Integrated silicon photonic MEMS

Abstract

Silicon photonics has emerged as a mature technology that is expected to play a key role in critical emerging applications, including very high data rate optical communications, distance sensing for autonomous vehicles, photonic-accelerated computing, and quantum information processing. The success of silicon photonics has been enabled by the unique combination of performance, high yield, and high-volume capacity that can only be achieved by standardizing manufacturing technology. Today, standardized silicon photonics technology platforms implemented by foundries provide access to optimized library components, including low-loss optical routing, fast modulation, continuous tuning, high-speed germanium photodiodes, and high-efficiency optical and electrical interfaces. However, silicon's relatively weak electro-optic effects result in modulators with a significant footprint and thermo-optic tuning devices that require high power consumption, which are substantial impediments for very large-scale integration in silicon photonics. Microelectromechanical systems (MEMS) technology can enhance silicon photonics with building blocks that are compact, low-loss, broadband, fast and require very low power consumption. Here, we introduce a silicon photonic MEMS platform consisting of high-performance nano-opto-electromechanical devices fully integrated alongside standard silicon photonics foundry components, with wafer-level sealing for long-term reliability, flip-chip bonding to redistribution interposers, and fibre-array attachment for high port count optical and electrical interfacing. Our experimental demonstration of fundamental silicon photonic MEMS circuit elements, including power couplers, phase shifters and wavelength-division multiplexing devices using standardized technology lifts previous impediments to enable scaling to very large photonic integrated circuits for applications in telecommunications, neuromorphic computing, sensing, programmable photonics, and quantum computing.

Keywords: NEMS; Nanoscale devices; Other photonics.

Fig. 1. Representative examples of recently demonstrated silicon photonic MEMS.

Individual components based on dedicated processes on silicon-on-insulator wafers include a tunable couplers, b phase shifters, and c ultrasound sensors, and silicon photonic MEMS based on specifically developed microfabrication processes using multiple layers have shown d beam steering for 3D imaging and e, f large-scale photonic switch matrices for optical circuit switching in datacentres^,. (Permissions: a Adapted with permission from ref. © The Optical Society. b Adapted with permission from ref. © The Optical Society. c Adapted with permission from ref. © Springer Nature. d Adapted from ref. under creative commons license CC BY 4.0. e Adapted with permission from ref. © The Optical Society. f Adapted from ref. under creative commons license CC BY 4.0)

Fig. 2. Integrated silicon photonic MEMS technology platform.

The platform is based on IMEC’s iSiPP50G platform, enhanced with custom postprocessing to produce suspended and movable MEMS structures alongside the platform’s standard components and with sealing caps above the MEMS structures to provide protection of the movable MEMS devices. a Schematic cross section of the technology platform and 3D perspective views of b in-plane and c out-of-plane movable devices to visualize conceptually the electrostatic actuators, the optical and electrical routing, as well as grating coupler and metal contact pads, serving as interfaces. d Photograph of a 200 mm silicon photonics wafer before MEMS release postprocessing and e microscope recording of a representative MEMS device integrated alongside high-performance grating couplers, low-loss waveguides for optical routing, and two metal levels for electrical routing. The final metallization layer serves as a bond pad for wirebonding, as well as an aluminium ring for the wafer-level hermetic sealing process

Fig. 3. Tunable power couplers and switches.

a 3-D schematic view of an out-of-plane actuated tunable power coupler indicating the input, through, and drop ports and the actuator. b SEM recording of the released device showing a cleanly suspended and initially well-aligned configuration of the couplers in the unactuated state. c Measurement results for the spectral characteristics of the tunable power coupler under DC actuation showing a maximum extinction ratio of 25 dB between the drop and through ports in the unactuated (0 V) state and a maximum extinction ratio of 30 dB in the actuated (27 V) state. The associated 3-dB bandwidth of this device is >30 nm. d 3-D schematic view of an in-plane actuated photonic 1 × 2 switch with the movable input and two fixed outputs. e SEM recording of the released device showing the curved electrodes, stoppers, and suspended waveguides. f Measurement results for the spectral characteristics of the device in all three functional states: ON1, OFF, and ON2, wherein the extinction ratio is >23 dB for both ON states and the bandwidth is >70 nm

Fig. 4. Tunable phase shifter.

a 3-D schematic view of an in-plane actuated phase shifter. b SEM recording of the phase shifter, comprising a suspended waveguide and a comb-drive actuator to tune the modal cross-section of the waveguide by pulling away a 220 nm wide silicon beam attached to the movable shuttle. c Characterization of the phase shifter within an unbalanced Mach Zehnder interferometer (MZI), displaying an over 2π phase shift and low insertion loss (large extinction ratio). d Extracted phase shift and e insertion loss versus voltage at a wavelength of 1550 nm. Dotted: single device, line: median for N = 10 devices, shaded region: +/−1σ. f Tunable basic unit composed of two MEMS phase shifters, one per arm of a suspended balanced MZI, here configured as an all-pass ring resonator. g Example characterization of the tunable basic unit with two MEMS phase shifters, showing versatile control over the ring filter response and a free spectral range of 0.93 nm at a wavelength of 1550 nm. h Actuation map of the corresponding filter extinction ratio and i resonance tuning at a wavelength of 1550 nm. For low values of the extinction ratio, no resonances were observed, and no resonance shift was extracted

Fig. 5. Tunable add-drop filter.

a 3-D schematic view of the out-of-plane actuated tunable add-drop filter depicting the resonant-mode behaviour of the engaged filter. b SEM recording of the released device showing the suspended ring and the two well-aligned and suspended bus waveguides on either side. c Measurement results for power transmission to the through and drop ports for both on- and off-resonance wavelengths in the two filter states. When the filter is ON (0 V), the free spectral range (FSR) is ~5 nm, and the port extinction ratio (ER) is 20.4 dB for resonant wavelengths and 13.2 dB for off-resonance wavelengths. Turning OFF the filter (27 V) maintains an ER > 20 dB for resonant wavelengths

Fig. 6. Optical crossings.

a Microscope image of a multimode interference (MMI)-based optical waveguide crossing. MMI length (L_MMI), trenches along the MMIs for penetration of hydrofluoric acid (HF) and the undercut regions are annotated. b Microscope image showing cascaded crossings to determine the insertion loss of the MMI-based waveguide crossings. c Plot showing the spread of the experimentally extracted insertion loss values over the measured wavelength range for different MMI lengths used for the crossing. The box extends from the lower to upper quartile values, the red dotted line represents the mean, and the light green line indicates the median. Inset shows the 3-D drawing

Fig. 7. Optical transitions.

a Scanning electron microscope (SEM) image of an optical transition from the oxide clad region to the cavity where the back end of line stack and the buried oxide have been removed using the developed wafer level postprocessing on the standard silicon photonic platform (iSiPP50G) chip. b Plot showing the spread of the experimentally extracted insertion loss values over the measured wavelength range for different core widths of the shallow etch waveguide-based optical transitions. The box extends from the lower to upper quartile values, the red dotted line represents the mean, and the light green line indicates the median. The inset shows a 3-d representation of the optimized optical transition

Fig. 8. Wafer-level hermetic sealing.

a Hermetically sealed silicon photonic MEMS devices on a 100 mm diameter wafer. The hermetic sealing approach uses wafer bonding for cap transfer and enables versatile shapes and designs of the sealed cavities with surrounding bond pads. b Top SEM and optical microscope images of the transferred caps on top of the hermetically sealed cavities. c Top microscope view of a MEMS phase shifter before wafer bonding and sealing, enclosed by the aluminium ring against which the cap will be bonded. d Successful transmission measurement of a MEMS phase shifter (see section ‘Tunable phase shifter’ above) after sealing and dicing, using an actuation voltage of up to 36 V

Fig. 9. Electronic-photonic assembly.

a Schematic cross-section representation and b photograph of the electronic-photonic assembly. c Subassembly of the silicon photonic MEMS die flip-chip bonded to the glass interposer. The assembly consists of the interposer-chip subassembly, attached to a mechanical support by silver epoxy and wirebonded to the printed circuit board with fan-out to high-density connectors, a 72-channel fibre-array attached by index matched epoxy to the PIC, and a metal housing to support the chip-interposer subassembly. d Using active alignment with shunt connectors on the chip allows for low insertion loss fibre-to-chip coupling over the telecom C-band

Fig. 10. Silicon photonics chip design.

Layout (left) of the full silicon photonics chip provided for fabrication at IMEC. Regions highlighted in green indicate the portions of the chip allocated to the testing of new geometries, functionalities, etc., at the individual device level. The red, yellow, blue, and black regions indicate the positions of four representative large-scale circuits utilizing a large number of individual devices. A microscope image (right) of the chip after fabrication and after custom MEMS postprocessing captures the full layout and scale. The aluminium rings are visible as bright contours, as is the exposed device layer silicon seen in pink inside the MEMS cavities. Note that the dark spots in the image are regions where the integrity of the alumina passivation was locally compromised and the vapour HF etchant could penetrate into the BEOL stack

Fig. 11. Simplified process flow indicating the main steps used in MEMS postprocessing.

Step a shows the initial cross-section of the sample after processing at the foundry. The first oxide removal step b clears any remaining oxide on top of and between the waveguides before an alumina passivation layer is conformally deposited over the entire sample using atomic layer deposition (ALD) in step c; this layer serves as a hard mask against the vapour phase hydrofluoric acid (vHF) etchant used to selectively remove the sacrificial buried oxide (BOx) layer. Following passivation, the alumina must be opened up inside the MEMS cavities in step d to provide vHF access to the BOx, as well as over the metallization in step e to ensure proper electrical contact. In a final step f, vHF is used to remove the BOx under the exposed device layer silicon, resulting in free-standing, movable silicon photonic MEMS structures. Arrows serve as visual guides on the locations where the corresponding process step is applied

Fig. 12. Schematic representation of the microfabrication process flow for interposer fabrication.

a Starting glass substrate that is cleaned in a piranha bath, b chromium–gold–titanium metal sputtering, c maskless lithography and subsequent ion beam etch (IBE), d sputtering of isolation oxide, e oxide patterning by maskless lithography and buffered-HF liquid etch, and f titanium etch by IBE

Fig. 13. Electronic–photonic assembly process.

a Flow chart for scalable electrical and optical interfaces by providing gold stud bumps on the PIC (b, c) for flip-chip bonding to a glass interposer with solder-jetted bumps (d, e), and subsequent active alignment (f) and epoxy attach of fibre arrays for the high count of optical interfaces, and finally wirebonding (g) to a printed circuit board (for the final assembly, see Fig. 9 in the main text)

Fig. 14. Schematic of a typical photonic MEMS test setup used here to characterize the power couplers and add-drop filter.

Light with wavelengths between 1460 and 1580 nm is injected into a fibre array from a tunable laser and passes through a polarization controller to ensure propagation of only the TE mode. The fibre array is in turn carefully aligned with on-chip grating couplers using a combination of the 2-axis stage on which the chip rests and the z-control on the fibre array holder. Following the optical alignment procedure, actuation voltages are applied via a set of electronic probe tips whose position can accurately be adjusted using micropositioners. For DC measurements, the actuation voltage comes from a DC power supply connected to a custom-built voltage distribution board, and for transient measurements, this power supply is replaced with a waveform generator. Optical power measurements are recorded using the integrated photodetectors, which are connected to other fibres in the fibre array. If the signal should be observed in the time domain, the photodetector can be connected to an oscilloscope

Fig. 15. Mechanical characterization of double-clamped suspended beams.

a 2D deflection map using an optical profilometer (Veeco Wyko NT9300). b Corresponding deflection profile of the different beams and c maximum deflection versus beam length