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• Photonics Research
• Vol. 10, Issue 2, 02000A14 (2022)
Gaehun Jo1, Pierre Edinger1, Simon J. Bleiker1, Xiaojing Wang1, Alain Yuji Takabayashi2, Hamed Sattari2, Niels Quack2, Moises Jezzini3, Jun Su Lee3, Peter Verheyen4, Iman Zand5, Umar Khan5, Wim Bogaerts5, Göran Stemme1, Kristinn B. Gylfason1、6、*, and Frank Niklaus1、7、*
Author Affiliations
• 1Division of Micro and Nanosystems, KTH Royal Institute of Technology, 11428 Stockholm, Sweden
• 2École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
• 3Tyndall National Institute, Lee Maltings Complex Dyke Parade, T12 R5CP Cork, Ireland
• 4imec vzw. 3DSIP Department, Si Photonics Group, Kapeldreef 75, 3001 Leuven, Belgium
• 5Department of Information Technology, Photonics Research Group, Ghent University - IMEC, 9052 Gent, Belgium
• 6e-mail: gylfason@kth.se
• 7e-mail: frank@kth.se
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Abstract

The emerging fields of silicon (Si) photonic micro–electromechanical systems (MEMS) and optomechanics enable a wide range of novel high-performance photonic devices with ultra-low power consumption, such as integrated optical MEMS phase shifters, tunable couplers, switches, and optomechanical resonators. In contrast to conventional $SiO2$-clad Si photonics, photonic MEMS and optomechanics have suspended and movable parts that need to be protected from environmental influence and contamination during operation. Wafer-level hermetic sealing can be a cost-efficient solution, but Si photonic MEMS that are hermetically sealed inside cavities with optical and electrical feedthroughs have not been demonstrated to date, to our knowledge. Here, we demonstrate wafer-level vacuum sealing of Si photonic MEMS inside cavities with ultra-thin caps featuring optical and electrical feedthroughs that connect the photonic MEMS on the inside to optical grating couplers and electrical bond pads on the outside. We used Si photonic MEMS devices built on foundry wafers from the iSiPP50G Si photonics platform of IMEC, Belgium. Vacuum confinement inside the sealed cavities was confirmed by an observed increase of the cutoff frequency of the electro-mechanical response of the encapsulated photonic MEMS phase shifters, due to reduction of air damping. The sealing caps are extremely thin, have a small footprint, and are compatible with subsequent flip-chip bonding onto interposers or printed circuit boards. Thus, our approach for sealing of integrated Si photonic MEMS clears a significant hurdle for their application in high-performance Si photonic circuits.

1. INTRODUCTION

Over the past decade, photonic integrated circuits (PICs) have evolved rapidly. They have already found widespread use in telecommunication systems, where they serve as highly miniaturized optical transceivers, and are being developed for emerging applications in medical technology and sensing [1]. Especially, silicon (Si) photonics has established itself as a scalable technology that is becoming widely available via commercial foundry platforms [25]. Si photonics foundries offer a wide range of devices, such as high-speed modulators and photodetectors, as well as high-quality passive waveguides. Moreover, the availability of Si photonics has catalyzed new research fields that leverage the excellent mechanical properties of Si in combination with its large refractive index. For example, integrated optical sensors based on suspended Si waveguides achieve high sensitivity, due to the high Si–air refractive index contrast that allows for high mode deconfinement [69]. Furthermore, optomechanical coupling and optical forces can be harnessed, for uses such as particle manipulation, telecommunication, or on-chip nonreciprocal transmission [1012]. Movable and tunable Si photonic micro–electromechanical (MEMS) components also show promise as low-power and compact reconfigurable photonic building blocks for large scale programmable photonic circuits that can be reprogrammed for a variety of optical functions [1319], i.e., generic field-programmable PICs (FP-PICs), similar to FP gate arrays (FPGAs) in electronics. In contrast to conventional $SiO2$-clad photonic devices, optomechanical and Si photonic MEMS devices feature suspended and movable parts, and exposed waveguide cores. These devices are much more susceptible to environmental influences such as exposure to dust, gas composition, and humidity levels, and, therefore, require a robust packaging solution to ensure reliable operation over extended time periods. Hermetic sealing in inert gas or vacuum, protecting the photonic MEMS from such environmental influence, is crucial for their reliable performance, and serves as a prerequisite for their commercialization [2022].