The Role of Oxygen in Zone-Melting Recrystallization of Silicon-On-Insulator Films
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THE ROLE OF OXYGEN IN ZONE-MELTING RECRYSTALLIZATION OF SILICON-ON-INSULATOR FILMS JOHN C. C. FAN,* B-Y. TSAUR,* C. K. CHEN,* J. R. DICK,** AND L. L. KAZMERSKI** *Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02173-0073; **Solar Energy Research Institute, Golden, CO 80401
ABSTRACT Using secondary-ion mass spectroscopy, we have found that oxygen is strongly concentrated at the sub-boundaries in zone-melting-recrystallized silicon-on-insulator films prepared by the graphite-strip-heater technique. This observation suggests that the formation of sub-boundaries during recrystallization may be caused by constitutional supercooling resulting from the presence of oxygen that is dissolved into the molten Si zone from the adjacent SiO2 layers. Since all zone-melting-recrystallized films to date have been bordered by SiO2 layers, regardless of the heating techniques employed, the sub-boundaries almost always present in these films may well have dissolved oxygen as their common origin. INTRODUCTION We have recently developed [1-41 a graphite-strip-heater technique for preparing large-area, high-quality silicon films on Si0 2 by zone-melting recrystallization (ZMR). The experimental configuration is shown in Fig. 1. The sample, which is placed on the stationary lower heater, consists of
MOVABLE UPPER STRIP HEATER LOWER STRIP HEATER
WAFER
Si 3 N4 (30 nm) MOLTEN ZONE .SiO 2 (2 mm)' Si (0.5 mm) $i02 (1 JAM)"
Si Fig. 1. Schematic diagram of experimental configuration used for zone-melting recrystallization of encapsulated Si films. The inset shows a cross section through a typical sample. Mat.
Res. Soc. Symp. Proc. Vol.
23 (1984) Published by Elsevier Science Publishing Co.,
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a fine-grained Si film on an insulating or insulator-coated substrate, together with an Si3N4/SiO2 encapsulation layer. The inset of Fig. 1 shows a cross section of a typical sample, with a -I-m-thick thermally grown SiO2 layer on a Si wafer, a 0.5-Pm-thick poly-Si layer deposited by low-pressure chemical-vapor deposition (LPCVD), a 2-Im layer of CVD Si0 2 , and a 30-nm layer of sputtered Si-rich Si3N4. The lower strip heater is used to heat the sample to a base temperature of 1100-1300"C, generally in a flowing Ar gas ambient at atmospheric pressure. Additional radiant energy, provided by the movable upper strip heater, is used to produce a narrow molten zone in the poly-Si layer (m.p. of Si = 1410C). The molten zone is then translated across the sample by scanning the upper heater, typically at 1/2 to 1 mm/sec, leaving a recrystallized Si film.
[41,
Almost all the recrystallized films, whether seeded [3] or unseeded contain sub-boundaries. (The only films without sub-boundaries are seeded ones with closely spaced
seed openings [1,51.)
y
source
t
The
sub-boundaries are fine-line defects, typically spaced 20-40 I'm apart, that run parallel to the direction of molten-zone motion (see Fig. 2). Transmission electron microscopy shows that these defects are dislocation arrays associated with angular deviations
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