General three-dimensional image simulation and surface reconstruction in scanning probe microscopy using a dexel representation

The ability to image complex general three-dimensional (3D) structures, including reentrant surfaces and undercut features using scanning probe microscopy, is becoming increasing important in many small length-scale applications. This paper presents a dexel data representation and its algorithm implementation for scanning probe microscope (SPM) image simulation (morphological dilation) and surface reconstruction (erosion) on such general 3D structures. Validation using simulations, some of which are modeled upon actual atomic force microscope data, demonstrates that the dexel representation can efficiently simulate SPM imaging and reconstruct the sample surface from measured images, including those with reentrant surfaces and undercut features.     

A simple method for AFM tip characterization by polystyrene spheres

An AFM image would not be the true topography of a surface because of the limitation of a finite size of the tip. The true topography of the surface can be deduced if we can know the tip shape. In this paper a simple method has been established to determine the profile of an AFM tip. A geometrical model for the tip and a spherical object has been proposed to show the procedure for deducing the tip shape from AFM images. Isolated spheres and closely packed spheres with different diameters have been observed to confirm the tip shape by this method. It is a non-destructive method to determine the tip shape and the results can be used for future reconstruction of an AFM image.     

光学原子力显微镜中的蒙特卡罗方法

扫描探针显微镜(Scanning probe microscopy,SPM)是显微镜的一个分支,它利用物理探针扫描标本形成样本表面图像.而原子力显微镜(Atomic force microscopy,AFM)是SPM中一种多功能的表面成像和测量工具,对导电、不导电、真空中、空气中或流体中的各种样本均可测量.原子力显微镜最面临的最大挑战之一是评估其在表面测量过程中所伴随的不确定度.本研究通过XYZ Phase的标定,对一台光学原子力显微镜进行了校准.该方法旨在克服在评估一些无法实验确定的不确定部件时遇到的困难,如尖端表面相互作用力和尖端几何.运用蒙特卡罗方法来确定根据相关容差和概率密度函数(PDFs)随机绘制参数而引起的相关不确定度.整个过程遵循《测量不确定度表示指南》(GUM)补编2.经本方法验证,原子力显微镜的评估不确定度为10nm左右.     

Analysis of AFM Images of Self-Structured Surface Textures by Directional Fractal Signature Method

A new method, called augmented blanket with rotating grid (ABRG), has been proposed in our recent work on characterizing roughness and directionality of self-structured surface textures. This is the first method that calculates fractal dimensions (FDs) at individual scales and directions for the entire surface image data and does not require the data to be Brownian fractal. However, before the ABRG method can be used in real applications, effects of atomic force microscope (AFM) imaging conditions on FDs need to be evaluated first. In this paper, computer-generated AFM images with three different resolutions, 48 combinations of tip radii and cone angles, and 15 noise levels were used in the tests. The images represent isotropic self-structured surface textures with small, medium and large motif sizes, and anisotropic surfaces exhibiting two dominating directions. For isotropic surfaces, the ABRG method is not significantly affected (i.e. FDs changes <5 %) by image resolution, tip size (for surfaces with large motifs) and noise (except the level above 8 %). For anisotropic surfaces, the method exhibits large changes in FDs (up to ?34 %). The results obtained show that the ABRG method can be effective in analysing the AFM images of self-structured surface textures. However, some precautions should be taken with anisotropic and isotropic surfaces with small motifs.     

Measuring the sizes of nanospheres on a rough surface by using atomic force microscopy and a curvature-reconstruction method

One of the advantages of atomic force microscopy (AFM) is that it can accurately measure the heights of targets on flat substrates. It is difficult, however, to determine the shape of nanoparticles on rough surfaces. We therefore propose a curvature-reconstruction method that estimates the sizes of particles by fitting sphere curvatures acquired from raw AFM data. We evaluated this fitting estimation using 15-, 30-, and 50-nm gold nanoparticles on mica and confirmed that particle sizes could be estimated within 5% from 20% of their curvature measured using a carbon nanotube (CNT) tip. We also estimated the sizes of nanoparticles on the rough surface of dried cells and found we also can estimate the size of those particles within 5%, which is difficult when we only used the height information. The results indicate the size of nanoparticles even on rough surfaces can be measured by using our method and a CNT tip.     

Traceable calibration of transfer standards for scanning probe microscopy

A Metrological Atomic Force Microscope (MAFM) has been constructed for the traceable calibration of transfer standards for scanning probe microscopy. It uses optical interferometry to generate image scales with direct traceability to the national standard of length. Three interferometers monitor the relative displacements of the AFM tip and sample in the x, y and z directions and the interferometer data is used directly to construct 3D images of sample surfaces. Traceable dimensional measurement of surface features may then be derived from the image data. This paper describes the MAFM instrument and presents a measurement uncertainty budget. Examples are given of measurements of pitch and step height on calibration transfer standards for scanning probe microscopy.     

运用碳纳米管原子力显微镜针尖减小长程力作用的研究

减小探针和样品表面之间的长程宏观力是原子力显微镜获得高分辨率成像的关键。首先通过理论分析得出影响长程力的主要因素是探针的几何形状和尺寸。然后分别运用几何形状和尺寸不同的原子力显微镜的传统Si针尖和碳纳米管针尖对样品进行扫描试验研究,结果显示碳纳米管针尖较传统针尖获得了高分辨率的图像。这一结果表明,碳纳米管针尖减小了成像中宏观长程作用力的影响,是理想的原子力显微镜针尖。     

WiFi‐controlled portable atomic force microscope

This article proposes to develop a WiFi‐controlled portable atomic force microscope (AFM). The AFM consists of a horizontal probe, controlling circuits, digital to analog (D/A) and analog to digital (A/D) interfaces, a microcomputer (Raspberry Pi, RPi), and a laptop. The proposed AFM uses a pocket‐size power supply to drive the controlling circuits, the D/A and A/D interfaces, as well as the RPi that constructs network hotspots and generates scanning signals. With special design and integration of the whole system, both of the AFM probe and electronic controlling system are portable. At a distance of 50 m from the proposed AFM, experiments in the constant height mode and the constant force mode are conducted to evaluate its performance. The results show that this WiFi‐controlled AFM has a maximum scan range of 3.6 × 3.6 μm² with nanometer order resolution. Meanwhile, it achieves satisfactory image contrast, stability, and repeatability. Compared with conventional AFMs, the AFM proposed in this paper no longer relies on commercial AC mains supply or high‐voltage DC power supply, and realizes WiFi‐controlled AFM scanning and imaging in 50 m or farther without wire or network cable connection to a laptop or a desktop computer. Given credits to these features, WiFi‐controlled AFMs are expected to own a wider range of application, especially in isolated environments, outdoor researches, or even fieldwork investigations.     

Polystyrene spheres on mica substrates: AFM calibration, tip parameters and scan artefacts

Atomic force microscopy (AFM), in various versions, has had major impact as a surface structural and spectroscopic tool since its invention in 1986. At its present state of development, however, the interpretation of AFM images is limited by the current state of methodologies for calibration over the wide dynamic range of magnification. Also, the parameters of individual tips, as well as the generic characteristics of different kinds of tips, affect both the quality of the images and their interpretation. Finally, the very nature of the tip-to-surface interaction will generate artefacts, in addition to those associated with tip shape, which need to be fully understood by the practitioners of force microscopy. This project seeks to address and shed light on some of these issues. Polystyrene beads deposited on mica substrates form hexagonal close-packed layers. The unit cell parameters are suitable for calibration of the AFM in the lateral plane, while the perpendicular spacing of the layers is appropriate for calibration along the vertical axis. Using different size fractions, it is straightforward to determine the extents of linearity, orthogonality, thermal and instrumental drifts over distances from 100 nm to tens of micrometres. The present results show that the methodologies for contact mode operation can be adapted to noncontact modes. It is known that an AFM image arises from a convolution of surface topography and tip shape, and is mediated by the interaction. In principle it is possible to carry out a deconvolution, if we have complete knowledge about two of the three elements (i.e. tip, surface and interaction). In practice we rarely have the requisite information. Prominent artefacts will occur when the characteristic parameters of the tip are comparable to those of the surface topography, and/or if there is a variable strength, or extent of localization, of the interaction. The present results demonstrate artefacts due to effects of geometry as well as interaction.     

一维矩形光栅AFM图像盲探针表面重建模拟研究

探针结构参数的合理选取将直接决定扫描图像及其盲探针修正图像的失真程度。基于此,以一维矩形模拟光栅为典型案例,对该模拟光栅的原子力显微镜(AFM)扫描成像过程与盲探针修正过程进行了仿真,阐明了探针结构参数对扫描成像过程与盲探针修正过程的影响规律。通过建立线宽变化度与半高宽相结合的图像重建误差评价指标,确定了针对该模拟光栅的AFM探针建议结构参数,并取得了良好的光栅图像重建效果。研究表明,应用线宽变化度结合半高宽来综合评价光栅的AFM测量和图像重建过程,有利于提升实际光栅AFM图像盲探针重建的准确度。     

一种基于SFS方法的含高光表面三维重构系统

针对含高光表面三维形状重构的需求,设计了一种基于从明暗恢复形状(SFS)方法的三维重构系统。在正交投影条件下由CCD相机获取点光源照射下的物体表面图像,使用Ward反射模型描述含高光表面的反射特性,建立物体表面图像辐照度方程。系统软件将该方程转化为包含物体高度信息的H-J偏微分方程,并计算此偏微分方程的解,得到物体的高度函数,进而恢复出物体的表面形状。实验表明,该系统可以有效地重构含高光表面的三维形状。     

Topographic characterization of unworn contact lenses assessed by atomic force microscopy and wavelet transform

This paper analyses the three‐dimensional (3‐D) surface morphology of optic surface of unworn contact lenses (CLs) using atomic force microscopy (AFM) and wavelet transform. Refractive powers of all lens samples were 2.50 diopters. Topographic images were acquired in contact mode in air‐conditioned medium (35% RH, 23°C). Topographic measurements were taken over a 5 µm × 5 µm area with 512 pixel resolution. Resonance frequency of the tip was 65 kHz. The 3‐D surface morphology of CL unworn samples revealed (3‐D) micro‐textured surfaces that can be analyzed using (AFM) and wavelet transform. AFM and wavelet transform are accurate and sensitive tools that may assist CL manufacturers in developing CLs with optimal surface characteristics. Microsc. Res. Tech. 78:1026–1031, 2015. © 2015 Wiley Periodicals, Inc.     

Direct deformation study of AFM probe tips modified by hydrophobic alkylsilane self-assembled monolayers

The in-use wear of atomic force microscopy (AFM) probe tips is crucial for the reliability of AFM measurements. Increase of tip size for several nanometers is difficult to monitor but it can already taint subsequent AFM data. We have developed a method to study the shape evolution of AFM probe tips in nanometer scale. This approach provides direct comparison of probe shape profiles, and thus can help in evaluation of the level of tip damage and quality of acquired AFM data. Consequently, the shape degradation of probes modified by hydrophobic alkylsilane self-assembled monolayers (SAMs) was studied. The tip wear length and wear volume were adopted to quantitatively verify the effectiveness of hydrophobic coatings. When compared with their silicon counterparts, probes modified by SAM materials exhibit superior wear-resistant behavior in tapping mode scans.     

Effect of tip shape on line edge roughness measurement based on atomic force microscopy

Atomic force microscopy (AFM) is an important tool in line edge roughness (LER) measurements, where accuracy for line edge identification is influenced by the shape of the tip. In this article, the effect of tip shape on LER measurement based on AFM is studied theoretically. The formulas for calculating the distance between the measured and actual line edge of the sample are presented. The effects of the three kinds of tips with different shapes are experimentally compared for validation. Suggestions on how to reduce measuring error caused by tip shape are also given.     

An intercepted feedback mode for light sensitive spectroscopic measurements in atomic force microscopy

In most atomic force microscopes (AFMs), the motion of the tip is detected by the deflection of a laser beam shining onto the cantilever. AFM applications such as scanning capacitance spectroscopy or photocurrent spectroscopy, however, are severely disturbed by the intense stray light of the AFM laser. For this reason, an intercepted feedback method was developed, which allows to switch off the laser temporarily while the feedback loop keeps running. The versatility of this feedback method is demonstrated by measuring tip-force dependent Schottky barrier heights on GaAs samples.     

Atomic force microscope tip cleaning by using gratings as micro‐washboards

The sharpness of atomic force microscope (AFM) tips is essential for acquiring high quality AFM images. However, AFM tips would easily get contaminated during scanning and storage at ambient condition, which influences image resolution and causes image distortion. Replacing the probe frequently is a solution, but uneconomical. To solve this problem, several tip cleaning methods have been proposed but there is space for further improvement. Therefore, this article developed a method of tip cleaning by using a one‐dimensional grating (600 lines/mm) as a micro‐washboard to “wash” contaminated tips. We demonstrate that the contaminants can be scrubbed away by rapidly scanning such micro‐washboard against the tip in the aids of Z‐dithering (10–20 Hz) exerted on the washboard. This method is highly efficient and proved to be superior to traditional ones. Experiments show that AFM images acquired with “washed” tips have higher resolution and less distortion compared with images acquired using contaminated tips, even comparable to those scanned by new ones. Microsc. Res. Tech. 76:1131–1134, 2013. © 2013 Wiley Periodicals, Inc.     

The substructure of immunoglobulin G resolved to 25 kDa using amplitude modulation AFM in air

Amplitude modulation (or tapping-mode) atomic force microscopy (AM AFM or TM AFM) in air can reveal sub-molecular details of isolated multi-subunit proteins, such as immunoglobulin G (IgG) antibodies, on atomically flat support surfaces such as mica [A. San Paulo, R. Garcia, Biophys. J. 78(3) (2000) 1599]. This is achieved by controlling the microscope imaging parameters (e.g. cantilever drive frequency and set-point amplitude) to keep the AFM tip predominantly in the attractive force regime. Under these conditions, the 50 kDa F(c) and F(ab) subunits can be resolved when the molecule has the appropriate orientation on the surface. The presence of a water layer on hydrophilic mica is an important factor affecting imaging contrast, a consequence of capillary neck formation between tip and surface [L. Zitzler, S. Herminghaus, F. Mugele, Phys. Rev. B 66(15) (2002) 155436]. Desiccation of samples to remove surface bound water layers can yield reproducible imaging of the IgG substructure [N.H. Thomson, J. Microsc. (Oxford) 217(3) (2004) 193]. This approach has also given higher resolution than previously achieved, down to about 25 kDa, and these data are detailed here. These subdomains are formed as two immunoglobulin folds from the light and heavy peptide chains of the IgG crossover. This result has been validated by comparing the AFM images with X-ray crystallography data from the protein data bank. These data show that the AFM can obtain 25 kDa resolution on isolated protein molecules with commercially available silicon tips, but, as expected for a local probe technique, resolution is highly dependent on the macromolecular orientation on the support surface.     

Unstable amplitude and noisy image induced by tip contamination in dynamic force mode atomic force microscopy

Liquid 1-decanethiol was confined on an atomic force microscope (AFM) tip apex and the effect was investigated by measuring amplitude-distance curves in dynamic force mode. Within the working distance in the dynamic force mode AFM, the thiol showed strong interactions bridging between a gold-coated probe tip and a gold-coated Si substrate, resulting in unstable amplitude and noisy AFM images. We show that under such a situation, the amplitude change is dominated by the extra forces induced by the active material loaded on the tip apex, overwhelming the amplitude change caused by the geometry of the sample surface, thus resulting in noise in the image the tip collects. We also show that such a contaminant may be removed from the apex by pushing the tip into a material soft enough to avoid damage to the tip.     

Complex wavelet enhanced shape from shading transform for estimating surface roughness of milled mechanical components

The metal surface topology contains abundant information related to the health states of the cutting tool as well as the cutting operation. In this paper, we attempt to adopt 2D digital images of the machined metal surface, acquired via non-contact photo-imaging techniques, as the monitoring media. A Wallis filter based dodging algorithm is applied to cure the uneven contrast phenomenon caused by imperfect lighting illumination. 3D digital models were derived and retrieved from the digital image using a wavelet enhanced Shape from shading (SFS) transform. The minimization based SFS is presented to retrieve the 3D digital surface from the milled workpiece. The dual tree complex wavelet transform is adopted to enhance SFS such that the interfering noise can be suppressed. In the end, quantitative surface roughness indicators are utilized to estimate the surface roughness numerically. A milling cutting experiment of aero-material of aluminum alloy 7075 was carried out to verify the effectiveness of the proposed approach. The comparison results demonstrate that the proposed approach was capable of retrieving 3D surfaces of high precision. With the approach, the digital image emerges as a promising vehicle for machining condition monitoring of CNC machines.