Commercial Atomic Force Microscope Research Articles

Single- and Multi-Frequency Detection of Surface Displacements via Scanning Probe Microscopy

Piezoresponse force microscopy (PFM) provides a novel opportunity to detect picometer-level displacements induced by an electric field applied through a conducting tip of an atomic force microscope (AFM). Recently, it was discovered that superb vertical sensitivity provided by PFM is high enough to monitor electric-field-induced ionic displacements in solids, the technique being referred to as electrochemical strain microscopy (ESM). ESM has been implemented only in multi-frequency detection modes such as dual AC resonance tracking (DART) and band excitation, where the response is recorded within a finite frequency range, typically around the first contact resonance. In this paper, we analyze and compare signal-to-noise ratios of the conventional single-frequency method with multi-frequency regimes of measuring surface displacements. Single-frequency detection ESM is demonstrated using a commercial AFM.

Ultrastable AFM: Improved Stability, Precision, and Bandwidth for Bio-AFM

Enhancing the short-term force precision and temporal resolution of atomic force microscopy (AFM) while maintaining excellent long-term stability would result in improved performance across multiple AFM modalities, including single molecule force spectroscopy. However, instrumental drift in AFM remains a critical issue that limits the precision and duration of experiments. Previously, we developed an active optical stabilization technique to improve tip-sample stability at ambient conditions. However, force drift also occurs via uncontrolled deflection of the zero-force position of the cantilever. We found that the primary source of force drift in liquid for a popular class of soft cantilevers is their gold coating, even though they are coated on both sides to minimize drift. While removing the gold led to ∼10-fold reduction in reflected light, we nonetheless achieved a 10-fold improvement in force stability of bioAFM, with a sub-pN force precision over a broad bandwidth (0.01-20 Hz) just 30 minutes after loading. We subsequently extended AFM's sub-pN bandwidth by a factor of ∼50 to span five decades of bandwidth (Δf ≈ 0.01-1,000 Hz by developing an efficient process to modify a short (L = 40 μm) commercially available cantilever (BioLever Mini) with a focused ion beam (FIB). Measurements of mechanically stretching individual proteins showed improved force precision coupled with state-of-the-art force stability and no significant loss in temporal resolution compared to stiffer, unmodified cantilevers. Ongoing work in our lab extends this concept down to ultrashort cantilevers (L =10 μm) along with instrumental development to detect these cantilevers in a commercial AFM. Such cantilevers enable sensitive detection of protein unfolding and refolding with ∼1 μs time resolution. Importantly, these cantilevers were robust and were reused for SFMS over multiple days. Hence, we expect these responsive, yet stable, cantilevers to broadly benefit diverse AFM-based studies.

Effects of Scratching Parameters on Fabrication of Polymer Nanostructures in Atomic Force Microscope Tapping Mode

The nano scratching with an oscillating Atomic Force Microscopy (AFM) tip in tapping mode is called as the dynamic ploughing. The tip is vibrated in a high frequency and scratches the surface which is similar to the conventional vibration-assistant machining process. In the present study, the dynamic ploughing technique is utilized to scratch PolymethylMethacrylate (PMMA) polymer surfaces forming nanostructures with a commercial AFM system and two kinds of cantilevers. Effects of scratching parameters of the dynamic ploughing including scratching velocity, driving amplitude, pitch and the cantilever's elastic constant on the machined results are studied in detail. Finally nano ring structures with different radius are achieved successfully. Video abstract [Display omitted]

A Novel Dog-Bone Oscillating AFM Probe with Thermal Actuation and Piezoresistive Detection

In order to effectively increase the resonance frequency and the quality factor of atomic force microscope (AFM) probes, a novel oscillating probe based on a dog-bone shaped MEMS resonator was conceived, designed, fabricated and evaluated. The novel probe with 400 μm in length, 100 μm in width and 5 μm in thickness was enabled to feature MHz resonance frequencies with integrated thermal actuation and piezoresistive detection. Standard silicon micromachining was employed. Both electrical and optical measurements were carried out in air. The resonance frequency and the quality factor of the novel probe were measured to be 5.4 MHz and 4000 respectively, which are much higher than those (about several hundreds of kHz) of commonly used cantilever probes. The probe was mounted onto a commercial AFM set-up through a dedicated probe-holder and circuit board. Topographic images of patterned resist samples were obtained. It is expected that the resonance frequency and the measurement bandwidth of such probes will be further increased by a proper downscaling, thus leading to a significant increase in the scanning speed capability of AFM instruments.

High aspect ratio AFM Probe processing by helium-ion-beam induced deposition.

A Scanning Helium Ion Microscope (SHIM) is a high resolution surface observation instrument similar to a Scanning Electron Microscope (SEM) since both instruments employ finely focused particle beams of ions or electrons [1]. The apparent difference is that SHIMs can be used not only for a sub-nanometer scale resolution microscopic research, but also for the applications of very fine fabrication and direct lithography of surfaces at the nanoscale dimensions. On the other hand, atomic force microscope (AFM) is another type of high resolution microscopy which can measure a three-dimensional surface morphology by tracing a fine probe with a sharp tip apex on a specimen's surface.In order to measure highly uneven and concavo-convex surfaces by AFM, the probe of a high aspect ratio with a sharp tip is much more necessary than the probe of a general quadrangular pyramid shape. In this paper we report the manufacture of the probe tip of the high aspect ratio by ion-beam induced gas deposition using a nanoscale helium ion beam of SHIM.Gas of platinum organic compound was injected into the sample surface neighborhood in the vacuum chamber of SHIM. The decomposition of the gas and the precipitation of the involved metal brought up a platinum nano-object in a pillar shape on the normal commercial AFM probe tip. A SHIM system (Carl Zeiss, Orion Plus) equipped with the gas injection system (OmniProbe, OmniGIS) was used for the research. While the vacuum being kept to work, we injected platinum organic compound ((CH3)3(CH3C5H4)Pt) into the sample neighborhood and irradiated the helium ion beam with the shape of a point on the apex of the AFM probe tip. It is found that we can control the length of the Pt nano-pillar by irradiation time of the helium ion beam. The AFM probe which brought up a Pt nano-pillar is shown in Figure1. It is revealed that a high-aspect-ratio Pt nano-pillar of ∼40nm diameter and up to ∼2000 nm length can be grown. In addition, for possible heating by the helium ion beam, it was observed that an original probe shape was transformed. AFM measurement of a reference sample (pitch 100-500 nm, depth 100 nm) of the lines and spaces was performed using the above probes. The conventional probes which did not bring up platinum was not able to get into the ditch enough. Therefore it was found that a salient was big and a reentrant was shallow. On the other hand, the probe which brought up platinum was able to enter enough to the depths of the ditch.jmicro;63/suppl_1/i30-a/DFU075F1F1DFU075F1Fig.1.SHIM image of the AFM probe with the Pt nano-pillar fabricated by ion-beam induced deposition.

Scanning thermoelectric microscopy of local thermoelectric behaviors in (Bi,Sb)2Te3 films

In this paper we develop scanning thermoelectric microscopy (STeM) on the basis of commercial atomic force microscope. The nanoscale thermoelectric behaviors of (Bi,Sb)2Te3 (BST) thin films were studied. 3ω-technique was used for thermal conductivity imaging and quantitative thermal characterization. By acquiring the unique Seebeck information from 2ω frequency component, nanoscale thermoelectric images were firstly obtained, exhibiting remarkably inhomogeneous distribution of local Seebeck coefficient in the thin films. Positive thermoelectric response is revealed by the modulation of temperature difference between thermal tip and sample, corresponding to p-type conduction within BST sample.

Design and fabrication of in-plane AFM probes with sharp silicon nitride tips based on refilling of anisotropically etched silicon moulds

In this paper a micromachining method for batch fabrication of in-plane atomic force microscope (AFM) probes that consist of a sharp silicon nitride tip on a monocrystalline silicon cantilever is presented. The tips are realized by conformal deposition of silicon nitride inside an anisotropically etched cavity inside a silicon wafer. The best measured radius of the sharp tips was 8 nm. Our fabrication method is fully compatible with silicon-on-insulator (SOI) micromachining, allowing a straightforward monolithic integration of the AFM probes with high-aspect-ratio monocrystalline silicon MEMS. The fabrication method allows for lateral cantilevers, which oscillate in the plane of the fabrication wafer. This allows for simple integration of micromechanical transducers, opening the way towards dedicated probes for high speed AFMs. To demonstrate the innovation potential of this method, three different probe designs were fabricated: a plane passive AFM probe, a probe with integrated electrostatic actuator, and a probe which allows scanning on vertical sidewalls. The passive probes were successfully tested in a commercial AFM set-up. Correct operation of the probes with integrated actuator was demonstrated by actuation under a laser vibrometer.

Fabrication of AFM probe with CuO nanowire formed by stress-induced method

A novel method to fabricate an atomic force microscope (AFM) probe with CuO nanowire was proposed using a stress-induced method which can form nanowire easily. By heating a commercial AFM probe which is eliminated its tip and is coated with Ta and Cu films, Cu hillock and CuO nanowires on the hillock can be formed at the probe end. To obtain the CuO nanowire of high aspect ratio that can use as an AFM probe tip, thickness of coating films, heating temperature and heating time were investigated. As the result, the probe tip approximately 5 μm high and the nanowire approximately 25 nm in diameter were successfully obtained. In the results that the measurement resolution was evaluated, the measurement resolution of the CuO nanowire probe slightly worse than that of a commercial AFM probe. However, from the viewpoint of the dimensional measurement, the precision was almost equal between both probes.

High-speed imaging upgrade for a standard sample scanning atomic force microscope using small cantilevers.

We present an atomic force microscope (AFM) head for optical beam deflection on small cantilevers. Our AFM head is designed to be small in size, easily integrated into a commercial AFM system, and has a modular architecture facilitating exchange of the optical and electronic assemblies. We present two different designs for both the optical beam deflection and the electronic readout systems, and evaluate their performance. Using small cantilevers with our AFM head on an otherwise unmodified commercial AFM system, we are able to take tapping mode images approximately 5-10 times faster compared to the same AFM system using large cantilevers. By using additional scanner turnaround resonance compensation and a controller designed for high-speed AFM imaging, we show tapping mode imaging of lipid bilayers at line scan rates of 100-500 Hz for scan areas of several micrometers in size.

Frequency function in atomic force microscopy applied to a liquid environment.

Scanning specimens in liquids using commercial atomic force microscopy (AFM) is very time-consuming due to the necessary try-and-error iteration for determining appropriate triggering frequencies and probes. In addition, the iteration easily contaminates the AFM tip and damages the samples, which consumes probes. One reason for this could be inaccuracy in the resonant frequency in the feedback system setup. This paper proposes a frequency function which varies with the tip-sample separation, and it helps to improve the frequency shift in the current feedback system of commercial AFMs. The frequency function is a closed-form equation, which allows for easy calculation, as confirmed by experimental data. It comprises three physical effects: the quasi-static equilibrium condition, the atomic forces gradient effect, and hydrodynamic load effect. While each of these has previously been developed in separate studies, this is the first time their combination has been used to represent the complete frequency phenomenon. To avoid “jump to contact” issues, experiments often use probes with relatively stiffer cantilevers, which inevitably reduce the force sensitivity in sensing low atomic forces. The proposed frequency function can also predict jump to contact behavior and, thus, the probe sensitivity could be increased and soft probes could be widely used. Additionally, various tip height behaviors coupling with the atomic forces gradient and hydrodynamic effects are discussed in the context of carbon nanotube probes.

Mirror control with strain feedback and charge drive for high-performance of scanning probe microscope positioners

An integrated control method based on mirror control with strain feedback and charge drive for piezoelectric actuators is presented in this paper. For mirror control, a target actuator is indirectly controlled and supposed to function simultaneously with a reference actuator. Based on this, a basic charge drive with good linearity and high resolution at high frequencies gains the ability to drive grounded load and output high voltages. Strain feedback is adopted to guarantee the low-frequency characteristic of the control system. This paper describes the integrated control method in detail. The experimental results demonstrated that high linearity, low hysteresis and good low-frequency response were achieved. The technique was also successfully implemented on a commercial atomic force microscopy for low-distortion scanning images.

An AFM-based methodology for measuring axial and radial error motions of spindles

This paper presents a novel atomic force microscopy (AFM)-based methodology for measurement of axial and radial error motions of a high precision spindle. Based on a modified commercial AFM system, the AFM tip is employed as a cutting tool by which nano-grooves are scratched on a flat surface with the rotation of the spindle. By extracting the radial motion data of the spindle from the scratched nano-grooves, the radial error motion of the spindle can be calculated after subtracting the tilting errors from the original measurement data. Through recording the variation of the PZT displacement in the Z direction in AFM tapping mode during the spindle rotation, the axial error motion of the spindle can be obtained. Moreover the effects of the nano-scratching parameters on the scratched grooves, the tilting error removal method for both conditions and the method of data extraction from the scratched groove depth are studied in detail. The axial error motion of 124 nm and the radial error motion of 279 nm of a commercial high precision air bearing spindle are achieved by this novel method, which are comparable with the values provided by the manufacturer, verifying this method. This approach does not need an expensive standard part as in most conventional measurement approaches. Moreover, the axial and radial error motions of the spindle can both be obtained, indicating that this is a potential means of measuring the error motions of the high precision moving parts of ultra-precision machine tools in the future.

Design and Fabrication of a Vacuum Chamber for a Commercial Atomic Force Microscope

A vacuum chamber for a commercial atomic force microscope (AFM) is designed and fabricated. Only minimal modifications were made to an existing microscope in an effort to work in a vacuum environment, while most of the available AFM functionalities were kept intact. The optical alignment needed for proper AFM operations including a SLD (superluminescent diode) and a photodiode can be made externally without breaking the vacuum. A vacuum level of 5×10 -3 torr was achieved with a mechanical pump. An enhancement of the quality factor was observed along with a shift in the resonance frequency of a non-contact-mode cantilever in a vacuum. Topographical data of a calibration sample were also obtained in air and in a low vacuum using the non-contact mode and the results were compared.

Dynamic speed control in atomic force microscopy to improve imaging time and quality

Quantitative, three-dimensional surface imaging is one of the most significant advantages of atomic force microscopy (AFM). The imaging speed, however, is its major issue, due to the limited response time of the feedback loop. We present a dynamically adjusted scanning speed feature implemented on a commercial AFM instrument. The signals available in the system are utilized for that purpose. The auxiliary module controls the scanning speed in order to provide the necessary time to restore the tip–sample distance, which may deviate due to insufficient settling time. The solution allows the measurement to be performed with a relatively fast scanning rate and the desired imaging quality. Quantitative analysis of the results shows the relation between the imaging error and the settings of the system. It is shown that the image is acquired faster and with better imaging quality than using the constant speed method. Also, the data are presented showing a reduction of the topographical crosstalk in the phase imaging feature, as an example of the utilization of this feature in advanced AFM modes.

Specific methodology for capacitance imaging by atomic force microscopy: A breakthrough towards an elimination of parasitic effects

On the basis of a home-made nanoscale impedance measurement device associated with a commercial atomic force microscope, a specific operating process is proposed in order to improve absolute (in sense of “nonrelative”) capacitance imaging by drastically reducing the parasitic effects due to stray capacitance, surface topography, and sample tilt. The method, combining a two-pass image acquisition with the exploitation of approach curves, has been validated on sets of calibration samples consisting in square parallel plate capacitors for which theoretical capacitance values were numerically calculated.

Microcantilevers with embedded accelerometers for dynamic atomic force microscopy

The measurement of the intermittent interaction between an oscillating nanotip and the sample surface is a key challenge in dynamic Atomic Force Microscopy (AFM). Accelerometers integrated onto AFM cantilevers can directly measure this interaction with minimal cantilever modification but have been difficult to realize. Here, we design and fabricate high frequency bandwidth accelerometers on AFM cantilevers to directly measure the tip acceleration in commercial AFM systems. We demonstrate a simple way of calibrating such accelerometers and present experiments using amplitude modulated AFM on freshly cleaved mica samples in water to study the response of the accelerometer.

Photothermal excitation setup for a modified commercial atomic force microscope

High-resolution imaging in liquids using frequency modulation atomic force microscopy is known to suffer from additional peaks in the resonance spectrum that are unrelated to the cantilever resonance. These unwanted peaks are caused by acoustic modes of the liquid and the setup arising from the indirect oscillation excitation by a piezoelectric transducer. Photothermal excitation has been identified as a suitable method for exciting the cantilever in a direct manner. Here, we present a simple design for implementing photothermal excitation in a modified Multimode scan head from Bruker. Our approach is based on adding a few components only to keep the modifications as simple as possible and to maintain the low noise level of the original setup with a typical deflection noise density of about 15 fm/√[Hz] measured in aqueous solution. The success of the modification is illustrated by a comparison of the resonance spectra obtained with piezoelectric and photothermal excitation. The performance of the systems is demonstrated by presenting high-resolution images on bare calcite in liquid as well as organic adsorbates (Alizarin Red S) on calcite with simultaneous atomic resolution of the underlying calcite substrate.

MEMS based low cost piezoresistive microcantilever force sensor and sensor module

In the present work, we report fabrication and characterization of a low-cost MEMS based piezoresistive micro-force sensor with SU-8 tip using laboratory made silicon-on-insulator (SOI) substrate. To prepare SOI wafer, silicon film (0.8µm thick) was deposited on an oxidized silicon wafer using RF magnetron sputtering technique. The films were deposited in argon (Ar) ambient without external substrate heating. The material characteristics of the sputtered deposited silicon film and silicon film annealed at different temperatures (400–1050°C) were studied using atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. The residual stress of the films was measured as a function of annealing temperature. The stress of the as-deposited films was observed to be compressive and annealing the film above 1050°C resulted in a tensile stress. The stress of the film decreased gradually with increase in annealing temperature. The fabricated cantilevers were 130μm in length, 40μm wide and 1.0μm thick. A series of force–displacement curves were obtained using fabricated microcantilever with commercial AFM setup and the data were analyzed to get the spring constant and the sensitivity of the fabricated microcantilever. The measured spring constant and sensitivity of the sensor was 0.1488N/m and 2.7mV/N. The microcantilever force sensor was integrated with an electronic module that detects the change in resistance of the sensor with respect to the applied force and displays it on the computer screen.

A Novel Platform for Simultanoues Mechanical Stimulation and Characterization of Single Cells Based on Dielectric Elastomers and Atomic Force Microscopy

We have developed a novel set-up to simultaneously 1) apply static and dynamic deformations to adherent cells in culture 2) optically image cells under fluorescence microscopy and 3) assay the near-membrane mechanical properties with atomic force microscopy. In this system, the cell culture substrate is formed by a film of dielectric elastomer which can be eletro-actuated. The geometry and position of the actuating electrodes and the applied potential can be manipulated to obtain specific strain fields over the cell culture chamber. We have modeled the electro-mechanical behavior of the actuated elastomer film and using optical markers we have established an experimental procedure to optimize and quantify the strain at the adherent cells. This cell culture device has been integrated together with a commercial atomic force microscope coupled with an inverted optical microscope equipped for fluorescence. This novel set-up allows us to temporally assess, with sub-micron spatial resolution, single cell topography and elasticity, as well as ion fluxes, all during static or cyclically applied deformations. Preliminary results on fibrobalsts (3T3 NIH) show reproducible and reversible increase in cell elastic modulus as a response to 4% applied uni-axial stretch; additionally high resolution elasticity maps of an area of 40x40 μm on a single fibroblast could be obtained while stretching a single cell. When measuring cardiomyocites from mouse embryo, profiles of Ca2+ intracellular concentration could be also monitored while applying static and dynamic stretches. This study provides proof-of-concept for this set-up as a flexible experimental platform to investigate mechano-transduction mechanisms at the single cell level.

Parallel nanoimaging and nanolithography using a heated microcantilever array

We report parallel topographic imaging and nanolithography using heated microcantilever arrays integrated into a commercial atomic force microscope (AFM). The array has five AFM cantilevers, each of which has an internal resistive heater. The temperatures of the cantilever heaters can be monitored and controlled independently and in parallel. We perform parallel AFM imaging of a region of size 550 μm × 90 μm, where the cantilever heat flow signals provide a measure of the nanometer-scale substrate topography. At a cantilever scan speed of 1134 μm s−1, we acquire a 3.1 million-pixel image in 62 s with noise-limited vertical resolution of 0.6 nm and pixels of size 351 nm × 45 nm. At a scan speed of 4030 μm s−1 we acquire a 26.4 million pixel image in 124 s with vertical resolution of 5.4 nm and pixels of size 44 nm × 43 nm. Finally, we demonstrate parallel nanolithography with the cantilever array, including iterations of measure-write-measure nanofabrication, with each cantilever operating independently.