research activities in 2007 - CSEM

MicroMec – Microtechnology for Silicon Compliant Structures
C. Verjus, J.-M. Major, T. Overstolz, A. Hoogerwerf, A. Ibzazene, A. Neels, A. Schifferle, A. Dommann
The target of this project is the development of a microfabricated silicon compliant structure for mechanical applications and its understanding of the
aging behaviour.
MEMS can be made highly reliable, but it must however be
noted that the failure modes of MEMS can be different from
those of solid-state electronics. Therefore testing techniques
must be developed to accelerate MEMS-specific failures [1] .
Monocrystalline material and, especially, silicon is
preferentially used due to its potential resistance against
aging. However, quantified results of this fact are rarely
published. The reasons are manifold; however they are also
related to the surface roughness as well as to the defect
concentration of the etched surfaces due to the ion
bombardment [2] .
Deep reactive ion etching (DRIE) of silicon-on-insulator (SOI)
substrates allows the fabrication of structures with arbitrary
shapes (2D) that are vertically extruded by removing excess
silicon.
Test structures can be built from single crystalline silicon by
DRIE processes. Mechanical tests on these structures in
relation with simulations of stresses and the experimental
determination of the strain / stress behavior and defect
analysis by High Resolution Diffraction Methods (HRXRD)
give very important information about the device and its long
term stability. A silicon beam structure processed by DRIE
(Figure 1) has been studied. Simulations have been done
related to mechanical shifts applied to the entire silicon
structure which results in the generation of strain in the small
silicon beams having a thickness of 50 µm. In dependance of
the position of the beam in the structure, stresses ranging
from about 60 to 1100 MPA are calculated by simulations.
Figure 1: Silicon beam structure processed by DRIE
High resolution x-ray diffractometry (HRXRD) measures the
strain of a crystal. This is an accurate, non destructive method
applied in the field of MEMS to obtain quantified results on the
crystalline disorder.
CSEM therefore applies an X-ray rocking curve method, which
measures the strain of a crystal as well as the defect
concentrations.
Applying a mechanical force to a perfect silicon single crystal
results in a deformation which is directly related to a change of
the crystal strain profile. In addition, reciprocal space mapping
(RSM) visualizes the strain generation related to the bending
of the thin Silicon beam. The appearance of diffused
scattering in the RSM (Figure 2) is related to the beam
bending strain. Elastic deformation in the test structure was
observed.
Figure 2: Diffused scattering in the RSM of compliant structure
The study of mechanical tests combined with simulations and
related HRXRD measurements results in a better
understanding of the material properties. The generation of
defects in the material and their increase related to
mechanical stresses or other environemental influences is an
important issue as it is directly related to the device
performance and its long term stability.
[1] A. Dommann, G. Kotrotsios, A. Neels, MEMS Reliability and
Testing, MST News, (2007)
[2] E. Mazza and J. Dual, Mechanical behavior of a µm-sized single
crystal silicon structure with sharp notches. J. Mechanics and
Physics of Solids 47 (1999) 1795-1821
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