Shintaro Itoh

Surfactant Adsorption on Single-Crystal Silicon Surfaces in TMAH Solution: Orientation-Dependent Adsorption Detected by In Situ Infrared Spectroscopy

This paper focuses on two aspects, macroscopic and microscopic, of pure and surfactant-added tetramethylammonium hydroxide (TMAH) wet etching. The macroscopic aspects deal with the technological/engineering applications of pure and surfactant-added TMAH for the fabrication of microelectromechanical systems (MEMS). The microscopic view is focused on the in situ observation of the silicon surface during etching in pure and surfactant-added TMAH solutions using Fourier transform infrared (FT-IR) spectroscopy in the multiple internal reflection geometry. The latter is primarily aimed at investigating the causes behind the change in the orientation-dependent etching behavior of TMAH solution when the surfactant is added. Silicon prisms having two different orientations ({110} and {100}) were prepared for comparison of the amount of adsorbed surfactant using FT-IR. Stronger and weaker adsorptions were observed on {110} and {100}, respectively. Moreover, ellipsometric spectroscopy (ES) measurements of surfactant adsorption depending on the crystallographic orientation are also performed in order to gain further information about the differences in the silicon-surfactant interface for Si{100} and Si{110}. In this paper, we determine the differences in surfactant adsorption characteristics for Si{110} and Si{100} using FT-IR and ES measurements for the first time, focusing both on the mechanism and on the technological/engineering applications in MEMS.

Fiber wobbling shear force measurement for nanotribology of confined lubricant molecules

A new method for measuring dynamic shear force called fiber wobbling method has been developed. This method enables us to clarify nanotribological properties of a confined lubricant at a high shear rate. The method shows that the viscosity of the lubricant confined at the gap of less than 10 nm is higher than that of the bulk lubricant.

Spreading properties of monolayer lubricant films: Effect of bonded molecules

Monolayer lubricant films applied to head-disk interface of hard disk drives consist of bonded and mobile molecules. We measured the diffusion coefficient of mobile molecules that spread through the spaces not covered with bonded molecules and revealed its dependence on the coverage fraction. The diffusion coefficient gradually decreased with increasing coverage from 0.2 to 0.8, and its trend was well represented by reptation theory. However, the diffusion coefficient significantly decreased at a coverage higher than 0.8, in contrast to the prediction of reptation theory. The reason for this decrease is thought to be the disappearance of minimum spaces required for mobile molecules to spread through.

Structure-based coarse-graining for inhomogeneous liquid polymer systems.

The iterative Boltzmann inversion (IBI) method is used to derive interaction potentials for coarse-grained (CG) systems by matching structural properties of a reference atomistic system. However, because it depends on such thermodynamic conditions as density and pressure of the reference system, the derived CG nonbonded potential is probably not applicable to inhomogeneous systems containing different density regimes. In this paper, we propose a structure-based coarse-graining scheme to devise CG nonbonded potentials that are applicable to different density bulk systems and inhomogeneous systems with interfaces. Similar to the IBI, the radial distribution function (RDF) of a reference atomistic bulk system is used for iteratively refining the CG nonbonded potential. In contrast to the IBI, however, our scheme employs an appropriately estimated initial guess and a small amount of refinement to suppress transfer of the many-body interaction effects included in the reference RDF into the CG nonbonded potential. To demonstrate the application of our approach to inhomogeneous systems, we perform coarse-graining for a liquid perfluoropolyether (PFPE) film coated on a carbon surface. The constructed CG PFPE model favorably reproduces structural and density distribution functions, not only for bulk systems, but also at the liquid-vacuum and liquid-solid interfaces, demonstrating that our CG scheme offers an easy and practical way to accurately determine nonbonded potentials for inhomogeneous systems.

Lateral force measurement using a probe fiber as a microlens

We present a lateral force sensing method for small lateral forces that uses an optical fiber probe as a microlens and measures displacement of the laser spot on a position sensitive detector. We demonstrate that arranging the focus point near the object focal point of the fiber improves the signal intensity and our method combined with synchronous detection can provide a detection limit of the order of 1 pm at a bandwidth of 1 Hz. This indicates that the method enables us to measure lateral force of the order of 1 pN. The method is not restricted by measurement frequency as is the case with tuning fork-based lateral force sensing, and will be a useful method in applications that aim to clarify the intermolecular interaction between the probe and sample by measuring the frequency response, such as nanotribology, nanorheology, interfacial science, and nanobiology.

Effect of Ultraviolet Irradiation on Adhesion of Nanometer-Thick Lubricant Films Coated on Magnetic Disk Surfaces

By measuring force-distance curves with an environmental scanning probe microscope (SPM), we quantitatively evaluated the effect of ultraviolet (UV) irradiation on the adhesive properties of nanometer-thick perfluoropolyether (PFPE) Z03 films coated on magnetic disks. The experimental results indicated that UV irradiation shows negligible influence on the adhesion of the disk surface and on the adhesion of the mobile fraction contained in the UV-irradiated Z03 films. The effect of UV irradiation on the adhesion of Z03 films is essentially achieved by its effect that improves the bonding of the Z03 molecules to the disk surfaces. As the bonding ratio increased with UV irradiation, the adhesive force measured on the 2-nm-thick PFPE Z03 films initially decreased, but it increased after reaching a minimum at a bonding ratio of about 0.6. This is possibly attributed to the increase of the meniscus force caused by decreased mobile lubricant thickness and the decrease of the van der Waals force between the SPM probe and the sample surfaces caused by increased bonded lubricant thickness.

Adhesion Properties of Monolayer Lubricant Films Coated on Magnetic Disk Surfaces: Contributions of Mobile and Bonded Molecules

Monolayer lubricant films on magnetic disk surfaces consist of bonded and mobile molecules whose different adsorption states may result in different adhesion properties, which directly impact the stability of ultra-low flying head-disk interfaces. By measuring the adhesion force and the surface coverage as a function of bonding ratio, we investigated the contributions of the bonded and mobile molecules to the adhesion of monolayer perfluoropolyether Zdol4000 and Z03 films. The experimental results, which agree with our theoretical results, indicate that bonded lubricant molecules decrease adhesion by increasing the surface coverage, whereas the ability of mobile lubricant molecules to reduce adhesion depends on their adsorption strength with the substrate. Mobile molecules with polar end groups tend to adsorb strongly on a disk, thereby increasing the surface coverage and decreasing the adhesion force.

Control of wettability of molecularly thin liquid films by nanostructures.

The patterning of liquid thin films on solid surfaces is very important in various fields of science and engineering related to surfaces and interfaces. A method of nanometer-scale patterning of a molecularly thin liquid film on a silicon substrate using the lyophobicity of the oxide nanostructures has recently been reported (Fukuzawa, K.; Deguchi, T.; Kawamura, J.; Mitsuya, Y.; Muramatsu, T.; Zhang, H. Appl. Phys. Lett. 2005, 87, 203108). However, the origin of the lyophobicity of the nanostructure with a height of around 1 nm, which was fabricated by probe oxidation, has not yet been clarified. In the present study, the change in thickness of the liquid film on mesa-shaped nanostructures and the wettability for the various combinations of the thickness of the liquid films and the height of ridge-shaped nanostructures were investigated. These revealed that lyophobicity is caused by a lowering of the intermolecular interaction between the liquid and silicon surfaces by the nanostructure and enables the patterning of a liquid film along it. The tendency of the wettability for a given liquid film and nanostructure size can be predicted by estimating the contributions of the intermolecular interaction and capillary pressure. In this method, the height of the nanostructure can control the wettability. These results can provide a novel method of nanoscale patterning of liquid thin films, which will be very useful in creating new functional surfaces.

Contributions of Mobile and Bonded Molecules to Dynamic Friction of Nanometer-Thick Perfluoropolyether Films Coated on Magnetic Disk Surfaces

To protect the interface against intermittent head–disk contact in hard disk drives, nanometer-thick perfluoropolyether (PFPE) films consisting of both “bonded” and “mobile” molecules are applied on the disk surfaces. Because of their different adsorption states and mobility, the bonded and mobile molecules are supposed to contribute differently to friction properties, which directly impact the stability of ultra-low flying head–disk interfaces. By measuring the friction force at light loads and low to high speeds as a function of bonded and mobile film thicknesses, we studied the contributions of bonded and mobile molecules to the dynamic friction of nanometer-thick PFPE films. We found that the friction coefficient of lubricant films without or with less bonded molecules increased as a power function of sliding speed, whereas that of lubricant films with more bonded molecules increased logarithmically with sliding speed. We suggest that these results can be explained by the following mechanisms: the dynamic friction of lubricant films without and with less bonded molecules is dominated by shear thinning behavior of mobile molecules, while that of lubricant films with more bonded molecules is governed by bonded molecules which lead to boundary lubrication.

Real-Time Visualization of a Shearing Nanometer-Thick Lubricant Film by Two-Stage Imaging Ellipsometric Microscopy

A two-stage imaging ellipsometric microscopy with high lateral resolution and a wide field of view has been developed that will enable real-time visualization of the dynamic behavior of the nanometer-thick lubricant film used in head-disk interfaces incorporating contact or intermittent contact recording technology when it is sheared by the head. The first stage forms an object image at a low magnification ratio, which reduces the angle between the object image and the image sensor. This enables the first object image to be positioned perpendicular to the optical axis of the second stage. The second stage forms a final image on the image sensor at a high magnification ratio. As a result, both submicrometer lateral resolution and a wide field of view (115 μm × 85 μm) were achieved. A lubricant film sheared by an optical fiber probe with a micro ball at its end was visualized, and a bump due to meniscus formation during the intermittent contact was observed. This ellipsometric microscopy can thus visualize nanometer-thick sheared lubricant film in real time, which will be useful in the development of hard disk drives with higher recording density.