ResearchFields

A Micromachined 2DOF Nanopositioner with Integrated Capacitive Displacement Sensor

With the development of the MEMS technology, MEMS-based positioning stages have attracted more and more attention in numerous applications in micro-/nano-scale positioning and manipulation systems due to their small size, low cost, fast response, and flexibility for system integration. Compared with piezoelectric stages, MEMS-based positioning stages have many advantages such as small size, high resonance frequency, precise positioning control, and flexibility in system integration. Among MEMS actuators, electrostatic comb-drive actuator is popular due to its fabrication compatibility with micromachining process. Advantages of electrostatic actuation are summarized as simplicity, low power consumption and fast response, while the well recognized disadvantage is high actuation voltage requirement. Recent research related to electrostatic actuation mainly focused on the optimization of structure shape to maximize electrostatic force and elimination of side effects. In this project, a MEMS-based 2DOF nanopositioning stage is developed, which integrates comb-drive actuators and capacitive displacement sensors to allow for simultaneous actuation and position sensing.

Fig. 1
Schematic diagram of the micromachined 2DOF nanopositioner.

The schematic diagram of the MEMS-based 2DOF nanopositioner is depicted in Fig. 1. On the silicon layer, distributed around the center stage, four sets of in-plane comb-drive actuators are employed to drive the center stage along the X and Y directions, with each actuator set consisting of four banks of combs. And, four sets of comb-drive sensors are designed to sense the corresponding displacements of the center stage, with each sensor set consisting of two banks of combs. Four tethering beams are used to suspend the center stage and transmit in-plane motions from the comb-drive actuators. The device was fabricated using the silicon-on-insulator (SOI) multi-user MEMS processes (SOIMUMPs) in a commercial foundry (MEMSCAP). The images of the whole device and the center stage taken under a Scanning Electron Microscope (SEM) are shown in Fig. 2. As depicted in Fig. 2 (b), patterned with blanket metal, the micron-arrays on top of the center stage were designed for Atomic Force Microscope (AFM) image scanning.

The static behaviour of the nanopositioner was measured using a PolytecTM Planar Motion Analyzer (PMA). Fig. 3 shows the static bi-directional displacement of the stage along the X direction against the applied DC voltage. The stage was actuated bi-directionally by applying static voltages ranging from 0 V to 100 V to the driving electrode on either side of the X direction, and the displacement was measured by the PMA under different driving voltages. The stage has a dynamic range from -6.27 um to +6.64 um along the X direction, which falls into the range of AFM scanning applications. A commercial capacitive readout IC (MS3110) is used for the open-loop capacitive sensing. Fig. 4 demonstrates the experimental setup for capacitive sensing using the evaluation board MS3110BDPC. Pin 4 and Pin 6 supply the capacitance bridge with AC carrier signals which are 100 kHz square wave differential signals with a peak-to-peak amplitude of 2.25 V. Pin 5 is kept at 2.25 V DC potential and connected to a common electrode that is the movable part of the MEMS structure. At every actuation voltage for the displacement measurement, the output of MS3110 (Pin 14) was measured using a digital oscilloscope (Tektronix TDS 3024B). Fig. 6 shows the measured sensor output voltage versus the stage displacement. Fig. 5 shows the measured sensor output voltage versus the stage displacement. Characterized using a spectrum analyzer (HP 35670A), the frequency response of the fabricated nanopositioner is presented in Fig. 6. The first resonance frequency is measured at 4.24 kHz, which is 15.5% higher than the simulated first undamped natural frequency of 3.67 kHz. This discrepancy is due to the fabrication imperfections. The phase delay from 10 Hz to 8 kHz is 200deg.

Fig. 2
a) SEM image of the 2DOF nanopositioner
b) Magnified view (center stage).

Fig. 3 (Left)
Static X-directional displacement as a function of applied voltage

Fig. 4 (Right)
Capacitive sensing measurement setup using MS3110.

Fig. 5 (Left)
Sensor output voltage as a function of X-directional displacement

Fig. 6 (Right)
Frequency response of the X-directional motion