Constrained-Film Sintering of a Borosilicate Glass: In Situ Measurement of Film Stresses

We have measured in-plane stresses developed in a borosilicate glass (BSG) film during its constrained sintering on a rigid substrate. Samples were prepared by casting BSG slurries on a silicon substrate and sintered inside a hot stage at 715°C just above the glass-softening temperature. Inplane stresses from the constrained-film sintering were determined by wafer-curvature measurements using an optical system. The measured stresses were tensile and rose rapidly from zero to a maximum level of 20 kPa during the initial stage of sintering and gradually decreased to zero at the final stage; these stresses were considerably smaller than those calculated from available microstructural models. We also measured the densification profiles of the free and constrained films. The stresses had no apparent effect on the densification profile of the constrained film up to 90% relative density; but beyond that, the densification kinetics were reduced in the constrained film. We believe that the stresses could have prevented a few large pores from shrinking during the initial stage of sintering, which then leads to an overall lower density and larger pores in the constrained film.