Integration of MEMS in Silicon Photonics

Like integrated electronics, integrated photonics such as Silicon Photonics benefit from increased device-density on a single chip. Silicon is an excellent material for integrated photonics because its high refractive index allows devices to be made small, and the established silicon CMOS fabrication infrastructure provides a convenient route towards scaling up production. However, standard Silicon Photonics faces a bottleneck in scaling up device-density due to excessive power consumption and high optical losses associated with individual devices. Microelectromechanical systems (MEMS) provide a unique solution to this problem by providing a physical redistribution of optical media to perform the necessary active functions in photonic integrated circuits (PICs) with minimal power consumption and low loss.This thesis tackles two important aspects needed for implementing large-scale MEMS-enabled PICs in silicon. First, the required microfabrication processes are developed for the first-time, by full process integration of MEMS in an established foundry platform, the Interuniversity Microelectronic Centre's (IMEC) iSiPP50G Silicon Photonics technology. By demonstrating MEMS-compatibility, the barrier to adopting this new technology is reduced, and the devices and circuits themselves benefit from co-integration with high-performance standard components. Second, a new class of electrostatic MEMS-enabled photonic building blocks are designed, simulated, and experimentally characterized. Demonstrated devices include a set of remarkably broadband (bandwidth > 80 nm) tunable couplers capable of continuous optical power tuning between output ports to produce extinction ratios greater than 20 dB with minimal insertion loss (reaching < 0.4 dB). In terms of switching, a very low-voltage, six-port count device and a particularly compact (65 um x 62 um) photonic MEMS switch with sub-microsecond switching time are also presented. Further MEMS-enabled functionality is demonstrated with a wavelength-selective add-drop filter and a discussion of phase-shifters and multi-device sub-circuits. These components can be repeated and combined with one another to create complex and reconfigurable networks, and therefore represent an essential stepping stone towards the realization of very large-scale PICs. Together, these key developments in microfabrication and device design promise PIC designers MEMS-enabled photonic components as standard elements in their circuits to efficiently implement functions such as switching, tuning, and filtering for application in photonic switch matrices, weighted interconnects for neural networks, and programmable PICs.

Wafer-Level Vacuum Sealing for Packaging of Silicon Photonic MEMS

Niels Quack, Hamed Sattari, Göran Stemme, Alain Yuji Takabayashi

Silicon (Si) photonic micro-electro-mechanical systems (MEMS), with its low-power phase shifters and tunable couplers, is emerging as a promising technology for large-scale reconfigurable photonics with potential applications for example in photonic accelerators for artificial intelligence (AI) workloads. For silicon photonic MEMS devices, hermetic/vacuum packaging is crucial to the performance and longevity, and to protect the photonic devices from contamination. Here, we demonstrate a wafer-level vacuum packaging approach to hermetically seal Si photonic MEMS wafers produced in the iSiPP50G Si photonics foundry platform of IMEC. The packaging approach consists of transfer bonding and sealing the silicon photonic MEMS devices with 30 mu m-thick Si caps, which were prefabricated on a 100 mm-diameter silicon-on-insulator (SOT) wafer. The packaging process achieved successful wafer-scale vacuum sealing of various photonic devices. The functionality of photonic MEMS after the hermetic/vacuum packaging was confirmed. Thus, the demonstrated thin Si cap packaging shows the possibility of a novel vacuum sealing method for MEMS integrated in standard Si photonics platforms.

SPIE-INT SOC OPTICAL ENGINEERING2021

Accordabilité de la réponse optique des cristaux photoniques par infiltration de matériaux organiques

Pascale El-Kallassi

Photonic crystals are stuctures in which the refractive index varies periodically on the wavelength scale in one, two or three dimensions. Due to interference effects, these structures can forbid light propagation over a large spectral range called "the photonic bandgap". Integrated optics is one of the potential applications of photonic crystals in which the information can be treated as a light signal that propagates through photonic crystal-based devices, such as filters, lasers, waveguides, bends and multiplexers. The photonic crystals structures studied for integrated optics applications consist of periodic arrays of holes etched through classical semiconducteur-based planar waveguides. For any practical application, the optical properties of the fabricated components have to be tuned by external means. In this thesis, we explore a possible approach to photonic crystal tuning : the infiltration of photoactive organic materials in the holes of these photonic structures. First we considered liquid crystals, which exhibit anisotropic optical properties like birefringence due to their long-range orientational order. This latter molecular organization can be modified by external factors, i.e. temperature and electric field. Nematic liquid crystals were infiltrated in the holes of the photonic crystals to tune their properties by changing the temperature and applying an electric field. Moreover, the organization of liquid crystals in the nanometric holes were studied by optical techniques. The demand for all-optical tunable devices in integrated optical circuits led us to consider the possibility of optically tuning the response of photonic crystal structures. For this purpose, we used photo-induced phase transitions of a liquid crystal blend doped with azobenzene photochromic molecules. Since the phase transitions in these mixtures depend on the concentration of the photochromic compound and on the temperature, we studied in detail the phase diagram of the blend. The last part of this thesis focused on one of the most innovative aspects in the domain of photonic crystal tuning : selective infiltration to locally modify the optical properties of the photonic structures. We used the photopolymerization of acrylate monomers by ultraviolet laser irradiation. Finally, with three clear examples, this thesis shows that organic photoactive materials are well-adapted to be combined with photonic crystals. We demonstrated that the obtained hybrid structures are very promising for integrated optics applications.

EPFL2009

A new versatile hybrid MEMS technology for high sensitivity, fluid proof sensing applications.

Matthias Neuenschwander

The field of micro electromechanical systems (MEMS) evolved from the microelectronic industry and the technologies developed to fabricate integrated circuits. As a result, MEMS are commonly fabricated on silicon wafers. The development of MEMS has been driven by three main merits: miniaturisation, microelectronics integration, and parallel fabrication with high precision. Many operational properties scale well with smaller sizes. In addition, integrated electronic circuitry allows embedding MEMS with computing or networking capabilities, while parallel manufacturing enables the fabrication of many identical devices on a single wafer, reducing the unit cost. The main MEMS application is transducers which transform signals from one form of energy to another, and can be used for perception (sensors) or to produce actions (actuators). Silicon and other microelectronic materials like metals have enabled very high-performance MEMS transducers because these materials can be used for a range of very effective actuating and sensing principles.More recently, polymers have started to be used in MEMS because of their unique electrical, physical and chemical properties which include biocompatibility, viscoelasticity and mechanical shock tolerance. They are also much softer than silicon or metals, and they can be processed using many techniques that allow unique low-cost, batch-style fabrication and packaging. However, polymers have relatively low glass-transition and melting temperatures. As a result, the advantages of polymers cannot easily be combined with high-performance actuating and sensing elements as these are based on silicon technology and require high-temperature fabrication steps. Here, a hybrid-MEMS fabrication process is used to address this issue. The developed devices are based on a trilayer structure, where a thick polymer core is sandwiched between two hard thin films, while electronic layers are embedded within in a fluid-compatible way.The objective of this thesis is to turn hybrid-MEMS into a benchmark MEMS technology. This involves many different aspects. First, sensing and actuation elements are integrated into trilayer devices, and their performance is analysed. Then, some more unique possibilities offered by hybrid-MEMS are exploited. A way to fabricate electronics on multiple layers is developed, which allows different electronic features to be integrated in parallel. In addition, three-dimensional bulk features fabricated at several levels are demonstrated. This is possible because the hybrid-MEMS process is based on the bonding of multiple wafers.All these new fabrication features are demonstrated in atomic force microscopy (AFM) applications. Self-actuated trilayer AFM cantilevers with sharp silicon tips are used to boost imaging speeds in the off-resonance tapping (ORT) mode, thanks to an order of magnitude faster surface probing rates. Furthermore, fully integrated ORT imaging is demonstrated with self-actuated piezoresistive cantilevers. Next, trilayer cantilevers with shielded conductive tips are introduced for electrical AFM modes. Finally, ongoing research projects are touched upon, including approaches to fabricate hybrid-MEMS with single crystal silicon piezoresistors for improved sensitivity, and an analysis of reproducibility of fabricated trilayer cantilevers.