Improving the accuracy of a quartz crystal microbalance with automatic determination of acoustic impedance ratio

A new method for automatic determination of acoustic impedance ratio, or z ratio, of a thin solid film deposited upon an AT-cut quartz crystal is described. The method uses frequency information from two piezoelectrically excitable resonant modes which are not harmonically related. This results in an improvement of accuracy for the quartz crystal microbalance (QCM) technique, requiring no prior knowledge of a material’s z ratio. The method is quite general and requires no adjustable parameter specific to a particular quartz crystal design. Results of extensive testing on wide ranging materials including metals, dielectrics, codeposited alloys, and sequential layers is presented. Excellent agreement is found between the predicted values and the gravimetrically measured mass of deposited films.