The use of design of experiments for the optimization of deposited glass on SAW filters

Vacuum deposited glass layers on piezoelectric crystals have been effectively used to improve the performance properties of surface acoustic wave (SAW) filters. These include, reducing the temperature coefficient of frequency, increasing the coupling factor, frequency trimming, and surface passivation for improved wafer yield. Using an orthogonal design of experiments (DOE) procedure, based upon a set of 9 experiments, the influence of process parameters on SAW properties were determined for RF diode sputtered glass on 128° Y-X lithium niobate. The glass was sputtered to a thickness around 600 nm for each experiment. There were four dependent sputter system variables, substrate temperature, background sputter gas pressure, RF power to the target, and gas flow rate. Three independent properties were measured, film stress, SAW velocity, and SAW propagation loss. The work is illustrative of a technique for optimizing the deposition conditions for films on SAW substrates using a minimum number of experiments. The DOE procedure can be applied to any combination of film and substrate with the results dependent upon the limits of the vacuum equipment and deposition technique used