KOKUSAI ELECTRIC 与横滨国立大学发现 ALD-Al2O3 自激活晶圆键合机制，无需等离子体预处理

September 15, 2026

Bonding Mechanism Discovered Between “Self-Activated” Thin Films
That Enable Wafer Bonding Without Surface Pre-treatment
—Next-Generation 3D Semiconductor Bonding Technology That Eliminates the Need
for Pre-Bonding Plasma Treatment—

Research Highlights
 Discovered that aluminum oxide (Al2O3) thin films formed by atomic layer deposition (ALD) are
self-activating, enabling strong, void-free wafer bonding without the plasma treatment
conventionally considered necessary.
 Revealed that the origin of this self-activating property lies in the high polarity of Al–O bonds, the
Lewis acidity of exposed or undercoordinated Al³⁺ sites, and the amorphous low-temperature
ALD-AlOₓ structure, its abundant hydroxyl (OH) network, and the heat-induced rearrangement
of the amorphous AlOx structure, through surface and interface analyses.
 Succeeded in bonding 300 mm wafers and confirmed that bonding performance remained
essentially unchanged for approximately 60 days, based on previously reported queue-time data.
This discovery is expected to streamline manufacturing processes, minimize damage, and
enhance process flexibility for next-generation 3D semiconductors.




Figure 1. Self-activated bonding using ALD-Al2O3 and its mechanism

Research Overview
A research group (Professor Fumihiro Inoue of the Institute of Multidisciplinary Sciences (IMS) at Yokohama
National University; IMS Visiting Assistant Professor Anton Myalitsin; Hayato Kitagawa, a doctoral student,
and Taisuke Yamamoto, a master’s student, in the university’s Graduate School of Engineering Science;
and Jenyu Lee, Shuntaro Machida, Kazuhiro Yuasa, and colleagues at KOKUSAI ELECTRIC
CORPORATION) has discovered that thin films formed by atomic layer deposition (ALD) [Term 1] of
aluminum oxide (Al2O3) [Term 2] can undergo direct bonding [Term 3] without plasma treatment or chemical
mechanical polishing (CMP), a property termed “self-activation” [Term 4] (Figure 1).

These results were published on September 3, 2026 (local time) in ACS Applied Materials & Interfaces, an
international journal in the materials science field published by the American Chemical Society.




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Figure 2. Self-activated bonding scheme omitting the pre-treatment steps (CMP/Plasma/Rinse)

Social Background
In three-dimensional (3D) integration [Term 5] technology, in which semiconductor devices are stacked
vertically, direct bonding and hybrid bonding, which join different wafers and chips at the atomic level, are
essential. Conventionally, bonding oxide films together with sufficient strength was believed to require an
additional step: irradiating the surfaces with plasma immediately before bonding to activate them. However,
plasma treatment can cause plasma-induced damage (PID) beneath the surface and introduce excess
moisture, and the waiting time from plasma treatment to bonding (queue time [Term 6]) has to be strictly
controlled (Figure 2).




Figure 3. Wafer-scale quality and processing robustness of ALD-Al2O3 self-activated bonding: (a) bond wave
propagating without plasma, (b) void-free bonding, (c) continuous interface without observable voids or
delamination; the original bonding plane was not resolved, and (d) strong, highly-uniform bonding that
withstands thinning to 10 µm

Research Findings
The research group formed ALD-Al2O3 thin films approximately 5 nm thick on 300 mm silicon wafers and
bonded the wafers without plasma treatment. As a result, bonding free of voids [Term 7] was achieved across
the entire bonding interface, and after post-bond annealing (PBA) at 300 °C for 2 h, sufficient bonding
strength was achieved for subsequent processes, such as wafer grinding and CMP (Figure 3). The group
also showed that bonding strength can be enhanced even further by controlling interfacial moisture through
post-deposition annealing (PDA) to degas residual water from the ALD-Al₂O₃ film before bonding or an
underlying thermally grown SiO₂ buffer layer that facilitates the redistribution and diffusion of interfacial water.

In addition, the surfaces and bonding interfaces were analyzed in detail using a combination of techniques
including sum-frequency generation (SFG) spectroscopy [Term 8], X-ray photoelectron spectroscopy (XPS),
thermal desorption spectroscopy (TDS), transmission electron microscopy (TEM), and electron energy-loss
spectroscopy (EELS). The combined analyses support a mechanism in which hydrogen-bonded OH/water-
related species associated with the surface or near-surface region promote initial adhesion at room
temperature. During PBA, interfacial dehydration/condensation and local rearrangement of the amorphous
ALD-AlOₓ network toward more highly coordinated Al–O environments are considered to contribute to strong
bonding.

Based on previously reported queue-time data, the bonding performance remained nearly constant for
approximately 60 days even when the queue time between film deposition and bonding is extended. Unlike
the active surfaces formed only temporarily by plasma, self-activation arises from the intrinsic polarity and
structure of Al2O3, and the bondable state is therefore thought to be maintained over long periods.
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This distinctive phenomenon exhibited by ALD-Al2O3 provides a new materials design guideline for
semiconductor bonding technology, which has traditionally depended on plasma treatment. In particular, it
can offer significant advantages over conventional plasma-based bonding for bonding to the carrier wafers
used to form backside power delivery networks in 3D-integrated logic semiconductors. In the future, the
approach is expected to lead to simplified, lower-temperature, and less damaging manufacturing processes
and to multifunctional bonding materials in 3D integration technologies that stack logic, memory, image
sensors, chiplets, and other devices.

Future Outlook
This research has demonstrated that ALD-Al2O3 is a self-activating material capable of direct bonding without
plasma treatment and has clarified key factors underlying this property.

If the plasma process can be omitted, expected benefits include simpler bonding equipment configurations,
lower manufacturing costs, avoidance of plasma-induced material damage, relaxed constraints on queue
times between processes, and reduced variability. Applications in miniaturized chiplets, backside power
delivery networks, logic-memory stacking, image sensors, and other semiconductor technologies are also
anticipated.

Going forward, the group will optimize the deposition conditions, thickness, heat treatment, and underlying
materials of the Al2O3 films, and will also pursue applications to hybrid bonding involving copper
interconnects and evaluate the electrical and thermal reliability of the bonding interfaces.

Furthermore, based on the materials design guidelines identified in this study, namely high polarity, Lewis
acidity [Term 11], and an amorphous structure deposited at low temperatures, the group will search for self-
activated bonding materials other than Al2O3. In this way, the group aims to expand the range of bonding
materials available for 3D integration and to build a new bonding platform suited to multilayer stacking and
heterogeneous material integration.

Acknowledgments
This research was conducted with support from the Japan Science and Technology Agency (JST) Adaptable
and Seamless Technology Transfer Program Through Target-driven R&D (A-STEP) under the project
“Research and Development of High-Energy-Efficiency Beyond-2nm Devices through the Integration of
Front-End and Back-End Processes” (Grant Number: JPMJTR241A).




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Glossary
[Term 1] Atomic layer deposition (ALD)
A thin-film deposition technique in which source gases are supplied alternately. It can control film thickness
at the atomic-layer level and can form uniform films even on surfaces with steps and fine structures.
[Term 2] Aluminum oxide (Al2O3)
An insulating material composed of aluminum and oxygen. In semiconductor manufacturing, it is used as a
thin-film material that offers high insulation, heat resistance, and chemical stability. In this study, amorphous
Al2O3 thin films formed by low-temperature ALD were used as the bonding material.
[Term 3] Direct bonding
A technique for bonding planarized material surfaces to each other at the atomic or molecular level without
using adhesives. It includes fusion bonding, in which insulating films are bonded together, and hybrid
bonding, in which insulating films and metal interconnects are bonded at the same time.
[Term 4] Self-activation
In this study, “self-activation” refers to the ability of a material to be directly bonded through its intrinsic
surface chemical state, without being externally activated using plasma or other means.
[Term 5] 3D integration
A technology for stacking multiple semiconductor chips or wafers vertically and connecting them with short
interconnects. In addition to horizontal miniaturization of semiconductor devices, it combines logic, memory,
sensors, and other components with different functions to form a three-dimensional structure, thereby
enhancing performance and reducing power consumption.
[Term 6] Queue time
The waiting time between a process step, such as film deposition, cleaning, or plasma treatment, and the
subsequent bonding step. When the surface activity decreases over time, strict time management is required.
[Term 7] Void
A gap or unbonded region remaining at the bonding interface. Because voids degrade bonding strength,
reliability, and thermal and electrical properties, void-free bonding is required in semiconductor
manufacturing.
[Term 8] Sum-frequency generation (SFG) spectroscopy
A nonlinear spectroscopy technique in which light of different frequencies is directed onto a material surface
and light at the sum frequency is detected. It can probe the bonding states of molecules present on surfaces
and interfaces with high sensitivity.
[Term 9] Hydroxyl (OH) network
A structure in which hydroxyl groups and water molecules on a material surface interact with one another
through hydrogen bonding. In this study, this network is believed to enable initial adhesion at room
temperature.
[Term 10] Dehydration condensation reaction
A reaction in which two hydroxyl groups or similar species react to release a water molecule and form a new
covalent bond. In this study, Al–OH groups and other species are believed to react to form strong Al–O–Al
and Al–O–Si bonds.
[Term 11] Lewis acidity
The ability to accept an electron pair. Because Al3+ ions in Al2O3 have high Lewis acidity, they are believed
to promote interactions with water molecules and hydroxyl groups.




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Publication Details
Title: “Self-Activated Direct Bonding with a Highly Polar Atomic-Layer-Deposited Film for 3D
Integration”
Authors: Hayato Kitagawa, Taisuke Yamamoto, Jenyu Lee, Shuntaro Machida, Kazuhiro Yuasa,
Anton Myalitsin, Fumihiro Inoue
Journal: ACS Applied Materials & Interfaces
DOI: 10.1021/acsami.6c08143


Contact for Research Inquiries
Institute of Multidisciplinary Sciences, Yokohama National University
Professor and Deputy Director, Semiconductor and Quantum Integrated Electronics Research Center
Fumihiro Inoue
79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa, 240-8501, Japan
Tel: +81-45-339-3861
E-mail: inoue-fumihiro-ty@ynu.ac.jp

Equipment Business Unit, KOKUSAI ELECTRIC CORPORATION
Kazuhiro Yuasa
1 Ebisu-cho, Kanagawa-ku, Yokohama, Kanagawa, 221-0024, Japan (Yokohama Technology Center)
Tel: +81-45-548-5366
E-mail: yuasa.kazuhiro@kokusai-electric.com

Contact for Media Inquiries
Public Relations and Endowment Division, General Planning Dept., Yokohama National University
Tel: +81-45-339-3027
E-mail: press@ynu.ac.jp

Corporate Communication Department, Corporate Strategy Division, KOKUSAI ELECTRIC CORPORATION
Tel: +81-3-6772-9655
E-mail: prir.ke@kokusai-electric.com




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