Absolute target thicknesses for calibration of accelerator experiments

For all quantitative ion beam analytical measurements, the target thickness is of essential significance. This thickness, however, must be calibrated in atoms per unit area. There are only two nondestructive and basic procedures in order to determine the thickness in units of atoms per area. One is weighing with a microbalance in connection with a precise determination of the relevant target area. The second one is a direct determination of a deposit on a quartz crystal resonator vibrating in its thickness shear mode. An intrinsic thickness accuracy of 0.5% can be obtained with a quartz crystal sensor (6 MHz) for target thicknesses smaller than 1 mg/cm2. In this paper the thickness accuracy of quartz crystal sensors for physical vapor deposition processes is discussed, thickness errors are pointed out and setups are described to minimize these errors.