Technology Overview

Silicon photonics is a transformative enabling technology that brings parallelism, high-throughput, power-efficiency, and cost-effectiveness to computing.  Integration and packaging between silicon photonics and silicon CMOS components can exploit 2D, 2.5D, or 3D as well as even monolithic integration technologies thanks to the compatibility between the two utilizing the form factors of silicon electronics.

Recently, both electronic integrated circuits (EICs) and photonic integrated circuits (PICs) have been seeking heterogeneous integration.  Modern EICs often integrate heterogeneous circuits involving analog and digital, or involving silicon CMOS and Ge bipolar transistor circuits. Likewise, photonic integration also benefits from integrating heterogeneous materials.  While silicon photonics is rapidly emerging as a viable and possibly ubiquitous photonic integration platform, silicon lacks optical gain, Pockel’s effect, and Faraday effect useful for realizing lasers, phase modulators, and non-reciprocal devices.  Hence, co-integration of silicon with III-V materials, electro-optical dielectrics, and magneto-optical materials can greatly enhance the functionalities of heterogeneously integrated microsystems.  Table 1 illustrates various photonic integration platforms indicating benefits from heterogeneous integration.

Table 1. Various photonic integration platforms

Heterogeneous integration can be achieved by either monolithic integration or hybrid integration.  In particular, monolithic integration by hetero-epitaxy is attractive in that it can exploit large host substrates (e.g. silicon wafers) and epitaxially grow III-V or other compound materials on a wafer-scale at dimensions defined by the host substrate. Overall, monolithic integration by heteroepitaxy remains challenging for achieving low-defect heterogeneous integration. In contrast to monolithic integration, hybrid integration methods do not attempt material growths but utilize various bonding mechanisms between dissimilar (or similar) materials already grown on individual substrates.  At UC Davis, we pursue heterogeneous integration by various methods including wafer bonding, micro-transfer printing, and special deposition techniques to integrate III-V, LiNbO3, and other specialty materials.  

Current Research Activities

Hybrid III-V/Si Lasers and Optical Amplifiers

Utilizing wafer-bonding methods, we designed, fabricated, and demonstrated the record efficiency semiconductor optical amplifiers at 1550 nm on silicon.  As shown in Figure 1, fabrication at UC Davis and UC Berkeley cleanrooms begins with a 150 mm diameter silicon-on-insulator (SOI) wafer which consists of a 500 nm thick top silicon layer and a 3 μm  buried oxide (BOX). The wafer goes through initial RCA-1 and RCA-2 cleaning and is then coated with 400 nm of Rohm Haas UV210-0.6 photoresist, soft-based at 130º C for 60 seconds and then exposed with a ASML PAS 5500 300 deep-UV lithography stepper using a 248nm-KrF light source at 18mJ/cm2 with appropriate focus correction. The detail of the fabrication steps are described in reference [1, 2].  Figure 2 shows completed hybrid InP/Si SOAs.

Figure 1. Fabrication flow for (a) 500nm tall silicon waveguide and VOC layers, (b) Cr/AuZn metal hard mask and III-V mesa definition, (c) N-contact metal lift-off, (d) additional p-contact metal , (e) H+ proton implantation at 120keV and 160keV, (f) removing 300nm n-InP layer from passive sections, (g) 1µm SiO2 isolation layer, (h) thick Ti/Au metal probing pads.
Figure 2. (a) Nomarski image of fabricated hybrid SOA, (b) Up-close image of III-V taper, and (c) cross-sectional SEM image

Hybrid LiNbO3 waveguides on silicon substrate

Lithium Niobate crystals offer a number of attractive properties for optical devices, including wide transparency windows, large electro-optic effect, high second-order susceptibilities [3]. Conventional LiNbO3 devices have been widely used for high-performance light modulators [4, 5]. However, these devices are fabricated by diffusion or ion-exchange waveguide fabrication processes, leading to relatively large footprints and large Vπ due to the small refractive index contrast between the core and cladding. Recently, heterogeneous integration of LiNbO3 thin-film on insulator [6, 7] has attracted strong interest as a new platform for compact photonic integrated circuits (PICs). It allows reducing the size of the devices thanks to stronger light confinement using higher refractive index contrast.  Motivated by these advantages, we design complementary building blocks oriented for light filtering compatible with large-scale integration. We can envisage to enable wavelength-division-multiplexing on-chip for this platform.

As illustrated in Figure 3[8] we designed, fabricated, and demonstrated low-loss Array Waveguide Gratings (AWG) on a compact LiNbO3 platform. Based on this platform, we designed a 8×500 GHz LiNbO3 AWG. The device is composed of two star-couplers and an array of waveguides with a linear increment of the optical path lengths. The typical footprint of the device is 5 × 3 mm2.

Figure 3 [8]: Designed TM mode AWGs at λ=1550 nm in z-cut LiNbO3 rib waveguide with w=2µm, h=400 nm, d=200nm and 70° angled sidewall, SEM images of cleaved LiNbO3 on SiO2 waveguides, and mcroscope pictures of a LiNbO3 AWG device after SiO2 cladding deposition

CMOS-Compatible Athermal Silicon Photonics

Silicon photonic devices are sensitive to environmental temperature variations since silicon has a relatively large thermo-optical coefficient (TOC) of ~1.84×10-4/°C. One method of thermal stabilization is to adopt an upper cladding made of a material with negative TOC to compensate for the positive TOC of silicon. Titanium dioxide has a strong negative TOC of ~-(1~2)×10-4/°C, and is compatible with CMOS processes offering superior reliability than polymers, thus it has been attracting strong interest. We designed, fabricated, and tested athermal silicon waveguides clad with TiO2  with athermal characteristics at 1550 nm and also additional devices with temperature-dependent resonant wavelength shifts less than 5 pm/°C exhibiting second order effects near 1310 nm{Feng, 2014 #3082;Shang, 2013 #2986;Djordjevic, 2013 #2994;Djordjevic, 2012 #2985;Zhou, 2009 #2317;Zhou, 2009 #2322}.

Figure 4 Illustration of fabricated devices: a) cross-section schematic of the phase tuning section in the ring resonator after filling the trench with TiO2; b) SEM image of the ring resonator with 275 nm wide waveguide, prior to TiO2 deposition; c) magnified view of the section of waveguide to ring directional coupler, illustrating the principle of trenching; d) transition of trenched waveguide to waveguide clad with SiO2 (not trenched).
Figure 5 a) evidence of blue shift with temperature increase in 250nm wide waveguide device b) summary of measured (square markers) and fitted values of resonant frequency shifts for different waveguide width devices. Inset marks the waveguide width.

References

[1]        S. Cheung, Y. Kawakita, K. Shang, and S. J. Ben Yoo, “Highly efficient chip-scale III-V/silicon hybrid optical amplifiers,” Optics Express, vol. 23, pp. 22431-22443, 2015/08/24 2015.

[2]        S. Cheung, S. Tiehui, K. Okamoto, and S. J. B. Yoo, “Ultra-Compact Silicon Photonic 512 x 512 25 GHz Arrayed Waveguide Grating Router,” Selected Topics in Quantum Electronics, IEEE Journal of, vol. 20, pp. 310-316, 2014.

[3]        R. S. Weis and T. K. Gaylord, “Lithium niobate: Summary of physical properties and crystal structure,” Applied Physics A, vol. 37, pp. 191-203, 1985.

[4]        E. L. Wooten, K. M. Kissa, A. Yi-Yan, E. J. Murphy, D. A. Lafaw, P. F. Hallemeier, D. Maack, D. V. Attanasio, D. J. Fritz, G. J. McBrien, and D. E. Bossi, “A review of lithium niobate modulators for fiber-optic communications systems,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 6, pp. 69-82, 2000.

[5]        D. Janner, D. Tulli, M. Garcı́a-Granda, M. Belmonte, and V. Pruneri, “Micro-structured integrated electro-optic LiNbO3 modulators,” Laser & Photonics Review, vol. 3, pp. 301-313, 2009.

[6]        G. Poberaj, H. Hu, W. Sohler, and P. Günter, “Lithium niobate on insulator (LNOI) for micro-photonic devices,” Laser & Photonics Reviews, vol. 6, pp. 488-503, 2012.

[7]        A. Rao, A. Patil, P. Rabiei, A. Honardoost, R. DeSalvo, A. Paolella, and S. Fathpour, “High-performance and linear thin-film lithium niobate Mach–Zehnder modulators on silicon up to 50GHz,” Optics Letters, vol. 41, pp. 5700-5700, 2016.

[8]        M. Prost, G. Liu, and S. J. B. Yoo, “A Compact Thin-Film Lithium Niobate Platform with Arrayed Waveguide Gratings and MMIs,” in 2018 Optical Fiber Communications Conference and Exposition (OFC), 2018, pp. 1-3.