Reflow bonding of borosilicate glass tubes to silicon substrates as fluidic interconnects

Abstract

The subject of the thesis was the use of borosilicate glass (Duran®) tubes as an interface to wafer-level microfluidic devices. The tubes are compatible with the standard fluidic connectors and can be used as a package for the so-called MEMS-ona- tube assembly. The connection technology is based on the brief reflow of the glass tubes after they are placed on single crystal silicon wafers. Because of the decreased viscosity of the glass at the bonding temperature, the glass at the interface with the silicon flows slowly over time enabling the voids at the interface to be filled, producing a permanent bond between the glass and the silicon substrate. The selection and cleaning of the silicon substrates, the selection and surface preparation of glass tubes, positioning of the samples in an oven and the formation of the bond at high temperature were explained and the selection of bonding parameters was discussed. The heterogeneous nucleation and growth of crystals in borosilicate glass during its heat treatment for the reflow bonding above its glass transition temperature were investigated, although the bulk of the borosilicate glass remained unaffected by crystallisation. The activation energy for crystal growth was estimated and attributed to the diffusion of boron being the rate-limiting step within the borosilicate framework. Interfacial effects on nucleation were shown and the implications of the devitrification of glass on the reflow bonding were discussed. The strength of the connections between borosilicate glass tubes and silicon substrates has been tested by pressurising it to failure. Failure occurred in the bonded materials rather than along the bond itself. The bond formed was hermetic. The only leakage was due to gas diffusion through the glass. The interface of borosilicate glass tube–silicon joins formed after the reflow bonding was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Amorphous borosilicate glass has been found to be fully intact with silicon even at the atomic scale. Cristobalite crystals trapped at the interface have been occasionally observed with cracks around them. The compositional analysis of glass–silicon interface has revealed the formation of a thin amorphous silica layer adjacent to the silicon. The incorporation of in-plane electrical interconnects to reflow bonding has been studied. After joining the tubes to silicon substrates, the electrically conductive lines will allow probing into the volume confined by the tube. Therefore methods of fabrication of electrical interconnects, which would survive the bonding and not alter the quality of the bond interface, were investigated. Lastly, the examples of applications of borosilicate glass tubes as package or fluidic interconnects were given and the concept of long term encapsulation of MEMS structures under vacuum or selected gas environments was explained. The thesis was concluded with a detailed summary of the results and an outlook for the reflow bonding process.