7.8 Peeling the crystal

To shrink a lithium niobate waveguide below a micrometer, the crystal itself has to be thin and sit on something with a much lower index. A related method came from silicon. In 1995 Michel Bruel at CEA-LETI in Grenoble implanted hydrogen into silicon, bonded it to another wafer, and heated it until a thin layer split off: Smart Cut [Bruel 1995 rec]. In 1998 a Columbia-led team implanted ions deep into lithium niobate and lifted off 9-micrometer single-crystal films [Levy et al. 1998 rec]. In 2004 ETH Zurich sliced films 680 nm thick onto silicon dioxide and found the electro-optic coefficient intact [Rabiei & Gunter 2004 rec]. Paderborn and Zurich used the name lithium niobate on insulator, LNOI, in a 2012 review, when it already came as 3-inch wafers [Poberaj et al. 2012 rec]. A company in Jinan, NANOLN, founded in 2010, sold the films [NANOLN 2026 rec].

Ion slicing. Levy et al.: “Deep-ion implantation is used to create a buried sacrificial layer in single-crystal c-cut poled wafers of LiNbO₃”; 9 µm films separated and bonded to Si and GaAs [Levy et al. 1998 rec]. The same group's patent gives 3.8 MeV for a damage layer near 10 µm depth and dilute HF etching, without tying them to the paper [Levy et al. 1998 rec]. Rabiei and Gunter: 680 nm films on SiO₂ by ion slicing and wafer bonding, crystalline and optical quality comparable to bulk, r₃₃ = 31 pm/V at 633 nm [Rabiei & Gunter 2004 rec]. CEA-LETI transferred LiNbO₃ layers by Smart Cut for RF acoustic resonators in 2008-2010 [Pijolat et al. 2008 rec; Moulet et al. 2008 rec; Pijolat et al. 2009 rec; Deguet et al. 2010 rec].

First devices. EO-tunable microrings in a bonded film [Guarino et al. 2007 rec]. Photonic wires by Ar milling down to 1 x 0.73 µm²; 9.9 dB/cm (qTM) and 12.9 dB/cm (qTE) at 1.55 µm [Hu et al. 2009 rec]. LNOI review: sub-micrometer films by ion slicing, two bonding techniques, 3-inch wafers, PP-LNOI SHG, microrings, photonic crystals [Poberaj et al. 2012 rec]. Harvard microdisks: Q about 10⁵, on-chip SHG 0.109 /W [Wang et al. 2014 rec].

Before thin-film wafers. Ridge waveguides were etched into epitaxial Zn-substituted LN by ion-beam enhanced etching: near-vertical sidewalls, 0.9 dB/cm at 1064 nm [Hartung et al. 2008 rec].

Loss, 2009 to 2017 (derived): 9.9 dB/cm [Hu et al. 2009 rec] to 2.7 dB/m = 0.027 dB/cm [Zhang et al. 2017 rec], a factor of about 370.

References

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3. Rabiei & Gunter, Applied Physics Letters 85 4603-4605 (2004). Optical and electro-optical properties of submicrometer lithium niobate slab waveguides prepared by crystal ion slicing and wafer bonding. Issued 2004-11-15. doi:10.1063/1.1819527
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