Hong Jing Chung

Native oxide decomposition and local oxidation of 6H-SiC (0001) surface by atomic force microscopy

We have observed the native oxide decomposition and local oxide growth on 6H-silicon carbide (0001) surface induced by atomic force microscopy (AFM). When the biased AFM probe was scanned over surface areas, native oxide was decomposed and assembled into protruded lines. The decomposition is accompanied by simultaneous graphitization of the scanned areas, leading to metal–semiconductor contact as evidenced in I–V characteristics. When the probe was immobilized and longer bias duration applied, direct oxidation of silicon carbide (SiC) surface was achieved. The dielectrical properties of AFM oxide on SiC were also investigated in terms of interface barrier height.

Spatially resolved diagnosis of stress-induced breakdown in oxide dots by in situ conducting atomic force microscopy

We report an investigation on the stress-induced breakdown (BD) in ultrathin oxide grown by atomic force microscopy (AFM oxide). A conducting atomic force microscopy (c-AFM) technique was employed to stress the AFM oxide and examine its BD behavior. It was found that thermal annealing has a strong impact on the dielectric strength of AFM oxide. The stress-induced trap generation probability, Pt, could be reduced by ∼50% after annealing the oxide at elevated temperatures. Such a thermal effect is related to the local structural relaxation and trap state minimization in AFM oxide upon annealing. The spatially resolved current images allow a microscopic diagnosis of the distribution of BD sites: isolated single BD spots and laterally propagated BD areas were observed in an oxide dot. Soft and hard breakdown sites were also distinguished on the current images.

Polymeric conical structure formation by probe-induced electrohydrodynamical nanofluidic motion

We report the creation of polymeric structures by atomic force microscopy (AFM) probe induced electrohydrodynamic (EHD) instability and nanofluidic flow. By biasing the AFM probe in a high field regime, single conical structure was produced on poly(methylmethacrylate) due to the initiation of strong EHD instability in the locally heated polymer melts. The pattern formation is dominated by the interplay of polymer EHD motion, polymer ablation, and AFM tip repulsion. The dependence of cone formation probability on the bending of AFM cantilevers with different stiffness was also discussed.

Field-induced meniscus dynamics and its impact on the nanoscale tip-surface interface

We describe the spatiotemporal evolution of the nanoscale tip-surface junction during field-induced water meniscus formation in the junction. The motion of the meniscus and tip was analyzed on the basis of typical parameters concerning the nanoscale meniscus and tip-surface configuration. Being attracted by the electric field, the meniscus generates a repulsive hydrodynamic impact force counteracting the electrostatic force on the tip. The imbalance of the forces leads to an increase of the tip-surface separation distance, and the increase is related to the initial experimental parameters including tip bias voltage and tip spring constant. An explicit equation was derived for the estimation of the tip-surface junction enlargement effect. The theoretical results were confirmed by atomic force microscope (AFM) in situ observations of tip repulsion under electric fields. The induced tip-surface junction enlargement has significant implications in AFM nanolithography, e.g., it could facilitate the formation o...

Chemical etching study of probe-grown ultrathin nano-oxides by atomic force microscopy

We report the study of the etching characteristics of atomic force microscopy (AFM) probe-grown ultrathin oxides (AFM oxides, up to 5nm thick). In our method, an AFM localized depth analysis technique was employed to monitor the atomic layer-by-layer etching of AFM oxides. Insights into the growth mode and etching mechanism of AFM oxides were acquired on the basis of the etching results: it was found that AFM oxide growth is related to Si out-diffusion. For the formation of ultrathin oxides in ambient conditions, it is evident that oxidation-enhanced Si diffusion facilitates the layer-by-layer oxide growth in AFM anodic oxidation. The etching rate of ultrathin AFM oxides is dependent on the SiOH silanol reactive sites. Thermal annealing could reduce the content of silanol groups and enhance the chemical stability of AFM oxides against etching.

Micro∕nanoscopic patterning of polymeric materials by atomic force microscope assisted electrohydrodynamic nanolithography

In this work, we address the spatiotemporal evolution of micro∕nanoscopic pattern formation on polymer films by atomic force microscope (AFM) assisted electrohydrodynamic (EHD) nanolithography. This patterning method combines the locality and site specificity of AFM and the surface wave amplification of EHD instability for pattern creation and is capable of generating a series of structures corresponding to the zeroth- to second-order wave patterns. Detailed simulations in the framework of nonlinear three-dimensional analysis are presented to elucidate the localized EHD pattern formation. The experimental and theoretical results are quantitatively compared to address the characteristics of the AFM-assisted EHD pattern formation. The impact of key experimental parameters such as the tip bias voltage and bias duration on the formation rate, morphology, and lateral dimension of the wave patterns are discussed. The AFM assisted EHD nanolithography would open new route to the fabrication of complex polymer nan...

Chemical and dielectrical characteristics of ultrathin oxides grown by atomic force microscopy and scanning electron beam

We report a comparative study on the chemical and dielectrical properties of ultrathin oxides grown by atomic force microscopy (AFM) and scanning electron beam (SEB) techniques. Oxide grown by AFM (AFM oxide) shows preferential etching as compared to oxide grown by SEB (SEB oxide). The structural and chemical features of these oxides were probed using time-of-flight secondary ion mass spectrometry (TOF-SIMS) time profiling. It was found that AFM oxide is richer in Si–H and Si–OH content, while SEB oxide is oxygen rich and relatively dense in structure. The dielectric strength of AFM and SEB oxides were further evaluated by conducting AFM (c-AFM). The current–voltage characteristics and dielectric breakdown probability of these oxides were compared. The correlation between Si–H and Si–OH site formation and its impact on the chemical and dielectrical stability of AFM and SEB oxides was discussed.

Enhanced probe nano-oxidation by charge pump effect in swept tip voltage cycles

A probe-based nano-oxidation method for enhanced vertical oxide growth on silicon is presented. The technique involves cycling the tip biases between positive and negative polarities to produce high-aspect-ratio nano-oxides. Enhanced oxidation was observed to take place in the positive tip bias region as opposed to the negative tip voltage required for anodic oxidation. A model based on interface space charge accumulation and neutralization, OH− reactant mobilization, and diffusion is proposed to account for the oxidation enhancement observed under positive tip voltages. The proposed model is analogous to the transient charge pump effect which is at work when the polarity of a capacitor is switched. The results reveal the dynamical behavior of nano-oxidation under nonstatic fields which can be harnessed for fabricating oxide nanostructures with improved aspect ratios.

SCANNING PROBE MICROSCOPY BASED NANOSCALE PATTERNING AND FABRICATION

Nanotechnology is vital to the fabrication of integrated circuits, memory devices, display units, biochips and biosensors. Scanning probe microscope (SPM) has emerged to be a unique tool for materials structuring and patterning with atomic and molecular resolution. SPM includes scanning tunneling microscopy (STM) and atomic force microscopy (AFM). In this chapter, we selectively discuss the atomic and molecular manipulation capabilities of STM nanolithography. As for AFM nanolithography, we focus on those nanopatterning techniques involving water and/or air when operated in ambient. The typical methods, mechanisms and applications of selected SPM nanolithographic techniques in nanoscale structuring and fabrication are reviewed.