Wafer-Level Vacuum Sealing for Packaging of Silicon Photonic MEMS

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.

Integration of MEMS in Silicon Photonics

Alain Yuji Takabayashi

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.

EPFL2021

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

Fabrication defects and grating couplers in III-nitride photonic crystal nanobeam lasers (Conference Presentation)

Raphaël Butté, Jean-François Carlin, Nicolas Grandjean, Ian Michael Rousseau

We report a numerical and experimental investigation of fabrication tolerances and outcoupling in optically pumped III-nitride nanolasers operating near lambda = 460 nm, in which feedback is provided by a one-dimensional photonic crystal nanobeam cavity and gain is supplied by a single InGaN/GaN quantum well. Using this platform, we [1] and others [2] previously demonstrated single-mu W lasing thresholds due to the high beta Q-product inherent to the nanobeam geometry (beta is spontaneous emission coupling fraction into desired mode). In this work, we improved the fraction of emission emitted into our microscope's light cone by combining a redesigned photonic crystal cavity (c.f. [3]) with a cross-grating coupler with period approximately twice the photonic crystal lattice constant [4]. The samples were fabricated in epitaxial III-nitride layers grown on (111) silicon substrates using metal organic vapor phase epitaxy. The photonic crystal and output couplers were patterned using a single electron beam lithography exposure and subsequently transferred to the underlying III-nitride layers using dry etching. The nanobeams were then suspended via vapor phase etching of silicon in XeF2 (Figure 1) [5]. Scanning electron microscopy cross-sections revealed high-aspect ratio (>5), sub-70 nanometer diameter holes with near-vertical sidewalls. Fabrication-induced geometry errors were characterized by processing scanning electron micrographs with custom critical dimension software (Figure 2). Using UV micro-photoluminescence spectroscopy at room temperature, we measured the nanobeams' emission intensity, far-field profile, and quality factor. By comparing more than ten nominally identical nanobeams for each geometry with finite-difference time-domain simulations taking into account the geometrical deviations measured during fabrication [8], we characterized the role of fabrication-induced imperfections. Finally, we explored the trade-off between the quality factor and collected signal via lithographic variations of the output coupler grating amplitude. Our results demonstrate the robustness of III-nitrides for short-wavelength photonic crystal applications, such as photonic integrated circuits, optoelectronics, and cavity quantum electrodynamics.

Spie-Int Soc Optical Engineering2016