Shear Force Microscopy Research Articles

Shear force imaging of soft samples in liquid using a diving bell concept

We present a reliable and easy-to-use system, with a perfect analogy to a diving bell, to perform tuning fork based shear force microscopy on soft cells in liquid. Using the diving bell concept the tuning fork vibrates in air, while the tip is immersed in solution. In this way Q factors of 200 and higher in liquid are routinely obtained. The force feedback is reliable and stable over hours requiring a minimum adjustment of the set-point during imaging. With this system, tip–sample interaction forces are kept below 350 pN, enabling us to image soft dendritic cells in a buffer solution.

Increasing shear force microscopy scanning rate using active quality-factor control

The use of an active quality-factor control device has been demonstrated to increase the scanning rate of a shear force microscope by lowering the quality factor of the oscillating probe. Shear force microscopy (often used in combination with scanning near-field optical microscopy) requires slow scanning rates due to the long (>1 ms) time constant of the oscillating element (optical fiber or tuning fork). By electronically reducing the quality factor of the probe, it was possible to tune the time constant and correspondingly reduce the overall scanning time by a factor of 4. It was also seen that probes with high quality factors could manifest an unstable behavior; by reducing the quality factor of the probe, it was possible to increase the signal-to-noise ratio and so improve the imaging resolution. Deoxyribonucleic acid molecules deposited onto mica and imaged in air were used to demonstrate the technique.

Shear force detection by using bimorph cantilever with the enhanced Q factor

An improved nonoptical shear force detection system based on a rectangular bimorph cantilever incorporating the force feedback technique has been developed for tip–sample distance regulation in shear force microscopy. The force feedback amplifier consisting of a phase shifter and a linear amplifier is adjusted in such a way that the motion of the cantilever is mechanically amplified, resulting in a great enhancement of quality factor Q. Driving a fiber attached bimorph cantilever at its first harmonic resonance, with a phase shift φ=π/2 and an appropriate amplifier gain, allows one to obtain a Q factor greater than 103 in air, which corresponds to a Q enhancement of more than 1 order of magnitude. The effect of Q enhancement leads to an increase in the signal to noise ratio and thus the force detection sensitivity of the system. Typically, the minimum interaction force that can be sensed by the system is about 2 pN/√Hz. Topographic images of a human aorta tissue section in its natural state, taken with the described shear force detection system, show that it is very sensitive, reliable, and particularly suitable for biological applications.

Near-field optical writing on azo-polymethacrylate spin-coated films

We report on near-field optical writing and topographic reading of spin-coated azo-polymethacrylate films with subwavelength resolution. The sample is functionalized with photosensitive side chain azobenzene groups. These photochromes are locally illuminated at 325 and 488 nm through the aperture of a metalized tapered optical fiber. The printing on subwavelength scale is due to optically induced cooperative rearranging of the polymeric chain. It is read in situ in the surface topography imaged by shear-force microscopy. The stability of the optically produced pattern and the cycling characteristics of the photoisomerization of the azobenzene molecules make these samples suitable for optical nanolithography and high density optical data storage.

Comparison Between Shear Force and Tapping Mode AFM ‐ High Resolution Imaging of DNA

An improved shear force microscope (ShFM) is presented, where the oscillation of the probe can be excited and quantitatively detected in two orthogonal directions in the sample plane. This set-up allows a complete control of the dynamic behaviour of the probe that is necessary in order to obtain reproducible results. Increasing evidence has been collected confirming that the shear force mechanism is due to the water layer confined between the tip and the sample and that, therefore, ShFM is a true non-contact technique. To confirm the reduced tip-sample interaction with respect to conventional scanning probe techniques, the height and the width of more than 100 double stranded DNA fragments (4.3 kbp) were measured in air using different ShFM probes. The measurements were then compared with tapping mode atomic force microscopy (TMAFM) on a similar number of DNA fragments. From the statistical analysis of the data consistently higher profiles were obtained using ShFM: (1.1+/-0.2) nm instead of (0.6+/-0.1) nm that is the expected DNA height when using TMAFM in air. When acquiring high resolution images of DNA, the lower force of interaction of the shear force technique allowed the observation of a higher order periodic structure, which may reflect a conformational aspect of DNA peculiar to its interaction with mica. By improving the shear force technique, we have shown that important intrinsic advantages, such as non-contact interaction, can be fully exploited and the ShFM can become a very useful tool in the study of biomolecules.

Near-field optical patterning on azo-hybrid sol–gel films

We report on the near-field optical patterning of photochromic sol–gel films with subwavelength resolution. The sample containing functionalized azobenzene species is locally illuminated in the visible absorption band of these photochromes through the aperture of a metallized tapered optical fiber. The surface topography imaged by in situ shear-force microscopy reveals that, due to repeated photoisomerization cycles of the azobenzene molecules, photoinduced matter migration occurs under the tip leading to the formation of a surface relief. The shape of this structure is characteristic of the electromagnetic field distribution and strongly depends on the tip-to-sample distance. In near-field illumination conditions, protrusions of lateral dimension as small as 60 nm (≈λ/10) are currently produced. When repeating this process, compact arrays of nanodots are optically inscribed.

Nanoscale morphology of polyelectrolyte self-assembled films probed by scanning force and near-field scanning optical microscopy

We applied shear force microscopy, an analog to attractive-mode atomic force microscopy, and near-field scanning optical microscopy (NSOM) to the study of surface roughness and nanoscale morphology in polyelectrolyte self-assembled layers. Our data show that the surface roughness of multilayer films on glass grows linearly with the number of polyelectrolyte bilayers for the first 10 bilayers, and then asymptotes at a surface roughness of 4 nm. The surface of these films is characterized by bumps of 100–500 nm in diameter and 25–50 nm tall. In addition, the size and density of the bumps for each bilayer are uncorrelated to the previous surface. This result is in sharp contrast to what is observed for other self-assembled layer structures, such as metal-phosphonate and Langmuir–Blodgett self-assembly, where the surface roughness linearly increases with the number of layers. Subsurface morphology in thin films was observed via fluorescence NSOM of a dye-doped polyelectrolyte film. The NSOM images show domains of higher and lower fluorescence intensity, which could be assigned to local changes in the film thickness, suggesting that the dye molecules are uniformly distributed within the film. We also show that both the initial and asymptotic surface roughness can be dramatically decreased if the bare glass surface is treated with a high charge-density polyelectrolyte, such as poly(ethyleneimine) (PEI), prior to the bilayer formation. Finally, we observed that if the initial substrate is rough, growth of the polyelectrolyte layers acts to smooth the surface until the equilibrium value is reached.

High spatial resolution imaging with near-field scanning optical microscopy in liquids.

The mechanism of tuning fork-based shear-force near-field scanning optical microscopy is investigated to determine optimal experimental conditions for imaging soft samples immersed in liquid. High feedback sensitivity and stability are obtained when only the fiber probe, i.e., excluding the tuning fork prongs, is immersed in solution, which also avoids electrical shorting in conductive (i.e., buffer) solutions. Images of MEH-PPV were obtained with comparable spatial resolution in both air and water. High optical resolution (approximately160 nm fwhm) was observed.

Quantitative photon tunneling and shear-force microscopy of planar waveguide splitters and mixers

A combined photon scanning tunneling and shear-force microscope has been developed to investigate the optical field distribution in a planar waveguide splitter and a multibranch mode mixer. The optical intensity distribution just above the surface of a planar waveguide is mapped with subwavelength resolution by a tapered optical fiber that probes the evanescent field. Simultaneously, the topography of the waveguide is recorded with subnanometer accuracy using a constant-distance feedback system based on shear-force detection with a tuning fork sensor. The experimental field patterns are quantitatively compared with field patterns simulated with the two-dimensional finite difference beam propagation method and a−mode solver. Good quantitative agreement between experiment and simulation is obtained. Moreover, the experiment reveals several details in the field distribution that results from incoupling conditions, mask imperfections, waveguide edges, and surface roughness. The surface effects in the optical field distribution are introduced by the use of the constant-distance feedback system.

Near-Field Scanning Optical Microscopy of Temperature- and Thickness-Dependent Morphology and Fluorescence in Alq3 Films

We use near-field scanning optical microscopy (NSOM) to probe the local optical and morphological properties in annealed, vacuum-deposited Alq 3 films (10-480-nm thick) with 10-100-nm resolution. We use concurrent shear force microscopy (an analogue to atomic force microscopy, AFM) to correlate the morphology of different regions to intensity variations in our fluorescence images as well as variations in the localized fluorescence spectra. We have observed nanoscale effects of annealing temperature on film morphology and fluorescence emission. Our studies show that Alq 3 films annealed below the glass transition temperature of Alq 3 (T < Tg = 172 °C) were very similar to unannealed films, except in very thin films (< 50 nm). Films annealed above Tg (T = 200 °C) for the same amount of time exhibit increased surface morphology and decreased fluorescence. In sufficiently thick films (500 nm) annealed at 200 °C, we observe the formation of microcrystalline domains.

Observation of molecular layering in a confined water film and study of the layers viscoelastic properties

A transverse dynamic force microscope, more commonly known as shear force microscope, has been used to investigate confined water films under shear. A cylindrically tapered glass probe was mounted perpendicularly to the sample surface. Pure water was confined between the probe and a freshly cleaved mica surface and a sinusoidal shear strain was applied by setting the probe into transverse oscillation. Repeated measurements of the probe oscillation amplitude and relative phase lag, at different tip-sample separations, exhibited a clear step-like behavior. The periodicity, recorded over several curves, ranged between 2.4 and 2.9 Å, which is similar to the diameter of the water molecule. The in-phase (elastic) and the out-of-phase (viscous) stress response of the confined water film was evaluated (from the experimental data) by assuming a linear viscoelastic behavior. Finally, by modeling the water film with the Maxwell mechanical model, the values for the shear viscosity and shear rigidity were obtained.

Directional shear force microscopy

We describe a technique, based on shear force microscopy, that allows one to detect shear forces in a chosen direction at the nanometer scale. The lateral direction of an oscillating probe tip is determined by selecting which of the four quadrants are excited on the piezo driver. The shear forces depend directly on this lateral direction if structural anisotropies are present, as confirmed with polydiacetylene monolayers.

A high-stiffness axial resonant probe for atomic force microscopy

A high-stiffness, (>500 N/m) resonant atomic force microscope probe was constructed to allow force measurements in the presence of large force gradients. The probe employs a piezoresistively detected, electrostatically driven resonant beam sensor oriented perpendicularly to the sample surface. This probe is distinguished from shear force microscopy and noncontact atomic force microscopy in that the design allows for a stationary probe tip for improved spatial resolution and measures forces rather than force gradients. Measured results show a force resolution of 9 nN in a 1-kHz bandwidth in air with an oscillation amplitude of 36 nm and a resonance quality of 20. In a 1-mtorr vacuum the force resolution in the same bandwidth improves to 200 pN with a resonance quality of 450 and oscillation amplitude of 53 nm. This resolution is limited by white noise of the piezoresistor, and scales as expected with amplitude and resonance quality.

Investigation of microcontact transfer of proteins from a selectively plasma treated elastomer stamp by fluorescence microscopy and force microscopy.

Selective plasma treatment of the recessed regions of the elastomer stamps is shown to alter the resultant protein patterns. Fluorescence microscopy is demonstrated to be an excellent tool to discriminate between regions of microcontact printed fluorescent dye-labelled albumin in polystyrene. Atomic force microscopy and shear force microscopy are used to provide high-resolution images of the patterned protein layers. The formation and characteristics of the patterns formed by these alternative strategies is discussed.

Fast-scanning shear-force microscopy using a high-frequency dithering probe

We have demonstrated high-speed scanning shear-force microscopy using a fiber tip attached to quartz crystal vibrating at 2.0 MHz resonance frequency. With a high-frequency dithering probe, we have obtained clear images at the scanning speed of 1.2 mm/s and it takes only 0.5 s to image the surface area of 55 μm2 for 64×64 pixels without any compromise of spatial lateral resolution. This speed is more than five times faster than that obtained by other topographic imaging methods and can be further improved to realize real-time shear-force probe microscopy.

Interpretation of Contrast in Tapping Mode AFM and Shear Force Microscopy. A Study of Nafion

The origin of phase contrast in tapping-mode atomic force microscopy has been investigated using two complementary scanning probe microscopy techniques, atomic force microscopy and shear force microscopy, which can be classified as a transverse dynamic force microscopy. The sample chosen for this study was Nafion, and specifically the membrane in different hydration states by virtue of its cation form. Differences in probe−sample adhesion throughout a sample, caused by an inhomogeneous distribution of surface water, were an important phase-contrast mechanism. A new variant in three-dimensional force imaging, phase-volume imaging has been a useful tool in the interpretation of phase contrast. With the use of transverse dynamic force microscopy, approach curves were obtained while the frequency spectrum around resonance was measured. This enabled the damping of the probe oscillation amplitude and the shift in its resonant frequency to be decoupled. Knowing the true oscillation amplitude of the probe, it was...

Interfacial shear force microscopy

We present an experimental investigation of lossy and reactive shear forces at the nanometer scale using quartz-crystal tuning-fork shear-force microscopy. We show that this technique allows us not only to quantitatively measure viscous friction and elastic shear stress with a combination of high spatial and force resolution (better than 10 nm, and less than 1 pN, respectively), but also to obtain such quantities with the tip positioned at any arbitrary distance away from direct electrical tunnel contact with the sample surface. We are proposing that, even under vacuum conditions, the measured viscous and elastic shear stress (i.e., velocity dependent) are directly attributable to a third body filling the tip-sample gap. A simple model is given that allows us to obtain its local viscosity and shear modulus as a function of the tip-sample distance, showing that tuning-fork shear-force microscopy can be applied to quantitative analysis in nanotribology.

Imaging soft samples in liquid with tuning fork based shear force microscopy

We present a study of the dynamic behavior of tuning forks and the application of tuning fork based shear force microscopy on soft samples in liquid. A shift in resonance frequency and a recovery of the tip vibration amplitude have been observed upon immersion into liquid. Conservation of the vibration mode is confirmed by both direct stroboscopic observation and by detection of the tip vibration amplitude of the tuning fork. Thanks to the partial recovery of the Q factor upon complete immersion into liquid, it is possible to obtain high-resolution images on soft samples in liquid. This opens a new domain of applications for tuning fork based near-field scanning optical microscopes.

Stencil printing process development for flip chip interconnect

Traditional methods for forming flip chip interconnection include evaporation and electroplating. While both options have proven performance records, their costs are too high for many of today's cost-sensitive applications. Motorola's Interconnect Systems Laboratory (ISL) developed a low cost flip chip interconnect alternative, which electroless plates Ni/Au as the under bump metallurgy (UBM), deposits solder paste using a stencil or photoresist mask, and reflows the paste to form solder bumps. This paper is focused on the stencil print process development for wafer solder bumping. Solder paste selection, characterization, stencil design, and process parameter optimizations are critical factors for successful fine pitch stencil printing. Fine mesh eutectic solder pastes (Type 5: -500+635, Type 6: -635) with different flux vehicles were evaluated for printability and reflow studies. Paste and flux modification and co-development with vendors for this specific application were conducted. An optimal paste, with specified viscosity and desired print, reflow, and cleaning behavior, was developed. Wafer stencil design rules were established to deposit the right amount of solder paste to form the required reflowed solder bump height without bridges between pads. Printing and reflow design of experiments were performed to establish the baseline and optimum process parameters. Solder bump characterizations, including bump height and uniformity, composition, shear force, and scanning electron microscope (SEM) bump profile and cross section microstructure analysis, were conducted. Development and characterization results are reported. This stencil print solder bump process is demonstrated on 4, 5, 6, and 8-in device wafers with pitches down to 200 /spl mu/m. Solder bumps are formed on wafers without bridges or missing bumps. Wafer level yield is >95% on die basis. Bump height standard deviation within die is less than 3%, and range is less than 15% of the average bump height. The 63 Sn/37 Pb eutectic-bumped functional device dice were flip chip assembled to test boards with Cu/Ni/Au pad metallization, and underfilled for interconnect reliability studies. All reliability requirements were met.

Optical microscopy using a single-molecule light source

Rapid progress in science on nanoscopic scales has promoted increasing interest in techniques of ultrahigh-resolution optical microscopy. The diffraction limit can be surpassed by illuminating an object in the near field through a sub-wavelength aperture at the end of a sharp metallic probe. Proposed modifications of this technique involve replacing the physical aperture by a nanoscopic active light source. Advances in the spatial and spectral detection of individual fluorescent molecules, using near-field and far-field methods, suggest the possibility of using a single molecule as the illumination source. Here we present optical images taken with a single molecule as a point-like source of illumination, by combining fluorescence excitation spectroscopy with shear-force microscopy. Our single-molecule probe has potential for achieving molecular resolution in optical microscopy; it should also facilitate controlled studies of nanometre-scale phenomena (such as resonant energy transfer) with improved lateral and axial spatial resolution.