Multiple Through-wafer Interconnects for MEMS Applications

Abstract

This chapter reports on the design and manufacturing of multiple through-wafer interconnects for stacking of microelectromechanical devices. The feedthroughs have been applied to an interconnect (intermediate) layer for an integrated microphone. The integrated microphone consists of the transducer part and an application specific integrated circuit (ASIC). The two parts are joined by stacking them on top of each other with the interconnect (intermediate) layer between them. The intermediate layer is a multifunctional layer. It provides the microphone with an acoustic frontchamber. The intermediate layer ensures the interchange of acoustical and electrical signals with the environment. Also, the intermediate layer protects the microphone during dicing, chip handling and in operation. The two active components are electrically connected through the intermediate layer by multiple wafer frontside to backside interconnects. The feedthrough interconnects are provided with under bump metallizations (UBM), solder bumps and sealing rings on the microphone side and top surface metallizations (TSM) on the ASIC side. The entire metallization system is based on electroplating techniques. The patterning of the metallizations has been done utilizing an electrodepositable photoresist (EDPR) as a plating mold. Several EDPR processes have been developed which are optimized for the respective metallization. The feedthrough interconnects have been optimized in terms of series resistance and parallel capacitance. Electrical characterization of the feedthrough interconnects has been carried out analytically and experimentally. The series resistance of one single interconnect wire is in the order of 100 mX. The parallel (parasitic) capacitance is in the order of 2 pF. Coupling between two adjacent feedthrough interconnects is negligible for low relative humidity (<50%). For high relative humidity (>90%) the impedance between two wires may be reduced due to a condensed water film. Sensitivity measurements performed before and after bonding of the interconnect layer to the microphone showed identical results which proves sufficient (TX) isolation between the feedthrough wires.