SPACE AND PHASE RESOLVED MODELING OF ION ENERGY ANGULAR DISTRIBUTIONS FROM THE BULK PLASMA TO THE WAFER IN DUAL FREQUENCY CAPACITIVELY COUPLED PLASMAS* Yiting Zhang a, Nathaniel Moore b, Walter Gekelman b Patrick Pribyl b and Mark J. Kushner a (a) Department of Electrical and Computer Engineering, University of Michigan, Ann Arbor, (b) Department of Physics, University of California, Los Angeles, ) October 23, 2012 * Work supported by National Science Foundation, Semiconductor Research Corp. and the DOE Office of Fusion Energy Science

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/O 2  Uniformity and edge effect  Experimental comparison  Higher frequency  IEADs and plasma properties for dual-frequency Ar/O 2  Sheath thickness and ion transit time  Voltage amplitude ratio  Concluding remarks YZHANG_GEC2012_01 University of Michigan Institute for Plasma Science & Engr.

DUAL FREQUENCY CCP SOURCES YZHANG_GEC2012_02  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.  A. Perret, Appl. Phys.Lett 86 (2005) University of Michigan Institute for Plasma Science & Engr.  LAMRC 2300 Flex dielectric etch tool  Coupling between the dual frequencies may interfere with independent control of plasma density, ion energy and produce non-uniformities.

ION ENERGY AND ANGULAR DISTRIBUTIONS (IEAD) YZHANG_GEC2012_03  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. S.-B. Wang and A.E. Wendt, J. Appl. Phys., Vol 88, No.2 B. Jacobs, PhD Dissertation University of Michigan Institute for Plasma Science & Engr.  Ion velocity trajectories measured by LIF (Jacobs et al.)

IEADs THROUGH SHEATHS YZHANG_GEC2011_04  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. University of Michigan Institute for Plasma Science & Engr.

HYBRID PLASMA EQUIPMENT MODEL (HPEM) YZHANG_GEC2012_05 Monte Carlo Simulation f(ε) or Electron Energy Equation  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. EETM Continuity, Momentum, Energy, Poisson equation FKM Monte Carlo Module PCMCM S e (r) N(r) E s (r) M. Kushner, J. Phys.D: Appl. Phys. 42 (2009) University of Michigan Institute for Plasma Science & Engr. Maxwell Equation Circuit Module I,V(coils) E EMM E (r, θ, z,φ ) B (r, θ,z, φ )

REACTOR GEOMETRY University of Michigan Institute for Plasma Science & Engr.  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/O 2 plasmas:  Ar, Ar*, Ar +, e  O 2,O 2 *, O 2 +, O, O*,O +, O - YZHANG_GEC2012_06

YZHANG_GEC2012_07 PLASMA PROPERTIES  Majority of power deposition producing ions comes from inductively coupled coils.  T e 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 [O 2 - ] /[M + ] =0.0175, due to dissociation of O 2 with ions pooling at the peak of the plasma potential. Discharge is electropositive.  Ar/O 2 =80/20, 2 mTorr, 50 SCCM  Freq=2 MHz, 500 V ppk  DC Bias=-400 V University of Michigan Institute for Plasma Science & Engr.

YZHANG_GEC2012_08 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.  Ar/O 2 =80/20, 2 mTorr, 50 SCCM  Freq=2 MHz, 1000 V ppk  DC Bias=-400 V 2 MHz ( b)

YZHANG_GEC2012_09 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. University of Michigan Institute for Plasma Science & Engr.  Ar/O 2 =0.8/0.2, 2 mTorr, 50 SCCM  Freq=2 MHz 1000 V ppk  DC Bias=-400 Volt 0.5 mm above wafer

LIF Measured YZHANG_GEC2012_10 COMPARISON WITH EXPERIMENTS: SHEATH THICKNESS Time Averaged Simulation Results  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.  Ar/O 2 =80/20, 0.5 mTorr, 50 SCCM  RF Freq=2 MHz, 900 Vppk (2.2MHz for experimental)  Coil Power=500W CW

 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. Phase COMPARISON WITH EXPERIMENTS: PRESHEATH & SHEATH Exp z=4.4mm above wafer Model z=4.4mm above wafer YZHANG_GEC2012_11  Ar/O 2 =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) R=11.2 mm Φ=π 1.2 mm 2.0 mm 2.8 mm 3.6 mm 4.4 mm R=11.2 mm Φ=π Simulated IEDFs LIF Measured 1.0 mm 1.4 mm 1.8 mm 2.2 mm 3.4 mm

YZHANG_GEC2012_12 IEADs vs. FREQUENCY University of Michigan Institute for Plasma Science & Engr.  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/O 2 =0.8/0.2, 2 mTorr, 50 SCCM  Freq = 2/10/30/60 MHz, 1000 V ppk  DC BIAS = -400 V, IEAD on wafer

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

 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.  Ar/O 2 =0.8/0.2, 2mTorr, 50 SCCM  HF = 10 MHz, 800 V ppk  LF = 2 MHz, 800 V ppk  DC BIAS = -100 Volt  IEAD 0.5mm above wafer  Sheath Potential  IEAD DUAL-FREQUENCY IEAD vs. SHEATH POTENTIAL:2/10 MHz YZHANG_GEC2012_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/O 2 =0.8/0.2, 2mTorr, 50 SCCM  HF = 20/30 MHz, 800 V ppk  LF = 2 MHz, 800 V ppk  DC BIAS = -100 Volt  IEAD 0.5mm above wafer University of Michigan Institute for Plasma Science & Engr. YZHANG_GEC2012_17

YZHANG_GEC2012_18 DUAL-FREQ IEAD vs. PHASES  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/O 2 =0.8/0.2, 2mTorr, 50 SCCM  DC BIAS = -100 Volt University of Michigan Institute for Plasma Science & Engr.

CONCLUDING REMARKS YZHANG_GEC2012_19  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. University of Michigan Institute for Plasma Science & Engr.