Stresses in SiC MEMS test structures

This work presents the growth and characterisation of b SiC films on silicon by CVD, performed in a horizontal, cold-walled reactor. Test structures of different geometries were used to determine the stresses in the film fabricated either by etching through the silicon from underneath the wafer to produce membranes or an attack from the front to reveal cantilevers. Membranes and cantilevers presented both compressive and tensile strain, depending on film thickness. The mechanisms for the cause of the strain, due to both thermal and lattice mismatch and chemical effects, are compared. The evolution of the stresses as a function of film thickness can be correlated with changes in morphology and crystal structure. For very thin films the crystal structure is highly defected as confirmed by X ray crystallography and electron microscopy. The XRD data gives ambiguous information regarding the crystallinity of the film; however, when this data was correlated with information from high resolution microscopy it was possible to interpret the diffraction data correctly. As the film becomes thicker the crystal lattice relaxes even though compressive stresses are still observed in the MEMS test structures. The use of µ-Raman spectroscopy as an auxiliary tool for assessing local stresses is also discussed.