Acoustic plate mode properties of rotated Y-cut quartz

Recent demands for compact, low cost, accurate sensors for fluid phase operation have been largely unmet. Among the most promising technologies are piezoelectric sensors. The piezoelectric sensors directly detect mechanical and electrical property changes caused by the analyte and are thus ammenable to continuous monitoring of fluid streams. The current effort is directed towards the development of trace ion (e.g. mercury) and biochemical (e.g. DNA, antibodies, toxins) detection. Several candidate structures have been proposed and many have been shown to be feasible for fluid phase sensing; however, the best experimental piezoelectric sensor results published to date employed the SHAPM structure [1]. The sensors detected approximately 10 ng/ml of such analytes as mercury, human IgG and cholera toxin and employed lithium niobate plate mode devices. The X-propagating Z-cut (ZX) lithium niobate wafers provide low propagation loss, high mass sensitivity, high electrical coupling and a single electrically-dominant acoustic mode. The principal drawback is the poor temperature stability of the material (-78 ppm/degrees C). In order to obtain better results the residual temperature instability of lithium niobate must be overcome while not substantially sacrificing its advantageous properties. In order to accomplish this, the current work analyzes potentially temperature stable plate modes in quartz crystals for dominant, temperature-stable, electrically-efficient, mass-sensitive acoustic modes with low propagation loss under fluid loading.