DIGITAL MIXER FOR QUARTZ-CRYSTAL THICKNESS MONITOR

A stable digital mixer used in a quartz crystal thickness monitor for monitoring the rate of evaporation and total thickness of films during thin film deposition is reported. The mixer circuit plays a vital rol e in the crystal monitor. A 'D' flip-flop is used as a mixer. The mixe r receives inputs both from the reference oscillator and the monitor o scillator and produces an output which is the difference of the two in put frequencies. The design aspects of the mixer, its stability and it s repeatability are described., The measured stability of the mixer sh ows a variation of 1.5Hz for 2 h at an ambient temperature of 30 degre es C (+/- 2 degrees) with a beat frequency of 151KHz. The performance of the mixer has been studied in the beat frequency range 0.5Hz-2.1MHz . The mixer is coupled to the crystal monitor and its performance is s tudied by evaporating different materials and the results are reported .