Characterizing Surface Energetics of Wafer Bonding of LiTaO₃ to Si and SiO₂ via Three Liquid Contact Angle Analysis (3LCAA) and Gibbs Free Energy

Monolithic integration of piezoelectrics is key in the Internet of Things.

This work investigates direct wafer bonding, or Nano-Bonding™ (NB), via Surface Energy Engineering (SEE) at RT to bond LiTaO₃ to Si and SiO₂, because the thermal expansion of LiTaO₃(110), is mismatched to Si by a factor 8 to α-quartz SiO₂ by 25. In addition, LiTaO₃ decomposes into Ta₂O₅ and Li vapor at T ≥ 500K.

SEE modifies surface energies, SE^T, and hydro-affinities to far-from-equilibrium states, based on Three Liquid Contact Angle Analysis (3LCAA) measurements. The SE^T of as received 6” LiTaO₃ averages 41.3 ± 2 mJ/m², and its water contact angle, 45.3± 6°. This is much more variable than as received 4” B-doped p+ Si, with 53 ± 0.2 mJ/m², and 38 ± 1°. After SEE, LiTaO₃ becomes uniformly super-hydrophilic.

If contacted to within 60 seconds of SEE, LiTaO₃ reverts in air to hydrophobic after minutes. Its water contact angle converges reproducibly to 66.3 ± 1°. At that point SE^T of LiTaO₃ returns 40.6 ± 2 mJ/m².

The dependence of contact angles with polar water and glycerin, and with the apolar alpha-bromo-naphthalene, as well the SE^T are mapped on 6” LiTaO₃ wafers using sets of 10 drops for each liquid. Their correlation with crystal direction will be discussed.