We describe an original integration technological platform for the miniaturization of micromachined on-chip optical microscopes, such as the laser scanning confocal microscope. The platform employs the multi-wafer vertical integration approach, combined with integrated glass-based micro-optics as well as micro-electro-mechanical systems (MEMS) components, where the assembly uses the heterogeneous bonding and interconnecting technologies. Various heterogeneous components are disposed in vertically stacked building blocks (glass microlens, MEMS actuator, beamsplitter, etc.) in a minimum space. The platform offers the integrity and potential of MEMS microactuators integrated with micro-optics, providing miniaturized and low cost solutions to create micromachined on-chip optical microscopes.

OCT instruments permit fast and non-invasive 3D optical biopsies of biological tissues. However, they are bulky and expensive, making them only affordable at the hospital and thus, not sufficiently used as an early diagnostic tool. Significant reduction of system cost and size is achieved by implementation of MOEMS technologies. We propose an active array of 4x4 Mirau microinterferometers where the reference micro-mirrors are carried by a vertical comb-drive microactuator, enabling the implementation of the phase-shifting technique that improves the sensitivity and eliminates unwanted interferometric terms. We focus on the design of the imaging system, the microfabrication and the assembly of the Mirau microinterferometer, and the swept-source OCT imaging.

In this paper, Ag-sinter and CuSn-transient liquid phase based joining technologies for attaching power die to lead frame are investigated. The fo-cus will be set on the evaluation of the process pa-rameters, such as sinter temperature and bonding pressures, with respect to their reliability. For this reason, characterization of the intermediate bond-ing layer and of the bonded die were performed and power cycling tests in conjunction with finite element analysis were conducted. The results will be detailed within the paper and compared with classical PbSn-solder joint technologies.

To increase the yield of the wafer-level Cu-Cu thermo-compression bonding method, certain surface pre-treatment methods for Cu are studied which can be exposed to the atmosphere before bonding. To inhibit re-oxidation under atmospheric conditions, the reduced pure Cu surface is treated by H2/Ar plasma, NH3 plasma and thiol solution, respectively, and is covered by Cu hydride, Cu nitride and a self-assembled monolayer (SAM) accordingly. A pair of the treated wafers is then bonded by the thermo-compression bonding method, and evaluated by the tensile test. Results show that the bond strengths of the wafers treated by NH3 plasma and SAM are not sufficient due to the remaining surface protection layers such as Cu nitride and SAMs resulting from the pre-treatment. In contrast, the H2/Ar plasma–treated wafer showed the same strength as the one with formic acid vapor treatment, even when exposed to the atmosphere for 30 min. In the thermal desorption spectroscopy (TDS) measurement of the H2/Ar plasma–treated Cu sample, the total number of the detected H2 was 3.1 times more than the citric acid–treated one. Results of the TDS measurement indicate that the modified Cu surface is terminated by chemisorbed hydrogen atoms, which leads to high bonding strength.

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.

This paper addresses both, the thermal and the thermo-mechanical performance of percolating thermal underfill applied flip-chip packages. We present a thermal test platform in flip-chip package design allowing the thermal conductivity of any underfill to be measured at package scale. We give details about design technology and current fabrication status. We benchmarked the thermal performance of the platform with percolated thermal underfill against the capillary thermal underfill case. In a layout with peripheral solder bumps the performance benefit related to heat dissipation of an overall system with percolating thermal underfill can be over 50% for liquid cooling and over 25% for enforced convection. The improvement for natural convection, typical in mobile environment, is 5% in the best considered case. We also present a study of the thermo-mechanical performance of the flip chip test platform during thermo-shock cycling by the aid of finite element (FE) tools at the critical regions of interest. We benchmarked the percolating thermal underfill against capillary mechanical underfill and capillary thermal underfill. For that investigation we used an effective material merging a visco-elastic polymer and a visco-plastic solder for saving computation time. In the percolating thermal underfill applied package, we've found slightly higher risk of delamination at the die | underfill interface but very low fillet loads compared to conventional underfill. The risk of solder fatigue is significantly lower and the risk of die cracking is 25% higher for percolating than for the capillary underfills.

Efficient heat dissipation is of major importance in advanced packages of high performance integrated circuits (IC's). To ensure and extend the integration density, 3D chip stacking with multiple silicon dies vertically arranged on top of each other is indispensable. However, these packages require an enhanced thermal management, in order to dissipate the heat from each stacked die to the heat sink. This report presents a process flow for an advanced concept of percolating thermal underfills using the sequential assembly of micron- and nano-sized particles. Due to an improved connection of the micron-sized particles by bridges of a nano-sized material, a highly percolated network with increased thermal paths is obtained. A thermal conductivity of up to 3.8 Wm-1K-1 was already demonstrated for the resulting composite material. This study focuses on three main process steps, the centrifugal filling of microparticles into a defined silicon cavity to form a percolating particle bed, the self-assembly of nanoparticles around the contact points of the microparticles by capillary bridging (so-called neck formation) and the capillary backfilling of the formed particle network with an epoxy. For the centrifugal filling, silica and alumina spheres with diameters ranging from 27 micrometer to 30 micrometer and 25 micrometer to 36 micrometer, respectively, were dispensed into a rotating filling plate. As a substitute for the solder ball interconnect bond line within a chip stack, fabricated silicon cavities with different pillar layouts are filled with microparticles. The dependencies of the fill fraction, fill front, packing structure and occurring defects on the rotational speed are studied. Particularly, an empty space in the particle bed behind the pillars in fill direction (referred to as shadowing) appeared as defect. Furthermore, the work discusses processing aspects to form necks by capillary bridging between the microparticles. For the initial experiments, metal-based nano-particle inks are assembled into the contact points of the micron-sized spheres, directed by the surface tension during drying. The concentration of the nanoparticle suspension, as well as the drying and sintering temperatures, and times are discussed. In addition, further defects in terms of microparticle rearrangements, voids and air inclusions were detected for different solvents and nanoparticle inks. Finally, the formed particle network is infiltrated by a two component epoxy system. X-ray computed tomography (CT) analysis und SEM images of the cross section of a cavity are used to evaluate the entire composite material.

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.

The paper presents the multi-wafer bonding technology as well as the integration of electrical connection to the z-scanner 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 focus-adjustment 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 degrees C. To be compatible with MEMS devices, bonding of different material stacks at temperatures lower than 400 degrees 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 degrees 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.

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.