Kwang Joo Kwak

Nanotribology and nanomechanics of AFM probe-based data recording technology

With the advent of scanning probe microscopes, probe-based data recording technologies are being developed for ultrahigh areal density recording, where the probe tip is expected to be scanned at velocities up to 100?mm?s?1. In one technique, a conductive atomic force microscope (AFM) tip is scanned over a phase change chalcogenide medium and phase change is accomplished by applying either a high or low magnitude of current which heats the interface. Another technique is ferroelectric data storage, where a conducting AFM tip is scanned over a lead zirconate titanate (PZT) film, a ferroelectric material. Ferroelectric domains can be polarized by applying short voltage pulses between the AFM tip and the bottom electrode layer that exceed the coercive field of the PZT film, resulting in nonvolatile changes in the electronic properties. Tip wear is a serious concern in both data storage methods. The understanding and improvement of tip wear, particularly at the high velocities needed and at high interface temperatures for high data rate recording, is critical to the commercialization of these data storage technologies. This paper presents a review of nanotribological and nanomechanical studies on the materials used in phase change and ferroelectric probe-based recording. Although this work is aimed at probe-based data recording, it is also relevant to the development of robust AFM probes and to the study of nanocontacts in general.

Nanoscale adhesion, friction and wear studies of biomolecules on silane polymer-coated silica and alumina-based surfaces

Proteins on biomicroelectromechanical systems (BioMEMS) confer specific molecular functionalities. In planar FET sensors (field-effect transistors, a class of devices whose protein-sensing capabilities we demonstrated in physiological buffers), interfacial proteins are analyte receptors, determining sensor molecular recognition specificity. Receptors are bound to the FET through a polymeric interface, and gross disruption of interfaces that removes a large percentage of receptors or inactivates large fractions of them diminishes sensor sensitivity. Sensitivity is also determined by the distance between the bound analyte and the semiconductor. Consequently, differential properties of surface polymers are design parameters for FET sensors. We compare thickness, surface roughness, adhesion, friction and wear properties of silane polymer layers bound to oxides (SiO2 and Al2O3, as on AlGaN HFETs). We compare those properties of the film–substrate pairs after an additional deposition of biotin and streptavidin. Adhesion between protein and device and interfacial friction properties affect FET reliability because these parameters affect wear resistance of interfaces to abrasive insult in vivo. Adhesion/friction determines the extent of stickage between the interface and tissue and interfacial resistance to mechanical damage. We document systematic, consistent differences in thickness and wear resistance of silane films that can be correlated with film chemistry and deposition procedures, providing guidance for rational interfacial design for planar AlGaN HFET sensors.

Velocity dependence of nanoscale wear in atomic force microscopy

The velocity dependence of nanoscale wear is studied over a wide range of velocities up to 100mm∕s. High sliding velocities are achieved by modifying an existing setup with a custom calibrated piezostage. Pt-coated atomic force microscope probes are scanned against uncoated diamondlike carbon-coated Si wafers and with an overcoat of a lubricant film. The wear rate increases with logarithm of velocity up to a certain value and then levels off. A wear model is developed to explain the velocity dependence of nanoscale wear based on thermally activated atomic-scale stick slip and tribochemical wear.

Noble metal-coated probes sliding at up to 100 mm s−1 against PZT films for AFM probe-based ferroelectric recording technology

With the advent of scanning probe microscopes, probe-based data recording technologies are being developed for ultra-high areal density. In the ferroelectric data storage being explored, a conductive atomic force microscope (AFM) tip is scanned over a lead zirconate titanate (PZT) film, which is a ferroelectric material. Ferroelectric domains can be polarized by applying short voltage pulses between the AFM tip and the bottom electrode layer that exceed the coercive field of the PZT film, resulting in nonvolatile changes in the electronic properties. A crucial reliability concern is the wear of the AFM tip and PZT film. The understanding and improvement of tip wear, particularly at the high velocities needed for high-data-rate recording, is critical to the commercialization of ferroelectric data storage. To this end, wear experiments are performed using various noble metal-coated tips sliding against a PZT film at velocities of 10 and 100 mm s−1. The noble metals that were used were Pt, Au–Ni, Pt–Ir and Pt–Ni. High sliding velocities are achieved by using a custom calibrated piezo stage in a commercial AFM. The Au–Ni and Pt–Ir tips are shown to exhibit the lowest wear. The tip wear mechanism is found to be primarily adhesive and abrasive wear, with some evidence of impact wear. The coefficient of friction increases during wear. This study advances the understanding of the physics of friction and wear of noble metal-coated AFM probes.

Platinum-coated probes sliding at up to 100 mm/s against lead zirconate titanate films for atomic force microscopy probe-based ferroelectric recording technology

With the advent of scanning probe microscopes, probe-based data recording technologies are being developed for ultrahigh areal density. In alternative ferroelectric data storage, a conductive atomic force microscope (AFM) tip is placed in contact on a lead zirconate titanate (PZT) layer as the ferroelectric film. Ferroelectric domains can be polarized by applying short voltage pulses between the AFM tip and the bottom electrode that exceed the coercive field of the PZT layer, resulting in local, nonvolatile changes in the electronic properties of the underlying film. By monitoring the piezoelectric vibration of the ferroelectric film caused by an external ac voltage, the domain structure can be visualized. A degradation due to a voltage pulse to the PZT film occurs and is one reliability concern, called ferroelectric fatigue. Another important reliability concern is tip wear during tip-sample contact. The understanding and the improvement of tip wear, particularly at high velocities needed for high data r...

The role of lubricants, scanning velocity and operating environment in adhesion, friction and wear of Pt?Ir coated probes for atomic force microscope probe-based ferroelectric recording technology

Probe-based data recording is being developed as an alternative technology for ultrahigh areal density. In potential ferroelectric data storage, a conductive atomic force microscope tip is scanned over a lead zirconate titanate (PZT) film, a ferroelectric material. The understanding and the improvement of wear of the tip during its contact with the ferroelectric materials is critical, particularly at the high scanning velocities needed for high data rate recording in the operating environments. To this end, adhesion, friction and wear experiments are performed using Pt–Ir coated tips sliding against unlubricated and lubricated PZT films at velocities ranging from 0.1 to 100 mm s−1, the maximum velocity corresponding to the expected recording rate. Two lubricants were used: perfluoropolyether and ionic liquid. The Pt–Ir tips are shown to exhibit lower wear against the lubricated PZT film. To study the role of the operating environment, experiments are also conducted at 80 and 120 °C and at 5–80% relative humidity. Relevant wear mechanisms are discussed.

Rational Enhancement of Nanobiotechnological Device Functions Illustrated by Partial Optimization of a Protein-Sensing Field Effect Transistor

Semiconductor field effect transistors(FETs) are widely used as biosensors, although a potentially powerful application of FET sensing technology (planar immunoFETs sensing proteins at physiological salt concentrations) has long been argued to be intrinsically infeasible. The infeasibility assessment has come under increasing scrutiny of late, and has been found to be lacking on conceptual and empirical grounds. This paper summarizes some, but, by no means all, of the strategies that have been pursued to render the use of immunoFETs, and analogous FET sensors that detect the electrical fields of proteins bound to affinity elements on FET sensing channels (protein-sensing bioFETs), practical in high-salt biological buffers. This paper provides original characterization of oxidized AlGaN surfaces and interfacial polymer/protein films of protein-sensing AlGaN/GaN HFETs. It shows those films to influence significantly FET sensitivity/signal accumulation. The data indicate that re-assessment of the classical assertion of immunoFET infeasibility is long overdue. Beyond substantiating the feasibility of immunoFET operation under solution conditions as found in vivo, data presented here also suggest that transition away from costly AlGaN/GaN HFETs to inexpensive silicon-based immunoMOSFETs may be possible. If so, immunoFETs, dismissed as infeasible 20 years ago, may yet become powerful clinical tools.