Rapid Silicon-to-Steel Bonding by Induction Heating for MEMS Strain Sensors

A rapid and manufacturable technique to bond silicon to steel for microelectromechanical system (MEMS) sensor applications is presented. Using an induction heating system, the process is simple, clean, inexpensive, and fast, with bonding occurring in 3-5 s. Heat generation is isolated to within several micrometers below the steel surface, allowing for a controllable thermal budget that does not damage the surface treatment of steel. The process requires minimal surface conditioning and may be used with a variety of different metal systems. Bonds formed with adhesion layers of gold, silver, and nickel showed excellent mechanical characteristics, surviving surface strains exceeding +/- 2000 με. The transferred strain was quantified by ANSYS simulation and measured experimentally for silicon substrates of varying thicknesses, with the 100 μm-thick substrates demonstrating the greatest strain sensitivity. A double-ended tuning fork MEMS resonant strain sensor, bonded to a steel parallel, demonstrated a repeatable 5.4-fold improvement in strain sensitivity compared to epoxy bonding. As such, this robust bonding process is well-suited for bonding MEMS sensors to monitor components in the automotive, manufacturing, and other harsh environment industries.