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Abstract
In this paper, we demonstrate a broadband Mach-Zehnder interferometer optical switch based on polycrystalline silicon (poly-Si), which enables the development of multilayer photonics integrated circuits. The poly-Si is deposited under a low temperature of 620 °C to avoid unexpected thermal stress and influence on optoelectronic performance. By introducing a π/2 phase shifter and a push-pull configuration, the switch achieved low power consumption and loss caused by carrier plasma absorption (CPA). The switch operates effectively in both “Bar” and “Cross” states at voltages of −3.35 V and 3.85 V. The power consumptions are 7.98 mW and 9.39 mW, respectively. The on-chip loss is 5.9 ± 0.4 dB at 1550 nm, and the crosstalk is below −20 dB within the C-band. The switch exhibits a 10%-90% rise time of 7.7 µs and a 90%-10% fall time of 3.4 µs at 1550 nm. As far as we know, it is the first demonstration of a poly-Si switch on an 8-inch wafer pilot-line. The low-temperature deposited poly-Si switch is promising for multilayer active photonic devices and photonic-electronic applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
To meet the ever-increasing demands for more data and information, optical interconnect, with high speed and low power consumption, is an attractive candidate in data centers (DCs) and high-performance computers (HPCs) [1–4]. Silicon photonics (SiPh) has been a topic of extensive research and is being implemented for optical interconnects [5–7], optical phase arrays (OPAs) [8–10] and optical processors [11,12]. One key advantage of SiPh is its compatibility with complementary metal-oxide-semiconductor (CMOS) technologies. These advancements have opened avenues for several new applications in the field of photonics. The maximum size of port count 128 in a single reticle is rapidly approaching its limit due to the constraints imposed by the reticle size of lithography [13]. Three-dimensional (3D) integration is a protentional method to further increase the integration density on optical devices. Crystalline silicon (c-Si), with low optical loss and excellent electronic properties, is the most widely used material for optical devices. However, the wafer bonding processes and equipment for multilayer c-Si structures are expensive and challenging [14,15]. Till now, several materials have been deposited and patterned above silicon-on-insulator (SOI) wafers with 220-nm top silicon, including silicon nitride (SiN) [16–18], amorphous silicon (a-Si) [19–21] and polycrystalline silicon (poly-Si) [22]. Thanks to the low deposition temperature and moderated carrier mobility, poly-Si is a promising material for realizing multilayer active photonic devices and photonic-electronic applications (EPICs) [23–25]. While the attenuation of poly-Si waveguide remains challenging, it is deemed suitable for on-chip optical interconnect purposes within short distances measured in mere millimeters. [2,23]. In this paper, we present a poly-Si 2 × 2 Mach-Zehnder interferometer (MZI) switch with p-i-n shifters fabricated on an 8 inch wafer through 180 nm pilot-line. A propagation loss of 65.63 dB/cm for ridge waveguides was calculated from microring resonators (MRRs) with an intrinsic quality factor around 9108. Multimode interferometer (MMI) used in the switch spits equally covering C-band. With a pre-biased π/2 phase shift, push-pull modulation is applied on the switch. With a 170-µm-length modulation arm, the on-off shift is realized with 8.32 mW (−3.4 V) and 9.31 mW (3.85 V). The switch's insertion loss (IL) is lower than 5.9 ± 0.4 dB at 1550 nm. The crosstalk is lower than −20 dB within the C-band. The switch exhibits a 10%-90% rise time of 7.7 µs and a 90%-10% fall time of 3.4 µs at 1550 nm, respectively. As far as we know, it is the first demonstration of a poly-Si switch on an 8-inch wafer through pilot-line. The low-temperature deposited poly-Si switch is promising for multilayer active photonic devices and photonic-electronic applications.
2. Design, fabrication, and characterization
The devices were fabricated with the 180 nm pilot-line at the Institute of Microelectronics of Chinese Academy of Sciences (IMECAS). To achieve large polysilicon grain boundaries and reduce propagation loss, 220-nm-thick poly-Si was deposited on the 2-µm-thick SiO2 buffer under a low temperature of 620 °C. In our pilot-line, we used rapid thermal processing (RTP) for doping activation at a temperature of 1050 °C. The findings suggest that doping evaporation, which occurs at this temperature, does not significantly affect the performance of active devices [26], which prove this technology could be used to realize multilayer active photonic devices. All the components were etched with a thickness of 150 nm silicon and followed by the deposition of 1 µm SiO2 cladding. A broadband tunable laser system (Santec full-band TSL-550) was used to characterize the fabricated devices. The resolution is 5 pm in the measurements. The measured spectral responses were normalized with respect to the transmission of a straight waveguide connected with grating couplers on the same wafer. Two grating couplers are connected with a 300-µm-length straight waveguide. The grating coupler is designed with a 670 nm period and 50% duty circle, which has a −15.96 ± 0.59 dB coupling efficiency at 1550 nm with an 8° coupling angle.
The optimized 2 × 2 MMI used in the switch is shown in Fig. 1(a). Based on the self-imaging theory [27], the MMI-based splitter is compact and fabrication tolerant while maintaining low crosstalk between the two output waveguides. Taper waveguides decrease the loss caused by the MMI region and input/output waveguides. In order to characterize the excess loss and the uniformity of our optimized MMI, we designed and fabricated 6 MMIs in cascade. The microscope views of a series of cascaded MMIs are shown in Fig. 1(b). Transmission spectra from ports 1-7 in C-band are shown in Fig. 1(c). A linear transmission fitting yields an insertion loss of 4.19 dB per port at 1550 nm, as shown in Fig. 1(d), which means the excess loss of the MMI is 1.19 dB. A good uniformity of the MMI’s output is observed from the transmission of ports 6 and 7 with a power imbalance of 0.2 dB. Therefore, the loss is mainly caused by the propagation of poly-Si waveguides.
Fig. 1. (a) Schematic of 2 × 2 MMI (b) Microscopic image of the cascade MMI (c) Measured transmission spectra of the cascaded 2 × 2 MMIs at the wavelength range of 1460-1580 nm (d) Linear fitting of the normalized transmission at 1550 nm wavelength.
The p++ and n++ regions are formed by doping impurities with densities of approximately 1020 cm−3 to form an ohmic contact. To conduct the MZI switch, two 2 × 2 MMIs are connected by two straight doping waveguides as modulation arms. The cross-section of modulation arms is depicted in Fig. 2(a). The distance Sdope between the doping region and waveguide is optimized to be 0.8 µm. The length of the modulation arm is 170 µm. The radius of the bent waveguide inside the MZI is 50 µm to avoid bending loss.
Fig. 2. Cross-section view of PIN modulation arm(b) Schematic of π/2 phase shifter (c) Schematic of 2 × 2 PIN switch (d) Microscopic image of the switch proposed
The electro-optic switches based on the free-carrier dispersion (FCD) effect have a faster switching time than thermo-optic switches. However, the free-carrier absorption (FCA) effect will introduce undesired loss with increasing power consumption. Meanwhile, when large power consumption is required to obtain a π phase shift, an obvious self-heating effect will enlarge the imbalance between the two modulation arms, reducing the extinction ratio of the switch. Therefore, push-pull operation with a preset π/2 phase shifter on one arm is a popular method to decrease the phase shift needed for switching, resulting in less optical absorption and high extinction ratio [28]. The parameter of π/2 phase shifter is shown in Fig. 2(b). The schematic of the MZI is shown in Fig. 2(c). After fabrication, the microscope view of the MZI switch is shown in Fig. 2(d).
The spectra from the switch output ports are shown in Fig. 3(a) vias different input ports. With the π/2 phase shifter introduced, the output spectra are almost the same without electric power applied. The switch is tuned by applying electric power using source meter instruments (Keithley 2450). As shown in Fig. 3(b), the power consumption in the junctions and the transmittance at 1550 nm are measured simultaneously, while the applied voltage is from −4.5 V to 4.5 V with a step of 0.01 V. It reveals the measured I–V curve with anti-symmetry, simultaneously proving the push-pull property, free-carrier injection, and free-carrier depletion. When sweeping the voltage from −4.5 V to 4.5 V, we could determine “Bar” and “Cross” points for the central wavelength. The switch works on “Bar” and “Cross” states while the voltages are −3.35 V and 3.85 V. The power consumptions are 7.98 mW and 9.39 mW. Then the normalized transmission spectra from different input ports are depicted in Fig. 3(c-d). Over C-band, the crosstalk is lower than −20 dB range. The on-chip loss of the switch is 5.9 ± 0.4 dB. The error 0.4 dB presents the measurement error while the switch is “on” state.
Fig. 3. (a) Normalized transmission of the switch without tuning (b) Measured Power-Voltage (P-V, red) and Transmission-Voltage (O-V, blue) curves of the switch. Measured transmission spectra under on-off voltages while the light is coupled from (c) input 1 and (d) input 2.
The microring resonator (MRR) with 40 µm radius is fabricated and measured on the same chip to characterize the propagation loss. The width of ring is 500 nm, which is the same with propagation waveguides. After normalized with reference waveguides, the extinction ratio of the MRR shows a depth of 33.57 dB in Fig. 4(a). The free spectral range (FSR) of 2.738 nm is measured. The normalized transmission is shown in Fig. 4(b). The Lorentzian fitting (see the solid blue curve) is well applied to the measured data shown by the solid red circles. The full width at half maximum (FWHM) of the resonance peak for the present resonator is about Δλ = 331 pm at 1507.492 nm, indicating a loaded Q factor Qload of 4554 is obtained. The group index ng is calculated to be 3.30 through ng≈λ02/(FSR × L), where L is the path length of the ring. A propagation loss in the ring αring = 65.63 dB/cm can be estimated by
Fig. 4. (a) Normalized transmission spectruml for MRR in dB scale; (b) Normalized transmission of the deepest resonance peak
For the dynamic characterization, an arbitrary waveform generator (AWG, Siglent SDG6032X-E) is utilized to apply the modulation signal. The optical output of the device is connected to a high-speed photodetector. The driving and optical signal are recorded by an oscilloscope (Siglent SDS5104X) in real time. The dynamic response of the switch is shown in Fig. 5(a). A set of zoom-in figures of rising and falling edge of switch are shown in Fig. 5(b) and Fig. 5(c) to show the response time effectively. The driving signal is a 5 kHz square wave signal with a duty cycle of 50%. The high- and low-level voltage of the driving signal are on-off voltages of the switch, respectively. From Fig. 5(b), the switch achieves a 10%-90% rise time of 7.7 µs and a 90%-10% fall time of 3.4 µs at the wavelength of 1550 nm. The switch times are faster than the switching time of thermo-optic switches. The significant reduction of carrier mobility by grain boundaries in p-Si is an important reason for the speed limitation [29]. Response time can be suppressed to nanosecond by subsequent processes such as laser annealing to achieve better crystallization [30].
Fig. 5. (a)Time response of the switch, the red curve is the driving signal; the blue curve is the time response of the MZI when the switch works on ‘Cross’; the green curve is the time response of the MZI when the switch works on ‘Bar’ ; (b) a set of zoom-in figures of rising and falling edge of switch when it works on ‘Cross’ (c) a set of zoom-in figures of rising and falling edge of switch when it works on ‘Bar’
3. Disscussion
Table 1 gives the comparison of the performances of MZI-based optical switches. Benefiting from injection of free-carriers and modulation inside waveguide directly, the power consumptions of c-Si switch and p-Si switch proposed in this paper are lower than 10 mW. The bandwidth and extinction ratio are even better than c-Si switches. The insertion loss of p-Si switch is higher than c-Si switch due to scattering and absorption at the p-Si grain boundaries. The response time of p-Si switch is slower compared to crystalline silicon because of reduction of carrier mobility by grain boundaries in p-Si platform. Both propagation loss and response time can be improved by laser annealing to achieve better crystallization [30,34]. Deposition at a low temperature of p-Si device is promising to realize multilayer active platform. Separation by implantation of oxygen (SIMOX) requires annealing at a very high temperature to cure the implantation damages, which is not compatible with doping activation [35]. In a conclusion, p-Si devices are promising to be stacked repeatedly and realize multilayer active photonics integrated circuits.
Table 1. The comparison of the performances of MZI-based optical switches.
4. Conclusion
In conclusion, we have demonstrated a broadband Mach-Zehnder interferometer optical switch in poly-Si. With the π/2 phase shifter, the push-pull modulation configuration is incorporated into the switch design. The 2 × 2 poly-Si switch works in “Bar” and “Cross” states while the applying voltages are −3.35 V and 3.85 V. The power consumptions are 7.98 mW and 9.39 mW. The crosstalk levels are below −20 dB, covering the whole C-band. The insertion loss of the switch is 5.9 ± 0.4 dB at 1550 nm. The demonstrated poly-Si switch is deposited under a temperature of 620 °C on an 8-inch wafer pilot-line, which is essential for realizing high-throughput, low-power, reconfigurable, short-range optical interconnects. Moreover, low temperature is possible for multilayer active photonic devices and photonic-electronic applications.
Funding
National Key Research and Development Program of China (2019YFB2203001); Open Fund of the State Key Laboratory of Integrated Optoelectronics (IOSKL2020KF06).
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.
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