Laser Microfabrication Technology and its Application to High Speed Interconnect of Gate Arrays
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LASER MICROFABRICATION TECHNOLOGY AND ITS APPLICATION TO HIGH SPEED INTERCONNECT OF GATE ARRAYS Anthony F. Bernhardt, Bruce M. McWilliams, Fred Mitlitsky and John C. Whitehead Lawrence Livermore National Laboratory, Livermore, CA 94550 Abstract Nickel and doped polysilicon lines can be written at speeds exceeding 1000 microns/sec using laser direct-write deposition. We explore the roles of gas pressure, composition, and laser power in determining writing speed and line morphology. The use of a surface layer of amorphous silicon provides optical absorption, thermal and electrical insulation which help to maintain high, relatively stable, surface temperature. Laser direct-write deposition is used to interconnect CMOS gate arrays by means of computer controlled laser pantography. Complex circuits, such as an array of five 16-stage shift registers and one 16-stage counter have been successfully fabricated and tested. I. Introduction A goal of the LLNL Laser Pantography (LP) program has been demonstrating processes in which a computer-steered and computer-modulated laser beam directly deposits or removes material onto or from a substrate such as a silicon wafer [1]. Substantial advantages could accrue from a fully developed set of such processes, including:
"* Lower
cost for prototyping and low volume manufacturing. The costly tooling phase and complex wafer processing sequence of conventional integrated circuit fabrication are eliminated, making limited-volume manufacturing less expensive.
"* Faster
fabrication. We estimate that two-level interconnect for very-large-scale integrated (VLSI) circuits can be fabricated on 36 hour time scales, with actual laser writing time of about one hour per interconnect level per die.
"* On-line
repair. Because LP is a serial process, defects observed during fabrication (for example, a shorted interconnecting structure) can be repaired under computer control.
"* Customized
computers. With simpler, faster, and more efficient manufacture, it becomes economically practical to fabricate computers customized for specific applications, even if only a few are built.
II. Laser Deposition Processes A variety of laser-induced interconnection schemes has been studied [2]. These fall into two broad classes: photolytic and pyrolytic. The photolytic processes are based on local laser-induced photodecomposition reactions occurring either at the substrate and/or in the gas phase. The pyrolytic processes are based on chemical vapor deposition reactions which occur at the laser heated substrate. Our research deals exclusively with pyrolytic processes. Some of the considerations leading to this choice include:
Mat. Res. Soc. Symp. Proc. Vol- 75. 11987 Materials Research Society
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Heat Transfer. Using pyrolysis, very rapid deposition rates are obtained due to the high working temperature at the laser-heated spot on the surface. Such high temperatures (> 1400 K) do not thermally compromise nearby structures due to the three-dimensional nature of the heat transfer to the substrate. (For exam
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