Effect of sol rheology on the uniformity of spin-on silica xerogel films

Abstract

Spin-on xerogels, which are promising candidates for use as interlayer dielectric materials in future microelectronic devices, change from a Newtonian liquid to a solid gel during processing. Since the rheology of the sol may affect the uniformity of the xerogel films produced, here we relate the rheology of a two-step, acid-base catalyzed, sol-gel system to the thickness and porosity profiles across xerogel films of importance to the microelectronics industry. We also analyze the effect of spin speed on the thickness and porosity of the films. Our rheological studies of the xerogel sol demonstrated that the sol changes from Newtonian far from the gel point, to shear thinning close to the gel point. On films spin coated with shear-thinning sols there is a region of uniformity extending for a distance of about 5 mm from the center. The film thickness and porosity are highest in this region and both quantities decrease towards the edge. If the sol is spun in its Newtonian regime, the resulting films are uniform (<3% thickness and porosity). The predictions of film thickness and uniformity based on simple models for spin coating a Newtonian and truncated power law fluid were found to be in good agreement with the experimental observations. The film thickness was varied from 0.485 to 1.9 μm by adjusting the spin coating speed from 5900 to 1000 rpm. Over this sixfold range of speed, the porosity was almost constant and varied by less than 10% while the thickness varied by about a factor of 4.