Condition optimization, reliability evaluation of SiO₂-SiO₂ HF bonding and its application for UV detection micro flow cell

In order to apply SiO₂-SiO₂ bonding with hydrofluoric acid (HF bonding) for micro-electro-mechanical systems (MEMS) fabrication, the optimal bonding conditions were examined under different temperature, HF concentration and bonding time. The necessary HF concentration and the necessary time for bonding are reduced by elevating the bonding temperature. The time for bonding was reduced from 24 h at room temperature to 30 min at 80 °C, 60 min at 60 °C under the conditions of 0.5 wt.% HF concentration and 1.3 MPa applied pressure. The bonding time is comparable to that of anodic bonding. Reliability of the HF bonding was confirmed by the results of temperature cyclic tests and thermal shock tests. A long term stability of the bonded sample was also evaluated by helium (He) leak detection. The measured He leak rate was less than 2.0×10^-9 atm cm³/s which is much smaller than that calculated value through component materials of the sample. A novel quartz UV detection micro flow cell for chemical analysis having Si shade structure was fabricated by the HF bonding. The absorbance unit for UV absorption detection of the cell was improved remarkably.