In this work, vertical integration of miniaturized array-type Mirau interferometers at wafer level by using multi-stack anodic bonding is presented. Mirau interferometer is suitable for MEMS metrology and for medical imaging according to its vertical-, lateral- resolutions and working distances. Miniaturized Mirau interferometer can be a promising candidate as a key component of an optical coherence tomography (OCT) system. The miniaturized array-type interferometer consists of a microlens doublet, a Si-based MEMS Z scanner, a spacer for focus-adjustment and a beam splitter. Therefore, bonding technologies which are suitable for heterogeneous substrates are of high interest and necessary for the integration of MEMS/MOEMS devices. Multi-stack anodic bonding, which meets the optical and mechanical requirements of the MOEMS device, is adopted to integrate the array-type interferometers. First, the spacer and the beam splitter are bonded, followed by bonding of the MEMS Z scanner. In the meanwhile, two microlenses, which are composed of Si and glass wafers, are anodically bonded to form a microlens doublet. Then, the microlens doublet is aligned and bonded with the scanner/spacer/beam splitter stack. The bonded array-type interferometer is a 7- wafer stack and the thickness is approximately 5mm. To separate such a thick wafer stack with various substrates, 2-step laser cutting is used to dice the bonded stack into Mirau chips. To simplify fabrication process of each component, electrical connections are created at the last step by mounting a Mirau chip onto a flip chip PCB instead of through wafer vias. Stability of Au/Ti films on the MEMS Z scanner after anodic bonding, laser cutting and flip chip bonding are discussed as well.
Some of the critical limitations for widespread use in medical applications of optical devices, such as confocal or optical coherence tomography (OCT) systems, are related to their cost and large size. Indeed, although quite efficient systems are available on the market, e.g. in dermatology, they equip only a few hospitals and hence, are far from being used as an early detection tool, for instance in screening of patients for early detection of cancers. In this framework, the VIAMOS project aims at proposing a concept of miniaturized, batch-fabricated and lower-cost, OCT system dedicated to non-invasive skin inspection. In order to image a large skin area, the system is based on a full-field approach. Moreover, since it relies on micro-fabricated devices whose fields of view are limited, 16 small interferometers are arranged in a dense array to perform multi-channel simultaneous imaging. Gaps between each channel are then filled by scanning of the system followed by stitching. This approach allows imaging a large area without the need of large optics. It also avoids the use of very fast and often expensive laser sources, since instead of a single point detector, almost 250 thousands pixels are used simultaneously. The architecture is then based on an array of Mirau interferometers which are interesting for their vertical arrangement compatible with vertical assembly at the wafer-level. Each array is consequently a local part of a stack of seven wafers. This stack includes a glass lens doublet, an out-of-plane actuated micro-mirror for phase shifting, a spacer and a planar beam-splitter. Consequently, different materials, such as silicon and glass, are bonded together and well-aligned thanks to lithographic-based fabrication processes.
We describe a technological platform developed for miniaturization of optical imaging instruments, such as laser scanning
confocal microscopes or Optical Coherence Tomography devices. The platform employs multi-wafer vertical integration
approach, combined with integrated glass-based micro-optics and heterogeneous bonding and interconnecting technologies.
In this paper we focus on the unconventional fabrication methods of monolithic micro-optical structures and components in
borosilicate glass (e.g. micro beamsplitters, refractive microlenses) for optical beam shaping and routing. In addition, we
present hybrid laser-assisted integration of glass ball microlenses on the silicon MEMS actuators for transmissive beam
scanning as well as methods of electrical signals distribution through thick glass substrates, based on HF etched via holes.
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.