Optical contacting of low-expansion materials

We report on optical contacting or "direct bonding" of glass to glass for optical and precision engineering applications. Fused silica (SiO2) and ultra-low-expansion (ULE) glass materials with low and extremely low coefficients of thermal expansion, respectively, were investigated. Large glass wafers of up to 150 mm diameter and about 1.5 mm thickness were bonded to each other and to plane glass substrates of up to 20 mm thickness. Successful bonding was achieved after extensive chemical cleaning and low pressure plasma surface activation, using a commercial wafer bonding equipment. High quality (optically transparent) bonds with a very low fraction of aerial defects were obtained at low bonding temperatures of about 250 °C, by applying compressive forces of several tens of kN in a high vacuum environment. Typically, only small unbound locations occurred at the rim, where insufficient pressure had been applied in the bonding process. Bonding strengths were estimated from destructive "razor-blade" testing of bonded wafer pairs, resulting in bond energies up to about 2 J/m2. For surface activation, Nitrogen-plasma was tested in comparison to Oxygen-Plasma without significant differences. However, ULE wafers were found to bond much stronger than fused silica wafers under nominally identical bonding conditions. An exemplary "sandwich" structure was generated from ULE materials by bonding wafers from both sides to a core structure, obtained by perforating a massive plane plate with bore holes. This illustrates possible use in light-weight and stiff construction for high precision opto-mechanical applications.