ION ENERGY AND ANGULAR DISTRIBUTIONS INTO SMALL FEATURES IN PLASMA ETCHING REACTORS:

1. ION ENERGY AND ANGULAR DISTRIBUTIONS INTO SMALL FEATURES IN PLASMA ETCHING REACTORS: THE WAFER- FOCUS RING GAP* 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 AVS 54th International Symposium October 2007 * Work supported by Semiconductor Research Corp., Applied Materials and NSF AVS2007_Natalie_01

2. AGENDA • Wafer edge effects • Description of the model • Ion energy and angular distribution on different surfaces in wafer-focus ring gap for focus ring: • Capacitance • Height • Conductivity • Concluding remarks Iowa State University Optical and Discharge Physics AVS2007_Natalie_02

3. PENETRATION OF PLASMA INTO WAFER-FOCUS RING GAP • Gap (< 1 mm) between wafer and focus ring in plasma tools for mechanical clearance. • Beveled wafers allow for “under wafer” plasma-surface processes. • Penetration of plasma into gap can deposit of contaminating films. • Orientation of electric field and ion trajectories, energy and angular distributions depend on details of the geometry and materials. Iowa State University Optical and Discharge Physics AVS2007_Natalie_03

4. INVESTIGATION OF IEADs INTO WAFER-FOCUS RING GAP • The ion energy and angular distributions (IEADs) into the wafer-focus ring gap are important; • Angular distribution determines erosion (e.g., maximum sputtering at 60o. • Time between replacement of consumable parts depends on erosion. • Spacing, materials (e.g., dielectric constant, conductivity) determine electric field in gap and so IEADS. • In this presentation, results from a computational investigation of IEADs onto surfaces in wafer-focus ring gap will be discussed. • Model: nonPDPSIM using unstructured meshes. • Goal: How does one control the IEADs? Iowa State University Optical and Discharge Physics AVS2007_Natalie_03

5. nonPDPSIM: BASIC EQUATIONS • Poisson equation: Electric potential • Transport of charged species j • Surface charge balance • Full momentum for ion fluxes • Neutral transport: Navier-Stokes equations. • Improvements to include Monte Carlo simulation of Ion Energy and Angular distributions (IEADs). Iowa State University Optical and Discharge Physics AVS2007_Natalie_04

6. MESHING TO RESOLVE FOCUS RING GAP • 2-dimensional model using an unstructured mesh to resolve wafer-focus ring gaps of < 1 mm. • Numbering indicates materials and locations on which IEADs are obtained. • Ar, 10 MHz, 100 mTorr, 300 V, 300 sccm Iowa State University Optical and Discharge Physics AVS2007_Natalie_05

7. MINMAX POTENTIAL, ELECTRIC FIELD, IONS Potential • Off-axis maximum in [Ar+] is due to electric field enhancement near focus ring and is uncorrelated to gap. • Ar, 10 MHz, 100 mTorr, 300 V • Gap: 1 mm E/N [Ar+] Iowa State University Optical and Discharge Physics AVS2007_Natalie_06

8. MINMAX Log scale POTENTIAL AND CHARGES (RF CYCLE)  1.0 mm Gap  Surface Charges  Cycle averaged potential -1.1 x 1011 cm-3 Powered Electrode Powered Electrode • Highly conductive wafer with small capacitance charges and discharges rapidly. • Focus ring acquires larger negative surface charges. • Large potential drop in focus ring. Animation Slide Iowa State University Optical and Discharge Physics AVS2007_Natalie_07

9. ION FLUX VECTORS (RF CYCLE)  1.0 mm Gap Powered Electrode • Directions of electric fields near surfaces evolve slowly during rf cycle due to slowly changing surface charge. • Direction of ion fluxes changes during rf cycle from nearly vertical to perpendicular to surface with transients in electric field. Iowa State University Optical and Discharge Physics Animation Slide AVS2007_Natalie_08

10. MINMAX Log scale ION ENERGY AND ANGULAR DISTRIBUTIONS • Broad IEAD on top bevel due to ions arriving during positive and negative parts of rf cycle. • Grazing angles for ions striking vertical surfaces. Iowa State University Optical and Discharge Physics AVS2007_Natalie_09

11. MINMAX Log scale ION FLUXES AT DIFFERENT PHASE OF RF CYCLE  Cathodic rf cycle  1.0 mm Gap Cathodic cycle: • High energy ions at grazing incident on side wall. • Near vertical to bevel. Anodic rf cycle: • Low energy ions near vertical on side wall. • High energy angles a large angle to bevel. 5 9  Anodic rf cycle 5 9 Iowa State University Optical and Discharge Physics AVS2007_Natalie_10

12. MINMAX Log scale CAPACITANCE OF FOCUS RING: ION DENSITY AND CHARGES • Wafer charges quickly (almost anti-phase with focus ring). • More surface charges collected on focus ring with larger capacitance. • Ions penetrate into gap throughout rf cycle with larger capacitance.  1.0 mm Gap -7.8 x 1010 cm-3 -1.2 x 1011 cm-3 Powered electrode Powered electrode Ar+ Ar+ Powered electrode Powered electrode Animation Slide  /o= 4  0.5 mm Gap  /o= 20 Iowa State University Optical and Discharge Physics AVS2007_Natalie_11

13. MINMAX Log scale CAPACITANCE OF FOCUS RING: IEAD  /o= 4 • Penetration of potential into focus ring with low capacitance produces lateral E-field. • IEAD on substrate is asymmetric.  /o= 20 Iowa State University Optical and Discharge Physics AVS2007_Natalie_12

14. MINMAX Log scale FOCUS RING HEIGHT: ION DENSITY AND FLUX  1.0 mm Gap • Ions do not fully penetrate into the gap with high focus ring. • Ion focusing on edges. • Substantial penetration of ion flux under bevel with low focus ring. Powered Electrode Powered Electrode Animation Slide Powered Electrode Powered Electrode Iowa State University Optical and Discharge Physics AVS2007_Natalie_13

15. MINMAX Log scale FOCUS RING HEIGHT: IEAD  1.0 mm Gap  0.25 mm Gap • “Open” edge produces skewed IEADs Iowa State University Optical and Discharge Physics AVS2007_Natalie_14

16. DESIGN TO CONTROL IEADs • Configuration of wafer-focus ring gap can be used to control IEADS. • Example: Extension of biased substrate under dielectric focus ring of differing conductivity. Iowa State University Optical and Discharge Physics AVS2007_Natalie_15

17. MINMAX Log scale EXTENDED ELECTRODE: CHARGE, E-FIELD AND ION FLUX • Same conductivity wafer and FR. • More uniform and symmetric sheath and plasma parameters. • 0.1 Ohm-1 cm-1 Powered Electrode Powered Electrode Powered Electrode Animation Slide Iowa State University Optical and Discharge Physics AVS2007_Natalie_16

18. MINMAX Log scale EXTENDED ELECTRODE: IEAD • Wafer: 0.1 Ohm-1 cm-1 • Ring: 10-8 Ohm-1 cm-1 • On all surfaces more narrow and symmetric IEAD with uniform electrical boundary condition. • Wafer and Ring: 0.1 Ohm-1 cm-1 Iowa State University Optical and Discharge Physics AVS2007_Natalie_17

19. MINMAX Log scale BROADENING OF IEAD ON TOP BEVEL: EFFECT OF FR  /o= 4  /o= 20  High FR  Low FR • FR Conductivity • Always broad and asymmetric IEAD on tilted surface. Iowa State University Optical and Discharge Physics AVS2007_Natalie_18

20. CONCLUDING REMARKS • Ion energy and angular distributions were investigated on surfaces inside wafer-focus ring gap. • Different regions of the IEADs are generated during different parts of the rf cycle. Even vertical surfaces receive some normal angle ion flux. • Narrow IEAD are obtained with • High focus ring • High focus ring capacitance • High focus ring conductivity. • Uniform electrical boundary conditions leads to more symmetric sheath over the gap and narrows IEADs. • On tilted surfaces broad and asymmetric IEADs are obtained for most conditions. Iowa State University Optical and Discharge Physics AVS2006_Natalie_19