Low cycle fatigue of single crystal silicon thin films

Hsien-Kuang Liu, BJ Lee, Pang-Ping Liu

Sensors and Actuators A 140 (2007) 257-265

 

Purpose: The work in this study was aimed at understanding the fatigue of single crystal silicon thin films by a cantilever microbeam.  SEM was used to observe the modes of fracture and deformation for various conditions of loading.

Methods: Fatigue testing was conducted using a tungsten probe such that the probe is held firmly against the specimen and subjected to a cyclic load. SEM imaging was used to determine the failure modes of the cantilever under various strain amplitudes. EDS (Engergy Dispersive Spectrometer) was used to analyze the chemical composition of the failed sample.

Key Findings: 

1. Single crystal silicon can fail at strains near one half of their static failure strain for cyclic loading in excess of a million cycles.

2. Dominant failure mode is cleavage at a surface flaw with striations parallel to the length direction.

3. Fatigue crack initiates from the top flat surface of the beam and propagates in the {1 1 1} plane.

4. Two sub-mechanisms are suggested for failure: 1st) fracture debris at incident point inhibits crack propgation, and 2nd) moisture enhances surface reaction resulting in crack formation and eventual failure.