Interfacial Electrical and Microstructural Characterization of a Conductive Polysilicon Direct Bonding for MEMS Wafer-Level Packaging

Abstract

Conductive direct bonding can simplify MEMS wafer level integration by creating electrical connections directly at the bonding plane, without adding dedicated metallization schemes or feedthrough modules. It also preserves a higher post bonding thermal budget, which is beneficial for devices requiring high temperatures steps such as resonator and piezoelectric film optimization. Achieving electrical conduction is difficult with hydrophilic direct bonding because the interfacial oxide is electrically insulating. In contrast, hydrophobic direct bonding can avoid forming a continuous oxide and enable direct Si-Si conduction paths, but is more challenging and requires tighter control of surface state and bonding conditions. This work electrically and structurally characterizes in situ doped polysilicon (ISDP) hydrophobic bonding interface. Dedicated electrical test structures show ohmic behavior with contact resistivities as low as $\rho _{c} = 4~\Omega $ . $\mu $ m² across multiple ISDP film variant (different thicknesses, with or without prebonding anneal). TEM reveals locally reconstructed regions where the interface behaves as a grain boundary and is fully closed. Across all ISDP thicknesses and thermal histories, the interfaces exhibit contact resistivities compatible with conductive wafer-level packaging, offering design flexibility for MEMS requiring an all-silicon solution.[2026-0053]