"Micromachined Hot-Filament Vacuum Devices"
Kirt Reed Williams

Dissertation for
Doctor of Philosophy in Electrical Engineering and Computer Sciences
University of California, Berkeley
Professor Richard S. Muller, Chair
1997


Abstract

We describe investigations on micromachined hot-filament vacuum devices, which are 
electronic devices made by micromachining an ohmically heated tungsten filament that 
acts as a source of blackbody-like radiation or of free electrons. The freestanding 
filaments, which are typically 200-500 um long, 5-20 um wide, and 0.7-2.3 um thick, 
are suspended over a cavity etched into a silicon substrate. 

When the hot filament is used as a source of radiation (visible or infrared), the 
device is a "microlamp." When used as the source of thermionically emitted electrons, 
circuit devices such as diodes and triodes can be made. The thermionically emitting 
devices have also been characterized as magnetic-field sensors and ionization pressure 
sensors (ion gauges). We have designed, modeled, fabricated, and tested all of the 
aforementioned devices.

We discuss at length the modeling of ohmic heating, thermionic emission, and electron 
flow through our devices. 

We describe the design and fabrication of vacuum-sealed and unsealed devices (which 
must be operated in a vacuum chamber). During the design of the device-fabrication 
processes, we developed a low-stress sputtered-tungsten-deposition method. We have 
also measured etch rates for more than 300 combinations of materials and etches used 
in micromachining and describe 20 sources of wet- and plasma- etch-rate variation.

We describe the microscale optical pyrometry technique used to measure hot-filament 
temperature. Lifetimes of 10 hours at 2200 K and 1 hour at 3000 K are typical. 

Anode current in the vacuum diodes varies with filament temperature, with 50 uA being 
a representative value. Common-cathode triode circuits with small-signal voltage 
gains on the order of 2 to 3 have been made.

The magnetic-field sensors are based on the Lorentz-force steering of electrons to a 
pair of anodes in a symmetrical device.

The micro ion gauges are a planar version of the macroscopic devices, with about 
10^-6 times their volume. They have a sensitivity of 0.43 Torr^-1, which is in the 
range predicted by our model.





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