Pulsed Laser Recrystallization of Polysilicon: Analysis Via a Novel SEM Technique
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Laser andElectron-Beam Solid Interactions and materials Processing
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PULSED LASER RECRYSTALLIZATION OF POLYSILICON: ANALYSIS VIA A NOVEL SEM TECHNIQUE RAJIV R. SHAH AND D. LLOYD CROSTHWAIT Semiconductor Research and Development Laboratories, Texas Instruments Incorporated, P.O. Box 225012, Dallas, Texas, 75265
ABSTRACT Large crystals of silicon were obtained via pulsed laser annealing of thin films of fine grain CVD polycrystalline silicon. These films were analyzed using a novel technique that provides rapid feedback of crystallographic and defect information. The technique uses very shallow angle metallurgical sections in conjunction with chemical decoration and scanning electron microscopy and results in excellent depth resolution (100 pm). The technique was used to reveal conversion of very large areas (- 1 mm 2 ) of polysilicon films deposited on silicon into single crystal film. albeit with point defects, without requiring the melt depth to reach the polysilicon/silicon
interface. A gradual transition from single crystal to increasingly polycrystalline material was observed going from top to bottom of the initially uniform polycrystalline film depending on the pulse energy used. Polysilicon on top of an oxide layer, on the other hand, transforms into large (- 10 pm) single crystals with grain boundaries penetrating the entire polysilicon film thickness. These experimental results shed new light on two very important questions: (1) Is laser annealing entirely an epitaxial process or is it strongly influenced by the thermal properties of the underlying substrate, and (2) Whether substantial regrowth of polysilicon requires equilibrium thermodynamics. INTRODUCTION Chemically vapor deposited polycrystalline silicon has been used extensively in semiconductor processing. These uses have been primarily in MOS technology for the fabrication of gate electrodes, interconnections and resistors. Conventional polysilicon, however, has thus far found only limited applications in the fabrication of bipolar devices and circuits. Even the highly prevalent use of polysilicon in MOS structures is fraught with limitations. These limitations in the use of polysilicon in conventional device structures stem, in one form or another, from its small grain size. The small grain size of polysilicon causes the polysilicon to have poor transport characteristics which also result in poor electrical performance of unconventional bipolar and MOS device structures fabricated in polysilicon. The advent of laser processing offers the promise of changing this situation by drastically improving the structural and electrical properties of CVD polycrystalline silicon. The potential applications of lasers to the processing of polysilicon may be divided into two broad categories: (1) Near-term applications that are compatible with existing device, circuit and process technologies and may be implemented relatively easily without a considerably better understanding of laser recrystallization of polysilicon, and (2) long-term applications th
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