Modifying Polycrystalline Films Through Ion Channelling
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MODIFYING POLYCRYSTALLINE FILMS THROUGH ION CHANNELLING
R. B. IVERSON AND R. REIF Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139
ABSTRACT A novel low-temperature process to enhance the grain size of a polycrystalline film on an amorphous substrate has been previously reported. In this process, ion implantation is used to selectively anorphize the film, and undamaged grains act as seed crystals in a subsequent low-temperature anneal. In this work, a 120 nm polycrystalline silicon film was implanted from three angles with pj 3 sph 2 rous at 150 K. The total dose was l.OxlO •cm . Transmission electron micrographs after a partial anneal (7000C for 30 minutes) indicate that some crystallites survived implantation due to ion channelling in the (111) plane. After a 60 minute anneal 0 at 700 C, 7 pm grains were observed.
INTRODUCTION In crystals, the rate of amorphization due to ion implantation decreases as the ion beam becomes parallel to a plane or axis of symmetry of the crystal [1]. By utilizing this ion channelling effect, we hope to produce a large-grain device-quality film on an amorphous substrate. The fabrication process, reported earlier [2,3], has three steps. First, a thin polycrystalline film is deposited. Next, the film is implanted to amorphize all grains except where significant ion channelling occurs. Surviving grains act as seed crystals in the final step, a low-temperature anneal. Combined with existing fabrication technologies for silicon devices, advantages of this process include: (a) the ability to build devices on mesas, eliminating parasitic capacitance associated with the reverse-biased PN junctions commonly used to isolate devices; (b) a much cheaper substrate than used in silicon-on-sapphire technology; (c) a technology which allows stacking of active layers of silicon on insulating or conducting films, making possible the fabrication of three-dimensional integrated circuits; and (d) the ability to fabricate efficient and inexpensive solar cells. The most successful techniques reported in the literature (laser recrystallization [4], graphoepitaxy [5], strip-heat recrystallization [6]) require melting of the silicon film. In this technique, the recrystallization is achieved by solid-phase epitaxial growth, fundamentally a low-temperature process requiring no melting of the silicon film. In this paper, the process is reviewed and some theoretical and experimental results are presented and evaluated.
Mat.
Res. Soc. Symp. Proc. Vol.
27 (1984) @Elsevier science Publishing Co.,
Inc.
544
PROCESS OVERVIEW The fabrication procedure presented here consists of three steps: deposition, implantation, and anneal. Deposition of Polycrystalline Film Any method of deposition of the polycrystalline film can be used as long as it allows sufficient control of the size and orientation of crystals in the film. (Grain size and orientation will affect the density and quality of the seed crystals remaining after implantation.) In this expe
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