Sealing of poly-SiGe surface micromachined cavities for MEMS-above-CMOS applications

This paper presents for the first time a study of different methods to seal SiGe surface micromachined cavities for above-CMOS MEMS applications. Four different sealing layers are proposed: sputter-deposited AlCu, sub-atmospheric pressure chemical vapour deposited Si-oxide and a porous microcrystalline-SiGe cover in combination with either high-density plasma chemical vapour deposited Si-oxide or AlCu. The maximum processing temperature is kept below 460 °C to allow for the post-processing on top of standard CMOS. To verify the sealing process, optical measurements of membrane deflection were carried out both in air and in vacuum. Analytical modelling and finite element analysis were used to study the load-deflection behaviour of the (poly-SiGe/sealing layer) composite membranes and derive the pressure inside the cavities. In order to study the behaviour of the diaphragms under 0-pressure difference, micro-venting holes were drilled, using focused ion beam, in some of the sealed membranes. The results indicate that both oxide and AlCu layers provide air-tight sealing and that, for AlCu direct sealing, a near-vacuum cavity pressure is obtained.