We describe investigations on micromachined hot-filament vacuum devices, which are electronic devices made by micromachining an ohmically heated tungsten filamentthat acts as a source of blackbody-like radiation or of free electrons. The freestanding filaments, which are typically 200-500μm long, 5-20μm wide, and 0.7-2.3μm 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 deviceshave 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, andelectron 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 50μA 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 about10-6 times their volume. They have a sensitivity of 0.43 Torr^-1, which is in the range predicted by our model.
April 30, 1997
Williams, K. R. (1997). Micromachined Hot-filament Vacuum Devices. United States: University of California, Berkeley.