Fracture toughness of low-pressure chemical-vapor-deposited polycrystalline silicon carbide thin films

The fracture toughness of thin-film polycrystalline silicon carbide (poly-SiC) deposited on silicon (Si) wafers via low-pressure chemical-vapor deposition (LPCVD) has been measured on a scale useful for micromachined devices; the results are compared to previous studies on poly-SiC thin films deposited by atmospheric pressure chemical-vapor deposition (APCVD) [Bellante , Appl. Phys. Lett. 86, 071920 (2005)]. Samples in this study included those with and without silicon dioxide (SiO2) sacrificial release layers. The LPCVD processing technique induces residual tensile stresses in the films. Doubly clamped microtensile specimens were fabricated using standard micromachining processes, and microindentation was used to initiate atomically sharp precracks. The residual stresses in the films create stress intensity factors K at the crack tips; upon release, the precracks whose K exceeded a critical value, K-IC, propagated to failure. The fracture toughness K-IC was the same for both types of devices, 2.9 +/- 0.2 MPa m(1/2) for the SiC on Si samples and 3.0 +/- 0.2 MPa m(1/2) for the SiC on SiO2/Si samples, and similar to that found for APCVD poly-SiC, 2.8 <= K-IC <= 3.4 MPa m(1/2) [Bellante , Appl. Phys. Lett. 86, 071920 (2005)], indicating that K-IC is truly a structure-insensitive material property. The fracture toughness of poly-SiC compares favorably with that for polysilicon, 0.85 +/- 0.05 MPa m(1/2) [Kahn , Science 298, 1215 (2002)].