  High Q-factor inductors are critical in designing high performance
RF/microwave circuits on SOI technology. Substrate losses is a key limiting
factor when designing inductors with high Q-factors. In this context, we report
a substrate engineering method that enables improvement of quality factors of
already fabricated inductors on SOI. A novel femtosecond laser milling process
is utilized for the fabrication of locally suspended membranes of inductors
with handler silicon completely etched. Such flexible membranes suspended
freely on the BOX show up to 92 % improvement in Q factor for single turn
inductor. The improvement in Q-factor is reported on large sized inductors due
to reduced parallel capacitance which allows enhanced operation of inductors at
high frequencies. A compact model extraction methodology has been developed to
model inductor membranes. These membranes have been utilized for the
improvement of noise performance of LNA working in the 4.9,5.9 GHz range. A 0.1
dB improvement in noise figure has been reported by taking an existing design
and suspending the input side inductors of the LNA circuit. The substrate
engineering method reported in this work is not only applicable to inductors
but also to active circuits, making it a powerful tool for enhancement of RF
devices.
