  A femtosecond laser is a powerful tool for micromachining of silicon. In this
work, large-area laser ablation of crystalline silicon is comprehensively
studied using a laser source of pulse width 300 fs at two wavelengths of 343 nm
and 1030 nm. We develop a unique approach to gain insight into the laser
milling process by means of detailed analysis of trenches. Laser scribed
trenches and milled areas are characterized using optical profilometry to
extract dimensional and roughness parameters with accuracy and repeatability.
In a first step, multiple measures of the trench including the average depth,
the volume of recast material, the average longitudinal profile roughness, the
inner trench width and the volume removal rate are studied. This allows for
delineation of ablation regimes and associated characteristics allowing to
determine the impact of fluence and repetition rate on laser milling. In a
second step, additional factors of debris formation and material redeposition
that come into play during laser milling are further elucidated. These results
are utilized for processing large-area (up to few mm2) with milling depths up
to 200 {\mu}m to enable the fabrication of cavities with low surface roughness
at high removal rates of up to 6.9 {\mu}m3 {\mu}s-1. Finally, laser processing
in combination with XeF2 etching is applied on SOI-CMOS technology in the
fabrication of radio-frequency (RF) functions standing on suspended membranes.
Performance is considerably improved on different functions like RF switch (23
dB improvement in 2nd harmonic), inductors (near doubling of Q-factor) and LNA
(noise figure improvement of 0.1 dB) demonstrating the applicability of milling
to radio-frequency applications.
