Solidification Interface Morphologies in Zone Melting Recrystallization
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SOLIDIFICATION INTERFACE MORPHOLOGIES IN ZONE MELTING RECRYSTALLIZATION
J.S. IM*, C.V. THOMPSON*, AND H. TOMITA** *Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; **Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139. (Permanent address: Sony Co. Research Center, Yokohama, Japan.)
ABSTRACT
Using in situ optical microscopy we have studied the morphology of the liquid-solid interface during zone melting recrystallization.
observed a variety of interface morphologies,
We have
each of which corresponds to
specific types and distributions of subboundaries in
the solidified material.
We have also observed a variety of morphologies for stationary interfaces. propose that perturbations in both the stationary and moving liquid-solid interfaces develop, at least in part, due to the spatial gradient in radiation intensity in the region of the interface.
We
the
INTRODUCTION Zone melting recrystallization [1] (ZMR) has been studied extensively as a means for producing device-quality semiconductor films on insulating substrates. While films can be readily produced without high angle grain The boundaries [2,3], low angle subboundaries [4] almost always remain. locations of these subboundaries have been clearly linked to the morphology of the liquid-solid interface during ZMR.[5,6] Several studies of interface morphology in ZMR have been undertaken. These studies have been based on in situ observations [7-9] as well as other methods [5]. A variety of interface morphologies have been observed and can be generally characterized as having regularly spaced protrusions into the Subboundary locations have been correlated with these protrusions.[5] liquid. Proposed explanations for the shapes and spacings of these protrusions have included discussions of growth velocity anisotropy [5,6], constitutional supercooling [7], and absolute undercooling. We have undertaken an experimental program aimed at better defining the In conditions required for various interface and subboundary morphologies. this first report we will focus on the influence of spatial gradients in radiation intensities. EXPERIMENTAL DESIGN We have constructed a graphite strip heater which has several important features. First, the thin film (and wafer) are moved while the upper and This approach is used in other techniques lower strips are held stationary. Also, for crystal growth [10] and allows better control of thermal profiles. because the liquid-solid interface remains stationary in space, this technique allows convenient microscopic observation of the interface during solidification. Our strip heater was also designed in such a way that the height of the upper strip can be externally controlled during solidification with an accuracy of 10 micrometers. Samples were prepared via low pressure chemical vapor deposition (LPCVD) from SiH4 of 0.5, and lpm-thick polycrystalline silicon (poly-Si) films on The poly-Si was capped with 2pm-thick thermally oxidized
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