Surface energy engineering for LiTaO3 and α-quartz SiO2 for low temperature (<220 °C) wafer bonding

Abstract

Wafer bonding can be substituted for heteroepitaxy when manufacturing specific heterojunction-based devices. Devices manufactured using wafer bonding include multijunction solar cells, integrated sensors, heterogeneously integrated photonic devices on Si (such as high-performance laser diodes), Mach-Zehnder modulators, photodetectors, optical filters, and surface acoustic wave devices. In these devices, creating heterointerfaces between different semiconductors with heavily mismatched lattice constants and/or significant thermal expansion mismatch presents significant challenges for heteroepitaxial growth. High costs and poor yields in heavily mismatched heteroepitaxy can be addressed by wafer bonding in these optoelectronic devices and sensors, including the LiTaO3/Si and LiTaO3/SiO2 heterostructures. In the present work, heterostructure formation between piezoelectric LiTaO3 (100) and Si (100) and α-quartz SiO2 (100) is investigated via wafer bonding. Direct bonding is selected instead of heteroepitaxy due to a significant thermal expansion mismatch between LiTaO3 and Si-based materials. The coefficient of thermal expansion (CTE) of LiTaO3 is 18.3 × 10−6/K. This is 1 order of magnitude larger than the CTE for Si, 2.6–2.77 × 10−6/K and 25–30 times larger than the CTE for fused SiO2 and quartz (which ranges 0.54–0.76 × 10−6/K). Thus, even at 200 °C, a 4 in. LiTaO3/Si bonded pair would delaminate with LiTaO3 expanding 300 μm in length while Si would expand only by 40 μm. Therefore, direct wafer bonding of LiTaO3/Si and LiTaO3/SiO2 is investigated with low temperature (T < 500 K) Nano-Bonding™, which uses surface energy engineering (SEE). SEE is guided by fast, high statistics surface energy measurements using three liquid contact angle analysis, the van Oss/van Oss–Chaudhury–Good theory, and a new, fast Drop Reflection Operative Program analysis algorithm. Bonding hydrophobic LiTaO3 to hydrophilic Si or SiO2 is found to be more effective than hydrophilic LiTaO3 to hydrophobic Si or SiO2 temperatures for processing LiTaO3 are limited by thermal decomposition LiTaO3 into Ta2O5 at T ≥ 180 °C due to Li out-diffusion as much as by LiTaO3 fractures due to thermal mismatch.

Dewetting behavior of Ge layers on SiO2 under annealing

“… = 1.5 J/m 2 can be taken as an average value. The surface energy of SiO 2 can be determined through the contact angles of droplets (particles), which are formed due to the dewetting of various liquids on SiO 2 70 , 71 , including melted Ge 34 . This gives ~ 0.05 J/m 2 .…”

GaAs to Si Direct Wafer Bonding at T ≤ 220°C in Ambient Air via Nano-BondingTM and Surface Energy Engineering (SEE)

“…NB is a new DWB process at T ≤ 220°C [9,18,19,[23][24][25]. NB uses SEE to cross-bond wafers at the nanoscale and 2D-PP to modify γ T0 and H-A 0 by bringing each surface in the wafer pair to synergistic 'farfrom-equilibrium' states [14,15,17,24,26].…”

“…They are characterized by 3LCAA, then a few 7 mm x 7 mm squares pieces for conducting HR-IBA and a few 10 mm x 10 mm square pieces for XPS. Wafers 3 The vOCG theory was proposed in 1989 as a three-component theory for γ T [9,18,19,[23][24][25]27]. The theory states in Eq.…”

“…The model for this work's approach is low temperature NB, which can lower thermal budgets for DWB to ≤ 220°C and wafer pair compression to P ≤ 60 kPa (9 psi). NB lowers thermal budget via Room Temperature Surface Energy Engineering (SEE) for compound semiconductors [9,18,19,[23][24][25]. SEE modi es initial total surface energies, γ T0 , to far-from-equilibrium states, γ T *, so direct cross-bonds between two different crystals can occur via direct donor-acceptor interactions between SEE-modi ed surfaces.…”

“…At the same time, optimizing SEE can raise the initial γ T0 to 'far-from equilibrium' γ T * and vice-versa. The NB model and SEE are designed by comparing surfaces and NB before and after SEE [24]. Figure 1 shows four such experiments.…”