1. WAFER EDGE EFFECTS CONSIDERING ION INERTIA IN CAPACITIVELY COUPLED DISCHARGES* Natalia Yu. Babaeva and Mark J. Kushner Iowa State University Department of Electrical and Computer Engineering Ames, IA 50011, USA natalie5@iastate.edu mjk@iastate.edu http://uigelz.ece.iastate.edu June 2006 * Work supported by Semiconductor Research Corp. and NSF ICOPS2006_Natalie_01

2. Iowa State University Optical and Discharge Physics AGENDA  Wafer Edge effects and their origin.  Description of the model:  Improvement of nonPDPSIM to include ion momentum equation  Effect of wafer-focus ring gaps on Ar and Ar/Cl 2 CCPs  Plasma penetration  Ion focusing  Concluding remarks ICOPS2006_Natalie_02

3. Iowa State University Optical and Discharge Physics WAFER EDGE EFFECTS  It is desirable to use wafer area to the edge of the wafer to maximize utilization.  Perturbation of fluxes may occur by method of terminating wafer and matching to tool material  Wafer is beveled at edge with small gap (< 1 mm) between wafer and focus ring.  Penetration of plasma into gap is bad due to formation of particles and deposition of contaminating films. ICOPS2006_Natalie_03

4. Iowa State University Optical and Discharge Physics ION MOMENTUM EQUATION IN nonPDPSIM  Goal is to computationally investigate edge effects and penetration of plasma into wafer-focus ring gap.  Large dynamic range (> 100) requires unstructured mesh.  Large Knudson number in gap requires accounting for inertia.  nonPDPSIM, a 2-dimensional plasma hydrodynamics model, was improved by adding ion momentum equations on unstructured mesh.  The coupling between the dynamics of charged and neutral transport is through the species resolved collision terms in momenta equations. ICOPS2006_Natalie_04

5. Iowa State University Optical and Discharge Physics nonPDPSIM CHARGE PARTICLE TRANSPORT Poisson equation for the electric potential Transport equations for conservation of the charged species j Surface charge balance Full momentum for ion fluxes of species j Equations are simultaneously solved using a Newton’s iterations. ICOPS2006_Natalie_05

6. Iowa State University Optical and Discharge Physics 2-D GEOMETRY AND CONDITIONS  Conditions:  Ar, 90 mTorr, 300 sccm, 500 V  Ar/Cl 2 = 70/30, 90 mTorr, 300 sccm, 500 V  Biased substrate, grounded housing  Showerhead to wafer distance = 4 cm  Transport of energetic secondary electrons from biased substrate is addressed with a Monte Carlo simulation. ICOPS2006_Natalie_06

7. MESHING TO RESOLVE WAFER-FOCUS RING GAP Iowa State University Optical and Discharge Physics  Unstructured mesh with multiple refinement zones was used to resolve wafer- focus ring gap.  Gaps of < 1 mm were investigated. ICOPS2006_Natalie_07

8. Iowa State University Optical and Discharge Physics MIN MAX Log scale  Electron penetration into the gaps is nominal due to surface charging and sheath formation.  Ar, 90 mTorr, 10 MHz, 300 sccm, 500 V Animation slide ELECTRON DENSITY NEAR THE GAPS  0.9 mm Gap 10 6 –10 8 cm -3 Electrons (10 6 – 3 x10 9 cm -3 ) ICOPS2006_Natalie_08  0.3 mm Gap

9. Iowa State University Optical and Discharge Physics EDGE REGION: NEGATIVE CHARGE  Negative charging of wafer surface (and focus ring) extends beyond edge of bevel in large gap.  Ar, 90 mTorr, 10 MHz, 300 sccm, 500 V ICOPS2006_Natalie_09  0.9 mm Gap  0.3 mm Gap MIN MAX Log scale

10. EDGE REGION: IONS Iowa State University Optical and Discharge Physics Animation slide 10 6 – 3x10 8 cm -3 10 8 –3 x10 8 cm -3 ICOPS2006_Natalie_10 MIN MAX Log scale  0.9 mm Gap  0.3 mm Gap  Ions are modulated by 10 MHz e-field variation.  Ions penetrate into the large gap reaching the biased substrate.  Ions do not penetrate into the small gap but do respond to “sentinal” surface charge.  Ar, 90 mTorr, 10 MHz,300 sccm, 500 V

11. Iowa State University Optical and Discharge Physics EDGE REGION: ELECTRON TEMPERATURE MIN MAX  T e is higher near the small gap due to overlapping os sheaths and higher local electric fields.  Electron temperature (and electron density) is negligibly small inside the gaps.  Ar, 90 mTorr, 10 MHz, 300 sccm, 500 V ICOPS2006_Natalie_11  0.9 mm Gap  0.3 mm Gap

12. Ar/Cl 2 DISCHARGE Iowa State University Optical and Discharge Physics  Maximum electron density shifts towards the focus ring.  Negative ion density comparable to electron density, though are trapped in the plasma bulk and do not reach the wafer  Ar/Cl 2 = 85/15, 90 mTorr, 300 sccm, 500 V ICOPS2006_Natalie_12 MIN MAX Log scale [e] [Cl 2 + ] [Ar + ] [Cl - ]

13. Iowa State University Optical and Discharge Physics EDGE REGION: Ar + AND Cl 2 + FLUXES  Cl 2 + flux is larger and less collisional than Ar + due to lower rate of charge exchange.  There is some focusing of flux to the corner of the bevel that could lead to excessive heating and sputtering.  Some ion trajectories terminate on the lower bevel.  Ar/Cl 2 = 85/15, 90 mTorr, 300 sccm, 500 V ICOPS2006_Natalie_13  0.9 mm Gap

14. Iowa State University Optical and Discharge Physics EDGE REGION: Ar + AND Cl 2 + FLUXES  Less focusing of ion fluxes to corner of bevel occurs with the smaller gap due to lack of charging of wafer into wafer-focus ring cavity.  Ar/Cl 2 = 85/15, 90 mTorr, 300 sccm, 500 V ICOPS2006_Natalie_14  0.3 mm Gap

15. Iowa State University Optical and Discharge Physics EDGE REGION: Ar + FLUX STREAMTRACES  Streamlines penetrate into large gap throughout rf cycle.  In small gap, momentary penetration occurs at peak of cathode cycle. Slightly conductive wafer is able to dissipate that charge.  Ar/Cl 2 = 85/15, 90 mTorr, 300 sccm, 500 V Animation slide ICOPS2006_Natalie_15  0.3 mm Gap

16. Iowa State University Optical and Discharge Physics EDGE REGION: Cl 2 + FLUX STREAMTRACES  Focusing of ion flux streamlines to edge of wafer is more severe for Cl 2 + than Ar + due to lower collisionality.  Periodic flux into gap is also larger.  Ar/Cl 2 = 85/15, 90 mTorr, 300 sccm, 500 V Animation slide ICOPS2006_Natalie_16  0.9 mm Gap  0.3 mm Gap

17. CONCLUDING REMARKS  Penetration of plasma into narrow wafer-focus ring gap of a capacitively coupled discharge was computationally investigated.  Gap sizes > 0.5 mm allow significant penetration of the plasma.  Charging and ion fluxes may penetrate to bottom side of bevel.  Focusing of ion flux to the corner of the bevel depends on the ion species and collisionality: chemically enhanced sputtering is problematic. Iowa State University Optical and Discharge Physics ICOPS2006_Natalie_17