A Compact, Dual-stage Actuator with Displacement Sensors for the Molecular Measuring Machine Open Access
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In this dissertation, we present the design, modification, optimization, assembly, performance characterization, calibration, and uncertainty analysis for a compact, for the Molecular Measuring Machine (M3) at the National Institute of Standards and Technology. The M3 is a scanning probe microscope (SPM) designed for making measurements with nanometer-level uncertainty over a working area of 50 mm by 50 mm. The design of the Z motion assembly is a particular challenge due to various constraints, especially a limited available volume of 25 mm in height and 35 mm in diameter, and the need for repeatable motion generation with integrated high resolution sensors. In the ultra limited space, the Z motion assembly is composed a coarse motion stage and a fine motion stage. The coarse motion stage is a piezoceramic inchworm like stepping motor with a potentiometer type position sensor. It is capable of translating the probe over a 3 mm range with overshoot-free steps ranging from 1 μm to 2 μm. The fine-motion stage is a flexure guided, piezoceramic-driven actuator to generate high-speed motion with a linear differential capacitive position sensor. A flexure hinge drive plate is designed as a motion amplifier to keep the stroke of the fine-motion actuator at more than 8 μm. An analytical solution is developed and optimization routines are used to optimize the design of the drive plate. The calculated deformations of the flexure amplifier show good agreement with experimental results. A differential capacitance gauge with high signal to noise ratio AC bridge is designed as the fine-motion position sensor, which has noise floor better than 0.1 nm. To validate the performance and calibration, a series of step height gratings with step heights ranging from 84 nm to 1.5 μm are measured using the Z-motion assembly and compared with the calibration results from NIST. The uncertainty budgets for measurements made with the Z motion assembly are evaluated and found to be about 1% with a coverage factor k = 2 (95 % confidence interval). Follow-up work to integrate the Z-motion assembly into M3 and use high accuracy step-height samples to calibrate the capacitance gauge in situ is suggested to reduce the uncertainty further.