(adhesion of suspended structures and the underlying surface) is
a considerable problem for batch fabricated MEMS devices. MEMS devices
are often released through wet etching. As the wafer is removed
from the wet etch liquid is trapped between the small space separating
the MEMS device and the substrate. This liquid, through capillary
forces, pulls the cantilevered MEMS device down to the substrate
where it remains. The purpose of this study was repair the stiction
by using a laser-induced stress wave.
Numerous techniques have been explored
to resolve this problem. However, most require significant trade-offs
e.g. non-standard silicon processing, additional design structures,
long release process times, etc.
Cantilevered beams were fabricated
using a standard MEMS process with the exception of the special
release process used after the final isopropyl rinse. The stiction-failed
beams varied from 100 –1000 microns in length, with their
intended substrate separation (after release) of 2 or 4.5 microns.
A Nd:YAG laser pulse was focused to a 3mm diameter spot on the backside
of the Si substrate whose front surface had the MEMS structures,
including the stiction-failed cantilevered beams. The cantilever
beams were examined before and after each laser exposure under an
optical microscope to spot for the released structures. Cantilevered
beams were released starting at 7kJ/m2 and additional beams were
released as the laser energy was increased. It was confirmed that
no adjoining structures were damaged by the stress wave. The whole
process of stiction repair is accomplished literally in few seconds.
This technology can thus be used to substantially increase the yield
of MEMS devices during fabrication, and also to repair in-use stiction.
The invention has been tested and
fully demonstrated on actual MEMS chips that had several cantilevers
failed due to stiction.
Because the process is very simple,
quick (takes few seconds at most) and can be implemented on-line
during batch processing of MEMS structures, operates at very safe
low laser energy densities, and does not involve direct contact
with the MEMS features. A United States Patent is pending for this