to enable the crucible-melt assembly to be X-rayed. The X-ray silhouette is displayed on a TV monitor and this image is processed to measure the diameter. The result of this measurement is fed back to the output of the rf generator to adjust the heating rate of the melt.
The visibility during the growth of GaP crystals from a liquid encapsulated melt is impaired by several factors, including the clouding of the B,O,, the unfavourable viewing angle, window tarnish and turbulent convection of the high pressure ambient gas. This lack of visibility makes it difficult to control the diameter of the growing crystal by manual action. It was the aim of this work to eliminate these difficulties by developing a method that would give accurate and instantaneous visual information about the diameter of the crystal, independent of the interfering circumstances. Furthermore it would be highly desirable to use this information for automatic diameter control. Careful consideration of earlier described methods for diameter control’5, made us decide to use an X-ray imaging system as the most suitable one for our own problem. The X-ray shadow image is displayed on a TV monitor. The line that is representative for the diameter is withdrawn from the TV picture and processed to give a voltage output signal which is proportional to the diameter. This signal is compared with a selectable set-value and the result is fed back via a conventional PID controller to the output of the rf generator in order to adjust the heating-rate of the melt. The use of X-rays to control the diameter of growing crystals has been described earlier’). This system implies that the X-ray absorption of all parts surrounding the melt is negligible compared with the absorption of the melt. In our particular case, the liquid encapsulated Czochralski (LEC) growth of GaP, some essential parts
of the
puller
Fig.
310
(Metals
Research
Cambridge
MSR4
system) had to be redesigned to fulfil this requirement. A new high-pressure chamber was constructed which has two opposite ports in the lower section of the chamber at crucible height (fig. 1). These ports were closed by aluminium windows of a special shape capable of withstanding the high pressure (tested up to 150 atm) and having a minimum thickness (8 mm) for low X-ray absorption. The effective diameter of the window is 50 mm. The windows are water-cooled; corrosion is prevented by anodization. One quartz window, as in the original chamber, is retained. The copper rf coil is replaced by an anodized aluminium coil. The X-ray system consisted of a commercially available medical
1.
Modified
puller
construction.
DIAMETER
CONTROL
OF LEC GROWN
Fig. 2. Cross-sectional view of the puller chamber with the X-ray system. (1) X-ray source, (2) aluminum windows, (3) Xray image intensifier, (4) TV camera, (5) radiation shields, (6) aluminum rf coil.
Fig. 3. X-ray image of the growing crystal displayed on a TV monitor.
Fig. 5.
Schematic representation
Schematic representation
of the X-ray image.
of h, CI and r (see text).
chain including an X-ray tube (Philips Practix XB 1020), an X-ray image-intensifier (Philips XG 2000) and a TV camera (fig. 2). The X-ray shadow image obtained with this system is shown in fig. 3. The left and right parts of the image are shadowed by vertical metal masks; the upper part (above the boron oxide layer) is shadowed too to avoid saturation of the X-ray image-intensifier.
311
Diameter measurement Fig. 4 is a schematic representation of the X-ray image of the growing crystal. An electronic system counts a preselected number of lines a (total image consists of 625 lines) starting at position A in the B direction. Position A is defined as the lowest position where the first white information on the TV monitor occurs. Number a is selected in such a way that the position of the last counted line is below the solid-liquid interface. The next line (a+ 1 at position B) is withdrawn from the image and the width of the black centre part (i.e. the crystal diameter) is measured. The output of the measuring device is a voltage signal proportional to the crystal diameter. The line that is withdrawn from the TV image leaves there a white line so that the position at which the diameter is measured can always be checked. The distance AB is about equal to but smaller than the height h of the solidification interface above the surface of the melt. The advantage of this system is that the distance AB is kept constant throughout the run and in a first approximation is independent of the drop of the melt level during growth. In practice, however, some corrections have to be made, arising from the dependence of h on two geometrical parameters, viz. the contact angle u and the crystal radius r “) (fig. 5). The height of the solidification interface above the melt changes markedly if the contact angle CI changes, i.e. if the diameter changes. Since the diameter should be constant within certain limits only small changes of TVwill be allowable, and in consequence the change of h will be small too. It should be clear that corrections to the diameter can only be made as long as the material has not yet solidified. To be sure that at least part of the diameter measurement is related to the liquid near the solid-liquid interface not one but several lines (typically 18) are used to measure the diameter. Diameter control can be performed from the beginning of a run, including the crystal cone. In this part of the growth procedure the number a has to be adapted since CIthen deviates significantly from zero and furthermore h is no longer independent of the (small) diameter. A further adaptation was necessary at the end of a run. Here the position A shifts due to an increasing curvature of the melt surface. For that reason the vertical metal masks are placed as close as possible to the silhouette of the growing crystal.
ma-:
Fig. 4
GaP CRYSTALS
312
H. J. A. VAN
DIJK,
C. M. G. JOCHEM,
G. J. SCHOLL
AND
P. VAN
DER
WERF
melt is adjusted for minimum off-set. The set-point signal is obtained from a motor-driven tenturn potentiometer. By proper selection of the motor speed the top cone, the cylindrical part and the bottom cone of the crystal can be grown automatically. Crystals up to 100 g are currently being grown. The diameter of the crystals is typically 20 + 0.5 mm (fig. 6). This work would not have been possible
without
the
great skill of the people in the design department and the workshop. The continuous support of Mr. J. Goorissen and Mr. A. F. Verkruissen and the valuable discussions with them are gratefully acknowledged. References Fig. 6. GaP crystals. Grown without trol (left) and with automatic diameter
automatic diameter control (right).
con-
Control and results The output signal of the diameter measurement is compared with a selectable set-point signal. The difference of these two signals is fed back to the rf generator and power output control is achieved by means of a conventional PID controller. The heating rate of the
nr. 1.154.240. 1) British patent specification J. Crystal Growth 15 (1972) 85. 2) U. Gross and R. Kersten, 3) D. F. O’Kane, T. W. Kwap, L. Gulitz and A. L .Bednowitz, J. Crystal Growth 13/14 (1972) 624. K. J. Gartner, K. F. Rittinghaus, A. Seeger and W. Uelhoff, 4, J. Crystal Growth 13/14 (1972) 619. 5) W. Bardsley, G. W. Green, C. H. Holliday and D. T. J. Hurle, J. Crystal Growth 16 (1972) 277. 6) G. K. Gaule and J. R. Pastore, in: Metallurgy of Elemental and Compound Semiconductors, Ed. R. 0. Grubel (Interscience, New York, 1961) p. 201.
Growth 21 (1974) 310-312
DIAMETER
H. J. A. VAN Philips Research Received
0 North-Holland
CONTROL
DIJK,
Co.
OF LEC GROWN
C. M. G. JOCHEM,
Laboratories,
21 September
Publishing
Eindhoven,
GaP CRYSTALS
G. J. SCHOLL
and P. VAN
DER
WERF
The Netherlands
1973
Gallium phosphide single-crystals have been pulled by the liquid encapsulated Czochralski technique. The process is remotely-controlled and observed by a closed circuit TV system. However, the visibility needed to permit the growth of crystals with a constant diameter is limited by several process conditions. Moreover an automatic diameter control was felt to be desirable. The X-ray shadowgraph method has been selected as the most appropriate one for this particular system. For that purpose the puller chamber was redesigned to enable the crucible-melt assembly to be X-rayed. The X-ray silhouette is displayed on a TV monitor and this image is processed to measure the diameter. The result of this measurement is fed back to the output of the rf generator to adjust the heating rate of the melt.
The visibility during the growth of GaP crystals from a liquid encapsulated melt is impaired by several factors, including the clouding of the B,O,, the unfavourable viewing angle, window tarnish and turbulent convection of the high pressure ambient gas. This lack of visibility makes it difficult to control the diameter of the growing crystal by manual action. It was the aim of this work to eliminate these difficulties by developing a method that would give accurate and instantaneous visual information about the diameter of the crystal, independent of the interfering circumstances. Furthermore it would be highly desirable to use this information for automatic diameter control. Careful consideration of earlier described methods for diameter control’5, made us decide to use an X-ray imaging system as the most suitable one for our own problem. The X-ray shadow image is displayed on a TV monitor. The line that is representative for the diameter is withdrawn from the TV picture and processed to give a voltage output signal which is proportional to the diameter. This signal is compared with a selectable set-value and the result is fed back via a conventional PID controller to the output of the rf generator in order to adjust the heating-rate of the melt. The use of X-rays to control the diameter of growing crystals has been described earlier’). This system implies that the X-ray absorption of all parts surrounding the melt is negligible compared with the absorption of the melt. In our particular case, the liquid encapsulated Czochralski (LEC) growth of GaP, some essential parts
of the
puller
Fig.
310
(Metals
Research
Cambridge
MSR4
system) had to be redesigned to fulfil this requirement. A new high-pressure chamber was constructed which has two opposite ports in the lower section of the chamber at crucible height (fig. 1). These ports were closed by aluminium windows of a special shape capable of withstanding the high pressure (tested up to 150 atm) and having a minimum thickness (8 mm) for low X-ray absorption. The effective diameter of the window is 50 mm. The windows are water-cooled; corrosion is prevented by anodization. One quartz window, as in the original chamber, is retained. The copper rf coil is replaced by an anodized aluminium coil. The X-ray system consisted of a commercially available medical
1.
Modified
puller
construction.
DIAMETER
CONTROL
OF LEC GROWN
Fig. 2. Cross-sectional view of the puller chamber with the X-ray system. (1) X-ray source, (2) aluminum windows, (3) Xray image intensifier, (4) TV camera, (5) radiation shields, (6) aluminum rf coil.
Fig. 3. X-ray image of the growing crystal displayed on a TV monitor.
Fig. 5.
Schematic representation
Schematic representation
of the X-ray image.
of h, CI and r (see text).
chain including an X-ray tube (Philips Practix XB 1020), an X-ray image-intensifier (Philips XG 2000) and a TV camera (fig. 2). The X-ray shadow image obtained with this system is shown in fig. 3. The left and right parts of the image are shadowed by vertical metal masks; the upper part (above the boron oxide layer) is shadowed too to avoid saturation of the X-ray image-intensifier.
311
Diameter measurement Fig. 4 is a schematic representation of the X-ray image of the growing crystal. An electronic system counts a preselected number of lines a (total image consists of 625 lines) starting at position A in the B direction. Position A is defined as the lowest position where the first white information on the TV monitor occurs. Number a is selected in such a way that the position of the last counted line is below the solid-liquid interface. The next line (a+ 1 at position B) is withdrawn from the image and the width of the black centre part (i.e. the crystal diameter) is measured. The output of the measuring device is a voltage signal proportional to the crystal diameter. The line that is withdrawn from the TV image leaves there a white line so that the position at which the diameter is measured can always be checked. The distance AB is about equal to but smaller than the height h of the solidification interface above the surface of the melt. The advantage of this system is that the distance AB is kept constant throughout the run and in a first approximation is independent of the drop of the melt level during growth. In practice, however, some corrections have to be made, arising from the dependence of h on two geometrical parameters, viz. the contact angle u and the crystal radius r “) (fig. 5). The height of the solidification interface above the melt changes markedly if the contact angle CI changes, i.e. if the diameter changes. Since the diameter should be constant within certain limits only small changes of TVwill be allowable, and in consequence the change of h will be small too. It should be clear that corrections to the diameter can only be made as long as the material has not yet solidified. To be sure that at least part of the diameter measurement is related to the liquid near the solid-liquid interface not one but several lines (typically 18) are used to measure the diameter. Diameter control can be performed from the beginning of a run, including the crystal cone. In this part of the growth procedure the number a has to be adapted since CIthen deviates significantly from zero and furthermore h is no longer independent of the (small) diameter. A further adaptation was necessary at the end of a run. Here the position A shifts due to an increasing curvature of the melt surface. For that reason the vertical metal masks are placed as close as possible to the silhouette of the growing crystal.
ma-:
Fig. 4
GaP CRYSTALS
312
H. J. A. VAN
DIJK,
C. M. G. JOCHEM,
G. J. SCHOLL
AND
P. VAN
DER
WERF
melt is adjusted for minimum off-set. The set-point signal is obtained from a motor-driven tenturn potentiometer. By proper selection of the motor speed the top cone, the cylindrical part and the bottom cone of the crystal can be grown automatically. Crystals up to 100 g are currently being grown. The diameter of the crystals is typically 20 + 0.5 mm (fig. 6). This work would not have been possible
without
the
great skill of the people in the design department and the workshop. The continuous support of Mr. J. Goorissen and Mr. A. F. Verkruissen and the valuable discussions with them are gratefully acknowledged. References Fig. 6. GaP crystals. Grown without trol (left) and with automatic diameter
automatic diameter control (right).
con-
Control and results The output signal of the diameter measurement is compared with a selectable set-point signal. The difference of these two signals is fed back to the rf generator and power output control is achieved by means of a conventional PID controller. The heating rate of the
nr. 1.154.240. 1) British patent specification J. Crystal Growth 15 (1972) 85. 2) U. Gross and R. Kersten, 3) D. F. O’Kane, T. W. Kwap, L. Gulitz and A. L .Bednowitz, J. Crystal Growth 13/14 (1972) 624. K. J. Gartner, K. F. Rittinghaus, A. Seeger and W. Uelhoff, 4, J. Crystal Growth 13/14 (1972) 619. 5) W. Bardsley, G. W. Green, C. H. Holliday and D. T. J. Hurle, J. Crystal Growth 16 (1972) 277. 6) G. K. Gaule and J. R. Pastore, in: Metallurgy of Elemental and Compound Semiconductors, Ed. R. 0. Grubel (Interscience, New York, 1961) p. 201.