Active Q factor control of MEMS cantilevers by integrated piezoelectric transducers for high-speed AFM applications under vacuum

Ahstract- For imaging nano-scaled samples, atomic force microscopy (AFM) and scanning electron microscopy (SEM) represent two complementary imaging techniques. In a hybrid SEM-AFM system, a compact AFM is installed inside the high vacuum chamber of an SEM, where SEM provides largely 2D imaging and material compositions of a sample while AFM is capable of complementarily measuring 3D topography of the sample. Although SEM can achieve real-time imaging (e.g., 20 Hz), AFM scan can take minutes to generate an image, demanding strategies for speeding up AFM measurement. In existing hybrid SEM-AFM systems, SEM and AFM measurements are made independently. This paper presents, for the first time, a technique of using SEM nanoscopic imaging to guide the scan speed of AFM imaging. The dynamic variation of AFM scan speed is based on features identified in SEM imaging. Information/features are extracted from real-time SEM images and quantitated using local entropy and other metrics. The generated feature metric map is used to produce a speed map for varying AFM scan speed at each position on the sample. Experiments were conducted with a new SEM-compatible AFM instrument we recently developed, as the test bed of the SEM-guided AFM scan technique. The results for the samples measured in this work demonstrate that time savings of this technique, compared to traditional AFM scan using a constant speed, were up to 66% with equivalent imaging accuracy obtained with traditional fine scan. With the same time cost of traditional fast scan, the SEM -guided AFM scan technique had an accuracy improvement of 47%.

Microdeformations observed in atomic force microscopy (AFM) images on the surface of metallic films when an electrical current (dc) flows through the film were analyzed on a film–substrate system. The electrical current flow causes an increase in temperature on the thin film and a step in the AFM topography while imaging. Experiments realized with AFM explain the nature of such a step and allow one to quantify these microdeformations. Theoretical bending models mainly derived from modifications of the Stoney equation support our experimental results. Theoretical predictions show reasonable agreement with AFM measurements, and demonstrate that the major contribution to the observed deformation step (about 90%) is due to the thermal bending effect of the AFM cantilever. The remaining deformation is due to the nature of bimaterial effects in the film–substrate system and thermal expansion of the substrate, with the expansion of the film being negligible. The possibility of electric and magnetic effects in the AFM cantilever (due to current flow through the film) is also discussed. The existence of a strong thermal effect on the AFM cantilever is outlined. Hence, a slight variation in temperature during AFM measurements could turn out to be misinterpretation of the results obtained. Consequently, we recommend strict control of the temperature during AFM imaging, in order to improve the reliability and accuracy of the instrument.

Currently, two types of direct methods to characterize and identify single virions are available: electron microscopy (EM) and scanning probe techniques, especially atomic force microscopy (AFM). AFM in particular provides morphologic information even of the ultrastructure of viral specimens without the need to cultivate the virus and to invasively alter the sample prior to the measurements. Thus, AFM can play a critical role as a frontline method in diagnostic virology. Interestingly, varying morphological parameters for virions of the same type can be found in the literature, depending on whether AFM or EM was employed and according to the respective experimental conditions during the AFM measurements. Here, an inter-methodological proof of principle is presented, in which the same single virions of herpes simplex virus 1 were probed by AFM previously and after they were measured by scanning electron microscopy (SEM). Sophisticated chemometric analyses then allowed a calculation of morphological parameters of the ensemble of single virions and a comparison thereof. A distinct decrease in the virions' dimensions was found during as well as after the SEM analyses and could be attributed to the sample preparation for the SEM measurements. Graphical abstract The herpes simplex virus is investigated with scanning electron and atomic force microscopy in view of varying dimensions.

Mounting the atomic force microscope (AFM) probe on the gripper of the robotic nanoscale manipulators system enables the combination of the AFM and scanning electron microscope (SEM). The AFM–SEM‐based method of determining Hamaker constant for carbon nanotube (CNT) and AFM cantilever in the vacuum condition of SEM, which is completed through a robotic nanoscale manipulators system, is presented for the first time. During Hamaker constant determining experiment, the elastic force is measured utilising integrated AFM–SEM imaging technique through detecting the deflection of AFM cantilever that actually equals van der Waals adhesion force. The van der Waals attraction force between CNT and AFM cantilever is calculated through the modelling of a cylindrical nanotube and a flat gold‐coated surface. The experiment results indicate that Hamaker's constant measured are comparable to typical values published by previous researchers, being of the order of 100 zJ, which indicates that the present approach is valid and reliable.

Doped diamond has found a commercial use for achieving durable and reproducible electrical measurements in atomic force microscopy (AFM). Yet so far it has not been used on self‐sensing AFM probes due to thermally and mechanically sensitive integrated detection circuits. Herein, conventional microwave plasma chemical vapor deposition (CVD) is employed while taking advantage of thermal conductivity along the silicon AFM cantilever probe for growing high‐quality B‐doped nanocrystalline diamond film on the probe apex that is selectively seeded by dip coating in nanodiamond solution. By investigating various CVD process parameters, it is shown that the detection circuit remains functional up to 400 °C for 2 h deposition or up to 8 h at 300 °C. Scanning electron microscopy and Raman spectroscopy corroborate quality of the diamond coating and doping. The self‐sensing probes are successfully tested in conductive AFM (C‐AFM) regime and surface spreading resistance regime, showing linear response in current–voltage spectroscopy and capability of conductivity mapping on metals and semiconductor devices. In the results, prospects for stable C‐AFM measurements when conventional optical detection is not suitable, such as on photosensitive materials or in probe‐electron microscopy, are opened.

Piezoelectric actuation of atomic force microscope (AFM) cantilevers often suffers from spurious mechanical resonances in the loop between the signal driving the cantilever and the actual tip motion. These spurious resonances can reduce the accuracy of AFM measurements and in some cases completely obscure the cantilever response. To address these limitations, we developed a specialized AFM cantilever holder for electrostatic actuation of AFM cantilevers. The holder contains electrical contacts for the AFM cantilever chip, as well as an electrode (or electrodes) that may be precisely positioned with respect to the back of the cantilever. By controlling the voltages on the AFM cantilever and the actuation electrode(s), an electrostatic force is applied directly to the cantilever, providing a near-ideal transfer function from drive signal to tip motion. We demonstrate both static and dynamic actuations, achieved through the application of direct current and alternating current voltage schemes, respectively. As an example application, we explore contact resonance atomic force microscopy, which is a technique for measuring the mechanical properties of surfaces on the sub-micron length scale. Using multiple electrodes, we also show that the torsional resonances of the AFM cantilever may be excited electrostatically, opening the door for advanced dynamic lateral force measurements with improved accuracy and precision.

The vibrational characteristics of an atomic force microscope (AFM) cantilever beam play a key role in dynamic mode of the atomic force microscope. As the oscillating AFM cantilever tip approaches the sample, the tip–sample interaction force influences the cantilever dynamics. In this paper, we present a detailed theoretical analysis of the frequency response and mode shape behavior of a cantilever beam in the dynamic mode subject to changes in the tip mass and the interaction regime between the AFM cantilever system and the sample. We consider a distributed parameter model for AFM and use Euler–Bernoulli method to derive an expression for AFM characteristics equation contains tip mass and interaction force terms. We study the frequency response of AFM cantilever under variations of interaction force between AFM tip and sample. Also, we investigate the effect of tip mass on the frequency response and also the quality factor and spring constant of each eigenmodes of AFM micro-cantilever. In addition, the mode shape analysis of AFM cantilever under variations of tip mass and interaction force is investigated. This will incorporate the presentation of explicit analytical expressions and numerical analysis. The results show that by considering the tip mass, the resonance frequencies of the cantilever are decreased. Also, the tip mass has a significant effect on the mode shape of the higher eigenmodes of the AFM cantilever. Moreover, tip mass affects the quality factor and spring constant of each modes.

Fabrication and analysis of cylindrical resin AFM microcantilevers

Atomic force microscopy (AFM) cantilevers are commonly made from two material layers: a reflective coating and structural substrate. Although effective, this can result in thermally induced cantilever deflection due to ambient and local temperature changes. While this has been previously documented, key aspects of this common phenomenon have been overlooked. This work explores the impact of thermally induced cantilever deflection when in- and out-of-contact, including the topographic scan artefacts produced. Scanning thermal microscopy probes were employed to provide direct cantilever temperature measurement from Peltier and microheater sources, whilst permitting cantilever deflection to be simultaneously monitored. Optical lever-based measurements of thermal deflection in the AFM were found to vary by up to 250% depending on the reflected laser spot location on the cantilever. This highlights AFM’s inherent inability to correctly measure and account for thermal induced cantilever deflection in its feedback system. This is particularly problematic when scanning a tip in-contact with the surface, when probe behaviour is closer mechanically to that of a bridge than a cantilever regarding thermal bending. In this case, measurements of cantilever deflection and inferred surface topography contained significant artefacts and varied from negative to positive for different optical lever laser locations on the cantilevers. These topographic errors were measured to be up to 600 nm for a small temperature change of 2 K. However, all cantilevers measured showed a point of consistent, complete thermal deflection insensitivity 55% to 60% along their lengths. Positioning the reflected laser at this location, AFM scans exhibited improvements of up-to 97% in thermal topographic artefacts relative to other laser positions.

This report details the measurement and data analysis protocol for Project 12 ‘Distribution of lateral size and thickness of few-layer graphene flakes using SEM and AFM’ of Versailles Project on Advanced Materials and Standards (VAMAS) Technical Working Area (TWA) 41 “Graphene and Related 2D Materials”, as part of the European Metrology Programme for Innovation and Research (EMPIR) 19NORM04 ISO-G-SCoPe project. This study is an international interlaboratory comparison of the measurement of the structural properties of graphene nanoplatelets (GNPs) deposited onto different Si/SiO2 substrates for scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements. Participants will be asked to measure these samples, along with test samples to verify the calibration of their instruments and report the lateral flake size and thickness of the measured GNPs. The protocols for SEM and AFM measurements and related data analysis are described, with an aim of the study being to understand the sources of uncertainty in these types of measurements, as well as within- and in-between laboratory measurement variability, for a group of laboratories spread across the globe.

Calibration of atomic force microscope (AFM) cantilevers is necessary for the measurement of nano-newton and pico-newton forces, which are critical to analytical application of AFM in the analysis of polymer surfaces, biological structures and organic molecules. Previously we have described microfabricated array of reference spring (MARS) devices for AFM cantilever spring-constant calibration. Hitherto, these have been limited to the calibration of AFM cantilevers above 0.03 N/m, although they can be used to calibrate cantilevers of lower stiffness with reduced accuracy. Below this limit MARS devices similar to the designs hitherto described would be fragile and difficult to manufacture with reasonable yield. In this work we describe a device we call torsional MARS. This is a large-area torsional mechanical resonator, manufactured by bulk micromachining of a “silicon-on-insulator” wafer. By measuring its torsional oscillation accurately in vacuum we can deduce its torsional spring constant. The torsional reference spring spans the range of spring constant (from 4 down to 0.01 N/m) that is important in biological AFM, allowing even the most compliant AFM cantilever to be calibrated easily and rapidly.

Calibration of the spring constants of atomic force microscopy (AFM) cantilevers is one of the issues in biomechanics and nanomechanics for quantified force metrology at pico- or nano Newton level. In this paper, we present an AFM cantilever calibration system: the Nano Force Calibrator (NFC), which consists of a precision balance and a one-dimensional stage. Three types of AFM cantilevers (contact and tapping mode) with different shapes (beam and V) and spring constants (42, 1, 0.06 Nm?1) are investigated using the NFC. The calibration results show that the NFC can calibrate the micro cantilevers ranging from 0.01 ~ 100 Nm?1 with uncertainties of about 2%.

We experimentally validate theoretical relation between the roughness power spectrum (PS) and electrochemical current transient for a reversible charge transfer system under a single potential step. Roughness features at the electrochemically roughened electrode are characterized using standard measurements such as scanning electron microscopy (SEM), atomic force microscopy (AFM) and cyclic voltammetry (CV). The PS obtained from AFM shows composite finite fractal and nonfractal nature in roughness, whereas the PS from SEM shows only a finite fractal nature. AFM or SEM measurements provide knowledge of fractal dimension (DH) and two fractal cutoff lengths (landL). Topothesy length (lτ) or the related proportionality factor (μ≡lτ2DH−3) from PS data of AFM requires extrapolation of data for unit wavenumber, but this method usually provides unphysical values of μ. We provide a novel method to determine the topothesy of electrodes from CV measurements of electroactive area in conjunction with SEM or AFM measurements. Chronoamperometric measurements were made on morphologically characterized Pt-electrodes for a solution of K4[Fe(CN)6] and K3[Fe(CN)6] in 3 M NaNO3. The transient response observed experimentally is validated using the measured PS in the theoretical equation for the current. The transient response does not show contributions from Gaussian PS in the low wavenumber region; this is due to the fact that the effective lower cutoff wavenumber is usually limited up to the inverse of the width of roughness (or topothesy length). Fractal dimensions obtained through chronoamperometric measurement on electrodes using Pajkossy's approach do not correspond to the one obtained from AFM and SEM measurements. Finally, the anomalous response in the Cottrell measurements can be understood through PS-based theory.

Micro Electro Mechanical Systems (MEMS) is developed based on the semi-conductor technology, however, relative material, design, fabrication, simulation, packaging and test are more complex than those in semi-conductor technology. In the primary stage, MEMS technology focused on the design and development, now on the commercialization and improving reliability and decreasing cost and price. So test is increasingly important to MEMS technology and testing cost is about 1/3 of the whole cost of MEMS. In order to improve the production and decrease the cost, producers and researchers pay more attention to MEMS test to solve all the testing problems from design to packaging process. There are a number of methods to carry out these measurements, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), stylus profiler, and optical profiler, etc. Every method has its advantages and disadvantages.  SEM is one of the most common measurement tools. However, nearly all nonconductive specimens examined using SEM need to be coated with a thin film of conducting material. This may result in bending or distortion of the device, especially where free structures such as cantilever beams. SEM tests are also time consuming and not suitable for a production environment.  AFM has been suggested as a MEMS measurement tool. As with SEM, analysis may be slow (about 20 min/device), and the limited measurement range of an AFM (100 ┤m×100 ┤m×5 ┤m, Veeco multimode AFM) means that it is unable to investigate large samples or out-of-plane devices such as the cantilevers. It is also difficult to examine packaged devices using an AFM.  Mechanical stylus surface profilers are commonly used for dimensional measurements in MEMS. While these can measure step heights with a high accuracy, they are not suitable for the analysis of freestanding structures, where the stylus may break the device under test. Deep, high aspect ratio devices also pose problems, as the stylus probe may be too large to accurately reproduce the surface profile. If MEMS devices need to fit the large-scale production, it is essential that these measurements must be cheaply and easily made at the wafer level, without the need for large space, expensive packaging or destructive test methods. Optical techniques can offer

This work presents micro-actuation of atomic force microscopy (AFM) cantilevers using piezoelectric Zinc Oxide (ZnO) thin film. In tapping mode AFM, the cantilever is driven near its resonant frequency by an external oscillator such as piezotube or stack of piezoelectric material. Use of integrated piezoelectric thin film for AFM cantilever eliminates the problems like inaccurate tuning and unwanted vibration modes. In this work, silicon AFM cantilevers were sputter deposited with ZnO piezoelectric film along with top and bottom metallic electrodes. The self-excitation of the ZnO coated AFM cantilever was studied using Laser Doppler Vibrometer (LDV). At its resonant frequency (227.11 kHz), the cantilever displacement varies linearly with applied excitation voltage. We observed an increase in the actuation response (131nm/V) due to improved quality of ZnO films deposited at 200 °C.