Technology of Cleaning and Activation Using Surface Wave Excited Plasma Downstream for Hybrid Bonding

Presented at the 2026 IEEE Hybrid Bonding Symposium, Jan 22-23, 2026 in Silicon Valley.  More information below.
(26:05 + Q&A) Yasuhiro Morikawa, ULVAC, Inc.
Summary: Wafer bonding has been attracting increasing attention as a next-generation technology to realize advanced 3D integration architectures. Achieving sufficiently high bonding strength, low distortion, and high alignment accuracy enables the realization of 3D device structures. To accomplish this, it is essential to reduce the overall thermal budget of the bonding process. Although numerous studies have been conducted on wafer bonding methods compatible with the latest semiconductor manufacturing processes, discussions regarding the optimal plasma source for pre-bonding plasma activation, as well as the optimal processes and surface management mechanisms, remain insufficient. In this study, compared to conventional plasma activation processes using capacitively coupled plasma (CCP) and inductively coupled plasma (ICP), advanced surface wave plasma (SWP) was employed, and bonding strength was evaluated under various plasma activation conditions using SiO2, a dielectric material for bonding. Downstream plasma ashing and cleaning which are applications of surface wave excite plasma for semiconductor device processing, is introduced. Plasma downstream treatments aim at treating surfaces by electrically neutral reactive species generated in plasma without influences of high energy particles such as electron, ion, and high frequency photons. Downstream processes are composed of generation of the reactive species in the plasma, transferring of the reactive species through the downstream, and reaction on the target surfaces. Through the ashing and cleaning process. It is shown that the downstream treatment performs high quality processing by control or alteration of the reaction in the plasma and the downstream. The plasma source to be mounted has the following three features; (1) High-density surface wave excited plasma with a non-slot antenna type, (2) Microwave transmission window material (introduction of a material replacing quartz), (3) Equipped with an ion, electron, and photon removal plate. Generally, the generation of high-density surface wave plasma is optimized by designing slot antenna and microwave transmission window. In contrast, our SWP technology, primarily aimed at mass production processes, achieves SWP discharge without using slot antenna to reproducibly obtain high-density microwave plasma. Furthermore, by using a material other than conventional quartz for the microwave transmission window, the use of Fluorine (Halogen) gases is also possible. Under the diffusion region of the microwave plasma, an ion, electron, and photon removal plate connected to ground (GND) is installed, allowing only radicals to flow downstream into the main chamber. Therefore, this downstream ashing system essentially meets the above-mentioned requirement (1) and has the scalability to address requirement (3). Additionally, a CCP can be applied on the substrate stage side. By combining this CCP with the microwave plasma, it is possible to accelerate the ashing rate and control the in-plane distribution on the wafer surface. Consequently, this system is applied not only to ashing but also to various surface treatment processes. In this study, a fundamental evaluation was conducted to apply the Advanced SWP + CCP process to the plasma activation step of hybrid bonding. The substrates used for bonding evaluation were 12-inch wafers with thermal SiO2. The bonding strength (Double Cantilever Beam: DCB) evaluation conditions were as follows: insertion length of 4 mm, insertion speed of 1 mm/sec, dew point of -75.1 °C, and oxygen concentration of 1.07 ppm. Bonding strength was measured immediately after activation (0 seconds) and after 180 seconds. The gas used for plasma activation was N2, and the irradiation time was 30 seconds. In the case of conventional CCP (RIE) only, the bonding strength decreased from 4.18 J/m2 at 0 seconds to 2.88 J/m2 after 180 seconds. Since this is a DCB measurement conducted under an anhydrous atmosphere, the observed reduction in bonding strength here indicates the detection of residual water at the bonding interface, known as internal water stress corrosion. For SWP only, the bonding strength was 1.61 J/m2 at 0 seconds and showed little change, measuring 1.34 J/m2 after 180 seconds. In the case of Ad-SWP + CCP, the bonding strength was 5.11 J/m2 at 0 seconds and remained high at 4.48 J/m2 after 180 seconds, showing minimal degradation. These results suggest that the optimal combination of Advanced SWP and CCP bias in the plasma activation process enables effective control of bonding strength.
Bio: Yasuhiro Morikawa joined ULVAC in 1997. He earned his Master’s degree in Electrical Engineering from the University of Toyo in 1997, and received a Ph.D. in Material Engineering from the University of Tokyo in 2003. He is currently working as a Technology Manager for global technology sensing strategy at the Institute of Advanced Technology at ULVAC. His main interests are the development of plasma technologies and equipment for the advanced packaging of integrated semiconductor chips. He is a member of the Japan Society of Applied Physics, the Japan Institute of Electronics Packaging (JIEP), the International Microelectronics Assembly and Packaging Society (IMAPS), and IEEE / IEEE EPS.

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