Unwanted transverse modes in SAW resonators caused by stitching errors and stripe nonlinearities

Abstract

High Q SAW and STW resonators utilize interdigital transducers with transverse weighting. This weighting is fit to the fundamental mode (M0) of the resonator cavity, so the transducer should not couple to the higher order transverse modes (M1, M2, ...). Errors in the actual resonator pattern caused by stitching errors in laser generated photomasks will introduce an asymmetry which can excite both higher order symmetric (M2, M4, ...) and antisymmetric (M1, M3, ...) transverse modes. Non-uniformities due to processing can also contribute to these higher order modes. Higher order modes in oscillators are undesirable because they degrade linearity or induce frequency popping. The majority of high resolution (0.5um or better) 1X photomasks are fabricated using laser mask-making machines. This approach is adequate for most SAW devices; however SAW resonators are very sensitive to small variations in linewidths and line positions - variations that cannot be detected by normal optical examination. A typical resonator pattern (2mm × 2mm active area) is too large to be exposed in a single laser stripe. The objective of this study is to determine the photomask fabrication method which can minimize the higher order transverse modes. A 695MHz STW resonator was designed and realized in an array of 26 rows and 16 columns on multiple 1x dark field quartz masks exposed with different photomask tools. The tools were Etec Alta 3500 and 3900 laser pattern generators, Micronic laser writer and an ASET 645 15 Image Repeater used with a 10x reticle. Multiple wafers were contact-printed from each mask, and all dies on the wafers were RF-probed to acquire S11 over a span of 40MHz centered at 695MHz. The data was converted to admittance, and the main response was fitted to a lumped-element model, which was then removed. The residual data, which contained the extraneous modes, was then analyzed for correlation to: position in the array of dies; mask technology; wafer number. Electrical measurements of STW resonators fabricated with different photomask tools do exhibit differences in transverse mode levels, the newer pattern generators having lower high order modes. Those fabricated with mask generated with the 10X stepper exhibited the best performances.