Magnetron Plasma Etching of SiC for Microfabrication

Etch rates are high, and etched surfaces are remarkably smooth.

Lewis Research Center, Cleveland, Ohio

Magnetron plasma etching has been found to be a promising technique for micromachining of single-crystal silicon carbide to fabricate microscopic structures comprising integrated mechanical, electronic, and/or optical devices. Silicon carbide offers several advantages over silicon for the development of future devices: In comparison with silicon, silicon carbide is harder and stiffer, is less chemically reactive, and has greater thermal conductivity; moreover, silicon-carbide-based electronic devices can operate at temperatures higher than silicon-based electronic devices can withstand. Etching techniques for micromachining of silicon are well known, but the lesser chemical reactivity of silicon carbide makes it necessary to devise alternative etching techniques.

In research that preceded the development of the present magnetron-plasma-etching technique, it had been shown that single-crystal 6H-SiC could be etched at high rates to very smooth final surfaces in an electron-cyclotron-resonance (ECR) plasma reactor, using a mixture of CF₄ and O₂ gases. An ECR reactor provides a high-density plasma at a low pressure, which facilitates the formation and removal of volatile reaction products. However, the sample to be etched is remote from the main body of the plasma.

In the present magnetron-plasma-etching technique as in the ECR etching technique, a magnetic field is used to increase the density of the plasma. Unlike in ECR etching, the main body of the plasma is confined in proximity to the sample. Such confinement results in a high concentration of reactive chemical species together with a high flux of bombarding ions at the sample, making it possible to achieve rapid etching.

Magnetron plasma etching of SiC was demonstrated in preliminary experiments in a vacuum chamber, using a magnetron sputter gun as a cathode. For each experiment, a sample of 6H-SiC n-doped to a concentration 6 × 10¹⁷ cm ^{– 3} was placed on a silicon target. The (0001) face of the sample was masked with a SiC fragment. A radio-frequency (RF) power supply was used, and the target was allowed to self-bias. CHF₃ and O₂ were fed into the chamber at rates fixed by mass-flow controllers. The pressure in the chamber was regulated by throttling the flow to the vacuum pump.

The duration of each etch was 7 minutes. At an RF power of 250 W, a maximum etch depth of 4.5 µm (see figure) was achieved with a gas mixture of 0.6 CHF₃ + 0.4 O₂ at a total pressure of 20 mtorr (about 2.7 Pa). This depth corresponds to an etch rate of about 640 nm per minute, which is a relatively high rate, significantly higher than that which has been achieved using an ECR plasma. The etched surface was found to have a root-mean-square surface roughness of only 20 Å. The reaction of aluminum and fluorine yields a nonvolatile product, which makes aluminum suitable for use as a mask material in order to provide selective etching in fluorine-based plasmas. At the RF power of 250 W, aluminum films were etched at a high rate. At an RF power of 50 W, silicon carbide was etched at a rate of 170 nm per minute, while aluminum was etched at 1/12 of that rate.

The Profilometer Trace was made after a 7-minute etch of 6H-SiC in 0.6 CHF₃ + 0.4 O₂ at a total pressure of 20 mtorr, at an RF power of 250 W. The scanning electron micrograph shows an edge of the etched region.

This work was done by Glenn Beheim of Lewis Research Center and Carl Salupo of Cortez III Service Corp. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Manufacturing/Fabrication category, or circle no. 188 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Lewis Research Center, Commercial Technology Office, Attn: Tech Brief Patent Status, Mail Stop 7 – 3, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16563.Download detailed Technical Support Package for this Brief