Implementation of atomically defined field ion microscopy tips in scanning probe microscopy

The field ion microscope (FIM) can be used to characterize the atomic configuration of the apices of sharp tips. These tips are well suited for scanning probe microscope (SPM) use since they predetermine the SPM resolution and the electronic structure for spectroscopy. A protocol is proposed for preserving the atomic structure of the tip apex from etching due to gas impurities during the period of transfer from the FIM to the SPM, and estimations are made regarding the time limitations of such an experiment due to contamination with ultra-high vacuum rest gases. While avoiding any current setpoint overshoot to preserve the tip integrity, we present results from approaches of atomically defined tungsten tips to the tunneling regime with Au(111), HOPG (highly oriented pyrolytic graphite) and Si(111) surfaces at room temperature. We conclude from these experiments that adatom mobility and physisorbed gas on the sample surface limit the choice of surfaces for which the tip integrity is preserved in tunneling experiments at room temperature. The atomic structure of FIM tip apices is unchanged only after tunneling to the highly reactive Si(111) surface.     

Reproducible fabrication of a fiber probe with a nanometric protrusion for near-field optics

We propose a new type of fiber probe with a nanometric protruding tip emerging from a metal film and describe a novel method, called the selective resin-coating method, for fabricating such probes. It is a reproducible etching process consisting of four steps and can be applied to silica fibers sharpened by selective chemical etching. With this method, we obtained tips with the apex diameter and the foot diameter of the protrusion being less than 10 and 30 nm, respectively, when the gold film was ~120 nm thick.     

Diamond-modified AFM probes: from diamond nanowires to atomic force microscopy-integrated boron-doped diamond electrodes

In atomic force microscopy (AFM), sharp and wear-resistant tips are a critical issue. Regarding scanning electrochemical microscopy (SECM), electrodes are required to be mechanically and chemically stable. Diamond is the perfect candidate for both AFM probes as well as for electrode materials if doped, due to diamond's unrivaled mechanical, chemical, and electrochemical properties. In this study, standard AFM tips were overgrown with typically 300 nm thick nanocrystalline diamond (NCD) layers and modified to obtain ultra sharp diamond nanowire-based AFM probes and probes that were used for combined AFM-SECM measurements based on integrated boron-doped conductive diamond electrodes. Analysis of the resonance properties of the diamond overgrown AFM cantilevers showed increasing resonance frequencies with increasing diamond coating thicknesses (i.e., from 160 to 260 kHz). The measured data were compared to performed simulations and show excellent correlation. A strong enhancement of the quality factor upon overgrowth was also observed (120 to 710). AFM tips with integrated diamond nanowires are shown to have apex radii as small as 5 nm and where fabricated by selectively etching diamond in a plasma etching process using self-organized metal nanomasks. These scanning tips showed superior imaging performance as compared to standard Si-tips or commercially available diamond-coated tips. The high imaging resolution and low tip wear are demonstrated using tapping and contact mode AFM measurements by imaging ultra hard substrates and DNA. Furthermore, AFM probes were coated with conductive boron-doped and insulating diamond layers to achieve bifunctional AFM-SECM probes. For this, focused ion beam (FIB) technology was used to expose the boron-doped diamond as a recessed electrode near the apex of the scanning tip. Such a modified probe was used to perform proof-of-concept AFM-SECM measurements. The results show that high-quality diamond probes can be fabricated, which are suitable for probing, manipulating, sculpting, and sensing at single digit nanoscale.     

10 micrometer-scale SPM local oxidation lithography

10 micrometer-scale scanning probe microscopy (SPM) local oxidation lithography was performed on Si. In order to realize large-scale oxidation, an SPM tip with a contact length of 15 microm was prepared by focused-ion-beam (FIB) etching. The oxidation was carried out in contact mode operation with the contact force ranging from 0.1 to 2.1 microN. The applied bias voltage was 50 V, and scanning speed was varied from 10 to 200 microm/s. The scan length was 15 microm for one cycle. The influence of contact force on the large-scale oxidation was investigated. At high contact force, the Si oxide with good size uniformity was obtained even with high scanning speed. The SPM tip with larger contact length may increase the spatial dimensions of the water meniscus between the SPM tip and sample surface, resulting in the larger dimensions of the fabricated oxide. Furthermore, the throughput of large-scale oxidation reached about 10(3) microm2/s by controlling the scanning speed and contact force of the SPM tip. It is suggested that SPM local oxidation can be upscaled by using a SPM tip with large contact length.     

Nanoscale capacitance imaging with attofarad resolution using ac current sensing atomic force microscopy

Nanoscale capacitance imaging with attofarad resolution (～1?aF) of a nano-structured oxide thin film, using ac current sensing atomic force microscopy, is reported. Capacitance images are shown to follow the topographic profile of the oxide closely, with nanometre vertical resolution. A comparison between experimental data and theoretical models shows that the capacitance variations observed in the measurements can be mainly associated with the capacitance probed by the tip apex and not with positional changes of stray capacitance contributions. Capacitance versus distance measurements further support this conclusion. The application of this technique to the characterization of samples with non-voltage-dependent capacitance, such as very thin dielectric films, self-assembled monolayers and biological membranes, can provide new insight into the dielectric properties at the nanoscale.     

Scanning probe microscope lithography at the micro- and nano-scales

Scanning probe microscopy (SPM)-based lithography at the micro- and nano-scales is presented. Our method in SPM local oxidation involves two SPM tips, one having a robust blunt tip, a "micrometer tip," and the other having a sharp tip, a "nanometer tip." In tapping-mode SPM local oxidation experiments, Si oxide wires with sub-10 nm resolution were produced by precisely tuning the dynamic properties of the nanometer tip such as drive amplitude and quality factor. On the other hand, in order to perform large-scale oxidation, SPM tip with a contact area of microm2, which is about 10(4) times larger than that of the conventional nanometer tip, was prepared. We propose and demonstrate a method of performing micrometer-scale SPM local oxidation using the micrometer tip under contact-mode operation. The width of the Si oxide produced was clearly determined by the contact length of the tip. Furthermore, we explore the possibility of performing the sub-20 nm lithography of Si surfaces using SPM scratching with a diamond-coated tip. The influence of various scan parameters on the groove size was investigated. The groove size could be precisely controlled by the applied force, scan direction, and the number of scan cycles. There is no effect of the scan speed on the groove size. It is concluded that high-speed nanolithography can be achieved without the degradation of patterns by SPM scratching. SPM-based lithography has the advantage of being able to fabricate a desired structure at an arbitrary position on a surface and plays an important role for bridging the gap between micro- and nano-scales.     

An atomic-force-microscopy study of the structure of surface layers of intact fibroblasts

Intact embryonic fibroblasts on a collagen-treated substrate have been studied by atomic-force microscopy (AFM) using probes of two types: (i) standard probes with tip curvature radii of 2–10 nm and (ii) special probes with a calibrated 325-nm SiO₂ ball radius at the tip apex. It is established that, irrespective of probe type, the average maximum fibroblast height is on a level of ~1.7 μm and the average stiffness of the probe–cell contact amounts to ~16.5 mN/m. The obtained AFM data reveal a peculiarity of the fibroblast structure, whereby its external layers move as a rigid shell relative to the interior and can be pressed inside to a depth dependent on the load only.     

Tour de force microscopy

Scanning probe microscopy (SPM) is capable of imaging synthetic polymers and biomolecular systems at sub-molecular resolution, without the need for staining or coating, in a range of environments including gas and liquid, so offering major advantages over other forms of microscopy. However, there are some limitations, which could be alleviated by (i) reducing the force interaction between the probe and specimen and (ii) increasing the rate of imaging. New developments in instrumentation from the SPM group at the University of Bristol to overcome these limitations are discussed and illustrated here.The invention of scanning tunneling microscopy (STM) in 1981 began a revolution in microscopy¹, which has led to a whole new family of about a dozen microscopies known collectively as scanning probe microscopy (SPM). The importance of this development is comparable to that of the invention of electron microscopy in the 1930s and arguably as fundamental as the development of the first optical microscopes, since SPM uses an entirely different principle from optical and electron microscopy to achieve imaging at high resolution.     

Surface imaging using holographic optical tweezers

We present an imaging technique using an optically trapped cigar-shaped probe controlled using holographic optical tweezers. The probe is raster scanned over a surface, allowing an image to be taken in a manner analogous to scanning probe microscopy (SPM), with automatic closed loop feedback control provided by analysis of the probe position recorded using a high speed CMOS camera. The probe is held using two optical traps centred at least 10 μm from the ends, minimizing laser illumination of the tip, so reducing the chance of optical damage to delicate samples. The technique imparts less force on samples than contact SPM techniques, and allows highly curved and strongly scattering samples to be imaged, which present difficulties for imaging using photonic force microscopy. To calibrate our technique, we first image a known sample--the interface between two 8 μm polystyrene beads. We then demonstrate the advantages of this technique by imaging the surface of the soft alga Pseudopediastrum. The scattering force of our laser applied directly onto this sample is enough to remove it from the surface, but we can use our technique to image the algal surface with minimal disruption while it is alive, not adhered and in physiological conditions. The resolution is currently equivalent to confocal microscopy, but as our technique is not diffraction limited, there is scope for significant improvement by reducing the tip diameter and limiting the thermal motion of the probe.     

Near-field optical apertured tip and modified structures for local field enhancement

We present experimental measurements and simulation of the spatial distribution of near-field light at the aperture of a Si micromachined near-field scanning optical microscopy (NSOM) probe. A miniature aperture at the apex of a SiO(2) tip on a Si cantilever was fabricated with the low temperature oxidation and selective etching technique. An optical transmission efficiency (optical throughput) of the fabricated probe was determined to be approximately 10(-2) when the aperture size was approximately 100 nm, which is several orders of magnitude higher than that for conventional optical fibers. A three-dimensional finite difference time domain (FDTD) simulation shows that the near-field light is well confined within the aperture area with a throughput of 1% for a 100-nm aperture, which is in good agreement with the measurement. The spatial distribution of the near-field light at an aperture of 300-nm diameter shows a full width at half-maximum of 250 nm with a sharp peak that is nearly 60 nm wide. The 2.4% throughput for a 300-nm aperture was estimated based on the measured spatial distribution of the near-field light that is almost the same as the experimental result. We also present the initial results of the fabrication of high throughput coaxial and surface plasmon enhancement NSOM probes.     

Scanning probe microscopy based on magnetoresistive sensing

Integrated sensors are essential for scanning probe microscopy (SPM) based systems that employ arrays of microcantilevers for high throughput. Common integrated sensors, such as piezoresistive, piezoelectric, capacitive and thermoelectric sensors, suffer from low bandwidth and/or low resolution. In this paper, a novel magnetoresistive-sensor-based scanning probe microscopy (MR-SPM) technique is presented. The principle of MR-SPM is first demonstrated using experiments with magnetic cantilevers and commercial MR sensors. A new cantilever design tailored to MR-SPM is then presented and micromagnetic simulations are employed to evaluate the achievable resolution. A remarkable resolution of 0.84?? over a bandwidth of 1?MHz is estimated, which would significantly outperform state-of-the-art optical deflection sensors. Due to its combination of high resolution at high bandwidth, and its amenability to integration in probe arrays, MR-SPM holds great promise for low-cost, high-throughput SPM.     

One nanometer resolution electrical probe via atomic metal filament formation

Scanning probe microscopy has been widely used to investigate various interactions in microscopic nature. Particularly, conductive atomic force microscopy (C-AFM) can provide local electronic signals conveniently, but the probe resolution of C-AFM has been limited by the tip geometry. Here, we improve the probe resolution greatly by forming an atomic-size metallic filament on a commercial C-AFM tip. We demonstrate ～1 nm lateral resolution in C-AFM using the metal filament tip. The filament tip is mechanically robust and electrically stable in repeated scans under ambient conditions since it is imbedded in a stable insulating matrix. The formation of the atomic filament is highly controllable and reproducible and can be easily integrated to existing AFM tip technologies to produce the next generation of high-resolution electrical and other scanning probes.     

Parameters of the magnetic force microscope probe optimized for high-resolution measurements

The resolving power of various magnetic probes, which depends on the probe shape and the amount of deposited magnetic material, has been determined by means of computer modeling. The minimum thickness of a cobalt film is experimentally determined, which must be applied to the surface of a filament crystal tip on a nonmagnetic cantilever in order to obtain a magnetic image. It is shown that the point probes with filament crystal tips coated by a thin magnetic film (obtained using a relatively simple preparation method) can provide a spatial resolution comparable with that achieved using cantilevers with magnetic nanoparticles, which require a significantly more complicated preparation procedures.     

Microscopic imaging with electrogenerated chemiluminescence

The use of electrogenerated chemiluminescence (ECL) at microelectrodes as a light source for scanning optical microscopy is demonstrated. Cone-shaped microelectrodes were constructed by flame etching carbon fibers to a fine point. ECL generated in solution at such electrodes was forced to the apex of the conducting surface by using high-frequency (20-kHz) potential pulses and high concentrations of ECL reagents in the solution. ECL arose from the reaction of 9,10-diphenylanthracene radical cation with the radical anion of benzonitrile, the solvent. The electrode/light source was raster-scanned a finite distance above the sample surface, and images were generated with standard scanning probe software by collecting the transmitted light with a microscope objective. These images compared favorably to optical images of the same sample. A resolution of approximately 600 nm was achieved with this arrangement even though a feedback loop was not employed to control the tip distance from the sample. The source was sufficiently bright (1.82 pW) that well-defined transmittance spectra could be obtained at individual locations on the sample.     

An electrospray ionization source for integration with microfluidics

We have demonstrated a new electrospray ionization (ESI) device incorporating a tip made from a shaped thin film, bonded to a microfluidic channel, and interfaced to a time-of-flight mass spectrometer (TOFMS). A triangular-shaped thin polymer tip was formed by lithography and etching. A microfluidic channel, 20 microm wide and 10 microm deep, was embossed in a cyclo olefin substrate using a silicon master. The triangular tip was aligned with the channel and bonded between the channel plate and a flat plate to create a microfluidic channel with a wicking tip protruding from the end. This structure aided the formation of a stable Taylor cone at the apex of the tip, forming an electrospray ionization source. This source was tested by spraying several solutions for mass spectrometric analysis. Because the components are all made by lithographic approaches with high geometrical fidelity, an integrated array system with multiple channels can be formed with the same method and ease as a single channel. We tested a multichannel system in a multiplexed manner and showed reliable operation with no significant cross contamination between closely spaced channels.     

Two methods to prepare nanorings/nanoholes for the fabrication of vertical nanotransistors

A self-assembled monolayer of polystyrene (PS) beads is formed on a silicon wafer by spin-coating. After drying at 80?°C, a thin film of metal/oxide is deposited. During the deposition, the PS beads are detached due to forces such as the inner stress induced by plasma sputtering deposition, mechanical vibration, and centrifugal shearing induced by substrate rotation, resulting in nanoring/nanohole formation. Further experiments demonstrate that the PS?detachment can be controlled by scanning probe microscopy (SPM) tip manipulation. We believe this is a promising set of processes for fabricating nanodevice structures such as those of vertical nanotransistors, which provides high flexibility for nanocrystal characterizations and application for single-electron devices.     

Fabrication and characterization of optical-fiber nanoprobes for scanning near-field optical microscopy

The current scanning near-field optical microscopy has been developed with optical-fiber probes obtained by use of either laser-heated pulling or chemical etching. For high-resolution near-field imaging, the detected signal is rapidly attenuated as the aperture size of the probe decreases. It is thus important to fabricate probes optimized for both spot size and optical transmission. We present a two-step fabrication that allowed us to achieve an improved performance of the optical-fiber probes. Initially, a CO(2) laser-heated pulling was used to produce a parabolic transitional taper ending with a top thin filament. Then, a rapid chemical etching with 50% buffered hydrofluoric acid was used to remove the thin filament and to result in a final conical tip on the top of the parabolic transitional taper. Systematically, we obtained optical-fiber nanoprobes with the apex size as small as 10 nm and the final cone angle varying from 15 degrees to 80 degrees . It was found that the optical transmission efficiency increases rapidly as the taper angle increases from 15 degrees to 50 degrees , but a further increase in the taper angle gives rise to important broadening of the spot size. Finally, the fabricated nanoprobes were used in photon-scanning tunneling microscopy, which allowed observation of etched double lines and grating structures with periods as small as 200 nm.     

Scanned probe imaging of quantum dots inside InAs nanowires

We show how a scanning probe microscope (SPM) can be used to image electron flow through InAs nanowires, elucidating the physics of nanowire devices on a local scale. A charged SPM tip is used as a movable gate. Images of nanowire conductance versus tip position spatially map the conductance of InAs nanowires at liquid-He temperatures. Plots of conductance versus backgate voltage without the tip present show complex patterns of Coulomb-blockade peaks. Images of nanowire conductance identify their source as multiple quantum dots formed by disorder along the nanowire--each dot is surrounded by a series of concentric rings corresponding to Coulomb blockade peaks. An SPM image locates the dots and provides information about their size. In this way, SPM images can be used to understand the features that control transport through nanowires. The nanowires were grown from metal catalyst particles and have diameters approximately 80 nm and lengths 2-3 microm.     

Artifacts in SPM measurements of thin films and coatings

Ideally, scanning probe microscopy (SPM) should generate a three-dimensional map of a sample surface such that the result is an exact replication of the actual sample. Any measurement data that result in an image differing from the actual sample surface are artifacts. The chief sources of SPM artifacts are mechanical systems, piezoelectric crystals, electronic scanners, tip-sample interaction, and image processing. For example, choosing the proper SPM probe for a specific sample is only the first step in minimizing probe-related artifacts. In fact, geometrical effects cause the largest number of artifacts. Good quality atomic SPM images can be clearly seen in raw data and should respond appropriately when the scan range or rotation is changed. Because SPM images are often periodic, it is possible for heavily filtered “data” to sometimes be misinterpreted as “atomic resolution images”. This paper presents SPM image studies using a range of materials from hard rough diamond films to soft nanometer smooth polyimide films. The investigation brings out the hidden sources of SPM artifacts for samples with different geometries and physical properties. Ten suggestions are presented which, if implemented/followed, should minimize the number of SPM image artifacts thereby assuring high quality images.     

Development of FBG probes for dimensional metrology with micro parts of high aspect ratio

The development is described of fiber Bragg grating (FBG) probes with single or four cores using spheres fixed onto optical fibers comprising Bragg gratings in their cores. Simulations and preliminary experiments were conducted to evaluate the performance of these FBG probes for dimensional metrology with micro parts of high aspect ratio. Simulation and experimental results indicate that a one-dimensional FBG probe with single core can be used to achieve an axial resolution of 100 nm at an aspect ratio of 15:1. A three-dimensional FBG probe with four cores can be used to achieve a theoretical axial resolution as high as that of a FBG probe with single core, with a capability of decoupling two-dimensional radial displacements with a resolution of 13 nm.