Steven Ian Moore

Displacement Measurement With a Self-Sensing MEMS Electrostatic Drive

This letter outlines a simultaneous actuation and displacement sensing technique applied to a microelectromechanical system (MEMS) electrostatic drive. Using the same electrostatic drive for both actuation and sensing allows more die space to be dedicated to the electrostatic drive, increasing the effective transduction efficiency of both functions and simplifying the mechanical design. Displacement sensing is performed with capacitive measurement implemented by incorporating the drive into an LC oscillator. This provides the mapping from displacement-to-capacitance to frequency-to-voltage. The technique was applied to a MEMS nanopositioner and the sensor exhibited no dynamics over the bandwidth of the device. The sensitivity of the sensor was 0.7551 V μm-1 and had a displacement noise floor of 0.00836 nmrms/√Hz.

Zero displacement microelectromechanical force sensor using feedback control

Conventional microscale force sensors use moving parts to infer applied forces. Whenever physical deformations are involved, the sensor characteristics become a function of mechanical parameters, and there is an inevitable trade-off between the sensitivity and measurement range. We developed a microfabricated force sensor that uses feedback control to nullify any displacements within the device, directly transducing forces as high as 1.5 mN with a 7.8 nN resolution. The range and sensitivity of the device no longer depend on mechanical parameters, which allow the same device to be used to test samples with a wide range of stiffnesses without loss of accuracy.

Vibration Control With MEMS Electrostatic Drives: A Self-Sensing Approach

Nanopositioning is the actuation and sensing of motion on the nanometer scale and recent nanopositioner designs have been utilizing microelectromechanical systems (MEMS). This brief demonstrates a simple method to implement vibration control on a MEMS nanopositioner. The actuation and sensing of the system are performed with a MEMS electrostatic drive. The electrostatic drive is arranged to be self-sensing, that is, the drives voltage is used to actuate the system and the drives current is used to observe the system. With this arrangement, the current is proportional to velocity at the resonance frequency and velocity feedback is used to damp the nanopositioner. To filter the current signal and recover a displacement signal, a charge measurement may be preferred to a current measurement. The self-sensing arrangement was modified to be a charge sensor and resonant control was applied to damp the nanopositioner. With this arrangement, the gain at the resonance frequency was attenuated by 18.45 dB.

Feedback-Controlled MEMS Force Sensor for Characterization of Microcantilevers

This paper outlines the design and characterization of a setup used to measure the stiffness of microcantilevers and other small mechanical devices. Due to the simplicity of fabrication, microcantilevers are used as the basis for a variety of mechanical sensor designs. In a range of applications, knowledge of the stiffness of microcantilevers is essential for the accurate calibration of the sensors in which they are used. Stiffness is most commonly identified through measurement of the microcantilevers resonance frequency, which is applied to an empirically derived model. This paper uses a microelectromechanical system (MEMS)-based force sensor to measure the forces produced by a microcantilever when deformed and a piezoelectric tube-based nanopositioner to displace the microcantilever. A method of calibrating the force sensor is presented that takes advantage of the lumped nature of the mechanical system and the nonlinearity of MEMS electrostatic drives.

Multimodal cantilevers with novel piezoelectric layer topology for sensitivity enhancement

Self-sensing techniques for atomic force microscope (AFM) cantilevers have several advantageous characteristics compared to the optical beam deflection method. The possibility of down scaling, parallelization of cantilever arrays and the absence of optical interference associated imaging artifacts have led to an increased research interest in these methods. However, for multifrequency AFM, the optimization of the transducer layout on the cantilever for higher order modes has not been addressed. To fully utilize an integrated piezoelectric transducer, this work alters the layout of the piezoelectric layer to maximize both the deflection of the cantilever and measured piezoelectric charge response for a given mode with respect to the spatial distribution of the strain. On a prototype cantilever design, significant increases in actuator and sensor sensitivities were achieved for the first four modes without any substantial increase in sensor noise. The transduction mechanism is specifically targeted at multifrequency AFM and has the potential to provide higher resolution imaging on higher order modes.

A switched actuation and sensing method for a MEMS electrostatic drive

MEMS technology is being investigated to improve the performance, integration and cost of nanopositioning systems. The most basic MEMS fabrication processes produce designs etched into a single layer of silicon and electrostatic transduction is often seen as the most viable actuation and sensing technology. This work provides a method to utilize the same electrostatic drive for both actuation and sensing functions. By combining both functions into the one drive, an effective increase in actuator force and sensor sensitivity can be achieved. The sensor utilizes a sigma-delta type arrangement to create a displacement-to-digital converter. With the actuator composed of a switching amplifier, this allows the nanopositioner to be controlled directly from a DSP platform. This work outlines the design of the nanopositioning system, provides the system modeling and identification and characterizes the open loop performance of the nanopositioner.

Simultaneous actuation and sensing for electrostatic drives in MEMS using frequency modulated capacitive sensing

Abstract This paper presents a displacement sensing technique that can be integrated into a microfabricated microelectromechanical system (MEMS) device. This sensor determines displacement by measuring the capacitance of a MEMS electrostatic drive, as the capacitance is a function of the displacement. The electrostatic drive is incorporated into an LC oscillator whose frequency varies with the capacitance. A lock-in amplifier is used to extract the frequency signal. The sensitivity of the sensor was –1.153 V μm –1 and exhibited no dynamics up to the 1.2 kHz bandwidth of the MEMS device it was implemented in. The electrostatic drive in this technique is used for both actuation and sensing. This effectively increases the transduction efficiency of both the actuator and sensor as more space on the die can be dedicated to the one drive. Additionally, the scheme allows for one terminal of the drive to be grounded. Thus, this scheme can be used on MEMS devices with more than one drive connected to a common mechanical structure which is electrically grounded.

Multimodal atomic force microscopy with optimized higher eigenmode sensitivity using on-chip piezoelectric actuation and sensing

Atomic force microscope (AFM) cantilevers with integrated actuation and sensing provide several distinct advantages over conventional cantilever instrumentation. These include clean frequency responses, the possibility of down-scaling and parallelization to cantilever arrays as well as the absence of optical interference. While cantilever microfabrication technology has continuously advanced over the years, the overall design has remained largely unchanged; a passive rectangular shaped cantilever design has been adopted as the industry wide standard. In this article, we demonstrate multimode AFM imaging on higher eigenmodes as well as bimodal AFM imaging with cantilevers using fully integrated piezoelectric actuation and sensing. The cantilever design maximizes the higher eigenmode deflection sensitivity by optimizing the transducer layout according to the strain mode shape. Without the need for feedthrough cancellation, the read-out method achieves close to zero actuator/sensor feedthrough and the sensitivity is sufficient to resolve the cantilever Brownian motion.

Switched self-sensing actuator for a MEMS nanopositioner

This work outlines the instrumentation and actuation of a MEMS nanopositioner, implementing a switching electronics based self-sensing actuation technique. Self-sensing actuation allows for optimal use of transducer die space in MEMS designs. The switching design accommodates actuation voltages of 50 V and is compatible with the silicon-on-insulator microfabrication process. The switching electronics are designed to be directly interfaced to a digital control platform. The actuator is based on the class D amplifier and the sensor is implemented using a ΣΔ modulator to create a displacement-to-digital type sensor that is operated at 1 MHz.

Design and analysis of piezoelectric cantilevers with enhanced higher eigenmodes for atomic force microscopy

Atomic force microscope (AFM) cantilevers with integrated actuation and sensing provide several distinct advantages over conventional cantilever instrumentation such as clean frequency responses, the possibility of down-scaling and parallelization to cantilever arrays as well as the absence of optical interferences. However, for multifrequency AFM techniques involving higher eigenmodes of the cantilever, optimization of the transducer location and layout has to be taken into account. This work proposes multiple integrated piezoelectric regions on the cantilever which maximize the deflection of the cantilever and the piezoelectric charge response for a given higher eigenmode based on the spatial strain distribution. Finite element analysis is performed to find the optimal transducer topology and experimental results are presented which highlight an actuation gain improvement up to 42 dB on the third mode and sensor sensitivity improvement up to 38 dB on the second mode.