Hydrophilic Bonding of SiO2/SiO2 and Cu/Cu using Sequential Plasma Activation
Authors: Takeuchi Kai , Ninomiya Takeki , Kubota Michitaka , Kawano Masaya , Takagi Takeshi , Masaaki Niwa , Kuroda Tadahiro , Suga Tadatomo 
Published: February 2024

To meet the increasing market demand for next-generation electronic devices, 3D system integration is getting greater and greater attention. However, traditional methods such as soldering or bump bonding cannot handle high density interconnection. Therefore, hybrid bonding technology, which simultaneously bonds Cu wiring layer and SiO2 dielectric layer, is indispensable. In the hybrid bonding process, hydrophilic bonding is commonly used to bond SiO2 to SiO2. This technique involves bonding of hydrophilic bonding surfaces, followed by post-bonding annealing to achieve a reliable bonding interface. For Cu bonding, it is necessary to break the oxide layer at the Cu bonding interface and allow Cu atoms to diffuse into each other to achieve electrical connection.

However, since post-bonding annealing at high temperatures can damage the device or induce residual stress at the bonding interface, low temperature process below 200°C is preferable. Currently, the most widely adopted process is hydrophilic bonding using N2 plasma, which also achieves hybrid bonding through heating at around 350°C to 400°C.

In this study, we propose to apply sequential plasma activation bonding (SPAB), which has been shown to be effective for low-temperature bonding of quartz glass, to the bonding of TEOS SiO2 and Cu. SPAB is a technique that forms a highly reactive oxynitrides on the substrate by continuously irradiating the bonding surface with O2 plasma, N2 plasma, and nitrogen radicals. This then adsorbs a large amount of OH groups by reacting with moisture, leading to a strong bond after bonding and post-bonding annealing at 200°C. Since this technique is effective for quartz, it is expected to be effective for TEOS SiO2 as well, and we also investigate its effect on Cu in this study.

For this, we investigated the effect of SPAB for blanket TEOS SiO2 and Cu wafer separately. Cu layers were deposited on 4 inch, 525 µm thick Si wafers, and TEOS SiO2 layers on 6 inch, 625 µm thick Si wafers. The Cu layers were deposited by sputtering with a 1 µm thickness, while the SiO2 layers were deposited using plasma-enhanced TEOS process for a 1 µm thickness. Both types of deposited layers were planarized by CMP.

The SPAB process was carried out using RIE plasma and microwave radicals of N2 and O2 gas. The bonding surfaces were activated by O2 plasma, N2 plasma, and N radicals sequentially. After the surface activation, the wafers were exposed to ambient air to adsorb water on the surface, followed by bonding in air and post-bonding annealing at 200°C for 2 hours.

For comparison, bonding was also carried out using only N2 plasma and only O2 plasma activation, respectively. Results show that when surface activation was performed using only N2 plasma, the bond strength was 0.75 J/m2 for TEOS SiO2 and 0.33 J/m2 for Cu. In the case of O2 plasma activation, the bond strength was 0.57 J/m2 for TEOS SiO2 and 0.98 J/m2 for Cu.

On the other hand, the SPAB technique improves the bond strength to 1.13 J/m2 for TEOS SiO2 and 1.03 J/m2 for Cu. Therefore, it can be concluded that the SPAB technique enhances the bond strengths of both TEOS SiO2 and Cu compared to single gas plasma activation. Since the SPAB improves the bond strength of both SiO2 and Cu in the same process condition with annealing temperature of 200°C, this technique will contribute low temperature hybrid bonding applications.