1. FLCC September 25, 2006 1 Fiona M. Doyle and Shantanu Tripathi* University of California at Berkeley Department of Materials Science and Engineering 210 Hearst Mining Building # 1760 Berkeley, CA 94720-1760 fmdoyle@berkeley.edu *Department of Mechanical Engineering TRIBO-CHEMICAL MECHANISMS AND MODELING IN COPPER CMP

2. FLCC September 25, 2006 2 FLCC CMP Approach Our approach is to develop integrated feature-level process models linked to basic process mechanicsOur approach is to develop integrated feature-level process models linked to basic process mechanics These models will drive process optimization and the development of novel consumables to minimize feature-level defects and pattern sensitivityThese models will drive process optimization and the development of novel consumables to minimize feature-level defects and pattern sensitivity Current effort aims to integrate mechanical and chemical phenomenaCurrent effort aims to integrate mechanical and chemical phenomena Need to capture synergism between the twoNeed to capture synergism between the two

3. FLCC September 25, 2006 3 CMP Overview ALUMINA PARTICLES average size ~ 120 nm from EKC Tech. Cross-sectional View of SUBA 500 Pad, Rodel Corp. (courtesy Y. Moon) SLURRY Abrasive particles Chemicals Wafer Carrier Slurry feeder Polishing Plate POLISHING PAD Pressure Rotation Polishing pad Pad asperities Patterned wafer

4. FLCC September 25, 2006 4 Kaufman’s Model for Planarization For effective planarization, must maintain higher removal at protruding regions and lower removal at recessed regions on the wafer 1 - removal of passivating film by mechanical action at protruding areas 3- planarization by repetitive cycles of (1) and (2) Metal Passivating film 2- wet etch of unprotected metal by chemical action. passivating film reforms Passive films, or corrosion inhibitors, are key to attaining planarization

5. FLCC September 25, 2006 5 Mechanical Phenomena Chemical Phenomena Interfacial and Colloid Phenomena Chemical Mechanical Planarization

6. FLCC September 25, 2006 6 Chemistry interacts synergistically with mechanical/colloidal phenomena Mechanical forces on copper introduce defects, increasing reactivity Mechanical properties of films appear to be strongly dependent on chemistry, and probably potential Chemistry affects degree of aggregation of abrasive particles. Coppernanoparticleshave dramaticeffect

7. FLCC September 25, 2006 7 Integrated Cu CMP Model Colloid Agglomeration Oxidizer Inhibitor Complexing agent Surface Film Pad Pressure/ Velocity Abrasive The Problem Needed: an Integrated Copper CMP Model Fluid Mechanics Mass Transfer Needed: understanding of the synergy between different components Interactions: Asperity-copperAsperity-copper Abrasive-copperAbrasive-copper Fluid pressure Contact pressure Integrated Cu CMP Model Colloid Agglomeration Oxidizer Inhibitor Complexing agent Surface Film Pad Pressure/ Velocity Abrasive The Problem Needed: an Integrated Copper CMP Model Fluid Mechanics Mass Transfer Needed: understanding of the synergy between different components Interactions: Asperity-copperAsperity-copper Abrasive-copperAbrasive-copper Fluid pressure Contact pressure

8. FLCC September 25, 2006 8 Tribo-Chemical Model of Copper CMP Synergism between frequent mechanical interactions and action of chemical slurry make copper CMP process electrochemically TRANSIENT; but to date NO study of transient behavior, focus on steady state. NO mechanistic models of tribo-chemical synergism. We must study: Transient passivation behavior of copper: first few moments of copper passivation.Transient passivation behavior of copper: first few moments of copper passivation. Abrasive-copper interactions: frequency, duration and force.Abrasive-copper interactions: frequency, duration and force. Properties of passive film: mechanical, electrical, chemicalProperties of passive film: mechanical, electrical, chemical

9. FLCC September 25, 2006 9 i active i passive Oxidationrate i0i0 Interval between two abrasive- copper contacts ( τ ): stochastic Abrasive- copper interaction: stochastic Bare copper Thick passive film Stochastic variation in i 0 t0t0 Time (t’) Copper oxidized Copper: transient passivation behavior i(t’) Copper oxidation influenced by abrasive interactions More frequent interactions Average removal rate between abrasive-copper contacts

10. FLCC September 25, 2006 10 Transient Passivation Behavior -2 -3 -4 -5 -6 Region I II III IV V -2 -1 0 1 2 Log i (A/cm 2 ) Log t (s) Log i Log t No direct study on copper CMP slurry constituents. Observed behavior for other metal-chemical combinations: log-log (oxidation rate – time) [Jones DA “Principles and prevention of corrosion” Prentice Hall; 2nd edition, 1995] Complex behavior observed for Cu-AHT (inhibitor) behavior [Beier M, Schultze JW, Electrochimica Acta 37 (12): 2299-2307 1992] Wide variation observed in decay kinetics for different systems: milliseconds to minutes. [Beier & Schultze]

11. FLCC September 25, 2006 11 Parabolic Rate Law for Corrosion Kinetics? Cu Film thickness x(t) Passive film CMP Slurry containing oxidant {oxidant} in slurry (fixed) {oxidant} in copper (fixed) Flux of oxidant =

12. FLCC September 25, 2006 12 Differing wear distance Relative motion Contact area in plan view Wear distance Pad asperity Abrasive Copper Passive film Abrasive Duration between contact events. Passive film thickness ↔ corresponding oxidation rate Duration/Force of contact ↔ Thickness of Passive film removed Mechanical Interactions

13. FLCC September 25, 2006 13 Interaction Frequency & Duration Elmufdi & Muldowney, Mater. Res. Soc. Symp. Proc. Vol. 91, 2006 Spring Interval between asperity-copper contact ≈ 1ms Duration of contact ≈ 10μs Needed: study of abrasive-copper interactions C-RICM image of real contact area

14. FLCC September 25, 2006 14 Tribological Properties of Passive Films Film thickness (nm) Wear Distance (μm) Film thickness (nm) Wear Distance (μm) Film thickness (nm) Wear Distance (μm) Linear wear till passive film removed Bi-layer passive film‘Loading’ of abrasive Passive film properties varying with slurry chemistry Wear of passive film depends on mechanical properties of passive film and abrasive particle, and force of contact. Mechanical properties of passive film affected by chemical conditions (inhibitor, oxidation potential) Wear distance (μm) Conditions (a) Conditions (b)

15. FLCC September 25, 2006 15 Quartz Crystal Microbalance Sauerbrey equation: where  q is the shear modulus of the quartz crystal,  q the density, and f 0 the resonant frequency for an AT-cut quartz crystal with a resonant frequency of 5 MHz gives that  m/  f is –1.77 x 10 -8 g/cm 2 Hz The changes in frequency of a piezoelectric quartz crystal,  f, are related to changes in mass,  m, of a substrate (e.g. Cu) that is attached to the quartz crystal:

16. FLCC September 25, 2006 16 EQCM Experimental apparatus and materials (a)Maxtek Research Quartz Crystal Microbalance (b)Maxtek 1-inch diameter quartz crystals and the electrode configuration (c)Maxtek crystal holder (d)Schematic diagram of experimental setup for EQCM measurements. (left) chemical reagents introduced against the wall of cell, (right) a tube 10 mm from the crystal) for injecting chemicals

17. FLCC September 25, 2006 17 pH 4, OCP, 0.01 M glycine premixed in acetate buffer Temporary loss in weight, followed by significant gain in weight, more pronounced at higher concentration of H 2 O 2.

18. FLCC September 25, 2006 18 pH 9, OCP, 0.01 M glycine added to carbonate buffer after stabilization Slow loss in weight upon adding glycine. Temporary sharp loss in weight after adding peroxide, followed by significant gain in weight.

19. FLCC September 25, 2006 19 Effect of adding additional glycine, after adding 2.09% hydrogen peroxide pH 9 Deionized water

20. FLCC September 25, 2006 20 Open circuit potential of copper, pH 9, 0.01 M glycine and 2.09% hydrogen peroxide No passivation without H 2 O 2. See that behavior is strongly dependent on history of glycine additions; oxidized layers must resist dissolution No H 2 O 2. Potential same as that induced by H 2 O 2

21. FLCC September 25, 2006 21 Effect of glycine and H 2 O 2 additions at different potentials, pH 9, 0.01 M glycine Iron disk-Au ring electrode. H 2 O 2 produced during reduction of O 2 is rapidly reduced at high and low potentials, but can escape electrode at intermediate potentials S. Zečević, D.M. Dražić, S. Gojkivić; J. Electroanal. Chem, 265 (1989) 179 At controlled potentials, either oxidizing or reducing, H 2 O 2 does NOT lead to weight increase. Protective film must be sensitive to potential However, this is not consistent with passivation at high concentrations of H 2 O 2

22. FLCC September 25, 2006 22 Environmental AFM AFM scanner Cu sample in a flow through cell Peristaltic pump 6 port valve in out 321 In-situ flow through experiment (flow rate = 0.675ml/min) Slurry constituents 1)DI water (introduced at time t = 0min) 2)Glycine in pH 4 acetic acid/acetate buffer (at time t = 22 min) 3)Glycine + Hydrogen Peroxide in pH 4 acetic acid/acetate buffer (at time t = 56 min)

23. FLCC September 25, 2006 23 AFM in Air of Copper Pre-exposed to Different Slurry Components Ex-situ Copper in Air Copper pre-exposed to 0.01M glycine @ pH4 for 1 minute Copper pre-exposed to 2% H 2 O 2 and 0.01M glycine @ pH4 for about 1 hour Glycine at pH 4 (albeit short exposure) does not affect surface morphology significantly With peroxide, original surface morphology is changed dramatically Although there is some ambiguity, peroxide is much more likely to be adding a surface film rather than etching, which would affect grain boundaries preferentially TopographyDeflectionx=y=1.13μm z range = 47.6nm z range = 0.74nm TopographyDeflectionx=y=1.13μm z range = 31.3nm z range = 0.59nm Topography Deflectionx=y=1.94μm z range = 320.1nm z range = 4.93nm

24. FLCC September 25, 2006 24 t=29min t=32min t=35min t=44min t=41min t=47min Corrosion of Copper in 0.01M glycine, pH 4 In-situ imaging: Buffered glycine solution introduced at t=22 min. See slight etching, correlates with very slightly negative gradient in EQCM work before peroxide addition x=y=1.13μm, Deflection images

25. FLCC September 25, 2006 25 Copper in 2% H 2 O 2, 0.01M Glycine at pH 4 Contact mode imaging gives very noisy AFM images Consistent with presence of very porous and mechanically weak film on copper Possible deterioration of AFM probe tips in this chemistry Effect of changing the flow through constituent: Instant drift and noise in AFM imaging, then stabilization. Transient noise prevents capturing any transient material removal upon adding peroxide Topography Deflectionx=y=1.13μm z range = 98.4nmz range = 1.2nm Flow through imaging, H 2 O 2 solution introduced at t=56min, after solution 1 & 2 Imaging in standing solution, no pre-exposure to solutions 1 & 2 Topography Deflection x=y=2.09μm z range = 65nmz range = 0.66nm t=68min Consistent with plateau in weight gain after adding peroxide, with passivation seen in glycine/peroxide chemistries, and with signficant acceleration of material removal

26. FLCC September 25, 2006 26 Future AFM Work Use of AFM tip to damage existing passive filmsUse of AFM tip to damage existing passive films Observe effect of chemistry on mechanical properties of filmsObserve effect of chemistry on mechanical properties of films Observe transient currents, and correlate with area of damaged surface to obtain current densities as a function of timeObserve transient currents, and correlate with area of damaged surface to obtain current densities as a function of time Study passive film formation kinetics, to identify best model for transient behaviorStudy passive film formation kinetics, to identify best model for transient behavior

27. FLCC September 25, 2006 27 Testing of Model Earlier electrochemical studies (under SFR, by Serdar Aksu) will be used to test model predictionsEarlier electrochemical studies (under SFR, by Serdar Aksu) will be used to test model predictions –Synergy between mechanical and chemical factors of particular interest EQCM work done under FLCC by Ling Wang will provide reference for short time framesEQCM work done under FLCC by Ling Wang will provide reference for short time frames

28. FLCC September 25, 2006 28 Polarization Curves in Cu-Glycine-H 2 O {Cu T } = 10 -5, {L T } = 10 -2 {L T } = 10 -2

29. FLCC September 25, 2006 29 In-situ Polarization Aqueous 10 -2 M glycine, 27.6 kPa, 200 rpm pH 4 pH 9 pH 12

30. FLCC September 25, 2006 30ConclusionsConclusions Earlier mechanistic studies of copper CMP are providing insight for coupling of chemical and mechanical modelsEarlier mechanistic studies of copper CMP are providing insight for coupling of chemical and mechanical models –Mechanistic approach is designed to capture the synergy between the two –Work on colloidal properties of abrasives will also be invoked FLCC CMP team well positioned to capture relevant developments in other fieldsFLCC CMP team well positioned to capture relevant developments in other fields In addition to the intrinsic utility of a combined chemical/mechanical model for CMP, this should resolve remaining questions on material removal mechanismsIn addition to the intrinsic utility of a combined chemical/mechanical model for CMP, this should resolve remaining questions on material removal mechanisms This in turn will allow more efficient developments in futureThis in turn will allow more efficient developments in future