The paper presents the multi-wafer bonding technology as well as the integration of electrical connection to the zscanner
wafer of the micromachined array-type Mirau interferometer. A Mirau interferometer, which is a key-component
of optical coherence tomography (OCT) microsystem, consists of a microlens doublet, a MOEMS Z-scanner, a focusadjustment
spacer and a beam splitter plate.
For the integration of this MOEMS device heterogeneous bonding of Si, glass and SOI wafers is necessary. Previously,
most of the existing methods for multilayer wafer bonding require annealing at high temperature, i.e., 1100°C. To be
compatible with MEMS devices, bonding of different material stacks at temperatures lower than 400°C has also been
investigated. However, if more components are involved, it becomes less effective due to the alignment accuracy or
degradation of surface quality of the not-bonded side after each bonding operation.
The proposed technology focuses on 3D integration of heterogeneous building blocks, where the assembly process is
compatible with the materials of each wafer stack and with position accuracy which fits optical requirement. A
demonstrator with up to 5 wafers bonded lower than 400°C is presented and bond interfaces are evaluated.
To avoid the complexity of through wafer vias, a design which creates electrical connections along vertical direction by
mounting a wafer stack on a flip chip PCB is proposed. The approach, which adopts vertically-stacked wafers along with
electrical connection functionality, provides not only a space-effective integration of MOEMS device but also a design
where the Mirau stack can be further integrated with other components of the OCT microsystem easily.