Residual stress in films is normally created in the process and results in the unwanted deformation or fail of structures. It could be characterized by measuring the changes in the radius of curvature of an overall stressed film on a substrate, i.e. global residual stress or by the curvature change of locally micromachined suspended cantilever beams, i.e. local residual stress. In this study, global and local residual stress behavior of PECVD silicon carbide films is investigated by deposition parameters and post rapid thermal annealing to obtain low stress and flat free-standing microstructure. In the as-deposited state, a low global compressive film with a stress level of -160 MPa has been obtained at medium pressure of 147 Pa and at low substrate temperature of 250 °C. Then the stress can be further reduced to less than 100 MPa with low local gradient stress by a post annealing at 420 °C. The final stress could be effectively controlled by a discrete annealing method, approaching a steady state only after several minutes. It is comparable with the complementary metal-oxide-semiconductor (CMOS) process. As the annealing temperature increases, a stress transition from the compressive region to the tensile one takes place. The stress relaxation is attributed to the dissociation of Si-H and C-H bonds, and incorporation of hydrogen molecules in the film for the formation of tensile Si-C bonds with large electronegativity difference or the outdiffusion of hydrogen molecules to increase the bond length between atoms for a more tensile stress. Self-deformed micromachined cantilevers are fabricated to exhibit the local gradient stress from the curvature of beams. The gradient stress is negative for the cantilevers with negative or downward curvature in the as-deposited state and then transforms to positive for the cantilevers with positive or upward curvature after annealing. The gradient stress varied from negative to positive is attributed to the difference of stress variation between the free surface and the interface of the film and substrate. A low stress film of less than 100 MPa with a near-zero gradient component could be adjusted by as-deposited process parameters and post annealing for the fabrication of the flat suspended MEMS structure.
All Science Journal Classification (ASJC) codes
- 化學 (全部)