The fracture toughness of polysilicon microdevices

The development of polysilicon fracture mechanics specimens with characteristic dimensions comparable to those of typical microelectromechanical systems (MEMS) devices is presented. The notched cantilever specimens are fully integrated with a simultaneously microfabricated electrostatic actuator, which allows on-chip resting of the specimens without the need of an external loading device, and without any possible influences from external sources. Under monotonic loading, the average maximum tensile stress (strength) and average nominal fracture toughness were measured as 4.2 GPa and 3.5 MPa-m(1/2) for boron-doped specimens, and 5.0 Cpa and 4.0 MPa-m(1/2) for undoped specimens. An average modulus of rupture of 3.3 GPa and average nominal toughness of 2.7 MPa-m(1/2) were measured for specimens cracked under cyclic resonance loading. The differences between the monotonic loading and cyclic loading data are attributed to fatigue initiation of a sharp crack from the 1 mu m radius notch. The experimental data is consistent with a critical flaw size in the fabricated devices, a, that is related to the fracture toughness K-Ic by K-Ic/a(1/2)=4600 MPa.