Effects of PVA Brush Shape and Wafer Pattern on the Frictional Attributes of Post-CMP PVA Brush Scrubbing

Transcription

1 Effects of PVA Brush Shape and Wafer Pattern on the Frictional Attributes of Post-CMP PVA Brush Scrubbing J. Weaver and A. Philipossian Department of Chemical Engineering University of Arizona, Tucson AZ & W. Li, K. Bahten and L. Curtis Rippey Corporation, El Dorado Hills, CA 1

2 Objectives Precisely quantify the extent of frictional forces between wafer & brush during scrubbing Low amounts of shear force will fail to remove particles High amounts of shear force may cause surface damage (especially on soft metals and ULK dielectrics) Determine how the following parameters affect coefficient of friction (COF) and total energy due to friction Brush rotational speed Brush shape factor Wafer surface Cleaning solution ph Brush pressure F COF avg = F Shear Normal 2

3 Lubrication Theory and PVA Brush Scrubbing Cleaning fluid viscosity Dependent on fluid type and additive concentrations 1 h ~ 0 Boundary Lubrication h ~ Ra Relative brush-wafer velocity Dependent on scrubber geometry & kinematics Coefficient of Friction (unitless) Partial Lubrication h >> Ra Hydro-dynamic Lubrication So = ( u ) ( µ ) ( P applied ) ( h eff Applied normal brush force divided by contact area Dependent on brush macro-texture, geometry and compliance ) E E E E+00 Sommerfeld Number 'So' (unitless) Effective fluid film thickness in the brush-wafer interface Dependent on applied pressure 3

4 Experimental Conditions Constants: Variables: Cleaning solution flow rate = 120 cc/min Brush rotational velocity 10, 20, 30, 40, 50 & 60 RPM Rippey Symmetry PVA-33 brush Brush pressure 0.25, 0.35, 0.45 & 0.55 PSI Frictional force acquisition frequency = 1,000 Hz Scrubbing time = 2 minutes Brush surface design With nodules Without nodules Cleaning solution ph 1.1, 7.0 and Wafer type: ILD

5 Effect of Pressure, Brush Rotation & Brush Shape on COF (ph ~ 1.1) Patterned Mixed-to-Hydrodynamic Lubrication ILD Mixed-to-Hydrodynamic Lubrication PSI 0.45 PSI PSI PSI With Nodules Without Nodules With Nodules Without Nodules E E E E E E E E E E E E E E+01 5 Sommerfeld Number Sommerfeld Number

6 The Complete Experimental Matrix (solid symbols = brushes without nodules ; open symbols = brushes with nodules) 1.00E+00 ph ~ 1.1 ph ~ 7.0 ph ~ 10.7 COF is not a strong function of wafer topography (i.e. blanket vs. ) Need for chemical action and electrostatic forces for improved wafer cleaning COF 1.00E-01 Cylindrical brush nodules increase COF by 2X (i.e. compared to brushes without nodules) Need for smart nodule designs for improved cleaning efficiency and fluid use reduction Low solution ph increases COF by 5X to 10X 6

7 Solubility & Gellation Characteristics of Silica (ILD) in the SiO2-Water System IEP Charge = Time to Gel (a. u.) Stable Sols Silica Dissolves ph = 1.1 High values of COF due to strong localized gelling in the brush-wafer interface Polymerization of silicic acid monomers to form high MW particles Collision and aggregation of above particles to form 3-dimensional networks (gels) ph = 7.0 Moderate values of COF due to increased stability of the system Little or no tendency to gel Increase in hydroxyl groups causing breakage of network-forming siloxane bonds Partial hydration of ILD surface with network-terminating Si-OH groups ph = 10.7 Low values of COF due to complete dissolution of silica ILD surface is continually dissolving and rejuvenating Complete elimination of silanol groups from the surface ph 7

8 Analyzing Raw Frictional Data Typical Scrubbing Process: 2-minute 1000 frictional force measurements per second 120,000 data points per run γ = γ = γ = Interfacial Interaction Index Area under the curve Total amount of mechanical energy caused by stick-slip F ( t) = F f ( t) shear shear + Frequency (Hz) 8 COF = avg F F Shear Normal Fast Fourier Transform

9 Contribution of Tool Vibration & Resonance to the Force Spectra brush nodule (macro-deformation) effects tool kinematics brush nodule (micro-deformation) effects γ = γ = Area under the curve Total amount of mechanical energy caused by stick-slip proportional to variance of frictional force brush is disengaged 9 rubber hammer

10 Effect of Pressure, Brush Rotation & Brush Shape on Gamma (ph ~ 1.1) Patterned Mixed-to-Hydrodynamic Lubrication ILD Mixed-to-Hydrodynamic Lubrication PSI 0.45 PSI 0.35 PSI 0.25 PSI With Nodules Without Nodules With Nodules Without Nodules E E E E-03 Sommerfeld Number 1.00E E E E E-03 Sommerfeld Number 1.00E-01

11 The Gamma Criterion after Philipossian et al. Proceedings of CMP-MIC, Marina del Rey, CA (2004) Gamma > Boundary Lubrication 1.00E-03 Gamma > Gamma > Partial Lubrication 1.00E E-05 Gamma < Hydrodynamic Lubrication 11

12 The Complete Experimental Matrix (solid symbols = brushes without nodules ; open symbols = brushes with nodules) Gamma 1.00E-03 ph ~ 1.1 ph ~ 7.0 ph ~ 10.7 Lowering the solution ph causes the lubrication mechanism to shift from Hydrodynamic Lubrication to Mixed Lubrication Wafer topography does not affect the lubrication mechanism 1.00E E-05 Only at low values of solution ph, cylindrical brush nodules shift the lubrication mechanism to Mixed Lubrication, otherwise lubrication mechanism is unaffected by nodule design 12

13 Summary of Observations The tribological mechanism ranges from mixed lubrication to hydrodynamic lubrication depending on brush pressure, rotational velocity, solution ph and nodule design Chemical model based on solubility and gellation characteristics of the silica-water system is assumed to be responsible for the observed trends in COF as a function of solution ph: Formation of 3-dimensional siloxane networks at low ph Silanol termination of the silica surface at intermediate ph Complete dissolution of silanol groups and continual formation of a pristine ILD surface at high ph COF is not a strong function of wafer topography, thus suggesting the need for chemical action and electrostatic forces for improved wafer cleaning Cylindrical brush nodules increase COF by 2X, thus suggesting the need for smart nodule designs for improved cleaning efficiency and fluid use reduction 13