A modular fabrication process for thin-film lithium niobate modulators with silicon photonics

Abstract

We report advancements in the fabrication of electro-optic Mach-Zehnder modulators made by bonding an unetched thin film of lithium niobate to a second chip with rib waveguides in another material, such as silicon. Devices were fabricated after storing bonded silicon-lithium niobate chips in a common laboratory environment for more than three years. The chips survived the full processing flow and yielded modulators with greater than 50 GHz 3-dB electro-optic bandwidth, and VπL less than 3 V-cm at 1550 nm and equivalent performance to freshly-bonded and processed chips. Furthermore, we demonstrate the co-integration of hybrid bonded thin-film lithium niobate modulators and silicon photonics based high quality-factor ring resonators and higher-order coupled microring optical filters. The silicon microring resonators are used for photon-pair generation at 1550 nm using spontaneous four-wave mixing. These results show the feasibility of a modular modulator fabrication procedure, where the planarization and bonding steps are performed for a batch of chips at one time and smaller sub-batches are customized by end users at a much later time according to their needs and convenience.

110 GHz, 110 mW hybrid silicon-lithium niobate Mach-Zehnder modulator

“…Figure 3(a) illustrates the direct bonding hybrid Si/LN modulator fabrication process. Before bonding, the surfaces of the LNOI and SOI singulated-dies were cleaned using an RCA-1 process followed by plasma surface activation 19 . After plasma surface activation, the dies were soaked in deionized water.…”

“…To improve the bond strength, the bonded sample was annealed using temperature cycles up to 300 • C under an applied pressure of 9.8 N.cm −2 . At this stage, as described in our earlier work 19 , the bonded samples can be stored for processing at a later stage, if required. A plasma-enhanced-chemical-vapor-deposition (PECVD) was used to deposit a few microns of thick SiO 2 as the top cladding.…”

“…The asymmetric MZM under test was measured using a tunable CW C-band laser source which resulted in a free spectral range of 2.3 nm, a mean extinction ratio (ER) of 28 dB over 50 nm (maximum of 31 dB at λ = 1560 nm), and a fiber-to-fiber IL of 12.2 dB as shown in Fig 4(a). The half-wave voltage of the Si/LN modulator with L ps = 0.5 cm was measured by overdriving it with a trapezoidal signal and projecting the overshoots of the modulated signal 19 , resulting in an average V π L of 3.1 V.cm in the range of 0.1 to 10 MHz as shown in Fig. 4(b).…”

“…TFLN-based waveguides have been made using various methods such as etching, blade-dicing or polishing of ridge or rib waveguides, and rib-loading of a TFLN slab using other materials 1,[9][10][11][12][13][14][15][16][17][18][19] . Our approach is based on hybrid optical modes, in which the waveguide core consists of a combination of silicon (Si) and LN, and the cladding consists of silicon dioxide (SiO 2 ).…”

“…The Si waveguides are formed in the crystalline layer of a silicon-on-insulator (SOI) wafer, and are tapered in the hybrid region to implement inter-layer optical mode transitions; the TFLN layer is not etched or patterned. The Si waveguides can be continuously connected, without breaks, to other photonic components outside the bonded region, and thus present a simple and scalable way to include ultrahigh bandwidth EOMs with other photonic components 19,20 .…”