Laser Growth of Thin Silicon Crystals in Patterned Structures
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LASER GROWTH OF THIN SILICON CRYSTALS IN PATTERNED STRUCTURES R. M. FASTOW*, H. J. LEAMY, G. K. CELLER, Y. H. WONG, and C. DOHERTY Bell Laboratories, Murray Hill, New Jersey 07974
J.
ABSTRACT Beam processing can be applied to thin, polycrystalline silicon films on amorphous substrates to: a) increase grain size, b) produce single, isolated crystals, and c) to produce oriented single crystals. Specific methods for these tasks are outlined. Crystal growth and heat flow consideration appropriate to each are presented. INTRODUCTION Dielectric isolation of active areas in silicon devices is common planar-technology practice. Typically, isolation is achieved only in the plane; i.e., laterally. Extension of isolation to the vertical dimension in order to produce fully isolated structures is practiced only rarely; e.g., in silicon-on sapphire (SOS) technology1 or "dielectric isolation" (DI) technologyz. Recently, however, laser and electron beam experiments have yielded material of interest in this connection. Essentially, laser and electron beams can be employed to perform, on a microscopic scale, crystal growth operations that are normally performed on bulk material. Although reports of such microfabrication by beam processing first appeared nearly 20 years 3 ago , experience with such techniques is still quite meager. Our aim here is to supplement current knowledge with an account of our recent experience. Various schemes for the production of isolated silicon for device fabrication are listed in Figure 1. The range of crystal quality represented in Fig. 1 is wide, and although the eventual applicability of the various schemes can be assessed only in connection with specific device requirements, none can be immediately rejected. Indeed, laser processed polycrystalline silicon has been found to be device-worthy in recent preliminary evaluations. Polycrystalline silicon can be deposited onto dielectric substrates to yield films of small grain size. Transistors fabricated in such material are vastly inferior to those made in single 4 crystals . The grain size of such films can be increased by both pulsed 5 and cw6 laser processing. Continuous (cw) laser beams produce local melting and resolidification in the wake of a For 500nm films scanned molten zone produces elongated grains. Present address: New York 14850.
Physics Department,
Cornell University,
Ithaca,
496 on SiO2 or Si 3 N4 coated silicon substrates, grains from 1-10bm wide and up to 20pm long are typically observed. Much larger grains have been produced on bulk fused silica substrates 7 . Pulsed laser processing also produces large (2-10bm) grained material via local melting, and resolidification, but the detailed mechanism of grain nucleation and growth is quite different 8 . If an overlapping scan pattern is employed in laser processing, continuous sheets of large grained polycrystalline silicon may be fabricated, and the surface state density at the polysilicon/SiO2 interface is satisfactory for device fabrication 9 . Both MOSFET5,10,11 and MESFET1 2
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