Atomic Force Microscope Nano-oxidation Research Articles

3-D finite element calculation of electric field enhancement for nanostructures fabrication mechanism on silicon surface with AFM tip induced local anodic oxidation

ABSTRACTThe atomic force microscope (AFM) can be used in dynamic tipping mode as an effective lithography technique capable of manufacturing nanometer sized devices on the surface of a silicon wafer with a higher resolution surface characterization. The most difficult challenge in fabrication of nanostructure based on AFM nano-oxidation approach is the way of controlling an electric field between a cantilever probe tip and a silicon wafer. A water bridge builds up between the tip and the wafer, resulting in the oxidation due to the high electric field in the region. A reconstructive AFM system for nano-oxidation with a tapping model, implemented in an air, was developed. The presented AFM implements the increasing of electric field intensity by analyzing the impact voltage from −1 V to −5 V, electric field, and ion concentrations at the ambient/oxide and oxide/silicon interfaces, while the growth of thin oxides assumes a single liquid/silicon interface, which is modeled as an infinitely long conducting plane. Therefore, particle distribution for the surface charge density is generated for topography simulations. Based on the control-parameters, an enhanced electrical field of up to 1010 V/m can be obtained, which provides a powerful support for controllable experimental study of nanostructures fabrication with AFM tip induced local anodic oxidation. This showed the dependence of applied voltage types and various nanostructures and life time of AFM tip, by controlling tip-sample position in nanolithography processes is an important factor for controlling the aspect electric field distribution. And the effect of different parameters on enhancement distribution was performed and analyzed in this paper.

Fabrication of single titanium oxide nanodot ultraviolet sensors by atomic force microscopy nanolithography

We report on the fabrication of single titanium oxide nanodot (ND) ultraviolet sensors by atomic force microscopy (AFM) nanolithography. A single titanium nanowire is first fabricated by AFM nanomachining and gold contact electrodes are then created by photolithography. By subsequent AFM nano-oxidation, a single titanium oxide ND sensor is produced. Two types of ND sensors, namely ohmic contact and Schottky contact, have been obtained and the sensitivities are around 0.25 and 320, respectively, under ultraviolet illumination. The rise and the reset times of the Schottky contact sensor are also significantly faster.

A method for simulating Atomic Force Microscope nanolithography in the Level Set framework

During the last decades it has been shown that the Atomic Force Microscope (AFM) can be used in non-contact mode as an efficient lithographic technique capable of manufacturing nanometer sized devices on the surface of a silicon wafer. The AFM nanooxidation approach is based on generating a potential difference between a cantilever needle tip and a silicon wafer. A water meniscus builds up between the tip and the wafer, resulting in a medium for oxyions to move due to the high electric field in the region. A simulator for nanooxidation with a non-contact AFM, implemented in a Level Set environment, was developed. The presented simulator implements the growth of thicker oxides by analyzing the potential, electric field, and ion concentrations at the ambient/oxide and oxide/silicon interfaces, while the growth of thin oxides assumes a single liquid/silicon interface, which is modeled as an infinitely long conducting plane. The nanodot shapes have been shown to follow the electric field and hence the surface charge distribution shape; therefore, a Monte Carlo particle distribution for the surface charge density is generated for two-dimensional and three-dimensional topography simulations in a Level Set framework.

Effect of the tip-sample contact force on the nanostructure size fabricated by local oxidation nanolithography

Nanofabrication technique based on atomic force microscope (AFM)/scanning tunneling microscope (STM) can be applied to the fabrication of semiconductor micro/nano devices. Here, many oxide wires on Si surface were fabricated using conductive contact mode and tapping mode AFM in ambient atmosphere. The experiments show that tip-sample contact force plays an important role in electric-field-induced anodization with AFM. It is proved that the height and aspect ratio of oxide structures fabricated by contact mode AFM nano-oxidation technique will decrease gradually with increasing contact force between the tip and the sample. Compared with oxide structures fabricated by contact mode AFM, nanofabrication based on tapping mode AFM gives higher aspect ratio and height of oxide structures and narrower oxide width because of decreased contact force. In addition, field-induced oxidation by tapping mode AFM gives higher flexibility on the control of oxide size and aspect ratio by varying the oscillation amplitude, the range of the damping ratio with about 10%~38% can optimize the height and aspect ratio of oxide structures.

Formation of Ni-Silicide Nanowires on Silicon-on-Insulator Substrates by Atomic Force Microscope Lithography and Solid Phase Reaction

Nickel silicide nanowires were fabricated by atomic force microscopy nano-oxidation on silicon-on-insulator substrates, selective wet etching, Ni deposition and a solid phase reaction. Si nanowires with various widths can be fabricated using oxide lines with various widths, which can be controlled by tuning the sample bias during nano-oxidation. When the ratio of the thickness of Ni films to the height of Si nanowires was 1.8, the Ni-rich silicide phases were obtained and were depended on the width of the Si nanowires. Ni atoms on top of and around the Si nanowires participated in the formation of silicide. NiSi nanowires were fabricated by measuring the width of nanowires accurately and tuning the thickness of Ni films that were estimated on the basis of 1 of Ni/Si atomic ratio. Resistivity of the NiSi nanowires increased as their width decreased, mainly because of the polycrystalline structure of nanowires and the electron scattering by grain boundaries.

Imaging resonant modes in photonic crystal nanocavity by atomic force microscope nano-oxidation

Electric field distributions of resonant modes in a photonic crystal nanocavity were imaged using atomic force microscope nano-oxidation. A grid pattern of nanosize oxides was grown on the nanocavity to perturb the resonant modes. The perturbation caused a shift in the resonant wavelength that was proportional to the local electric field intensity of the resonant mode. The experimentally obtained field intensity images agreed excellently with the finite-difference time-domain calculations. The measured resonant mode images had high spatial resolution and image contrast, owing to the extremely local perturbation of the atomic force microscope oxidation technique.

Patterning at the nanoscale: Atomic force microscopy and extreme ultraviolet interference lithography

We present our recent results achieved using atomic force microscopy nano-oxidation and nano-indentation of thin layers of polymers, and using extreme ultraviolet interference lithography. We focus our attention on the advantages and drawbacks of such lithographic techniques. In particular, we show the possibility to create patterns on polymers compatible with the conventional metal deposition and lift-off processes on submicrometer scale (EUV-IL and nano-indentation) and we experimentally confirm the theoretical prediction on the strong dependence of the oxidation process to the sample conductivity.

Growth behavior of oxide nanostructures by electrical and thermal conductivities of substrate in atomic force microscope nano-oxidation

We report the growth behavior of oxide nanostructures according to physical properties such as work function, electrical and thermal conductivities, and roughness for high resolution nanostructure fabrication. Among these factors, threshold voltages, in particular, which induced the formation of a water meniscus and driving voltage, which drive oxyanions for oxidation, decreased as the mobility of electrons increased by the increasing electrical conductivity. Oxide growth increased as the diffusion of OH radical increased by the increasing conductivity of thermal energy. The high electrical and thermal conductivities imply that the reaction of the OH radical and surface was more easily activated over a wide reaction region (in the parallel direction of substrate) by the conductivity of the generated thermal energy at a low driving voltage. On the basis of these conductivity effects, the Cr film, which is the most sensitive to electron transport and conductivities, had hill-shaped nanostructures and could be applied as a candidate for high-speed atomic force microscope lithography at the lowest driving voltage. In addition, Ta and Ti, which are less sensitive, can be used to fabricate nanostructures with a high aspect ratio (spike shape).

Tuning photonic nanocavities by atomic force microscope nano-oxidation

The authors demonstrate a technique to achieve high-precision tuning of photonic crystal nanocavities by atomic force microscope nano-oxidation of the cavity surface. Relative tuning between two nanocavity modes is achieved though careful choice of the oxide pattern, allowing them to restore the spectral degeneracy conditions necessary to create polarization-entangled quantum states. Tuning steps less than the linewidth (1Å) of the high quality factor modes are obtained, allowing for virtually continuous tuning ability.

Mechanisms of p-GaAs(100) surface by atomic force microscope nano-oxidation

Nanopattering using atomic force microscope (AFM) has become an important area of research, for both fundamental research and future nanodevice applications. Local oxidation of p-GaAs(100) surface by using a negatively biased conductive AFM tip is a universal method for this purpose. The dependences of the height, aspect ratio and volume on applied anodization times and voltages during which the anodization voltage is applied were studied. We explore the kinetics and mechanisms of the anodization process and how factors such as the electric field strength and the relative humidity influence its growth rate and the contribution of ionic diffusion. The results revealed that the protruding oxide dot's height, aspect ratio and volume increase during longer anodization time and at larger anodization voltage as well as in higher relative humidity conditions. The high initial growth rate (∼300 nm s−1 for 10 V) decreases quickly with decreasing electric field strength and the oxide practically ceases to grow at an order of (2–3) × 107 V cm−1. Auger electron spectroscopy measurements confirm that the modified structures take the form of anodized p-GaAs(100). Also, the contribution of ionic diffusion increases by about 80% at a higher relative humidity. In addition, the nanohardness of the oxide structures was measured with the aid of an AFM-based nanoindentation technique.

Selective growth of carbon nanotubes on nickel oxide templates created by atomic force microscope nano-oxidation

The selective growth of carbon nanotubes on nickel oxide templates of square blocks of size 2×2 μm 2 on Si by a microwave-heated CVD has been successfully demonstrated. By applying a negative bias to a conductive tip, nickel oxide patterns were first created by the process of nano-oxidation. The unoxidized nickel film was then removed using a wet chemical etch technique, thereby forming nickel oxide templates over which the carbon nanotubes were grown. We also obtained single carbon nanotubes on nickel oxide nanodots that have diameters as small as 100 nm. The selective growth of CNTs over nickel oxide templates could be also fulfilled on not nickel-etched substrate by appropriately controlling the growth temperature and growth time.

Selective growth of vertically aligned carbon nanotubes on nickel oxide nanostructures created by atomic force microscope nano-oxidation

We report the selective growth of carbon nanotubes (CNTs) on nickel oxide catalytic templates created by atomic force microscope nano-oxidation. Nickel oxide patterns are first produced on a nickel film by applying a negative bias to the tip. After removing unoxidized nickel, oxide patterns or nanodots with a smallest size of around 30 nm are successfully fabricated. Vertically aligned bunched or single CNTs with diameters of 30–80 nm are selectively grown on the oxide nanostructures by inductively coupled plasma chemical vapor deposition. It is also shown that the tube diameter can be effectively controlled by the dot size.

Magnetic nanostructures fabricated by the atomic force microscopy nano-lithographytechnique

A local oxidation technique using atomic force microscopy (AFM) was performed inorder to modify magnetic domain structures in ferromagnetic nanostructures.Co-based nanostructures with a rectangular shape were fabricated by using electronbeam lithography followed by AFM nano-oxidation. After fabricating a Co-oxidenanowire across a Co nanodot by AFM nano-oxidation, a domain structure ofthe dot observed before the nano-oxidation was divided into two parts of thedomain. AFM nano-oxidation for a higher aspect ratio of height/width of thefabricated nanostructures was also investigated. By applying a dc bias between anAl2O3/Ni double layer film and the AFM tip, the Ni layer was locally oxidized through the cappedAl2O3 insulating layer. The nanowires of oxide obtained on theAl2O3/Ni double layer film were thicker and had a higher aspect ratio of height/width than thoseobtained on a Ni single layer film.

Fabrication of nickel oxide nanostructures by atomic force microscope nano-oxidation and wet etching

We report the fabrication of nickel oxide nanostructures by atomic force microscope nano-oxidation and subsequent wet etching. By applying a negative bias to a conductive tip, nickel oxide patterns are first created by the process of nano-oxidation. The unoxidized nickel film is then etched away in a diluted nitric acid solution. Auger electron spectroscopy measurements confirm the complete removal of the nickel film and the preservation of the oxide patterns. Nickel oxide nanodots with diameters as small as 100 nm are reliably produced by the present method.

Room Temperature Coulomb Diamond Characteristic of Single Electron Transistor Made by AFM Nano-Oxidation Process

To obtain a clear signal of the single electron transistor (SET) at room temperature, low capacitance of the SET is indispensable. This SET also has need of the fabrication of the tunnel junction and island with extremely small size. In this paper, three new processes will be proposed to achieve these requirements. First, we applied a single-wall carbon nanotube (SWNT) atomic force microscopy (AFM) cantilever in the AFM nano-oxidation process to obtain less than 10-nm-wide tunnel junctions and a smaller island size of the SET. Secondly, we have succeeded in developing an atomically flat quartz (SiO2) substrate with a smaller relative dielectric constant, which could reduce the SET capacitance to half that of our previous SET on the Al2O3 substrate. Finally, we established the thermal oxidation squeezing process, which can make the island smaller in size after the fabrication of the SET. The thermally squeezed SET showed a clearer Coulomb diamond characteristic than the previously fabricated SET at room temperature.

Single Wall Carbon Nanotube Cantilever: Fabrication and Application.

A single wall carbon nanotube (SWNT) has an extremely small diameter of 1∼2 nm and a high aspect ratio. For the use of the SWNT as an atomic force microscopy (AFM) cantilever, the SWNT was grown directly onto the top of the conventional silicon (Si) AFM cantilever using chemical vapor deposition (CVD) at 900oC in flowing CH4 gas. The length of the SWNT was hardly controlled in the growing process. Therefore, we cut the SWNT and adjusted its length at the top of the cantilever by bending and by applying the bias between the cantilever and the conductive substrate with monitoring the force curve and the vibration amplitude of the cantilever. The resolution of AFM using the optimized SWNT cantilever was compared with the one using conventional Si cantilever by observing the Au surface and TiOx lines fabricated utilizing the AFM nano-oxidation process. The radius of the SWNT cantilever was estimated by observing the width of the DNA in air.

Experimental and theoretical results of room-temperature single-electron transistor formed by the atomic force microscope nano-oxidation process

The single-walled carbon-nanotube (SWNT) was grown directly onto the top of the conventional Si atomic force microscopy (AFM) cantilever. This SWNT AFM cantilever was introduced into the AFM nano-oxidation process, which oxidized the titanium (Ti) metal film on the atomically flat α-Al2O3 substrate and formed the ultranarrow oxidized titanium (TiOx) line of 5 nm width. This TiOx line was used as the tunnel junction of the single-electron transistor (SET), and the SET fabricated by this process showed room-temperature Coulomb oscillation with periods of 1 V. It was determined by three-dimensional simulation that the tunnel-junction capacitance shows only weak dependence on the tunnel-junction width.

Room-temperature single-electron memory made by pulse-mode atomic force microscopy nano oxidation process on atomically flat α-alumina substrate

A single-electron memory was fabricated using the improved pulse-mode atomic force microscopy nano oxidation process which oxidized the surface of the thin titanium (Ti) metal on the atomically flat α-alumina (α-Al2O3) substrate and formed the narrow oxidized titanium (TiOx) line that works as a tunnel junction for the device. This single-electron memory consists of the multitunnel junction and a memory capacitance. The single-electron transistor, which works as an electrometer, was connected to the memory node of the single-electron memory to detect the potential change of the memory node by the injection of the individual electrons. The fabricated single-electron memory showed the hysteresis loop even at room temperature by the return trip of the memory bias when starting from 0 to 10 V and again coming back to 0 V. About 25 electrons were stored at the memory node.

Metal-Based Room-Temperature Operating Single Electron Devices Using Scanning Probe Oxidation

Coulomb oscillation was clearly observed at room temperature in the single electron transistor fabricated by atomic force microscopy (AFM) nano-oxidation process. In order to obtain a clear Coulomb oscillation at room temperature, new and improved fabrication processes and measurement systems such as a pulse-mode AFM nano-oxidation process and a triaxial active feedback measurement system are introduced. The Coulomb oscillation peaks appear with the period of 1.9 V at the drain bias conditions of 0.25 V and 0.3 V. The current modulation rate ranges from 20% to 30%.