Effects of Electrode Spacing on Reactive Ion Etching of 4H-SiC
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Effects of Electrode Spacing on Reactive Ion Etching of 4H-SiC Janna R. Bonds, Geoff E. Carter, Jeffrey B. Casady, James D. Scofield1 Electrical & Computer Engineering Department, Mississippi State University, MS 39762-9571, U.S.A. 1 U.S.A.F. Research Laboratory, Wright-Patterson AFB, OH 45433, U.S.A. ABSTRACT 4H-SiC was selectively etched in a Reactive Ion Etch (RIE) system using a nickel mask. The power, pressure, and electrode spacing were varied within a RF generated SF6:O2 (1:2) plasma. Peak etch rates of up to 2600 Å/min. were obtained at a pressure of 350 mT, power of 90 W (2 W/cm 2 ), and electrode spacing of 3.180 cm. Etches were all residue-free, although power levels above 60 W (1.36 W/cm 2 ) resulted in the SiC surface being roughened, which limited smooth surface etch capability to 2000 Å/min. When comparing electrode spacing from 3.180 cm to 1.270 cm, the 3.180 cm spacing was found to have the highest etch rate at pressures ranging from 250 mT to 500 mT. INTRODUCTION 4H-SiC is now commonly known as the most mature of wide bandgap semiconductors, with 50 mm diameter substrates commercially available since 1997. Applications in high-power and high-frequency such as radar transmit/receive modules, high-power RF amplifiers, DC motor control, and DC-DC converters are now being developed commercially [1-3]. Because of its chemical inertness, the only practical SiC etch process for deep via holes in monolithic microwave integrated circuits (MMIC’s) and micro machining of microelectromechanical systems (MEMS) is through the use of dry (or plasma) etching [4]. No room temperature wet etch of SiC is known, and wet chemical etches which do exist are not suitable for small feature size, anisotropic etching. For deep dry etching in SiC, it must be feasible (have high etch rate), be able to be masked, and cost-effective. For 100 µm etches, this requirement practically would require an etch rate of at least 2500 Å/min., and a mask which could survive the long (400 minutes at 2500 Å/min.) etch. Dry etching of single crystalline SiC using an Inductively Coupled Plasma (ICP) has now been demonstrated up to 8000 Å /min. [5], but this process may have difficulty in being a practical etch due to mask erosion. However, vias in SiC have been demonstrated using an ICP system [6], with the ICP system via etch process having a slower etch rate (only 300 minutes to etch a 97 µm deep via or 3200 Å/min effective etch rate). Vias in SiC have now been demonstrated using a reactive ion etching (RIE) system [4] as well, but with much slower etch rates of 1500 Å/min., which would require too lengthy of a process (over 11 hours). However, the selectivity of the etch to the nickel mask (20) was sufficient for such a deep etch, so increasing the etch rate while maintaining selectivity is key to making RIE useful for deep via etching.
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Of the dry etching techniques, RIE is the lowest cost technique for batch, high-throughput, high-yield processing. RIE systems, while having relatively low ion and free radical densities, are less c
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