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Experimental Analysis on CMP Mechanism of Single Crystal SiC

Experimental Analysis on CMP Mechanism of Single Crystal SiC

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Internati<strong>on</strong>al C<strong>on</strong>ference <strong>on</strong> Planarizati<strong>on</strong>/<strong>CMP</strong> Technology · October 25 – 27, 2007 DresdenVDE VERLAG GMBH · Berlin-Offenbach<str<strong>on</strong>g>Experimental</str<strong>on</strong>g> <str<strong>on</strong>g>Analysis</str<strong>on</strong>g> <strong>on</strong> <strong>CMP</strong> <strong>Mechanism</strong> <strong>of</strong> <strong>Single</strong> <strong>Crystal</strong> <strong>SiC</strong>Houjun Lee 1 , Sukho<strong>on</strong> Je<strong>on</strong>g 1 , He<strong>on</strong>deok Seo 1 , Beomyoung Park 1 , Jihe<strong>on</strong> Oh 1 ,Haedo Je<strong>on</strong>g 1,# , and Hyoungjae Kim 21 Dept. <strong>of</strong> Precisi<strong>on</strong> & Mechanical Engineering, Pusan Nati<strong>on</strong>al University,Busan 609-735, Korea2 Busan R&D Center, Korea Institute <strong>of</strong> Industrial Technology, Busan 609-735, KoreaE-mail: hdje<strong>on</strong>g@pusan.ac.krSilic<strong>on</strong> carbide (<strong>SiC</strong>) is expanding its applicati<strong>on</strong> field as a next generati<strong>on</strong>compound semic<strong>on</strong>ductor because <strong>of</strong> its significant advantages in high power, highfrequency, low coefficient <strong>of</strong> thermal expansi<strong>on</strong> and high thermal c<strong>on</strong>ductivity. For themanufacturing <strong>of</strong> <strong>SiC</strong> to semic<strong>on</strong>ductor substrate, many researchers have studied <strong>on</strong>the subject <strong>of</strong> <strong>SiC</strong> polishing. However, it is difficult to polish without defects such asscratches and subsurface damages, because <strong>SiC</strong> is a chemically and mechanicallystable material. Therefore, hybrid process, chemical mechanical polishing (<strong>CMP</strong>) hasbeen proposed to achieve epi-ready surface followed by fine diam<strong>on</strong>d polishing.In this paper, the material removal rate (MRR) is investigated to recognize how l<strong>on</strong>gthe <strong>CMP</strong> process c<strong>on</strong>tinues to remove a damaged layer by mechanical polishing using100 nm sized diam<strong>on</strong>d, and find the dependency <strong>of</strong> mechanical factors such aspressure, velocity and abrasive c<strong>on</strong>centrati<strong>on</strong> 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 <strong>CMP</strong> process.Keywords: <strong>SiC</strong>, <strong>CMP</strong>, epi-ready surface, mixed abrasive slurry (MAS)1. Introducti<strong>on</strong><strong>SiC</strong> expands its applicati<strong>on</strong> area as a major compound semic<strong>on</strong>ductor because <strong>of</strong> its hightemperaturecapabilities and excellent mechanical properties[1]. Since high level commercialdevices using <strong>SiC</strong> substrates requires the perfect surface without defects, the <strong>CMP</strong> process hasbeen c<strong>on</strong>sidered to take a key role as a final wafering process. Several studies in <strong>CMP</strong> processhave been already reported using colloidal silica with varying <strong>of</strong> pH, chemicals andabrasives[2,3]. However, due to its mechanical hardness and chemical resistance the polishing<strong>of</strong> <strong>SiC</strong> wafer leaves scratches and subsurface damages, and the its MRR has not beenimproved more than 200 nm/hour[4,5]. Mechanical polishing <strong>of</strong> <strong>SiC</strong> is typically d<strong>on</strong>e withdiam<strong>on</strong>d based slurries, where the abrasive size is successively reduced and eventually endingwith a submicr<strong>on</strong> 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 <strong>of</strong> scratches and defects are revealed[6].


Internati<strong>on</strong>al C<strong>on</strong>ference <strong>on</strong> Planarizati<strong>on</strong>/<strong>CMP</strong> Technology · October 25 – 27, 2007 DresdenVDE VERLAG GMBH · Berlin-Offenbach( a )( b )( c )( d )( e )Fig. 4. Optical microscope and AFM images : (a) before <strong>CMP</strong>, and after <strong>CMP</strong> using (b) 20wt% colloidal silica slurry, (c) DIW + 25 nm diam<strong>on</strong>d, (d) MAS-I (10 wt% colloidal silicaslurry + 25 nm diam<strong>on</strong>d), and (e) MAS-II (20 wt% colloidal silica slurry + 25 nm diam<strong>on</strong>d)Table 2. <str<strong>on</strong>g>Experimental</str<strong>on</strong>g> c<strong>on</strong>diti<strong>on</strong>sWafer2 inch silic<strong>on</strong> carbide wafer (6H-<strong>SiC</strong> <strong>on</strong>-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 diam<strong>on</strong>dMAS-I(10 wt% Colloidal silica +25 nm diam<strong>on</strong>d)125 ml/min2 hoursMAS-II(20 wt% Colloidal silica+ 25 nm diam<strong>on</strong>d)


Internati<strong>on</strong>al C<strong>on</strong>ference <strong>on</strong> Planarizati<strong>on</strong>/<strong>CMP</strong> Technology · October 25 – 27, 2007 DresdenVDE VERLAG GMBH · Berlin-OffenbachTable 3. Measurement results after <strong>CMP</strong> <strong>of</strong> each c<strong>on</strong>diti<strong>on</strong>Before test(a)20 wt% Slurry(b)DIW + diam<strong>on</strong>d(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 <strong>SiC</strong> <strong>CMP</strong> is 8, and the deepest scratch depth(Rv) is 6 nm <strong>on</strong> n<strong>on</strong>-<strong>CMP</strong> tested <strong>SiC</strong> wafer. The Ra and Rv <strong>of</strong> the measured window range(10 10 ) <strong>of</strong> AFM increased up to 10.5 and 12 nm in the case <strong>of</strong> (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 <strong>of</strong> (c) case (diluted suspensi<strong>on</strong> with diam<strong>on</strong>d abrasives) dramatically increased. Itis becuase the diam<strong>on</strong>d abrasives have <strong>on</strong>ly mechanical effect <strong>on</strong> the material removal. Whenthe <strong>SiC</strong> wafer was polished by using mixed abrasive slurry (MAS) which is high mechanicaland high chemical c<strong>on</strong>diti<strong>on</strong>, 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 comparis<strong>on</strong> with other experimental c<strong>on</strong>diti<strong>on</strong>s. It is because the diam<strong>on</strong>dabrasives easily remove the reacted layer <strong>on</strong> 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 <strong>of</strong> the drop <strong>of</strong> ability to remove shallow scratchesaccording to the decrease <strong>of</strong> silica abrasives. Therfore, the balance am<strong>on</strong>g abrasivec<strong>on</strong>centrati<strong>on</strong>, abrasive type and based chemicals in slurry is important to ensure the materialremoval rate for <strong>SiC</strong> <strong>CMP</strong> process.4. C<strong>on</strong>clusi<strong>on</strong>In order to make the surface <strong>of</strong> <strong>SiC</strong> wafer which finished mechanical polishing process int<strong>of</strong>ine surface, it goes through the <strong>CMP</strong> process. Using colloidal silica slurry, if pressure,velocity and c<strong>on</strong>centrati<strong>on</strong> <strong>of</strong> 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 <strong>of</strong> silica, MAS (colloidal silica + diam<strong>on</strong>d) was used in <strong>SiC</strong><strong>CMP</strong>. Increasing <strong>of</strong> a kind energy am<strong>on</strong>g various energies did not work. So, to increasemechanical and chemical energy, MAS was used. The scratch was removed and the surfaceroughness improved <strong>on</strong>ly when using MAS with both mechanical and chemical energiesraised. Through this experiment, the importance <strong>of</strong> mechanical and chemical energy balancewas c<strong>on</strong>firmed. Research <strong>on</strong> finding out the influence through diverse mixed c<strong>on</strong>diti<strong>on</strong>s isneeded.References[1] L. T<strong>on</strong>g, M. Mehregany, L. G. Matus, 5th Technical Digest, IEEE, 198-201 (1992)[2] C. L. Neslen, W. C. Mitchel, R. L. Hengehold, J. Electr<strong>on</strong>. 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. Skowr<strong>on</strong>ski, MRS Bulletin, Vol. 30, 273 April (2005)[5] J. A. Powell, D. J. Larkin, P. B. Abel, J. Electr<strong>on</strong>. Mater. Vol. 24, 295 (1995)[6] C. Martin, T. M. Kerr, W. Stepko, T. Anders<strong>on</strong>, Compound semic<strong>on</strong>ductor manufacturingtechnology, .291-294 (2004)

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