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MEMS are tiny devices (with dimensions measuring less then 100 microns) that are generally integrated on a larger silicon substrate and controlled by electronic circuits to perform mechanical functions.
Many MEMS devices involve the binding of thin films of distinct materials.
The drive to create smaller new devices to perform critical functions requiring high stress levels makes mechanical characterization at nanoscales a design priority.
As device dimensions decrease, the number of intrinsic volume defects such as dislocations and grain boundaries (for poly crystalline materials) decreases.
However, with decreasing sizes, the surface-to-volume ratio increases.
Which makes the MEMS structure more prone to stress. Thus the fracture.
CRACK DEFORMATION MODES
The stress field has a tendency to deform the crack in three distinct modes.
A)Mode-1 (The Opening Mode)
B)Mode-2 (The Sliding Mode)
C)Mode-3 (The Tearing Mode)
The opening mode (Mode-1) is associated with local displacements in which the crack surfaces tend to move apart in a direction perpendicular to their surfaces.
The edge-sliding mode (Mode-2) is characterized by displacements in which the crack surfaces slide over one another and remain perpendicular to the leading edge of the crack.
The tearing mode (Mode-3) is defined by the crack surfaces sliding with respect to one another parallel to the leading edge of the crack.
The stress component ( φ ) is given by,
The displacement component ( ρ ) is given by,
= stress intensity factor to asses the stress field surrounding
r and = depends on crack length and geometry of solids
HOW TO TEST FRACTURE ?
The methods proposed for fracture testing at small size scales consisted of modiﬁcations to well-established macro-scale methods such as
A) Tensile Testing
B) Fatigue Testing
C) Torsion Testing
In order to determine critical stresses for microstructures, were subjected to a tensile test
The force acting on the specimen is determined using a high resolution balance, while the strain is determined using a video camera and a least square template matching algorithm (LSM) which yields a super resolution in the specimen extension around 10nm under an optical microscope.
It is based on the fact that upon crack propagation the resonance frequency of a structure in bending decreases in a monotonous way with increasing crack length.
By using a phase-locked feedback loop, the resonance frequency can be tracked, such that frequency vs. time curves result
The torque is determined from the rotation of a bar, which is supported with a calibrated torsional spring.
The rotation of the bar is measured with a two point optical fiber interferometer .
FINITE ELEMENT STRESS ANALYSIS (FEA)
almost all MEMS and micro-systems involve complex 3-d geometry.
These devices are fabricated under severe thermal and mechanical conditions and subject to harsh loading during operation.
Credible stress analysis of these complex systems cannot achieved by conventional methods.
The finite element analysis (FEA) is the only way to attain credible solutions.
PRINCIPLE OF FEA
The principle of FEA can be summarized in two headings.
Place denser and smaller elements in the parts of the structure with an abrupt change of geometry where high stress or strain concentrations are expected.
Avoid using elements with high aspect ratio, which is defined as the ratio of the longest dimension to the shortest one in the same element.(the user is advised to keep this ratio below 10)
In addition to these techniques, a number of other methods for
material testing of micro-materials exist, which have received increasing attention.
They offer insight into many interesting phenomena (like bending, micro-tensile and torsion tests) Together with elaborate manufacturing possibilities as known from MEMS fabrication, they offer possibilities of probing material behavior in the nm range.
With this it is hoped that an improved understanding of processes important for the life time and reliability of MEMS components will be possible in the near future.