We review our recent work on compact and low power silicon photonic components based on microelectromechanical systems (MEMS) and implemented in imec’s iSiPP50G foundry platform. Large scale reconfigurability is attractive for emerging applications such as photonic accelerators for AI workloads. However, the large power consumption and footprint of current components prohibits scaling to large circuits. Silicon MEMS offer 10000x lower power consumption, a small footprint, and excellent mechanics. We show phase shifters, couplers, and wavelength filters . The devices have small footprints of a few tens of micrometers per side, low insertion losses of the order of 0.1 dB and time constants of the order of 1 µs.
We present the concept and detailed design of a Smart Slit Assembly for next generation spectrometers, and we experimentally demonstrate operation of an individual 221 μm × 111 μm smart slit channel employing a MEMS actuated shutter to continuously modulate the intensity of the optical input signal. The MEMS actuated shutter is fabricated in a 211 μm thick device layer of a Silicon-On-Insulator wafer by Deep Reactive Ion Etching. Electrostatic comb drive actuators allow an absolute displacement of 52 μm at 74 V, resulting in a continuously tunable shutter efficiency of up to 99.97% at an operating wavelength of 532 nm.
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 μm-thick Si caps, which were prefabricated on a 100 mm-diameter silicon-on-insulator (SOI) 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.
In the European project MORPHIC we develop a platform for programmable silicon photonic circuits enabled by waveguide-integrated micro-electro-mechanical systems (MEMS). MEMS can add compact, and low-power phase shifters and couplers to an established silicon photonics platform with high-speed modulators and detectors. This MEMS technology is used for a new class of programmable photonic circuits, that can be reconfigured using electronics and software, consisting of large interconnected meshes of phase shifters and couplers. MORPHIC is also developing the packaging and driver electronics interfacing schemes for such large circuits, creating a supply chain for rapid prototyping new photonic chip concepts. These will be demonstrated in different applications, such as switching, beamforming and microwave photonics.
We present the design of a non-volatile, bistable silicon photonic MEMS switch. The switch architecture builds on our previously demonstrated silicon photonic MEMS switch unit cell, using vertically movable adiabatic couplers. We here propose to exploit compressive stress in the movable polysilicon waveguides in a controlled manner, to intentionally displace the movable waveguides out of plane upon release. We design the waveguide suspensions to achieve close alignment with the fixed bus waveguide in the ON state, and positioning of the movable waveguide far from the fixed waveguide in the OFF state. Both ON and OFF positions are stable mechanically, without the need for maintaining an actuation voltage. In order to actuate the movable waveguide, we design vertical comb drive actuators that allow to commutate between both stable ON and OFF positions. Finite Element simulations predict electrostatic switch actuation with less than 30 V for compressive stress typically accessible in deposited polysilicon thin films. We validate the bistability mechanism by comparison with a representative experimental demonstrator. The demonstrator consists of a structured 100 nm poly-Si layer, deposited by chemical vapor deposition onto a thermally oxidized (1 μm) silicon wafer, exhibiting a compressive intrinsic stress of 275 MPa. Upon direct writing laser based photolithography, etching and final HF vapor release, the suspended structures bend into either stable position, and we measure a total buckling amplitude of 800 nm, sufficient to entirely de-couple the waveguides optically in the OFF state.
We present a design for an on-chip MEMS-actuated Variable Optical Attenuator (VOA) based on Silicon Photonic MEMS technology. The VOA consists of 30 individual mechanically movable MEMS cantilevers, suspended over an integrated, 1 μm wide bus waveguide, each terminating with two optical attenuation bars. By exploiting the pull-in instability, electrostatic actuation allows to move the individual cantilevers into proximity of the waveguide, leading to scattering of the evanescent field and thus attenuation of the remaining optical power in the waveguide. Electrodes are placed below the cantilevers for electrostatic actuation. Mechanical stoppers are used to avoid contact between the cantilevers and the electrodes and to keep the bars at a precisely defined distance of 60 nm away from the bus waveguide. The attenuator provides nearly zero insertion loss in OFF state, while in ON state, the attenuation range is defined by the number of actuated digital attenuation cantilevers and can be adjusted in discrete increments of only 1.2 dB. Owing to the small size, fast microsecond scale response time can be achieved, and electrostatic MEMS actuation allows for broadband and low-power operation. Our design exhibits a compact footprint of 30 μm × 45 μm, attenuation from 0 dB to 36 dB, while keeping return loss below 27 dB. To the best of our knowledge, this is the first presentation of a design of a VOA in Silicon Photonic MEMS technology.