Glass Bonding Method Utilizing Relatively Mildly Acidic Treatment and Fluorinated Plasma for Stable Micro/Nano Device Fabrication

The bonding process is one of the most critical processes in the fabrication of stable, long-term, glass-based micro/nano devices for various applications, including biosensors. However, the current bonding process causes delamination and topographical damage to the detection unit of a biosensor, such as electrodes patterned on a glass substrate, owing to the strong acid treatment used for surface cleaning and activation. Herein, we propose a low-temperature glass bonding method that utilizes a relatively mildly acidic treatment without sacrificing bonding strength. To explore this method, we examined the effects of various acids, such as piranha solution, fuming nitric acid, and hydrochloric acid, on the bonding strength. To measure bonding strength, we treated fused-silica glass substrates with an acid solution, activated with oxygen plasma, bonded to each other, and annealed at 110°C. A crack-opening test using bonded glass substrates revealed that acid treatment is necessary to achieve reliable bonding strength. Relatively mildly acidic treatment, such as with hydrochloric acid, exhibited comparable bonding strength to strong acid treatment, such as piranha solution. Furthermore, we found that the selection of the bonding surface (top or bottom) and the use of fluorinated plasma with polytetrafluoroethylene pieces during oxygen plasma significantly enhance the bonding strength, e.g., 0.76 J/m 2 with hydrochloric acid. We believe that our findings will contribute to the development of reliable long-term glass-based biosensors.