Electromagnetic modeling of an integrated micromachined inductive microphone

This paper presents a detailed electromagnetic modeling for a new structure of a monolithic CMOS micromachined inductive microphone. This study is of capital importance for the optimization of this new microphone structure based on the variation of mutual inductance between an external fixed inductor and an internal suspended inductor. A prototype fabrication of this microphone has been done in a CMOS compatible process using a 0.6 mum technology. The modeling results show that the use of an AC current in the primary fixed inductor gives a much larger induced voltage in the secondary inductor comparing to the case when a DC current is used. The induced voltage evaluated, in the muV range using a DC current solution, is multiplied by a factor of 3 to 6 when using the AC alternative current. We find a value of 1 nA for the current induced in the secondary inductor, which indicates that the Laplace force generated will have a negligible effect on the membrane displacement. We have proposed the use of the symmetric dual-layer spiral inductor structure that can increase the induced emf value by a factor of 4.