Hydroxide-catalyzed bonding of sapphire and quartz glass enabled by plasma activation

Bonding of sapphire and quartz glass is essential for aerospace and quantum technologies. Since the coefficient of thermal expansion (CTE) of sapphire is more than ten times that of quartz, direct bonding may fail due to excessive interfacial thermal stress. Hydroxide-catalyzed bonding (HCB) can join silicon-based materials through chemical reactions with alkaline solutions and relieve interfacial stress. However, sapphire is difficult to react due to its chemical inert, which is challenging to combine with quartz glass achieving high bonding strengths and excellent optical interfaces. Here, a reactive ion etching (RIE) plasma was employed to improve surface roughness and chemical activity. It promotes the spontaneous spreading and infiltration of Na₂SiO₃ solution on both sapphire and quartz surfaces. The HCB solution formulation was optimized so that high bonding strength over 8 MPa was obtained while maintaining excellent optical transmittance (∼95% theorical value). Moreover, the bridging interlayer of transparent inorganic networks can relieve thermal stress. The bonded sample was thus completely survived after −55 ∼ +125 °C thermal cycles. It has great potential for hybrid integrated optical and transparent encapsulation applications.