# Mechanisms of sodium hydroxide (NaOH)-catalyzed sealing of SiO2/SiO2 direct bonding interfaces

P. Noel, F. Rieutord, V. Larrey, S. Tardif, C. Morales, H. Hijazi, V. Thoréton, A.-M. Papon, Z. Saghi, D. Landru, et al.

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P. Noel, F. Rieutord, V. Larrey, S. Tardif, C. Morales, et al. Mechanisms of sodium hydroxide (NaOH)-catalyzed sealing of SiO2/SiO2 direct bonding interfaces. Journal of Applied Physics, 2025, 138, pp.055307. 10.1063/5.0277335. cea-05206330

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# Mechanisms of sodium hydroxide (NaOH)-catalyzed sealing of SiO₂/SiO₂ direct bonding interfaces

P. Noel ⚙️; F. Rieutord ⚙️; V. Larrey ⚙️; S. Tardif ⚙️; C. Morales; H. Hijazi ⚙️; V. Thoréton; A.-M. Papon ⚙️;
Z. Saghi; D. Landru ⚙️; F. Fournel ⚙️

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J. Appl. Phys. 138, 055307 (2025)

https://doi.org/10.1063/5.0277335

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# Mechanisms of sodium hydroxide (NaOH)-catalyzed sealing of SiO₂/SiO₂ direct bonding interfaces

Cite as: J. Appl. Phys. **138**, 055307 (2025); doi: 10.1063/5.0277335

Submitted: 23 April 2025 · Accepted: 15 July 2025 ·

Published Online: 6 August 2025

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P. Noel,<fnref n="1" />,ᵃ⁾ ID F. Rieutord,<fnref n="2" /> ID V. Larrey,<fnref n="1" /> ID S. Tardif,<fnref n="3" /> ID C. Morales,<fnref n="1" /> H. Hijazi,<fnref n="1" /> ID V. Thoréton,<fnref n="1" /> A.-M. Papon,<fnref n="1" /> ID
Z. Saghi,<fnref n="1" /> D. Landru,<fnref n="2" /> ID and F. Fournel<fnref n="1" /> ID

## AFFILIATIONS

## ABSTRACT

The introduction of NaOH catalysts at an SiO₂/SiO₂ hydrophilic bonding interface is investigated. Combining bonding energy measurements, interface hard x-ray reflectometry, transmission electron microscopy, and time of flight secondary ion mass spectroscopy, the change of interface morphology, and composition as a function of surface preparation is evidenced. The hydroxide catalyzed interface shows the formation of interface nanoclusters, associated with the low temperature interface sealing. A reference interface (bonded without a catalyst) is sharper. These contrasting behaviors highlight the issue of water management at the interface and the balance between interface sealing and water out-diffusion. The additional impact of oxide thickness in interfacial water management is further investigated. Finally, the catalytic interface sealing is compared with other surface preparations that achieve high bonding energies at low temperatures.

© 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0277335

## INTRODUCTION

Direct wafer bonding is a well-known method used since the 1980s to build hetero-structures in microelectronics.<fnref n="1" />,<fnref n="2" /> This method enables the bonding of two surfaces spontaneously and is mostly used for Silicon-On-Insulator (SOI) or advanced substrates manufacturing with the Smart-Cut™ technology<fnref n="3" /> for hybrid bonding,⁴ die-to-wafer bonding,⁵,⁶ or MEMS manufacturing.⁷ After the spontaneous bonding at room temperature, a thermal annealing is applied to the bonded substrates to enhance their mechanical coherence. The bonding energy increase is due to an enhanced density of covalent bonds.⁸,⁹ Associated with this bond formation, contact points between the two substrates widen. One of the challenges encountered in today's technologies is to increase the mechanical coherence of the bonded structures with a minimal thermal budget. Thus, understanding bonding interface closure mechanisms is key. Low temperature adherence booster solutions have been developed to address this issue, such as plasma activation¹⁰ and chemical catalyst addition.¹¹ The impact of a spin-drying of organic and inorganic basic solutions on the silicon dioxide (SiO₂) surface prior to

SiO₂/SiO₂ bonding was studied by Calvez et al.¹²,¹³ This surface preparation relies on the deposition of catalytic molecules or ions onto the surface, prior to the bonding. Details of the deposition process have been given by Noel et al.¹⁴ It was demonstrated that such surface preparations have a significant beneficial impact on bonding energies. Based on bonding energy results, several physicochemical mechanisms were proposed to explain the bonding energy increase at lower temperatures after such surface preparation. They involve the presence of hydroxide ions (OH⁻) at the bonding interface, which have been generated during surface preparations. Three mechanisms have been proposed in the literature to explain such an improvement:¹⁵ silica hydrolysis,¹⁶,¹⁷ silanol condensation,¹⁸ and silica dissolution/deposition.

Hypothesis (i), e.g., “silica hydrolysis,” postulates that OH⁻ ions would catalyze the hydrolysis of the oxide layer in accordance with Eq. (1). The catalysis of the hydrolysis reaction would allow water to penetrate interface asperities, leading to a softer mechanical behavior of the subsurface, which would be favorable for contact point widening during annealing,¹⁷

<footnote n="1">¹University Grenoble Alpes, CEA-LETI, 17 rue des Martyrs, 38000 Grenoble, France</footnote>

<footnote n="2">²SOITEC, Parc Technologique des Fontaines, 38190 Bernin, France</footnote>

<footnote n="3">³University Grenoble Alpes, CEA-IRIG, 17 rue des Martyrs, 38000 Grenoble, France</footnote>

<footnote>ᵃ) Author to whom correspondence should be addressed: paul.noel@cea.fr</footnote>
$$ \begin{array}{l} (1) \mathrm{Si} - \mathrm{O} - \mathrm{Si} + \mathrm{H}_2\mathrm{O} + \mathrm{OH}^{-}, \\ \rightarrow (2) \mathrm{Si} - \mathrm{OH} + \mathrm{Si} - \mathrm{O}^{-} + \mathrm{H}_2\mathrm{O}, \\ \rightarrow (3) \mathrm{Si} - \mathrm{OH} + \mathrm{Si} - \mathrm{OH} + \mathrm{OH}^{-}. \end{array} \qquad (1) $$

A comparison with a plasma layer after plasma activation was performed.¹⁶ It would seem that this subsurface hydrolyzed layer would play the same role as a plasma layer subsurface or amorphous subsurface using Surface Activated Bonding (SAB).¹⁹ Indeed, surface asperities need to be softened enough to allow the formation of covalent bonds. The plasma layer, the SAB amorphous layer, and the hydrolyzed layer would play the same role in the mechanism of interface sealing.

Hypothesis (ii), e.g., “silanol condensation,” explains the bonding energy increase by the direct formation of covalent bonds through a silanol condensation reaction. It is catalyzed by $OH^{-}$ ions in accordance with Eq. (2),

$$ \begin{array}{l} (1) \mathrm{Si} - \mathrm{OH} + \mathrm{Si} - \mathrm{OH} + \mathrm{OH}^{-}, \\ \rightarrow (2) \mathrm{Si} - \mathrm{OH} + \mathrm{Si} - \mathrm{O}^{-} + \mathrm{H}_2\mathrm{O}, \\ \rightarrow (3) \mathrm{Si} - \mathrm{O} - \mathrm{Si} + \mathrm{H}_2\mathrm{O} + \mathrm{OH}^{-}. \end{array} \qquad (2) $$

Direct formation of covalent bonds would then allow the widening of contact points driven by the gain in adhesive energy. In this case, elastic deformation of asperities should occur to enable contact point widening. The main difference between hypotheses (i) and (ii) is the root cause of contact point widening, mechanical softening with (i) or increased condensation increasing reaction rate with (ii).

Hypothesis (iii), e.g., “silica dissolution and re-deposition,” states that hydroxide ions are able to locally dissolve the silica matrix and condense silicates on areas profitable to contact point widening. This hypothesis is contradicted by the following results: $F^{-}$ ions at neutral pH are as effective as $OH^{-}$ ions with respect to the bonding energy increase.¹¹,²⁰,²¹ However, it is known that, at neutral pH, $F^{-}$ ions do not dissolve silica.²² The catalysis mechanism is attributed to the activity of a nucleophilic agent ($F^{-}/OH^{-}$) on siloxane and silanols bonds at the interface as introduced by Noel et al.²⁰ Silicon bonds reactivity would be enhanced by the presence of the nucleophilic agent, thus increasing condensation and/or hydrolysis rates.²²⁻²⁴

This article reports on several characterizations of the bonding interface in order to improve our understanding of the sealing of $SiO_2/SiO_2$ direct bonding interfaces. At first, we focus on symmetric 100 nm $SiO_2$ on Si bonding interfaces, then we address the effect of the oxide thickness on the bonding interface morphology, and finally, the interface composition is explored.

The choice of carrying out this study with NaOH catalyst stems from its high pKa (the highest pKa is able to dissociate more water molecules into hydroxide ions, thus it allows us to highlight the most significant effect on the bonding interface¹⁵). However, this solution may be unsuitable for semiconductor front-end processes due to sodium metallic contamination. In spite of that, organic chemistry, which leads to a similar effect, is usable, one example is Ethanolamine chemistry.¹⁵

## EXPERIMENTAL

### Sample preparation

In this study, samples are 200 mm diameter and 725 µm thick (100) silicon substrates with a chosen thermal oxide thickness (dry oxidation for 10 nm thick layers and wet oxidation processes for thicker oxides). Prior to any treatment, a standard RCA (Radio Corporation of America) SC1/SC2 wet cleaning is used on all substrates.²⁵ A final $DiO_3$ treatment is performed to obtain clean hydrophilic surfaces. De-Ionized Water (DIW) samples and NaOH samples are then manufactured. Surface preparation consists of pouring DIW or a $10^{-4}$ M sodium-hydroxide (NaOH) solution followed by a spin-drying step. The NaOH solution is obtained by the dilution of a 50 wt. % solution supplied by Merck. This step allows the deposition of catalytic ions on the surface before bonding.¹⁴ For a $10^{-4}$ M concentration and a rotation speed set to 1500 rpm, the order of magnitude of deposited Na measured by Total X-Ray Fluorescence (TXRF) is in the order of $10^{13}$ at cm$^{-2}$, which corresponds to 1% surface coverage. This deposition method allows a good uniformity of deposited ions on wafer surfaces with a standard deviation of 20% across the whole wafer, measured by TXRF. Finally, the bonding step is performed, with a spontaneous bonding wave propagation. DIW surface preparation is taken as the catalyst-free reference. In this work, only symmetrical $SiO_2$ samples are studied (10–10 nm/50–50 nm/100–100 nm/500–500 nm).

### Bonding energy measurements

The bonding energy (the so-called “adherence energy”) represents the energy required to separate the interface into two surfaces. The measurement is performed with the Double Cantilever Beam (DCB) technique under prescribed displacement requiring a blade insertion at the bonding interface. The debonded length a resulting from blade insertion is measured with an infrared camera, and the bonding energy, E, evaluated with El-Zein’s formula given in Eq. (3),

$$ E = \frac{3.\delta^2 \cdot t^3}{16.\beta_{11} \cdot a^4} \frac{1}{1 - \frac{t^3 \cdot \beta_{26}}{8 \cdot \beta_{11} \cdot a^4}} ; \quad \beta_{ij} = S_{ij} \frac{S_{j3} \cdot S_{j3}}{S_{33}}, \qquad (3) $$

where $δ$ is the blade thickness, $t$ the wafer thickness, and $S_{ij}$ the compliance tensor factors of silicon. A 10% error bar is considered for measurements given the equipment limitations. Before measurements, bonded wafers are annealed at 100 °C for 2 h and sawn into beams. Beam samples are then annealed at the desired temperature for 2 h. The DCB measurements are performed in an anhydrous atmosphere, avoiding water stress corrosion from the clean room atmosphere humidity. This method also enables an investigation of the bonding energy relaxation. A slow increase of the debonded length is indeed observed after blade stop, corresponding to a slow energy reduction occurring during the measurement. This relaxation is attributed to the only reactive compound available: interfacial water. The bonding energy relaxation is calculated using Eq. (4), with $t=0$ corresponding to the blade stop. Figure 1 is an example of relaxation decrease during the DCB measurement in an anhydrous
<figure><img src="image_17.png" /><figcaption>FIG. 1. Bonding energy as a function of measurement time; the relaxation is the dynamic loss of energy.</figcaption></figure>

atmosphere,

$$ \text{Relaxation} = E(t = 0) - E(t = 300 \text{ s}). \quad (4) $$

The final given bonding energy values are measured 300 s after blade stop E (t = 300 s), when relaxation dynamics are supposed to be over and no further evolution is observed.

## Hard x-ray reflectometry (HXRR)

HXRR measurements are performed with a 27 keV (λ = 0.4592 Å) 50 µm wide x-ray beam on the BM32 beamline of the European Synchrotron Radiation Facility (ESRF).²⁶ Characterizations are performed on the 4 mm wide samples, sawn in the bonded wafers. Normalization to monitor and direct beams is performed, as well as footprint corrections due to beam size. The $q^4R(q)$ reflectivity data are then extracted. Figure 2 shows a standard bonding interface $q^4R(q)$ scan.

<figure><img src="image_18.png" /><figcaption>FIG. 2. $q^4R(q)$ signal for a DIW 10–10 nm oxide thick sample annealed at 100 °C.</figcaption></figure>

One large contribution is observed due to the presence of the narrow bonding interface. Multiple short-period interference fringes are observed due to the presence of the thermal oxide layer. To extract the electron density profile from these data, the inverse Fourier transform of $\sqrt{q^4R(q)}$ is calculated.²⁷ This calculation is valid when the following conditions are met: sample electron density profiles are symmetric (symmetric bonding), refraction effects can be neglected (due to low refractive index differences), and no multiple reflections occur at the interfaces. In our case, the reflected intensity never touches the zero line, meaning that no sign allocation due to phase change at some q value is required. Thus, in our case, no reflectivity fit is performed; electron density profile is directly calculated from the reflectivity data.²⁶ Data treatment and electron density profile extraction have been performed using a homemade Python program.

## Time of flight - secondary ion mass spectroscopy (ToF-SIMS)

ToF-SIMS measurements are performed on thinned substrates to analyze the interfacial chemical composition. Substrate thinning is achieved with grinding and silicon wet etching. The thinning process requires a minimum bonding energy. Pre-annealing at 200 °C is, thus, performed on NaOH samples and at 300 °C on DIW samples prior to grinding. The resulting structure is a Silicon-On-Insulator (SOI) stack with a 150 nm thick top silicon layer and a 200 nm thick buried oxide (BOX) layer. The bonding interface is located in the center of the BOX, e.g., at a 250 nm depth. ToF-SIMS chemical profiling is conducted with a $Cs^+$ 2 keV abrasion beam and a $Bi_3^+$ 30 keV analysis beam. The intensity of collected $MCs^+$ ions (e.g., ion clusters with M being the ion of interest) is normalized to $Cs_3^+$ (point-to-point normalization) to minimize ion yield variations with sample composition, especially at the interfaces.²⁸ The plotted ToF-SIMS will be labeled as intensities, even if the values displayed are actually the point-to-point normalization.

## RESULTS AND DISCUSSION

### 100–100 nm Thermal oxide bonding interfaces

Bonding energies after DIW and NaOH surface preparations of 100 nm thick thermal oxides were measured. Figure 3 shows the bonding energy as a function of annealing temperature.

In agreement with the literature, NaOH surface preparation is very effective in increasing the bonding energy of $SiO_2$ bonded substrates at low temperatures. For the NaOH sample at 500 °C, a fracture occurs at the $SiO_2/Si$ interface rather than the debonding of the $SiO_2/SiO_2$ bonding interface (represented with the dotted line). The fracture location has been determined with post-fracture visual inspection, as a $SiO_2$ transfer changes the color of the sample surface. This behavior indicates that the mechanical coherence of the $SiO_2/SiO_2$ bonding interface is higher than that of the $SiO_2/Si$ one. The energy measured at 250 °C for NaOH surface preparation reaches 6 J/m², a value which is slightly higher than the silicon fracture energy (around 5 J/m²). We would, at first glance, have expected sample breakage to occur before reaching such an energy level. A plausible explanation is that some plastic energy loss in the
<figure><img src="image_19.png" /><figcaption>**FIG. 3.** Bonding energies as functions of annealing temperatures (2 h) for NaOH and DIW surface preparation (empty markers represent a breakage at SiO₂/Si oxidation interface).</figcaption></figure>

oxide happens during measurement. The extracted bonding energies and relaxations are shown in Fig. 4.

A significant increase in the bonding energy relaxation is observed at low annealing temperatures for NaOH samples, reaching a maximum at 250 °C. In comparison, DIW samples show a slight relaxation increase only. Normalized to the bonding energies, relaxation rates are similar, however. The bonding energies relaxation is driven by the relative concentration of water, silanols, and siloxane at the bonding interface, according to Birgckigt et al.²⁹ An increase of the initial concentration of siloxane groups and/or water is expected to result in an increase of the relaxation rate. For NaOH samples, the bonding energy relaxation increase is consistent with the enhanced bonding energies via the formation

of siloxane covalent bonds, whatever the water concentration. However, this increase should be observable for DIW samples (especially at 500 °C). With such a surface preparation, as the energy increase occurs at a higher temperature, water exo-diffusion may occur, minimizing the bonding energy relaxation increase. No relaxation is otherwise observed for NaOH samples at 300 and 500 °C at the SiO₂/Si interface, meaning that this interface is completely dry. In that case, even if the water diffuses inside the oxide, there is not enough water reaching this interface to induce internal water stress corrosion. However, more information is required to fully understand water management at the bonding interface.

Electron density profiles of the samples were extracted from HXRR measurement using the previously described technique. The density profiles of DIW samples after 100, 200, 300, and 500 °C anneals are plotted in Fig. 5.

Away from interfaces, we have the bulk density of oxide layers, e.g., $ρ = 0.7 e⁻/Å³$. The presence of an interfacial gap at $z = 0$ results in an electron density reduction because of the bonding interface composition. As the annealing temperature increases over the 100–500 °C range, a slight electron density decrease is measured at the interface, while an important bonding energy increase is evidenced with the DCB technique. This result can be explained by electron density exchanges: contact point widening (gap densification) in opposition to water exo-diffusion (loss of interfacial electrons). Furthermore, a slight gap widening is also observed between 100 and 200 °C. Those DIW results were compared with electron density profiles for NaOH surface preparation, as shown in Fig. 6.

NaOH samples show a very different electron density profile evolution. NaOH electron densities at interfaces are indeed higher and increase faster with the annealing temperature compared to DIW samples. This is likely due to a densification of the interface. Already at 100 °C, the gap density is $0.45 e⁻/Å³$ for DIW samples, while it reaches $0.53 e⁻/Å³$ for NaOH samples. This result is coherent with the higher energies measured by DCB and a gap that started to close earlier for NaOH. Note, however, that the gap densities cannot be directly linked to the bonding energies as

<figure><img src="image_20.png" /><figcaption>**FIG. 4.** Bonding energy relaxations as functions of annealing temperature (2 h) for NaOH and DIW surface preparation (empty markers represent a breakage at the thermal oxide/silicon interface).</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**FIG. 5.** Electron density profile of the DIW bonding interface for 100, 200, 300, and 500 °C anneals (2 h) (100 nm thick oxides).</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**FIG. 6.** Electron density profile of the NaOH bonding interface for 100, 200, 300, and 500 °C anneals (2 h–100 nm thick oxides).</figcaption></figure>

the formation of covalent bonds does not change the total number of electrons.

A definite gap widening is observed, as the annealing temperature increases, for NaOH prepared samples. This widening is interpreted as water displacement at the bonding interface. Competition between interface sealing and incompressible water presence is expected to occur at the bonding interface. The system stores water in the form of nano-clusters at the interface, whose projection along the surface normal results in a wider gap by HXRR. Cross-sectional Transmission Electron Microscopy (TEM) was performed at bonding interfaces to identify the gap morphology. Images shown in Fig. 7 confirm the presence of clusters at the NaOH bonding interface. The measured size of the biggest clusters is 7 nm wide along the depth, a result consistent with the measured width of the interface gap in Fig. 6.

<figure><img src="image_23.png" /><figcaption>**FIG. 7.** Cross-sectional TEM imaging of SiO₂/SiO₂ interfaces after NaOH and DIW surface preparation, bonding, then 300 °C annealing for 2 h.</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>**FIG. 8.** Electron density profile of the bonding interface after NaOH surface preparation, then some anneals at 500, 600, 700, and 1200 °C for 2 h (100 nm thick oxides).</figcaption></figure>

At first glance, bonding energies and TEM images are at variance, since the most porous and defective interface observed by TEM has the highest mechanical coherence. Bonding energy and HXRR measurements suggest that the toughness of the contacted areas for NaOH samples is higher than that of the DIW bonding interface. TEM observation is done in transmission and defocusing modes, on TEM lamella that are ~100 nm thick. The nanoclusters observed are present over the entire depth of the sample. The cluster interface density has been calculated to be only ~1% of the overall interface surface, which remains a weak proportion. High temperature resistance of these clusters has been studied by HXRR. Figure 8 electron density profiles show that nano-clusters are still present for high annealing temperatures (T > 500 °C). A slight trace of these clusters is still present even after annealing at 1200 °C, at a temperature at which oxide creep begins.

We have seen that the structure of the bonding interface is quite different depending on surface preparation and interface sealing. There are two competitive kinetics that govern the bonding interface morphology: interface sealing and water exo-diffusion into the oxide layer. In the case of a NaOH surface preparation, the interface sealing is accelerated. This forces the system to store water in nanoscale clusters at the bonding interface. Even if such a competition was already highlighted with DIW-like bonding interfaces by Ventosa et al.³⁰ for a thermal oxide that was much thinner (10 nm, to be compared with 100 nm so far here), it was not anticipated that surface preparation would play such a role. In fact, the only difference between Ventosa's study and current data is the thermal oxide thickness, which might have a significant impact on the bonding interface structure.

## Effect of oxide thickness on bonding interface structures

The impact of oxide thickness on bonding interface structures was, thus, investigated by HXRR and DCB. Figure 9 shows the
<figure><img src="image_25.png" /><figcaption>FIG. 9. Electron density profile of the bonding interface for different oxide thicknesses with (a) DIW and (b) NaOH surface preparations and 300 °C 2 h anneals.</figcaption></figure>

electron density profiles for different oxide thicknesses with NaOH and DIW surface preparations and 300 °C 2 h anneals.

Figure 9 results highlight the very significant impact of the oxide thickness on SiO₂/SiO₂ bonding interfaces. With 10 nm thick oxides and a DIW surface preparation, water nano-clusters are formed, with a broad electronic density profile at the interface. This is not the case anymore for thicker oxides, with much more abrupt interfaces. This result is coherent with Ventosa's data on thin oxides.³⁰ With a NaOH surface preparation, density profiles are different as interface sealing occurs at lower temperatures, with wider interfaces formed. Water clusters are still visible with the thick oxide layers, and their sizes decrease as the oxide thicknesses increase, with narrower interfaces. Bonding energies of such samples have been measured. Results are shown in Fig. 10.

We show that, for a standard DIW surface preparation, thinner oxide layer samples have a significant higher bonding energies above 200 °C. This energy difference may be explained by the higher water concentration in the oxide layer, which would be favorable to interface closure. Such results are at variance with the bonding energies of NaOH catalyzed samples, as shown in Fig. 11.

We have a rather opposite phenomenon with NaOH catalyzed interfaces, where higher bonding energies are found for thicker oxides. The energy increase as the annealing temperature increases from 100 to 300 °C is otherwise much faster for NaOH compared to DIW samples. All oxide thicknesses samples indeed reach at least 5 J/m² at 300 °C.

## Characterization of the bonding interface's composition

Structure differences between DIW and NaOH catalyzed bonding interfaces have been identified; the atomic composition and

<figure><img src="image_26.png" /><figcaption>FIG. 10. Bonding energies for different oxide thicknesses with a DIW surface preparation and various annealing temperatures (2 h anneals).</figcaption></figure>

chemistry of the bonding interface were then investigated. Thinned bonded samples were studied with ToF-SIMS. The thermal oxide thickness is chosen first as 100–100 nm for bonding. OH and Na were tracked to identify their diffusion at the bonding interface and in the oxide nearby. Cs₂OH⁺ ToF-SIMS intensities for both samples are plotted in Fig. 12 for different annealing temperatures.

As samples were thinned with 150 nm silicon left on top, a higher intensity of Cs₂OH⁺ ions is detected from 150 nm depth, e.g., the location of the Si/SiO₂ interface. A peak is then located at the bonding interface (100 nm thick SiO₂ films) and attributed to enclosed water at the bonding interface. Peaks become less intense as the annealing temperature increases. Measured peaks for NaOH surface preparation are otherwise wider than DIW peaks

<figure><img src="image_27.png" /><figcaption>FIG. 11. Bonding energies for different oxide thicknesses with NaOH surface preparation as a function of annealing temperature (2 h anneals).</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>**FIG. 12.** ToF-SIMS profile of Cs<sub>2</sub>OH<sup>+</sup> intensities of 100–100 nm samples for (a) NaOH catalyzed and (b) DIW after 2 h anneals at various temperatures.</figcaption></figure>

(FWHM<sub>NaOH</sub> = 12 nm at 200 °C vs FWHM<sub>DIW</sub> = 3.5 nm at 300 °C). Such a result is consistent with previous results on bonding interface morphology with water nanoclusters (see Fig. 7). The interfacial peak of Cs<sub>2</sub>OH<sup>+</sup> ions in NaOH samples decreases from 200 to 500 °C, showing the water exo-diffusion. The drop is important for temperatures higher than 300 °C. At 500 °C, the maximum of the interfacial peak is close to the steady level in the thermal oxide. This drop is consistent with a lack of energy relaxation from DCB shown in Fig. 4. However, the Cs<sub>2</sub>OH<sup>+</sup> peak maximum does not really change between 200 and 300 °C, although we had some relaxation increase in Fig. 4. The higher energy relaxation should then be attributed to higher stress on the crack tip during DCB measurements. Samples with reference DIW surface preparations were not evaluated in ToF-SIMS for anneals below 300 °C because of their low bonding energy causing difficulties in sample preparation. However, the Cs<sub>2</sub>OH<sup>+</sup> interfacial peak maximum drops as the annealing temperature increases from 300 to 500 °C. The interfacial peak maximum is otherwise lower in NaOH than in the reference, e.g., DIW samples at such temperatures. Such a result may evidence the promotion of water diffusion in the bulk of the oxide layers with a NaOH surface preparation. To assess the oxide thickness parameter, the same samples with 10 nm oxide thickness have been manufactured. In this case, the bonding interface is located at 200 nm depth (the depth change is due to the silicon consumption differences during the oxidation process). The resulting profiles are shown in Fig. 13.

Notable changes are measured within thin SiO<sub>2</sub> layers. For DIW samples, the CsOH<sup>+</sup> peak is wider, which is consistent with the HXRR measurement shown in Fig. 9, this measurement confirms the presence of nano-cluster. However, the intensity peak is higher, which tends to indicate that the interfacial water concentration is higher with thin dielectric layers. For NaOH, the same tendency is observed, a wider peak (also coherent with HXRR measurements in Fig. 9) and higher peak intensity.

<figure><img src="image_29.png" /><figcaption>**FIG. 13.** ToF-SIMS profile of Cs<sub>2</sub>OH<sup>+</sup> intensities for 100–100 nm and 10–10 nm samples: (a) NaOH catalyzed and (b) DIW after 2 h anneals at various temperatures.</figcaption></figure>

Sodium diffusion through the oxide layers has been assessed, too. CsNa<sup>+</sup> intensities for NaOH samples are plotted in Fig. 14. DIW samples are not shown here as no sodium was expected to be detected. No major amounts of sodium are detected in the top 100 nm of SiO<sub>2</sub> for NaOH samples annealed at 100, 200, and 300 °C. Meanwhile, there are major sodium peaks at the bonding interfaces. In the bottom 100 nm of SiO<sub>2</sub>, sodium is detected along the oxide depth, whatever the anneal. It is most likely a measurement artifact as sodium is expected to migrate under abrasion beam of SIMS.<sup>31,32</sup> Moreover, diffusion phenomena are expected to be symmetric. This is the reason why we considered that sodium diffusion occurred only if sodium was detected in the top oxide layer (e.g., from 150 to 250 nm). Figure 14 shows that sodium diffusion does not happen for anneals in the 25–300 °C range and is significant at 400 °C (not shown here) and 500 °C only.

For 10–10 nm samples, it was not possible to discern sodium diffusion because the nanocluster size equals the oxide thickness.

<figure><img src="image_30.png" /><figcaption>**FIG. 14.** ToF-SIMS profile of CsNa<sup>+</sup> intensities vs sample depth in NaOH samples after 2 h anneals at various temperatures.</figcaption></figure>
Mechanism

Current results shed light on mechanisms for bonding inter-
face sealing that can be found in the literature. We showed that
with NaOH catalyzed interface sealing, the bonding energy quickly
increased with the temperature, reaching 5 J/m² at 300 °C for thin
and thick oxide layers (Fig. 11). For DIW surface preparation, a
rather slow bonding energy increase with the temperature is
observed, with an impact of starting oxide thickness. We highlighted
the formation of water nano-clusters at the bonding interface for
specific oxide thickness or surface preparations. The different energy
evolutions with temperature for DIW samples with different oxide
thicknesses (highlighted in Fig. 10) are hypothetically explained by
different hydration levels. We assume that oxide layers act as storage
layers with limited water solubility. This interpretation is supported
by the calculated diffusion lengths for 2 h anneals of molecular water
in silica. These values were calculated using Jones³³ diffusion
coefficients and activation energy at low temperatures. They are
shown in Fig. 15.

Water solubility in glass is expected to be from a few tenths of
a percent to a few percent of the mass.<sup>34-36</sup> A mass fraction compu-
tation for a 4.5 Å thick water layer (approximated value of water
concentration enclosed at the interface<sup>37</sup>) dissolved in a 200 Å total
oxide layer yields roughly a 1%, e.g., a value roughly above that
found in the literature. As nanoclusters are no more observed with
a 100 nm total oxide thickness, this limited hydration theory would
be supported if the solubility of water in silicon thermal oxide is
between 1% and 0.2% of the mass (respectively, for 20 and 100 nm
total oxide thicknesses), which seems plausible based on the litera-
ture's data. A 20 nm oxide overall layer (after bonding) would,
thus, not be a large enough reservoir to contain all the interfacial
water. However, a 100 nm overall layer would be enough, as experi-
mentally demonstrated. This mechanism explains the formation of
nano-clusters with standard DIW surface preparation with thin
oxide layers. This result is also supported by the ToF-SIMS mea-
surement with thin SiO<sub>2</sub> layers. It shows that higher water concen-
tration is measured in the case of 10–10 nm than 100–100 nm. The
hydration level and the density of interfacial SiOH are, thus, higher
for lower oxide thickness, meaning easier siloxane bond formation
when conditions (temperature) for silanol condensation are met.

<figure><img src="image_31.png" /><figcaption>FIG. 15. Calculated diffusion length of hydroxide, molecular water, and sodium in the SiO₂ matrix for 2 h anneals.</figcaption></figure>

An easier formation of covalent bonds (i.e., surfaces made adhesive sooner) may explain the formation of interfacial droplets for low oxide thickness (with NaOH or DIW surface preparation). When bonding two rough surfaces, a balance between attractive and repulsive forces of the contacted asperities¹⁸ occurs. The ratio of these two forces determines the final sealing of the interface. When surfaces have low adhesive energies, the repulsive contact force will dominate, a contact on a few highest asperities will occur and an interfacial gap will be visible. On the contrary, when the adhesive energy is strong, contact points will spread and the interfacial gap will close. The transition between these two regimes can be described by different adimensional parameters such as the Johnson parameter,³⁸ the Tabor parameter, or the so-called adhesion parameter θ of Fuller and Tabor³⁹ given in Eq. (5),

$$
\theta = \frac{E^* \sigma^2}{\Delta\gamma R^2}, \qquad (5)
$$

where E* is the effective Young modulus, σ the surface roughness,
Δγ the gain in surface energy, and R the radius of the asperities.
These numbers will be impacted by the ratio between adhesion
energy (contact surface area) and elastic repulsive energy (due to
asperity compression). The calculation with SiO₂ Young modulus
E* = 47.5 GPa, radius of asperities taken as 10 nm, and a 2.2 Å
roughness gives θ = 1 for Δγ = γc = 1.5 J/m².

In our case, water is present at the interface, probably up to
temperatures of 300 °C (cf. Fig. 12). When the contact adhesion
energy exceeds γc, contact point widening begins and concentrates
water into interfacial droplets. For NaOH assisted bonding, this
critical energy for interface sealing is exceeded in any conditions at
300 °C (cf. Fig. 11). For DIW bonding, only the lowest oxide thick-
ness (10 nm) exceeds this threshold (cf. Fig. 10). At higher temper-
ature, the energies of thick oxide layers get higher than γc, but
interfacial water exo-diffusion toward the oxide on both sides has
started, explaining why cluster formation does not occur.

Nano-clusters are still present at the interface at high tempera-
ture when the water has out-diffused in the oxide layer (cf. Fig. 8).
The clusters' presence despite the water exo-diffusion can lead to
two hypotheses: (i) the deformation induced is plastic and not elastic
or (ii) the elastic deformation may be irreversible because the strong
interface bonding around clusters is irreversible. However, these
assumptions remain speculative, and dedicated experimental studies
or mechanical modeling calculations would provide a deeper under-
standing of the nanocluster's behavior at high temperatures.

We observe different cluster sizes depending on the oxide
thickness, with larger sizes for thin oxides. Again, water diffusion
into the oxide should play a role. Indeed, a lower amount of inter-
facial water would result in lower size nano-clusters. These results
remain compatible with condensation and hydrolysis catalysis
mechanisms. However, the presented results lead to the conclusion
that both bulk hydrolysis and interface silanol condensation may
occur near the bonding interface.

Without additional catalysts, the higher interfacial water con-
centration for thin oxide layers results in a bonding energy increase.
This phenomenon would be more compatible with a hydrolysis
mechanism with a higher OH concentration in the oxide layer.
<figure><img src="image_32.png" /><figcaption>FIG. 16. Cross-sectional TEM image of a Si/100 nm SiO₂ bonding interface (with O₂ plasma activation on both surfaces) after an anneal at 300 °C for 2 h.</figcaption></figure>

With NaOH catalysts, all samples reach high bonding energies at low temperature, whatever the oxide thickness. This would be compatible with both hydrolysis and condensation mechanisms. SIMS profiles show that high water and sodium concentrations remain at or close to the interface. Thus, the hydrolyzed layer should be very thin (below 10 nm) to explain such a profile, with most of the water stored in nano-clusters at the bonding interface. However, this mechanism seems to be applicable to every surface preparation yielding higher bonding energies at low temperature, such as plasma activation. A cross-sectional TEM image of an oxygen plasma activated bonding interface with similar interfacial nano-clusters is shown as an illustration in Fig. 16.

## CONCLUSION

Multiple SiO₂/SiO₂ bonding interface characterizations have been performed after de-ionized water surface preparation (e.g., the reference bonding interface) and NaOH surface preparation (considered as OH⁻ catalyzed bonding interface). We showed the presence of different interface structures and composition evolutions depending on the annealing temperature. Nano-cluster formation at the interface of catalyzed NaOH samples highlights the competition between interface sealing and water exo-diffusion kinetics. This phenomenon is also visible with standard hydrophilic bonding interfaces for low oxide thicknesses. The formation of clusters was linked to the presence of trapped water at the interface, which prevented full interface sealing when the adhesion energy is large enough. This mechanism of interface sealing with the formation of interfacial nano-clusters seems applicable to other surface

preparations (such as plasma activation), yielding low temperature bonding energy increases.

## ACKNOWLEDGMENTS

The authors would like to thank SOITEC S.A. for support and the European Synchrotron Radiation Facility (ESRF) for provision of beam time on BM32. S. Tardif and F. Rieutord acknowledge the Agence Nationale de la Recherche for its financial support of the MAGNIFIX Project No. ANR 21-ESRE-0011.

## AUTHOR DECLARATIONS

### Conflict of Interest

The authors have no conflicts to disclose.

### Author Contributions

P. Noel: Conceptualization (equal); Data curation (lead); Formal analysis (lead); Investigation (lead); Methodology (lead); Resources (lead); Software (equal); Visualization (lead); Writing – original draft (lead); Writing – review & editing (lead). F. Rieutord: Conceptualization (equal); Formal analysis (equal); Methodology (equal); Supervision (supporting); Writing – original draft (supporting); Writing – review & editing (equal). V. Larrey: Conceptualization (equal); Formal analysis (supporting); Methodology (equal); Supervision (equal); Writing – review & editing (equal). S. Tardif: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Resources (equal); Software (equal); Writing – review & editing (equal). C. Morales: Investigation (equal); Resources (equal). H. Hijazi: Investigation (equal); Resources (equal); Writing – review & editing (equal). V. Thoréton: Data curation (equal); Investigation (equal); Resources (equal); Writing – review & editing (equal). A.-M. Papon: Investigation (equal); Resources (equal). Z. Saghi: Investigation (equal); Resources (equal). D. Landru: Conceptualization (equal); Supervision (equal); Writing – review & editing (equal). F. Fournel: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Methodology (equal); Supervision (lead); Writing – review & editing (equal).

## DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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