Modified Hanma-Hunsinger Cell Geometry For Improved Manufacturability Of Saw Filters

High performance SAW filters typically incorporate cell structures with acoustic reflection for insertion loss reduction, triple transit suppression and size reduction. The electrical characteristics of the filters are sensitive to the reflection strength of the metallic electrodes in the cells. Variations in the reflection due to manufacturing can contribute to low device yield in production.Filter yield may be improved by frequency trimming techniques, e.g. reactive ion etching (RIE) of the substrate; however this only modifies the effective metal thickness of the electrodes but has no effect on the line widths - both dimensions having an impact on the reflection strength of the cell.Velocity and reflection of a modified Hanma-Hunsinger (HH) cell structure described in ref. 1 were computed for a range of line widths with both raised and buried electrodes on a quartz substrate using the original coupled FEM/BIE numerical model of P. Ventura [2]. From the results a nominal cell geometry was identified that possesses a cell reflection strength insensitive to the HH cell electrode widths for manufacturing variations of +/- 10%.The optimum geometry for the modified HH cells insensitive to electrode widths is determined from analysis using the original coupled FEM/BIE numerical model. Filters designed with this cell structure can achieve high yields with RIE trimming at the wafer stage of the manufacturing process.