Silicon Nitride Cantilever Research Articles

Fabrication of PdIr-Coated Conductive Atomic Force Microscope Tip and its Application in Nanofabrication

We report a new conductive AFM tip, which is fabricated by thermal evaporation of 20–30nm PdIr alloy onto commercial silicon nitride cantilevers. With well-controlled evaporation conditions, a tip with nice electrical conductivity, good mechanical and chemical stabilities has been prepared. Thus fabricated AFM tips were used to measure electrical properties of materials with spatial resolution higher than 2nm, and to fabricate nanostructures on Zn-doped p-type GaAs (100) and chromium surfaces via field-induced oxidation.

Scanning electron microscopy studies of protein-functionalized atomic force microscopy cantilever tips.

Protein-functionalized atomic force microscopy (AFM) tips have been used to investigate the interaction of individual ligand-receptor complexes. Herein we present results from scanning electron microscopy (SEM) studies of protein-functionalized AFM cantilever tips. The goals of this study were (1) to examine the surface morphology of protein-coated AFM tips and (2) to determine the stability of the coated tips. Based on SEM images, we found that bovine serum albumin (BSA) in solution spontaneously adsorbed onto the surface of silicon nitride cantilevers, forming a uniform protein layer over the surface. Additional protein layers deposited over the initial BSA-coated surface did not significantly alter the surface morphology. However, we found that avidin-functionalized tips were contaminated with debris after a series of force measurements with biotinylated agarose beads. The bound debris presumably originated from the transfer of material from the agarose bead. This observation is consistent with the observed deterioration of functional activity as measured in ligand-receptor binding force experiments.

Novel thin membrane probe and a new twisting modulation force detection method of an atomic force microscope

For inspection of high aspect ratio structures like narrow semiconductor trenches, a thin membrane probe and a new force detection method have been proposed. Instead of conventional conical and pyramidal tips, a thin silicon nitride cantilever was set up vertically, and its edge was used as a tip. The membrane probe named as twist-probe (TP) was oscillated in the twisting resonance to detect a force from both vertical and lateral directions. About 100 μm long, 0.7 μm thick TP was fabricated as a trial. Amplitude versus distance curve measurements showed that the TP has a high spacing change sensitivity between the tip and a sample in both vertical and lateral directions. A trench cross-section imaging was demonstrated successfully with a TP and the twist resonant force detection method.

Development of a metal patterned cantilever for scanning capacitance microscopy and its application to the observation of semiconductor devices

The performance of scanning capacitance microscopy (SCM) strongly depends on the probe used. We have developed an original probe suitable for SCM (SCM probe), which uses a pyramid-shaped metal tip and metal lead line patterned on a silicon nitride cantilever. We installed the SCM probe on a SCM based on a commercial atomic force microscope (AFM). Differences in silicon oxide thickness and differences of dopant types and densities in a silicon substrate with a thermal oxide layer were successfully imaged by SCM simultaneously with AFM using the SCM probe. Boundaries between different dopant types and densities, which were not recognizable by AFM, were clearly observed by SCM. Signal-to-noise ratio and reproducibility were improved in the SCM images obtained with the SCM probe when compared with images obtained with a metal-coated silicon nitride cantilever.

Multi-mode noise analysis of cantilevers for scanning probe microscopy

A multi-mode analysis of micro-cantilever dynamics is presented. We derive the power spectral density of the cantilever displacement due to a thermal noise source and predict the cantilevers’s fundamental resonant frequency and higher harmonics. The first mode in the multi-mode model is equivalent to the traditional single-mode model. Experimental results obtained with a silicon nitride cantilever at 300 K are in excellent qualitative agreement with the multi-mode model. The multi-mode model may be used to obtain accurate values of the cantilever properties such as the elastic modulus, effective mass, thickness and moment of inertia.

Nanomechanical Properties of SiC Films Grown From C60 Precursors Using Atomic Force Microscopy

The mechanical properties of SiC films grown via C60 precursors were determined using atomic force microscopy (AFM). Conventional silicon nitride and diamond-tipped steel AFM cantilevers were employed to determine the film hardness, friction coefficient, and elastic modulus. The hardness is found to be 26 GPa by nanoindentation of the film with a Berkovich diamond tip. The friction coefficient for the silicon nitride tip on the SiC film is about one half to one third that for silicon nitride sliding on a silicon substrate. By combining nanoindentation and AFM measurements an elastic modulus of ~300 GPa is estimated for these SiC films.

Improvements to atomic force microscopy cantilevers for increased stability

A modification of commercially manufactured atomic force microscopy cantilevers which reduces the bending of the V-shaped legs due to changes in temperature is described. Gold-coated silicon nitride cantilevers are a bimorph system in which the different thermal expansion coefficients of the materials comprising the system can produce a temperature-dependent change in curvature. Other stress-related effects might also be responsible for the observed bending. By removing the gold film and redepositing gold only at the end of the V-shaped legs, a reduction in the bending of the cantilever is accomplished while the required optical reflectivity for the laser deflection system is retained. Imaging x-ray photoelectron spectroscopy, scanning electron microscopy, and changes in the detector photodiode signal related to bending of the cantilever are shown for modified and unmodified tips.

Study of molecular scale friction on stearic acid crystals by friction force microscopy

We observed two types of stearic acid single crystals having different lattice constants with a friction force microscope to study the molecular scale friction. Molecularly resolved friction force microscope images and molecularly periodic stick-slip motions of the cantilever were observed. The stick-slip motion of the cantilever was measured in terms of the normal load acting on the silicon nitride cantilever and scanning directions over the single crystals.

Microfabrication of near-field optical probes

Near-field optical microscopy generally uses a tapered optical fiber, which is metal coated, to form a sub-wavelength sized light source. Here, a technique for the fabrication of a new type of probe is described. The new design is based on atomic force microscope probes and consists of a silicon nitride cantilever with a solid transparent conical tip. The probes are made using micromechanical techniques, which allow batch fabrication of the probes. A near-field scanning optical microscope system was built to test the probes. This system features force detection by a beam deflection technique and subsequent force feedback together with a conventional optical microscope. A major advantage of the apparatus is the ease at which images are obtained. Results on a test sample show that an optical resolution of 300 nm can be obtained together with a simultaneous height image.

Three-dimensional imaging with a nuclear magnetic resonance force microscope

A magnetic resonance force microscope was used to demonstrate three-dimensional nuclear magnetic resonance imaging with micrometer-scale spatial resolution. The sample was mounted on a silicon nitride cantilever that served as a micromechanical force sensor. A nearby magnetic tip generated a field gradient of 22 G/μm. A three-dimensional magnetic resonance force map of the 1H spins in the sample was produced by lateral scanning of the magnetic tip relative to the sample and by varying the rf frequency of the spin excitation. The real-space spin density of the sample was reconstructed from the force map by means of a deconvolution technique. The spatial resolution achieved in the experiment was ∼3 μm in the axial direction.

Imaging domains in Co/Pt multilayers by magnetic force microscopy

The magnetic microstructure in a Co/Pt multilayer has been imaged by magnetic force microscopy employing a novel tip structure. Magnetic tips have been fabricated by evaporating a thin film onto a single facet of the pyramidal tip of a silicon nitride cantilever. Nanolithographically defined elements have been used to simulate the micromagnetic structure of the tip and have been investigated by the Lorentz mode of electron microscopy. We demonstrate that stray fields from the Co/Pt multilayer can be sensed by the tip producing images of the magnetic microstructure with a fine structure on a scale of 30–40 nm.

Study of aging rat tail collagen using atomic force microscope.

Collagen structure of young and old rats was examined by using atomic force microscope (AFM) images. Rat tail tendons of eight and twenty-four month-old Wistar rats were digested enzymatically (pepsin), and allowed to refibrillate for 24 hours at 37 degrees C. The samples were examined using a Nanoscope III (Digital Instruments, Santa Barbara, CA, U.S.A.) with a J scanning head and a 200 microns silicon nitride cantilever. The study was performed in air and without filters. The AFM inspection of refibrillated collagen produced images showing long fibrils with relatively homogeneous heights and widths, characterized by clear banding with a periodic interval (D band) of 67 nm. With respect to collagen extracted from young rats, collagen extracted from old rats revealed fibrils exhibiting the same band interval, but with lower widths and heights. Furthermore, the depth of gap between two overlaps showed a higher mean value in the aged rats. These data are consistent with biochemical reports of collagen modifications during aging; we suggest that post-synthetic reactions might be responsible for this as they interfere with the refibrillation process and also modify the three-dimensional structure of fibrils.

Atomic force microscope lithography using amorphous silicon as a resist and advances in parallel operation

Lithography on (100) single-crystal silicon and amorphous silicon is performed by electric-field-enhanced local oxidation of silicon using an atomic force microscope (AFM). Amorphous silicon is used as a negative resist to pattern silicon oxide, silicon nitride, and selected metals. Amorphous silicon is used in conjunction with chromium to create a robust etch mask, and with titanium to create a positive AFM resist. All lithographies presented here were patterned in parallel by arrays of two piezoresistive silicon or two silicon-nitride cantilevers. Parallel arrays of five piezoresistive cantilevers were fabricated and used in imaging and lithographic applications. A 400 μm×100 μm parallel image is obtained in the time it would normally take to obtain a 100 μm×100 μm image. In our method of parallel operation, it is only possible to image and lithograph in modes that do not require feedback. In imaging, this limits the possible applications of the parallel AFM. During parallel lithography, discrepancies are seen between the tip in the feedback loop and those that are not. To overcome these differences it will be necessary to devise a system where each of the tips in the array are controlled by individual feedback loops.

Improvement of thermally induced bending of cantilevers used for atomic force microscopy

AbstractThe microfabricated silicon nitride cantilevers that are used for atomic force microscopy (AFM) are, unfortunately, sensitive thermometers. They bend with ambient temperature changes and those due to laser heating. The bend can result in displacements for the silicon nitride cantilevers of an order several hundred nanometers at the tip of the cantilever. If, however, the silicon nitride cantilevers are treated by removing the metallization and annealing at 500°C for 30 min, these displacements can be reduced by one or two orders of magnitude. Silicon cantilevers have approximately a one order of magnitude smaller drift than silicon nitride cantilevers as received from vendors and are improved less by treatment.

Method for evaluating sharpness of tip apex of a cantilever for the atomic force microscope

A chemically textured alumite disk with micropillar posts on its surface is found to be a good standard sample to evaluate sharpness of atomic force microscope tips in the range of a few to a few tens of nm. The tips evaluated were silicon nitride cantilever with a normal pyramidal tip, one with a carbon microtip deposited by the electron beam method and one with a microtip formed by the focused ion beam method (FIB tip). The FIB tip has a fairly sharp tip apex so it can show very fine ridges around micropillars precisely.

Atomic Force Microscope-Based Lithography of Titanium

ABSTRACTAnodization of Ti by high electric fields at the tip of a scanned probe can be used to produce nanoscale features consisting of oxides of Ti. In this manner, Ti can be used as a sacrificial resist for nanoscale lithography by exploiting the etching selectivity differences between Ti and anodized Ti. The anodization was accomplished with an atomic force microscope using Ticoated silicon nitride cantilevers. The anodizing bias voltage is applied to the tip and is independent of the feedback, unlike the scanning tunneling microscope. With this setup we were able to fabricate sub-40 nm lines by direct anodization of Ti. It is also shown that once tip and sample are brought into hard contact, subsequent bending of the cantilever has little effect on the linewidth or thickness of the anodized material.

Viscoelasticity of living cells allows high resolution imaging by tapping mode atomic force microscopy

Application of atomic force microscopy (AFM) to biological objects and processes under physiological conditions has been hampered so far by the deformation and destruction of the soft biological materials invoked. Here we describe a new mode of operation in which the standard V-shaped silicon nitride cantilever is oscillated under liquid and damped by the interaction between AFM tip and sample surface. Because of the viscoelastic behavior of the cellular surface, cells effectively "harden" under such a tapping motion at high frequencies and become less susceptible to deformation. Images obtained in this way primarily reveal the surface structure of the cell. It is now possible to study physiological processes, such as cell growth, with a minimal level of perturbation and high spatial resolution (approximately 20 nm).

Force Detection of Nuclear Magnetic Resonance

Micromechanical sensing of magnetic force was used to detect nuclear magnetic resonance with exceptional sensitivity and spatial resolution. With a 900 angstrom thick silicon nitride cantilever capable of detecting subfemtonewton forces, a single shot sensitivity of 1.6 x 10(13) protons was achieved for an ammonium nitrate sample mounted on the cantilever. A nearby millimeter-size iron particle produced a 600 tesla per meter magnetic field gradient, resulting in a spatial resolution of 2.6 micrometers in one dimension. These results suggest that magnetic force sensing is a viable approach for enhancing the sensitivity and spatial resolution of nuclear magnetic resonance microimaging.

Tapping mode atomic force microscopy in liquid

We show that standard silicon nitride cantilevers can be used for tapping mode atomic force microscopy (AFM) in air, provided that the energy of the oscillating cantilever is sufficiently high to overcome the adhesion of the water layer. The same cantilevers are successfully used for tapping mode AFM in liquid. Acoustic modes in the liquid excite the cantilever. On soft samples, e.g., biological material, this tapping mode AFM is much more gentle than the regular contact mode AFM. Not only is the destructive influence of the lateral forces minimized, but more important, the intrinsic viscoelastic properties of the sample itself are effectively used to ‘‘harden’’ the soft sample.

AFM of polymers using force spectroscopy modes

Several novel imaging modes in scanning-probe microscopy are capable of imaging the surface compliance properties of polymers. The atomic-force microscope is used with a silicon nitride cantilever, in contact mode. While scanning, the tip can be modulated with a low amplitude (25 Å) and low frequency (5 kHz), and the amplitude of tip deflection is compared with the input modulation signal. This mode, called modulated force, maps out the surface compliance of a sample, and gives pixel-to-pixel matching with a topography mode image. Alternatively, while scanning a topography mode image, a force-distance curve can be performed at each x-y pixel location. Several data points in the z-direction of the dF-dS curve can therefore be collected at each x-y data position. In result, one obtains a 3D dataset of force-distance curves with corresponding topography data. The dF-dS images that result are slices through the force-distance regime, each with pixel-to-pixel correspondence to the topography image. In this study, various polymer systems are examined with AFM force imaging modes.