SPM Probes Research Articles

Polymer Graphite Pencil Lead as a Cheap Alternative for Classic Conductive SPM Probes.

This paper presents polymer graphite (PG) as a novel material for the scanning tunneling microscopy (STM) probe. Conductive PG is a relatively modern nanocomposite material used for micro-pencil refills containing a polymer-based binding agent and graphite flakes. Its high conductivity and immunity against surface contamination, with a low price, make it seem like a highly suitable material for electrode manufacturing in general. For the tip production, three methods were developed and are further described in the paper. For the production, three commercially available polymer graphite rods were used. Each has been discussed in terms of performance within the tunneling microscope and within other potential applications.

AFM Integrated with SEM/FIB for Complete 3D Metrology Measurements

Scanning electron microscopy (SEM) and ion beam milling (FIB) techniques are mature nanoscale measurement technologies, while Atomic Force Microscopy (AFM) is a developing technology with intense interest in the scientific community for basic research and development. We discuss the recent integration of these techniques into a single instrument enabled by technological innovation in AFM instrument design, and applications of this instrument for characterization of FIB milled trenches. An AFM integrated into the SEM/FIB has specific design constraints that need to be met involving the AFM scanner, probe design, and feedback loop so that it geometrically fits into the chamber without interfering with the electron and ion beams. An ultraflat scanning stage was thus developed for precise sample motion that enable large ranges (85 microns or greater) in x,y, z. SPM probes that do not interfere with the electron beam were manufactured from fused silica in a cantilevered geometry (see Fig 1) where there is full visualization of the tip, without any loss of SPM functionality. These probes are combined with a tuning fork based feedback mechanism to maintain accurate tip-sample control; this mechanism does not involve lasers or optical beam deflection that interferes with the electron and ion beams. The many benefits of this combined approach to three-dimensional nanoscale characterization are illustrated in the imaging of a FIB milled trench. Figure 2a shows an AFM image profiling a 25-µm-deep trench milled in silicon with FIB imaged with the large Z range of the system. In this measurement, the FIB beam was used to mill this feature, and it was followed immediately by AFM imaging without having to remove the specimen from the FIB chamber and search in the AFM. Furthermore, the true depth of the trench was measured by the AFM in a crosssectional profile (Figure 2b) while a 3D reconstruction revealed the geometry of the sidewall (Figure 2c). Sidewall imaging with such long and exposed tips now are feasible with this instrument. The red arrow in this image clearly shows a Pt decoration on this structure, and the white arrow shows an undercut. Additionally, the ability of AFM to quickly and easily follow FIB milling with high Z resolution provides a straightforward and convenient method to check on FIB milling results. This capability is demonstrated in Figure 3 where several FIB-milled features in Si were monitored during the milling process. The grayscale SEM image in Figure 3a can easily locate these features and measure their widths but cannot provide direct information on their depths. The AFM delivers a 3D reconstruction (Figure 3b), and a cross sectional profile that together provide a true topographic map of specimen features. These depth profiles demonstrate the linearity of the etching process with time. This instrument incorporates important innovations in the design and technology of atomic force microscopy enabling its integration with SEM and FIB into a single powerful capability.

Multiprobe NSOM fluorescence

AbstractIn this paper, we demonstrate simultaneous AFM/NSOM using a dual-tip normal tuning-fork based scanning probe microscope. By scanning two SPM probes simultaneously, one dedicated for AFM with a standard tip diameter of 20 nm, and the second having a 150 nm aperture NSOM fiber with 200 nm thick gold coating, we combine the benefits of ∼20 nm spatial resolution from the AFM tip with the spectral information of a near-field optical probe. The combination of simultaneous dual-tip scanning enables us to decouple the requirements for high resolution topography and probe functionality. Our method represents a marked shift from previous applications of multi-probe SPM where essentially a pump-probe methodology is implemented in which one tip scans the area around the second. As a model system, we apply dual-tip AFM/NSOM scanning to a sample of spin-cast nano-clustered Lumogen dyes, which show remarkable brightness and photochemical stability. We observe morphology features with a resolution of 20 nm, and a near-field optical resolution of 150 nm, validating our approach.

A reproducible electropolishing technique to customize tungsten SPM probe: From mathematical modeling to realization

The present work is carried out with the aim of developing an optimized electropolishing technique for the process of electrochemical etching from mathematical modeling to the final realization of the well-prepared tungsten SPM probes. Based on the axisymmetric shapes of bounded interfaces, a calculation scheme of successive approximation is derived, which facilitates the numerical solutions for a wide range of parameter values which will cover most practical situations. The proposed art of electrochemical etching technique and process are experimentally confirmed to have advantages for the tungsten probe fabrication of the stabilized stylus contour, ultra-sharp apex radius and high production reproducibility. The tungsten probes with profiles of exponential, conical, and multi-diameter with stabilized stylus contour, and ultra-sharp apex radius are fabricated. The characteristic parameters of the probe including length, aspect ratio and tip apex radius can also be well defined. The reproducible technique opens a way of batch customization of metallic SPM probes with high quality.

Application of Carbon Nanofibers to SPM Probes

Application of Carbon Nanofibers to SPM Probes

Self-assembled polystyrene nanospheres for the evaluation of atomic force microscopy tip curvature radius

In this paper, self-assembled polystyrene nanospheres are proposed as a shape characterizer sample for SPM tips. Ordered arrays or 2D islands of polystyrene spheres may be prepared either by sedimentation or by crystallization of the colloidal spheres' suspension. The self-assembling mechanism guarantees high reproducibility; thus the characterizer sample can be ‘freshly’ prepared at each use, avoiding the problem of time and use deterioration and reducing the problem of sample structure fidelity that occurs when lithographic structures are employed. The spheres could also be deposited on the sample itself in order to speed up the characterization process in applications requiring frequent tip characterizations. We present numerical calculations of geometrical convoluted profiles on the proposed structures showing that, for a variety of different tip shapes, at the border between a couple of touching spheres the tip flanks do not come into contact with the spheres. Due to this behaviour, touching spheres are an optimum characterizer sample for SPM tip curvature radius characterization, enabling a straightforward procedure for calculating the curvature radius from the amplitude of tip oscillation along profiles connecting spheres' centres. The new procedure for the characterization of SPM probes was assessed exploiting different kinds of self-assembled structures and comparing results to those obtained by spiked structures and SEM observations.

Analysis on vibration rejection ratio of scanning probe microscope

In order to investigate the influence of external vibration on the tip-sample interaction in scanning probe microscope, a concept, vibration rejection ratio (VRR), was proposed by Thompson et al. In the present article, a concise theoretical background was introduced, a vibration stimulating and measuring system for SPM was built up, and an experimental research of the influence on SPM VRR was carried out with different SPM probes, samples, tip preloads, and open/close loops of tip-sample interaction. Based on the experiments, it was proved that SPM VRR had correlations with the spring constant, the air damping constant, the effective mass of the cantilever, the contact spring constant, the contact damping constant between tip and sample, etc. Being different from the traditional theoretical analysis, other than the resonant frequency, a “leading coefficient” was proposed to describe the VRR. A conclusion was drawn after an experimental analysis that SPM’s vibration rejecting ability can be improved by longer cantilever probe, softer sample surface, greater tip preload, and close-loop control for tip-sample interaction. More attention should be paid to a proper combination of the above parameters.

Characterization of small-scale batch-fabricated carbon nanofiber probes

Linear-shaped single carbon nanofibers (CNFs) were batch-grown onto commercially available Si cantilevers for atomic force microscope by the Ar+-ion-irradiation method (9 cantilevers/batch). The force-curve measurements revealed that the long CNF probes (~ 1 μm in length) were as flexible as the carbon nanotubes probes, whereas the short CNF probes (~ 400 nm in length) were characterized by the rigid nature similar to the Si probes. Thus, the mechanical properties of CNF probes were controllable by the CNF length. The ion-induced CNF probes were metallic in electrical property, and the higher resolution images in scanning spreading resistance microscopy was attained by the CNF probes than by conventional conductive diamond probes, due to the small tip radius, high aspect ratio and the durability of the CNF probes. Because the small-scale batch-fabricated CNF probes showed good uniformity in the size, mechanical and electrical properties, it was concluded that they are promising as practical conductive SPM probes.

One-Step Polymer Grafting from Silicon Nitride SPM Probes: From Isolated Chains to Brush Regime

We show that a reactive polymer can be directly grafted from commercial Si3Nx SPM tips, without any intermediate layer. The conditions can be chosen to achieve either an isolated mushroom regime, with only one or a few chains accessible at the tip apex, or a brush regime.

3D Reconstruction of SPM Probes by Electron Tomography

Three-dimensional morphological and compositional structures of tungsten tips consisting of layered amorphous oxide shell and crystalline W core are reconstructed by electron tomography using both coherent and incoherent imaging modes. The fidelity of the reconstruction is dependent on three criteria, suppression of unwanted crystal orientation contrast in the crystalline core, nonlinear intensity-thickness relations above a certain thickness limit, and artefacts due to missing angular ranges when acquiring a tilt series of images. Annular dark field (ADF), and EDX chemical mapping are discussed as alternatives to standard bright field (BF) TEM imaging.

A quantitative comparison of resolution, scanning speed and lifetime behavior of CVD grown Single Wall Carbon Nanotubes and silicon SPM probes using spectral methods

Due to their extreme aspect ratios and exceptional mechanical properties Carbon Nanotubes terminated silicon probes have proven to be the ‘‘ideal’’ probe for Atomic Force Microscopy. But especially for the manufacturing and use of Single Walled Carbon Nanotubes there are serious problems, which have not been solved until today. Here, Single and Double Wall Carbon Nanotubes, batch processed and used as deposited by Chemical Vapor Deposition without any postprocessing, are compared to standard and high resolution silicon probes concerning resolution, scanning speed and lifetime behavior.

Electron Tomography of SPM Probes, Nanoparticles and Precipitates

ABSTRACTNanoscale tomographic reconstructions from objects with diameters of 100nm or smaller can only be achieved non-destructively with transmission electron tomography. The application of this technique to W tips, which are common probes for scanning tunneling microscopy and nanoindentation, is demonstrated with emphasis on visualizing oxide layers and functionally attached nanoparticles. For the reconstruction of facetted free-standing catalyst nanoparticles, such as CeO2 octahedra, we propose a combination of energy-filtered (EF) and bright field (BF) TEM tomography to achieve high fidelity of the projection relationship via EFTEM, due to its incoherent imaging mode, and high resolution definition of the particle circumference from the edge-enhancement effects of the BF tomogram. Finally, electron tomography applications to CeO2 nanoprecipitates embedded in a multicomponent oxide glass matrix are shown, which comprises the first tomographic 3D reconstruction of a nanoscale dendrite.

STM carbon nanotube tips fabrication for critical dimension measurements

Reconstruction errors due to the finite shape of the tip remain the major limitation of scanning probe microscopy for nanometer scale measurements. Carbon nanotubes are ideal SPM probes thanks to their nanometric diameter, high aspect-ratio and strength. In this work, the feasibility of a method to attach CNT on the apex of a Tungsten tip has been tested. By the fabricated CNT tip we successfully imaged a metalized rectangular grating obtaining a profile nearly rectangular in comparison with classic STM-W tip. The metallic behavior of CNT was also analyzed from I– V characteristics of the tunneling current.

Micromachined SPM probes with sub-100 nm features at tip apex

We have developed a fabrication process that allows thin films to be patterned at the end of a sharp tip. Using this method, many different material contrasts can be achieved on the apex of a scanning probe microscope tip. For example, small apertures down to 30 nm have been patterned into an aluminium film covering quartz tips to form scanning near-field optical microscopy probes. Also, small well-defined metal electrodes can be fabricated on the tip apex, which can be used for electrochemical measurements. The tip structuring process is a batch fabrication method and is based on CMOS-compatible technologies, which means that it can be integrated easily into an existing microfabrication process. Furthermore, it allows the patterning of a wide range of tip heights, materials or geometries. Copyright © 1999 John Wiley & Sons Ltd.

Conductive SPM probes of base Ti or W refractory compounds

The results of investigations of , thin films as prospective materials for conductive SPM probes in a silicon cantilever are presented. The ultrathin (1.5-10 nm) films are characterized by high conductivity, increased adhesion to silicon and chemical passivity. It is shown by means of conductive SPM measurements that there is no dielectric layer on the film surface and the conductive metal-coated silicon cantilevers were wear-proof.