Heinz H. Busta

Vacuum microelectronics-1992

Vacuum microelectronics is a new interdisciplinary field utilizing the mechanism of electron emission into a vacuum for devices requiring bulk and surface micromachining. By fabricating arrays of conductive or semiconductive structures that are either gated or ungated, a multitude of devices can be formed that utilize cold emission and ballistic transport of electrons from emitting cathodes to appropriate collector electrodes. Potential applications include flat panel vacuum fluorescent displays, ultrahigh-frequency power sources and amplifiers, high-speed logic and signal processing circuits, and sensors. The technology of vacuum microelectronics may be capable of operation within extremely harsh environments in terms of temperature and radiation, assuming the availability of compatible packaging materials. The author summarizes the state of the art for vacuum microelectronics, with special emphasis on operation, device fabrication, testing, experimental results, and potential applications.

Electron emission from a laser ablated and laser annealed BN thin film emitter

Electron emission from a ∼100-nm-thick, laser ablated and laser annealed, carbon-doped BN film deposited on polycrystalline diamond was measured at room temperature and at pressures of about 1×10−8 Torr. For a 500-μm-diam extraction electrode, currents of several mA were obtained which corresponds to current densities of >1 A/cm2. At low currents, the current–voltage characteristics follow the classical Fowler–Nordheim behavior. At higher currents, deviations occur which are correlated to a nonlinear BN film resistance which ranges from 3×106 Ω to 5×104 Ω. For comparison, similar measurements were performed for the polycrystalline diamond film. Current densities of only 1 mA/cm2 were obtained which is attributed to a much higher sample resistance of about 1×109 Ω.

Critical point drying and cleaning for MEMS technology

A critical step in surface micromachining of microelectromechanical systems (MEMS) is the process that releases, cleans, and dries the flexible structures that are crucial to MEMS functionality. Standard release methods employed today can leave residue particles and can cause sticking because of surface tension. Aggressive design requirements, liquid processing, packaging, handling, transportation, and device operation etc., can contribute to device failure due to stiction. The use of supercritical carbon dioxide has been proven in various industries to achieve ultra-clean surfaces. Recent critical research studies by academia, research laboratories and industry have shown that supercritical carbon dioxide can be successfully used to alleviate the stiction problem and provide a clean and dry surface. The absence of surface tension in the supercritical phase of a fluid provides an excellent means to overcome stiction. The advantages of supercritical carbon dioxide include its relatively low critical temperature and pressure, its high diffusivity, low surface tension, and environmentally friendly (non-ozone depleting, non- hazardous). This paper reviews the stiction problem for MEMS, and the application of critical point drying for MEMS technology.

Performance of laser ablated, laser annealed BN emitters deposited on polycrystalline diamond

Thin BN films of about 100 nm were deposited on 25 μm thick n-type polycrystalline diamond by laser ablation and annealed with a 25 ns pulse from an excimer laser at 0.1 J. Current densities of about 2 A/cm2 were obtained with an extraction electrode probe area of 2×10−3 cm2 (500 μm diameter). Emission is governed by a Fowler–Nordheim-type behavior with some deviation at higher currents due to the voltage dependency of the BN resistance. From present experiments it cannot be deduced if the films exhibit negative electron affinity. Emission currents are stable and independent of pressure to about 10−4 Torr. The magnitude of the current fluctuations are similar to Spindt-type devices. Compared to Mo and Si emitters, no careful high vacuum conditioning procedure is needed prior to operation. At current densities above 1 A/cm2, restructuring of the emission area took place at some locations of the 0.5 cm × 1 cm sample, leaving craters in the diamond substrate. After restructuring, emission still took place wi...

The field emitter triode as a displacement/pressure sensor

By operating the field emitter triode in the collector-assisted field emission mode, a strong dependency of the collector current as a function of emitter-to-collector distance is obtained. This property can be used in the construction of displacement and/or pressure sensors. Experimental results are presented for a silicon emitter array. These include displacement measurements under DC and AC conditions, sensitivity, and temperature dependency from room temperature to 200 degrees C. From experimental data, model parameters for the Fowler-Nordheim equation are deduced. These parameters can then be used to calculate the performance of the device as a function of gate and collector voltages and of deflection.

Self-aligned bottom-gate submicrometer-channel-length a-Si-:H thin-film transistors

Fully self-aligned bottom-gate thin-film transistors (TFTs) fabricated by using a back substrate exposure technique combined with a metal lift-off process are discussed. Ohmic contact to the sources and drains is accomplished by a 40-nm-thick layer of phosphorous-doped microcrystalline silicon. Devices with channel lengths ranging from 0.4 to 12 mu m are processed with overlap dimensions between the gate and the source and the gate and the drain ranging from 0.0 to 1.0 mu m. Analysis of the conductance data in the linear voltage regime reveals a parasitic drain-to-channel and source-to-channel resistance that is 14% of the channel resistance for a 10- mu m device and 140% for a 1- mu m device. Thus, increase in the device speed caused by reducing the channel length does not follow expected behavior. A similar situation exists in the nonlinear regime. The on-current of the devices starts to saturate below channel lengths of 2 mu m. Current on/off ratios taken at V/sub d/=5 V and V/sub G/=15 V and 0 V, respectively, are approximately 1*10/sup 6/ for the 1- and 12- mu m-long devices. The on/off ratio is reduced to 1*10/sup 5/ for the 0.4- mu m device. >

Performance of nanocrystalline graphite field emitters

Nanocrystalline graphite field emitters fabricated on silicon substrates have been characterized in terms of current–voltage, pressure dependency, long term stability, work function and lateral momentum. The work function is 4.0–4.2 eV and the cone angle of emission due to the lateral momentum of the emitting electrons in less than 1°. It is shown that with proper pre-testing treatment and copper anodes, these emitters can operate for over 5000 h without significant changes in emission current. They also operate at pressures of 5×10−5 Torr with current fluctuations ΔI/I of less than 1%. A quite complex emission pattern develops in conjunction with CRT phosphors. The transmission coefficient of electrons exiting a grid can vary from 1–50% depending on which phosphor is being used.

Emission characteristics of silicon vacuum triodes with four different gate geometries

Arrays of 10*10, 30*30, and 50*50 phosphorus-doped 0.005-0.025 Omega -cm, monocrystalline silicon field emitters have been fabricated with an emitter height of approximately 4.5 mu m, a cone angle of 110 degrees , and four gate openings ranging from 1.8 to 5.3 mu m. The placement of the rims of the gates range from coplanar with the apexes of the emitters for the 1.8- mu m devices to fully recessed for the 5.3- mu m devices. The devices have been characterized in terms of geometry-dependent beta factors, scaling of emission currents with array size, temperature dependency from room temperature to 48 K, pressure dependency from 2.5*10/sup -9/ to 0.8*10/sup -5/ torr, current fluctuations at room temperature and at 48 K, and image formation. All of the measurements have been performed by operating the devices in the gate-induced field emission mode. >

Atomically sharp silicon and metal field emitters

A method is described for forming atomically sharp silicon tips of less than 10-15 degrees half-angle by utilizing a known oxidation inhibition at regions of high curvature; equally sharp silicon wedges are now made in a similar fashion. The sharp silicon tips serve as the starting point for forming sharp tips of W, beta -W and gold. Field emission data from silicon emitters are compared with Fowler-Nordheim modelling and emission as a function of emitter-anode distance is described. >

Collector-assisted operation of micromachined field-emitter triodes

The field at the tip of a field emitter triode can be expressed by E= beta V/sub g/+/sub gamma /V/sub c/, where V/sub g/ and V/sub c/ the gate and collector voltages, respectively. For small gate diameters and tips below or in the plane of the gate and/or large tip-to-collector distances, /sub gamma /V/sub c/ >