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Patterned Electrodes for Thickness Shear Mode Quartz Resonators to Achieve Uniform Mass Sensitivity Distribution

Anthony Richardson1, Venkat Bhethanabotla1, and Allan L. Smith2. (1) Department of Chemical and Biomedical Engineering, University of South Florida, ENB 118, Tampa, FL 33620, (2) Masscal Scientific Instruments, 12565 Research Parkway, Suite 300, Orlando, FL 32826

Summary: Development of an electrode-modified thickness shear mode (TSM) quartz resonator that is responsive to nanogram mass loadings, while exhibiting a mass sensitivity profile that is independent of material placement on the sensor platform, is the aim of this project. A nanogram balance would revolutionize the field of mass measurement especially in processes like droplet gravimetry (1), the study of non-volatile residue (NVR) contamination in solvents. Mass sensitivity of current commercial analytical and mechanical balances is limited to the microgram scale which is inadequate as scientific and technological research continues to progress towards further minute magnitudes. Fortunately, theoretical research allowing for the utilization of TSM quartz resonators as nanoscale mass balances is available (2). However, a substantial shortcoming of past research is the establishment of a flat radial mass sensitivity distribution across the sensor platform. Non-uniformity of mass sensitivity on the TSM resonator surface prohibits the utilization of this technology in the measurement of absolute mass. To achieve a uniform distribution requires the design and testing of various electrode patterns. Presented in this study are the radial mass sensitivity distributions of TSM devices having different electrode geometries. Nanogram droplets were deposited at specific radial locations along the resonator surface and the resulting frequency shifts were measured. Experimental mass sensitivity distributions were then compared to theoretical models (2) to assess their viability as a tool to predict the geometry producing a flat sensitivity distribution. Experimental: In conducting the experiments, the gold electrodes along with the chromium adhesion layer were deposited using simple photolithography and evaporative deposition on 5 MHz blank AT-cut quartz wafers and the wafers placed inside an experimental apparatus specifically constructed to extract the mass sensitivity across the resonator surface (Figure 1). This instrument permits the extraction of the frequency shift of the crystal associated with the perturbation caused by nanogram droplet deposition from a microvalve at specific radial locations extending from the center of the resonator. Results: The radial mass sensitivity distribution for an arbitrarily chosen solid n-m' electrode design is presented in Figure 2. This design was fabricated simply to present the viability of the apparatus as an experimental technique to extract the mass sensitivity of any TSM device. It was not optimized or driven by theory. Comparison of this result to the theoretical distribution of a similar geometry in Figure 3 demonstrates this functionality of the apparatus. Simultaneous work involves the utilization of the theoretical modeling1 to predict the mass sensitivity distribution of various electrode designs; as well, as their fabrication and testing. Figure 4 demonstrates the viability of using the theoretical modeling as a tool to predict the design yielding a uniform mass sensitivity across the resonator surface. By reducing the electrode factor, R, a decrease in electrode thickness, the theoretical mass sensitivity for a 7-10 mm ring electrode configuration flattens. References: [1] Smith, A. Gravimetric analysis of the non-volatile residue from an evaporated droplet, using the quartz crystal microbalance/heat conduction calorimeter. J. ASTM Intl. 2006, 3, 1-5. [2] Josse, F.; Lee, Y.; Martin, S. J.; Cernosek, R. W. Analysis of the radial dependence of mass sensitivity for modified-electrode quartz crystal resonators. Analytical Chemistry 1998, 70, 237-247.