Silicon Field Emitter Arrays Research Articles

A field effect spacecraft neutralizer for the LISA Pathfinder mission

We outline the design, construction and testing of a field effect neutralizer, which provides a source of up to 6 mA of electron emission to maintain charge neutrality for the LISA Pathfinder mission spacecraft. The low mass, low power neutralizer uses silicon field emitter arrays and has been engineered for integration into the LISA Pathfinder micro-propulsion system. The silicon emitters are constructed using micro-fabrication techniques and electron beam lithography to ensure uniformity and precise control of emitter location. Control of the manufacture processes, plasma-enhanced vapour deposition, wet and dry plasma etch and various coatings, has been optimized to achieve robust reproducible devices suitable for space use.

Fabrication and characterization of p-type silicon field-emitter arrays for lithography

p -doped silicon field emitters were studied experimentally to assess their usefulness in multibeam electron lithography. Both individual emitters and emitter arrays were fabricated from single crystal Si wafers with several doping levels. Current-voltage curves were measured for different temperatures and illumination conditions. The typical plateaus in the I-V curves and the sensitivity to light known from the literature were reproduced. Stability measurements showed a very stable total emission current even while the angular emission distribution fluctuated strongly, giving unstable currents in apertured beams. Measured light response times varied between 34 ns and 20 μs, depending on experimental conditions. It was found that in the plateau of the I-V curve, the energy of the electrons shifts over up to 100 eV when changing the extraction voltage over a few kilovolts. In operation, when the current is stable, the energy shift is rather unstable. The experimental results are discussed within a model of the emission process involving an induced p-n junction inside the tip. The conclusion is that p-doped silicon field emitters are not particularly useful for applications in electron beam lithography.

シリコン電界放出電子源から放出される電子のエネルギー分析

We propose the application of silicon field emitter array (Si-FEA) to resolve the problem of wafer charging in the ion implantation process. This article reports experiments conducted to review the basic property of Si-FEA. This time, we measured the energy distribution of electrons emitted from three types of Si-FEA with different number of tips and gate apertures. The result is different from that of platinum field emitter array (Pt-FEA) in some points. First, half bandwidth of Si-FEA distribution is a little smaller than that of Pt-FEA. Second, the energy peak of Si-FEA shifts to lower energy side from Fermi level. Such phenomenon doesn't happen in the energy distribution of Pt-FEA.

Fabrication of Polycrystalline Silicon Field Emitter Arrays with Hafnium Carbide Coating for Thin-Film-Transistor Controlled Field Emission Displays

Hafnium Carbide (HfC)-coated polycrystalline silicon (poly-Si) field emitter arrays (FEAs) were fabricated on an oxidized silicon wafer. The HfC coating layer was deposited by inductively coupled plasma-assisted magnetron sputtering. The effect of the HfC thickness on the emission characteristics was investigated by depositing 15-nm- and 30-nm-thick HfC on the tip. The thinner HfC could effectively improve the emission characteristics of poly-Si FEA, while the thicker one degrades the emission. Two methods of tip sharpening were also investigated: one is thermal oxidation sharpening which is commonly used for Si FEA fabrication, and the other is Ar ion sputtering. The apex radius of the thermally oxidized FEA was much smaller than that of the sputter-sharpened one. However, some irregularly shaped tips were found in the oxidized FEA, which are responsible for gate current. The detailed fabrication and emission characteristics, including gate current, will be reported.

Field emission study of titanium silicide array

AbstractA silicon field emitter array (FEA) was fabricated on n‐type, low‐resistance (5Ω·cm) Si(100) wafer by anisotropic etching (dipping in 40 wt.% KOH solutions). To increase the thermochemical durability of the tip, the long‐term stability and the current density, Ti (30 nm) film was deposited with evaporation and the silicide was formed by annealing at 650°C. The C54–TiSi2 phase of the sample was observed by x‐ray diffraction analysis. The emission characteristics of the titanium silicide FEA were compared with those of an Si FEA under high vacuum conditions of ∼10−7 Torr. The turn‐on voltage of the titanium silicide FEA is ∼1500 V whereas that of the Si FEA is found to be 1950 V. In addition, the emission stability of the titanium silicide FEA proved to be much better than that of the Si FEA. Copyright © 2004 John Wiley & Sons, Ltd.

Optimization of silicon field-emission arrays fabrication for space applications

Optimization of fabrication of silicon field-emission arrays has been carried out on 4 in. silicon wafers for field-emission electric propulsion neutralizer application. A mixture of SF6, O2, and Ar for silicon tip etches has been optimized to improve the uniformity and process repeatability. A thin aluminum nitride (AlN) film has been coated on gated emitter arrays to enhance the field at the tip and to protect tips from ion bombardment. A statistical analysis of emission characteristic shows a narrow distribution of the turn-on voltage from array to array, which makes it possible to achieve a few milliamperes current emission by connecting several arrays in parallel. Lifetime tests over 1000 h have been carried out on single arrays, producing in excess of 10 μA of emission current under continuous mode of operation.

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.

Effect of gases on the field emission properties of ultrananocrystalline diamond-coated silicon field emitter arrays

We performed studies of electron emission from ultrananocrystalline diamond (UNCD)-coated, ungated silicon field emitters as a function of in situ exposure to various gases during current versus voltage and current versus time measurements. The emitter arrays were fabricated by a subtractive tip fabrication process and coated with UNCD films using microwave plasma chemical vapor deposition with a CH4/Ar plasma chemistry. The emission characteristics of the coated tip arrays were studied in the diode configuration; using a 2 mm diameter anode with rounded edges to suppress arcing. Significant enhancement of the electron emission was observed, increasing from 35% to 100%, after the emitting surface was exposed to H2 at pressures in the 10−5 and 10−4 Torr range. Upon termination of the H2 exposure, the current decreased to the initial value of 2 μA. The emission current subsequently remained stable at 2 μA upon continued evacuation down to the base pressure below 10−9 Torr. The emission current variation is repeatable with ensuing hydrogen exposure, indicating that the enhancement is due to the hydrogen exposure. Negligible emission current variations are observed at pressures less than 10−5 Torr. Exposure to either Ar or N2 resulted in a reduction of the emission current for ambients of up to 10−5 Torr. This effect is reversible. The effect of the investigated gases on the emission characteristics of UNCD-coated Si tip arrays is attributed to a modification of the effective work function at the localized sites from where electrons are being emitted.

Field-emission characteristics of nitrogen-doped diamond-like carbon film deposited by filtered cathodic vacuum arc technique

We have fabricated silicon field-emitter arrays coated with nitrogen-doped diamond-like carbon (DLC) films which were deposited by a filtered cathodic vacuum arc technique. The DLC coating on silicon emitters improved the emission characteristics including turn-on electric field and emission current. The turn-on electric fields of DLC-coated silicon emitters are 6.8, 4.0, 1.9, and 0.8 V/μm when the nitrogen flow rates are at 0, 5, 7, and 10 sccm, respectively. The turn-on electric field of the non-coated silicon emitter is 16.0 V/μm. The work functions of DLC-coated silicon emitters have been estimated from Fowler–Nordheim plots and were reduced from 1.86 eV in undoped DLC films to 0.47 eV in 10 sccm of nitrogen doped DLC-coated emitter. As the nitrogen doping concentration of the DLC-coated silicon emitters is increased, the field emission properties improved.

Wavelength Dependence of UV Laser Cleaning for Silicon Field Emitter Arrays

The effect of laser irradiation on Si field emitter arrays has been investigated as a function of wavelength using different harmonics of a diode-pumped Q-switched Nd:YLF laser. The laser wavelength was varied as 1047 nm, 523 nm, 349 nm and 262 nm. Infrared (λ=1047 nm) or visible (λ=523 nm) laser irradiation did not change the emission current, while UV (λ=349 nm) laser irradiation increased the emission current due to the cleaning of the emitter tip surface. On the other hand, UV (λ=262 nm) laser irradiation resulted in a temporary reverse current. Desorption gas species were found to be H2O gases during UV (λ=349 nm) laser irradiation. These gas species were related to with the improvement in emission current.

Low leakage current optically gated silicon field emitter arrays

Gated silicon emitters (50×50 emitters at 20 μm spacing) having a “volcano” shape were fabricated by a dry etching-based technique. The insulating oxide layers formed by wet oxidation and low pressure chemical vapor deposition were ∼1 μm thick, and the Cr/Au film was deposited to function as the gate electrode. Quantum efficiency and dynamic range of photoenhanced field emission from gated silicon field emitter arrays improve at the expense of higher dark current (current with no photoexcitation) as the gate voltage is increased. For 830 nm photoexcitation, efficiency approached 29% at 130 V, and ratio of photocurrent to dark current peaked at 5.5 at 80 V. Measured saturation behavior was attributed to screening of the applied field by photogenerated electrons. Optical excitation with ∼10 ns pulses at 1064 nm revealed shorter anode current pulses at increasing gate bias consistent with an increase in collected drift electrons due to an increase in depletion layer thickness. Also, the ratio of anode current to gate current reached a high of 820 at 30 V.

Characterization of electron emitters for miniature x-ray sources

Single tip tungsten emitters, nanocrystalline graphite (NCG) emitters, gated silicon field emitter arrays, thoriated tungsten filament emitters, and micromachined n-type polycrystalline silicon thermionic emitters were investigated as potential electron sources for miniature x-ray devices. The emission sources were characterized in a demountable 3.5 mm diameter Schott glass 8250 tube using a 45° cone angle, 1 mm diameter tungsten anode. X-ray characterization was performed in a custom-built shielded system surrounding the tube using an AMPTEX, model XR-100T-cadmium zinc telluride crystal detector. The best results in terms of reliability and recovery from flashover events were obtained with the NCG emitter. Lateral, distal, and rotational x-ray spectra were obtained for the NCG emitter. Initial integration efforts of mounting the x-ray source on the end of a cardiac catheter yielded 15 kV standoff voltages for a 0.2 mm catheter wall thickness.

Silicon metal–oxide–semiconductor field effect transistor/field emission array fabricated using chemical mechanical polishing

An integrated lightly doped drain-metal–oxide–semiconductor field effect transistor (LD-MOSFET) is used to control the electron supply of a silicon field emission array (FEA), resulting in low voltage switching and stabilization of emission current. The LD-MOSFET-driven field emission array was fabricated using isotropic and anisotropic etching of silicon, oxidation sharpening, chemical vapor deposition, and chemical mechanical polishing. Current–voltage characterization of the LD-MOSFET-driven FEA demonstrated modulation of electron emission by the MOSFET gate voltage and the reduction of current fluctuation.

Silicon field emission arrays with atomically sharp tips: turn-on voltage and the effect of tip radius distribution

We report 1 nm tip radius, 1 /spl mu/m gate-aperture silicon field emission arrays (FEAs) with turn-on voltage as low as 14 V. The low turn-on voltage is attributed to the small emitter tip radius, which was achieved by isotropic etching of silicon and low-temperature oxidation sharpening. Optimization of the oxidation sharpening process reduced the tip radius to less than 1 nm and was confirmed by transmission electron microscopy (TEM). The tip radius has a log-normal distribution with a peak at 0.75 nm, an expected value of 1.8 nm, and shape parameter of 0.74 nm. Current-voltage (I-V) characteristics of the field emission devices are in agreement with Fowler-Nordheim (FN) theory. The extracted tip radius using two-dimensional (2-D) numerical simulation showed good agreement with the TEM measurements.

Fabrication of Single–Crystal Silicon Field Emitter Array on Glass Substrate

The fabrication process of a single–crystal silicon field emitter array on a glass substrate has been investigated. Single–crystal silicon emitter tips are prepared by anodization of a silicon wafer having an n/p junction. Emitter tips are transferred to the glass substrate by anodic bonding. The anodic bonding is carried out at temperatures lower than 300°C. The conditions of the fabrication process and the field emission characteristics are reported. It is shown that the temperature of the emitter tips rises to the melting point of silicon by Joule heating. This phenomenon is shown to be peculiar to the emitter tips on a glass substrate which has a low thermal conductivity. The influence of Joule heating at the emitter tip, during electron emission, on the emission characteristics is also reported.

Anomalous field emission from Al2O3 coated Si tips

The emission properties of Al2O3 coated Si field emitters were found to be radically different from any others reported to date. Silicon field emitter arrays were coated with ultrafine aluminum oxide powders by dielectrophoresis. The emission threshold, corresponding to a current of 10pA, was 0.25V/mm. The current–voltage (I–V) plots of the Al2O3 coated Si field emitter arrays were steep, with a distinct deviation from Fowler–Nordheim (F–N) behavior. The emission showed a slow but distinct increase or decrease of current with time depending on the magnitude of the current. This behavior could be approximated with an exponential function. We suggest a tentative explanation in terms of “field emission induced secondary electron emission”, i.e. the generation of a secondary electron cascade by the interaction of the field emitted electrons with crystallites comprising the porous aluminum oxide coating.

Recent Developments of Field-Emission Research. Improvement of Silicon Field Emitters by Surface Modification.

Field emission from individual tips in n- and p-type silicon field emitter arrays (FEAs) has been directly evaluated by using an electrostatic lens projector. The number of the emission spots in the p-type FEA increased with increasing total emission current, and the p-type FEA exhibited better uniformity of the emission than the n-type FEA. In order to improve the emission uniformity, the operation of the FEAs under an atmosphere of ambient gas was executed. The emission uniformity was improved dramatically by exposing C2H4 gas. Also, surface modification of Si field emitters was investigated for damageless vacuum sealing. The vacuum sealing process reduced the emission currents from the nontreated Si FEA by a factor of 10. On the contrary, the emission current from the CHF3-plasma-treated FEA did not change before and after the vacuum sealing process.

Emission-uniformity improvement and work-function reduction of Si emitter tips by ethylene gas exposure

Emission uniformity improvement of a silicon field emitter array (FEA) by ethylene (C2H4) gas exposure has been investigated in detail especially about its origin. Scanning Maxwell-stress microscopy has been utilized to evaluate the surface potential at the Si tip apex in the FEA before and after the improvement. Careful consideration to eliminate the influence of the surrounding gate has been made in order to estimate work function at the tip apex. It has been found that the work function at the tip apex decreased upon C2H4 exposure. It has been clarified that the uniformity improvement by C2H4 exposure is mainly due to the reduction in the tip work function.

Synthesis of Ordered Single Crystal Silicon Nanowire Arrays

The fast and inexpensive production of single-crystal silicon nanowires, which circumvents the use of lithography, is achieved by pyrolysis of silane in nanochannel alumina with a gold catalyst. The nanowires (see Figure for an SEM image) are very straight, not intertwined, and uniform in diameter and length. They should be good candidates for silicon field emitter arrays with high tip density.

Fabrication and electrical characterization of high aspect ratio silicon field emitter arrays

Micromachined high aspect ratio field emitter areas have been realized to obtain stabile and uniform emission current. Dry etching based on gas chopping method was used to etch the 42-μm-high ultra-sharp emitter tips in single-crystal silicon. Diamond-like carbon (DLC) films of 25–100 nm thickness were deposited from hydrocarbon precursor gas on the Si tips applying inductively coupled plasma enhanced chemical vapor deposition (ICPECVD) technique. Partial stabilization of emission current versus time due to oxygen or nitrogen plasma treatment could be related to surface passivation and/or formation of stable DLC/nanolayer on the emitter surface. It has been shown that DLC-coated emitters provide a comparable good long-term emission stability—the emission current did almost not decrease during a 46 h operation cycle. Summarizing we can say that high aspect ratio field emitter areas seem to have favorable field emission properties in respect to long-term stability.