Reactive Ion Etching Techniques for Silicon Sidewall Angle Control in Microengineering

Abstract

This paper describes reactive ion etching (RIE) techniques with silicon substrates for initial processing of semiconductor field emitter array cathodes for vacuum microelectronics. A secondary focus is process research for optical reflecting gratings for opto‐ and micromechanical devices. These applications each require control of the sidewall angle and apex radius. In this paper, we describe micromachining control of trenches with remainder sidewall angles varying from 15–60° and resulting apex ridge radii of as small as 40 nm. A fluorine‐based chemistry with oblique angles for the incident beam (tilted substrates) and overetching is used. The use of both deep UV‐hardened photoresist and aluminum as RIE shadowmasks is compared. Unannealed aluminum as a shadowmask for RIE micromachining has the advantage of lower etch/sputtering rates and higher process temperature tolerance compared to photoresist in the RIE system. Etch environments in the pressure range 30–80 mTorr with flow rates of 20/2 seem, RF power 100–200 W, and etch duration 30 min are described. Both tilted and untilted substrates mountings were studied. Under these conditions, the surface erosion is primarily a combination of ion milling and chemical etch mechanisms. Both P‐type and N‐type substrates of 100 nm diam wafers were used. Relatively sharp single‐ and double‐ridged silicon structures were obtained using the photoresist shadowmask on untilted substrates. The photoresist shadowmask provides an advantage over Al only for the sidewall orientation where we were able to specify a condition for obtaining a smooth optically reflective surface. The more vertical sidewall angles obtainable with the unannealed Al shadowmask should permit fabrication of field emitter cone or ridge cathodes on pedestals with higher height‐to‐width ratios. A field cathode pedestal with a more vertical sidewall results in a higher electric field at the electron emission tip or ridge.