ECS Journal of Solid State Science and Technology, 3 (8) P277-P284 (2014) P277 2162-8769/2014/3(8)/P277/8/$31.00 © The Electrochemical Society Effect of Polishing Parameters on Chemical Mechanical Planarization of C-Plane (0001) Gallium Nitride Surface Using SiO 2 and Al 2 O 3 Abrasives Khushnuma Asghar, Mohd Qasim, and D. Das z School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad 500046, India In this study, Chemical Mechanical Planarization (CMP) of (0001) GaN surface, using two different slurries containing colloidal silica and alumina as abrasive nanoparticles, has been reported. Effect of processing parameters, such as concentration of the oxidizer, downward pressure, head rpm, base rpm, pH of the slurries, and type and concentration of abrasive particles, on the material removal rate (MRR) and surface quality (roughness) have been studied in details. The maximum MRR has been found to be ∼39 nm/hr and ∼85 nm/hr for slurries containing SiO 2 and Al 2 O 3 abrasives, respectively, under 38 kPa pressure, 90 rpm base speed (100 rpm for Al 2 O 3 containing slurry), 30 rpm carrier speed, slurry pH 1 (2 for Al 2 O 3 containing slurry), 0.3 M Oxidizer concentration, and 3.75 wt% abrasive particle concentration. RMS surface roughness of 1.3 Å and 0.7 Å, over scanning area of 10 μm × 10 μm and 5 μm × 5 μm, respectively, has been achieved on polished Ga-faced GaN surface for SiO 2 containing slurry using optimized slurry chemistry and processing parameters. © 2014 The Electrochemical Society. [DOI: 10.1149/2.0181407jss] All rights reserved. Manuscript submitted March 11, 2014; revised manuscript received June 11, 2014. Published June 21, 2014. Gallium nitride and related iii-v nitride semiconductors are be- coming increasingly popular as the candidate material for use in opto- and high power and high frequency electronic devices because of their wide bandgaps, high carrier mobility, high saturated electron drift ve- locity and high breakdown field. 1,2 GaN, in particular, is suitable for variety of electronic and optoelectronic applications because of its unique capabilities of amplifying high-frequency RF signals without distortion, can withstand high temperatures, and emits blue and green lights. However, an important issue in development and commercial- ization of GaN based power or optoelectronics is the unavailability of suitable device grade (epi-ready) wafer. The ideal choice would be to use homoepitaxial substrate. But, due to difficulties in growth of bulk GaN wafer these substrates are not readily available. The most im- portant challenge for further development of GaN based technology is the need to improve the quality of GaN films by reducing defects caused by the heteroepitaxial growth. GaN, most commonly, is grown heteroepitaxially on sapphire substrate. 1,3 Due to large difference in thermal expansion and lattice mismatch between sapphire and GaN, a large number of crystal defects and rough surface is generated dur- ing the growth process, which limits further application of the grown film. 4,5 It is essential to produce an atomically flat, smooth and defect free GaN surface to realize the full potential of this material. Conven- tional planarization techniques cannot be effectively applied to GaN surfaces because of its extreme mechanical hardness and chemical in- ertness toward many chemicals. Mechanical polishing using diamond abrasive produces scratches on the surface. Chemical mechanical pla- narization (CMP) using colloidal silica has been proven successful for preparing defect free GaN surfaces. Numerous reports are available on CMP of several other materials including Al 2 O 3 , 6 SiO 2 , 7 and GaAs 8 using colloidal SiO 2 based slurries. CMP of polar GaN faces, using colloidal SiO 2 abrasives, was re- ported by Tavernier et al. 9 Polishing of N polar (000–1) GaN face was found to be more effective than that of Ga polar (0001) GaN face, where no visible change after CMP was observed. Hideo Aida et al demonstrated the CMP of Ga polar GaN surface using colloidal SiO 2 slurry under varying pH conditions. The difference in material removal rates (MRR) on Ga- and N- polar GaN faces was attributed to the non-reverse crystallographic symmetry. 10 A material removal rate of 17 nm/hr was demonstrated on Ga polar GaN surface, using col- loidal SiO 2 , under typical polishing conditions. 11 Using a mixture of sodium-hypochlorite and alumina abrasives as polishing slurry, CMP of Ga polar GaN surface has resulted in a MRR of 50 nm/min. 12 The higher MRR of polar GaN surfaces, with colloidal Al 2 O 3 containing slurries, indicates higher hardness of the abrasive Al 2 O 3 nanoparti- cles compared to that of SiO 2 nanoparticles. Although colloidal Al 2 O 3 z E-mail: ddse@uohyd.ernet.in abrasive nanoparticles result in higher MRR in conventional CMP pro- cess, it creates polishing related defects on the surface. The surface damages can be controlled by appropriate slurry chemistry and pol- ishing conditions. In general, a lower MRR is obtained on polar GaN surfaces compared to that on non-polar (11–20, 10–10) or semi-polar (11–22) planes, which is related to the higher hardness of these planes (0001, 000–1), revealed in an elasto-plastic mechanical behavior study of the GaN planes carried out recently. 13 From a critical review of the literature it has been observed that the CMP of Ga polar GaN surface is equally affected both by the chemical (use of different oxidizers in the literature) and mechanical (use of different abrasive particles) effect of polishing. It is seen from the literature that either only colloidal silica, 10,11 with varying pH and very high solid loading (∼40%) or a mixture of sodium-hypochlorite and alumina abrasives, 12 was used in CMP of polar GaN surfaces. In the present work, however, either a mixture of colloidal silica and KMnO 4 or colloidal alumina and KMnO 4, with very low abrasive particle concentrations (1.25–5.00 wt%), have been used as the polishing chemical. In the present investigation, we report on the comparative study of CMP processes carried out on Ga polar (0001) GaN surface using colloidal Al 2 O 3 and SiO 2 abrasive nanopar- ticles with appropriate slurry chemistry and experimental conditions. The effect of CMP process parameters, such as abrasive particles and its concentration, polishing pressure, platen velocity, carrier velocity, and slurry chemistry, on the material removal rate (MRR) and surface finish of Ga polar GaN surface have been reported. An atomically flat and smooth surface with RMS surface roughness of ∼1 Å has been obtained on the CMP processed Ga- polar GaN surface. Experimental Commercially available GaN film grown on 2 inch sapphire sub- strate was used in this study. All the CMP experiments have been carried out on a Buehler Ecomet 250 variable speed Grinder Polisher with Automet 250 power head. Cabot D100 pad was used for all the CMP experiments. The GaN wafer was mounted on a 2 ′′ stainless steel carrier using Crystal Bond 509 Clear epoxy resin. The wafer was char- acterized for crystal structure and surface characteristics before the CMP experiments. CMP slurries consisted of oxidizer and abrasive particles, silica (SiO 2 ) or alumina (Al 2 O 3 ). Potassium permanganate (KMnO 4 ) has been used as oxidizer in this investigation. The colloidal abrasive particles used in the experiments had a mean size of ∼50 nm with a solid loading of ∼40% for colloidal Al 2 O 3 slurry and mean size of ∼20 nm with a solid loading of ∼20% for colloidal SiO 2 slurry. The slurry was delivered on the polishing pad by a peristaltic pump (Ravel, Model No. RH-P100VS-100) at a flow rate of 10 mL/min. The typical time of each polishing experiment was 15 minutes, with ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 14.139.69.199 Downloaded on 2014-10-15 to IP