Abstract
Residual stress in films is normally created in the process and results
in the unwanted deformation or fail of structures. It could be characterized
by measuring the changes in the radius of curvature of an overall
stressed film on a substrate, i.e. global residual stress or by the
curvature change of locally micromachined suspended cantilever beams,
i.e. local residual stress. In this study, global and local residual
stress behavior of PECVD silicon carbide films is investigated by
deposition parameters and post rapid thermal annealing to obtain
low stress and flat free-standing microstructure. In the as-deposited
state, a low global compressive film with a stress level of - 160
MPa has been obtained at medium pressure of 147 Pa and at low substrate
temperature of 250 ?C. Then the stress can be further reduced to
less than 100 MPa with low local gradient stress by a post annealing
at 420 ?C. The final stress could be effectively controlled by a
discrete annealing method, approaching a steady state only after
several minutes. It is comparable with the complementary metal-oxide-semiconductor
(CMOS) process. As the annealing temperature increases, a stress
transition from the compressive region to the tensile one takes place.
The stress relaxation is attributed to the dissociation of Si-H and
C-H bonds, and incorporation of hydrogen molecules in the film for
the formation of tensile Si-C bonds with large electronegativity
difference or the outdiffusion of hydrogen molecules to increase
the bond length between atoms for a more tensile stress. Self-deformed
micromachined cantilevers are fabricated to exhibit the local gradient
stress from the curvature of beams. The gradient stress is negative
for the cantilevers with negative or downward curvature in the as-deposited
state and then transforms to positive for the cantilevers with positive
or upward curvature after annealing. The gradient stress varied from
negative to positive is attributed to the difference of stress variation
between the previous termfreenext term surface and the interface
of the film and substrate. A low stress film of less than 100 MPa
with a near-zero gradient component could be adjusted by as-deposited
process parameters and post annealing for the fabrication of the
flat suspended MEMS structure.
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