QUARTZ-CRYSTAL MICROBALANCE - A NEW DESIGN ELIMINATES SENSITIVITY OUTSIDE THE ELECTRODES, OFTEN WRONGLY ATTRIBUTED TO THE ELECTRIC FRINGINGFIELD

Sensitivity outside the electrodes of a quartz crystal microbalance (Q CM) is typically 5%-10% of the total sensitivity for the popular plana r AT-cut type, and can be a problem in many types of adsorption measur ement. This off-electrode sensitivity has sometimes been assumed incor rectly to be due mainly to the electric ''fringing field'' at the elec trode edge, despite the fact that no quantitative theory has been put forward as to how the fringing field could lead to off-electrode sensi tivity. Tn fact, off-electrode sensitivity results from the mass distr ibution of the electrodes, and is virtually independent of the electri c field. Recognition of this fact leads directly to a new QCM design w hich virtually eliminates the problem altogether; in the new design se nsitivity outside the electrode covered area has been reduced from 6% to about 0.3%. Equations previously derived to predict the differentia l mass sensitivity of existing planar AT-cut QCM sensors are applied t o the new design, and compared to experimental calibration measurement s obtained using the ink-dot method. Agreement with theory is excellen t. (C) 1997 American Vacuum Society.