Comparison Of F 2 Plasma Chemistries For Deep Etching Of SiC
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Comparison Of F2 Plasma Chemistries For Deep Etching Of SiC K.P. Lee 1, P. Leerungnawarat 1*, S.J. Pearton 1, F. Ren 2, S.N.G. Chu 3, and C.-M. Zetterling 4 1 Materials Science and Engineering University of Florida, Gainesville FL 32611 USA * Current address: Lucent Technologies, Reading, PA. 2 Chemical Engineering University of Florida, Gainesville FL 32611 USA 3 Bell Laboratories, Lucent Technologies, Murray Hill NJ 07974 USA 4 Department of Electronics, Royal Institute of Technology (KTH), Kista, Sweden ABSTRACT A number of F2-based plasma chemistries (NF3, SF6, PF5 and BF3) were investigated for high rate etching of SiC. The most advantageous of these is SF6, based on the high rate (0.6 µm· min-1) it achieves and its relatively low cost compared to NF3. The changes in electrical properties of the near-surface region are relatively minor when the incident ion energy is kept below approximately 75 eV. At a process pressure of 5 mTorr, the SiC etch rate falls-off by ~15 % in 30 µm diameter via holes compared to larger diameter holes (> 60 µm diameter) or open areas on the mask. INTRODUCTION There is considerable interest in developing a capability for deep etching in SiC single crystal substrates for applications ranging from via hole formation for power transistors (1,2) to microelectromechanical systems (MEMS)(3). While this technology exists for both Si and GaAs substrates, the much higher bond strength of SiC means that wet etching is impractical and dry etching is limited to relatively low rates. Previous results on reactive ion etching of SiC have employed F2-based plasma chemistries such as CHF3, SF6, CH4 and NF3 with resultant etch rates ≤ 1500 Å·min-1. Relatively rough surfaces are often observed under these conditions due to sputtering of the electrode material onto the SiC sample, leading to micromasking (4-13). With the advent of high density plasma sources, including Electron Cyclotron Resonance (ECR), Inductively Coupled Plasma (ICP) and Helicon, much higher SiC etch rates have been reported (14-25). The key advantage of these sources is decoupling of ion energy and ion flux, so that relatively low ion energies can be employed. This reduces the electrode sputtering problem and in addition the plasma chemistries for high density sources generally involve gases that do not contain CHx because of the extensive polymer deposition that can occur within the source at high applied powers. The absence of these two sources of redeposition onto the SiC generally leads to good surface morphologies. In this paper we report an investigation of ICP etching of 4H-SiC using four different F2based plasma chemistries and we compare these results with previous data in the literature. We demonstrate that under optimized conditions, ICP etching with SF6 or NF3 is capable of creating through-wafer via holes in SiC substrates in practical times. EXPERIMENTAL
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The samples used were bulk 4H-SiC substrates (N-doped, n ~ 5x1016 cm-3) with Si face (0001) orientation. These were cleaved into 5x5 mm2 sections for the etch
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