Photonic integrated circuits (PICs) are a maturing technology with foundries enabling wafer-scale PIC fabrication. At the same time, optomechanics, in which micro-/nano-optical and -mechanical structures are coupled, is well-established with many basic research and practical applications. However, optomechanical devices have so far required highly-customized fabrication that limits their inclusion in foundry-processed PICs. To address this need, we design optomechanical PICs using standard low-loss process design kit (PDK) components. Our approach ensures access to the foundry’s low-loss PDK components and enables process compatibility. As a demonstration, we design a foundry-processed optomechanical Mach-Zehnder interferometer (MZI). Measurements demonstrate that a π-phase shift can be accumulated over an optomechanical interaction length of only 60 µm and tunable phase shifting can be achieved using gradient electric force actuation. We further demonstrate all-optical excitation and readout of mechanical resonances for sensing applications. Our PDK-focused optomechanics design approach enables the co-integration of optomechanics, photonics, and electronics in a single PIC.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Optical micro-electro-mechanical systems (MEMS) and micro-/nano-opto-electro-mechanical systems (MOEMS/NOEMS) [1,2] enable coupling of optical and mechanical degrees of freedom. These new chip-scale structures have led to many exciting advances ranging from optical switches  to laser cooling of mechanical oscillators  to all-optical reconfigurable filters . In particular, optomechanics enables large phase shifts [5–8] in passive optical materials including in ultra-low-loss silicon nitride waveguides. However, optomechanical integration is limited by the highly customized fabrication processes required.
Recently, optical foundries (e.g. [9,10]) enabled low-loss, batch fabricated photonic integrated circuits (PICs) [11–13]. The development of standardized optical components using process design kits (PDKs) enables end users to design complex PICs without requiring detailed knowledge of the fabrication process. The emergence of photonic PDKs is expected to lead to a large increase in the design and adoption of PICs, similar to conventional electronic ICs. However, photonic PDKs have so far not included MEMS, despite the many applications of optomechanical devices [1–4].
This work presents a PIC architecture enabling co-integrated opto-electro-mechanical structures to be fabricated in a silicon foundry . We utilize low-loss standard PDK components and realize opto-mechanical structures by designing a MEMS element directly in the foundry’s existing PDK to demonstrate optical phase shifters and MEMS resonators. This work presents a step towards full photonic, mechanical, and electronic co-integration by incorporating optomechanical components directly within the foundry’s existing PDK without the need for customized layers and processes that may be incompatible with other on-chip photonic components. The inclusion of MEMS in foundry PDKs, importantly, will give PIC designers access to a new and low-power phase shifting technology.
2. PDK-focused design of optomechanical photonic integrated circuits
The optomechanical architecture is illustrated in Fig. 1(a) showing an optomechanical Mach-Zehnder Interferometer (MZI) as a demonstration device. Other optical MEMS devices, e.g. optomechanical microring cavities , can be realized in a similar manner. The optomechanical MZI consists of foundry-supplied c-band silicon (Si) components (AIM Photonics ) including input/output taper couplers at the chip edges (not shown in Fig. 1(a)), full-etched Si waveguides, 2 × 2 multi-mode interference (MMI) splitters, and Si rib waveguides. We design a custom transition waveguide (“transition WG” in Fig. 1(a)) by modifying a PDK Si rib waveguide to enable low-loss coupling between the full-etched and rib waveguides (low-loss transition confirmed by measurements). Aside from the transition waveguide we use only standard PDK silicon waveguide components to ensure process compatibility while also guaranteeing well-known loss and device characteristics. The foundry offers a standard “trench” in which the top silicon oxide (SiO2 cladding) is removed for evanescent field sensing applications (Fig. 1(a,(b))). Additionally, the PDK also includes silicon nitride (SiN) components. These SiN waveguides are situated above the Si waveguides (Fig. 1(b)) and are separated by a thin SiO2≈100 nm layer that enables coupling between Si/SiN devices. We modify the SiN waveguide to design a pre-release optomechanical (MEMS) structure consisting of a suspended SiN microbridge and four anchor pads—all within the foundry’s existing PDK (Fig. 1(c)).
Individual chips are received from the foundry in a pre-release state. The aforementioned trench leaves the optomechanical structures exposed while other PDK components are protected by the thick top SiO2 cladding. We perform a short post-process back end of line (BEOL) buffered oxide etch (BOE) and critical point dry—both steps could be incorporated in a future standard foundry PDK process—to remove the 100 nm SiO2 layer to suspend the SiN MEMS (wMEMS=550 nm) above the Si rib waveguide. Note that we do not require any additional masking layer or other specialized processes before performing the release etch since all PDK photonic components are protected by the thick top SiO2 cladding and the optomechanical structures are exposed via the foundry trench region. In contrast, previous foundry-processed optomechanical devices with a stacked waveguide-MEMS configuration as reported here have required customized layer depositions deviating from the standard foundry process . Consequently, a key benefit of our approach is that the MEMS is incorporated into and entirely compatible with the existing PDK components and process.
The foundry PDK does not specifically include optomechanical devices; however, it does include vertically-coupled Si- and SiN waveguides. These waveguides are separated by a 100 nm thick SiO2 layer to enable optical coupling between Si and SiN photonic components. Removal of the SiO2 layer results in weaker optical coupling while also enabling optomechanical coupling between a suspended SiN MEMS structure and an underlying Si waveguide. Simulations show strong optomechanical coupling between the SiN MEMS and the Si waveguide mode (TE-polarization, λ=1550 nm) as exhibited by the large neff-tuning in Fig. 1(d). Assuming a nominal SiN MEMS-Si waveguide separation of gap=100 nm (dictated by the 100 nm thick SiO2 spacer layer), a mode effective index tuning of Δneff≥0.03 is possible for vertical actuation over gap=0-100 nm. Although the Δneff is smaller than in SiN waveguides coupled to SiN MEMS , the Si-SiN configuration still enables large index tuning exceeding thermo-optic approaches while being entirely foundry compatible.
In the following sections we describe and experimentally demonstrate two applications of our PDK-focused design approach for optomechanical PICs. Section 3 describes an optical phase shifter in which electrostatic actuation of the MEMS structure enables tunable optomechanical interaction, mode effective index tuning (Δneff), and phase shifting. Section 4 describes the optical excitation (via optical forces) and readout of mechanical resonances of the optomechanical PIC; such all-optical mechanical resonators may have application in fiber-coupled and stand-off sensors.
3. Optomechanical (MEMS) phase shifters
Upon successful release of the optomechanical devices we first measured the MZI spectra using a tunable laser and photodetector that were coupled to the device using lensed optical fibers. A measured MZI spectrum is shown in Fig. 2(a) (lMEMS=40 µm). The peak transmission near λ=1550 nm results from the Si PDK MMI splitters, which are optimized for the c-band. Additional measurements on devices with lMEMS=0-60 µm show that the SiN MEMS does not induce substantial optical loss as exhibited by the near constant extinction of >25 dB. This low MEMS loss likely benefits from the Si-SiN (waveguide-MEMS) foundry process that results in a gentle mode perturbation even without nanotapers to adiabatically transition  from waveguide to MEMS-waveguide. The phase shift in Fig. 2(b) is Δϕ≈π for lMEMS=60 µm (green curve) vs. the no MEMS case (black curve) indicating that a π-phase shift can be accumulated over short (60 µm) interaction lengths using optomechanical phase shifting. According to the mode perturbation simulations in Fig. 1(d) the MEMS effective index tuning is Δneff≈0.012 at a gap=60-70 nm, which gives a phase shift Δϕ=2π(Δneff lMEMS/λ)≈π, consistent with our measurements. Although the SiO2 spacer layer is 100 nm thick, the post-release etch may result in gap<100 nm. Additionally, the static fringe shift may also result from non-uniformities in the trench that may induce phase errors between devices.
The measurements in Fig. 2(b) indicate a constant phase shift. Future devices require electrodes to enable dynamically-tunable phase shifts using actuated MEMS [5–8]. The passive foundry run did not include metallization, although active foundry runs can include both doping and metallization. We post-processed gold (Au) electrodes on pre-release devices using electron beam lithography followed by a release etch as described above (Fig. 3(a),(b),(c)). We then measured the MZI spectrum as the MEMS was actuated by applying a bias across the electrodes to generate gradient electric forces [7,8]. The clear observed fringe shift confirms tunable phase shifting (Fig. 3(d)). As expected, longer MEMS structures result in substantially lower actuation voltages (e.g. 15 V for lMEMS=60 µm) compared with shorter structures (e.g. lMEMS=20 µm). The lower extinction for the lMEMS=60 µm device is likely the result of fabrication imperfections in the electrodes which were observed to be within ≈100 nm of the Si rib waveguide (Fig. 3(c)); in contrast, the lMEMS=20 µm device had well-defined electrodes with >500 nm distance from the rib waveguide (Fig. 3(b)). Beyond the extinction, the electrodes on the lMEMS=60 µm device may also potentially affect the actuation mechanism and total displacement (Δgap) thereby limiting the observed MZI fringe and phase shift (Fig. 3(d), inset).
For practical PIC phase shifters the actuation voltage needs to be decreased. Simulations indicate that low-voltage actuation is possible (10 V for longer lMEMS≈100 µm; others have predicted <1 V actuators ). MEMS electrostatic actuators and gradient electric force actuators are capacitive and require minimal electric power during tuning/switching (pW-level electrical power predicted from simulations) and virtually no power during steady-state operation. Therefore, MEMS phase shifters are a key technology for ultra-low-loss (SiN) large-scale PICs since they enable sizable phase shifts (Δneff>0.01) at minimal electrical power compared to thermo-optic devices.
4. All-optical excitation and measurement of mechanical resonances
The optomechanical MZI can also be used to excite and measure the mechanical resonances of the suspended MEMS structure. High mechanical Q-factor resonators may find application in chemical sensors  as well as accelerometers  and inertial sensors. Rather than electrostatic actuation we utilize an all-optical MEMS excitation and measurement scheme . The ability to excite and measure mechanical resonances all-optically enables our optomechanical PICs to be used in fiber-coupled (standoff) sensors in which no on-chip electrical power is required.
We place our sample in a vacuum chamber (P≈100 mTorr) and use a pump-probe setup (Fig. 4(a)) with free-space optical coupling to and from the sample similar to our previous work . The pump laser is tuned to a MZI fringe minimum (λ=1552.28 nm) and is modulated and swept in frequency to excite the fundamental mechanical resonance using the gradient optical force . The probe laser is tuned to a MZI fringe 3 dB transmission point (λ=1449.33 nm or 1452.90 nm) so that the MEMS oscillation results in a phase modulation (via Δneffective in Fig. 1(d)) and an amplitude modulation of the MZI signal. Although the pump is set to a fringe minimum, we use a bandpass filter (BPF, Fig. 4(a)) to remove any pump trace so that the measured signal is purely the CW probe whose modulation represents the MEMS oscillation.
We excited the fundamental mechanical resonance of an lMEMS=60 µm optomechanical MZI using different pump powers (laser set point Ppump=0.125-6 mW) and constant probe laser power (1 mW). The measurements show a clear resonance at f0=2.961 MHz and Qmech∼104 (Fig. 4(b)). Tuning the probe laser to the two 3 dB transmission points of an MZI fringe (λ=1449.33 nm or 1452.90 nm) results in identical MEMS displacement amplitude; however, the phase response is shifted by 180° (Fig. 4(b), inset) which is consistent with a MEMS displacement-induced phase modulation of the MZI fringe.
Beyond the 3 dB transmission wavelengths we also tuned the probe laser across one MZI fringe (Fig. 4(c)) and performed pump-probe measurements (Fig. 4(d)). The mechanical resonance shows a clear probe wavelength-dependent amplitude. Closer inspection confirms that the signal amplitude peaks at the 3 dB points (λ=1449.33 nm or 1452.90 nm) and is minimized at the MZI fringe peak and valley (Fig. 4(e)) as would be expected from a MEMS displacement phase modulation. Finally, we also performed pressure-dependence measurements of the MEMS oscillation (not shown) and found a clear dependence of the displacement amplitude with pressure. These results confirm that the 2.961 MHz signal results from optically driven mechanical oscillation and not, e.g. a thermo-optic effect in the silicon waveguide.
Next, we performed pump-probe measurements at a fixed probe wavelength and power (Pprobe=1 mW, λ=1449.20 nm) while varying the pump power (Ppump=0.125-6 mW, λ=1552.28 nm) in order to extract the optical force that drives the MEMS resonance. Although the device in Fig. 5 is the same lMEMS=60 µm device as in Fig. 4, the resonance is shifted slightly to 2.959 MHz (Fig. 5(a)). The frequency shift is likely due to small variations in the ambient conditions (temperature and vacuum level). We convert the measurements in Fig. 5(a) to a displacement by calibrating the signal using the thermal mechanical displacement noise spectrum [14,18]. The peak displacement is Δz≈200 pm at a calibrated pump power of ≈9.3 µW in the BAR waveguide (Fig. 5(b)). The optical force is extracted using the approach in [14,18] and found to be Foptical≈0.13 pN, or 0.24 pN/mW-µm (force per unit power and unit lMEMS), in general agreement with calculations. The low pump power in the waveguide (<1 µW) required to excite a mechanical resonance bodes well for fiber-coupled and networked sensors.
Our optomechanical PIC architecture requires no change in the foundry’s fabrication process. As such, any standard and low-loss PDK components (e.g. waveguides, splitters, microring cavities, etc.) can be co-integrated with optomechanical structures. The short release etch is performed in-house without the need for additional processing steps (e.g. deposition of etch stop layers) and could in the future be performed on full wafers in a foundry. Foundry processing of PICs is now becoming more common [9,12] and enables the co-integration of photonic and electronic components. Our optomechanical design approach, in which MEMS structures are realized within the foundry’s existing PDK, enables the seamless integration of optomechanics with photonics and electronics on a single chip.
Although MEMS optical phase shifters and optically-actuated MEMS resonators have been demonstrated previously, our PDK-focused approach and foundry-processed optomechanical PICs have several advantages. The use of standard low-loss PDK components including edge couplers enables the optomechanical PICs to be readily fiber-coupled using the foundry’s packaging facility. Furthermore, the use of high-index Si-waveguides (nSi≈3.45) and modest index SiN-MEMS (nSiN≈2.0) enables adiabatic mode perturbation and effective index tuning as exhibited by the nearly constant MZI extinction in Figs. 2 and 3. In contrast, prior work in which the waveguide and MEMS were both made of Si  or SiN  requires careful design and nanoscale geometries in order to achieve adiabatic mode perturbation. The reduced loss in the Si/SiN optomechanical structure also has implications for optical force actuation of MEMS resonators, and the optical power required for actuation (Fig. 5) is lower than our previous devices  even without cavity enhancement.
In future designs a number of improvements can be made. The demonstrated optomechanical MZI was processed using a passive foundry multi-project wafer (MPW) run. We post-processed metal actuation electrodes using in-house custom fabrication. Future active foundry MPW runs can enable these actuation electrodes to be included in the delivered optomechanical PICs. Alternatively, the silicon waveguide can be doped to create a PIN diode directly at the waveguide core enabling ultra-low-voltage electrostatic actuation . Additional consideration needs to be given to the electrostatic design to enable tuning over the full gap=0-100 nm range since electrostatic actuators commonly have a limited range due to pull-in phenomena. Having access to the full actuation range (gap=0-100 nm) will maximize the mode effective index tuning achievable.
In conclusion, we demonstrated MHz-rate optomechanical devices with standard foundry PDK components. Our foundry-compatible architecture enables opto-electro-mechanical PICs for efficient optical phase shifters and MEMS resonators for sensing. With the inclusion of a foundry release etch, our approach will enable a future “optical MEMS PDK” to be realized giving users access to an efficient and low-power phase shifting technology that can be implemented in many photonics platforms including silicon and SiN. This PDK-focused approach streamlines PIC development enabling large-scale optics, MEMS, and electronics integration towards photonic systems on a chip.
USNRL base 6.1 program and NISE-219 EDA program.
The authors thank USNRL Nanoscience staff for cleanroom access and assistance with post-processing. M. W. Pruessner thanks S. Preble (RIT) for helpful feedback regarding foundry process compatibility. The devices presented in this work were fabricated at the AIM Photonics foundry, which is supported in part by OUSD(R&E) through its AIM Photonics Institute.
The authors declare that there are no conflicts of interest related to this article.
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