Direct wafer bonding is replacing hetero-epitaxy in semiconductor-based hetero-structures such as tandem solar cells and sensors. Bonding of Li-based piezo-electronic perovskites, specifically LiTaO3 (100) and LiNbO3 (100) to Si (100) and a-quartz SiO2 (100) is investigated because these perovskites exhibit unique acoustic and piezoelectric properties used in Surface Acoustic Wave (SAW) devices. A direct wafer bonding process called NanoBondingTM  can bond LiTaO3 (100) or LiNbO3 (100) to Si using surface energy modification in ambient, low temperature (<220°C) conditions with neither plasma activation nor UHV.
The principle underlying NanoBondingTM is to cross-bond at the molecular scale two surfaces over large interfacial domains, creating a 2-D bonding interphase. NanoBonding™ uses surface engineering to modify surface energy to promote electronic exchange during contact and bond activation.
Electron exchange is promoted between the surfaces by engineering them as hydrophilic-hydrophobic pairs, with each surface in a far-from-equilibrium state. The hydro-affinity of the initial surfaces is measured quantitatively using surface energy analysis via 3LCAA and the van Oss-Chaudhury-Good (vOCG) theory. Next, the hydro-affinity is modified to bring the surface far-from-equilibrium - from a hydrophobic to a hydrophilic state or vice-versa to bring the paired complementary far-from-equilibrium states.
NanoBonding™ includes planarization of both surfaces at three scales - nano-, micro-, and macro-scale. Creating extended atomic terraces by etching and reducing macroscopic wafer warp enable for direct mechanical contacting over large surface areas (Nano-contacting).
After Nano-contacting, cross-bonding is activated via electromagnetic radiation, such as heat, or ultra-violet illumination, or via isotropic pressurization using steam .
The surface energy ��T is thus measured by 3LCAA on the “as received” LiTaO3(100), LiNbO3(100), Si(100), and a-quartz SiO2(100) 4” wafers from TDC Corp. The uniformity of ��T across the 4” LiTaO3 wafer is mapped in 10 locations in a Class 100 hood using 10 µL droplets. The three liquids used are 18 MΩ DI water, glycerin, and α-bromo-naphthalene. ��T is then computed via the vOCG theory from the three interaction components that yield ��T, the LifeChat-Van der Waals molecular interaction energy ��LW, the interaction energy of the surface with donors, ��+, and with acceptors (��-). Increasing the values of ��- and ��+ promotes ionic bonding, electronic exchange and molecular cross-bonding.
��T across the 4” LiTaO3 wafer averages 43.3 ± 2 mJ/m2, thus native LiTaO3 surfaces are hydrophobic. Native Si(100) surfaces, by comparison, are always hydrophilic and have a typical surface energy of 53.0 ± 0.2 mJ/m2, as measured over several 4” wafers. However, ��T uniformity Si(100) 4” wafers is better by an order of magnitude than on LiTaO3. Several NanoBonding™ experiments were then conducted with LiTaO3 (100) on Si (100), and a-quartz SiO2 (100) where the hydroaffinity of LiTaO3 was reversed to hydrophilic by etching with aqueous HF and the hydroaffinity of native Si (100) reversed to hydrophobic.
Ion Beam Analysis and XPS is used to investigate the surface stoichiometry of LiTaO3(100) before and after etching to correlate to ��T and NanoBonding™ results.
 US Patents #9,018,077; #9,589,801, Herbots N. et al. (2015); (2017)
5:00 PM–7:00 PM Apr 23, 2019 (US - Arizona)
PCC North, 300 Level, Exhibit Hall C-E