Quasi-Static Characteristics of Microstrip on an Anisotropic Sapphire Substrate

The well-defined and repeatable electrical properties of single crystal sapphire make it an attractive substrate material for microstrip, but its dielectric anisotropy constitutes an important design complication. This paper describes investigations into the quasi-static characteristics of single microstrip lines on sapphire substrates cut with a specified orientation. To account for anisotropy, a new permittivity parameter epsilonreq is introduced, which is a function of the Iinewidth to substrate-height ratio W/h. The variation of epsilonreq with W/h is derived by finite-difference methods. Universal curves for microstrip on correctly orientated sapphire are presented, showing 1) Epsilonreq, 2) The low-frequency limit of effective microstrip permittivity epsilone0, and 3) The characteristic impedance of the line Z0, all as functions of W/h.

A method was developed for depositing silicon films by the pyrolytic decomposition of SiH4 on single crystal sapphire. Electron diffraction and Laue reflection examinations of the films shows single-crystal patterns. The silicon film has a Hall mobility of 135 cm²/volt-second at a hole density of 1017/cm³. Insulated-gate field-effect transistors with a transconductance of 1000 µmho at 5 ma were made with dimensions of 10 µ source-to-drain spacing and an active distance of 120 µ. This value compares favorably with similar units made on bulk silicon and is very encouraging for the possibilities of thin-film silicon devices.


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