Date of Award

Spring 1992

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering - (Ph.D.)

Department

Electrical and Computer Engineering

First Advisor

William N. Carr

Second Advisor

Robert Joseph Zeto

Third Advisor

Robert Boris Marcus

Fourth Advisor

Kenneth Sohn

Fifth Advisor

Marek Sosnowski

Sixth Advisor

Durgamadhab Misra

Abstract

Vacuum microelectronics is a new research field which applies semiconductor process technology to the fabrication of micron-dimensioned electron devices. Vacuum microelectronics is made possible by advances in microstructures and nanofabrication technology. Vacuum microelectronic devices are further characterized by a wide operating temperature range, nuclear radiation immunity, higher emission current density potential, and lower power consumption than that of thermionic emitters. The low mass of the electron provides a higher carrier mobility than GaAs or any solid state device. These features offer the potentials for wide variety of applications.

In this dissertation. electron field emission structures applicable to a variety of vacuum microelectronic devices have been fabricated and characterized. The cathodes are micromachined of N-type silicon and tungsten using a combination of ultraviolet liftoff lithography and reactive ion etching. A minor emphasis has been placed on micromachining surface-grooved structures for applications that include both vacuum microelectronics and optical microsystems. Optimized processing, device modeling, and physical/electrical characterization are key elements in the research described.

The control of sidewall angle, cavity depth, and apex radius for ridge structures in silicon has been a major focus of this thesis. Reactive ion etching techniques have been studied for sidewall angles up to 45° and ridge apex radii of approximately 40nm.

A fluorine-based chemistry (CF4/O2) with oblique angles ( tilted wafers) for the incident beam electric field and overetching is used in separate experiments. The use of deep UV-hardened photoresist and image-reversal aluminum liftoff for reactive ion etching masking are compared. Aluminum as a shadow mask for reactive ion etching micromachining has the advantage of lower etch/sputtering rates and higher temperature tolerance compared to photoresist in the CF4/O2 system. Typical etch conditions used were CF4/O2 flow rates of 20/2 sccm, pressure 10 to 40 mTorr. and etch duration 30 min. This thesis is one of the first detailed studies of reactive ion etching comparing tilted and untilted wafer substrates.

A new process technology for vacuum microelectronic diodes is shown, and device design with a knife-edge cathode and a lateral electron trajectory is implemented as a characterization tool. The cathode structure for these devices consists of a titanium:tungsten /tungsten film sandwich overlaying an aluminum adhesion and sacrificial film. The aluminum film is partially sacrificed to achieve the necessary sharp edge of tungsten metal surface for field emission. The tungsten and titanium metals are deposited by dc magnetron sputtering followed by liftoff lithography and thermal annealing. Selected devices with a cathode to anode spacing of 0.8 μm are electrically characterized in 2 to 5 x10-9 Torr vacuum. A maximum static current of 26μA current is obtained. The I-V characteristics of lateral trajectory devices with a knife-edge cathode are compared with Fowler Nordheim theory. Good agreement occurs if the cathode apex radius is approximately l0nm. Based on a Fowler Nordheim model the effective fraction γ of the knife-edge emitting electrons is in the range 3 to 80%.

This research includes the first experimental verification of the effect of deflection electrodes to confine the electron beam. Characterization and modeling comparisons for vacuum microelectronic devices with a knife-edge cathode and lateral electron trajectory are described for the first time.

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