Detection of buried reference structures by use of atomic force acoustic microscopy

The miniaturization of micro- and nanoelectronic components requires new methods for the inspection of buried inner structures at the nanoscale. We used the atomic force acoustic microscopy technique (AFAM) to image subsurface defects. This technique combines high lateral resolution with the capability to determine local elastic properties of materials near the surface. As the structures buried near the surface change the effective tip-sample contact stiffness it is possible to detect them. For the verification of the detection capabilities of AFAM we fabricated well-defined buried void structures with different geometries and dimensions. Large, thin, plate like structures of silicon nitride with a local filling were our first test samples. Then, sets of nine small, square, thin plates with thicknesses increasing stepwise from 30 to 270 nm were etched in a thinned silicon wafer. The last two samples contained wedge structures of widths varying between 1.6 and 10 μm. Our results showed that it was possible to detect buried void structures at depths between 180 and 900 nm. We also observed that the depths at which the buried defects can be detected by the use of the AFAM method depend on the defect dimensions and geometry, and on the mismatch in the elastic properties of the sample and the defects. The experimental results obtained for the groups of small, thin plates were verified by quantitative analysis via finite element method (FEM) simulations.     

基于AFM的微位移测量新方法研究

提出了一种测量物体微位移的新方法。原子力显微镜作为测量工具,样品和扫描器置于待测物体上,物体每移动一定距离就由AFM扫描获得一幅样品图像,由此获得一系列连续的序列图像。采用模板匹配方法检测相邻序列图像的偏移,从而可计算出物体的微位移。实验结果表明,用该方法还可实现物体二维方向的微位移测量,且精度达到纳米量级。     

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.     

Visualization of cellular DNA crosslinks by atomic force microscopy

Cellular DNA crosslinks are a type of DNA damage induced by toxic chemicals or high‐energy radiation. If damaged DNA is not rapidly repaired, cells will die or mutate. To evaluate the types of DNA damage and their influence on vital cell activities, it is necessary to be able to detect DNA crosslinks. To date, indirect methods such as alkaline elution, potassium chloride–sodium dodecyl sulfate assay and comet assay have been used to detect DNA damage. Direct morphological observation, on the other hand, may be a useful tool to differentiate the types of DNA damage. In this report, atomic force microscopy (AFM) has been employed to visualize the breakage and crosslinking of cellular DNA strands in cells treated with formaldehyde and hydrogen peroxide. Our results showed that toxic chemical‐induced crosslinking of cellular DNA occurred in a dose‐dependent manner. DNA conglomerates were observed with high concentrations of formaldehyde, and the AFM observations were consistent with those of a comet assay. Our experiments demonstrate that AFM is an efficient method to differentiate the types of DNA damage. SCANNING 31: 75–82, 2009. © 2009 Wiley Periodicals, Inc.     

A convenient and rapid sample repositioning approach for atomic force microscopy

A novel repositioning approach is described for repeated observations of a specimen at a close proximal location in the atomic force microscope. The approach is similar to keystone architecture, whereby the repositioning is achieved by forming a male structured base for the specimen, and a corresponding female counterpart as the frame. For the combination of an acrylic acid frame and a metal base, 90% translation shifts are less than 10 µm, and almost all angular disorientations are within +3° to ?3°. Nanometre‐scale surface features can be relocated easily and reliably even after 40 imaging–removal–imaging cycles, dipping the specimen in solutions or heating up to 500 °C.     

基于平板扫描器的新型AFM系统

研制了一种基于平板扫描器的新型原子力显微镜(AFM)系统。该系统创新地把二维平板扫描器和一维反馈控制器相结合,有效地克服了传统扫描器Z向反馈控制与XY扫描平面之间的非线性交叉耦合误差,同时保证了大范围扫描时检测光路的稳定性。利用该系统与传统AFM作了氧化铝薄膜和光栅对比扫描实验,结果表明这种AFM系统能够获得无扭曲、规则的理想图像。     

Capturing the elasticity and morphology of live fibroblast cell cultures during degradation with atomic force microscopy

Atomic force microscopy, in a liquid environment, was used to capture in vitro the morphological and mechanical changes that cultured fibroblasts undergo as time elapses from the completion of the cell culture. Topography images illustrated that initially, the nucleus had a height of 1.18 ± 0.2 μm, and after 48 h it had decreased to 550 ± 60 nm; similarly, the cell membrane exhibited significant shrinkage from 34 ± 4 to 23 ± 2 μm. After each image scan, atomic force microscopy indentation was performed on the centre of the nucleus, to measure the changes in the cell elasticity. Examination of the force‐distance curves indicated that the membrane elastic modulus at the nucleus remained the same within the time frame of 48 h, even though the cell morphology had significantly changed.     

A new calibration method of the lateral contact stiffness and lateral force using modulated lateral force microscopy

We describe a new calibration method for lateral contact stiffness using modulated lateral force microscopy, a technique that offers some advantages with respect to the more classical friction force microscopy currently used for characterizing the friction properties of materials. The calibration method is based on the study of the lateral contact stiffness versus applied load and on the use of elasticity contact theories to determine by fit the calibration coefficient, allowing the scaling of experimental data. The method is tested by measuring the friction coefficient and shear strength of silicon and mica samples, respectively, and compared with results from the literature. This revised version was published online in August 2006 with corrections to the Cover Date.     

Application of atomic force microscopy to the study of natural and model soil particles

The structure and surface chemistry of soil particles has extensive impact on many bulk scale properties and processes of soil systems and consequently the environments that they support. There are a number of physiochemical mechanisms that operate at the nanoscale which affect the soil's capability to maintain native vegetation and crops; this includes soil hydrophobicity and the soil's capacity to hold water and nutrients. The present study used atomic force microscopy in a novel approach to provide unique insight into the nanoscale properties of natural soil particles that control the physiochemical interaction of material within the soil column. There have been few atomic force microscopy studies of soil, perhaps a reflection of the heterogeneous nature of the system. The present study adopted an imaging and force measurement research strategy that accounted for the heterogeneity and used model systems to aid interpretation. The surface roughness of natural soil particles increased with depth in the soil column a consequence of the attachment of organic material within the crevices of the soil particles. The roughness root mean square calculated from ten 25 microm(2) images for five different soil particles from a Netherlands soil was 53.0 nm, 68.0 nm, 92.2 nm and 106.4 nm for the respective soil depths of 0-10 cm, 10-20 cm, 20-30 cm and 30-40 cm. A novel analysis method of atomic force microscopy phase images based on phase angle distribution across a surface was used to interpret the nanoscale distribution of organic material attached to natural and model soil particles. Phase angle distributions obtained from phase images of model surfaces were found to be bimodal, indicating multiple layers of material, which changed with the concentration of adsorbed humic acid. Phase angle distributions obtained from phase images of natural soil particles indicated a trend of decreasing surface coverage with increasing depth in the soil column. This was consistent with previous macroscopic determination of the proportions of organic material chemically extracted from bulk samples of the soils from which specimen particles were drawn. Interaction forces were measured between atomic force microscopy cantilever tips (Si(3)N(4)) and natural soil and model surfaces. Adhesion forces at humic acid free specimen surfaces (Av. 20.0 nN), which are primarily hydrophilic and whose interactions are subject to a significant contribution from the capillary forces, were found to be larger than those of specimen surfaces with adsorbed humic acid (Av. 6.5 nN). This suggests that adsorbed humic acid increased surface hydrophobicity. The magnitude and distribution of adhesion forces between atomic force microscopy tips and the natural particle surfaces was affected by both local surface roughness and the presence of adsorbed organic material. The present study has correlated nanoscale measurements with established macroscale methods of soil study. Thus, the research demonstrates that atomic force microscopy is an important addition to soil science that permits a multiscale analysis of the multifactorial phenomena of soil hydrophobicity and wetting.     

Long-term structural changes of plasmid DNA studied by atomic force microscopy

Long-term stability of plasmid DNA (pDNA) conformations is critical in many research areas, especially those concerning future gene therapy. Despite its importance, the time-evolution of pDNA structures has rarely been studied at a molecular resolution. Here, the time-evolution of pDNA solutions spanning four years was observed with atomic force microscopy (AFM). The AFM data show that the pDNA molecules changed over time from isolated supercoiled structures, to aggregated supercoiled structures, to thin, branched network structures, and finally to wider, branched network structures. Additional topographical analysis of the AFM data suggests that the actions of residual proteins could be the main mechanism for the structural changes in our laboratory-prepared pDNA.     

Analysis of simulated scanning of atomic-scale silicon surface by atomic force microscopy

This study constructs a contact-mode atomic force microscopy (AFM) simulation measurement model with constant force mode to simulate and analyze the outline scanning measurement by AFM. The simulation method is that when the probe passes the surface of sample, the action force of the atom of sample received by the atom of the probe can be calculated by using Morse potential. Through calculation, the equivalent force on the cantilever of probe can be acquired. By using the deflection angle equation for the cantilever of probe developed and inferred by this study, the deflection angle of receiving action force can be calculated. On the measurement point, as the deflection angle reaches a fixed deflection angle, the scan height of this simulation model can be acquired. By scanning in the right order, the scan curve of the simulation model can be obtained. By using this simulation measurement model, this study simulates and analyzes the scanning of atomic-scale surface outline. Meanwhile, focusing on the tip radii of different probes, the concept of sensitivity analysis is employed to investigate the effects of the tip radius of probe on the atomic-scale surface outline. As a result, it is found from the simulation on the atomic-scale surface that within the simulation scope of this study, when the tip radius of probe is greater than 12 nm, the effects of single atom on the scan curve of AFM can be better decreased or eliminated.     

Study of SU-8 photoresist cross-linking process by atomic force acoustic microscopy

In this paper, a method is presented to detect the different phases of epoxy cross-linking process and the subsurface structures of SU-8 thin films by atomic force acoustic microscopy (AFAM). The AFAM imaging of SU-8 thin films was investigated under different exposure and bake conditions. Optimized conditions were obtained for the cross-linking of SU-8 thin film at the exposure does of eight laser pulses with the laser fluence 10 mJ cm^–2 per pulse and the post exposure bake (PEB) time at 90 s. The subsurface structures of undeveloped SU-8 thin films were visible in the AFAM images. This method provides an effective and low-cost way for the determination of different phases of epoxy cross-linking process in nanostructured compounds, for the non-destructive testing of subsurface defects, and for the evaluation of the quality of patterned structures.     

Improved lateral force calibration based on the angle conversion factor in atomic force microscopy

A novel calibration method is proposed for determining lateral forces in atomic force microscopy (AFM), by introducing an angle conversion factor, which is defined as the ratio of the twist angle of a cantilever to the corresponding lateral signal. This factor greatly simplifies the calibration procedures. Once the angle conversion factor is determined in AFM, the lateral force calibration factors of any rectangular cantilever can be obtained by simple computation without further experiments. To determine the angle conversion factor, this study focuses on the determination of the twist angle of a cantilever during lateral force calibration in AFM. Since the twist angle of a cantilever cannot be directly measured in AFM, the angles are obtained by means of the moment balance equations between a rectangular AFM cantilever and a simple commercially available step grating. To eliminate the effect of the adhesive force, the gradients of the lateral signals and the twist angles as a function of normal force are used in calculating the angle conversion factor. To verify reliability and reproducibility of the method, two step gratings with different heights and two different rectangular cantilevers were used in lateral force calibration in AFM. The results showed good agreement, to within 10%. This method was validated by comparing the coefficient of friction of mica so determined with values in the literature.     

The study on the atomic force microscopy base nanoscale electrical discharge machining

This study proposes an innovative atomic force microscopy (AFM) based nanoscale electrical discharge machining (AFM-based nanoEDM) system which combines an AFM with a self-produced metallic probe and a high-voltage generator to create an atmospheric environment AFM-based nanoEDM system and a deionized water (DI water) environment AFM-based nanoEDM system. This study combines wire-cut processing and electrochemical tip sharpening techniques on a 40-μm thick stainless steel sheet to produce a high conductive AFM probes, the production can withstand high voltage and large current. The tip radius of these probes is approximately 40 nm. A probe test was executed on the AFM using probes to obtain nanoscales morphology of Si wafer surface. The silicon wafer was as a specimen to carry out AFM-base nanoEDM process in atmospheric and DI water environments by AFM-based nanoEDM system. After experiments, the results show that the atmospheric and DI water environment AFM-based nanoEDM systems operate smoothly. From experimental results, it can be found that the electric discharge depth of the silicon wafer at atmospheric environments is a mere 14.54 nm. In a DI water environment, the depth of electric discharge of the silicon wafer can reach 25.4 nm. This indicates that the EDM ability of DI water environment AFM-based nanoEDM system is higher than that of atmospheric environment AFM-based nanoEDM system. After multiple nanoEDM process, the tips become blunt. After applying electrochemical tip sharpening techniques, the tip radius can return to approximately 40 nm. Therefore, AFM probes produced in this study can be reused.     

Sample preparation for the quick sizing of metal nanoparticles by atomic force microscopy

Two alternative pretreatment methods for depositing metal nanoparticles on mica for atomic force microscopy (AFM) imaging are presented. The treated substrates are flat and clean, thus they are amenable of use to characterize very small nanoparticles. The methods do not require any instrumentation or particular expertise. As they are also very quick, the need for storage of the prepared substrates is avoided altogether. These proposed methods, which are compared with the results of transmission electron microscopy analysis, allow the quick sizing and characterization of nanoparticles with the atomic force microscope and could thus help expanding the user community of nanoparticle researchers who could use the AFM for their characterization needs.     

Towards quantification of doping in gallium arsenide nanostructures by low-energy scanning electron microscopy and conductive atomic force microscopy

We calculate a universal shift in work function of 59.4 meV per decade of dopant concentration change that applies to all doped semiconductors and from this use Monte Carlo simulations to simulate the resulting change in secondary electron yield for doped GaAs. We then compare experimental images of doped GaAs layers from scanning electron microscopy and conductive atomic force microscopy. Kelvin probe force microscopy allows to directly measure and map local work function changes, but values measured are often smaller, typically only around half, of what theory predicts for perfectly clean surfaces.     

Characterization of polymer thin films by phase-sensitive acoustic microscopy and atomic force microscopy: a comparative review

The potential of phase-sensitive acoustic microscopy (PSAM) for characterizing polymer thin films is reviewed in comparison to atomic force microscopy (AFM). This comparison is based on results from three-dimensional vector contrast imaging and multimodal imaging using PSAM and AFM, respectively. The similarities and differences between the information that can be derived from the AFM topography and phase images, and the PSAM phase and amplitude micrographs are examined. In particular, the significance of the PSAM phase information for qualitative and quantitative characterization of the polymer films is examined for systems that generate surface waves, and those that do not. The relative merits, limitations and outlook of both techniques, individually, and as a complementary pair, are discussed.     

Improvement of DNA-visualization in dynamic mode atomic force microscopy in air

Near-contact mode atomic force microscopy (AFM) imaging leads to sharper representations of DNA double strands on mica imaged at ambient conditions compared with noncontact mode AFM. Phase shift was used for feedback control yielding height information using a simple model calculation. No contact between tip and sample occurs. Measured DNA widths were up to four times smaller than measured with the same AFM tip in noncontact mode at ambient condition.     

Assembling and imaging of his-tag green fluorescent protein on mica surfaces studied by atomic force microscopy and fluorescence microscopy

The adsorption of his-tag green fluorescent protein (GFPH(6)) onto the mica surfaces has been studied by atomic force microscopy (AFM) and laser confocal fluorescence microscopy. By controlling the adsorption conditions, separated single GFPH(6) and GFPH(6) monolayer can be adsorbed and formed on mica surfaces. In present experiments, based on the AFM measurement, we found that the adsorbed GFPH(6) was bound on the mica surface with its beta-sheets. The formed GFPH(6) monolayer on mica surfaces was flat, uniform, and stable. Some applications of the formed monolayer have been demonstrated. The formed monolayer can be used as a substrate for DNA imaging and AFM mechanical lithography.     

Imaging and nano-dissection of tobacco mosaic virus by atomic force microscopy

Tobacco mosaic virus (TMV) has been deposited on freshly cleaved mica substrates. The topography was investigated by contact, non-contact and lateral-force microscopy under ambient conditions in air. The results were in accord with known dimensions of TMV (i.e. 18 nm in diameter and 300 nm in length). However, convolution of tip shape with TMV morphology resulted in an apparent width of 80–140 nm in the lateral plane, a factor of 4–7 greater than the known diameter. Other artefacts - broadening and double images - were observed and ascribed to tip anomalies. High force loadings and slow repetitive scanning resulted in controlled removal of parts of the TMV structure. Accordingly, it was possible to reveal and image the central core channel of the TMV. The precision and resolution of dissection induced by AFM is currently limited by the shape of the tip, having a 40-nm radius of curvature for standard Si₃N₄ tips. It is estimated that sharper tips, with a radius of curvature of less than 10 nm, should be able to resolve, non-destructively, the protein subunits in the non-contact mode, and selectively remove single subunits in the contact mode.