Si Tip Arrays Research Articles

Quasi-Saturated Arsenic Concentration and Uniform Electron Emission by Regulating Thermal Oxidation of Si Nanotips

The improving of tip-to-tip electron emission uniformity in an array is one of the essential issues for the vacuum microelectronic/nanoelectronic applications. Here, we report the achieving of quasi-saturated ultrahigh arsenic concentration ( $\sim 6.7\times 10^{{21}}$ /cm3) at the Si nanotip apex, in the mechanisms of dopant segregation during the thermal oxidation and the limitation of arsenic solubility in Si. The tips with quasi-saturated high-level arsenic concentration possess a well tip-to-tip uniformity in surface work function. The measured work function for the tips in an array with saturated dopant concentration is in a narrow range of 4.347–4.371 eV (~2.1% fluctuation). This merit not only resulted in high emission current density (~418.36 mA/cm2) from the devices at a relatively lower gate voltage but also brought well spatially uniform electron emission in an array. The work inspired vital insight into the interpretation for mechanisms in dopant density control and the improving of emission uniformity of the Si-tip array. It could be used as an essential approach to acquire high-performance Si-based vacuum electronic devices.

Investigations on the long-term performance of gated p-type silicon tip arrays with reproducible and stable field emission behavior

The authors report on the fabrication and characterization of p-type Si tip arrays with an integrated gate electrode for applications as field emission electron sources. After the reactive ion etching of the emitters, the combined thermal dry and wet oxidation was used for both the sharpening of the emitters and for the realization of an enhanced insulation layer. Au was evaporated in a self-aligned process as gate electrode. Arrays of 16 Si tips were fabricated with tip heights of about 3 μm and tip radii of about 20 nm with integrated gate electrode concentrically positioned ≈2 μm below the tip apex. Integral measurements with an additional anode showed improved field emission properties with a reproducible and stable emission behavior. A fast activation of the tips, low onset voltages of about 30 V, and moderate field emission currents up to 0.55 μA were noticed. The field emission parameters were calculated using the Fowler–Nordheim characteristics. A pronounced saturation regime was observed, and current fluctuations of less than ±1% were investigated for 30 min. Long-term measurements were carried out for a period of more than 8 h. In the first 6 h of operation, the authors observed a drift of the emission current from 0.35 to 0.55 μA caused by an increased emission surface.

Role of carbon nanotube interlayer in enhancing the electron field emission behavior of ultrananocrystalline diamond coated Si-tip arrays.

We improved the electron field emission properties of ultrananocrystalline diamond (UNCD) films grown on Si-tip arrays by using the carbon nanotubes (CNTs) as interlayer and post-treating the films in CH4/Ar/H2 plasma. The use of CNTs interlayer effectively suppresses the presence of amorphous carbon in the diamond-to-Si interface that enhances the transport of electrons from Si, across the interface, to diamond. The post-treatment process results in hybrid-granular-structured diamond (HiD) films via the induction of the coalescence of the ultrasmall grains in these films that enhanced the conductivity of the films. All these factors contribute toward the enhancement of the electron field emission (EFE) process for the HiDCNT/Si-tip emitters, with low turn-on field of E0 = 2.98 V/μm and a large current density of 1.68 mA/cm(2) at an applied field of 5.0 V/μm. The EFE lifetime stability under an operation current of 6.5 μA was improved substantially to τHiD/CNT/Si-tip = 365 min. Interestingly, these HiDCNT/Si-tip materials also show enhanced plasma illumination behavior, as well as improved robustness against plasma ion bombardment when they are used as the cathode for microplasma devices. The study concludes that the use of CNT interlayers not only increase the potential of these materials as good EFE emitters, but also prove themselves to be good microplasma devices with improved performance.

Enhanced electron field emission properties from hybrid nanostructures of graphene/Si tip array

High efficiency with excellently stable electron field emitters based on monolayer graphene coated on well-aligned Si tip (graphene/SiT) arrays fabricated by a simple transfer method is demonstrated.

An optically switchable emitter array with carbon nanotubes grown on a Si tip for multielectron beam lithography

In this paper we report the design, fabrication and evaluation of a field emitter array of carbon nanotubes (CNTs) on a Si tip with a pn junction. Electron beam emission can be switched on by laser irradiation. The Si tip array is formed on a 5 μm-thick Si membrane. Each emitter consists of CNT emitter tips and a gate electrode. On the apex of the Si tip, CNTs are grown in order to emit electrons at a low extraction voltage. Additionally, the pn junction is formed into emitter tips. Optical switching of an array consisting of nine emitters is demonstrated. The electron beam switching is synchronized with laser irradiation successfully. The emission current and its on/off ratio are approximately 40 nA and 4.

Stable field emission of single B-doped Si tips and linear current scaling of uniform tip arrays for integrated vacuum microelectronic devices

Advanced Si-based semiconductor technology is most suitable to fabricate uniform nanostructures as integrated field emitter arrays for novel vacuum electronic devices. In order to improve the field emission homogeneity and stability of p-type silicon tip arrays for pulsed sensor applications, the authors have systematically studied the influence of the fabrication parameters on the tip shape and on the specific operating conditions. Based on detailed design calculations of the field enhancement, they have fabricated two series of hexagonal arrays of B-doped Si-tips in a triangular arrangement. The first (second) type contains three (four) patches with different number of tips (1, 91, 547 and 1, 19, 1027, 4447 for the first and second type, respectively) of about 1 (2.5) μm height, ∼20 (20) nm apex radius, and 20 (10) μm pitch. The field emission properties of both individual tips and complete arrays were investigated with a field emission scanning microscope at a pressure of 10−9 mbar. The current plateau of these tips typically occurs at about 10 (3) nA and around 65 (25) V/μm field level. In this carrier saturation range, single tips provide the highest current stability (<5%) and optical current switching ratio (∼2.5). Fairly homogeneous emission of the tip arrays leads to an undershooting of the expected linear scaling of the mean plateau current as well as to a much improved current stability (<1%).

Electron spectrometer in adjustable triode configuration for photo-induced field emission measurements

We have constructed a new ultrahigh vacuum apparatus with a triode configuration for the systematic investigation of photo-induced field emission (PFE) from metallic or semiconducting cathodes. These are exposed to electric fields up to 400 MV∕m and laser irradiation by means of hole or mesh gates. Cathodes and gates are in situ exchangeable and adjustable with high precision to ensure a homogeneous extraction of electrons which are partially transmitted to the fixed electron spectrometer. Its hemispherical sector analyzer provides an energy resolution limit of 8 meV. The commissioning of the measurement system has been performed with a tungsten needle. Its temperature showed up in the high-energy tail of the electron spectrum, while its work function was derived from the spectral low-energy part combined with the integral current-voltage curve. First PFE measurements on B-doped Si-tip arrays yielded a small field emission current increase under green laser illumination. A shift and splitting of the energy spectra was observed which revealed different emission regimes as well as the photosensitivity of the cathode due to carrier excitation into the conduction band. For the full exploitation of the PFE system, a tunable laser over a wide eV-range is required.

Field emission properties of Si tip arrays coated with N-doped SrTiO3 thin films at different substrate temperature

The effect of substrate temperature (TS) on the behavior of field emission, microstructure, optical band gap, and the surface energy of N-doped SrTiO3 thin films coated on silicon tip arrays has been examined in detail. Results indicate that the TS dominates the chemical states of nitrogen added to the sputtered SrTiO3 films and hence the observations. At the critical temperature of 600 °C, nitrogen atoms incorporate into the oxide film with sp-hybridization features. The generation of the nonbonding lone pair states narrows the optical band gap and the lone pair induced antibonding dipoles lower the threshold field for electron emission substantially. At lowered TS, molecular adsorption of nitrogen dominates. Contact angle measurements further evidence for the presence of antibonding dipole states at the surfaces which is responsible for the adsorbate-induced surface stress.

Effects of sputtering pressure on the field emission properties of N-doped SrTiO 3 thin films coated on Si tip arrays

The influence of sputtering pressure on the electron emission properties of Si tips coated with N-doped SrTiO 3 ultrathin films was investigated. X-ray diffraction studies revealed that the N-doped SrTiO 3 films deposited at different pressures remain the perovskite structure. However, the threshold electric field of electron emission decreased markedly when the sputtering pressure is increased, and reached a minimum value of 17.37 V/μm while deposited at 1.6 Pa. The decrease in the threshold field is attributed to the narrowed band gap and the lowered surface energy of SrTiO 3 thin films with nitrogen doped, as confirmed using spectroscopic ellipsometry and water contact angle measurement. Furthermore, it is revealed using XPS that such sputtering pressure dependence is accompanied with the change of nitrogen bonding state in the films, which changes from poorly screened γ-N 2 state to atomic β-N state when the sputtering pressure is increased. A mechanism of bonding and band-forming was proposed for the enhanced electron emission with nitrogen incorporation in the sputtered SrTiO 3 films.

Electrical and emission properties of nanocomposite SiOx(Si) and SiO2(Si) films

The electrical and emission properties of as deposited and annealed SiOx(Si) films have been investigated. The films with thicknesses of 10–100nm were obtained by thermal evaporation of silicon powder in vacuum (2.7–4.0)×10−3Pa on flat Si substrates and Si tip arrays. The atomic force microscopy investigations of the surface morphology indicated the presence of nanoprotrusions on surface of the initial SiOx(Si) films with height up to 20nm and curvature radius of the nanoprotrusions about 3–5nm. As a result of the thermal annealing, the film surface becomes more uniform and its morphology is characterized by nanoprotrusions with height in the range of 1–3nm. At low electric fields the I-V characteristics of dark current through the initial SiOx(Si) films correspond to Poole-Frenkel transport mechanism. The Fowler-Nordheim tunneling dominates at higher electric fields. As to annealed SiO2(Si) films, the modified Fowler-Nordheim electron tunneling through trapezoidal SiO2 barrier between silicon nanoclusters restricts the current flow. The effective electron field emission from Si tip arrays coated with nanocomposite SiOx(Si) and SiO2(Si) films was observed. The results of the electron field emission from surface of SiOx(Si) films into vacuum show an emission current density of 0.25×10−5A∕cm2 at macroscopic electric fields of (5–6)×105V∕cm. The field emission from thermally annealed samples was not observed in the whole range of the applied voltages. In the contrary, for the case of thermally annealed samples subjected to the following partial etching in HF:H2O solution, the emission increases in comparison with initial samples. In this case the field emission appears already at an electric field of 1.5×105V∕cm and the maximum current density increases up to the value of ∼3×10−5A∕cm2. The current peaks are revealed on emission I(V) characteristics built in J(E) coordinates. The models for explanation of peculiarities of the electrical conductivity and electron field emission from nanocomposite films are discussed.

Electron field emission from silicon tip arrays coated by magnetron sputtering carbon nitride film

The field emission properties of silicon tip arrays coated by magnetron sputtering carbon nitride films (CN x ) were investigated. The coated Si tip arrays with the height of 3 μm and the density of 10 6 tips/cm 2 exhibit good field emission properties with the lowest threshold field of 2 V/μm, which is much better than that of uncoated Si tip arrays. The results demonstrate that CN x films effectively lower a surface potential barrier to improve the electron field emission properties of silicon tips. In particular, it was found that the variation of deposition conditions of CN x films coated on the Si tips had a significant influence on the field emission property of Si tip arrays. The dc substrate bias could reduce the field emission property of silicon tips, but a higher nitrogen partial pressure and moderate input power are favorable to the enhancement of the field emission property of Si tips. The effects of deposition conditions on the electron field emission are attributed to the change of sp 2 C–N bonds content in the CN x films and the apex shape of Si tip.

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.

Effect of oxygen on field emission properties of ultrananocrystalline diamond-coated ungated Si tip arrays

The effect of O2 exposure on field emission properties of ultrananocrystalline diamond (UNCD) coated ungated Si tip arrays is reported. The behavior of current degradation during O2 exposure and current recovery following evacuation of O2 strongly depended on the initial current/electric field applied to the emitters. At a low initial current of 50 μA/electric field of 5.8 V/μm, O2 exposure (1×10−7, 5×10−7, and 1×10−6 Torr) resulted in a rapid decrease in current in the beginning, followed by a slow decrease. The current made a strong recovery toward the original value when O2 was pumped out. With an increased initial current of 180 μA/electric field of 10.3 V/μm, the currents recovered only to a lesser extent after O2 was evacuated. At an even higher initial current of 930 μA/electric field of 20.5 V/μm, the current decreased steadily and finally stabilized after an exposure of 720 L with only 34% degradation. To explain these observations, we propose a hypothesis of oxygen interaction with the UNCD surface.

CMOS Application of Single-Grain Thin Film Transistor Produced Using Metal Imprint Technology

A novel technology which is termed metal imprint has been developed for positioning and growing large thin-film Si grains on amorphous substrates. The metal imprint is carried out using Ni coated Si tip arrays and crystallization is carried out in solid phase. This technology enables us to fabricate a grain-boundary-free thin-film transistor (TFT) in a single-grain by aligning the position of the channel and that of the imprinted site. Single-grain TFTs having the channel in the single-grains which were fabricated using a high temperature process showed the field effect mobility up to 450 cm2/Vs for n-channel and up to 260 cm2/Vs for p-channel. Single-grain TFTs showed better threshold controllability than TFTs fabricated on Si films prepared by conventional solid phase crystallization (SPC). Complementary-metal-oxide-semiconductor (CMOS) circuits composed of single-grain TFTs have been fabricated. CMOS inverters composed of single-grain TFTs showed superior transfer characteristics to that composed of SPC-polycrystalline Si TFTs. The 3-µm CMOS ring oscillator which operated with the supply voltage down to 2 V showed the propagation delay time of less than 6.7 ns/stage at 7 V.

Uniformity and stability of field emission from bare and metal coated Si tip arrays

Field emitter arrays of tips of some micrometers in height have been fabricated from p- and n-doped Si substrates by photolithographic techniques and partially coated with 7.5 nm thick W, Pt, or Cr layers by magnetron sputtering. The emission uniformity of these tip arrays was investigated with a field emission scanning microscope. The local current stability was measured with a spectrum analyzer. The bare Si tips show low average extraction fields (∼35 V/μm for 10 nA) but moderate uniformity and pronounced current noise and switches. In comparison, the metal-coated Si tips yield improved emission uniformity with up to 100% of emitting tips and sufficient stability up to 700 nA (1 μA) in the case of Pt (W) coating, but also a factor of 2 higher extraction fields. Noise power spectra of W and Pt coated samples showed a typical 1/fγ dependence up to the kHz range with a spectral density index γ of about 3.

Electron field emission characteristics of electrochemical etched Si tip array

Two-dimensional arrays of Si tips for electron field emission were fabricated using combination of photolithography and electrochemical etching. The array has good uniformity and near optimum shape for field emission. The current–voltage characteristics were measured and fitted with the Fowler–Nordheim model. The derived field enhancement factors are among the best figures of merit for electron field emitters. The result shows that Si tip array fabricated with this process has potential applications in vacuum microelectronics.

Microscopic field emission investigation of nanodiamond and AlN coated Si tips

We have investigated the suitability of nanodiamond (ND) and AlN coated Si tip arrays for cold cathodes by means of a field emission scanning microscope combined with scanning electron microscopy. The ND coated tips required an average extraction voltage U(10 nA) of 360 V for 10 μm electrode distance and yielded stable emission up to currents of 10–50 μA. The rather nonuniform emission distribution of the arrays (80 V<U(10 nA)<1800 V) was correlated with the varying morphology of the ND tips. Field-enhancing nanoprotrusions were observed at the apex of the strongest emitters. The AlN coated tips showed strong activation effects at emission currents below 10 nA or after annealing at 250 °C resulting in an average U(10 nA) of 410 V, improved lateral emission uniformity (150 V<U(10 nA)<600 V), and stable currents up to 1–10 μA. Direct high current processing of such emitters led to a partial destruction of the apex. Different field emission mechanism will be discussed for ND and AlN coatings.

Review of synthesis of low-work function Cu–Li alloy coatings and characterization of the field emission properties for application to field emission devices

Alkali metals have extremely low-work functions and are therefore expected to result in significant enhancement of the electron emission if they are used as coatings on Mo or Si microtip field emission arrays (FEAs). However, the alkali metals are physically and chemically unstable in layers exceeding a few nanometers in thickness. Maximum enhancement of electron emission occurs for alkali metal layers 0.5–1 monolayer thick, but it is extremely difficult to fabricate and maintain such a thin alkali metal coating. We review here our work of the last three years focused on developing Cu–Li alloy coatings for application to field emitter Spindt-type tip arrays and fabrication of edge emitters with low-voltage high electron emission characteristics for field emission devices. The Cu–Li alloy coatings provide a chemically and thermally stable, self-replenishing, segregated lithium monolayer on the surface of the host Cu layer, resulting in a substantial reduction of the work function of the alloy, compared to that of a thick Li layer or a pure Cu surface. We present results that show a 13-fold reduction in the threshold voltage for electron emission, compared with uncoated Si FEAs. The Cu–Li alloy coatings yielded tip and edge emitters with threshold voltages of 2–10 V/μm, depending of the coating thickness and Li concentration in the alloy. New results are presented that show that we can now coat gated Si tip arrays with Cu–Li alloy films, which results in very low emission voltage.

Fabrication of sub-10nm Si-tip array coated with Si 3N 4 thin film for potential NSOM and liquid metal ion source applications

A nanosize Si tip array for various purposes has been fabricated on the n-type (100) low resistive (0.05 ohm-cm) Si wafers using electron beam direct writing method. The Si pillars with diameter less than 0.1 μm can be potentially utilized for a near field optical microscope tip or a liquid metal ion source application. SiO 2 layers with different thicknesses of 500nm and 1.5μm were initially thermally grown and followed by the fabrication of the 500nm dot array pattern with an electron beam(EB) resist SAL60. The oxide etch-mask layers with two different thicknesses of 500nm and 1.5μm were patterned with a magnetic field enhanced reactive ion etcher. The 500nm EB resist layer (SAL601) was used as an etch-mask for pattern formation during oxide etching. The complete removal of the EB resist and the erosion of the 1.5μm thick oxide dot-patterns resulted from the ∼270 second etching period was observed. On the other hand, no erosion of the oxide mask with the EB resist remaining was found after 90 sec. oxide etching by scanning electron microscope. The two step RIE processes on the sample with a 500nm oxide etch mask were carried out with an isotropic SF 6 etching followed by the anisotropic (Cl 2 + He) etching in order to fabricate tall Si-pillar arrays with ∼4 μm height. Finally, the 35nm Si 3N 4 coated Si-tip array using a low pressure chemical vapor deposition (LPCVD) was fabricated. The LPCVD Si 3N 4 coated Si-tip array will be micromachined into sub-100nm metal aperture for the potential near field optical sensor or into a sub-10nm Si or Si 3N 4 aperture for liquid metal ion source.

High current density field emission from arrays of carbon nanotubes and diamond-clad Si tips

Arrays of carbon nanotubes (CNT) and diamond-clad Si tips were grown by microwave plasma-enhanced chemical vapor deposition. The former ones were grown directly on prepatterned cobalt-coated silicon substrate, while the latter ones were grown on Si-tip arrays. Each array contains 50×50 emitting cells and each individual cell is 3 μm square. A maximum effective emission current density of about 17 A/cm2 (at a macroscopic field of 17.5 V/μm) has been demonstrated, while a macroscopic emission current density of 10 mA/cm2 with operating fields around 10 V/μm can be routinely achieved from an array of CNT emitters. In contrast, operating fields above 20 V/μm were needed to draw a comparable emission current density from all of the diamond-clad Si tips arrays. Emission stability test performed at 40 mA/cm2 for CNT arrays also showed little sign of degradation. Due to the high efficiency of electron emission, simple sample process, and large area growth capability, field emitter arrays based on CNT are attractive for flat panel display applications.