Scanning Probe Oxidation Research Articles

Application of scanning probe oxidation lithography in fabrication of micro-nano devices

Scanning probe lithography technology is mainly used the electrical, mechanical or thermal interaction between the atomic force microscope tip and substrate to perform imaging, manipulation and modification of the nanoscale surface. It is a simple, rapid and accurate nanostructure preparation technique. The scanning probe oxidation lithography technology has a good prospect, using highly localized water bridge formed between the tip and the sample surface to prepare micro-nano scale structures on material surface through electrochemical reactions. It has been widely used to produce nano-scale functionalized patterns and micro-nano devices. The mechanism of the patterning process and influencing factors such as voltage, tip-sample force, duration, relative humidity, and scanning speed are described in detail in this paper. Then, the work on the preparation of micro-nano devices using this technology is summarized. Finally, we discussed the advantages and problems of this technology, as well as the potential solution and development of it.

Silica coating of PbS quantum dots and their position control using a nanohole on Si substrate

We succeeded in controlling the apparent size of a colloidal PbS quantum dot (QD) in the range of 20 to 140 nm by coating with silica and trapping the coated QDs in a nanohole prepared by scanning probe microscope lithography. Photoluminescence intensity was improved by controlling the process of adding the silica source material of tetraethoxysilane for the coating. Nanoholes of different sizes were formed on a single substrate by scanning probe oxidation with the combination of SF6 dry etching and KOH wet etching. QDs having an arbitrary energy structure can be arranged at an arbitrary position on the semiconductor substrate using this technique, which will aid in the fabrication of future nanosize solid devices such as quantum information circuits.

Scanning probe oxidation of SiC, fabrication possibilities and kinetics considerations

We report the outcome of atomic force microscopy local anodic oxidation experiments on 6H-SiC in air. Oxide thickness can be easily tuned by varying applied voltage and pulse duration. The height and the aspect ratio of single dots produced by single DC pulses are remarkably higher than what was reported previously, with self limiting heights exceeding 100 nm. We propose that the diminished density and the change in chemical composition of the oxide grown on SiC with respect to oxide grown under similar condition on Si cause a drop in the activation energy of oxanions diffusion within the newly formed oxide layer.

Local interdiffusion at buried TiN/Si interfaces with scanning probes

This study proposes a novel scanning-probe-based approach to induce local interdiffusion at buried interfaces. Scanning-probe oxidation (SPO) was performed on a thin TiN layer on a Si substrate with ethanol menisci. The diffusion of Si and Ti at the interface was observed using Auger electron analysis and photoelectron spectroscopy and mapping. The results of photoelectron spectroscopy revealed that SPO converted the TiN layer into TiNxOy. The significant conductivity of TiNxOy was verified by conductive atomic force microscopy. Joule heating in the conductive TiNxOy induced Si diffusion in the amorphous TiNxOy layer.

Possibility of Pulsed Electrochemical Processes to Nanofabrication Using Scanning Probe Oxidation

This study demonstrates nano-scale lithography process on localized (100) silicon (p-type) surface using modified AFM apparatuses and controlling methods. AFM-based experimental apparatuses are connected the customized pulse generator that supplies electricity between conductive tip and silicon surface maintaining constant humidity during processes. Then pulse durations are controlled according to various experimental conditions. The pulsed electrochemical reaction within the gap between conductive tip and silicon surface induces the formation of oxide with nano-scale topographies. Various heights and widths of oxides can be created by AFM surface modification according to various pulse durations and applied electrical conditions under humidity environment. In addition, it can be known that oxides are completely removed after sample surface is etched in diluted HF solution, which shows micro/nano-scale grooves can be fabricated after predefined chemical treatment. They are wider than oxides widths and have several nanometer depths. Nano patterning technique from this experiment suggests that pulse electrochemical machining process has bright potential for advancing nano machining technologies.

Characterization and molecular engineering of surface-grafted polymer brushes across the length scales by atomic force microscopy

With the advent of regulated, surface initiated polymerizations, specifically using controlled radical approaches, the choice of polymerizable compounds and the control over grafting chemistry have seen tremendous advancement. New analysis techniques and approaches are now needed to characterize these brushes with molecular precision. In addition, spatial structure control at the nanoscale, and tuning of thickness as well as composition of the brushes, have become feasible by utilizing recently developed enabling molecular nanofabrication approaches. Atomic force microscopy (AFM) is a powerful analytical tool for the characterization of polymer brushes, as well as for the fabrication of brush structures across the length scales. AFM has been used to investigate polymer brushes in a number of ways including imaging surface morphologies, measuring brush thickness, estimating the value of number average molar mass, observing stimulus responsive behavior and probing surface mechanical properties. In addition, AFM based methods such as nanoscratching, dip-pen nanolithography (DPN) and scanning probe oxidation (SPO) have been also employed for the nanofabrication of patterned polymer brushes. This feature article gives a short account of this field and highlights recent advances.

PH Responsive Polymeric Brush Nanostructures: Preparation and Characterization by Scanning Probe Oxidation and Surface Initiated Polymerization

pH-responsive PHEMA-based polymeric nanostructures were grown in a controlled manner by ATRP-based surface-initiated polymerization. Initiator nanopatterns were obtained on silicon wafers covered with OTS resists made by AFM scanning probe oxidation lithography. AFM images confirmed isolated grafting of stimuli-responsive hedge and dot brush structures exhibiting dimensions corresponding to a few tens of chains.

Scanning Probe Oxidation Lithography on Ta Thin Films

A Semi-Contact Scanning Probe Lithography Technique (SC-SPL) has been applied to create nano-oxide patterns on Ta thin films grown by DC magnetron sputtering method on SiO2/Si substrates. The height and linewidth profiles of nano-oxide lines created by a conductive AFM tip on Ta film surfaces were measured as a function of applied voltage, oxidation time, humidity, and tip apex curvature. The AFM surface measurements show that the height of the oxides increases linearly with increasing voltage; but there was no oxide growth, when less than 4 V was applied even at 85% relative humidity. Electrical measurements were performed and the resistivities of the TaOx layer and Ta film were obtained as 5.76 × 108 and 1.4 × 10−5 Ohm-cm, respectively.

Single-hole transistor in p-type GaAs∕AlGaAs heterostructures

A single-hole transistor is patterned in a p-type, C-doped GaAs∕AlGaAs heterostructure by scanning probe oxidation lithography. Clear Coulomb blockade resonances have been observed at Thole=300mK. A charging energy of ∼1.5meV is extracted from Coulomb diamond measurements, in agreement with the lithographic dimensions of the dot. The absence of excited states in Coulomb diamond measurements, as well as the temperature dependence of Coulomb peak heights indicate that the dot is in the multilevel transport regime. Fluctuations in peak spacings larger than the estimated mean single-particle level spacing are observed.

Nanometre resolution metrology with the Molecular Measuring Machine

Nanometre accuracy and resolution metrology over technically relevant areas is becoming a necessity for the progress of nanomanufacturing. At the National Institute of Standards and Technology, we are developing the Molecular Measuring Machine, a scanned probe microscope (SPM) and Michelson interferometer based metrology instrument, designed to achieve nanometre measurement uncertainty for point-to-point measurements over a 50 mm by 50 mm working area. The salient design features are described, along with example measurements that demonstrate the measurement capabilities so far achieved. Both long-range measurements of sub-micrometre pitch gratings over 10 mm, and short-range, high-resolution measurements of a molecular crystal lattice have been accomplished. The estimated relative measurement uncertainty so far attained for pitch measurements is 6 × 10−5, coverage factor k = 2. We have also used this instrument and scanning probe oxidation lithography for creating some simple nanometre dimension patterns that could serve as prototype calibration standards, utilizing the SPM probe tip positioning accuracy.

Surfactant-Resisted Assembly of Fe-Containing Nanoparticles for Site-Specific Growth of SWNTs on Si Surface

This paper describes a facile approach to the site-specific growth of single-walled carbon nanotubes (SWNTs) on silicon surfaces by chemical vapor deposition (CVD). The approach is based on the use of a surfactant as a resist to define patterns of silicon oxide nanodomains onto which nanoparticles of iron hydroxide (Fe(OH)3), 1-5 nm diameter, could be deposited. In base growth mode, the SWNTs can grow from the oxide nanodomains. By controlling the location of oxide nanodomains, site-specific growth could be obtained. The iron hydroxide nanoparticles were prepared by hydrolysis of ferric chloride (FeCl3). Patterned hydroxylated silicon oxide nanodomains were created by scanning probe oxidation (SPO) of silicon substrates modified with aminopropyltrimethoxysilane (APTMS, H2N(CH2)3Si(OCH3)3). Due to electrostatic interaction, Fe(OH)3 nanoparticles can be selectively deposited on hydroxyl groups present on silicon oxide nanodomains. To inhibit the assembly of the nanoparticles on a APTMS-coated silicon surface, sodium dodecyl sulfate (SDS) was introduced, which restricted deposition to the hydroxylated nanodomains. A model mechanism for the selective deposition mechanism has been proposed. It was possible to convert the patterned Fe(OH)3 nanoparticles to iron oxide, which served as a catalyst for the site-specific growth of SWNTs. Raman spectroscopy and AFM were used to characterize the nanotubes on the Si substrate. This will offer the possibility for future integration with conventional microelectronics as well as the development of novel devices.

Reproducible Patterning of Single Au Nanoparticles on Silicon Substrates by Scanning Probe Oxidation and Self-Assembly

This paper describes a rational approach for reproducibly patterning single Au nanoparticles, 15-20-nm diameter, on silicon wafer substrates. The approach uses scanning probe oxidation (SPO) to pattern silicon oxide nanodomain arrays on silicon substrates modified with octadecyltrimethoxysilane (OTS). It was usually found using aminopropyltrimethoxysilane (APS) that Au nanoparticles only assembled at the domain boundaries probably because of asymmetrically distributed hydroxyl groups. To generate uniformly distributed hydroxyl groups on oxide domains, we employed a two-step treatment to etch and oxidize the substrate. With this treatment, oxide domains consistently attached Au nanoparticles to maximum capacity. Single Au nanoparticles were readily patterned by fabricating oxide nanodomains with a diameter below 30 nm. We also investigated the deposition of APS on OTS monolayers, which resulted in the assembly of Au nanoparticles outside of the oxide domains, and proposed two alternative methods to inhibit it.

Current, charge, and capacitance during scanning probe oxidation of silicon. I. Maximum charge density and lateral diffusion

A comprehensive analysis of the electrical current passing through the tip-substrate junction during oxidation of silicon by scanning probe microscopy (SPM) is presented. This analysis of experimental results under dc-bias conditions resolves the role of electronic and ionic contributions, especially for the initial stages of the reaction, determines the effective contact area of the tip-substrate junction, and unifies the roles of space charge and meniscus formation. In Part I of this work, we demonstrate that SPM oxidation is governed by a maximum charge density generated by electronic species within the junction at the onset of the oxidation process. Excess charge is channeled into lateral diffusion, keeping the charge density within the reaction zone constant and reducing the aspect ratio of the resulting oxide features. A uniform charge density implies that SPM oxides contain a fixed defect concentration, in accordance with the space-charge model. The effective (electrical) thickness of SPM oxides determined by these defects is investigated by Fowler-Nordheim analysis. We conclude that most of the electrical current involved in high voltage SPM oxidation of Si does not actually induce surface oxide growth, and that lateral diffusion and small aspect ratios are unavoidable aspects of contact-mode conditions.

Current, charge, and capacitance during scanning probe oxidation of silicon. II. Electrostatic and meniscus forces acting on cantilever bending

A comprehensive analysis of the electrical current passing through the tip-substrate junction during oxidation of silicon by scanning probe microscopy (SPM) is presented. This analysis identifies the electronic and ionic contributions to the total current, especially at the initial stages of the reaction, determines the effective contact area of the tip-substrate junction, and unifies the roles of space charge and meniscus formation. In this work, we concentrate on noncontact SPM oxidation. We analyze simultaneous force-distance and current-distance curves to demonstrate that total current flow during noncontact oxidation is significantly less for noncontact mode than for contact oxidation, although the resulting oxide volume is nearly identical. Ionization of water layers and mobile charge reorganization prior to and following meniscus formation is also shown to alter the tip-substrate capacitance and, therefore, the bending of the SPM cantilever.

Measuring electrical current during scanning probe oxidation

Electrical current is measured during scanning probe oxidation by performing force versus distance curves under the application of a positive sample voltage. It is shown how the time dependence of the current provides information about the kinetics of oxide growth under conditions in which the tip–surface distance is known unequivocally during current acquisition. Current measurements at finite tip–sample distance, in particular, unveil how the geometry of the meniscus influences its electrical conduction properties as well as the role of space charge at very small tip–sample distances.

Silicon nanostructures fabricated by scanning probe oxidation and tetra-methyl ammonium hydroxide etching

Fabrication of silicon nanostructures is a key technique for the development of monolithically integrated optoelectronic circuits. We demonstrated that the process of scanning probe microscope (SPM) oxidation and anisotropic tetra-methyl ammonium hydroxide (TMAH) etching is a low-cost and reliable method to produce smooth and uniform silicon nanostructures on a variety of silicon substrates. Etched structures with a pitch of 100 nm, positive- and negative-contrast structures, and features height greater than 100 nm have been produced on bare silicon, and Si3N4-coated and silicon-on-insulator wafers. Evolution of hexagonal pits on two-dimensional grid structures were shown to depend on the pattern spacing and orientation with respect to Si(110) crystal directions. We successfully combined SPM oxidation with traditional optical lithography in a mixed, multilevel patterning method for realizing micrometer-and nanometer-scale feature sizes, as required for photonic device designs. The combination of SPM oxidation and TMAH etching is a promising approach to rapid prototyping of functional nano-photonic devices.

Nanometer-Scale Conversion of Si3N4 to SiOx for Applications in Lithography, Micromachining, and Selective-Area CVD

We present the use of scanning probe oxidation of Si3N4 masks for performing nanolithography, nanomachining, and nanoscale epitaxial growth on silicon. Three most unique features of this approach are presented: (i) exceptionally fast oxidation kinetics using silicon nitride masks (∼30 μm/s at 10 V for a ≈5 nm thick film), (ii) selective-area anisotropic etching of Si using a Si3N4 etch mask, (iii) selective-area chemical vapor deposition of Si using a SiO2/Si3N4 bilayer growth mask.

Nanometer-Scale Conversion of Si3N4 to SiOx for Applications in Lithography, Micromachining, and Selective-Area CVD

We present the use of scanning probe oxidation of Si3N4 masks for performing nanolithography, nanomachining, and nanoscale epitaxial growth on silicon. Three most unique features of this approach are presented: (i) exceptionally fast oxidation kinetics using silicon nitride masks (∼30 μm/s at 10 V for a ≈5 nm thick film), (ii) selective-area anisotropic etching of Si using a Si3N4 etch mask, (iii) selective-area chemical vapor deposition of Si using a SiO2/Si3N4 bilayer growth mask.

Scanning probe oxidation of Si 3N 4 masks for nanoscale lithography, micromachining, and selective epitaxial growth on silicon

For studying the physical, chemical, and electronic properties of ultrasmall man-made structures, the major challenge is to fabricate highly uniform structures and control their positions on the nanometer length scale. Local oxidation of metals and semiconductors using a conductive-probe atomic force microscope (AFM) or other scanning probe microscopes in air at room temperature has emerged as a simple and universal method for this purpose. Here the uses of scanning probe oxidation of Si 3N 4 masks for performing nanolithography, nanomachining, and nanoscale epitaxial growth on silicon are reviewed. The three most unique features of this approach are presented: (1) exceptionally fast oxidation kinetics using silicon nitride masks (∼30 μm/s at 10 V for a ∼5-nm-thick film); (2) selective-area anisotropic etching of Si using a Si 3N 4 etch mask; and (3) selective-area chemical vapor deposition of Si using a SiO 2/Si 3N 4 bilayer growth mask.