CW Laser-Recrystallized Polysilicon as a Device-Worthy Material
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449
and Materials Processing
CW LASER-RECRYSTALLIZED POLYSILICON AS A DEVICE-WORTHY MATERIAL
JAMES F. GIBBONS Stanford Electronics laboratories,
Stanford,
California 94305
ABSTRACT The basic crystallographic and electronic properties of cw laser recrystallized thin polysilicon films are presented and shown to be suitable for fabrication of MOSFET devices. Devices and simple integrated circuits fabricated to explore the potential of this material have electrical characteristics similar to devices fabricated on single crystal material and offer significant promise for future applications. INTRODUCTION Heavily doped polycrystalline silicon (polysilicon) is a material that is widely used in present day silicon integrated circuit technology for gates and interconnection lines in MOS integrated circuits. Initial interest in the beam annealing of polysilicon arose because of its potential as a process that could be used to promote grain growth and hence obtain resistivity reduction in these thin, heavily-doped films. Experiments undertaken to explore that potential [1] show that substantial reductions in resistivity can indeed be obtained and further that the electronic properties of cw beam-recrystallized, heavily doped polysilicon films closely approximate those of single crystal material. This latter result led us to what proved to be successful attempts to fabricate MOS transistors and integrated circuits directly in beam recrystallized polysilicon [2-4]. A substantial interest now exists in the potential of beam recrystallized silicon-on-insulators (SOI) as a substrate for integrated cirThe purpose of this paper is to review the present state of cuit fabrication. this field. Our development of this subject will begin with a discussion of the mechanism by which thin films of polycrystalline Si are recrystallized by a scanning cw beam. In Section 2 we discuss carrier velocity-vs-electric field measurements made on laser recrystallized films. We then describe the oxidation kinetics and interface properties of oxides grown on recrystallized polysilicon films, leading to a discussion of the electrical characteristics of MOSFETs fabricated directly on this material. 1. BEAM RECRYSTALLIZATION OF THIN POLYCRYSTALLINE FILMS We describe first the annealing of thin polysilicon films irradiated by a cw argon laser, with emphasis on the relation between laser power and the surface morphology, dopant distribution and crystal structure of the annealed films. Sample Preparation. The polysilicon films used for these studies were 0.57Mm thick layers deposited by low pressure chemical vapor deposition (LPCVD) on a 1000 A thick layer of either thermally grown Si0 2 or CVD Si3 N4 on a Si substrate. In most cases the depositions were performed in commercial LPCVD reactors currently used for integrated circuit fabrication. After deposition, the wafers were doped by either ion implantation or thermal diffusion and then subjected to the beam processing operations. Beam processing was performed in the scanning apparatus described in [5]
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