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Abstract
Photonic system component counts are increasing rapidly, particularly in CMOS-compatible silicon photonics processes. Large numbers of cascaded active photonic devices are difficult to implement when accounting for constraints on area, power dissipation, and response time. Plasma dispersion and the thermo-optic effect, both available in CMOS-compatible silicon processes, address a subset of these criteria. With the addition of a few back-end-of-line etch processing steps, silicon photonics platforms can support nano-opto-electro-mechanical (NOEM) phase shifters. Realizing NOEM phase shifters that operate at CMOS-compatible voltages (≤ 1.2 V) and with low insertion loss remains a challenge. Here, we introduce a novel NOEM phase shifter fabricated alongside 90 nanometer transistors that imparts 5.63 radians phase shift at 1.08 volts bias over an actuation length of 25μm with an insertion loss of less than 0.04 dB and 3 dB bandwidth of 0.26 MHz.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Intensity and phase modulation in silicon is generally achieved with the thermo-optic effect [1,2] or plasma dispersion [3], effects that are slow and lossy, respectively. Further, plasma-dispersion-effect modulators typically require phase-shifter lengths of 1 mm or more [3,4], while thermo-optic modulators exhibit significant crosstalk when in close proximity [2,5], preventing the use of such modulation mechanisms for the realization of large, dense arrays. Recent work has shown that large electric fields can be used to break the centro-symmetry of the silicon crystal lattice, enabling a non-zero, DC electro-optic coefficient [6]. There is ongoing work towards integrating novel materials like barium titanate [7], but with major hurdles yet to be addressed including integration of these (exotic) electro-optic materials in high-volume commercial CMOS processes. Even still, most $\chi ^{(2)}$ materials (including lithium niobate [8] and organic polymers [9]) do not have a large enough electro-optic coefficient at CMOS compatible voltages to enable $\pi$ radians of phase shift on length scales that would enable systems with $10^5$ or more phase shifters. While nano-opto-electro-mechanical (NOEM) phase shifters are promising, they typically require high, CMOS-incompatible bias voltages for actuation [10–15]. Further, scattering at SiO$_2$-air interfaces gives rise to significant insertion loss in many NOEM devices. Here, we overcome both of these problems with a NOEM phase shifter design that achieves 5.63 radians phase shift at 1.08 volts bias and avoids insertion loss using a well-confined, partially-etched waveguide mode at the input and output to minimize scattering from the SiO$_2$-air interface, and a novel double-slot waveguide for efficient coupling over a short distance [16,17]. Cascades of these devices can be realized while still satisfying loss budget requirements for large systems. These devices enable large-scale applications of silicon photonics to artificial intelligence acceleration [18,19], LiDAR [20,21], quantum computing and simulation [22,23] and more.
2. Principle of operation
The device design is shown in Fig. 1(a). The device is fabricated in a 90 nanometer silicon-on-insulator process with a 160nm thick silicon device layer clad in SiO$_2$. The device is actuated by applying a voltage between the n-doped region labeled "signal" and the p-doped region labeled "ground". Dopants are used so that the applied bias can generate fields near the gap region—resulting in an inward electrostatic force that bends the lateral beams toward the center beam. Dopants in the suspended region of the device do not significantly contribute to propagation loss because the optical mode is largely confined in the slot region.
Fig. 1. Dual-slot NOEM phase shifter. (a) The phase shifter with two electrical terminals (signal and ground), an optical input and an optical output. The device is actuated by applying a voltage difference between signal and ground. The signal terminal is doped n-type and indicated in red and the ground terminal is doped p-type and indicated in blue. (b) Zoom-in of the actuation (phase shift) region. Symbol denoting location of dominant capacitance contributing to force generation overlaid. (c) Effective index versus slot gap width. (d) Plot of the normalized electric-field intensity overlaid on the device in the slot region indicated by solid green line in (b).
Electrically, the actuation regions (dual slots) are capacitors that are charged and discharged during operation. In order for the speed of the system not to be limited by the charging of these capacitors, dopants are used to create a conductive path from the device terminals through the silicon sections of the actuation region. We use n-type doping for the signal path and p-type doping for the ground path. When the device is actuated with a positive voltage, the p-i-n junction near the device terminals is reverse-biased and does not conduct current.
The optical design of the device can be broken down into 3 regions: the ridge-to-rib waveguide region, the dual-slot mode converter region, and the actuation region. The input and output region is designed to minimize scattering loss at the SiO$_2$-air interface—we use a 650nm-wide, partially-etched waveguide to confine the mode to the silicon. The simulated mode in the wide rib in oxide immediately prior to the SiO$_2$-air interface is shown in Fig. 2(b). FDTD simulation (Fig. 2(a)) predicts a scattering loss of 0.013 dB from the interface.
Fig. 2. (a) FDTD simulation of the coupling region from the oxide to air. Partially etched silicon not shown. (b) Optical mode in the rib waveguide immediately inside the oxide interface (position P1 in figure a). (c) Optical mode in the rib waveguide immediately after the oxide layer (position P2 in figure a). (d) Optical mode at the beginning of the dual-slot coupler (position P3 in figure a). (e) Optical mode halfway down the dual-slot coupler (position P4 in figure a).
The dual-slot mode converter is used to implement a compact and low-loss mode conversion between the ridge-to-rib waveguide region and the dual-slot region. It is formed from two distinct coupling regions. The first region couples from the partially-etched waveguide to fully-etched waveguide. The second region couples from fully-etched ridge waveguide to dual-slot waveguide. This second region begins with a center beam width of 600 nm, lateral beam width of 100 nm and gap with of 240 nm. It smoothly tapers down to the actuation region cross-section (360 nm/340 nm/100 nm). Each of these sections is only 7.5 um long—minimizing the length of the suspended waveguide and reducing the likelihood of stiction during release as well as unwanted, out-of-plane vibrational modes. Our simulations indicate that, for similar coupling efficiency, a single-slot coupler is twice the length of a dual-slot coupler. To meet our required efficiency, this would mean adding 15 um to the total length of the central suspended beam. Simulated modes at several points along the dual-slot mode conversion are shown in Figs. 2(b-e). The simulated (FDTD) field intensity in the plane of the device is shown for a region including the oxide/air interface and the entire dual-slot mode coupler in Fig. 2(a).
The actuation region consists of a suspended dual-slot waveguide. The primary design goal for this region is to maximize the effective index ($n_{eff}$) sensitivity to slot width ($w$): $\frac {dn_{eff}}{dw}$. Figure 1(c) plots effective index as a function of gap width. The presented device used a center-beam width of 360 nm, a lateral-beam width of 340 nm and an unactuated gap width of 100 nm. The mechanical design is aimed at maximizing the resonant frequency of the lateral beams, while allowing enough movement to achieve a target phase shift. Mechanical resonance is the limiting factor on the modulation speed of the device, and is primarily determined by the stiffness of the spring sections and the mass of the lateral beams.
3. Fabrication
The majority of device fabrication was performed in a CMOS foundry [24], yielding 160-nm thick silicon waveguides with 60-nm thick partially etched features, embedded in a dielectric stack. A custom step was used to remove the dielectric stack above the free-standing portion of the device. As fabricated by the foundry, the device is initially enclosed in SiO$_2$. To release the NOEMS device, we use the process outlined in Fig. 3. First, the chip is coated in 100nm of Al$_2$O$_3$ using atomic layer deposition [25]. This is followed by ion milling normal to the surface—removing the Al$_2$O$_3$ coating from horizontal surfaces while leaving side-walls covered. The chip is then exposed to 17 minutes of 6:1 buffered oxide etch to release the actuation region. Evaporation of fluids between free-standing surfaces (beams) typically generates attraction forces large enough to cause stiction—we avoid this by drying the chip through the supercritical phase of CO2 using a Tousimis (Autosamdri-931) critical point dryer [26].
Fig. 3. Device release process. (a) Starting device topology with 100nm thick thermal oxide cladding above the device layer. (b) Device is coated in 100 nm Al$_2$O$_3$ via atomic layer deposition. (c) Device is ion milled normal to surface, removing Al$_2$O$_3$ coating from surface while leaving side-walls covered. (d) 17 minute buffered oxide etch (6:1) followed by critical point drying.
4. Experimental results
We embed the NOEM phase shifter in a path-length unbalanced Mach-Zehnder interferometer to determine the phase shift imparted as a function of applied bias. The test structure is shown in Fig. 4(a). A tunable laser (Keysight 8164B) is coupled via optical fiber to the test structure using grating couplers with an insertion loss of approximately 9 dB at 1550 nm. The test setup used to extract the static phase shift versus applied bias is shown in Fig. 4(b). A 50GHz 3dB bandwidth, 100$\mathrm{\mu}$m pitch, ground-signal-ground probe (Cascade Microtech Z50-X-GSG-100) is used to electrically contact the test structure. The voltage bias between the signal and ground pads is set using a Keithley 2400 SMU. A spectrum of the test structure is taken by sweeping the tunable laser wavelength and detecting the output with a power meter. The NOEM phase shifter step response is measured using the setup shown in Fig. 4(c). The laser wavelength is set to quadrature at 1550.1 nm and a signal generator (Keysight 33600A) is used to electrically excite the test structure with a step signal with a 4 ns rise time.
Fig. 4. Test structure and measurement setups. (a) Path-length unbalanced Mach-Zehnder interferometer test structure (TS), used for static and dynamic measurements. TS is optically excited with a tunable laser and light is detected with a power meter. (b) Measurement setup used to obtain spectra at various applied biases. (c) Step response electro-optic measurement setup showing a signal generator connected to the TS that is optically excited with a tunable laser set to the TS quadrature point near 1550 nm. Optical signal is detected with a high bandwidth photodetector (HBPD).
The test structure spectra measured using the setup in Fig. 4(b) is shown in Fig. 5(a). Each curve in Fig. 5(a) is fit to $T=a(\cos {(b\lambda +\theta _0)})^2$, which describes the transmission of light through a Mach-Zehnder Interferometer. The phase shift $\theta _0(v)$ describes the phase shift applied by the NOEM phase shifter, and its value at each bias voltage is plotted in Fig. 5(b). The NOEM phase shifter achieves 5.63 radians of phase shift at 1.08 volts applied bias, and $\pi$ radians at 850 mV. This latter result corresponds to a V$_{\pi }$L value of 0.002 Vcm, based on the primary actuation length of 25 um. As a final, destructive test and to explore the threshold at which stiction occurs, we increased the bias from 1.08 to 1.2 volts (highlighted in red in Fig. 5(b)). A slope change is present, indicating an irreversible, partial stiction of the device.
Fig. 5. Device performance data. (a) Spectra at various bias voltages. (b) Calculated phase shift versus bias voltage. (c) Step response of the test structure. (d) Fourier transform of the step response exhibiting resonances near 0.1MHz and 1.1MHz.
In order to estimate the insertion loss of the NOEM phase shifter, we normalize the spectra at 0 volts bias to that of test structures that contain the same coupling and routing components (waveguides, bends, y-junctions, grating couplers). This method yields an upper-bound estimate for the insertion loss of the phase shifter of 0.04 dB. For details on the loss estimation methodology, see the Supplement 1.
The test structure step response measured using the setup in Fig. 4(c) is shown in Fig. 5(c). A low quality-factor (0.72) mechanical resonance is visible in the data indicating that the NOEM phase shifter is under-damped. The Fourier transform of the temporal response is shown in Fig. 5(d). Two resonances are evident, corresponding to vibrational modes of the lateral beams: one at 0.1 MHz (fundamental mechanical resonance) and another at 1.1 MHz (higher-order mechanical resonance). The 3-dB bandwidth of the device is approximately 0.26 MHz.
5. Conclusion
Phase shifters are critical components in photonic systems. With component counts scaling rapidly, the development of compact, low loss, and energy efficient phase shifting technologies is of increasing importance. Electro-optic materials such as lithium niobate and barium titanate are promising avenues forward, but face integration challenges with high-volume CMOS manufacturing processes. We have demonstrated a novel nano-opto-electro-mechanical phase shifter fabricated in a 90 nanometer hybrid photonics and transistor process. Because of the small size of the device, low power dissipation, CMOS-voltage compatibility, and low loss, it is an excellent candidate for large-scale photonic systems with cascades of hundreds of phase shifters. Future work towards increasing the mechanical bandwidth of the device could broaden the number of applications for this technology.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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