Experimental Analysis on CMP Mechanism of Single Crystal SiC

International Conference on Planarization/CMP Technology · October 25 – 27, 2007 DresdenVDE VERLAG GMBH · Berlin-Offenbach<strong>Experimental</strong> <strong>Analysis</strong> on CMP Mechanism of Single Crystal SiCHoujun Lee 1 , Sukhoon Jeong 1 , Heondeok Seo 1 , Beomyoung Park 1 , Jiheon Oh 1 ,Haedo Jeong 1,# , and Hyoungjae Kim 21 Dept. of Precision & Mechanical Engineering, Pusan National University,Busan 609-735, Korea2 Busan R&D Center, Korea Institute of Industrial Technology, Busan 609-735, KoreaE-mail: hdjeong@pusan.ac.krSilicon carbide (SiC) is expanding its application field as a next generationcompound semiconductor because of its significant advantages in high power, highfrequency, low coefficient of thermal expansion and high thermal conductivity. For themanufacturing of SiC to semiconductor substrate, many researchers have studied onthe subject of SiC polishing. However, it is difficult to polish without defects such asscratches and subsurface damages, because SiC is a chemically and mechanicallystable material. Therefore, hybrid process, chemical mechanical polishing (CMP) hasbeen proposed to achieve epi-ready surface followed by fine diamond polishing.In this paper, the material removal rate (MRR) is investigated to recognize how longthe CMP process continues to remove a damaged layer by mechanical polishing using100 nm sized diamond, and find the dependency of mechanical factors such aspressure, velocity and abrasive concentration using NaOH based colloidal silica slurry.Especially, the authors tried to get an epi-ready surface with mixed abrasive slurry(MAS), in order to increase the removal activity and balance between mechanical andchemical factors in CMP process.Keywords: SiC, CMP, epi-ready surface, mixed abrasive slurry (MAS)1. IntroductionSiC expands its application area as a major compound semiconductor because of its hightemperaturecapabilities and excellent mechanical properties[1]. Since high level commercialdevices using SiC substrates requires the perfect surface without defects, the CMP process hasbeen considered to take a key role as a final wafering process. Several studies in CMP processhave been already reported using colloidal silica with varying of pH, chemicals andabrasives[2,3]. However, due to its mechanical hardness and chemical resistance the polishingof SiC wafer leaves scratches and subsurface damages, and the its MRR has not beenimproved more than 200 nm/hour[4,5]. Mechanical polishing of SiC is typically done withdiamond based slurries, where the abrasive size is successively reduced and eventually endingwith a submicron slurry to achieve the desired roughness. It is not relatively difficult toachieve the surface with low average roughness value. This surface can be seen defect-free bythe optical microscope, however shows some fine polishing damages under AFM microscopy.Moreover, after high-temperature thermal processing prior to or during epitaxial growth, adense network of scratches and defects are revealed[6].

International Conference on Planarization/CMP Technology · October 25 – 27, 2007 DresdenVDE VERLAG GMBH · Berlin-Offenbach( a )( b )( c )( d )( e )Fig. 4. Optical microscope and AFM images : (a) before CMP, and after CMP using (b) 20wt% colloidal silica slurry, (c) DIW + 25 nm diamond, (d) MAS-I (10 wt% colloidal silicaslurry + 25 nm diamond), and (e) MAS-II (20 wt% colloidal silica slurry + 25 nm diamond)Table 2. <strong>Experimental</strong> conditionsWafer2 inch silicon carbide wafer (6H-SiC on-axis)Pad Felt type pad (Suba 800)VelocityHead 120 rpm / Platen 120 rpmPressure 1.2 kg/cm 2SlurryFlow rateProcess time20 wt% Colloidal silica(KOH base, 120 nm)DIW + 25nm diamondMAS-I(10 wt% Colloidal silica +25 nm diamond)125 ml/min2 hoursMAS-II(20 wt% Colloidal silica+ 25 nm diamond)

International Conference on Planarization/CMP Technology · October 25 – 27, 2007 DresdenVDE VERLAG GMBH · Berlin-OffenbachTable 3. Measurement results after CMP of each conditionBefore test(a)20 wt% Slurry(b)DIW + diamond(c)MAS-I(d)MAS-II(e)Ra () 8 10.5 24.8 15.5 2.4Rv (nm) 6 12 23 12 1.2The average roughness value (Ra) before SiC CMP is 8, and the deepest scratch depth(Rv) is 6 nm on non-CMP tested SiC wafer. The Ra and Rv of the measured window range(10 10 ) of AFM increased up to 10.5 and 12 nm in the case of (b) (20 wt%colloidal silica slurry), respectively. But, the Ra without scratches was down to 2.5,because the shallow scratch after the mechanical polishing is removed by silica abrasives. TheRa and Rv of (c) case (diluted suspension with diamond abrasives) dramatically increased. Itis becuase the diamond abrasives have only mechanical effect on the material removal. Whenthe SiC wafer was polished by using mixed abrasive slurry (MAS) which is high mechanicaland high chemical condition, the Ra and Rv value were 2.4 and 1.2 nm in MAS-II case,respectively. Also, Fig. 4 (e) shows that the scratch effectively reduced. This result isoutstanding in comparison with other experimental conditions. It is because the diamondabrasives easily remove the reacted layer on deeper scratches in KOH based slurry and thesilica abrasives remove the shallow scratches at the same time. However, the MAS-I couldnot improve the surface roughness because of the drop of ability to remove shallow scratchesaccording to the decrease of silica abrasives. Therfore, the balance among abrasiveconcentration, abrasive type and based chemicals in slurry is important to ensure the materialremoval rate for SiC CMP process.4. ConclusionIn order to make the surface of SiC wafer which finished mechanical polishing process intofine surface, it goes through the CMP process. Using colloidal silica slurry, if pressure,velocity and concentration of abrasive increased, the MRR goes up as well. However the subsurfacedamage such as deep scratches could not removed, the surface roughness did notimprove. To solve these problems of silica, MAS (colloidal silica + diamond) was used in SiCCMP. Increasing of a kind energy among various energies did not work. So, to increasemechanical and chemical energy, MAS was used. The scratch was removed and the surfaceroughness improved only when using MAS with both mechanical and chemical energiesraised. Through this experiment, the importance of mechanical and chemical energy balancewas confirmed. Research on finding out the influence through diverse mixed conditions isneeded.References[1] L. Tong, M. Mehregany, L. G. Matus, 5th Technical Digest, IEEE, 198-201 (1992)[2] C. L. Neslen, W. C. Mitchel, R. L. Hengehold, J. Electron. Mater. Vol. 30, 1271 (2001)[3] L. Zhou, V. Audurier, P. Pirouz, J. A. Powell, J. Electrochem. Soc. Vol. 144, L161 (1994)[4] J. C. Zolper, M. Skowronski, MRS Bulletin, Vol. 30, 273 April (2005)[5] J. A. Powell, D. J. Larkin, P. B. Abel, J. Electron. Mater. Vol. 24, 295 (1995)[6] C. Martin, T. M. Kerr, W. Stepko, T. Anderson, Compound semiconductor manufacturingtechnology, .291-294 (2004)