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Small Feature Reproducibility A Focus on Chemical Mechanical Polishing

Small Feature Reproducibility A Focus on Chemical Mechanical Polishing. UC-SMART Major Program Award A. Chang, J.F. Luo, I. Hwang, and David A. Dornfeld Second Annual Workshop 11/8/99. Agenda. 8:30 – 9:00 Introductions, Overview / Spanos

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Small Feature Reproducibility A Focus on Chemical Mechanical Polishing

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  1. Small Feature ReproducibilityA Focus on Chemical Mechanical Polishing UC-SMART Major Program Award A. Chang, J.F. Luo, I. Hwang, and David A. Dornfeld Second Annual Workshop 11/8/99 SFR Workshop - CMP

  2. Agenda 8:30 – 9:00 Introductions, Overview / Spanos 9:00 – 10:15 Lithography / Spanos, Neureuther, Bokor 10:15 – 10:45 Break 10:45 – 12:00 Sensor Integration / Poolla, Smith, Solgaard, Dunn 12:00 – 1:00 lunch, poster session begins 1:00 – 2:15 Plasma, TED / Graves, Lieberman, Cheung, Aydil, Haller 2:15 – 2:45 CMP / Dornfeld 2:45 – 3:30 Education / Graves, King, Spanos 3:30 – 3:45 Break 3:45 – 5:30 Steering Committee Meeting in room 775A / Lozes 5:30 – 7:30 Reception, Dinner / Heynes rm, Men’s Faculty Club SFR Workshop - CMP

  3. Overview • Review of work in progress • Outline of future work • Specific efforts to address chemical effects SFR Workshop - CMP

  4. Key Issues The following key issues drive this work: • Need for in-process monitoring and control means; sensor and signal processing technology • Need for comprehensive process model • Consideration of both mechanical and chemical effects SFR Workshop - CMP

  5. Chemical & Mechanical Effects The importance of considering both chemical and mechanical influences on the process was pointed out. This is being addressed as follows: • Chemical issues in consumable design (e.g. pad polarity) being studied with LLNL (in progress, first study almost completed); This is part of a larger focus on consumable design, • Inclusion of chemical effects in process model (to start) • Collaborative research with Chemical Engineering (starting) SFR Workshop - CMP

  6. Acoustic Emission Process Monitoring • Pad condition monitoring • End Point Detection (EPD) - Acoustic signature of material removal changes when polishing different materials • Scratch Detection – Catastrophic scratching of the wafer surface caused by impurities embedded in the pad surface can be detected. Acoustic Emission Sources in CMP SFR Workshop - CMP

  7. ASL HFpeak Dt AE Sensor High Pass Filter >100 kHz HFpeak Ratio = ASL LFpeak LFpeak Raw AE Signal Dt Low Pass Filter 20-60 kHz AE Ratio Signal Processing AE Ratios—A signal processing scheme that separates the raw signal into high and low bandpass regimes. During each sampling period (Dt), an average signal level (ASL) calculation is performed, and the ratio of the two peak values is calculated. SFR Workshop - CMP

  8. Experimental Setup Sensor Integration – A laboratory-scale cmp machine was modified to attach the AE sensor on the backside of the wafer. SFR Workshop - CMP

  9. Experimental Setup SFR Workshop - CMP

  10. Bare and Oxide Wafers Polished with Slurry 3.0 2.5 Average HF/LF Ratio 2.0 Air Polish 1.5 Bare Wafer Oxide Wafer 1.0 0 20 40 60 80 100 120 140 160 Linear Velocity (mm/s) Results • AE data obtained from polishing in the absence of abrasive particles. • “Air polish” data point represents background level “noise” of environment without contact between pad and wafer SFR Workshop - CMP

  11. Bare Wafer Polished with Slurry and DI Water 3.0 2.5 2.0 Average HF/LF Ratio Air Polish 1.5 Wafer 1 with Slurry Wafer 2 with Slurry Wafer 2 with DI Water 1.0 0 20 40 60 80 100 120 140 160 Linear Velocity (mm/s) Results • AE data differentiating between polishing with (Nalco slurry) and without (DI Water) abrasive particle slurry. SFR Workshop - CMP

  12. Progress vs Milestones Process Monitoring Year 1 • Develop Acoustic Emission sensing for CMP. (Done) Year 2 • Evaluate Acoustic Emission sensing for pad degradation, micro-scratching, and end-pointing for multi-layer stacks. (on-going) SFR Workshop - CMP

  13. Basic Idea of the CMP Model Basic Equation of Material Removal:MRR= NwVol where N is the number of active abrasives, w the density of wafer and Vol the mean volume of material removed by a single active abrasive per unit time. Velocity Vol Softened wafer surface with density w Wafer Abrasive particles in Contact area Abrasive particles in Fluid(All inactive) Pad asperity Polishing pad Active abrasives in Contact area Inactive abrasives in Contact area Schematic of material removal mechanism SFR Workshop - CMP

  14. Wafer-Pad Contact Model: Real Contact Area A and Real Contact Pressure P Modeling of Pad and Wafer Surface ( A simplification from G-W Contact Model (Greenwood & Williamson, 1967): Pad Surface: i. Uniform distribution of Asperity with Density DSUM ii. Spherical Asperity Tip with Radius R iii. Equal Height of Asperities ( All asperities are in contact with wafer) Wafer Surface: Smooth in Comparison with Pad Surface Conclusions Based on Contact Mechanics (Johnson, 1987): i. Apparent Contact Area A0= 0.25 D2 ii. Real Contact Area A= bA0= iii. Real Contact Pressure P= P0A0/A= (1/b1) E*2/3P01/3 where P0is the down pressure, D the diameter of wafer, E* the effective Young’s modulus, b contact area ratio, and b1a constant value introduced for simplification. Pad surface Wafer-Pad Contact under Down Pressure P0 Area in Contact (Micro-Scale Size) R An Asperity with spherical tip under Load F (Johnson, 1987) Before deformation After deformation SFR Workshop - CMP

  15. Plastic Deformation over Wafer-Particle and Pad-Particle Interfaces Relative Velocity V • Assumption of Spherical Abrasive Particles • Indentation Force F on abrasive particles: Determined by contact pressure P and abrasive size X. • Deformation over Wafer-Particle Interface is sliding-plastic deformation: • Radius a1 of the projected circle of the indentation and indentation depth 1 can be determined according to F and hardness of Wafer Hw • Mean Volume Vol removed by a single particle in unit time: Determined by a1, 1 and relative velocity V. • Static-Plastic Deformationover Pad-Particle Interface: Indentation depth 2 is determined by hardness Hp of pad and indentation force F. Softened Wafer surface with Hardness Hw Down Pressure P0on Wafer Top Surface Indentation Depth 1 a1 a X Contact Pressure P Indentation Force F Indentation Depth 2 Pad Asperity with Hardness Hp Schematic of Wafer-Particle, Pad-Particle and Wafer-Pad Contact • A Gap X- 1 -2is introduced between the wafer and pad where the abrasive sits. The gap determines the chance for other abrasives to be involved in material removal. SFR Workshop - CMP

  16. Number nof Abrasives (Both Active and Inactive) Captured in Contact Area A0 Slurry out • Total Number nallof Abrasives in Wafer and Pad Interface: Determined by G, the concentration of abrasives in the slurry, A0 , the area of wafer surface and L, the height of the asperity after deformation • Number nf of Abrasives in Fluid: Determined by G, A0 L and Vola where Vola is the volume of all asperities, if the concentration of abrasives in fluid kept as G. • Vola is a constant independent of down pressure. • Number n of abrasives Captured in the Contact Area is determined by G and Vola n is dependent on the roughness of pad but independent of down pressure. Slurry in Wafer Pad L Slurry out with Concentration G Slurry in with Concentration G Abrasives Captured in Contact Area with Number n Abrasives in Fluid (inactive) with Concentration G Area in Contact (Micro-Scale Size) Constant Volume of An Asperity Before Deformation and After Deformation L Before deformation After deformation SFR Workshop - CMP

  17. Size Distribution of Abrasives and Active Abrasive Number Down Pressure P0on Wafer Top Surface  Portion of Active Abrasives Portion of Inactive Abrasives 1+ 2 Xavg Xmax Xmax- 1- 2 a Xmax Xmax- 1- 2 Xmax Contact Pressure P Indentation Force Fmax Active Abrasive Size Distribution of Abrasives Only abrasives larger than the gap Xmax- 1- 2 introduced by the indentation of largest abrasives can be Involved in material removal. So active abrasive number N= where n is the number of abrasives in contact area and  the standard deviation if normal distribution is assumed. N is a function of size distribution and hardness of wafer and pad. Inactive Abrasive Pad Asperity Largest Abrasive Schematic of Wafer-Particles, Pad-Particles and Wafer-Pad Contact SFR Workshop - CMP

  18. Model Verification • Proof 1. Two sets of Oxide CMPexperimental MRR results, Fig. 1, under different down pressures are used to verify the pressure dependence in the MRR formulation. Lines 9 and 10 in Figure 1 show the prediction. The pressure dependence is correct for both oxide CMP and metal CMP. Table 1 shows the probability of active abrasives for data set 1 calculated using the experimental data. P0 5psi 7psi 9psi 11psi % 0.896 1.092 1.287 1.474 Table 1. Probability of active abrasives Fig. 1 Oxide CMP • Proof 2. Estimation of MRR by estimating input parameters in the MRR formulation. The same order of MRR with that of experimental MRR can be obtained. The same range (0.5%~ 1.7%)of active abrasive probability as shown in Table 1 can be obtained by substituting typical abrasive size and pad hardness values into the formulation. SFR Workshop - CMP

  19. Conclusion • A model was developed to explain CMP material removal mechanism based on assumptions of plastic contact over wafer-abrasive and pad-abrasive interfaces, the normal distribution of abrasive size and the periodic roughness of the pad surface. • Compared with previous models (e.g. Preston’s equation) the model integrates process parameters including pressure and velocity as well as wafer hardness, pad hardness, pad roughness, and abrasive size the to predict the material removal rate. • The model may provide a quantitative tool for consumable design • Better process control may be realized using the proposed model SFR Workshop - CMP

  20. Progress vs Milestones Process Modeling Year 1 • Develop experimental database for CMP modeling and sensitivity analysis. (Done) Year 2 • Develop integrated CMP model and evaluate planarization efficiency predictions. (On-going) SFR Workshop - CMP

  21. Future Work During the 2000-2002 period, we will extend the evaluation of the use of acoustic emission as applied to: • AE Ratio sensitivity to CMP operating conditions • Pad Type • Slurry Type • Pressure • Correlation of AE to CMP material removal mechanism, multi-stack polishing, metal polishing • Development of endpoint detection methodology SFR Workshop - CMP

  22. Future Work During the 2000-2002 period, we will extend the process modeling work to include: - Further experimental verification of the model. - Investigation of influence of CMP process variables based on the model including: pad hardness, contact area ratio, and abrasive size distribution. - Modeling of Step Reduction Mechanism of Patterned Wafer. - Comprehensive Study of WIWNU. SFR Workshop - CMP

  23. Future Work During the 2000-2002 period, we will begin the exploration of “consumable design”: • Consumable design from a chemistry viewpoint (i.e. how to best deliver the chemicals to the wafer surface). • Consumable design from a mechanical viewpoint (i.e. what is the best geometry for mechanical material. removal given the material and geometry of the wafer) • Prototyping and evaluation of idealized “pads” designed via this approach. SFR Workshop - CMP

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