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THE WAFER- FOCUS RING GAP*

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Presentation on theme: "THE WAFER- FOCUS RING GAP*"— Presentation transcript:

1 THE WAFER- FOCUS RING GAP*
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 AVS 54th International Symposium October 2007 * Work supported by Semiconductor Research Corp., Applied Materials and NSF AVS2007_Natalie_01

2 Optical and Discharge Physics
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 Optical and Discharge Physics
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 Optical and Discharge Physics
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 POTENTIAL, ELECTRIC FIELD, IONS Optical and Discharge Physics
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 MIN MAX AVS2007_Natalie_06

8 POTENTIAL AND CHARGES (RF CYCLE) Optical and Discharge Physics
 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 MIN MAX Log scale AVS2007_Natalie_07

9 ION FLUX VECTORS (RF CYCLE) Optical and Discharge Physics
 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 ION ENERGY AND ANGULAR DISTRIBUTIONS Optical and Discharge Physics
Broad IEAD on top bevel due to ions arriving during positive and negative parts of rf cycle. Grazing angles for ions striking vertical surfaces. MIN MAX Log scale Iowa State University Optical and Discharge Physics AVS2007_Natalie_09

11 ION FLUXES AT DIFFERENT PHASE OF RF CYCLE
 1.0 mm Gap  Cathodic rf cycle 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 MIN MAX Log scale Iowa State University Optical and Discharge Physics AVS2007_Natalie_10 11

12 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 MIN MAX Log scale AVS2007_Natalie_11

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

14 FOCUS RING HEIGHT: ION DENSITY AND FLUX Optical and Discharge Physics
 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 MIN MAX Log scale AVS2007_Natalie_13

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

16 DESIGN TO CONTROL IEADs Optical and Discharge Physics
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 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 MIN MAX Log scale AVS2007_Natalie_16

18 EXTENDED ELECTRODE: IEAD Optical and Discharge Physics
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 MIN MAX Log scale Iowa State University Optical and Discharge Physics AVS2007_Natalie_17

19 BROADENING OF IEAD ON TOP BEVEL: EFFECT OF FR
 High FR  Low FR FR Conductivity Always broad and asymmetric IEAD on tilted surface. MIN MAX Log scale Iowa State University Optical and Discharge Physics AVS2007_Natalie_18

20 Optical and Discharge Physics
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

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