A technique of gentle, highly selective gas-phase etching of silicon has been devised to enable the fabrication of microelectromechanical devices integrated with electronic circuits. For example, newly fabricated complementary metal oxide/semiconductor (CMOS) integrated-circuit chips can be micromachined by use of this technique to incorporate microsensors, without damaging the circuitry already present.

Figure 1
Microscopic SiO2 Plates and Beams were fabricated in place on a silicon substrate, then suspended over a pit in the substrate by using XeF2 gas to etch the silicon out from under them. Aluminum pads are visible at the upper right.

The technique is based on the fact that at room temperature, xenon difluoride (XeF2) gas etches silicon preferentially to almost all other materials that are likely to be encountered in processing of semiconductor devices. Even when silicon is etched by XeF2 to a depth of hundreds of microns, there is no appreciable etching of adjacent uncured or cured photoresist, oxide, metal, or polymeric structures with dimensions down to fractions of a micron.

Another notable advantage of gas-phase etching with XeF2 is that unlike liquid-phase etching, it does not involve hydrodynamic forces that could damage fragile micromechanical structures like those shown in the figure. Still another advantage of etching with XeF2 is that since it can be done at room temperature, there is no risk of the diffusion that can occur at high temperature, ruining the devices being fabricated.

At room temperature and atmospheric pressure, XeF2 is a white solid. However, at room temperature, it sublimates at a vapor pressure of several torr (several hundred pascals). Thus, XeF2 can easily be converted to the gas phase by placing it in a vacuum system of modest capability.

Etching by XeF2 is performed in a chamber connected by valves to a vacuum pump and to a source of XeF2. First, the chamber is pumped down to a pressure of a few millitorr (a few tenths of a pascal). Then the valve to the source of XeF2 is opened and adjusted to maintain the pressure in the source at about 1 torr (≈133 Pa), so that XeF2 gas is released at a suitable rate and concentration. Under these conditions, etching of single-crystal silicon has been observed to proceed at typical rates between 3 and 5 µm/min, and occasionally at rates as high as 10 µm/min. The etch is nearly isotropic and insensitive to the type and concentration of dopant.

This work was done by Michael H. Hecht of Caltech and Kristofer S. Pister and Ezekiel Kruglick of UCLA for NASA's Jet Propulsion Laboratory.For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Materials category, or circle no. 114 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).


NASA Tech Briefs Magazine

This article first appeared in the February, 1998 issue of NASA Tech Briefs Magazine.

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