Fracture and Fatigue of MicroEletroMechanical Systems 

Primary Researchers:

Robert L. Mullen
Department of Civil Engineering
Case Western Reserve University, Cleveland, OH 44106, USA
E-mail: rlm@po.cwru.edu, Tel. (216) 368-2423

Roberto Ballarini
Department of Civil Engineering
Case Western Reserve University, Cleveland, OH 44106, USA
E-mail: rxb7@po.cwru.edu, Tel. (216) 368-2963

National Science Foundation award information
 

Abstract: 
The reliability of Micro Electro Mechanical Systems (MEMS) must determine for robust performance of MEMS devices.  We addressed the mechanical reliability of MEMS. Specific activities conducted under this project include: 
1) Calculation of behavior of polycrystalline materials when the component contains a small number of grains. 
2) Development of a self-contained fracture device for static and fatigue measurements of surface micromachined films. 
3) Initial measurements of fracture properties and microstructural characterizations for various processing parameters used to deposit films. 

The behavior of polycrystalline films was analyzed using a Poisson-Vorinoi tessellation to construct polycrystalline domains.  Finite elements were then used to calculate the elastic properties of the ensemble when each grain had a uniformly distributed random orientation.  From Monte-Carlo calculations, mean, variances and higher order moments were calculated as a function of anisotropic crystal properties and number of grains in a domain.  This work was then extended to calculation of local stress intensity factors in a polycrystalline domain.  The coefficient of variations for highly anisotropic materials was found to be 40%.

A micro fracture device with on-chip loading was designed and fabricated.  The device can statically load to failure a beam with a 2.0 micrometer notch.  Dynamic fatigue loading can also be applied.  The successful device consists of an electrostatic loading frame 1.5 millimeters in length that can produce forces in the hundreds of micro-Newton range.  The fracture specimen is a modified notched cantilever beam, designed to provide mechanical amplification of the loading. 

Fracture properties of polysilicon films were measured using both on-chip and external loading methods. 
Both monotonic and cyclic resonance loading have been applied to the fracture specimen.  A reduction in the nominal toughness under cyclic loading has been observed.  Fatigue fractures has also been observed in specimens subject to as many as 109 cycles.
 

Acknowledgment: The authors acknowledge support from the National Science Foundation's Grant CMS 9416752

CWRU Department of Civil Engineering Communication