Silicon Microtips Research Articles

Fabrication of Silicon Microtips with Integrated Electrodes

This paper describes a simple method to fabricate silicon microtips with integrated self-aligned polarization electrodes for development of MEMS and electron field emission devices. The method is based on the wet bulk micromachining of the silicon substrate in KOH solutions and utilizes low stress PECVD SiOxNy films obtained at low temperatures (320°C) as structural material for both mechanical support and electrical insulation of the electrodes. For the electrodes sputtered Cr films were utilized. The microtip formation process was studied by optical and electronic microscopy to analyze tip geometry and the characteristic etch rate of the different stages during the tips formation. The fabricated devices were matrixes with different numbers of microtips, each with a typical height of 52 μm and with diameter at the apex below 1 μm. In the best case, the distance between the apex of the tips and the metallic electrode was lower than 5 μm. The results also show that the low stress SiOxNy film is essential to attain the necessary flatness required by the process.

Field-Emission Cathodes Based on Microchannel Plates

The existing methods of fabricating low-field cathodes do not permit the development of device structures that comply with the requirements of developers of systems. In this work, large-area field-emission cathodes with homogeneous emission properties of the working surface and low working voltages (<1 kV) are considered. A Spindt cathode with a number of silicon microtips up to 6000 and a packing density of ~1 × 105 cm–2 is investigated. Titanium nitride and carbon films are deposited onto microtips using the electric-arc method. It is shown that the cathode has low emission homogeneity due to the problem of reproducing microtips of the same shape and size. A cathode based on a microchannel plate with channels 6 μm in diameter, inside which graphite-like nanostructures are formed by the electric-arc method, is fabricated. It is found that an increase in the electron flux in the channels of a microchannel plate can result in a considerable decrease in the operating voltage (<1 kV) and attain high emission homogeneity at the highest admissible output current.

Fabrication of hollow microtip electrode array with variable diameter using self-aligned etching process

In this paper, we propose the fabrication of a hollow microtip electrode array with variable diameter. The intended application of the fabricated electrode is the perfusion or aspiration at the localized area of a biological sample in a microfluidic system. The conductive electrode covered with an insulating layer formed the hollow microtip structure. The lower diameter and height of the fabricated hollow microtip electrode array were measured as approximately 4 and 35 μm, respectively. A fabrication process for controlling the hollow diameter is proposed based on the self-aligned etching process of the sharp silicon microtip array without an additional photolithography mask. The silicon microtip was exposed only at the tip end by using the self-aligned etching process after coating the conductive and insulating layer. The area of the exposed silicon tip could be controlled by varying the self-aligned etching time, which determined the hollow dimeter after removing the silicon structure. The hollow diameter was adjusted from 0.94 to 2.82 μm depending on the self-aligned etching time. Moreover, the proposed hollow microtip electrode array was characterized as the electrode to be used in electrochemical reaction measurements using cyclic voltammetry. Additionally, the perfusion of the fluid was demonstrated through the hollow microtip electrode assembled with a microfluidic channel.

Microfluidic platform for cell analysis using through-polydimethylsiloxane micro-tip electrode array

In this study, we present the fabrication and measurement of a microfluidic system combined with a through-polydimethylsiloxane (PDMS) microtip electrode that can be used to measure the photosynthetic reaction of green algae cells in the microfluidic channel. The conventional single-cell based method has a disadvantage of low efficiency. To solve this problem, we propose a method of electrical feed-through interconnection of a microfluidic channel platform using a microtip electrode array. Silicon microtip arrays were fabricated using a combination of processes using deep reactive ion etching (DRIE) and reactive ion etching (RIE) steps. In the RIE process, the shape of the cylindrical silicon pillar changed from a cylinder to a probe shape according to the local change of the side-wall etching rate of the silicon pillar. The local ultra-micro electrode (UME) structure was fabricated using a self-alignment fabrication method without additional photolithography steps, and its electrochemical properties were confirmed using a cyclic voltammetry (CV) experiment. The photosynthetic algae cells were stacked in the PDMS microfluidic chip and then combined with the microtip electrode array using an acrylic jig. After assembling the microtip electrode array, the microtip was confirmed to penetrate the PDMS thin film through the CV experiment. When irradiated with light, the current induced by the oxygen produced by a photosynthetic reaction in the algae cells (stacked in the PDMS microfluidic chip) was measured using the microtip electrode array. Further, experiments were conducted to measure the force applied by the acrylic jig when combining the microtip electrode array with the PDMS microfluidic chip.

Development and Investigation of a Field Emission Medium for Autocathodes of Mobile Power Microwave Devices

Silicon-diamond heterostructure based field emission media with silicon microtip arrays at the heterointerface were proposed and experimentally studied. The architecture of the heterostructures is optimized for the applications as an active medium for the field emission cathodes of mobile power microwave devices.

Fabrication and measurement of microtip-array-based LSPR sensor using bundle fiber

In this paper, we present a micro electro mechanical systems (MEMS)-technology-based localized surface plasmon resonance (LSPR) sensor using a uniformly distributed noble metal nanostructure array on a silicon microtip. The conventional LSPR sensor based on chemically synthesized metal nanoparticles exhibits a lack of reproducibility, which is caused by rearrangement and irregular sizes of metal nanoparticles. This problem strongly affects the LSPR sensing performance and reliability. To overcome this problem, we propose a novel fabrication process of the LSPR sensor using the microtip. The deposited gold is exposed using a self-aligned etching process at the tip end and it is used as gold nanoparticles. The whole process for the fabrication of the microtip is possible using a single photolithography mask. The detailed fabrication process is introduced and analyzed. In addition, the improved reproducibility in terms of fabrication and measurement of the fabricated sensor is assessed by reliability tests. Moreover, the LSPR signals with varying refractive indices of media are measured to verify the feasibility of the microtip-based LSPR sensor system.

Conductive microtip electrode array with variable aspect ratio using combination process of reactive ion etching

A micromachining process for a conductive microtip electrode array has been developed using a combination process of reactive ion etching. A localized conical ultra-micro electrode (UME) with submicron width is realized on the tip end for electrochemical measurements. A height of several tens of microns of the silicon microtip is achieved, and the area beyond the UME is covered with dielectric material for electrical insulation. A high-aspect-ratio silicon microtip is fabricated using anisotropic and isotropic etching with varying the gap and diameter of the etch masks. Cyclic voltammetry is conducted to characterize the electrochemical properties of the UMEs. The results indicate that the electrical connection on the UME works successfully, and the measured peak currents were found to be in good agreement with the values estimated from analysis based on conical UME theory.

Silicon Microtips with Self-Asligned Integrated Electrodes

In this work we present a method for the fabrication of silicon microtips with self-aligned anodes in order to obtain field emitting devices. The method is based on the anisotropic corrosion of the silicon substrate in KOH solutions and utilizes low stress SiOxNy films obtained by PECVD as masking material. These films are also utilized as structural material for mechanical support and electrical insulation of the electrodes. For the electrodes sputtered Cr films are utilized. Optical microscopy was utilized to determine the etch rate of the different stages during the microtip formation, in order to enable us to ascertain the total development time versus the initial mask dimensions. Matrixes with different number of microtips, 52 μm height and with diameter at the vertex bellow 1 μm, were fabricated and characterized. The results also show that the low stress SiOxNy film is essential to endure the corrosion process without structural damage.

Silicon Microtips Arrays Fabricated by HI-PS Technique for Application in Field Emission Devices

This work presents results of silicon field emission devices fabrication from HI-PS technology. A dedicated system based on virtual instrumentation was developed to characterize samples electrically. From this system, it was possible to extract I-V curves and to analyze the behavior of devices related to the electrical emitting field obtained for each one. Si microtips arrays were compared by extracting the field enhancement factor g from the curves and applying it as a quality parameter. The g factor differences between the samples (about 0.46 and 0.59) indicate a good improvement of emission characteristics of devices in comparison with Si planar samples.

Fabrication of self‐aligned gated silicon microtip array using electrochemical silicon etching

AbstractIn this paper we report a new approach to fabricate gated silicon microtip arrays for field emission applications using a single mask process. The key role in the fabrication process is played by the photoelectrochemical etching of silicon in HF‐based electrolytes. This technique is here exploited to produce highly uniform silicon microtip arrays. The fabrication process is detailed and the dependence of the tip geometry on the electrochemical etching parameters is discussed. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Application of ion beam etching technique to the direct fabrication of silicon microtip arrays

A relatively simple and effective method to directly fabricate large-area field emission microtip arrays on n-type silicon wafers is proposed. The presented fabrication approach of silicon microtip devices mainly involves photolithography, thermal shaping and consolidation, argon ion beam etching and sputtering deposition. The measurements show that the silicon field emission microtip devices fabricated have good structural uniformity and electrical characteristics. The center-to-center spacing of microtips fabricated is 50μm and the typical microtip height about 11μm. The scanning electron microscope analysis and the surface style measurements are carried out for the surface morphologies of silicon microtips, such as square-bottom pyramid-shaped microtips, cone-shaped microtips, square-bottom circle-microtips, and circle-bottom circle-microtips. Electrical measurements are performed to obtain the typical field emission properties of the devices. The experiment results show that the method utilized can be applied to fabricate silicon field emission microtip arrays and circle-microtip arrays of larger area.

Mechanism responsible for initiating carbon nanotube vacuum breakdown.

We report a physical mechanism responsible for initiating a vacuum breakdown process of a single carbon nanotube (CNT) during field emission. A quasidynamic method has been developed to simulate the breakdown process and calculate the critical field, critical emission current density and critical temperature beyond which thermal runaway occurs before the CNT temperature reaches its melting point. This model is in good agreement with experiments carried out with a single CNT on a silicon microtip.

Amorphous hydrogenated carbon films used as masks for silicon microtips fabrication in a reactive ion etching with SF 6 plasma

A room temperature fabrication process for silicon microtips has been developed using amorphous hydrogenated carbon films as masks for silicon etching. Reactive ion etching using an SF 6 plasma has been employed to sharpen the microtips without any thermal oxidation technique.

Silicon field emission arrays coated with CNx thin films

Arrays of silicon micro-tips were made by etching the p-type (1 0 0) silicon wafers which had SiO2 masks with alkaline solution. The density of the micro-tips is 2×104 cm−2. The Scanning Electron Microscope (SEM) photos showed that the tips in these arrays are uniform and orderly. The CNx thin film, with the thickness of 1.27 μm was deposited on the silicon micro-tip arrays by using the middle frequency magnetron sputtering technology. The SEM photos showed that the films on the tips are smoothly without particles. Keeping the sharpness of the tips will benefit the properties of field emission. The X-ray photoelectron spectrum (XPS) showed that carbon, nitrogen and oxygen are the three major elements in the surfaces of the films. The percents of them are C: 69.5%, N: 12.6% and O: 17.9%. The silicon arrays coated with CNx thin films had shown a good field emission characterization. The emission current intensity reached 3.2 mA/cm2 at 32.8 V/μm, so it can be put into use. The result showed that the silicon arrays coated with CNx thin films are likely to be good field emission cathode. The preparation and the characterization of the samples were discussed in detail.

Enhancement in field emission of silicon microtips by bias-assisted carburization

Ultrathin carbon layers with thicknesses below 50 Å have been deposited on silicon microtip arrays by bias-assisted carburization (BAC) using microwave plasma chemical vapor deposition. The tip radius of these silicon tips is reduced below 55 nm under low deposition temperature. The field emission characterization has been performed in a high-vacuum environment. An enhancement in the field emission is observed of about 3 orders of magnitude in BAC silicon microtips over untreated silicon microtips. With an applied voltage of 1100 V, emission currents of 80 and 120 μA have been achieved for the films grown (at dc bias of −200 V for 40 min) with 15% and 25% CH4/H2 gas ratio, respectively. An emission current of 40 μA has been achieved for the film grown (at dc bias of −300 V for 40 min) with 3.5% CH4/H2 ratio. The BAC silicon emitter has good emission stability at a constant voltage of 1100 V. These investigations indicate that further improvement of this technology will lead to simple and inexpensive field emission display devices.

Atomic level analysis of silicon emitters utilizing the scanning atom probe

In order to examine the effect of chemical treatment on silicon emitter surfaces, photolithographically processed, mechanically grooved, and crushed silicon specimens were atom-by-atom mass analyzed before and after the soakage in hydrofluoric acid (HF) by the scanning atom probe. Silicon microtips fabricated by the photolithography were found to be most heavily contaminated before the HF treatment. After the acid soakage, the (111)-oriented silicon surface is found to be most resistive to the HF treatment while other specimens are vulnerable to contaminants, possibly due to defects and disorders on the surface layers. The Fowler–Nordheim (FN) curves were plotted for all specimens. Generally, the HF treatment lowered the work function of the specimens which field evaporate hydrogen as H+ and H2+.

Process development of gated field emitter arrays with dry etched amorphous silicon microtips on glass substrates

A low temperature fabrication process for gated field emitter arrays is developed with etched amorphous silicon microtips on glass substrates. Reactive ion etching using a SF6/O2 plasma is employed to sharpen microtips without any thermal oxidation or wet etching technique. Field emitter arrays with a triode structure are successively fabricated by utilizing plasma enhanced chemical vapor deposition and electron beam evaporation for a SiO2 gate oxide and a niobium gate, respectively.

Fabrication and Characterization of Various Carbon-Clad Silicon Microtips with Ultra-Small Tip Radii

Various types of ultra sharp Si microtips and multitips with carbon-clading films were fabricated by microwave plasma chemical vapor deposition (MPCVD). The radii of these Si tips prepared by bias assisted carburization (BAC) can be reduced below 300 Å under a low deposition temperature (<550°C). Field emission characterization was performed in a high vacuum environment. With an applied anode voltage of 1100 V, emission currents of 169 µA, 198 µA, and 385 µA can be achieved from an array of 50×50 BAC-clad Si monotips, Si multitips via high bias, and Si multitips via the Ar presputtering technique, respectively. Both the auger electron spectroscopy (AES) and X-ray photo-electron spectroscopy (XPS) studies of the C 1 s peak suggest that the BAC-cladding is more likely to be a carbon-rich SiC layer or a SiC layer mixed with a small amount of diamond nuclei. This BAC-carbon can be used as an effective nucleation layer for further diamond nuclei. Due to the low field emission, low temperature, and large area growth capability, the sharp BAC-clad Si multitip field emitter arrays are attractive for flat panel display applications.

Development of the Scanning Atom Probe and Atomic Level Analysis

A scanning atom probe (SAP) was developed by modifying an ultrahigh vacuum scanning tunneling microscope. A unique feature of the SAP is the introduction of a funnel-shaped scanning microextraction electrode to a conventional atom probe. The electrode confines the high electric field required for field evaporation of the apex atoms in an extremely small space between the open hole of the extraction electrode and the apex of a micro cusp on a specimen surface. The incomparable capability of the SAP was successfully demonstrated by atom-by-atom analysis of microcusps of a thin diamond layer grown by chemical vapor deposition and microsilicon tips fabricated by a lithographic process. The study has shed light on the distributions and binding states of hydrogen in the diamonds and of oxygen and carbon in the silicon microtips for the first time.

Atom-by-atom analysis of microtip emitter surfaces by the scanning atom probe

Unique capabilities of the scanning atom probe, atomically high resolution and atom-by-atom mass analysis, were utilized to investigate the compositional distribution in the individual apexes of a silicon microtip array and fine grains of chemical vapor deposition diamonds. The dry-etched silicon tips contain a large amount of carbon and hydrogen and the HF-treated tips contain even oxygen and fluorine. Although the carbon concentration of the uppermost surface layer is as high as 50% for the dry-etched tip, it decreases to less than 10% for the second layer and approaches to the constant concentration of less than 2% at the depth of 20 nm suggesting the carbon intermixture during the fabrication and/or etching process of the microtip array. The carbon concentration in the HF-treated tip decreases from 18% to 8% at the depth of 30 nm. On the other hand, the oxygen concentration stays fairly constant at around 25% throughout the analysis. The high hydrogen concentration in the diamonds is attributed to the large difference between the activation energies for hydrogen desorption, 21 kcal/mol, and for methane adsorption, 7.3 kcal/mol.