UW-MEMS - Strain Diagnostic Microstructures



Strain Diagnostic Microstructures
The strain diagnostic microstructures designed here allow an accurate
experimental evaluation of the local strain level. Arrays of these structures
can be used to visually evaluate material stress without destructive procedures.

Compressive stress can be measured with a buckled beam method. In this
method free-standing doubly supported beams fabricated from the material to be
evaluated are fabricated at various lengths but identical cross sections. A
visual inspection of the free beams may be made to determine the smallest length
beam which buckles. This critical length represents a strain related threshold
allowing the film strain to be estimated from beam length and thickness.

The Euler buckling criterion determines the strain in a film for a
clamped-clamped beam: where h is the thickness of the film and lcr is the critical length at which buckling occurs.

Tensile stress can be measured with a ring and beam method. In this
method a doubly supported ring with a weak cross member is fabricated from the
material to be evaluated at various ring diameters (beam lengths). The
cross-member buckles under film tension. The smallest diameter ring whose cross
member buckles can be used to estimate the strain field in the film.

The buckling of a ring and beam structure depends on the conversion
efficiency in transferring tensile strain in the film to compressive strain on
the center beam.

The tensile film stress has been derived as where g(R) is the conversion
efficiency (function of ring radius, ring and beam width, and Poisson's ratio)
and Rcr is the critical radius at which the center beam
buckles.

Thick films can be measured using a similar technique. With thick
films, buckling occurs in the plane of the substrate.

References
H. Guckel, T. Randazzo, D.W. Burns, "A Simple Technique for the Determination
of Mechanical Strain in Thin Films with Applications to Polysilicon," Journal
Applied Physics, 57 (5), March 1985, pp. 1671-1675.
H. Guckel, D. W. Burns, C. R. Rutigliano, D. K. Showers, J. Uglow, "Fine
Grained Polysilicon and its Application to Pressure Transducers," Proceedings of
the 4th International Conference on Sensors and Actuators, Tokyo, Japan, June
1987, pp. 277-282 (Invited Paper).
H. Guckel, D. Burns, C. Rutigliano, E. Lovell, B. Choi, "Diagnostic
Microstructures for the Measurement of Intrinsic Strain in Thin Films," Journal
on Micromechanics and Microsystems, Vol. 2, No. 2, June 1992, pp. 86-95.
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