Heterogeneous material integration for MEMS

Abstract: This thesis describes heterogeneous integration methods for the fabrication of microelectromechanical systems (MEMS). Most MEMS devices reuse the fabrication techniques that are found in the microelectronics integrated circuit industry. This limits the selection of materials and processes that are feasible for the realization of MEMS devices. Heterogeneous integration methods, on the other hand, consist of the separate pre-fabrication of sub-components followed by an assembly step. The pre-fabrication of subcomponents opens up for a wider selection of fabrication technologies and thus potentially better performing and more optimized devices. The first part of the thesis is focused upon an adhesive wafer-level layer transfer method to fabricate resistive microbolometer-based long-wavelength infrared focal plane arrays. This is realized by a CMOS-compatible transfer of monocrystalline silicon with epitaxially grown silicon-germanium quantum wells. Heterogeneous transfer methods are also used for the realization of filtering devices, integration of distributed small dies onto larger wafer formats and to fabricate a graphene-based pressure sensor. The filtering devices consist of very fragile nano-porous membranes that with the presented dry adhesive methods can be transferred without clogging or breaking. Pick-and-place methods for the massive transfer of small dies between different wafer formats are limited by time and die size-considerations. Our presented solution solves these problems by expanding a die array on a flexible tape, followed by adhesive wafer bonding to a target wafer. Furthermore, a gauge pressure sensor is realized by transferring a graphene monolayer grown on a copper foil to a micromachined target wafer with a silicon oxide interface layer. This device is used to extract the gauge factor of graphene. Adhesive bonding is an enabling technology for the presented heterogeneous integration techniques. A blister test method together with an experimental setup to characterize the bond energies between adhesives and bonded substrates is also presented.