Effects on silanol condensation during low temperature silicon fusion bonding

This paper presents the results of surface energy measurements performed in situ during annealing of silicon wafers. The method allows conclusions to be drawn about the progress of silanol condensation while surface energy increases. The effects of wafer conditioning by atmospheric pressure plasmas, chemical post-treatments after plasma exposure but before bonding, and annealing temperatures on silanol condensation is investigated. Using nitrogen or nitrogen/oxygen gas mixtures for plasma activation, higher fracture surface energies gamma are obtained at room temperature compared with oxygen plasma activation. Upon annealing, all increase in surface energies starts below 100 degrees C. Possible reasons responsible for the effects of nitrogen plasma treatment and post-treatments with ammonia are discussed. Upward bent gamma(t) curves are obtained during annealing at 200 degrees C, independently of the type of wafer preconditioning, in contrast with the implications of existing chemical-kinetic models of the bonding process. Aside from it self-enhancement of the process due to facilitated water removal from the interface, the hypothesis of the nucleation and growth of covalently bonded microareas is able to explain the observed behavior qualitatively.