Experimental Quantization of Surface and Sub-Surface Structure in Float Polished Crystalline Quartz

Abstract

Float polishing produces extremely flat (<λ/20) surfaces to atomic conformity over large areas. This process was applied to develop robust AT-cut, premium-Q, cultured, crystalline-quartz acoustic oscillators, which are normally highly susceptible to fracture from mechanical shock. The fabrication process was optimized to eliminate sub-surface damage, and to maximize the flexural strength of the crystals. We report premium results of a near-bulk value of stress-fracture threshold of 620 MPa, and a surface roughness of less than 0.2nm rms for 6.35mm diameter, 103.8µm thick samples float polished with colloidal silica. The remarkable surface quality produced by float polishing is combined with a 10-fold improvement in strength over blanks manufactured by the conventional CeO₂ process. We also report on the characterization of the surfaces by atomic force microscopy, scatterometry, and photo acoustic spectroscopy where we see that deep 100µm removal eliminates damage from processing prior to the final float polish. These results are a significant improvement over the performance of commercially available resonators in equivalent tests.