Plasma assisted polishing of single crystal SiC for obtaining atomically flat strain-free surface K. Yamamura (2) a, *, T. Takiguchi a , M. Ueda a , H. Deng a , A.N. Hattori b , N. Zettsu a a Research Center for Ultra-precision Science and Technology, Osaka University, Suita, Osaka, Japan b Nanoscience and Nanotechnology Center, The Institute of Scientific and Industrial Research, Osaka University, Japan 1. Introduction Silicon carbide (SiC) is a promising next-generation semicon- ductor power device material for high-power and high-tempera- ture applications, owing to its excellent properties, such as wide energy band gap, excellent thermal conductivity, high saturated electron drift velocity, and good chemical stability. To apply SiC as a material for high-performance electronic devices, an atomically smooth and damage-free surface is essential, but SiC is one of the most difficult-to-machine materials because of its hardness and chemical inertness. In the present machining process for SiC wafers, a crystal ingot grown by a modified Lely method is first sliced by a wire saw and then planarized by lapping. Because of the high hardness of SiC, diamond abrasives are generally used in the slicing and lapping processes. These mechanical removal processes inevitably introduce microscratches and subsurface damage to the workpiece surface, which deteriorate surface integrity. To prepare a SiC wafer with a smooth surface, there are several approaches under development. Chemical mechanical polishing (CMP) is now widely used as a finishing process for the surfaces of single-crystal SiC and/or GaN substrates. Zhou et al. proposed the process of colloidal silica polishing, which requires a concentrated colloidal silica slurry at temperatures higher than 55 8C and a pH value higher than 10 [1,2]. The surface finished by CMP has no subsurface damage, as evaluated by cross-sectional transmission electron microscopy (XTEM), and leads to the formation of a high- integrity epitaxial growth layer in terms of surface morphology [1– 3]. However, the material removal rate (MRR) of CMP is very low (<0.5 mm/h). Recently, Jeong et al. have proposed a hybrid CMP process, which uses a mixed abrasive slurry (MAS) composed of a mixture of colloidal silica slurry and nano-diamond abrasive, to increase the MRR keeping a defect-free surface [4,5]. Some abrasive-free chemical finishing techniques have been developed to obtain high-integrity SiC surfaces. Yamauchi et al. proposed the technique of catalyst-referred etching (CARE) [6,7]. This technique uses a Pt polishing plate as a catalyst to activate hydrofluoric acid to react with the workpiece surface, enabling the removal of SiC. In their experiments, an atomically flat surface, which was crystallographically well ordered, was obtained. Watanabe et al. proposed a photo chemical polishing technique [8]. In their research, SiC was polished with chromium oxide abrasives by the irradiation of ultraviolet (UV) light. The irradiation of UV light enhanced the oxidation of the SiC surface. The hardness of Cr 2 O 3 is similar to that of SiO 2 ; thus, subsurface damage is minimal in this process. In a later research study, Watanabe et al. utilized TiO 2 and CeO 2 (ceria) as abrasive materials. The use of ceria abrasives greatly improved the removal rate and roughness [9]. On the other hand, we proposed a novel abrasive polishing technique combined with the irradiation of atmospheric-pressure plasma, which is named plasma assisted polishing (PAP), to realize the high-efficiency and high-integrity finishing of difficult-to- machine materials such as single-crystal SiC, sintered SiC, and tungsten carbide (WC) [10,11]. The aim of this paper is to clarify the polishing performances for 4H–SiC (0 0 0 1) substrate, such as the surface roughness, surface chemical structure, and subsurface damage, using a newly proposed PAP technique. 2. Concept of plasma-assisted polishing and procedure Mechanical lapping using a diamond abrasive has a high removal rate and a high flattening ability in a planarization process for SiC wafers. However, subsurface damage, which deteriorates the electronic property and/or quality of an epitaxial layer, is inevitably introduced owing to its removal mechanism, such as plastic deformation or brittle fracture. On the other hand, in the case of a plasma etching process, such as plasma chemical vaporization machining (PCVM), a damage-free surface can be CIRP Annals - Manufacturing Technology 60 (2011) 571–574 ARTICLE INFO Keywords: Polishing Single crystal Surface integrity ABSTRACT A novel polishing technique combined with the irradiation of atmospheric pressure plasma was proposed for the finishing of a silicon carbide material. The irradiation of helium-based water vapor plasma efficiently oxidized the surface of single-crystal 4H–SiC (0 0 0 1), and a nanoindentation test revealed that the hardness of SiC decreased by one order of magnitude compared with that of the unprocessed surface. Plasma-assisted polishing using a CeO 2 abrasive enabled us to improve the surface roughness of a commercially available SiC wafer without introducing crystallographical subsurface damage, and a scratch-free atomically flat surface with an rms roughness of 0.1 nm level was obtained. ß 2011 CIRP. * Corresponding author. Contents lists available at ScienceDirect CIRP Annals - Manufacturing Technology journal homepage: http://ees.elsevier.com/cirp/default.asp 0007-8506/$ – see front matter ß 2011 CIRP. doi:10.1016/j.cirp.2011.03.072