Fabrication and stress relief modelling of GaN based MEMS test structures grown by MBE on Si(111)

The fabrication of III-N MEMS test structures, such as cantilevers, beams and stress-pointers, and the modelling of their deformation due to residual stress relief, is presented. GaN and AlGaN/GaN structures were fabricated, either with one end and both ends clamped to the Si substrate (asymmetrical and symmetrical mechanical boundary conditions, respectively). The residual stress in the III-N layer was measured by photoluminescence and X-ray diffraction, and the stress relief induced deformation was analysed by a finite element method model. The deformations of the MEMS structures were used to calculate both the residual strains and the Young’s modulus of the material. One-end-clamped structures suffer from large deformations due to the uneven stress relaxation. During micromachining, the relaxation induces large upward buckling, as measured for fabricated devices and fitted by the FEM model. Two-end-clamped structures were also studied using different topologies and under-etching lengths of the clamped region. It is concluded that the deformation of such structures may be reduced with symmetrical mechanical boundary conditions and a small under-etched clamping region compared to the device dimensions