The effect of the physico-chemical properties of cellulosic

J Electroceram(2006)17:835–839DOI10.1007/s10832-006-9334-1The effect of the physico-chemical properties of cellulosic polymers on the Si wafer polishing processSang-Kyun Kim·Ye-Hwan Kim·Ungyu Paik·Takeo Katoh·Jea-Gun ParkReceived:29June2005/Revised:15May2006/Accepted:26May2006CSpringer Science+Business Media,LLC2006Abstract Cellulosic polymers were used as organic addi-tives to reduce the wafer surface roughness during the polish-ing process.The physico-chemical affinity of the polymer for the wafer and the formation of a hydro-plane on the Si wafer surface were investigated to identify the role of the poly-meric additives in the wafer-polishing process.Two different polymers,hydroxyethyl cellulose(HEC)and carboxymethyl cellulose(CMC),were employed as a surface modifier for the wafer.The wettability of the Si surface varied markedly between the different suspensions prepared with HEC and CMC due to different adsorptive behaviors.As a result,the suspension prepared with HEC reduced the haze level and micro-roughness of the wafer,enhancing overall polishing performance.Keywords CMP.Wafer polishing process.Cellulosic polymers.Micro scratch.Topography1IntroductionThe development of the next generation LSI devices now re-quires smaller device dimensions below50nm.To achieve multilevel-interconnections,a wide range of global pla-narization is required to attain a sufficient depth of focus (DOF)with the process margin of exposure equipment for the fine patterns of process technology[1,2].The manufacture S.-K.Kim·Y.-H.Kim·U.Paik( )Division of Advanced Materials Science Engineering,Seoul133-791,Koreae-mail:upaik@hanyang.ac.krT.Katoh·J.-G.ParkNano-SOI Process Laboratory,Hanyang University,Seoul133-791,Korea of semiconductor devices is significantly affected by the sur-face roughness of the Si wafer.Chemical mechanical pla-narization(CMP)is one of the key processes used for the planarization of the wafer surface[3–5].In the wafer polishing process,slurry conditions are crit-ical for the acquisition of high quality planarized wafers through chemical and mechanical interactions.However,pol-ishing slurries include abrasives which cause severe damage to the wafers after polishing.The problems commonly found in thefinal CMP process include surface scratching,polish-ing haze and residual particles[6–11].These defects gener-ated during the CMP process,including pin-holes,a lower punch-through voltage and the increase of current leakage, can directly influence the electrical characteristics and pro-ductivity of the devices in ULSI.In the wafer polishing process,the improper contact of abrasive particles on the wafer surface inevitably induces micro-scratches and a rugged topography on the wafer sur-face.Generally,organic additives such as cellulosic polymers have been used to resolve these problems.The formation of a hydro-plane resulting from the physico-chemical affinity of these polymeric molecules for the wafer is a key factor in a successful wafer polishing process[12].This hydro-plane prevents the direct contact of the abrasive on the wafer surface,and thus the polishing is conducted primarily by a chemical reaction and only indirectly by mechanical re-moval.Consequently,the interaction between the polymeric additives and wafer surface should be understood in order to manipulate the interaction force between the abrasive and the wafer surface.In this study,two different colloidal silica slurries were prepared containing two cellulosic polymers,hydroxyethyl cellulose(HEC)and carboxymethyl cellulose(CMC).The effect of the physico-chemical properties of cellulosic poly-mers on the wafer polishing process was investigated withthe aim of reducing roughness,micro-roughness,and haze on the wafer surface after polishing.2Experimental procedureNanometer-sized colloidal silica(PL-3,Fuso chemical Inc., Japan)was used in the present investigation.Three different colloidal silica slurries were prepared with hydroxyethyl cel-lulose(HEC)(Mw=25000–30000,Polysciences,U.S.A.) and carboxymethyl cellulose(CMC)(Mw=25000–30000, Polysciences,U.S.A.).The slurries were composed of0.4 wt%HEC,0.4wt%CMC,and a combination of0.2wt% HEC and0.2wt%CMC.The solid content of the suspension wasfixed at a weight fraction of10%.Suspension pH was adjusted to10using a1M ammonium hydroxide solution.The surface tension of the0.4wt%cellulosic polymer suspensions,which were prepared at pH10,was measured by a dynamic contact angle analyzer(DCA)(DCA-312,CAHN Instruments,Inc.,U.S.A.).The contact angle was measured by a drop shape analysis system(DSA)(FTA100,First Ten Angstroms,Inc.,U.S.A.).The contact angle measurement was obtained by the sessile drop method[13].To analyze the adsorption behavior of the cellulose-based polymer on the Si wafer,force-separation measurements were conducted using a commercial multimode atomic force microscope(AFM)(XE-150,PSIA Inc.,Seoul,Korea).A contact mode Si cantilever was used for investigating the interactinon forces of the bare AFM tip.To test wafer polishing,8-inch wafers were used after the stock polishing process.A diluted slurry was used for wafer polishing,with thefinal solid contentfixed at a weight fraction of0.3%.Polishing was performed on a Strasbaugh model6EC polisher with a single polishing head and a pol-ishing platen which used a Cegal pad(Toray Coatex Co., Japan).The polishing pressure,applied as a down force,was 3psi(pounds per square inch)and the back pressure was0 psi.The rotation speed of both the head and the table was 60rpm.Theflow rate was200cm3/min and the polishing time was5min.The surface roughness before and after pol-ishing was measured by AFM and an interferometric surface profiler(MP-2200,Chapman).The wafer surface haze was measured by a surfscan SP1(KLA-Tencor,U.S.A.).3Results and discussionThe surface tension(γ),wetting angle(θ),and spreading coefficient(S LS)of the cellulosic polymer suspensions are listed in Table1.Wettability can be quantitatively evaluated by the spreading coefficient,which is the energy difference between the solid substrate and the contacting liquid phase.It is well known that a large spreading coefficient indicates highTable1Surface tensions(γ),contact angles(θ)and spreadingcoefficients(S LS)of the cellulosic polymer suspensionsSuspensionsγ(dynes/cm)θ(degree)S LS(dynes/cm)DIW(H2O)73.1331.50−10.77HEC40.218.15−1.99CMC68.3229.22−8.68HEC+CMC53.5224.13−4.67 wettability.The wettability of the HEC solution was larger than that of de-ionized water and CMC.This phenomenon was due to the difference in the physicochemical properties of the cellulosic polymers.HEC is prepared by substituting OC2H4OH for hydroxyl groups(OH)in cellulose which consisted of a repeating structure of glucose and three re-placeable hydroxyl groups for each unit.Its functional group is a nonionic hydroxyl group.CMC is produced by reacting cellulose with sodium chloroacetate(ClCH2COONa).Its functional group is COOH which is ionized to COO−and Na+in aqueous solutions[14].Differences in the polymeric functional groups can result in different polymer adsorption behaviors on the wafer surface.When the wafer polishing was performed at alkaline pH,the Si surface had a negative charge[15].Due to the dissociation of the carboxylic acid groups,CMC presented a negative charge at alkaline pH. Therefore,the adsorption of CMC on the Si wafer surface was retarded by the presence of repulsive forces between negatively charged carboxyl group and negatively charged Si wafer surface.On the other hand,HEC can be adsorbed on the Si wafer either through hydrophobic interactions or through hydrogen bonding due to the fact that the hydroxyl group in the HEC was a non-dissociated ionizable functional group.Accordingly,nonionic polymers decreased the surface tension and enhanced wettability on the Si surface through polymer adsorption.These results are in general agreement with results reported previously[16,17]that describe an enhancement of wettability by the addition of polymeric sur-factants.The force-separation profile between the AFM tip and Si wafer was measured to analyze the interaction force between the cellulosic polymer and the Si wafer surface(Fig.1).It is well known that the Si surface has a negative charge at alkaline pH[15].As expected,it was shown that a repulsive force due to electrostatic interactions occurred between the bared Si wafer and the AFM Si tip at pH10.The range of interaction was about30nm.It is of interest that there was no significant difference of the surface forces between the tip andfilm,even in the presence of CMC,which indicated that as a result of the electrostatic repulsion between CMC and the Si wafer surface,CMC was only slightly adsorbed onto the Si wafer surface.At alkaline pH,the carboxylic acid of CMC is almost fully ionized and CMC has an extended-chain configuration[18].Therefore,it was suggested that the510F o r c e (n N )Separation (nm)Fig.1The force-separation profile between the AFM tip and Si wafer0.00.20.40.60.81.0slurriesh a z e (p p m a )Fig.2The ppma value after touch polishing measured by SP1formation of a hydro-plane on the Si wafer surface did not take place using the suspension prepared with CMC.In the case of HEC,a substantial change in the interac-tion forces between the Si wafer and AFM Si tip was ob-tained.The repulsive force apparently dissipated and then a weak attractive force appeared.This indicated that the max-imum attractive separation force was about 0.5nN at 5nm of separation.It was reported that poly(vinyl alcohol)(PV A)is adsorbed onto Si 3N 4or SiO 2,and the primary adsorption sites are silanols (Si OH)[19–21].For PV A adsorption ontoFig.3The surface roughness of the wafer after touch polishing;(a)Rms values after touch polishing measured by chapman and(b)The surface roughness profile of the AFM imagesSiO2,the deprotonation of Si OH is proposed.The adsorp-tion behavior of HEC on the wafer was believed to be similar to the adsorption of PV A on SiO2,because both polymeric molecules have the same hydroxyl functional groups.Thus,HEC could be adsorbed on the Si wafer via hydrogen bond-ing,leading to a decreased net surface charge,due to the shielding of the surface charge by HEC[20].In the case of a mixture of CMC and HEC,the interaction force measuredwas between the values obtained for CMC and HEC alone.Figure2shows the haze level of the wafer surface as a function of cellulosic polymer.The haze level indicating the surface roughness results in background scattering noise.The values of the dark narrow normal(DNN)and the dark wide normal(DWN)channels were calculated by a laser scatter-ing tool.The signal levels of the DNN and DWN channels corresponded to a longer and a shorter wavelength compo-nent of surface roughness,respectively.The suspension of CMC did not improve the haze compared to the raw wafer surface,while the suspension containing HEC had the lowest haze value amongst the slurries used.The adsorbed HEC on the wafer surface prevented silicon dissolution and the im-proper contact of the abrasive;thus,the surface roughness was reduced after polishing.In addition,the surface roughness of the wafer after touch polishing was measured by chapman and AFM.As shown in Fig.3(a),these results gave conclusive evidence of the effect that the adsorption of the cellulosic polymer has on the wafer polishing process.Analyses were performed over the follow-ing wavelength regions:(a)1–25μm(a short wavelength), (b)25–80μm(a medium wavelength)and(c)80–250μm(a long wavelength).For each wavelength range,the suspen-sion containing the HEC exhibited excellent surfaceflatness, which correlated well with the haze profiles.In Fig.3(b),the suspension containing HEC resulted in high surface planarity as a result of AFM.Consequently,it can be concluded that the hydro-plane layer on the wafer surface,which was formed by HEC,suppresses the mechanical polishing of silicon wafer.4ConclusionsThe effect of the physico-chemical properties of the cellu-losic polymer on the wafer polishing process was investi-gated.The surface wettability of the Si wafer was enhanced by the adsorption of HEC as a nonionic polymer.However, CMC as an anionic polymer was hardly adsorbed on the sur-face due to the repulsive force between the CMC and the Si wafer surface.This resulted in a low wettability of the Si wafer for CMC suspensions.The suspension prepared with HEC yielded a high planarity-surface profile for the wafer after the polishing process.The formation of a hydro-plane by the adsorption of the polymeric additive on the Si wafer was determined to be a key factor for the successful wafer polishing.References1.S.Wolfe,Silicon Processing(Lattice Press,CA,2002),p.313.2.S.K.Kim,U.Paik,S.G.Oh,Y.K.Park,T.Katoh,and J.G.Park,Jpn.J.Appl.Phys.,42,1227(2003).3.C.Teichert,J.F.Mackay,D.E.Savage,gally,M.Brohl,andP.Wagner,Appl.Phys.Lett.,66(18),2346(1995).4.C.F.Lin,W.T.Tseng,and M.S.Feng,J.Electrochem.Soc.,146(5),1984(1999).5.S.Sivaram,H.Bath,R.Leggeti,A.Maury,K.Monnig,and R.Tolles,Solid State Technol.,35–5,87(1992).6.T.Detzel,S.Hosali,A.Sethuraman,J.F.Wang,and L.Cook,inthe CMP-MIC Conference(Institute for Microelectronics Inter-Connection,Tampa,1997),p.202.7.G.B.Basim,J.J.Adler,U.Mahajan,R.K.Singh,and 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