A SURVEY OF THE PRESENT STATUS OF VACUUM MICROELECTRONICS

''Vacuum state'' technology appears ready for a comeback in microelectronics form, after years of domination of solid state devices. It will do so, however, based on the developments fostered by solid state technology, which has made available, during 30 years of uninterrupted progress, very sophisticated micromachining techniques, in a large variety of materials, that can be used to advantage for the new developments. Central to ''vacuum state'' microelectronics is the availability of efficient ''cold cathodes'', i.e. of electron sources that do away with thermionic electron generation, which would be incompatible in many ways (temperature, power requirements) with microelectronics. Field emission of electrons is used instead; and it is curious to note that the basic idea on which vacuum microelectronics is founded, namely to produce electron emission by the use of ''points'' or tips that locally enhance electric field, dates to 1961, i.e. exactly to the time when the IC era was just beginning. Evidently, little attention was paid to the proposal by the technological community, perhaps totally absorbed at that time by the exploding interest in semiconductors. A few scientists, however, and particularly C.A. Spindt of SRI Int., liked the idea first proposed by K. R. Shoulders, also of SRI Int., and quietly but tenaciously developed it, and the first interesting results were obtained. More researchers were attracted, and finally the technology turned into something that appears very promising. This paper presents the current state of the technology. After reviewing the basic structures, and some information on the physics of field emission, the ''Spindt cathode'' and its many variations are examined in detail. Several of the many proposed alternatives to the Spindt cathode are successively reviewed, including that using a p-n junction biased into avalanche breakdown, which appears at present the most promising after the Spindt cathode. Turning then to practical applications, those concerning the use of the vacuum microelectronic technology for the realization of cathode emitters are first examined, with reference in particular to their use in displays, which appear the closest to industrial exploitation at this time. More complicated applications, such as complete vacuum microelectronic triodes and IC's are successively considered, and the problems that up to now have hindered the achievement of successful results in this area briefly examined. Finally a comparison is attempted between vacuum state and solid state devices.