Jianmin Song

Optimizing the Quality Factor of Quartz Tuning Fork Force Sensor for Atomic Force Microscopy: Impact of Additional Mass and Mass Rebalance

A force sensor in the heart of an atomic force microscope (AFM) plays a key role in the AFM measurements. Quartz tuning fork (QTF) based force sensor is attracting huge attention due to its peculiar traits such as self-actuating and sensing capability, high quality factor and high force sensitivity. Unfortunately, mounting a tip on a tine of the QTF degrades its quality (Q)-factor and sensitivity. Attaching an equivalent counter mass on the opposite tine (mass rebalance) can improve the Q-factor. We investigate the impact of the attached mass and counter mass on different traits of the QTF such as Q-factor, inherent relationship between excitation voltage and output as well as shift in the resonance frequency. We propose straight forward strategies to rebalance the QTF force sensor. Experimental results demonstrate that by attaching a counter mass (at the parallel position) on the opposite tine, the tip mass can be rebalanced. Q-factor is significantly improved after mass rebalance. The increase in the Q-factor depends on the mass of the tip, counter mass and position of the counter mass relative to the position of the tip (

Broad modulus range nanomechanical mapping by magnetic-drive soft probes

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Multiparametric Kelvin Probe Force Microscopy for the Simultaneous Mapping of Surface Potential and Nanomechanical Properties

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Atomic Force Microscopy Sidewall Imaging with a Quartz Tuning Fork Force Sensor

Stiffness matching between the probe and deformed portion of the sample in piezo-drive peak force modulation atomic force microscopy (AFM) limits the modulus measurement range of single probes. Here we develop a magnetic drive peak force modulation AFM to broaden the dynamic range of the probe with direct cantilever excitation. This approach not only successfully drives the softest commercial probe (6 pN nm−1) for mapping extremely soft samples in liquid but also provides an indentation force of hundreds of nanonewtons for stiff samples with a soft probe. Features of direct measurements of the indentation force and depth can unify the elastic modulus range up to four orders of magnitude, from 1 kPa to 10 MPa (in liquid) and 1 MPa to 20 GPa (in air or liquid) using a single probe. This approach can be particularly useful for analysing heterogeneous samples with large elastic modulus variations in multi-environments.Force-distance curve-based atomic force microscopy can measure material nanomechanics, but only if the probe and material stiffness match, which limits the measurement range. Here, the authors broaden the dynamic range of the probe by up to four orders of magnitude using magnetic drive peak force modulation.

Fast Specimen Boundary Tracking and Local Imaging with Scanning Probe Microscopy

We report high-resolution multiparametric kelvin probe force microscopy (MP-KPFM) measurements for the simultaneous quantitative mapping of the contact potential difference (CPD) and nanomechanical properties of the sample in single-pass mode. This method combines functionalities of the force-distance-based atomic force microscopy and amplitude-modulation (AM) KPFM to perform measurements in single-pass mode. During the tip-sample approach-and-retract cycle, nanomechanical measurements are performed for the retract part of nanoindentation, and the CPD is measured by the lifted probe with a constant tip-sample distance. We compare the performance of the proposed method with the conventional KPFMs by mapping the CPD of multilayer graphene deposited on n-doped silicon, and the results demonstrate that MP-KPFM has comparable performance to AM-KPFM. In addition, the experimental results of a custom-fabricated polymer grating with heterogeneous surfaces validate the multiparametric imaging capability of the MP-KPFM. This method can have potential applications in finding the inherent link between nanomechanical properties and the surface potential of the materials, such as the quantification of the electromechanical response of the deformed piezoelectric materials.

Amplitude calibration of quartz tuning fork (QTF) force sensor with an atomic force microscope

Sidewall roughness measurement is becoming increasingly important in the micro-electromechanical systems and nanoelectronics devices. Atomic force microscopy (AFM) is an emerging technique for sidewall scanning and roughness measurement due to its high resolution, three-dimensional imaging capability and high accuracy. We report an AFM sidewall imaging method with a quartz tuning fork (QTF) force sensor. A self sensing and actuating force sensor is fabricated by microassembling a commercial AFM cantilever (tip apex radius ≤10 nm) to a QTF. The attached lightweight cantilever allows high-sensitivity force detection (7.4% Q factor reduction) and sidewall imaging with high lateral resolution. Owing to its unique configuration, the tip of the sensor can detect sidewall surface orthogonally during imaging, which reduces lateral friction. In experiments, sidewalls of a micro-electro-mechanical system (MEMS) structure fabricated by deep reactive ion etching process and a standard step grating are scanned and the sidewall roughness, line edge roughness and sidewall angles are measured.

Calibration of atomic force microscope probes using a pneumatic micromanipulation system

An efficient and adaptive boundary tracking method is developed to confine area of interest for high-efficiency local scanning. By using a boundary point determination criterion, the scanning tip is steered with a sinusoidal waveform while estimating azimuth angle and radius ratio of each boundary point to accurately track the boundary of targets. A local scan region and path are subsequently planned based on the prior knowledge of boundary tracking to reduce the scan time. Boundary tracking and local scanning methods have great potential not only for fast dimension measurement but also for sample surface topography and physical characterization, with only scanning region of interest. The performance of the proposed methods was verified by using the alternate current mode scanning ion-conductance microscopy, tapping, and PeakForce modulation atomic force microscopy. Experimental results of single/multitarget boundary tracking and local scanning of target structures with complex boundaries demonstrate the flexibility and validity of the proposed method.

Measurement of surface potential and adhesion with Kelvin Probe Force Microscopy

Amplitude calibration of the quartz tuning fork (QTF) sensor includes the measurement of the sensitivity factor (αTF). We propose, AFM based methods (cantilever tracking and z-servo tracking of the QTFs amplitude of vibration) to determine the sensitivity factor of the QTF. The QTF is mounted on a xyz-scanner of the AFM and a soft AFM probe is approached on the apex of a tine of the QTF by driving the z-servo and using the normal deflection voltage (Vtb) of position sensitive detector (PSD) as feedback signal. Once the tip contacts the tine, servo is switched off. QTF is electrically excited with a sinusoidal signal from OC4 (Nanonis) and amplitude of the QTFs output at transimpedance amplifier (Vtf) and amplitude of VTB (Vp) is measured by individual lock-in amplifiers which are internally synchronized to the phase of the excitation signal of the QTF. Before, the measurements optical lever is calibrated. By relating the both voltages (Vp & Vtf), sensitivity factor of the QTF (αTF) is determined. In the second approach, after the tip contacts the tine, the z-servo is switched off firstly, then the feedback signal is switched to Vp and frequency sweep for the QTF, Vtb as well as z-servo are started, instantaneously. To keep the Vp at set-point the feedback control moves the z-servo to track the vibration amplitude of the QTF and thus the distance traveled by the z-servo (Δζ) during sweep is equal to the forks amplitude of vibration (ΔxTF). αtf is determined by relating Δz and VTF. Both approaches can be non-destructively applied for QTF sensor calibration. AFM imaging of the AFM calibration grating TGZ1 (from NT-MDT Russia) has been performed with a calibrated QTF sensor.

Advances in the atomic force microscopy for critical dimension metrology

This paper presents an easy-to-use and efficient implementation of the added mass method for calibrating various atomic force microscope (AFM) probes without damage to the probe, which is based on the added mass method. The method is achieved by using a pneumatic control system and a home-made glass micro-pipette. Compared with the conventional added mass method, this method achieves the operation of the loading mass by negative pressure rather than the capillary force/electrostatic force, which is more stable and reliable. Therefore, it is efficient and reliable for placing/removing the loading mass to/from the AFM probe. It is a major significance for the calibration of unconventional shape cantilever. To confirm its ability, the polystyrene (PS) sphere is used as the loading mass to calibrate various AFM probes. The above experimental results show that this method can accurately and conveniently calibrate various AFM probes with less time consuming.

Publisher Correction: Broad modulus range nanomechanical mapping by magnetic-drive soft probes

We report a Kelvin Probe Force Microscope (KPFM), which can scan surface potential and adhesion. Single-pass pulsed force mode is proposed, to measure topography, contact potential difference (CPD) and adhesion force. For each measurement point, the tip perform an approach and retract from the sample. Therefore, the averaging effect which is an issue in amplitude modulation (AM)-KPFM, is greatly reduced. In addition, it has advantage of mechanical characterization of the sample at the same time as compare to frequency modulation (FM)-KPFM and AM-KPFM. Electrical and adhesion force of few-layer graphene on the n-doped silicon have been measured. The results show that the performance of proposed KPFM is better than AM-KPFM in measuring CPD while frequency modulation (FM)-KPFM has higher CPD resolution due to being sensitive to force gradient. The electrostatically caused adhesion errors in measuring adhesion force are compensated by the kelvin testing loop.