A methodology for quantitative evaluation of local electrical conductivity: from micron to submicron

The local electrical conductivity of aluminum thin film with dimensions from micron to submicron was quantitatively measured by a four-point atomic force microscope (AFM) technique. The technique is a combination of the principles of four-point probe method and standard AFM. A silicon nitride based AFM probe with a V-shaped two-dimensional sliced structure tip was patterned by using conventional photolithography method. The probe was then etched to four parallel electrodes isolated from each other, for the purpose of performing current input and electrical potential drop measurement. The spacing between electrodes is smaller than 1.0 μm, which facilitates the quantitative electrical conductivity measurement of ultrathin film. The four-point AFM probe technique is capable of measuring surface topography together with local conductivity simultaneously. The technique was applied to a series of 99.999% aluminum thin films with thicknesses from micron to submicron. The repeatable measurements demonstrate the capability of this technique and its possible extension to be used for fast in situ electrical properties characterization of submicron interconnects that widely applied in nanosensors and nanodevices.     

Micromachining of diamond film for MEMS applications

We realized two diamond microdevices: a movable diamond microgripper and a diamond probe for an atomic force microscope (AFM), consisting of a V-shaped diamond cantilever and a pyramidal diamond tip, using a microfabrication technique employing semiconductive chemical-vapor-deposited diamond thin film. The microgripper was fabricated by patterning the diamond thin film onto a sacrificial SiO ₂ layer by selective deposition and releasing the movable parts by sacrificial layer etching. The diamond AFM probe was fabricated by combining selective deposition for patterning a diamond cantilever with a mold technique on an Si substrate for producing a pyramidal diamond tip. The cantilever was then released by removing the substrate. We report the initial results obtained in AFM measurements taken using the fabricated diamond probe. These results indicate that this diamond probe is capable of measuring AFM images. In addition, we have developed the anodic bonding of diamond thin film to glass using Al or Ti film as an intermediate layer for assembly. This bonding technique will allow diamond microstructures to be used in many novel applications for microelectromechanical systems     

基于AFM探针的电晕放电研究

针对扫描探针显微镜与质谱联用系统中的采样方式,提出了利用原子力显微镜(AFM)探针进行电晕放电解吸附的采样方案.运用ANSYS软件对AFM导电探针进行了有限元仿真,电场分析表明间距100 μm加1 kV高压时的AFM探针周围场强在0.32 ～62.4 V/μm间,验证了利用其产生电晕放电的可行性;通过实验观察了电晕放电...     

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

A novel method has been proposed to fabricate an atomic force microscope (AFM) probe using CuO nanowire and a stress-induced method that can form the nanowire easily. By heating a commercial AFM probe with a film coating of Ta and Cu, a Cu hillock with CuO nanowires on its surface could be formed at the end of the probe. The thickness of the coating films, the heating temperature, and the heating time were investigated to obtain CuO nanowires with a high aspect ratio for use as an AFM probe tip. It was found that a suitable probe tip can be fabricated using the a Cu film thickness of 700 nm, a heating temperature of 380 °C and a heating time of 6 h. Probe tips (~5 μm high) and nanowires of ~25 nm diameter were obtained successfully. In the range evaluated, the measurement resolution of the CuO nanowire probe was slightly worse than that of a commercial AFM probe. However, both probes had almost the same dimensional measurement precision.     

Molecular dynamics prediction of nanofluidic contact angle offset by an AFM

This paper investigates the nano-fluidic contact angle measurement by performing molecular dynamics simulations. The contact angle between a nano-water droplet and a platinum surface is important for the design of the porous catalyst layer in low-temperature fuel cells. The measurement can generally be conducted by an atomic force microscope (AFM). However, the interaction force between the water droplet and the probe tip of the microscope may influence the measurement results. This paper employs the molecular dynamics technique to investigate the offset of the contact angle measurement. Calculations are in two sets, one simulated the water molecules clustering on the platinum surface, and the other involved the AFM measurement of the contact angle. The former case presents the original contact angle between the nano-scale water droplet and the platinum surface; the offset of the contact angle measurement due to intrusion of the AFM probe is predictable from the latter case. For engineering purposes, we present a correlation between the offset angle and the AFM measurement locations.     

Bifurcation and chaos analysis of atomic force microscope system

Atomic force microscope (AFM) is a very high-resolution type of scanning probe microscope, which is an essential characterization and actuation tool in modern nanoscience or engineering. This paper investigates the bifurcation and chaos behavior of the probe tip from AFM system by the differential transformation method (DTM). The dynamic behavior of the probe tip is characterized by reference to bifurcation diagrams, phase portraits, power spectra, Poincaré maps and maximum Lyapunov exponent plots produced using the time-series data obtained from DTM. The results indicate that the probe tip behavior is significantly dependent on the magnitude of the vibrational amplitude. Specifically, the probe tip motion changes from T-periodic to 3T-periodic, then from 2T-periodic to multi-periodic, and finally to chaotic motion with windows of periodic motion as the vibrational amplitude is increased from 0 to 2.0. Furthermore, it is demonstrated that the DTM is in good agreement for the considered system.     

Multiwalled carbon nanotube AFM probes for surface characterization of micro/nanostructures

A multiwalled carbon nanotube (MWNT) probe was used as a scanning probe in an atomic force microscope (AFM) to obtain surface height maps of micro/nano structures. The surface height maps acquired by the MWNT probe are compared with those by a conventional silicon probe on the four samples: silicon ruler, polymer microchannels, silicon nanomembrane and nanocomposite metal particle (MP) tapes. The results of the silicon ruler, microchannels and membrane samples show that the surface height maps by the MWNT probe have a better resolution than those by a conventional silicon tip due to the sharper tip with the larger aspect ratio of the MWNT. A MWNT probe is especially useful to observe surface height maps of the structures that have larger aspect ratio.Financial support for this work was provided by the National Science Foundation (contract no. ECS-0301056). The authors are grateful to Prof. Derek Hansford and Nick Ferrell of the Micro MD laboratory for providing the samples and fruitful discussions, and Imation Corporation for the samples provided. Work by C.V.N. was supported by a NASA contract to ELORET Corporation.     

Fabrication of nanoscale tungsten tip arrays for scanning probemicroscopy-based devices

A new fabrication process for nanoscale tungsten tip arrays was developed for scanning probe microscopy-based devices. It is suitable to make a huge array on a device chip and is potentially compatible with CMOS technology. In this study, tungsten was selected as a tip material because of its hardness and conductivity. The newly developed fabrication process mainly consists of several important techniques: a combination of optical lithography and electron beam (EB) lithography to reduce the total exposure time with high resolution and chromium/tungsten/chromium (Cr/W/Cr) sandwich deposition and etching in which the first chromium layer is used as a mask and a second one is used as an etch stop. A periodic array of dots in an EB resist with a spot diameter of less than 50 nm was obtained by a combination of optical lithography and EB lithography with a positive resist (polymethylmethacrylate) in which all processing conditions were optimized carefully. A thin and uniform chromium film, deposited by ion-beam sputtering, allowed the use of thin polymethylmethacrylate (PMMA) film which led to the high resolution. The conditions of dc magnetron sputtering were also optimized in order to deposit a densely packed and low-resistivity film. The resulting tungsten tip arrays had a cylindrical shape with diameters of less than 60 nm and heights of 300 nm     

Cantilever probes for high speed AFM

Since the invention in 1986 atomic force microscopy (AFM) has become the most widely used scanning probe microscopy (Binnig et al. in Phys Rev Lett 56:930–933, 1986). The microscope images the interaction of forces like Van der Waals or Coulomb forces between a sample and the apex of a small tip integrated near the free end of a flexible cantilever. But as all other scanning probe techniques the AFM requires serial data acquisition and suffers therefore from a low temporal resolution. Enhancing the speed to video rate imaging makes high demands on scanner technology, control electronics and on the key feature the cantilever with integrated sharp stylus. For the cantilever probes, fundamental resonance frequencies in the MHz regime are envisaged while the force constant of a few nN/nm shall be maintained. We present different novel AFM probes with ultrashort cantilevers and integrated sharp tips for high speed AFM while focusing on widely dispersed applications and on aspects of mass fabrication.     

Integrated SPM probes with NEMS technology

Two kinds of integrated scanning probe microscope (SPM) probes are developed. The first kind is AFM probes realized with a novel masked–maskless combined etching process. Both the nano-tips for scanning and the bending cantilevers are simultaneously formed with the masked–maskless combined anisotropic etching technique. The simultaneous formation method effectively avoids damage to the previously formed tips when the cantilever shaping is processed. The testing results for the probes show the imaging quality comparable with commercial probes. The second kind of probes is an integrated probe with both a piezoresistive sensor and an electric-heated tip. This kind of probe is used for thermal–mechanical data storage, with the pulse-heated tip for data writing and the piezoresistive sensor for data reading. Nano-sized bumps have been formed by probe scanning on PMMA thin film, resulting in a storage density beyond 30 GB/in.².     

Surface modification with self-assembled monolayers for nanoscalereplication of photoplastic MEMS

A release technique that enables to lift microfabricated structures mechanically off the surface without using wet chemistry is presented. A self-assembled monolayer of dodecyl-trichlorosilane forms a very uniform ~1.5-nm-thick anti-adhesion coating on the silicon dioxide surface, on full wafer scale. The structural layers are formed directly onto the organic layer. They consist here of a 100-nm-thick aluminum film and a high-aspect ratio photoplastic SU-8 structure. After the microfabrication the structure can be lifted off the surface together with the aluminum layer. This generic technique was used to make a variety of novel structures. First, aluminum electrodes that are embedded in plastic are made using lithography, etching and surface transfer techniques. Second, using a patterned monolayer as defined by microcontact printing, resulted in a spatial variation of the surface adhesion forces. This was used to directly transfer the stamped pattern into a metal structure without using additional transfer etching steps. Third, the monolayer's ability to cover surface features down to nanometer scale was exploited to replicate sharp surface molds into metal coated photoplastic tips with ~30-nm radii for use in scanning probe instruments such as near-field optical techniques. The advantage compared to standard sacrificial layer techniques is the ability of replication at the nanoscale and the absence of etchants or solvents in the final process steps     

Microwave atomic force microscope: MG63 osteoblast-like cells analysis on nanometer scale

In this paper, we report a non-invasive and non-destructive probing method for analyzing the MG63 osteoblast-like cells. High frequency microwave atomic force microscope (M-AFM) can be used to measure the surface topography and microwave image of MG63 cells simultaneously in one scanning process. Under the frequency modulation AFM mode, the M-AFM probe tip can scan above the cell surface, maintaining a constant stand-off distance and the created lateral forces were small enough as not to sweep away or deform the fragile biomolecules. By analyzing the results, quantification such as, the number and distribution of organelles and proteins of MG63 cells as well as their dimension and electrical property information can be characterized. The unique potentials of that M-AFM imaging biological substrates with no damaging manner and nanometer scale resolution, while the original structure and function of the biomolecules during the investigation are preserved, make this technique very attractive to biologists.     

Influence of sputtering power on properties of ZnO thin films fabricated by RF sputtering in room temperature

High mobility and c-axis orientated ZnO thin films were deposited on glass substrates using RF sputtering method at room temperature.Structural properties of ZnO thin films were investigated by X-ray diffraction (XRD).Surface morphology and roughness were studied with scanning electron microscopy (SEM) and atomic force microscopy (AFM).Electrical properties were measured at room temperature using a Hall effect measurement system.The influence of sputtering power on characteristics of ZnO thin films is studied.The results indicate that the sputtering powers have great influence on the crystal quality and mobility of ZnO thin films.By using optimized sputtering conditions,high crystal quality ZnO thin films with Hall mobility of 34 cm 2 /V·s at room temperature were obtained.     

A microwave probe nanostructure for atomic force microscopy

An atomic force microscope (AFM) probe on a GaAs wafer was studied as a new microwave probe structure. A waveguide was created by evaporating an Au film on the top and bottom surfaces of the GaAs AFM probe. The fabricated AMF probe’s tip is 8 μm long and has a radius of curvature of about 50 nm. The open structure of the waveguide at the tip of the probe was generated by using focused ion beam (FIB) fabrication. AFM topography of a grating sample was created by using the fabricated microwave AFM probe. The fabricated probe exhibits nanometer-scale resolution, and microwave emission was successfully detected at the tip of the probe by approaching Cr–V steel and Au wire samples.     

Molecular modeling of enzyme attachment on AFM probes

The immobilization of enzymes on atomic force microscope tip (AFM tip) surface is a crucial step in the development of nanobiosensors to be used in detection process. In this work, an atomistic modeling of the attachment of the acetyl coenzyme A carboxylase (ACC enzyme) on a functionalized AFM tip surface is proposed. Using electrostatic considerations, suitable enzyme–surface orientations with the active sites of the ACC enzyme available for interactions with bulk molecules were found. A 50 ns molecular dynamics trajectory in aqueous solution was obtained and surface contact area, hydrogen bonding and protein stability were analyzed. The enzyme–surface model proposed here with minor adjustment can be applied to study antigen–antibody interactions as well as enzyme immobilization on silica for chromatography applications.     

Mechanical and tribological characterization of a thermally actuated MEMS cantilever

The temperature effect on the mechanical and tribological behaviors of a microelectromechanical systems cantilever is experimentally investigated using an atomic force microscope. A nonlinear variation of the bending stiffness of microcantilevers as a function of temperature is determined. The variation of the adhesion force between the tip of atomic force microscope (AFM) probe (Si₃N₄) and the microcantilever fabricated in gold is monitored at different temperatures. Using the lateral mode operation of atomic force microscope, the influence of temperature on friction coefficient between the tip of AFM probe and microcantilever is presented. Finite element analysis is used to estimate the thermal field distribution in microcantilever and the axial expansion.     

Properties of M-AFM probe affected by nanostructural metal coatings

In order to develop a new structure microwave probe, the fabrication of the atomic force microscope (AFM) probe on a GaAs wafer was studied and characteristics of the AFM probe with different nanostructural metal coating were evaluated in order to understand the performance of the probe for the topography of materials and the propagation of microwave signals. A waveguide was introduced by the sputtering and the electron beam (EB) evaporation technique on the top and bottom surfaces of the GaAs AFM probe with Au or Al film. The open structure of the waveguide at the tip of the probe was introduced by using focused ion beam fabrication. It was found that the fabricated probes coated with the Au or Al film have nanometer order resolution. Moreover, using the Au-coating probe formed by the EB evaporation technique, microwave emission was detected successfully at the tip of the probe by approaching an Au film sample.     

Pattern recognition methods for thermal drift correction in Atomic Force Microscopy imaging

Atomic Force Microscopy (AFM) is a fundamental tool for the investigation of a wide range of mechanical properties on nanoscale due to the contact interaction between the AFM tip and the sample surface. The information recorded with AFM is stored and synthesized by imaging the sample properties to be studied. Distortions and unwanted effects in AFM images can be produced both due to instrumental sources or sample unknown bad responses. The focus of this paper is on an algorithm for distortion corrections for AFM recorded images due to the convolution of thermal drift and unknown polymer compliance. When a sequence of AFM images correspondent to the same polymeric area is acquired, it is common to observe the convolution of thermal drift with surface modifications due to the AFM tip stresses. The surface modifications are material properties and add knowledge to the response of the materials on nanoscale. As a consequence, a suitable de-convolution of the thermal drifts on the recorded images needs to be developed. Because soft polymeric samples can present unwanted height alteration due to the stressing AFM tip contact, we present a method that combines a thermal drifts correcting tool (where the original images are modified using a suitable mapping function) with a height rescaling method. In turn, an image matching method based on a Tikhonov functional is developed between topography data and the surface elastic maps, respectively. The precision achieved and the fast computation time required make our methods particularly useful for image analysis on soft polymeric samples as well as in a wide range of other scanning probe microscopy applications.     

Nonlinear and hysteretic influence of piezoelectric actuators in AFMs on lateral dimension measurement

Piezoelectric actuators that are used in atomic force microscopes (AFM) have undesirable properties. The nonlinear and hysteretic characteristics of piezoelectric actuators introduce geometric deformations in the reconstructed AFM images. Due to these deformations, the quantitative interpretation of the absolute dimensions of surface features is difficult and often not accurate.A real-time measuring ‘Nano-metrological Atomic Force Microscope’ system equipped with an ultra-high resolution three-axis laser interferometer system is developed, in which the undesirable properties of piezoelectric actuators are compensated completely. Using this AFM and a one-dimensional (1D) grating reference standard with pitches of 240 nm, which is one of the widely used reference standards as nano-metrological lateral scales, the influences of nonlinear and hysteretic characteristics of piezoelectric actuators on image reconstruction and lateral dimension measurement are examined and compared quantitatively among three different measurement methods. The three measurement methods are: (1) the relative movement between probe tip and sample is controlled and measured directly by voltage signals applied on the XYZ scanner, the nonlinear and hysteretic characteristics of piezoelectric actuators are not compensated; (2) the relative movement between probe tip and sample is controlled by voltage signals applied on the XYZ scanner, but it is measured accurately by interferometers; (3) the relative movement between probe tip and sample in lateral directions are both controlled and measured accurately by interferometers. According to the comparison results, an accurate displacement control system is key to reduce the influences of undesirable properties of piezoelectric actuators and the developed AFM system with three-axis laser interferometer system is proved to eliminate the nonlinear and hysteretic characteristics of piezoelectric actuators completely.     

Magnetic lithography for servowriting applications using flexible magnetic masks nanofabricated on plastic substrates

Magnetic lithography (ML) is a process qualitatively analogous to contact optical lithography which transfers information from a nanopatterned magnetic mask (analog of optical photomask) to magnetic media (analog of photoresist), and is interesting for applications in instantaneous parallel magnetic recording, in particular for servowriting applications. The magnetic mask consists of nanopatterned magnetically soft material (FeNiCo, FeCo) on a thin flexible plastic substrate, typically Polyethylene teraphtalate (PET) or polyimide. When uniformly magnetized media is brought into intimate contact with the magnetic mask, an externally applied magnetic field selectively changes the magnetic orientation in the areas not covered with the soft magnetic material. Flexible substrate of the magnetic mask offers superior compliance to magnetic media which is likely to have imperfect flatness and surface particulate contamination. We discuss nanofabrication challenges of magnetic masks on plastic substrates, including electron beam lithography, electroplating and lift-off processing on the nanometer scale, adhesion of metal thin films on PET and polyimide substrates, and release of plastic films from rigid substrates used during the processing. We present results on fabricated magnetic masks, magnetic force microscopy images of the magnetic transition patterns and disk spinstand tests of servowritten patterns.