FLCC March 19, 2007 CMP 1 FLCC Seminar Title: Effects of CMP Slurry Chemistry on Agglomeration of Alumina Particles and Copper Surface Hardness Faculty: Jan B. Talbot Student: Robin Ihnfeldt Department: Chemical Engineering University: University of California, San Diego

FLCC March 19, 2007 CMP 2 Introduction Integrated Circuit manufacturing requires material removal and global planarity of wafer surface – Chemical Mechanical Planarization (CMP) –Material Removal Rate (MRR) is affected by: Abrasive size and size distribution Wafer surface hardness –Cu is the interconnect of choice- our research focus –CMP slurries provide material removal by: Mechanical abrasion –Nanometer sized abrasive particles (alumina) Chemical reaction –Chemical additives (glycine, H 2 O 2, etc.)

FLCC March 19, 2007 CMP 3 slurry wafer polishing pad platen polishing pad wafer slurry wafer carrier P = psi ( ml/min) V= rpm (polyurethane) Particle concentration = wt% Particle size = nm dia Cu MRR= nm/min Planarization time = 1- 3 min RMS roughness = < 1 nm CMP Schematic

FLCC March 19, 2007 CMP 4 Motivation –Better process control Understand role of slurry chemistry (additives, pH, etc.) Develop slurries to provide adequate removal rates and global planarity –Prediction of material removal rates (MRR) Predictive CMP models - optimize process consumables Improve understanding of effects of CMP variables Reduce cost of CMP –Reduce defects Control of abrasive particle size Control of interactions between the wafer surface and the slurry

FLCC March 19, 2007 CMP 5 Research Approach Experimental study of colloidal behavior of CMP slurries –Zeta potential and particle size distribution measurements Function of pH, ionic strength, additives –Alumina particles in presence of common Cu CMP additives –Alumina particles in presence of copper nanoparticles Measurement of surface hardness as function of slurry chemistry Develop comprehensive model (Lou & Dornfeld, IEEE, 2003) –Mechanical effects (Dornfeld et al., UCB) –Electrochemical effects (Doyle et al., UCB) –Colloidal effects (Talbot et al., UCSD )

FLCC March 19, 2007 CMP 6 Common Cu Slurry Additives AdditivesNameConcentration Buffering agent KOHHNO 3 NH 4 OH, KOH, HNO 3 bulk pH 3-8 Complexing agent - bind with partial or fully charged species in solutionGlycine, Ethylene-diamine-tetra-acetate EDTA (EDTA), citric acid M Corrosion inhibitor - protect the wafer surface by controlling passive etching or corrosion BTA Benzotriazole (BTA) 3-amino-triazole (ATA) KI wt% Oxidizer - cause growth of oxide film H 2 O 2 H 2 O 2, KIO 3, K 3 Fe(CN) citric acid 0-2 wt% Surfactant - increase the solubility of surface and compounds SDS Sodium-dodecyl-sulfate (SDS), cetyltrimethyl-ammonium- bromide (CTAB) 1-20 mM Robin Ihnfeldt and J.B. Talbot. J. Electrochem. Soc., 153, G948 (2006). Tanuja Gopal and J.B. Talbot. J. Electrochem. Soc., 153, G622 (2006).

FLCC March 19, 2007 CMP 7 Cu CMP Chemical Reactions Dissolution: Cu(s) + HL  CuL + (aq) + H + + e Oxidation: 2Cu + H 2 O  Cu 2 O + 2H + + 2e Oxide dissolution: Cu 2 O + 3H 2 O  2CuO H + + 2e Complexation (to enhance solubility) Cu 2+ + HL  CuL + + H + Cu CuO, Cu 2 O, CuL 2 CuL +, Cu 2+, Cu +

FLCC March 19, 2007 CMP 8 Chemical Phenomena Chemistry of Glycine-Water System copper-water system [Cu T ]=10 -5 M Ref.: Pourbaix (1957); (Aksu and Doyle (2002) copper-water-glycine system [L T ]=10 -1 M, [Cu T ]=10 -5 M

FLCC March 19, 2007 CMP 9 Colloidal Aspects of CMP 1)Particle – particle 2)Particle – surface 3)Particle – dissolution product 4)Surface – dissolution product Surface Abrasive particle Dissolution product

FLCC March 19, 2007 CMP 10 Slurry Abrasives 40 wt%  -alumina slurry (from Cabot Corp.) 150nm average aggregate diameter – 20nm primary particle diameter Common Copper CMP Slurry Additives Glycine, EDTA, H 2 O 2, BTA, SDS Copper nano-particles Added 0.12 mM to simulate removal of copper surface during CMP <100 nm in diameter (from Aldrich) Zeta Potential and Agglomerate Size Distribution Brookhaven ZetaPlus –Zeta Potential – Electrophoretic light scattering technique (±2%) –Agglomerate Size – Quasi-elastic light scattering (QELS) technique (±1%) All samples diluted to 0.05 wt% in a 1 mM KNO 3 solution Solution pH adjusted using KOH and HNO 3 and ultrasonicated for 5 min prior to measuring Experimental Procedure

FLCC March 19, 2007 CMP 11 Electrical Double Layer a Distance Potential  1/  Diffuse Layer Shear Plane Particle Surface Potential at surface usually stems from adsorption of lattice ions, H + or OH - Potential is highly sensitive to chemistry of slurry Slurries are stable when all particles carry same charge; electrical repulsion overcomes van de Waals attractive forces If potentials are near zero, abrasive particles may agglomerate Zeta Potential = ionic strength

FLCC March 19, 2007 CMP 12 Zeta Potential - Potential at the Stern Layer Electrophoresis – Zeta potential estimated by applying electric field and measuring particle velocity Surface charge on metal oxides is pH dependant: IEP at  = 0 Slurries are stable when |  | > 25 mV M-OH + OH - → M-O - + H 2 O M-OH + H + → M-OH 2+ Cabot alumina without additives in M KNO 3 solution (bars indicate standard deviation of agglomerate size distribution) Zeta Potential

FLCC March 19, 2007 CMP 13 Cabot alumina in M KNO 3 solution with and without 0.12mM copper IEP ~6.5 with and without copper IEP~9.2 for  -alumina from literature* Impurities (NO 3 -, SO 4 2-, etc.) may lower IEP** At high pH values magnitude of zeta potential lower with copper than without *M.R. Oliver, Chemical-Mechanical Planarization of Semiconductor Material, Springer-Verlag, Berlin (2004). **G.A. Parks, Chem. Tevs., 65, 177 (1965). Zeta Potential

FLCC March 19, 2007 CMP 14 Agglomerate Size Distribution pH 2 – presence of copper causes decrease in agglomeration pH 7 – presence of copper causes increase in agglomeration Cabot alumina dispersion in 1mM KNO 3 solution with (red) and without (blue) 0.12 mM copper and without chemical additives

FLCC March 19, 2007 CMP 15 Potential-pH for Copper-water System [Cu]=10 -4 M at 25 0 C and 1atm (M. Pourbaix 1957) ■ Agglomeration behavior is consistent with the Pourbaix diagram Copper-Alumina-Water System Average agglomerate size of bimodal distributions in a 1 mM KNO 3 solution IEP of CuO ~ 9.5* *G.A. Parks, Chem. Tevs., 65, 177 (1965). Robin Ihnfeldt and J.B. Talbot. J. Electrochem. Soc., 153, G948 (2006).

FLCC March 19, 2007 CMP 16 Zeta Potential Cabot alumina in 0.1M glycine and M KNO 3 solution with and without 0.12mM copper IEP ~6.5 without copper IEP~9.2 increased with copper *M.R. Oliver, Chemical-Mechanical Planarization of Semiconductor Material, Springer-Verlag, Berlin (2004). **G.A. Parks, Chem. Tevs., 65, 177 (1965).

FLCC March 19, 2007 CMP 17 Potential-pH for Copper- Glycine-Water System* [Cu]=10 -4 M, [Glycine]=10 -1 M at 25 0 C and 1atm Agglomeration behavior is consistent with Pourbaix diagram Average agglomerate size of bimodal distributions in a 1 mM KNO 3 solution with various additives Copper-Glycine-Water System *S. Aksu and F. M. Doyle, J. Electrochemical Soc., 148, 1, B51 (2006).

FLCC March 19, 2007 CMP 18 Measuring Wafer Hardness TriboScope Nanomechanical Testing system from Hysitron Inc. ■ Considerations –Large applied load will increase indentation depth – more likely for underlying layer to affect nanohardness measurements –Slurry solutions with high etch rates will decrease copper thickness – thinner copper layer more likely for underlying layer to affect measurements 1 cm 2 silicon wafer pieces sputter deposited with 30 nm Ta nm Cu 10 min exposure in 100 ml of slurry solution (without abrasives), then removed and dried with air and measured Robin Ihnfeldt and J.B. Talbot. 210th Meeting Electrochem. Soc., Cancun, Mexico, Oct. 29-Nov. 3, 602, 1147 (2006).

FLCC March 19, 2007 CMP 19 ■ pH 2 – appears that state of surface is Cu metal with increase in nanohardness from underlying layer ■ pH 7 and 12 – hardness less than that of bulk metallic Cu –Cupric hydroxide, Cu(OH) 2, is most likely forming Copper Surface in Solution Bulk metallic Cu H~ 2.3 GPa* Surface nanohardness of Cu on Ta/Si (100uN applied load) after exposure to 1mM KNO 3 solution *S. Chang, T. Chang, and Y. Lee, J. Electrochemical Soc., 152, (10), C657 (2005). Ta 2 O 5 H~9 GPa

FLCC March 19, 2007 CMP 20 Glycine Surface hardness is less than that of bulk Cu at pH 2 and 12 – – Glycine may interact with surface layer to decrease compactness pH 7 appears to be Cu metal with increase due to underlying layer Glycine + H 2 O 2 H 2 O 2 increases solubility of Cu-glycinate complex or increases Cu oxidation Surface is less than bulk Cu at pH 2 and 7 – decrease in compactness due to glycine pH 12 appears to be cuprous oxide, Cu 2 O Copper Surface in Solution Surface nanohardness of Cu on Ta/Si (100uN applied load) after exposure to 1mM KNO 3 solution and other additives Film Growth Increased Hardness

FLCC March 19, 2007 CMP 21 CMP Experiments Toyoda Polishing apparatus (UC Berkeley) –IC1000 polishing pad pre- conditioned for 20 minutes with diamond conditioner –Polished 2 min with Cabot alumina Silicon wafers (100 mm dia.) with 1  m copper on 30 nm tantalum –Total of 18 wafers polished with various slurry chemistries and at various pH values

FLCC March 19, 2007 CMP 22 Experimental Copper CMP MRR MRR is <20 nm/min for all pH values without additives, with 0.1M glycine MRR is >100 nm/min for several pH values where both glycine and H are present

FLCC March 19, 2007 CMP 23 Lou and Dornfeld CMP Model Slurry Concentration C Average Abrasive Size X avg Proportion of Active Abrasives N Force F & Velocity Active Abrasive Size X act Wafer hardness H w / Slurry Chemicals & Wafer Materials Vol Basic Eqn. of Material Removal: MRR = N x Vol

FLCC March 19, 2007 CMP 24 Conclusions Colloidal Behavior pH has greatest effect on colloidal behavior Glycine acts as a stabilizing agent for alumina Presence of Cu nanoparticles can increase or decrease agglomeration depending on the state of copper in solution Agglomeration behavior with copper is consistent with potential- pH diagrams Nanohardness of Copper Surface pH of the slurry affects copper surface hardness Addition of chemical additives has large effect on the surface hardness State of copper on surface is consistent with potential-pH diagrams Under certain conditions glycine may cause decrease in copper surface hardness

FLCC March 19, 2007 CMP 25 Future Work Continue to investigate effect of copper on zeta potential and particle size –Determine state of Cu in solution –Study agglomeration as a function of time Initial hardness measurements show large differences in copper surface with pH and chemical addition –Determine reproducibility of hardness measurements –Determine state of Cu on surface Modeling – Luo and Dornfeld Model* –Incorporate experimental measurements (hardness and agglomerate size distribution) into model and compare with experimental CMP data *J. Luo and D. Dornfeld, IEEE Trans. Semi. Manuf., 14, 112 (2001).

FLCC March 19, 2007 CMP 26 Funded by FLCC Consortium through a UC Discovery grant. We gratefully acknowledge the companies involved in the UC Discovery grant: Advanced Micro Devices, Applied Materials, Atmel, Cadence, Canon, Cymer, DuPont, Ebara, Intel, KLA- Tencor, Mentor Graphics, Nikon Research, Novellus Systems, Panoramic Technologies, Photronics, Synopsis, Tokyo Electron Prof. Dornfeld and his research group at UC Berkeley for use of the CMP apparatus and model program Prof. Talke and his research group at UCSD for the use of the Hysitron Instrument. Acknowledgments