MAGNETICALLY ENHANCED MULTIPLE FREQUENCY CAPACITIVELY COUPLED PLASMAS: DYNAMICS AND STRATEGIES Yang Yang and Mark J. Kushner Iowa State University Department of Electrical and Computer Engineering Ames, IA October 2005 GEC05_MJK_01

Iowa State University Optical and Discharge Physics AGENDA  Introduction to Magnetically Enhanced Reactive Ion Etching (MERIE) reactors and two-frequency plasma sources.  Description of Model  Scaling parameters for single frequency MERIE  Scaling of 2f-MERIE Properties  Concluding Remarks  Acknowledgement: Semiconductor Research Corp., National Science Foundation, Applied Materials Inc. GEC05_MJK_02

Iowa State University Optical and Discharge Physics MERIE PLASMA SOURCES GEC05_MJK_03  Magnetically Enhanced Reactive Ion Etching plasma sources use transverse static magnetic fields in capacitively coupled discharges for confinement to increase plasma density.  D. Cheng et al, US Patent 4,842,683  M. Buie et al, JVST A 16, 1464 (1998)

Iowa State University Optical and Discharge Physics SCALING OF MERIE SYSTEMS General scalings: More confinement due to B-field has geometric and kinetics effects. GEC05_MJK_04 More positive bias with B-field G. Y. Yeom, et al JAP 65, 3825 (1989) Larger [e], T e with B-field S. V. Avtaeva, et al JPD 30, 3000 (1997)

Iowa State University Optical and Discharge Physics MULTIPLE FREQUENCY CCPs Dual frequency CCPs: goals of separately controlling fluxes and ion energy distributions; and providing additional tuning of IEDs. GEC05_MJK_05 Even with constant LF voltage, IEDs depend on HF properties due to change in sheath thickness and plasma potential V. Georgieva and A. Bogaerts, JAP 98, (2005) Ar/CF 4 /N 2 =80/10/10, 30 mTorr

Iowa State University Optical and Discharge Physics MULTIPLE FREQUENCY MERIEs Question to answer in this presentation: What unique considerations come to light when combining magnetic enhancement, such as in a MERIE, with dual- frequency excitation? Ground Rules: A computational investigation to illuminate physics. Ar only in this presentation. Mixtures for another talk. Power vs Voltage is important! We are varying power not voltage. GEC05_MJK_06

Iowa State University Optical and Discharge Physics MODELING OF DUAL FREQUENCY MERIE  2-dimensional Hybrid Model  Electron energy equation for bulk electrons  Monte Carlo Simulation for high energy secondary electrons from biased surfaces  Continuity, Momentum and Energy (temperature) equations for all neutral and ion species.  Poisson equation for electrostatic potential  Circuit model for bias  Monte Carlo Simulation for ion transport to obtain IEADs GEC05_MJK_07

Iowa State University Optical and Discharge Physics ELECTRON ENERGY TRANSPORT S(T e )=Power deposition from electric fields L(T e ) =Electron power loss due to collisions  =Electron flux  (T e )=Electron thermal conductivity tensor S EB =Power source source from beam electrons GEC05_MJK_08  All transport coefficients are tensors:

Iowa State University Optical and Discharge Physics PLASMA CHEMISTRY, TRANSPORT AND ELECTROSTATICS  Continuity, momentum and energy equations are solved for each species (with jump conditions at boundaries) GEC05_MJK_ 09  Implicit solution of Poisson’s equation

 Poisson’s equation is solved using a semi-emplicit technique where charge densities are predicted at future times.  Predictor-corrector methods are used where fluxes at future times are approximated using past histories or Jacobian elements are used. IMPROVEMENTS FOR LARGE MAGNETIC FIELDS Iowa State University Optical and Discharge Physics GEC05_MJK_10

Iowa State University Optical and Discharge Physics MERIE REACTOR  The model reactor is based on a TEL Design having a transverse magnetic field. GEC05_MJK_11  K. Kubota et al, US Patent 6,190,495 (2001)

Iowa State University Optical and Discharge Physics MERIE REACTOR: MODEL REPRESENTATION  2-D, Cylindrically Symmetric  Magnetic field is purely radial, an approximation validated by 2-D Cartesian comparisons. GEC05_MJK_12

Iowa State University Optical and Discharge Physics MERIE: Ar + DENSITY vs MAGNETIC FIELD  Ar, 40 mTorr, 100 W, 10 MHz  Increasing B-field shifts plasma towards center and increases density.  Large B-fields (> 100 G) decrease density.  Plasma is localized closer to wafer. GEC05_MJK_13

Iowa State University Optical and Discharge Physics  The localization of plasma density near the powered electrode with large B-fields is due to the confinement of secondary electrons and more localized heating of bulk electrons. MERIE: CONFINEMENT OF IONIZATION  Ionization by secondary electrons is uniform across the gap at low B- field; localized at high B-field.  Ar, 40 mTorr, 100 W, 10 MHz GEC05_MJK_14  Secondary Electrons  Bulk Electrons  Ionization Sources

Iowa State University Optical and Discharge Physics MERIE: SHEATH REVERSAL AND THICKENING  Ar, 40 mTorr, 100 W, 10 MHz  As the magnetic field increases, the electrons become less mobile than ions across the magnetic field lines.  The result is a reversal of the electric field in the sheath and sheath thickening. GEC05_MJK_15

Iowa State University Optical and Discharge Physics  The dc bias generally becomes more positive with increasing B-field as the mobility of electrons decreases relative to ions.  Constant power, decreasing ion flux, increasing bias voltage  More resistive plasma.  V Plasma – V dc decreases with bias (sheath voltage….) MERIE dc BIAS,RF VOLTAGE  Ar, 40 mTorr, 100 W, 10 MHz GEC05_MJK_16

Iowa State University Optical and Discharge Physics GEC05_MJK_17 Ar + ENERGY AND ANGLE DISTRIBUTIONS  Ar, 40 mTorr, 100 W, 10 MHz  The more positive dc bias reduces the sheath potential.  The resulting IEAD is lower in energy and broader.

Iowa State University Optical and Discharge Physics 2 FREQUENCY MERIE: GEOMETRY  Ar, 40 mTorr, 300 sccm  B (radial)  Base Case Conditions:  Low Frequency: 5 MHz, 500 W  High Frequency: 40 MHz, 500 W GEC05_MJK_18

Iowa State University Optical and Discharge Physics 2-FREQUENCY CCP (B=0): ELECTRON SOURCES  Mean free paths are long and thermal conductivity is high (and isotropic).  T e is nearly uniform over wafer. Bulk ionization follows electron density.  Secondary electrons penetrate through plasma.  Ar, 40 mTorr, 300 sccm, 0 G, 5 MHz, 40 MHz  LF: 500W, 193 V (dc: -22 V)  HF: 500 W, 128 V GEC05_MJK_19

Iowa State University Optical and Discharge Physics GEC05_MJK_20 2-FREQUENCY MERIE (B=150G): ELECTRON SOURCES  Short transverse mean free paths (anisotropic transport).  T e, bulk ionization peak in sheaths; convect in parallel direction.  Secondary electrons are confined near sheath (trapping on B-field).  dc bias more positive; voltages larger.  Ar, 40 mTorr, 300 sccm, 150 G, 5 MHz, 40 MHz  LF: 500W, 202 V (dc: -1 V)  HF: 500 W, 140 V

Iowa State University Optical and Discharge Physics ION DENSITIES: 2f-CCP vs 2f-MERIE  MERIE achieves goal of increasing ion density due to confinement of beam electrons and slowing transverse diffusion loss.  Spatial distribution changes due to both transport and materials effects. GEC05_MJK_21  B = 0 G (max 9 x cm -3 )  Ar, 40 mTorr, 300 sccm, 5 MHz, 40 MHz  LF: 500W, HF: 500 W  B = 150 G (max 1.3 x cm -3 )

Iowa State University Optical and Discharge Physics GEC05_MJK_22 2-FREQUENCY CCP (B=0): PLASMA POTENTIAL  Sheaths maintain electropositive nature through LF and HF cycles.  Bulk plasma potential is nearly flat and oscillates with both LF and HF components.  Ar, 40 mTorr, 0 G, 5 MHz, 40 MHz  LF: 500W, 193 V (dc: -22 V)  HF: 500 W, 128 V  Time dependent  Low Frequency  High Frequency

2-FREQUENCY MERIE (B=150G): PLASMA POTENTIAL Iowa State University Optical and Discharge Physics GEC05_MJK_23  Sheaths are reversed through portions of both LF and HF cycles.  Bulk electric field is significant to overcome low transverse mobility. Plasma potential oscillates with both LF and HF components.  Ar, 40 mTorr, 150 G, 5 MHz, 40 MHz  LF: 500W, 202 V (dc: -1 V)  HF: 500 W, 140 V  Time dependent  Low Frequency  High Frequency

Iowa State University Optical and Discharge Physics GEC05_MJK_24 2f-CCP vs 2f-MERIE: ION FLUXES  Larger electric fields to transport electrons results in significantly larger variations in ion flux through cycles.  Ar, 40 mTorr, 5 MHz, 40 MHz  LF: 500W, HF: 500 W  B = 0 G  B = 150 G

Iowa State University Optical and Discharge Physics MATERIALS AFFECT UNIFORMITY: PLASMA POTENTIAL  Low mobility of electrons prevent “steady state” charging of dielectrics.  Surface potential of dielectrics is out of phase with plasma potential. GEC05_MJK_25  Ar, 40 mTorr, 5 MHz, 40 MHz  LF: 500W, HF: 500 W  B = 0 G  B = 150 G View Animation-GIF

Iowa State University Optical and Discharge Physics GEC05_MJK_26 SECONDARY EMISSION: IMPORTANT TO SCALING  Scaling of ion flux with HF power is sublinear though better w/B-field.  Increasing HF power reduces LF voltage for constant power.  Poor utilization of secondary electrons.  Power lost to excitation that does not translate to ionization.  Ar, 40 mTorr, 5 MHz, 40 MHz  LF: 500W, HF: 500 W  B = 0 G  B = 100 G

Iowa State University Optical and Discharge Physics  B=0: Increasing  produces nominal increase in ion density and decrease in power as secondary electrons are poorly utilized.  B=100 G: Increasing  produces more ionization, larger ion density and increase in power.  Ar, 100 mTorr, 10 MHz PLASMA PARAMETERS: MERIE B=0, 100 G, V=constant  B = 0  B = 100 G 340 V (p-p)400 V (p-p) GEC05_MJK_27

Iowa State University Optical and Discharge Physics IEDS vs B-FIELD GEC05_MJK_19  IEDs broaden and move to lower energy with increase in B-field and more positive dc bias.  Reversal of sheaths slows ions, broaden angle.  Ar, 40 mTorr, 300 sccm, 150 G, 5 MHz, 40 MHz  LF: 500W  HF: 500 W GEC05_MJK_28

Iowa State University Optical and Discharge Physics IEDS vs LF POWER  Ability to control IED with LF power is compromised in MERIE.  Redistribution of voltage dropped across sheath and bulk  Change in angular distribution.  Ar, 40 mTorr, 300 sccm,  5 MHz, 40 MHz  HF: 500 W GEC05_MJK_29

Iowa State University Optical and Discharge Physics  Maximum ion energy is V(LF)+V(HF)-V(dc).  Increasing HF power increases V(HF) and ion current. For constant LF power, V(LF) decreases.  The maximum IED depends on relative increase in V(HF) and decrease in V(LF). Except that….. VOLTAGES vs HIGH FREQUENCY POWER  B = 0  Ar, 40 mTorr, 5 MHz, 40 MHz  LF: 500W  B = 100 G GEC05_MJK_30

Iowa State University Optical and Discharge Physics  More resistive plasma and field reversal in HF sheath consum voltage otherwise be available for ion acceleration in LF sheath.  The result is a decrease in sheath voltage with a B-field.  Ar, 40 mTorr, 5 MHz, 40 MHz  LF: 500W VOLTAGES vs HIGH FREQUENCY POWER GEC05_MJK_31  LF Sheath Potential  B = 100 G

Iowa State University Optical and Discharge Physics  It appears that ability to maintain IED while changing HF power is better without B-field.  That is generally true….but you just got lucky. IEDs vs HIGH FREQUENCY POWER  B = 0  B = 150 G  Ar, 40 mTorr, 5 MHz, 40 MHz  LF: 500W GEC05_MJK_32

Iowa State University Optical and Discharge Physics IEDS vs LF FREQUENCY B=0 GEC05_MJK_33  IED narrows in energy as LF decreases while maintaining nearly the same average energy.  Scaling does not significantly differ from single frequency system.  Ar, 40 mTorr, 300 sccm,  LF: 500 W  HF 40 MHz: 500 W

Iowa State University Optical and Discharge Physics PLASMA POTENTIAL vs LF FREQUENCY (B=100 G) GEC05_MJK_34  As the low frequency increases…  The fraction of the cycle during which the LF sheath is reversed increases.  Field reversal occurs in the bulk as well as sheath to attract sufficient electrons across B-field.  More phase dependent.  Ar, 40 mTorr, 300 sccm,  LF: 500 W  HF 40 MHz: 500 W  LF = 2.5 MHz  LF = 40 MHz

Iowa State University Optical and Discharge Physics IEDS vs LF FREQUENCY B=100 G GEC05_MJK_35  As the low frequency increases…  The window for allowing ions out of plasma narrows.  The IED narrows and broadens to a greater degree than without B-field.  Ar, 40 mTorr, 300 sccm,  4 MHz, 40 MHz  HF: 500 W

Iowa State University Optical and Discharge Physics CONCLUDING REMARKS  Scaling laws for an industrial MERIE reactor using 2-frequency excitation were investigated.  Reversal of sheaths LF and HF electrodes dominate behavior.  IED shifted to lower energy  Broadened in angle  Increasing (more positive) bias  Sensitivity to sheath reversal increases with increasing LF.  Ability to maintain constant IED when varying HF power is diminished in MERIE system  Larger voltage drop across bulk plasma and HF sheath leaves less voltage at LF electrode.  Larger plasma resistance with B-field increases RC time constant for charging surfaces thereby impacting uniformity. GEC05_MJK_36