Around 10% of the population will have to go through a catheterization procedure for the treatment of a cardiovascular disease at a certain stage of their lives. During such a procedure, smart catheters will be the "eyes and ears" of the surgeons, significantly improving the diagnosis and treatment. However, there have been very limited improvements and innovations in smart catheters over the past decade, as most smart catheters are manufactured with technical point solutions, and therefore cannot sustain themselves with enough production volume for continuous innovation. Consequently, Flexto- Rigid (F2R) was developed as an interconnect platformfor heterogeneous integration of electronic components in submillimeter formfactors. F2R is an open technology platformthat can serve many smart catheter applications from a variety of manufactures. It consists of multiple small and thin silicon islands connected by thin flexible interconnects, which allows devices and components to be mounted with standard assembly techniques or directly fabricated onto the F2R platform. This thesis presents innovations in F2R-based applications, integration, and process optimization for smart catheters. The first part of the thesis is an example of applying F2R for making a miniaturized device, a submillimeter optical data link module (ODLM). With smart catheters migrating from analog to digital instruments, an optical interposer is needed to realize highspeed optical data transmission. The biggest challenge is the form factor of the optical interposer, as it needs to fit into a catheter tip that is inserted inside human veins. This challenge falls exactly in the scope of F2R. The ODLM was fabricated, assembled, and integrated into an ICE catheter demo system. The second part of the thesis presents high-density embedded trench capacitor integration in the F2R platform. Compared to assembling discrete capacitors on F2R, embedded capacitors in the F2R substrate save space in the catheter tip and bring the decoupling capacitors directly underneath the ASICs, resulting in better performance. The work involved the trench capacitor process development, especially the high-aspect ratio (HAR) DRIE trench etching process. More importantly, the trench capacitor process was optimized to be compatible with the standard F2R process. The last part of the thesis presents the work on improving the fabrication process of the F2R platform. The largest bottleneck and most critical step of F2R is the "buried trench" process, which creates separated thin silicon islands. The buried trenches consist of thin oxide membranes, that are very sensitive to thin-film stress and other mechanical forces, resulting in reduced production yield. Cavity-BOX SOI eleminates the "buried trench" process by introducing a patterned buried oxide layer. The patterned buried oxide mask allows an intact wafer surface during the process until the final DRIE process, which separates the wafer in one go using this oxide mask. The production yield can be significantly improved using the cavity-BOX SOI for the F2R process. A deep brain stimulation (DBS) probe test structure was fabricated with the cavity-BOX SOI based F2R process to demonstrate the technology concept. A method to align the patterns on the wafer to the patterned buried oxide mask was developed. @en

Photonic applications often face industry-specific assembly requirements that can be challenging to overcome. There is a need for micron-accurate alignment, device miniaturization and integration in challenging form factors. Special materials and packaging must be developed for the best performance of the devices in exacting environmental conditions. In the medical field, these requirements are additionally complicated by the demanding ISO 13485 regulations. The article demonstrates how solutions are found in a combination of new and existing technology in microelectronics packaging and now made suitable for photonics.@en

The digitization of smart catheters will dramatically increase the demand for reliable and high data transmission in the distal tips. Optical fiber is a good candidate to provide high-speed data transmission. However, the extremely small size of the smart catheter tip, with less than a few millimeters in diameter, hampers the integration of optical fiber connections in the catheter tip. Our work presents a stand-alone optical data link module (ODLM) with a dimension of 240 μm × 280 μm × 420 μm for use in a 1 mm diameter intravascular ultrasound (IVUS) smart catheter. The fabrication of the ODLM is based on the Flex-to-Rigid (F2R) integration technology. In the ODLM, the flexible interconnects reroute the electrical contacts of the flip-chipped vertical-cavity sur-face-emitting laser (VCSEL) to the side of the device. This design enables the ODLM to be mounted on a flex-PCB and fit into a 200-300 μm gap in the IVUS catheter tip. An optical fiber that runs parallel to the catheter shaft is self-aligned to a commercially available VCSEL by inserting it into the through-silicon hole (TSH) of the ODLM. Clear eye diagrams prove the stand-alone ODLM can transmit 25.8 Gb/s, 231-1 Pseudo-Random Binary Sequence (PRBS) when driven through a high-speed bias-tee. The BER test indicates that error-free operation can be achieved at an optical output of around -4 dBm.

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Several Silicon on Insulator (SOI) wafer manufacturers are now offering products with customer‐defined cavities etched in the handle wafer, which significantly simplifies the fabrication of MEMS devices such as pressure sensors. This paper presents a novel cavity buried oxide (BOX) SOI substrate (cavity‐BOX) that contains a patterned BOX layer. The patterned BOX can form a buried microchannels network, or serve as a stop layer and a buried hard‐etch mask, to accurately pattern the device layer while etching it from the backside of the wafer using the cleanroom microfab-rication compatible tools and methods. The use of the cavity‐BOX as a buried hard‐etch mask is demonstrated by applying it for the fabrication of a deep brain stimulation (DBS) demonstrator. The demonstrator consists of a large flexible area and precisely defined 80 μm‐thick silicon islands wrapped into a 1.4 mm diameter cylinder. With cavity‐BOX, the process of thinning and separating the silicon islands was largely simplified and became more robust. This test case illustrates how cavity‐BOX wafers can advance the fabrication of various MEMS devices, especially those with complex geometry and added functionality, by enabling more design freedom and easing the optimization of the fabrication process.

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We demonstrate a stand-alone optical data link module (ODLM) that fits in the limited space budget of smart imaging catheters. The module is based on an extension of the Flex-to-Rigid (F2R) technology platform for miniaturized system integration. The ODLM is a silicon-based interposer that comprises a commercially available Vertical Cavity Surface Emitting Laser (VCSEL), which has its electrical contacts and laser emitting spot on the same surface. With the flexible interconnects, the ODLM reroutes the flip-chipped VCSEL electrical contacts to the side that is perpendicular to the surface of the VCSEL. This enables the ODLM to be mounted on a flex-PCB and fit into the limited space in the distal tip of the smart catheter. An optical fiber that runs in parallel to the catheter shaft is inserted into the through-silicon hole (TSH) of the ODLM and self-aligned to the VCSEL for optical data transmission. The design of the ODLM and the F2R technology platform are introduced, and an ODLM demonstrator is fabricated and presented.@en

A polymer-based wafer level integration technology suitable for miniaturized and multi-functional systems integration was developed and demonstrated in this work. Wafer scale flexible interconnects were firstly fabricated on one wafer, and then transferred to another wafer. Such transfer process involved wafer bonding and application of sacrificial materials. A sacrificial layer was firstly placed on the surface of the transfer wafer, and the sandwich interconnect structures were then manufactured on top of the sacrificial layer. With the help of the sacrificial layer, the flexible interconnects were transferred to another wafer through wafer bonding process. Contact resistance structures were fabricated with the help of wafer bonding process, connecting and aligning metal contact layer on device wafer and metal layer embedded in transferred flexible interconnects. Such transferred contact resistance was measured through designed testing structures as a demo for wafer level heterogeneous integration.

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Our work presents embedded high-density oxide-nitride-oxide (ONO) trench capacitors for power supply decoupling in the next generation of smart catheters. These millimeter-scale smart catheters are using a novel integration platform, Flex-to-Rigid (F2R). In the F2R platform, various functional modules are fabricated or assembled on thin silicon islands. They are connected by flexible interconnects and can be folded into arbitrary shapes to facilitate small form-factor integration. Trench decoupling capacitors have the advantage of being integrated into the thin silicon islands of F2R to reduce the parasitic inductances and space consumption. Additionally, their small surface openings can be closed by layer deposition to enable follow-up processes on the closed-up surface. For demonstration, high aspect ratio (1.1:25 and 1.2:30) ONO trench capacitors with total areas of 300x300 µm 2 and 1000x1000 µm 2 are fabricated on planar wafers, and a 700 nm and a 1 µm thick plasma-enhanced chemical vapor deposition (PECVD) SiO2 layers are deposited to test the trench closing process. The F2R compatible ONO trench capacitors have capacitance densities of 6.17 nF/mm 2 and 10.12 nF/mm 2 , combined with breakdown voltages ranging from 28 to 30 V.@en