QUARTZ CRYSTAL THICKNESS SENSOR DESIGNED FOR THE PREPARATION OF ISOTOPE TARGETS

In almost all commercial quartz crystal thickness monitors the period-thickness relationship is used. The accuracy and resolution of such systems, equipped with 5 MHz AT-cut quartz crystals vibrating in the thickness shear mode, is sufficient for the isotope target preparation. The main disadvantage is the relatively big crystal holder, which covers a large solid angle when evaporating at a short source-to-substrate distance. We have developed a quartz crystal thickness sensor with a diameter of 9 mm using an 8 mm diameter 5 MHz crystal with an accurate AT-cut (125°9′-10′). The thickness sensor is supplied with a remote electronic oscillator circuit and can be used with a Kronos or similar thickness monitor. In the design care had to be taken to avoid irreproducible changes of the mass sensitivity by coupling of other modes. First results of this thickness sensor are reported. © 1979.