Effect of alternating a-C:H multilayer full coating on fracture behavior of single-crystal silicon-based microstructure in tensile and toughness tests

The use of alternating hydrogenated amorphous carbon (a-C:H) full coatings on all sides of a single-crystal silicon (SCS) based microstructure was proposed to explore the enhancement of the tensile strength and fracture toughness of a coating-substrate system. By tailoring the individual layer thickness (lambda) from 25 to 150 nm, we synthesized a series of multilayer coatings deposited alternately by controlling bias voltage between -200 and -600 V with the total thickness maintained at 300 nm by plasma enhanced chemical vapor deposition (PECVD). The tensile strength and fracture toughness of coating-substrate system were investigated by a quasi-static tensile test and pillar splitting, respectively. A -37.8% increase in the tensile strength (maximum of 4.34 GPa) and -41.5% increase in the fracture toughness (maximum of 1.33 MPa m1/2) of coated samples were achieved as compared with a bare silicon microstructure. The highest tensile strength was found at a lambda of 75 nm, whereas fracture toughness increased monotonically as lambda dropped from 150 to 25 nm. We attributed this lambda dependence to a combined effect of toughness enhancement and crack shielding. These new findings were useful for the design of alternating multilayer coated micro electro mechanical system (MEMS) devices with enhanced mechanical reliability.