Measurements for a capacitive incremental position sensor for microactuators

Integrated high accuracy long-range position sensing will be of paramount importance for high potential applications like future probe memories [1, 2] and probemicroscopy scanners provided that nm position accuracy can be obtained over 10’s of μm displacement range. In this work design, fabrication and measurements of an integrated incremental capacitive long-range position sensor for microactuators are presented. The sensor consists of two periodic geometries (period ≈ 8-16 μm) on a slider (connected to a microactuator) and one on a sensestructure (fixed) with gap-distance of ~ 1 μm. A relative displacement between the two results in a periodic change in capacitance. In normal operation (i.e. the Incremental Capacitance Measurement Mode (ICMM)) the change in capacitance vs. slider displacement is measured directly using a charge amplifier and synchronous detection at 1 MHz. Adjusting the minimal gap-distance by additional sense-actuators increases the capacitance and the S-N Ratio. In a second mode of operation, Constant Capacitance Measurement Mode (CCMM), the gap between sense-structures and slider is actively controlled to keep the sensor-capacitance at a pre-set value for all positions of the slider. Thus, the control signal (i.e. the voltage for the sense-actuator) becomes a measure for the position of the slider. Our results indicate that the position measurement accuracy is increased to ~10 nm in CCM operation compared to 46 nm in normal operation [3]. Further results (using normal operation) show that the realized capacitive sensor can be used in ICMM for frequencies above the resonance frequency (fres~1.6KHz) of the device and therefore also characterizes the important dynamic properties of the test-vehicle.