Self-Activated Direct Bonding with a Highly Polar Atomic-Layer-Deposited Film for 3D Integration

By Hayato Kitagawa ¹, Taisuke Yamamoto ¹, Jenyu Lee ², Shuntaro Machida ², Kazuhiro Yuasa ², Anton Myalitsin ³, and Fumihiro Inoue ^1,3
1 Graduate School of Engineering Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama, Kanagawa 240-850, Japan
² Advanced Technology Development Division, KOKUSAI ELECTRIC CORPORATION, 1 Ebisu-cho, Kanagawa, Yokohama, Kanagawa 221-0024, Japan
³ Semiconductor and Quantum Integrated Electronics Research Center (SQIE), Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama, Kanagawa 240-8501, Japan

Abstract

We report a previously unknown bonding regime: “self-activated, plasma-free” direct wafer bonding, enabled by atomic-layer-deposited (ALD) Al₂O₃ on 300-mm wafers. Direct bonding underpins all advanced three-dimensional (3D) complementary metal-oxide-semiconductor (CMOS) and heterogeneous chiplet architectures, yet dielectric surfaces have been assumed to require aggressive plasma activation or chemical mechanical polishing (CMP) to achieve sufficient reactivity. Our results overturn this assumption. ALD-Al₂O₃ spontaneously forms strong, void-free bonds without any pre-treatment, delivering practical bonding performance sufficient for subsequent wafer processing. Using a comprehensive suite of interfacial probes (double-cantilever-beam measurements, X-ray photoelectron spectroscopy, X-ray reflectivity, thermal desorption spectroscopy, water-contact-angle dynamics, ultra-sensitive sum-frequency generation spectroscopy, and transmission electron microscopy), we uncover dense, highly polar Al–OH networks, stabilized by the Lewis acidity of Al³⁺ and the amorphous low-temperature ALD structure, as the intrinsic driver of bond formation. These networks and their rearrangement persist irrespective of queue time, creating a robust, activation-free pathway from hydrogen-bond attachment to covalent bonding. Process-level engineering, including post-deposition annealing (PDA) and oxide-buffer insertion, further boosts strength and reliability. This discovery demonstrates the potential of self-activated, plasma-free bonding using low-temperature ALD-Al₂O₃ as a new route toward simplified and damage-mitigated 3D integration and advanced semiconductor packaging.

Keywords: direct bonding, atomic layer deposition, plasma-free, hydrophilic, 3D integration, Al₂O₃ 

To read the full article, click here