NB uses SEE to remove native oxides and planarize wafer surfaces by reducing macro-scale wafer warp and both nano- and micro-scale roughness. At the same time, SEE modifies γT0 and H-A0 to bring surfaces into a ‘far-from equilibrium’ state and terminate surfaces with 2D-PPs. When 2D-PPs come into nano-contact under mechanical compression between 35 to 60 kPa, molecular cross-bonding is catalyzed via electron and/or ionic species exchange and form a cross-bonding interphase.
The initial, “as-received” γT0 and HA0 values and their modification into far-from-equilibrium γT* and H-A* values are measured via Three Liquid Contact Angle Analysis (3LCAA). Oxygen coverage is measured via High Resolution Ion Beam Analysis (HR-IBA), which combined < 111 > ion channeling with the 3.039 ± 0.01 MeV α(16O,16O)α nuclear resonance to achieve a threshold detection of about 0.2 oxygen monolayers (~ 1014 at/cm2) by increasing the signal-to-noise ratio for O in Si by a factor of 1600. Surface oxidation states are measured via X-Ray Photoelectron Spectroscopy (XPS). Two identical wafers are processed for each experimental condition to establish reproducibility.
4.1 Surface Energy Engineering of Si (100) and GaAs (100)
A boat of twenty-five 2′′ Te-doped n + GaAs (100) wafers cut from the same Czochralski-grown GaAs single crystal ingot labeled numbers 1 to 25 according to their slot location is used. As stated above, wafers are grouped into sets of identical processing conditions to establish reproducibility.
The design of the experiment is as follows. For identification, the simple sequential integer numbers from the boat are assigned to each wafer.
“As-received” n + GaAs (100) wafers are labeled 1 and 2 and come from sequential boat slots 1 and 2. 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 and 4 undergo SEE via etching with the most dilute aqueous NH4OH:H2O (1:10) solution for 10 minutes, then rinsed for 10 minutes in 18 MΩ deionized H2O. The aqueous NH4OH:H2O solution is effective in minimizing surface roughening and preserving the GaAs surface stoichiometry.
Similarly, numbers 5 to 8 are assigned to four 4′′ Boron-doped p + Si (100) wafers taken from slot #5–8 in a 25 wafers boat. Two “as-received” p + Si (100) are labeled 5 and 6 and characterized via 3LCAA. The two next wafers undergo identical SEE via a modified Herbots-Alturi process where the final 'passivation' etch is conducted with aqueous HF:H2O (1:20) instead of HF:CH3OH (1:20) [14, 18, 19].
SEE removes native oxides and passivates Si and GaAs surfaces with OH− and H+ termination, respectively. To minimize contamination, “as-received” wafers undergoing SEE are kept and processed in a Class 10/ISO 4 laminar flow hood in a Class 100/ISO 5 clean room. In addition, surface energy measurements via 3LCAA on “as-received” wafers that have undergone SEE are also done in a class 10/ISO 4 laminar flow hood in a Class 100/ISO 5 clean room. Following surface energy measurements, samples are characterized using IBA and XPS a few weeks later.
4.2 Three Liquid Contact Angle Analysis, Surface Energy vOCG theory and SEE
The vOCG theory was proposed in 1989 as a three-component theory for γT [9, 18, 19, 23–25, 27]. The theory states in Eq. (1) that γT is a simple sum of the surface energy component due to molecular interactions, or Lifshitz-van der Waals interaction energy (γLW) with the square root of the product of the surface energy component of interactions due to electron donor (γ+) and the surface energy component due to interaction with electron acceptors (γ−):
γ T = γLW + 2 √(γ+ γ− ) (1)
The vOCG theory was selected for the GaAs/Si pair as the best model to analyze the γT of semiconductor surfaces such as Si and GaAs. It can also reliably be used to analyze native oxides like SiO2 and GaAs oxides because these solids can exhibit molecular interactions, such as acceptor/donor interactions.
3LCAA can characterize γT because in the vOCG theory, the three unknown components γLW, γ+, and γ− used to compute γT can be measured via three different contact angles from three different liquids interactions with the surface, provided these three liquids have well-known surface energies and different molecular dipole moments.
The three liquids used in the present work are (i) 18 M deionized water with one hydroxyl (OH−) radical per molecule, 1.8546 D, which has a very high dipole moments (ii) glycerin with three OH− radicals per molecule, which has a dipole moment of 0.02 D and (iii) non-polar α-bromo-naphthalene, a geometrically flat molecule with strong donor/acceptor interactions due to Bromine. Figure 2.a shows the molecular structure of all three liquids.
3LCAA is performed in a class 100/ISO 5 laminar flow hood as shown in Fig. 2. All containers and surfaces are made of electronic-grade poly-propylene, boro-silicate glass, or Teflon to avoid organic contaminants.
Contaminant particulate filtration is critical to both SEE and 3LCAA. Figure 2 shows a schematic of the 3LCAA optical bench inside a Class100/ISO 5 laminar flow hood.
To capture high-resolution images of drops and their reflections, a 20 MP reflex digital camera is mounted on an adjustable polypropylene platform inside the Class 100/ISO 5 laminar flow hood. The platform supporting the wafer is a 6" Si wafer made level via four supporting adjustable nylon bolts (Fig. 2.b). Subsequently, three rows metered 10 µL liquid drops mapping the largest diameter of each wafer measured are photographed under precise illumination from a full solar spectrum light with ultra-high dark/white contrast and high resolution and analyzed via a specifically developed image analysis software called DROP™ . To extract contact angles from the drop contour and the drop reflection on the wafer polished surfaces, DROP™ uses fifth- order polynomial recursive correlated fits of both the 2D-drop boundary with air and of its reflection, as depicted in Fig. 2.a . DROP™ then computes contact angles as the angle made by the first order computed derivative at the air-liquid solid triple point as shown in Fig. 2.b.
A set of three Young-Dupré equations for the three unknowns, γLW, γ+ and γ−, can be derived by combining Eq. (1) with Young's Equation written for a planar surface geometry in (2):
Equation (2) expresses the relationships between the surface total surface energies at the triple point where the solid, gas (air) and liquid phases meet.
The total surface energies for the three surface phases are γTS for the solid surface, γTSl for the solid-liquid interface, and γTL for the liquid phase.
Now, Young's Equation (2) for a fully planar surface can then be rewritten for each of the identified components of the total surface energy according to the vOCG theory as shown in Equations (3) and (4).
The component identified as the surface energy of molecular interactions, γLW, is written as:
The energy of so-called polar interactions, γPOLAR is decomposed into the surface energies of interaction for electron donors γ+ and acceptors γ- as:
The total surface energy γT can then be computed via Equation (5) at a given position via the known γL of the three liquids components, γWater, γGlycerin, and γaBr-n, and by averaging the four extracted contact angles for each of the 3-10 drops.
For each of the three interacting liquids with the solid surface, the γLW, γ+ and γ− components of a solid surface can be mapped, and its γT can be computed via Eq. (1). Young-Dupré equation combines Young's equation and the vOCG model to correlate surface energy components of the solid, from the surface energy components of the interacting liquid, and the contact angle made by the interacting liquid with the solid surface. Hence, the solution of three sets of Young-Dupré equation yields three unknown surface energy components of the solid surface namely, Lifshitz-van der Waals interaction energy γLW, the energy of interaction of electron donors, γ+, and energy of interaction with electron acceptors γ−) to compute total surface energy γT of the solid by using vOCG.
DROP™ with an average error of ≤ ± 1° enables precise computation of solid surface energy with an average error ≤ ± 1 mJ/m2. Measuring water contact angles to ± 1° makes possible precise characterizations of hydro-affinities as well.
4.3 Ion Beam Analysis
Absolute Oxygen surface coverage is measured by High Resolution IBA, which combines < 111 > channeling with the 3.039 ± 0.01 MeV α(16O, 16O)α Oxygen Nuclear Resonance (NR), in a 10− 7 to 10− 8 Torr vacuum. IBA measure absolute 16O coverage via Rutherford Backscattering Spectrometry (RBS) using α-particles elastic collisions with atomic nuclei. The α backscattering energy scales with the mass of the scattering nucleus. 16O has a lighter mass than Si and GaAs, which limits 16O detection in RBS as the large Si and GaAs substrate signals overlap with the small O16 signal from native oxides. However, High Resolution in IBA can be achieved, by channeling the particle beam along Si and GaAs < 111 > crystal axis. <111 > channeling decreases the backscattering yield of Si and GaAs by a factor 40 to a channeling yield which is 2.5 % of the backscattering yield in an un-channeled, Rotating Random (RR) spectrum. Hence channeling can increase the signal to noise ratio for 16O by 1/2 order of magnitude RR spectra are acquired by rotating the crystal about ~ 5o off the < 111 > axis to average backscattering yield in all crystal directions.
Moreover, since the crystal (100) surface plane is tilted to an angle of ~ 54.75° to achieve channeling along the < 111 > axis, the geometric thickness sampled within the native oxide by the beam, increases by a factor (1/cos 54.75°) = 1.73. Thus, the magnitude of the O16 increase the signal to noise ratio by another 75%. Finally, conducting IBA with 3.039 ± 0.010 MeV α(16O, 16O)α NR further increases the yield of the 16O signal by an additional NR factor of ~ 23. NR profiling is conducted in incremental 3 keV steps around the 3.039 MeV resonance energy. At resonance, the 16O signal is simulated by iteratively by matching the 16O signal channeling counts in NR to simulated RR 16O counts, via the simulation software SIMNRA .
In summary, HR IBA increases the signal-to-noise ratio by a factor 40 x 23 = 910, thus about three order of magnitudes over RBS. Using < 111 > channeling instead of < 100 > improves O detection by a factor 1600, as can be seen in Fig. 4.a. and Fig. 7.a.
4.4 X-Ray Photoelectron Spectroscopy
XPS is used to measure the bonding states of Ga and As and their relative ratio at the surface before and after SEE. XPS spectra are obtained by irradiating the surface with a 1486 ± 0.7 eV monochromatic X-ray beam. Photoelectrons are emitted from the top ~ 5–10 nm of the surface and analyzed as a function of their binding energy. XPS spectra are fitted via Casa XPS 2.3.19, which computes the relative amounts of an atomic species in its various chemical states via tabulated Relative Sensitivity Factors . XPS detects quantitatively changes in oxidation states of As, by measuring the relative ratio As2O5 and As2O3. Four data points are collected on two different samples cut from GaAs wafers before and after SEE to establish reproducibility .
4.5. Nano-Bonding and its Characterization
NB uses nano-contacting followed by direct mechanical compression via an optically polished compression disk. The pressures experimented with during NB optimization are varied between 5 to 15 psi (~ 35–100 kPa), with the aim to minimize GaAs wafer breakage while maximizing nano-contacting and reducing and eliminating wafer warp. In addition, steam pressurization at about 1–3 psi (~ 7–20 kPa) is used. After NB, bonding between GaAs and Si is tested by debonding under 1–20 psi (7- 150 kPa). SAM is used to quantify the interface areas bonded. Bond gaps are characterized via Cross-Section TEM.