AGENDA

1. SPACE AND PHASE RESOLVED MODELING OF ION ENERGY ANGULAR DISTRIBUTIONS FROM THE BULK PLASMA TO THE WAFER IN DUAL FREQUENCY CAPACITIVELY COUPLED PLASMAS* Yiting Zhanga, Nathaniel Mooreb, Walter Gekelmanb Patrick Pribylb and Mark J. Kushnera (a) Department of Electrical and Computer Engineering, University of Michigan, Ann Arbor, 48109 (yitingz@umich.edu, mjkush@umich.edu) (b) Department of Physics, University of California, Los Angeles, 90095 (moore@physics.ucla.edu, pribyl@physics.ucla.edu, gekelman@physics.ucla.edu ) October 23, 2012 * Work supported by National Science Foundation, Semiconductor Research Corp. and the DOE Office of Fusion Energy Science

2. University of Michigan Institute for Plasma Science & Engr. AGENDA • Introduction to dual frequency capacitively coupled plasma (CCP) sources and Ion Energy Angular Distributions (IEADs) • Description of the model • IEADs and plasma properties for single rf bias Ar/O2 • Uniformity and edge effect • Experimental comparison • Higher frequency • IEADs and plasma properties for dual-frequency Ar/O2 • Sheath thickness and ion transit time • Voltage amplitude ratio • Concluding remarks YZHANG_GEC2012_01

3. University of Michigan Institute for Plasma Science & Engr. DUAL FREQUENCY CCP SOURCES • Dual frequency capacitively coupled discharges (CCPs) are widely used for etching and deposition in the microelectronics industry. • High driving frequencies produce higher electron densities at moderate sheath voltage and higher ion fluxes with moderate ion energies. • A low frequency contributes to the quasi-independent control of the ion flux and energy. • Coupling between the dual frequencies may interfere with independent control of plasma density, ion energy and produce non-uniformities. • LAMRC 2300 Flex dielectric etch tool  A. Perret, Appl. Phys.Lett 86 (2005) YZHANG_GEC2012_02

4. University of Michigan Institute for Plasma Science & Engr. ION ENERGY AND ANGULAR DISTRIBUTIONS (IEAD) • Control of the ion energy and angular distribution (IEAD) incident onto the substrate is necessary for improving plasma processes. • A narrow, vertically oriented angular IEAD is necessary for anisotropic processing. • Edge effects which perturb the sheath often produce slanted IEADs. • Ion velocity trajectories measured by LIF (Jacobs et al.) • S.-B. Wang and A.E. Wendt, • J. Appl. Phys., Vol 88, No.2 • B. Jacobs, PhD Dissertation YZHANG_GEC2012_03

5. University of Michigan Institute for Plasma Science & Engr. IEADs THROUGH SHEATHS • Results from a computational investigation of ion transport through RF sheaths will be discussed. • Investigation addresses the motion of ion species in the RF pre-sheath and sheath as a function of position in the sheath and phase of RF source. • Comparison to experimental results from laser induced fluorescence (LIF) measurements by Low Temperature Plasma Physics Laboratory at UCLA. • IEDFs with single high frequency (10-60MHz), dual frequency effects will also be discussed. YZHANG_GEC2011_04

6. University of Michigan Institute for Plasma Science & Engr. HYBRID PLASMA EQUIPMENT MODEL (HPEM) EMM EETM FKM E(r,θ,z,φ) B(r,θ,z,φ) PCMCM Maxwell Equation Monte Carlo Simulation f(ε) or Electron Energy Equation Se(r) Continuity, Momentum, Energy, Poisson equation Monte Carlo Module I,V(coils) E N(r) Es(r) Circuit Module • Electron Magnetic Module (EMM): • Maxwell’s equations for electromagnetic inductively coupled fields. • Electron Energy Transport Module（EETM): • Electron Monte Carlo Simulation provides EEDs of bulk electrons. • Separate MCS used for secondary, sheath accelerated electrons. • Fluid Kinetics Module (FKM): • Heavy particle and electron continuity, momentum, energy and Poisson’s equations. • Plasma Chemistry Monte Carlo Module (PCMCM): • IEADs in bulk, pre-sheath, sheath, and wafers. • Recorded phase, submesh resolution. • M. Kushner, J. Phys.D: Appl. Phys. 42 (2009) YZHANG_GEC2012_05

7. University of Michigan Institute for Plasma Science & Engr. REACTOR GEOMETRY • Inductively coupled plasma with multi-frequency capacitively coupled bias on substrate. • 2D, cylindrically symmetric. • Base case conditions • ICP Power: 400 kHz, 480 W • Substrate bias: 2 MHz • Pressure: 2 mTorr • Submesh covers wafer center to edge, presheath and sheath region. • Ar/O2 plasmas: • Ar , Ar*, Ar+, e • O2 ,O2*, O2+, O, O*,O+, O- YZHANG_GEC2012_06

8. University of Michigan Institute for Plasma Science & Engr. PLASMA PROPERTIES • Majority of power deposition producing ions comes from inductively coupled coils. • Te is fairly uniform due to high thermal conductivity - peaking near coils where E-field is largest. • Peak gas temperature is > 460 K. • Small amount of electro- negativity [O2-] /[M+] =0.0175, due to dissociation of O2 with ions pooling at the peak of the plasma potential. Discharge is electropositive. • Ar/O2=80/20, 2 mTorr, 50 SCCM • Freq=2 MHz, 500 Vppk • DC Bias=-400 V YZHANG_GEC2012_07

9. Ar+ IEAD FROM BULK TO SHEATH vs PHASE • In the bulk plasma and pre-sheath, the IEAD is essentially thermal and broad in angle. Boundaries of the pre-sheath are subjective…. • In the sheath, ions are accelerated by the E-field in vertical direction and angular distribution narrows. 2 MHz • Ar/O2=80/20, 2 mTorr, 50 SCCM • Freq=2 MHz, 1000 Vppk • DC Bias=-400 V ( b) YZHANG_GEC2012_08

10. University of Michigan Institute for Plasma Science & Engr. IEAD NEAR EDGE OF WAFER • IEADs are separately collected over wafer middle, edge and focus ring. • Non-uniformity near the wafer edge and focus ring - IEAD has broader angular distribution - though focus ring helps improve uniformity. • Maximum energy consistent regardless of wafer radius. • 0.5 mm above wafer • Ar/O2=0.8/0.2, 2 mTorr, 50 SCCM • Freq=2 MHz 1000 Vppk • DC Bias=-400 Volt YZHANG_GEC2012_09

11. COMPARISON WITH EXPERIMENTS: SHEATH THICKNESS • Both simulated and measured IEDF shows sheath thickness are about 4 mm compared with a predicted value of 3.2 mm. • Both results also observe non-uniformity near the edge by ion energy drop. • Time Averaged Simulation Results • LIF Measured • Ar/O2=80/20, 0.5 mTorr, 50 SCCM • RF Freq=2 MHz, 900 Vppk (2.2MHz for experimental) • Coil Power=500W CW YZHANG_GEC2012_10

12. COMPARISON WITH EXPERIMENTS: PRESHEATH & SHEATH • Simulated IEDFs R=11.2 mm Φ=π • In the presheath, small ion drift cause the IEDFs to slightly change vs. phase. • In the sheath during the ion accelerate phase, the ion quickly gain higher energy. • Experimental results show the same trend. 2.2 mm 1.4 mm 3.4 mm 1.8 mm 1.0 mm • LIF Measured R=11.2 mm Φ=π Model z=4.4mm above wafer Phase Exp z=4.4mm above wafer 3.6 mm 2.0 mm 4.4 mm 2.8 mm 1.2 mm • Ar/O2=80/20, 0.5 mTorr, 50 SCCM • RF Freq=2 MHz, 900 Vppk (2.2MHz for experimental) • DC Bias=-405 V • Coil Power=500W CW • Each phase measured in ~500ns (30ns for experimental) YZHANG_GEC2012_11

13. University of Michigan Institute for Plasma Science & Engr. IEADs vs. FREQUENCY • With increase of frequency, width of ion energy E decreases. • 30 MHz and 60 MHz show similar properties for IEAD. Due to ion’s high inertia, fails to respond to both frequencies. • Ar/O2=0.8/0.2, 2 mTorr, 50 SCCM • Freq = 2/10/30/60 MHz, 1000 Vppk • DC BIAS = -400 V, IEAD on wafer YZHANG_GEC2012_12

14. Model DUAL-FREQUENCY IEAD vs. PHASE University of Michigan Institute for Plasma Science & Engr. • With dual frequency (LF = 2 MHz, HF = 30 MHz), the extra HF produces additional peaks in IEADs • Experiments show similar trend. • LIF Measured • B.Jacobs, W.Gekelman, PRL 105, 075001(2010) • Ar/O2=0.8/0.2, 0.5 mTorr, 50 SCCM • LF=600kHz, 425W HF=2MHz, 1.5kW Phase refers to LF • Ar/O2=0.8/0.2, 2mTorr, 50 SCCM • HF = 30 MHz, 100 V LF = 2 MHz, 400 V • DC BIAS = -100 V, Phase refers to LF • IEAD 0.5mm above wafer YZHANG_GEC2012_15

15. DUAL-FREQUENCY IEAD vs. SHEATH POTENTIAL:2/10 MHz University of Michigan Institute for Plasma Science & Engr. • Sheath Potential • LF = 2 MHz, HF = 10 MHz • IEADs show general LF modulation of sheath potential. • Ions are able to respond to HF though there is a time delay that is not consistent across the phases. • The different IEAD time delays shows the sheath thickness is not constant. • Results are sensitive to relative amplitudes and phases. • IEAD • Ar/O2=0.8/0.2, 2mTorr, 50 SCCM • HF = 10 MHz, 800 Vppk • LF = 2 MHz, 800 Vppk • DC BIAS = -100 Volt • IEAD 0.5mm above wafer YZHANG_GEC2012_16

16. University of Michigan Institute for Plasma Science & Engr. DUAL-FREQUENCY IEAD vs. SHEATH POTENTIAL: 2/20 MHz, 2/30 MHz • LF = 2 MHz, HF = 20/30 MHz • As HF increases modulation during RF cycle decreases. • Modulation during the LF may also lessen. • Results are sensitive to relative amplitudes of LF/HF • Ar/O2=0.8/0.2, 2mTorr, 50 SCCM • HF = 20/30 MHz, 800 Vppk • LF = 2 MHz, 800 Vppk • DC BIAS = -100 Volt • IEAD 0.5mm above wafer YZHANG_GEC2012_17

17. DUAL-FREQ IEAD vs. PHASES University of Michigan Institute for Plasma Science & Engr. • The sheath thickness scales inversely with electron density. • There is some modulation of [e] at the sheath edge (and so sheath thickness) even during the HF period. • Varying the ratio of HF/LF voltage amplitudes gives control over the angular spread of the IEADs. • The ratio of HF/LF=1.0/0.5/2.0 • Ar/O2=0.8/0.2, 2mTorr, 50 SCCM • DC BIAS = -100 Volt YZHANG_GEC2012_18

18. University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS • In the pre-sheath, the IEAD is thermal and broad in angle. When the ion flux is accelerated through the sheath, the distribution increases in energy and narrows in angle on a phase dependent basis. • Multiple peaks in IEADs come from IEADs alternately accelerated by rf field during the whole RF period. • Both experiment and simulation results shows a decay of energy near the edge. The ion sinusoid behavior in sheath and Maxwellian distribution in pre-sheath are also observed in both. • There is modulation in the sheath thickness during the LF and HF period. This will affect ion transit time and result in different ion response delay times at different phases. • The ratios of HF/LF voltage and driving frequency are critical parameters in determining the shape of the IEADs. • Dual Frequency enhance electron and ion densities, provide flexibility of control of ion distribution while adding modulation to the IEAD. YZHANG_GEC2012_19