Molybdenum Field Emitter Research Articles

Design, Fabrication and Characterization of Molybdenum Field Emitter Arrays (Mo-FEAs)

We report on the fabrication of highly uniform field emitter arrays (FEAs) with an integrated self-aligned extraction gate from bulk molybdenum. All critical dimensions of the emitter tip were determined by a single process step of Inductively Coupled Plasma (ICP) etching. In addition, the height difference between the emitter tip and the gate plane was controlled by the thickness of the SiO2 dielectric layer. A 10 µm gate aperture molybdenum-FEAs (Mo-FEAs) at a typical 20 µm pitch with 6 µm height was achieved with 8.4 mA/cm2 current density at gate voltages of 110 V and the turn-on field of 1.4 V/µm. These self-aligned Mo-FEAs could be expanded to active larger areas to increase the emission current.

Bulk molybdenum field emitters by inductively coupled plasma etching.

In this work we report on the fabrication of inductively coupled plasma (ICP) etched, diode-type, bulk molybdenum field emitter arrays. Emitter etching conditions as a function of etch mask geometry and process conditions were systematically investigated. For optimized uniformity, aspect ratios of >10 were achieved, with 25.5 nm-radius tips realised for masks consisting of aperture arrays some 4.45 μm in diameter and whose field electron emission performance has been herein assessed.

Stable, ruggedized, and nanometer-order size transfer mold field emitter array in harsh oxygen radical environment

Extremely sharp, uniform, stable, rugged, low operating voltage, and nanometer size molybdenum field emitter arrays (Mo FEAs) were developed by transfer mold fabrication. These can be used in stable and reliable vacuum nanoelectronic devices such as those used in electric propulsion engines and in environmentally demanding applications. Nanometer size transfer mold Mo FEAs were fabricated with base lengths of 36–370 nm. The 36 nm base length transfer mold Mo FEAs are the smallest FEA's ever reported. The tip radii were as sharp as 2.8–3.5 nm. The turn-on fields of the transfer mold Mo FEAs with a base length of 36–370 nm were found to be as low as 9.0–23.6 V/μm for the short anode-to-apex distance of less than 10 μm. These are lower than those of conventional FEA's such as carbon nanotube FEAs, Spindt-type FEAs, and Gray-type FEAs with approximate turn-on fields of 50–600 V/μm. They can thus be used in reliable vacuum nanoelectronic devices. The emission fluctuation of the 36 nm base length transfer mold Mo FEAs without a resistive layer were found to be as low as ±1.6% and this value is the lowest ever reported. Conventional FEAs with and without resistive layers have values of 5%–100% and higher than 100%, respectively. The field emission characteristics of the FEAs have been evaluated by in-situ oxygen radical treatment with a flux of 1015 atoms cm−2 s−1. This value is 107–108 times higher than that of typical low earth orbit fluxes. The environmental characteristics of the transfer mold Mo FEAs were compared with those of transfer mold Ni FEAs. The turn-on fields of the transfer mold Mo FEAs changed slightly from 10.8 to 22.5 V/μm at 1 min from the initiation time. It was almost saturated at 29.4 V/μm upon 5 min of in-situ oxygen radical treatment. A rapid increase in the turn-on field from 14.9 to 59.4 V/μm upon in-situ radical treatment for 3 min was observed for the Ni FEAs. The experimental work functions measured by ultraviolet photoelectron spectroscopy and the work functions calculated by the slope of Fowler–Nordheim plots for the transfer mold Mo FEAs changed slightly from 4.3 to 5.2 eV and from 4.3 to 5.3 eV, respectively, after oxygen radical treatment for 5 min. The calculated work functions are similar to the experimental work functions because the transfer mold Mo FEAs have extremely uniform geometric factors and stable emission characteristics. The emission fluctuation rates determined without the radical treatment of the transfer mold Mo FEAs without a resistive layer were found to be as low as ±1.7%. Even with in-situ radical treatment for 30 min the transfer mold Mo FEAs exhibited remarkably stable emission without spike noise and with fluctuations as low as ±4.5%. Therefore, the extremely stable and nanometer size transfer mold Mo FEAs are expected to be useful in the production of vacuum nanoelectronic devices.

Nanosecond pulsed field emission from single-gate metallic field emitter arrays fabricated by molding

Electrically gated pulsed field emission from molybdenum field emitter arrays was studied. Single-gate field emitter array devices supported by metallic substrates were fabricated by a combination of molding and a self-aligned gate process. Devices were tested in a low-inductance cathode holder compatible with the high-acceleration electric field of a pulsed diode gun. Pulsed field emission down to 1.1 ns was observed for single-gate devices with 1.2×103–1.2×105 emitter tips with 5 μm array pitches. Integrating the field emitter arrays in a high-voltage pulsed diode gun, the authors demonstrated nanosecond field emission at an acceleration field of 30 MV/m at the cathode surface and acceleration of the field emission electron beam up to 300 keV. In addition, transverse beam emittance of the single-gate devices was measured with two different array sizes.

Fabrication of all-metal field emitter arrays with controlled apex sizes by molding

The authors proposed a method to fabricate field emitter arrays with uniform apex diameters in tens of nanometer scale based on the molding technique and apply it to fabricate molybdenum field emitter arrays. Apex diameter equal to 23±5nm was observed in a 6×6 tip array by high-resolution scanning electron microscope. They also studied the field-emission characteristics in devices with gate electrodes fabricated on top of the arrays by a self-aligned process. In single-gate devices, emission current of up to 20μA per tip with negligible gate leak current was observed. The gate-fabrication process was extended to fabricate double-gated emitters. Further optimization of the fabrication process for higher emission current, together with metallurgical and lithographic methods, is discussed.

Application of Anisotropic Conductive Film to Fabrication of Molybdenum Field Emitter Arrays Using Transfer Mold Technique

In the fabrication of molybdenum field emitter arrays (Mo FEA) by the transfer mold technique, anisotropic conductive film (ACF) was applied to the bond between the inverted mold structure and the transferred glass substrate. Without any electrical treatment of electrostatic bonding, the inverted mold was successfully bonded to an indium tin oxide (ITO) glass substrate under optimized thermal and pressure conditions. No additional conductive layers were used in the bonding process, and the bonded ACF was not chemically affected in the wet-etch process of the silicon inverted mold structure. The fabricated Mo FEA was structurally and electrically investigated and an anode current of 10 nA per emitter was obtained at a gate bias of 94 V. The results demonstrate the possibility of selective conduction in the fabrication of transfer mold FEA using ACF bonding.

Effect of ageing process on performance of molybdenum field emission arrays

The paper describes the beneficial effect of application of ballast resistors and an ageing process on the stability of field emission from Mo-tips. High-resistant silicon substrates were used as ballast resistor and better emission uniformity was obtained as compared with low-resistant silicon. The emission current of the field emitter arrays (FEAs) increased by 43% and 50% by DC and pulse mode ageing, respectively, and the emission current fluctuation as a function of time was decreased to 3.7%.

Gas-induced current decay of molybdenum field emitter arrays

Field emission displays can be seriously limited by current degradation of Mo field emitter arrays, primarily due to inadequate vacuum conditions inside their thin vacuum packages. Most lifetime studies describe the deleterious effects of short exposures to oxygenic gases on device performance. Here, we review experimental results on the emission characteristics of molybdenum field emitter arrays in a variety of gas ambients for extended periods. We find that even in inert gas, the emission current degrades rapidly, comparable to that observed in oxygenic gas ambients. Surprisingly, after exposure when the device is again operated under high vacuum conditions, various levels of current recovery can be observed. No evidence for ion sputter damage is observed. Existing degradation models do not provide an adequate explanation for this behavior. Rather, we suggest that these results point toward a model common for all gaseous ambients based on shallow implantation of ions into the field emitter tips. This creates a near surface layer that modifies the tunneling barrier, and thus subsequently the emission current. Support for this mechanism comes from several experimental observations. The implant degradation model is also consistent with devices that incorporate a field emitter ion pump process as a means to reduce pressure in the sealed vacuum package.

Laser-induced surface cleaning of molybdenum field emitter arrays for enhanced electron emission

Visible laser irradiation on molybdenum field emission arrays (Mo–FEAs) was performed as one efficient cleaning method in order to etch off any unnecessary oxidation layers on the FEA surface. Scanning electron microscopy and high-resolution transmission electron microscopy showed clear removal of ultrathin MoO oxide layers at the emitter edge through a photoinduced thermal process. A sharp surface morphology of the emitter tips was also observed due to the crystallization or thermalmigration effect during the laser exposure. The structural enhancement of the FEA was strongly confirmed by a remarkable increase of 40% in the emission current after laser exposure.

Electrical aging of molybdenum field emitters

As an approach to improve the stability and reliability of molybdenum field emitters with rough surfaces and impurities at tip apex, electrical aging was studied on the basis of the important tip heating mechanism, Nottingham heating, in dc and pulse mode. The effects of electric field in dc mode were investigated using single tips to avoid the averaging effect commonly arising in arrays. The fluctuations of emission currents were remarkably reduced after the exposure to high electric field (∼100 MV/cm). It was responsible for surface migration and impurity desorption at tip apex achieved under high electric field. Since the continuous exposure to high electric field can bring undesirable effects such as tip failures, electrical aging in pulse mode was also investigated. As the peak voltage of pulse was increased, the fluctuation of emission current was rapidly reduced. From the electrical aging experiments for a 0.7 in. field emission display (FED), it was proven that the poor uniformity of FED could be considerably improved by electrical aging.

Emission statistics and the characterization of array current

The current from an array of field emitters is often dominated by a subset of tips. The impact of processes such as sputtering damage on the evolution of array current can be estimated based on a hyperbolic tip model if the distribution of the emitters in both work function and apex radius can be approximated. In this work, we describe the characterization of emission under the assumptions that (i) the emission site radii are log-normal distributed and (ii) that the degree to which work-function-increasing adsorbates are present is related to background pressure. These assumptions generate statistical factors that may be used to generalize a single tip theory to array current estimates via the statistical hyperbolic model. A derivation of the statistical factors are given and the methodology of the hyperbolic model’s application explained. The methodology is then applied to analyze experimental data for molybdenum field emitters coated with ZrC under development for spacecraft electric propulsion and electrodynamics tethers devices.

Double-gated Spindt emitters with stacked focusing electrode

Arrays of molybdenum field emission cones with integrated focusing electrode (IFE-FEA) were studied as a potential device structure for field emission displays (FEDs) that achieve high brightness, high luminous efficiency, and longer lifetime, without loss of resolution. Device operation and optimization were examined both qualitatively and analytically. Double-gated devices were fabricated using the Spindt cone process. Extensive electrical characterization of IFE-FEAs was conducted. Preliminary optical measurements demonstrated the desired focusing effect.

Dynamic stability of field emission from molybdenum microtips exposed to oxygen

The emission current in a molybdenum field emission array can decrease by 50% in 1000 s at an oxygen pressure of 10−6 Torr. To overcome this disadvantage of molybdenum microtips, the effectiveness of dynamic surface cleaning has been investigated in a single-aperture gated-diode configuration. For dynamic surface cleaning, tip surface oxide buildup is balanced by tip oxide removal due to sputtering by ions created in ionizing collisions with field-emitted electrons. The present results demonstrate stable dynamic cleaning with clean and partially oxidized molybdenum tips for currents ranging from 10−11 to 10−9 A with oxygen exposures of up to 1000 L. For currents above 10−9 A, ion bombardment causes the tip shape to become unstable leading to failure with increasing oxygen exposure.

8.3: Operation of FEAs in Hydrogen and the Development of Thin Film Metal Hydrides for Integration into FEDs

AbstractWe demonstrate that having hydrogen inside the operational ambient of field emission displays results in significantly improved display life. However, improving device performance by incorporation of gas phase hydrogen into thin glass packages is a difficult task. In this paper, we report on the development of solid state hydrogen sources utilizing thin film metal hydrides, which can be easily integrated into field emission displays. We show that these metal hydride thin films provide a practical means to incorporate solid state hydrogen sources into field emission displays. Using stainless steel anodes coated with TiH2 thin films, we demonstrate the successful operation of molybdenum field emitter arrays without emission current loss for 400 hours of continuous operation. Comparable arrays operated in the absence of hydrogen, but in an ultrahigh vacuum had emission current degradation of over 50% in less than 100 hours of operation.

Argon inclusion in sputtered films and the effect of the gas on molybdenum field emitter arrays

Residual gas analysis of a number of field emission displays showed that argon desorbed from molybdenum metal lines was the dominant gas in sealed vacuum packages. We present experimental results on the emission characteristics of molybdenum field emitter arrays in argon ambient. In argon, the emission current dropped rapidly similar to that in oxygenic gas ambients. Existing degradation models do not provide an adequate explanation for this behavior. Rather, we suggest a model based on shallow implantation of argon into the field emitter tips that increases the effective width of the tunneling barrier. Experimental support for this model comes from the following observations: emission current degraded only when the device was turned on; after gas exposure, significant current recovery which followed diffusion type behavior was noted; degradation and recovery rates were functions of partial pressure; and no detectable effects associated with sputtering were observed. This mechanism is also consistent with ion pumping known to occur in field emission displays.

Oxidation of Molybdenum Thin Films and its Impact on Molybdenum Field Emitter Arrays

AbstractOxidation of emitter surfaces can be a serious problem for Mo field emitter arrays. We studied the oxidation and related changes in the electronic properties of Mo thin films as a function of annealing temperature. Experiments were done on Mo thin films prepared on Si and sodalime glass substrates. These films were thermally oxidized and characterized using a variety of techniques including x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and thermal desorption spectroscopy (TPD) methods. For films oxidized below 400°C, partial oxidation was observed, with MoO3(110) being the principal oxide phase. However, at a temperature of 500°C and above, oxidation of the film was complete. Electrical characteristics of the films undergo a rapid transition from semiconductive to highly insulating at temperatures between 475 to 500°C. Temperature programmed desorption spectra showed that the oxides are stable at elevated temperature with only a principal O2 desorption peak at approximately 786°C.

P‐39: Degradation Mechanism of Spindt‐Type Molybdenum Field Emitter Arrays by Oxidation and Surface Chemical Modification

AbstractOxidation and chemical modification of molybdenum micro tip surface have been investigated to understand the performance degradation mechanism of field emitter arrays(FEAs). Molybdenum FEAs could be easily oxidized due to their interaction with oxygen‐containing species including O2, CO2 and H2O during cathode fabrication or thermal sealing processes of field emission displays(FEDs). During device operation, outgassing from phosphors due to electron‐stimulated desorption and from other components inside a panel such as cathode, spacers, and sealant can lead to the chemical modification of the emitter surface. The oxidation and chemical modification of the emitter surface degrade the emission characteristics, resulting in the instability of an emission current, an increase of an electron extraction voltage for a given current, and lifetime reduction of the device. After the vacuum sealing and operation of the FED device, a micro tip is observed, and compared with an as‐grown one by using scanning electron microscopy(SEM) and transmission electron microscopy(TEM). Chemical composition analyses of the degraded emitter surface by nano‐probed Auger electron spectroscopy(AES) suggest that molybdenum micro tip is contaminated with S, In, C, and O after the device operation, where S and In seem to originate from the dissociation of phosphors by electron bombardment.

Study on Depletion Mode Operation of Protruding-Tip Field Emitter Triodes

ABSTRACTElectrical characteristics of Spindt-type Molybdenum (Mo) field emitter triode devices with varied emitter tip-height have been studied based on device modeling and experiment. Potential and electric field distributions with varied the emitter tip-height has been simulated. It is observed that the electric field of the top of the higher emitter tip was strongly affected with the anode-gate distance and the anode voltage compared to conventional field emitter triode device. Experimental results with varied different tip-height were in good agreement with that of calculated results. We present the possibility of “depletion mode” field emitter triode device.

Growth of a near-atomic protrusion on molybdenum field emitter tips under argon ion bombardment

Ar + ion bombardment of a molybdenum field emitter is shown to grow a single protrusion with a near-atomic dimension at the very tip of the emitter. This kind of protrusion is generated from a thick amorphous layer covering the monocrystalline emitter tip, and lengthens toward the incoming ions. The protrusion apex is roughened, resulting in atomic projections on its surface. This surface phenomenon may be developed into a technique to prepare metal tips with atomically small dimensions.

Field emitter based extractor gauges and residual gas analyzers

Attempts at using the Spindt-type molybdenum field emitter arrays in the extractor gauges and a residual gas analyzer are presented in this article. The sensitivity of the field emitter gauge is as high as 11 Torr−1. The departure from linearity of the pressure versus ion current measurements did not exceed 10% over the pressure range of 10−10–10−6 Torr. Stable sensitivities for nitrogen, helium, and hydrogen were achieved below 10−7 Torr with the field emitter residual gas analyzer. The slightly reduced emission current and sensitivity, after long-term operation, are of concern and need to be addressed. Residual gas spectra indicate that when using field emitters, the electron stimulated desorption ions (O+, F+, and Cl+) are reduced as compared to those made using a hot filament source.