Elemental Adsorption and Interface Reaction in Direct Active Bonding of GaAs to GaAs using Sn-Ag-Ti Solder filler
In this work, there are three research hypothesizes: First, the active element Ti only adsorbs at the interface of the joint but not involved in chemical reactions to form new substances; Second, Titanium does not form obvious adsorption at the interface, but takes part in reaction with GaAs substrate to form new phases; Third, at the bonding interface, there are both obvious titanium adsorption and new phases formation, which may be continuous or intermittent. It is very necessary to analyze the experimental data to understand the way and role of titanium in low temperature bonding. Although disrupt opinions have been given on the role of Ti in active bonding of filler alloys on ceramics, most of these experimental data are obtained from the samples welded at high temperature and the mechanism of wetting and welding is discussed at high temperature. However, first of all, the bonding temperature is a key parameter of the active solder wetting the substrate, whether the active solder following the high temperature wetting mechanism to achieve wetting with the substrate at low temperature needs to be further studied and analyzed; secondly, the reaction environment temperature between the active solder and the substrate at low temperature is also different from high temperature. Therefore, the behavior of active elements in the interface may be affected, which may cause some difference from that of high temperature brazing. And there will also be great differences in welding dynamic process. All of these are needed to be analyzed and discussed in detail both experimentally and theoretically. In our experimental data, titanium element is found to obviously take part in the active bonding between GaAs substrate and Sn3.5Ag4Ti(Ce,Ga) alloy filler. According to the transmission electron microscopy (TEM) analysis, the titanium elements segregate successively at the interface and the element gallium included in GaAs is dissolved into the molten alloy strongly. In addition, there is a resultant formed discontiniously along the interface which is identified as Ga4Ti5, but no arsenic compounds were found. Underlying wetting and reaction mechanism were elucidated based on the thermodynamics theories and the molecular dynamic (MD) model. The fitted results analyzed are all based on theoretical basis and experimental data which show that the chemical reaction and the adsorption of active elements may control the reactive wetting of Sn3.5Ag4Ti(Ce,Ga) filler alloy on Gallium arsenide together. The wetting behavior should be the result of the interaction of titanium adsorption and chemical reaction together.