The Effect of Pad Grooving and Texturing on CMP Process ...

(1) Slurry Dispensed onto <strong>Pad</strong>- Texture controls slurry retention on platen- Grooves influence initial film thickness(5) Slurry Held by <strong>Pad</strong> is Recycled- Texture controls slurry retention on platen- Grooves protract residence time on padΩ WaferΩ Platen(4) <strong>Pad</strong>-Wafer-Slurry Contact- Texture controls material removal(2) Slurry Spreads across <strong>Pad</strong>- Grooves/texture balance controls- <strong>Pad</strong> texture affects wetting anglewafer-scale transport- Grooves dictate spread direction(3) <strong>Pad</strong> Draws Slurry under Wafer- <strong>Pad</strong> texture moves slurry against drag <strong>of</strong> wafer- Grooves augment slurry carrying capacityFigure 1: Role <strong>of</strong> <strong>Pad</strong> Grooves <strong>and</strong> Texture in CMP Slurry Flow <strong>and</strong> Transport<strong>The</strong> present work compares several groove arrays <strong>and</strong> texture variants to examine theresponse <strong>of</strong> transport phenomena in the pad-wafer gap. Predicted wafer pr<strong>of</strong>iles areexamined to quantify the potential impact <strong>of</strong> pad grooving <strong>and</strong> texturing on processperformance <strong>and</strong> to identify features that favor efficient slurry mixing <strong>and</strong> heat removal,that in turn lead to uniform defect-free polishing.2. BackgroundWafer-scale CMP models published pre-2003 represent transport in the pad-wafergap using the Reynolds hydrodynamic equation [1,2,3], sometimes with “flow factors”to account for the effect <strong>of</strong> rough surfaces on the conveyed flow <strong>of</strong> slurry [4]. Whileusually adequate for predicting global velocity <strong>and</strong> temperature fields under the waferfor an ungrooved CMP pad, the lubrication approximation cannot capture the sharpdisparities in transport fields that prevail on grooved pads [5] nor inertial flow effectswithin the texture <strong>of</strong> a pad that may affect polishing [6].A porous-media flow approach was introduced [7] in 2003 to characterize CMP padsby forcing fluid over the surface as compressed between two flat plates <strong>and</strong> measuringthe resulting pressure loss pr<strong>of</strong>iles. <strong>Pad</strong> texture is thus quantified as a population <strong>of</strong>flow obstacles, which gives a more realistic picture <strong>of</strong> pad-slurry interaction under thewafer in a typical polish process (Fig. 2). Application <strong>of</strong> the Ergun equation [8] to thepressure pr<strong>of</strong>iles produces “porous-media” texture descriptors suitable for direct use incomputational fluid dynamics (CFD) simulations.Subsequent CFD studies using flow-based texture descriptors [9,10] have shown thattransport fields in the pad-wafer gap may depart significantly from those predicted bylubrication theory. For example, it has been found that pad texture interacts stronglywith pad-wafer relative velocity to determine how effectively fresh slurry replacesspent slurry [11]. In addition, grooves mitigate the effect <strong>of</strong> wafer tilt <strong>and</strong> curvatureon fluid pressures in the pad-wafer gap [5]. As a result, dual-axis polisher modelsthat omit grooves overestimate the angles <strong>of</strong> attack <strong>and</strong> pitch required to satisfy force<strong>and</strong> torque equilibria on a gimbaled wafer.32

Gap HeightHCharacteristicLength D EVoid FractionεHFluid In, WTop Plate withPressure SensorsCMP <strong>Pad</strong>SampleHr 0rp(r)WaferUniform PressureBottom PlateFluid Out<strong>Pad</strong>RadialPressurePr<strong>of</strong>ilesp(r)p 0FluidInExperimentalDataCFD Resultfor Chosenε <strong>and</strong> D EFluidOutSlurry Flow0r 0rFigure 2: Porous-Media Representation<strong>of</strong> CMP <strong>Pad</strong> Asperity LayerFigure 3: Flow-Based <strong>Pad</strong> TextureCharacterization Experiment3. Experimental Apparatus <strong>and</strong> Data Analysis<strong>The</strong> flow-based texture characterization apparatus, detailed elsewhere [7], consists<strong>of</strong> two 200-mm diameter flat plates between which a pad sample <strong>of</strong> the same size ismounted (Fig. 3). A hydraulic lift applies a controlled pressure to raise the bottomplate <strong>and</strong> bring the pad into contact with the top plate. Fluid enters through a centralinlet in the top plate, flows radially outward through the compressed asperity layer <strong>of</strong>the sample, <strong>and</strong> exits at the perimeter. A polar sensor grid on the top plate registersthe pressure field p(r,θ) due to fluid flow over the pad surface. Sensors accurate to0.1 psi are located every 9.5 mm along twelve equally-spaced radii. Pressure loss dataare taken at multiple downforces within the 0.5 to 8-psi range <strong>of</strong> industrial CMPpractice <strong>and</strong> at several flow rates spanning the range <strong>of</strong> viscous <strong>and</strong> inertial fluid forcesin commercial polishers. Plotting the difference <strong>of</strong> p 2 between two radii against fluidflow rate determines the asperity layer flow resistance based on the Ergun equation [6]:p ( r+ BW2223) − p(r10) = AW. (1)Thus A <strong>and</strong> B are constants for a given pad surface compressed under a fixed downforce.Generally the AW (viscous) term is larger, <strong>and</strong> A is taken as the surface flow resistance.4. Experimental Results <strong>and</strong> DiscussionFig. 4 shows fluid pressure drops in the pad-plate gap for three roughness variants <strong>of</strong>a hard (Type III [12]) polyurethane pad [6]. At 1.2 psi downforce, pressure drop <strong>and</strong>surface flow resistance (slope <strong>of</strong> ∆(p 2 ) versus W) are similar across all roughnesses.At 3 psi <strong>and</strong> higher, the low-roughness pad resistance becomes progressively largerthan the mid-roughness, which remains slightly larger than the high-roughness.Lower surface flow resistance at higher roughness was also observed for s<strong>of</strong>t pads [13]<strong>and</strong> illustrates that rougher surfaces have larger void fractions. However in s<strong>of</strong>t padsthe roughness influence was strong at 1-2 psi <strong>and</strong> weak at 4-6 psi (where it wastheorized that surface voids were flattened), while in Fig. 4 the effect is stronger athigher downforce. <strong>The</strong> contrast highlights two different regimes in the continuum <strong>of</strong>pad compliance. Being stiffer, the hard pad is less compliant at lower downforce <strong>and</strong>surface roughness effects are minimal, while compliance increases at higher downforce<strong>and</strong> roughness begins to manifest in the flow resistance. Under similar downforces as<strong>of</strong>t pad is almost fully compliant. Surface void deformation proposed in s<strong>of</strong>t padslikely occurs in hard pads only at very high downforces uncommon to CMP.33 PacRim-CMP 2004

1.2E+101.2E+10DF = 1.2psiLow Mid HighDF = 3.0 psiLow Mid Highp(3) 2 - p(10) 2 , Pa 28.0E+094.0E+09p(3) 2 - p(10) 2 , Pa 28.0E+094.0E+090.0E+000.0 1.0 2.0 3.0W, Fluid Flow Rate, kg/s x 10 60.0E+000.0 1.0 2.0 3.0W, Fluid Flow Rate, kg/s x 10 61.2E+101.2E+10DF = 5.8 psiLow Mid HighDF = 8.0 psiLow Mid Highp(3) 2 - p(10) 2 , Pa 28.0E+094.0E+09p(3) 2 - p(10) 2 , Pa 28.0E+094.0E+090.0E+000.0 1.0 2.0 3.0W, Fluid Flow Rate, kg/s x 10 60.0E+000.0 1.0 2.0 3.0W, Fluid Flow Rate, kg/s x 10 6Figure 4: Surface Flow Resistance for Circular Grooved Hard <strong>Pad</strong> Variants <strong>of</strong> Low, Mid,<strong>and</strong> High Roughness at Four Applied DownforcesFig. 5 summarizes Cu, TEOS, <strong>and</strong> TaNremoval rates for the hard pad roughnessvariants used in a 3-psi CMP process [6].<strong>The</strong> observed rates do not correlate withpost-polish pad roughness measurements,but they form a clear <strong>and</strong> linear responseto surface flow resistance. Highersurface flow resistance implies a largernumber <strong>of</strong> asperities in contact ornear-contact with the confining surface,a state that also favors greater materialremoval rate under a fixed downforce.However removal rate is also affected bypad texture through transport phenomena<strong>and</strong> the resulting distribution <strong>of</strong> heat <strong>and</strong>fresh slurry chemistry under the wafer,as shown in the computational resultsthat follow.Removal Rate, A 0 /min950820690560430HighRoughnessMidRoughnessLowRoughness3002 2.5 3 3.5 4 4.5<strong>Pad</strong> Surface Flow Resistance,Pa 2 s/kg x 10 9CuTEOSTaNFigure 5: Wafer Polish Results for CircularGrooved Hard <strong>Pad</strong> Variants <strong>of</strong> Low, Mid,<strong>and</strong> High Roughness at 3 psi Downforce5. Computational ModelEarlier publications have presented the CFD model built for a commercial dual-axisrotary CMP tool polishing a 200-mm wafer on a 508-mm pad. Fixed meshes were usedfor concentric circular grooves [5,7,9] <strong>and</strong> sliding meshes for radial <strong>and</strong> XY [10] <strong>and</strong>spiral grooves [11]. In all cases the flow domain was the thin gap bounded above bythe wafer <strong>and</strong> below by the textured pad areas <strong>and</strong> groove surfaces (Fig. 6). FLUENT6.1 was used to solve the 3D Navier-Stokes <strong>and</strong> continuity equations with a no-slip <strong>and</strong>no-penetration condition at the moving pad <strong>and</strong> wafer surfaces <strong>and</strong> a pressure boundary34

at the wafer edge. Fluid cells were used in the grooves <strong>and</strong> porous-media cells in thetextured areas with ε <strong>and</strong> D E values for the subject pad material <strong>and</strong> downforce. <strong>Pad</strong>rotation was 33 rpm; wafer speeds from 12 to 61 rpm were studied. Polish downforcewas 2 or 3 psi in the cases presented here. <strong>Effect</strong>s <strong>of</strong> pad texture were examined byusing measured porous-media parameters (H, ε, <strong>and</strong> D E in Fig. 2) for pads subjected todifferent levels <strong>of</strong> conditioning to produce surface textures ranging from coarse to fine.Temperature <strong>and</strong> concentration fields were generated by solving the energy transport<strong>and</strong> mass transport equations for porous media coupled with the momentum equations.Frictional <strong>and</strong> chemical heat source terms [9] were included with a pad thermalconductivity estimated at 0.30 W/m/K°. Consumption <strong>of</strong> fresh slurry <strong>and</strong> generation<strong>of</strong> polish debris <strong>and</strong> spent slurry were dictated by pseudo-first order surface kineticswith a rate constant <strong>of</strong> 1.08 x 10 -3 s -1 at 25°C <strong>and</strong> activation energy <strong>of</strong> 10-15 kcal/gmol.20” <strong>Pad</strong><strong>Pad</strong> Edge200 mm WaferArea <strong>of</strong>Detail20” <strong>Pad</strong>Grooves<strong>Pad</strong>CenterShape <strong>of</strong>FluidVolumeΩ WaferLocation <strong>of</strong>Fresh SlurryInfusion Front200 mmWafer0.0 sec0.1 sec0.2 sec0.3 secΩ PlatenGap Height43 µmL<strong>and</strong> Area(Porous Cells)0.4 secGroove(Fluid Cells)Figure 6: Computational Model <strong>of</strong> Slurry Flowbetween 200-mm Wafer <strong>and</strong> Grooved <strong>Pad</strong>Figure 7: CFD Transient Slurry MixingAnalysis for 33 rpm Platen Speed6. Computational Results <strong>and</strong> Discussion<strong>The</strong> impact <strong>of</strong> pad grooving <strong>and</strong> texturing is illustrated via the transient dynamics bywhich fresh slurry displaces spent slurry under the wafer (Fig. 7). At time zero, spentslurry occupies the pad-wafer gap <strong>and</strong> fresh slurry is released at the wafer leading edge.As the pad rotates, the fresh/spent slurry interface proceeds under the wafer where it issubject to the complicating influence <strong>of</strong> wafer rotation <strong>and</strong> the unequal flow resistances<strong>of</strong> grooves <strong>and</strong> l<strong>and</strong> areas. Slurry renewal is gauged at fixed time intervals to comparethe effectiveness <strong>of</strong> different groove <strong>and</strong> texture variants. Were the wafer absent, theinterface would advance at the speed <strong>of</strong> the pad, <strong>and</strong> for a 33-rpm platen the pad-wafergap would infuse with fresh slurry in 0.42 s. Actual renewal times are longer becauseat many locations the wafer surface moves more slowly or even in an opposite directionthan the pad. Regions <strong>of</strong> delayed slurry renewal signal slower transport phenomenafavoring locally higher temperature <strong>and</strong> lower concentration <strong>of</strong> fresh chemistry.Comparison <strong>of</strong> Groove PatternsSlurry mixing dynamics are shown in Figs. 8, 9, 10, <strong>and</strong> 11 for concentric circular,radial, XY grid, <strong>and</strong> spiral groove patterns respectively at 33 rpm platen speed <strong>and</strong> 33rpm wafer speed. In all cases the principal mixing interface advances under the waferas anticipated in Fig. 7, but each groove pattern interacts in a particular way with thedirection <strong>of</strong> local wafer motion <strong>and</strong> imparts to the flow field a different departure fromrigid-body movement <strong>of</strong> the slurry.35 PacRim-CMP 2004

Circular grooves (Fig. 8) lead to a broad b<strong>and</strong> <strong>of</strong> transitional slurry concentrationwith long <strong>and</strong> distinct trailing wakes in each groove. This flow field results becausethe textured areas act as momentum sources throughout, whereas the grooves can drivefluid only via drag at the groove walls. Fluid slip in circular grooves is thus a directconsequence <strong>of</strong> their exact <strong>and</strong> constant alignment with the direction <strong>of</strong> pad rotation.Velocities in the grooves are up to 50% less than in the immediately adjacent l<strong>and</strong> areas,<strong>and</strong> regions <strong>of</strong> 100% fresh slurry are separated by tracts <strong>of</strong> as low as 55% fresh slurry.Extended wakes form <strong>and</strong> persist along the line linking the pad <strong>and</strong> wafer centersbecause only at this location does the wafer velocity oppose the pad velocity <strong>and</strong> haveno component transverse to the groove to drive mixing in the groove cross-section.Fresh Slurry Fractionin Plane 5µ below WaferΩ PlatenFreshSlurryFreshSlurryFreshSlurryTexture AreaGrooveSpentSlurryΩ PlatenΩ Wafer Ω WaferSpentSlurrySpentSlurry0.08 sec 0.22 secCase:Table 33 rpmWafer 33 rpmDownforce 2 psiMedium ConditioningFlat 43 µm GapDetail at 0.22 secFigure 8: Slurry Mixing Dynamics in <strong>Pad</strong>-Wafer Gap for Concentric Circular GroovesRadial grooves (Fig. 9) create small but distinct jumps in an otherwise very broadb<strong>and</strong> <strong>of</strong> transitional slurry concentration. <strong>The</strong> mixing dynamics differ only marginallyfrom those <strong>of</strong> an ungrooved pad except near the pad center, where the diffusion <strong>of</strong> freshslurry under the wafer proceeds in discrete patches between each pair <strong>of</strong> grooves.<strong>The</strong>re is very little evidence <strong>of</strong> wafer motion. Being oriented perpendicular to the padrotation, radial grooves advance the slurry by positive displacement rather than surfacedrag, forestalling the fluid slip observed with circular grooves especially in the region<strong>of</strong> opposed pad <strong>and</strong> wafer velocities. Extended wakes also cannot form because aradial groove always has a transverse component <strong>of</strong> pad-wafer relative velocity tomotivate top-to-bottom mixing. However this same orientation prevents radialgrooves from relieving pressure extremes developed in the slurry during transit fromthe leading to trailing edge <strong>of</strong> the wafer, leading to a higher tendency for hydroplaning.Fresh Slurry Fractionin Plane 5µ below WaferΩ PlatenΩ Wafer Ω WaferΩ PlatenFreshSlurryFreshSlurrySpentSlurryFreshSlurrySpentSlurrySpentSlurryTexture AreaGroove0.08 sec 0.22 secCase:Table 33 rpmWafer 33 rpmDownforce 2 psiMedium ConditioningFlat 43 µm GapDetail at 0.22 secFigure 9: Slurry Mixing Dynamics in <strong>Pad</strong>-Wafer Gap for Radial Grooves36

Cartesian grid or XY grooves (Fig. 10) achieve a very sharp mixing front betweenfresh <strong>and</strong> spent slurry with only small regions <strong>of</strong> transitional concentration extendinginto groove segments ahead <strong>of</strong> the principal interface. Slurry renewal is efficientbecause XY grooves have a time-variant orientation relative to pad rotation that avoidsthe fluid slip <strong>of</strong> circular grooves, plus interconnections that limit fluid pressureextremes in the groove network. Moreover, textured areas <strong>and</strong> grooves are physicallyexchanged as the grid rotates, accelerating the process <strong>of</strong> fluid replacement. Detachedisl<strong>and</strong>s <strong>of</strong> relatively fresh slurry downstream <strong>of</strong> the principal interface (circled in Fig.10) reveal that fluid in the bottom <strong>of</strong> the grooves advances ahead <strong>of</strong> that near the wafersurface as the pad turns, <strong>and</strong> is raised by vertical mixing in the groove cross-section.This underscores the fully 3-D character <strong>of</strong> the flow field in many CMP pad grooves.Fresh Slurry Fractionin Plane 5µ below WaferΩ PlatenFreshSlurryFreshSlurryFreshSlurryTexture AreaΩ PlatenΩ WaferΩ WaferSpentSlurrySpentSlurryGrooveSpentSlurry0.08 sec 0.22 secCase:Table 33 rpmWafer 33 rpmDownforce 2 psiMedium ConditioningFlat 43 µm GapDetail at 0.22 secFigure 10: Slurry Mixing Dynamics in <strong>Pad</strong>-Wafer Gap for XY Grid GroovesSpiral grooves (Fig. 11) show a b<strong>and</strong> <strong>of</strong> transitional concentration similar in width tothat <strong>of</strong> circular grooves, but with extending mixing regions appearing in advance <strong>of</strong> theprincipal interface rather than trailing behind it. Relatively fresh slurry projects intothe spent slurry along every groove immediately after the interface begins from theleading edge. As the interface approaches the trailing edge, detached regions <strong>of</strong> freshslurry appear downstream in the grooves <strong>and</strong> bridge upstream to meet the interface.This unexpected result illustrates the same slurry tunneling that was observed in onlyisolated locations <strong>of</strong> the XY grid. Both slurry tunneling <strong>and</strong> the forwardly extendedmixing regions occur because slurry in a groove, upon entering the pad-wafer gap,decelerates sharply at the wafer surface due to drag <strong>and</strong> must accelerate in the bottom<strong>of</strong> the groove to maintain volume flow, that is, to satisfy the continuity constraint.Fresh Slurry Fractionin Plane 5µ below WaferFreshSlurryΩ PlatenΩ WaferΩ WaferΩ PlatenFreshSlurryDirection<strong>of</strong> WaferVelocityFreshSlurryTexture AreaGrooveSpentSlurrySpentSlurrySpentSlurry0.08 sec 0.22 secCase:Table 33 rpmWafer 33 rpmDownforce 2 psiMedium ConditioningFlat 43 µm GapDetail at 0.22 secFigure 11: Slurry Mixing Dynamics in <strong>Pad</strong>-Wafer Gap for Spiral Grooves37 PacRim-CMP 2004

<strong>The</strong> foregoing results illustrate that groove orientation relative to the local direction<strong>of</strong> wafer motion is a critical feature controlling fluid dynamics in the pad-wafer gap.Since each <strong>of</strong> the conventional groove designs has at least one advantage but most fallshort <strong>of</strong> ideal slurry renewal, advanced groove designs have been developed to combine<strong>and</strong> amplify the preferred features <strong>of</strong> simpler groove geometries.Comparison <strong>of</strong> <strong>Pad</strong> Surface TexturesThree texture variants <strong>of</strong> circular-grooved hard pads are compared in Fig. 12 interms <strong>of</strong> slurry mixing dynamics at 33/61 rpm platen <strong>and</strong> carrier speed under 3 psidownforce. Coarse texture, having high void fraction <strong>and</strong> low surface flow resistance,imparts little momentum to the slurry. L<strong>and</strong> areas <strong>and</strong> grooves are indistinguishableas both allow significant slip between the slurry <strong>and</strong> pad, <strong>and</strong> the last region to infusewith fresh slurry is just outside the wafer center where pad <strong>and</strong> wafer angular velocitiesbalance. Medium texture has higher surface flow resistance <strong>and</strong> imparts somemomentum to the slurry in the l<strong>and</strong> areas, more strongly countering the influence <strong>of</strong>wafer motion. Contrasting wakes are left in the (untextured) grooves <strong>and</strong> persistdisproportionately along the line linking the pad <strong>and</strong> wafer centers. <strong>The</strong> last point <strong>of</strong>slurry renewal is at the wafer edge because at this location the wafer angular velocity isopposed to the pad rotation <strong>and</strong> large enough to force the slurry in the reverse direction.Fine texture has the highest flow resistance <strong>and</strong> essentially grips the slurry in nearlyrigid-body rotation, rapidly infusing the l<strong>and</strong> areas under the wafer but allowing noslurry migration even to adjacent grooves. A sharply partitioned pr<strong>of</strong>ile thus occurs<strong>and</strong> slurry renewal is slowest in individual grooves near the pad center. It isnoteworthy that in all three texture cases, over two seconds are required to fully infusethe pad-wafer gap with fresh slurry, but the shape <strong>and</strong> location <strong>of</strong> the last-renewed areais entirely different.Coarse Texture Medium Texture Fine TextureArea <strong>of</strong>DetailMass Fraction<strong>of</strong> Fresh Slurryat Wafer Surface0.50 sec0.50 sec0.50 secCase:IC1000 TM K-Groove<strong>Pad</strong> 33 rpmWafer 61 rpmDownforce 3 psiVarious <strong>Pad</strong>Textures1.40 sec1.40 sec1.40 secFigure 12: Comparison <strong>of</strong> Transient Slurry Mixing in <strong>Pad</strong>-Wafer Gap for Three <strong>Pad</strong>Textures having Concentric Circular Grooves38

<strong>The</strong> impact <strong>of</strong> pad texture on mixing patterns directly influences the steady-statepr<strong>of</strong>iles <strong>of</strong> temperature <strong>and</strong> spent slurry concentration under the wafer during polishing(Fig. 13). Coarse texture leads to a hot zone 17 C above ambient centered halfwaybetween the wafer center <strong>and</strong> edge, <strong>and</strong> a broad spent slurry region across the trailingthird <strong>of</strong> the wafer. Medium texture induces a smaller hot spot 17 C above ambient justinside the wafer edge <strong>and</strong> a like-sized spent slurry zone along the line linking the pad<strong>and</strong> wafer centers. Fine texture results in spent slurry <strong>and</strong> temperatures 32 C aboveambient in each l<strong>and</strong> area at the trailing edge <strong>of</strong> the wafer, illustrating the extremes thatoccur in the absence <strong>of</strong> fluid exchange between textured areas <strong>and</strong> grooves. <strong>Pad</strong>textures <strong>of</strong> progressively smaller scales thus produce proportionally smaller features inthe steady transport fields in the pad-wafer gap, with sharper distinction betweentextured <strong>and</strong> untextured regions. Figs. 12 <strong>and</strong> 13 clearly show that pad conditioningshifts wafer polish pr<strong>of</strong>iles not only by improving pad-wafer contact, but also byrearranging the zones <strong>of</strong> highest temperature <strong>and</strong> most active slurry chemistry underthe wafer via the interaction <strong>of</strong> pad texture <strong>and</strong> pad-wafer relative velocity.Coarse Texture Medium Texture Fine TextureSlurry Temperature (°C) Slurry Temperature (°C) Slurry Temperature (°C)Mass Fraction Spent Slurry Mass Fraction Spent Slurry Mass Fraction Spent SlurryCase:IC1000 TM K-Groove<strong>Pad</strong> 33 rpmWafer 61 rpmDownforce 3 psiVarious <strong>Pad</strong> TexturesFigure 13: Comparison <strong>of</strong> Steady-State Slurry Temperature <strong>and</strong> Concentration Fields in<strong>Pad</strong>-Wafer Gap for Three <strong>Pad</strong> Textures having Concentric Circular GroovesComputational results for circular-grooved hard pads over a range <strong>of</strong> wafer rotationspeeds demonstrate that wafer rpm has far less impact on transport phenomena in thepad-wafer gap than does pad texture. Low wafer speed leads to accumulation <strong>of</strong> heat<strong>and</strong> spent chemistry at the trailing edge <strong>of</strong> the wafer, similar to fine texture. Highwafer speed produces localized hot spots <strong>and</strong> spent slurry at the 6 o’clock position,similar to medium texture. At fixed pad speed, pad texture <strong>and</strong> wafer speed determinethe relative influence <strong>of</strong> pad <strong>and</strong> wafer motion. However, pad texture is the strongerdriver because wafer speed dictates the fluid velocity only at the upper surface <strong>of</strong> thepad-wafer gap, whereas texture (through its effective porous media properties) controlsfluid conveyance throughout the volume <strong>of</strong> the asperity layer.39 PacRim-CMP 2004

Model cases were analyzed at combinations <strong>of</strong> characteristic length <strong>and</strong> surface voidfraction describing textures from very coarse to very fine covering a wide range <strong>of</strong> padsurface flow resistance (Table I). Both ε <strong>and</strong> D E are larger for coarse textures <strong>and</strong>smaller for fine textures. Fig. 14 compares the steady-state slurry temperature fieldscorresponding to all texture cases, three <strong>of</strong> which appeared in Fig. 13. At voidfractions close to unity, the length scale has no influence because the texture does notfill enough space to impart much momentum to the slurry. In these cases the hottestzone is near the wafer center with no apparent distinction between grooves <strong>and</strong> l<strong>and</strong>areas. At smaller void fractions, reducing the texture scale increases the influence <strong>of</strong>pad motion, moving heat from the wafer center to the edge. Long length-scaletextures concentrate the heat strongly where pad <strong>and</strong> wafer velocities are opposed, asthis is the most difficult region to cool with slurry flow. Short length-scale texturescollect the heat in each b<strong>and</strong> at the wafer trailing edge with minimal losses to coolerslurry in the grooves. Fig. 14 demonstrates that while no texture variant gives anisothermal pad-wafer gap, texture provides a degree <strong>of</strong> process control to shift the zone<strong>of</strong> highest temperature along the wafer radius.Table I: Viscous Flow Resistances <strong>of</strong> Various Surface Texturesε , void D E , characteristic length, micronsfraction 500 150 50 150.99 6.20E+04 6.90E+05 6.20E+06 6.90E+070.70 1.60E+08 1.70E+09 1.60E+10 1.70E+110.50 1.20E+09 1.30E+10 1.20E+11 1.30E+120.35 5.90E+09 6.60E+10 5.90E+11 6.60E+12Area <strong>of</strong>DetailD E= 500 D E= 150 D E= 50 D E= 15CoarseSlurryTemperatureT MAX= 37 Cε = 0.99MediumT MAX= 44 Cε = 0.70T MAX= 50 Cε = 0.50ε = 0.35Case:IC1000 TM K-Groove<strong>Pad</strong> 33 rpmWafer 61 rpmDownforce 3 psiFineT MAX= 52 CFigure 14: Impact <strong>of</strong> <strong>Pad</strong> Texture on Steady-State Slurry Temperature Field in <strong>Pad</strong>-WaferGap for Concentric Circular Grooves (cases from Fig. 13 are boxed)40

Fig. 15 compares wafer pr<strong>of</strong>iles obtained by angular integration <strong>of</strong> the point-to-pointArrhenius temperature term e -Ea/RT as a driver <strong>of</strong> chemical removal rate, based on thesteady temperature fields <strong>of</strong> Fig. 14 <strong>and</strong> an activation energy <strong>of</strong> 10 kcal/gmol. <strong>The</strong>flattest pr<strong>of</strong>iles <strong>and</strong> highest average removal rates are achieved with the finest padtextures, having a void fraction <strong>of</strong> 35% <strong>and</strong> characteristic lengths <strong>of</strong> 15 to 50 microns.Medium textures, such as 50% void <strong>and</strong> 150 microns, give 30 to 40% lower rates <strong>and</strong>overpolish the near-edge region due to the localized hot spot at the 6 o’clock position.Coarse textures, having 99% void <strong>and</strong> characteristic lengths <strong>of</strong> 150 to 500 microns,give slightly higher removal than medium textures <strong>and</strong> a high-polish region across theouter half <strong>of</strong> the wafer radius, both due to the broad hot area created when wafer motionhas a significant influence. It is seen that very fine textures <strong>and</strong> slurry movementapproaching rigid-body rotation are the most desirable for good removal rate <strong>and</strong>uniformity, <strong>and</strong> that the best pad texturing <strong>and</strong> grooving designs are those thatminimize the influence <strong>of</strong> wafer rotation on slurry flow.3500<strong>Pad</strong> Texture CasesRelative Removal Rate, A/min30002500200015001000Medium<strong>Pad</strong> TexturesFinest <strong>Pad</strong> TexturesCoarsest<strong>Pad</strong> Textures0 20 40 60 80 100Vo id = 0.35, DE = 15Vo id = 0.35, DE = 50Vo id = 0.35, DE = 150Vo id = 0.35, DE = 500Vo id = 0.50, DE = 15Vo id = 0.50, DE = 50Vo id = 0.50, DE =150Vo id = 0.50, DE = 500Vo id = 0.70, DE = 15Vo id = 0.70, DE = 50Vo id = 0.70, DE = 150Vo id = 0.70, DE = 500Vo id = 0.99, DE = 15Vo id = 0.99, DE = 50Vo id = 0.99, DE = 150Vo id = 0.99, DE = 500Distance from Wafer Center, mmFigure 15: Impact <strong>of</strong> Surface Texture Properties on Removal Rate <strong>and</strong> Wafer Pr<strong>of</strong>ile for<strong>Pad</strong>s having Concentric Circular GroovesIn light <strong>of</strong> the results <strong>of</strong> Fig. 15, the relationship found experimentally between padsurface flow resistance <strong>and</strong> CMP removal rate (Fig. 5) becomes clearer. Higher flowresistance in the asperity layer implies better pad-wafer compliance, <strong>and</strong> from thest<strong>and</strong>point <strong>of</strong> contact mechanics this is sufficient to explain an increase in polish rate.However there is a potentially larger impact <strong>of</strong> transport phenomena. Surface flowresistance dictates how strictly the slurry is conveyed with the pad movement <strong>and</strong>against the complicating local velocity <strong>of</strong> the wafer. For the circular grooved pads inFig. 5, higher flow resistance limits fluid migration between the textured areas <strong>and</strong>grooves, leading to increased slurry temperatures at the points <strong>of</strong> pad-wafer contact <strong>and</strong>accelerated chemical removal rates. <strong>The</strong> results clarify that when considering padtexture variants, CMP contact mechanics <strong>and</strong> transport phenomena are coupled becausesurface architecture affects both pad-wafer <strong>and</strong> pad-slurry interactions.41 PacRim-CMP 2004

7. Conclusions<strong>Pad</strong> grooving <strong>and</strong> texturing have been shown to be highly influential CMP processcontrol features due to their strong impact on slurry fluid mechanics <strong>and</strong> heat <strong>and</strong> masstransfer in the pad-wafer gap. Description <strong>of</strong> pad texture as a porous media providesnot only a straightforward surface characterization method but also a direct linkbetween physical topography <strong>and</strong> pad-slurry interaction during polishing. <strong>The</strong> presentwork <strong>of</strong>fers both a basis on which to quantify conditioned pad surfaces <strong>and</strong> a modelframework to predict trends in wafer pr<strong>of</strong>iles across pad groove <strong>and</strong> texture variants.Such results enable advanced CMP pad engineering to promote fresh slurry mixing <strong>and</strong>optimal heat distribution in the pad-wafer gap, contributing to improved removal rate<strong>and</strong> uniformity.<strong>The</strong> industry trend to lower-downforce polishing has increased the importance <strong>of</strong>precise pad texture control <strong>and</strong> efficient slurry distribution in the pad groove network.Next-generation consumables based on the fundamental underst<strong>and</strong>ing presented hereare expected to reach the higher performance st<strong>and</strong>ards <strong>of</strong> more dem<strong>and</strong>ing CMPapplications <strong>and</strong> to extend the utility <strong>of</strong> this versatile process.8. AcknowledgmentsThis study benefited from contributions by Ravi Palaparthi, Dimitri Tselepidakis,Birendra Kumar David, Hemant Devendra Punekar, <strong>and</strong> collaboration with David James<strong>and</strong> Lee Cook. Polish results cited in this study were obtained by Todd Crkvenac <strong>and</strong>Scott Koons, who also assisted with data interpretation. <strong>The</strong> author is grateful toMasaharu Kinoshita for the invitation to present at the PacRim-CMP 2004 conference,<strong>and</strong> to Rohm <strong>and</strong> Haas for permission to publish this work.References[1] S. Sundararajan, D. G. Thakurta, D. W. Schwendeman, S. P. Murarka, <strong>and</strong> W. N. Gill, J.Electrochem. Soc., 146 (2) 761 (1999).[2] R. S. Subramanian, L. Zhang, <strong>and</strong> S. V. Babu, J. Electrochem. Soc., 146 (11), 4263(1999).[3] D. G. Thakurta, C. L. Borst, D. W. Schwendeman, R. J. Gutmann, <strong>and</strong> W. N. Gill, J.Electrochem. Soc., 148 (4) G207 (2001).[4] N. Patir <strong>and</strong> H. S. Cheng, Trans. ASME, 100, 12 (1978).[5] G. P. Muldowney <strong>and</strong> D. P. Tselepidakis, Proceedings <strong>of</strong> CMP-MIC (2004).[6] R. Palaparthi <strong>and</strong> G. Muldowney, Proceedings <strong>of</strong> AIChE Annual Meeting (2004).[7] G. P. Muldowney, Proceedings <strong>of</strong> AIChE Annual Meeting (2003).[8] S. Ergun, Chem. Engr. Prog., 48, 89 (1952).[9] G. P. Muldowney, Proceedings <strong>of</strong> MRS Spring Meeting (2004).[10] G. P. Muldowney <strong>and</strong> D. P. Tselepidakis, Proceedings <strong>of</strong> ECS Spring Meeting (2004).[11] G. P. Muldowney, Proceedings <strong>of</strong> CAMP 9 th International Symposium on CMP (2004).[12] L. Cook, in Semiconductors <strong>and</strong> Semimetals, 63, Academic Press (2000).[13] G. P. Muldowney <strong>and</strong> D. B. James, Proceedings <strong>of</strong> MRS Spring Meeting (2004).IC1000 TM is a trademark <strong>of</strong> Rohm <strong>and</strong> Haas Company or its affiliates. FLUENT is atrademark <strong>of</strong> Fluent Inc.42