Interfacial studies of a metallurgical bond between "activated" ultrasonically applied solder and high purity fused silica

Packaging of optical devices often requires the need for creating strong bonds between metal and silica. The most convenient and cost-effective approach would be to directly solder to both silica and metal without requiring premetallization of the silica. Soldering to oxides and oxidized surfaces has been accomplished with various solders containing metals with strong affinity for oxygen. In this work we investigate solders based upon a tin-bismuth eutectic with potential activating additives of cerium, gallium, and titanium. Each of these metals are energetically capable of competing for the oxygen in silica and are therefore capable of reducing or forming mixed oxides with silica under appropriate conditions. The bond between such an "activated" solder and high purity fused silica (HPFS) has been characterized by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Two variations of solder produced by SBond Technologies, S-Bond 140 and S-Bond 140 M1, were bonded to silica using a fluxless ultrasonic technique. TOFSIMS was then used to characterize the bond interface by measuring the distribution of elements as a function of depth through the interface. The results show that the presumed activating elements concentrate at the interface and that their oxides form the interfacial layer between the HPFS and the bulk solder. The efficacy of these additives was established by demonstrating that the block shear strength of the bond to HFPS was increased by seven times through the addition of the aforementioned reactive metals to the base Sn-Bi solder.