Diffusivity measurements in polymers: III. Quartz crystal microbalance techniques

Abstract

A quartz crystal microbalance (QCM) system has been designed and constructed in order to measure the diffusion rates of several different casting solvents in a photoresist film. The QCM employs an oscillating quartz crystal that is coated with a photoresist film. Changes in mass caused by sorption or desorption of the casting solvent are detected as changes in the resonant frequency of the crystal. One advantage of the QCM is the high sensitivity with which this device can make weight measurements. The sensitivity is on the order of nanograms. This accuracy, along with the ease of data analysis, makes the QCM a particularly attractive apparatus for diffusion measurements. The QCM system consists of a piezoelectric crystal, a frequency counter, an oscillator unit, a detector, and a recorder. During the experiment, the shift in resonant frequency of the crystal with time is monitored, and the diffusion coefficient is extracted by using the method described by Crank. A related QCM technique has been used to study the dissolution kinetics of photoresists. The QCM system has been employed to measure the diffusion coefficients of common casting solvents in novolac films as a function of temperature. Acid diffusion in poly(hydroxystyrene) was also studied by this method.