Aligned, Coexisting Liquid and Solid Regions in Pulsed and CW Laser Annealing of Si
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ALIGNED, COEXISTING LIQUID AND SOLID REGIONS IN PULSED AND CW LASER ANNEALING OF Si
R. J. Nemanich, D. K. Biegelsen, and W. G. Hawkins,* Xerox Palo Alto Research Center, Palo Alto, CA 94304
ABSTRACT Aligned, coexisting liquid and solid regions are observed in cw laser annealing of polycrystalline Si films on quartz substrates. These stripe patterns are the precursors of surface topography that exists after cooling. It is proposed that a similar situation exists in the pulse annealing process. A calculation of the temperature evolution which assumes stripe symmetry and kinetic restraints of the crystallization process has been carried out. These calculations indicate a lattice temperature of between 1100 and 1300 K, 10 nsec after the sample has fully solidified.
INTRODUCTION In the studies of pulsed laser annealing of semiconductors, two areas of special interest are the observation of oriented surface topography (i.e. ripples) [1,2,3] and the Raman measurements of a low lattice temperature shortly after the annealing pulse [4]. By and large the experimental and theoretical investigations have treated these aspects separately. In this study it is shown that aligned liquid and solid regions are responsible for the surface topography of cw laser annealed Si. Furthermore, it is suggested that this configuration also exists in pulse laser annealing and can account for the Raman observation through a purely thermal analysis. The occurrence of surface topography has been observed for pulse laser annealing with pulse energies from - .4 to 2 J/cm 2. When higher pulse energies are used, the surface is meniscus like, indicating clearly that the annealed region of the sample has totally melted. An unusual characteristic of the surface topography is the formation of parallel linear ridges or ripples. While several spacings have been reported [3], for annealing with normally incident polarized light, the predominant pattern exhibits ripples aligned perpendicular to the polarization which are separated by a distance equal to the free space wavelength of the incident laser. Several analyses have indicated that the ripples are due to a standing electromagnetic wave at the surface and this pattern arises from an interference between the incident laser and a surface wave [1,2]. The origin of the surface wave and the coupling to the semiconductor are aspects yet to be defined. With respect to a purely thermal analysis of the laser annealing process, it is difficult to understand how the ripple structures could persist in the molten state after the incident radiation is removed. Of course, the low lattice temperature of 700 K deduced from Raman measurements [4] is also difficult to reconcile with the purely thermal analysis. *Permanent address: Mat. Res.
Soc.
Symp. Proc.
Vol.
Xerox, Webster Research Center, Webster, NY 13 (1983)
®Elsevier Science Publishing Co.,
Inc.
14580
212
EXPERIMENTAL To understand the formation of the surface ripples, cw laser annealing of Si has been monitored in situ. This has been accomplished by
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