Paul Schwoebel

Vacuum microelectronic devices

In this review/tutorial paper, we cover the history, physics, and current status of vacuum microelectronic devices. First we overview the performance requirements of vacuum microelectronic devices necessary for them to replace, or fill voids left by, solid state devices. Next we discuss the physical characteristics of micro-field-emission sources important to device applications. These characteristics include fundamental features, such as current-voltage data and noise, in addition to engineering considerations, such as life expectancy and procedures for tube assembly. We conclude with a review of a wide variety of demonstrated and proposed devices based on vacuum microelectronic principles, including electron guns, microwave tubes, and flat-panel displays. >

Field emitter array development for microwave applications. II

Microfabricated field emitter arrays are being used in an ongoing DARPA/NRL program as a means for gating or prebunching electrons in a microwave amplifier tube. The goals of the program are to demonstrate 10 dB gain at 50 W and 10 GHz in a gated Klystrode amplifier tube with 50% efficiency. The proposed cathode specifications call for 160 mA peak emission and 10 GHz emission modulation from an annular emitter array having a 600 μm outer diameter and an inner diameter to be determined by transconductance and capacitance requirements. Experimental results have shown an average array capacitance of 6 nF/cm2, and that a transconductance of 1 μS/tip can be achieved at emitter–tip loadings of 10 μA/tip. Calculations based on these results show that emitter arrays having 0.4 μm diam gate apertures on 1 μm centers, a 600 μm outer diameter, and a 560 μm inner diameter should meet the tube specifications. Such arrays have been fabricated and shown to have essentially the same characteristics (Fowler/Nordheim coeff...

High current, high current density field emitter array cathodes

Microfabricated field emitters have shown the potential for very high current densities (>100A∕cm2) and total emission currents (>1A). However, realizing this potential has been elusive, primarily because these cathodes exhibit insufficient emission uniformity over an emitter array. In this article we report the development of an in situ processing method based on emitter tip self-heating during operation that is shown to improve emission uniformity between emitter tips. Two tips differing in emission current by three orders of magnitude for a given voltage as fabricated are shown to be essentially identical in their emission characteristics after controlled pulsing to very high emission current. When the method was applied to a 50 000 tip array, it produced 300 mA of emission (40A∕cm2). The experimental arrangement prevented advancing to higher emission levels due to space charge limitations. It is expected that 1 A of emission at ∼100A∕cm2 is possible with appropriate modifications to the experimental a...

Field emission current cleaning and annealing of microfabricated cold cathodes

Thermal cleaning and annealing by the extracted field emission current has been used to achieve and maintain stable emission characteristics with microfabricated cold field emission cathodes. Our studies show that the cathode tip surface can be smoothed, recrystallized, and at least partially cleaned of surface adsorbates. The result is a significant decrease in emission current noise and recovery of the initial current–voltage characteristics of the cathode following surface contamination.

Spindt cathode tip processing to enhance emission stability and high-current performance

The extracted field emission current can be used to controllably heat microfabricated cold field emission cathode tips. The heating can be sufficient to smooth and recrystallize the tip surface by surface self-diffusion, and at least partially clean the surface of contaminants by thermal desorption. Self-heating not only allows for the achievement and maintenance of stable emission characteristics, but can be used to make the current-voltage characteristics of microfabricated field emitter tips nearly identical to one another. The resulting improvement in emission uniformity will allow for more reliable array operation at increased electron emission current densities.

Emission uniformity enhancement between microfabricated tips in cold cathode arrays

The current–voltage characteristics can vary significantly between individual microfabricated tips in as-fabricated arrays of cold cathodes. Such emission nonuniformity is undesirable in device applications. We have shown that in situ tip self-heating by the emitted electron current can be used to make the current–voltage characteristics of microfabricated field emitter tips nearly identical to one another. The improved emission uniformity will allow for more reliable array operation at increased electron emission current densities.

Electron emission enhancement by overcoating molybdenum field‐emitter arrays with titanium, zirconium, and hafnium

Overcoating Spindt‐type field‐emitter‐array cathodes with several monolayers of Ti, Zr, or Hf leads to a decrease in the voltage for the same emission current by 30 to 40%. This change is entirely ascribable to a 1 eV decrease in surface work function and an increase by a factor of 10 to 100 in the pre‐exponential term of the Fowler–Nordheim relation. The postdeposition current voltage characteristics have been observed to remain essentially unchanged for periods of greater than 100 hours at current levels of 10 μA/tip. Field‐emission micrographs indicate that the increase in pre‐exponential term cannot be attributed to an increase in the emitting area. A plausible explanation is that the electron supply function for these overcoatings is larger than that of the as‐fabricated emitter.

Observations of work function changes in field-emitter arrays

Abstract The current-voltage ( I–V ) characteristics for arrays of microfabricated field emitter tips have been measured as a function of cathode temperature in a range of 30 to 300°C in an turbomolecular pumped vacuum system with a base pressure of -10 Torr. The current at a fixed voltage was found to increase with increasing temperature. The change was reversible provided the temperature was held constant at its new value for about 40 min. This implies that the current increase with temperature is due to a change in work function rather than a change in the emitting area or tip shape. The I–V characteristic at each temperature followed the Fowler-Nordheim field emission equation over five orders of magnitude in current and indicated the occurence of work function changes between 0.5 and 2.0 eV. Such changes could be due to adsorption on the tip of electropositive atoms originating at the anode, which reached temperatures of approximately 670°C due to the electron bombardment at high anode voltages. Conversely, the lower work function could be due to desorption of electronegative atoms (such as oxygen) under the action of the heat and the increased H 2 and CO pressure associated with the relatively high anode temperature. Reversible desorption has been previously observed from the field emitting whiskers associated with high voltage vacuum breakdown between large area metal electrodes.

11.1: A reliable improved Spindt cathode design for high currents

Recent work with Spindt cathodes has shown that past unreliable behavior has not been due to poor tip uniformity and the failure of over-achieving tips as has been widely believed. Rather the failures have been due to flashover along oxide walls in the cathode cavities. A new cavity architecture has solved the problem, and enabled greatly improved performance.

An integrated field emission array for ion desorption

Field emission arrays that are used for ion desorption must be capable of operating at high applied voltages. The large electric fields can lead to dielectric breakdown or electron emission from the gate, both of which may result in catastrophic failure. Methods were developed to fabricate tip arrays with integrated gate electrodes, separated from the substrate with sufficient dielectric to sustain high voltages. To suppress gate electron emission, processes were developed to fabricate geometries that favor high fields at the tip while minimizing the field at the gate.