ResearchFields

Design, Characterization, and Control of a Micromachined Nanopositioner with On-chip Electrothermal Actuation and Sensing

High precision nanopositioners have been used extensively in many applications such as scanning probe microscopy (SPM), atomic force microscopy (AFM), and data storage. Closed-loop feedback control of the positioners is highly desirable for high degrees of displacement precision, and needs an accurate source of position information. However, the in-plane movements are mostly measured by laser reflectance microscope, making the footprint of the system fairly huge. In this research, a novel thermal position sensor is integrated with a thermal actuator in the same MEMS chip.

The displacement information of the positioner stage can be detected by measuring the resistance difference between the two sensors. The differential changes of the resistance result in current variations in the beam resistors, and the currents are converted to an output voltage using the trans-impedance amplifiers and an instrumentation amplifier. To suppress the common-mode noise, the gains of these two trans-impedance amplifiers must be well matched. Employing the differential topology allows the sensor output to be immune from undesirable drift effects due to changes in ambient temperature or aging effects.

The nanopositioner was calibrated using a PolytecTM Planar Motion Analyzer (PMA). Digital image capture and analysis methods were used to determine the displacement of the positioner stage. With the actuation voltage of 9 V, the thermal actuator can achieve a maximum displacement of 14.4 um. Meanwhile, at every actuation voltage, the instrumentation amplifier outputs were recorded for calibration of the position sensors. The sensors were biased with 6 V, and the instrumentation amplifier gain was set at 90.3 V/V. At this bias voltage, the sensors have a power consumption of 120 mW and a sensitivity of 0.27 mV/nm.

The dynamic characterization was conducted using a HP35670A spectrum analyzer. A voltage of 4.5 V dc plus 1 V ac was applied to the actuator and swept sinusoidal measurements were obtained from 1 Hz to 51.2 kHz. The frequency response shows the open-loop bandwidth of the positioner is 101Hz.

The open-loop sensor drift was measured at the output of the instrumentation amplifier over a period of 2000 seconds under normal laboratory conditions. The yellow/grey line is the moving average of the measured data, which indicates the low frequency drift. Thanks to the differential sensing of the sensor pair, the open-loop amplifier output has a low drift of 2.4 mV over 2000 seconds, which corresponds to 8.9 nm displacement.

The proportional-integral (PI) closed-loop feedback control of the developed positioner was investigated to improve positioning accuracy and robustness of the system. Due to the nonlinear nature of the thermal actuator, a nonlinear inversion block was added to the feedback loop to linearize the plant. Based on this control scheme, a controllable desired response of 2.5 um steps over a 10 um range was obtained with a positioning resolution of 7.9 nm and a time constant of 1.6 ms, as illustrated in Fig.6. As a comparison, a similar open-loop seek operation resulted in a maximum positioning error of 0.62 um.