Mathias Rohde

Direct Bonding of Glass Substrates

We report on investigations of direct bonding of glass materials for application as optical devices and high precision mechanical stages under vacuum conditions. Thin SiO2 (fused silica) wafers of about 1mm thickness were bonded to massive SiO2 substrates of up to 20mm thickness at diameters up to 200mm. Low Temperature Glass Bonding (LTGB) was performed under moderate vacuum, using a commercial bonding equipment of EVG (Austria). We report on our experience with suitable cleaning and low pressure plasma surface activation technologies to achieve high quality (optically transparent) bonds with a very low fraction of arial defects. Wetting angles were measured to monitor surface conditions during various steps of cleaning and low pressure plasma activation. Lowest defect levels were achieved with a combination of wet cleaning and N2-plasma processing, immediately proceeding the bonding process. Successful bonding was achieved in a moderate vacuum by exerting compressive forces of several kN to the glass stack at temperatures of about 250°Celsius. In the sandwich composite, essentially two classes of defects were discernible from interference fringes or haze: small circular defects –apparently from finite particle contamination– and slightly more extended edge related features –apparently from insufficient compressive forces– during the bonding process at these locations. The remaining bonding defects were analyzed by transmitting polarized light and measuring stress birefringence with equipment from ILIS (Germany). Particle related internal defects in the bonding area revealed a typical stress level of the order of 1 MPa. The unbounded edge regions showed no particular stress above the noise level. An exemplary 200mm diameter glass bond is documented in Figs. 1 and 2 below. Wafer (substrate) thickness was about 1mm (15mm), respectively, in this particular bond. This work was supported by DLR/Germany under contract No. 50YB0814.

Adjustment robot for fiber optics assemblies

Precise adjustment of the optical components may be achieved by stepped transfer of momentum via special stroke actuators (impulse hammers), which act onto a pre-stressed fiber- optical component. The motion of the component is controlled by a computer and a measurement device. The present paper discusses theory and experiment of this adjustment method, in particular motion behavior of pushed components under the influence of applied momentum, pre-stressing and frictional forces. Additionally it describes generically the wide application range of this adjustment method. In particular the article describes an innovative, automatic adjustment machine (robot) for the alignment of a single- mode fiber assembly, which was developed by the German Fraunhofer-Institute for Applied Optics and Precision Engineering (IOF) in collaboration with Agilent Technologies Inc., a global technology leader in communications, electronics and life sciences. The achieved adjustment accuracy for the fiber optical assembly is in a low micron range for the focusing motion and in a sub-micron for entering of the optics.