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Structure of platelike oxygen precipitate in Czochralski-grown silicon
Materials Science and Engineering, 92 (1987) L9-L10
L9
Letter
Structure o f platelike oxygen precipitate in Czochralski-grown silicon XIAO ZHIGANG Department of Metal Physics, Beijing University of Iron and Steel Technology, Beijing (China)
QIN LUCHANG Laboratory of Atom Images of Solids, Institute of Metal Research, Academia Sinica, Shenyang (China)
LIN XIWEI* Department of Metal Physics, Beijing University of Iron and Steel Technology, Bei]ing (China) (Received May 4, 1987)
long annealing at such temperatures are mainly platelike amorphous SIO2, but the mechanism of their formation remains uncertain. The present work was carried o u t to study the structure of the precipitates especially during early stages of their formation. Slices were cut from a CZ silicon single crystal containing 1.5 × 1018 oxygen atoms cm -3. After heat treatment at 750 °C under purified nitrogen, transmission electron microscopy specimens were prepared by argon ion milling. The observation was carried out in a 200CX electron microscope operating at 200 kV. A point resolution of 0.25 nm was attained with the electron beam parallel to the (110) direction of a silicon film of a b o u t 10 nm thickness and suitable defocusing.
ABSTRACT High resolution electron micrographs of an oxide precipitate in a slice cut from Czochralski-grown silicon crystal and annealed at 750 °C for 50 h are presented. The precipitate is about 0.8 nm thick and 15 nm wide, has { 100} habit planes and a (1 O0)si//(1 O0)ppt [011]si//[01 O]ppt coheren t in terface with the matrix and appears to be cristobalite ft. A mechanism o f formation is suggested.
Recently the problem of oxygen precipitation in Czochralski-grown (CZ) silicon has attracted a great deal of attention. This is because oxygen precipitates and their induced lattice defects can be used to absorb undesirable impurities, t h e r e b y improving the large scale integration (LSI) device'production yield. In order to raise the rate of oxygen precipitation silicon wafers are often subjected to preannealing at temperatures near 750 °C. From published reports [1-3] it is known that oxygen precipitates created after
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*Present address: Institute of Iron and Steel Research, Ministry of Metallurgical Industry, Beijing, China. 0025-5416/87/$3.50
Fig. 1. (a) Lattice image of a platelike precipitate on (100) silicon; (b) enlarged image of precipitate shown in Fig. l(a). © Elsevier Sequoia/Printed in The Netherlands
LIO A typical cross-sectional view of a platelike precipitate is illustrated in Fig. i which was taken after annealing the specimen for 50 h. The plate (thickness, 0.8 nm; diameter, 15 nm) is end-on in the specimen, From Fig. 1 and other high resolution images the following model is proposed. In the early stages of formation the platelike oxide precipitates appear to be cristoba~ite fi (an allotrope of SiO2 with a diamond lattice, the lattice parameter is 0.716 nm) [ 4], habit plane (100) and orientation relationship (100) silicon//(100) cristobalite and [011] silicon//[010] cristobalite. The mismatch on the interface is 7%. This is in good agreement with other observations. Meanwhile we f o u n d that half the silicon atoms on the (100) interface are situated on coherent positions providing lower interfacial energy. The other half of the silicon atoms have two unpaired electrons each available to produce a new donor which develops at the same temperature range, the origin of which is n o t y e t completely understood. Furthermore the process of the formation of platelike oxide precipitates might be as follows. At first oxygen atoms cluster along the (100) plane of silicon. In the cluster region Si-Si bonds then turn into Si-O-Si bonds and gradually lead to the appearance of distinct structure of cristo-
balite ft. As a result of crystal symmetry, precipitate plates tend to take a square shape with their sides aligned along the (110) directions of the silicon matrix. The volume effect induced by precipitation is very large (VsioJVsi ~ 2.3), although part of it can be relaxed by the emission of silicon interstitial atoms. As the precipitates grow, the accumulated transformation stress and the weakening of interfacial coherency will eventually induce an amorphous state.
ACKNOWLEDGMENT The atomic images were taken at the Laboratory of A t o m Images of Solids under the direction of Professor K. H. Kuo. His support is gratefully acknowledged.
REFERENCES
1 F. A. Ponce, S. Hahn, T. Yamashita, M. Scott and J.R. Carruthers, in A. G. Cullis and D. C. Joy (eds.), Microscopy of Semiconducting Materials, in Inst. Phys. Conf. Ser., 67 (1983) 65. 2 H. Bender, Phys. Status Solidi A, 86 (1984) 245. 3 W. A. Tiller, S. Hahn and F. A. Ponce, J. Appl. Phys., 59 (1986) 3255. 4 R. W. G. Wyckoff, Crystal Structure, Vol. 1, Wiley, New York, 2nd edn., 1963, p. 318.
L9
Letter
Structure o f platelike oxygen precipitate in Czochralski-grown silicon XIAO ZHIGANG Department of Metal Physics, Beijing University of Iron and Steel Technology, Beijing (China)
QIN LUCHANG Laboratory of Atom Images of Solids, Institute of Metal Research, Academia Sinica, Shenyang (China)
LIN XIWEI* Department of Metal Physics, Beijing University of Iron and Steel Technology, Bei]ing (China) (Received May 4, 1987)
long annealing at such temperatures are mainly platelike amorphous SIO2, but the mechanism of their formation remains uncertain. The present work was carried o u t to study the structure of the precipitates especially during early stages of their formation. Slices were cut from a CZ silicon single crystal containing 1.5 × 1018 oxygen atoms cm -3. After heat treatment at 750 °C under purified nitrogen, transmission electron microscopy specimens were prepared by argon ion milling. The observation was carried out in a 200CX electron microscope operating at 200 kV. A point resolution of 0.25 nm was attained with the electron beam parallel to the (110) direction of a silicon film of a b o u t 10 nm thickness and suitable defocusing.
ABSTRACT High resolution electron micrographs of an oxide precipitate in a slice cut from Czochralski-grown silicon crystal and annealed at 750 °C for 50 h are presented. The precipitate is about 0.8 nm thick and 15 nm wide, has { 100} habit planes and a (1 O0)si//(1 O0)ppt [011]si//[01 O]ppt coheren t in terface with the matrix and appears to be cristobalite ft. A mechanism o f formation is suggested.
Recently the problem of oxygen precipitation in Czochralski-grown (CZ) silicon has attracted a great deal of attention. This is because oxygen precipitates and their induced lattice defects can be used to absorb undesirable impurities, t h e r e b y improving the large scale integration (LSI) device'production yield. In order to raise the rate of oxygen precipitation silicon wafers are often subjected to preannealing at temperatures near 750 °C. From published reports [1-3] it is known that oxygen precipitates created after
~
! ~
~
~,
~
R
I nm
*Present address: Institute of Iron and Steel Research, Ministry of Metallurgical Industry, Beijing, China. 0025-5416/87/$3.50
Fig. 1. (a) Lattice image of a platelike precipitate on (100) silicon; (b) enlarged image of precipitate shown in Fig. l(a). © Elsevier Sequoia/Printed in The Netherlands
LIO A typical cross-sectional view of a platelike precipitate is illustrated in Fig. i which was taken after annealing the specimen for 50 h. The plate (thickness, 0.8 nm; diameter, 15 nm) is end-on in the specimen, From Fig. 1 and other high resolution images the following model is proposed. In the early stages of formation the platelike oxide precipitates appear to be cristoba~ite fi (an allotrope of SiO2 with a diamond lattice, the lattice parameter is 0.716 nm) [ 4], habit plane (100) and orientation relationship (100) silicon//(100) cristobalite and [011] silicon//[010] cristobalite. The mismatch on the interface is 7%. This is in good agreement with other observations. Meanwhile we f o u n d that half the silicon atoms on the (100) interface are situated on coherent positions providing lower interfacial energy. The other half of the silicon atoms have two unpaired electrons each available to produce a new donor which develops at the same temperature range, the origin of which is n o t y e t completely understood. Furthermore the process of the formation of platelike oxide precipitates might be as follows. At first oxygen atoms cluster along the (100) plane of silicon. In the cluster region Si-Si bonds then turn into Si-O-Si bonds and gradually lead to the appearance of distinct structure of cristo-
balite ft. As a result of crystal symmetry, precipitate plates tend to take a square shape with their sides aligned along the (110) directions of the silicon matrix. The volume effect induced by precipitation is very large (VsioJVsi ~ 2.3), although part of it can be relaxed by the emission of silicon interstitial atoms. As the precipitates grow, the accumulated transformation stress and the weakening of interfacial coherency will eventually induce an amorphous state.
ACKNOWLEDGMENT The atomic images were taken at the Laboratory of A t o m Images of Solids under the direction of Professor K. H. Kuo. His support is gratefully acknowledged.
REFERENCES
1 F. A. Ponce, S. Hahn, T. Yamashita, M. Scott and J.R. Carruthers, in A. G. Cullis and D. C. Joy (eds.), Microscopy of Semiconducting Materials, in Inst. Phys. Conf. Ser., 67 (1983) 65. 2 H. Bender, Phys. Status Solidi A, 86 (1984) 245. 3 W. A. Tiller, S. Hahn and F. A. Ponce, J. Appl. Phys., 59 (1986) 3255. 4 R. W. G. Wyckoff, Crystal Structure, Vol. 1, Wiley, New York, 2nd edn., 1963, p. 318.