Fabrication Tolerant Heterogeneously Integrated Lithium Niobate Modulator on Bi-Layer Silicon Nitride Using Micro Transfer Printing

The integration of Lithium Niobate (LN) on low-loss, CMOS-compatible SiN platforms combines the high Pockels effect of LN with SiN's low loss and CMOS compatibility, enabling efficient modulation and processing. Micro transfer printing (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu$</tex> TP) facilitates the integration of functional materials such as LN, III-V, and BTO, enabling the creation of compact, multifunctional chips. LN-SiN Mach-Zehnder Modulators (MZMs) require long interaction lengths (in the cm range) for low-voltage operation, necessitating <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu$</tex> TP of em-long LN coupons. These coupons should ideally cover the MZM arms, including signal and ground electrodes <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\sim 100\mu \mathrm{m}$</tex> wide). However, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu$</tex> TP of long coupons faces challenges such as limited rotational alignment accuracy, especially if the electrode incorporated on the LN coupon. Additionally, hybrid SiN-LN modulators encounter mode transition loss and reflections at the SiN-LN interface, which are typically mitigated with long tapers. However, tapering LN coupons for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu$</tex> TP often causes cracks and fabrication complexities [1]. This work presents design (Fig. 1(a)) and fabrication of LN-SiN modulator with simulated <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{V}\pi\mathrm{L}=3.6\mathrm{V}$</tex> -cm (Fig. 1(b)) and 3-dB bandwidth of over 100 GHz (Fig. 1(d)). The design features bi-layer SiN waveguides (750 nm and 200 nm thick) for adiabatic optical mode coupling [2], achieving ultra-low transition loss, and minimal reflection without LN tapers thereby alleviating the alignment accuracy requirement for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu$</tex> TP [3]. We also present the fabrication of the device, including the high-yield fabrication and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu$</tex> TP of LN coupons up to 1 cm long, 100 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu\mathrm{m}$</tex> wide.