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DEVELOPMENT OF ION ENERGY DISTRIBUTIONS THROUGH THE PRE-SHEATH AND SHEATH IN DUAL-FREQUENCY CAPACITIVELY COUPLED PLASMAS

DEVELOPMENT OF ION ENERGY DISTRIBUTIONS THROUGH THE PRE-SHEATH AND SHEATH IN DUAL-FREQUENCY CAPACITIVELY COUPLED PLASMAS* Yiting Zhang a , Nathaniel Moore b , Walter Gekelman b and Mark J. Kushner a (a) Department of Electrical and Computer Engineering,

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DEVELOPMENT OF ION ENERGY DISTRIBUTIONS THROUGH THE PRE-SHEATH AND SHEATH IN DUAL-FREQUENCY CAPACITIVELY COUPLED PLASMAS

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  1. DEVELOPMENT OF ION ENERGY DISTRIBUTIONS THROUGH THE PRE-SHEATH AND SHEATH IN DUAL-FREQUENCY CAPACITIVELY COUPLED PLASMAS* YitingZhanga, Nathaniel Mooreb, Walter Gekelmanb and Mark J. Kushnera (a) Department of Electrical and Computer Engineering, University of Michigan, Ann Arbor, MI 48109 (yitingz@umich.edu , mjkush@umich.edu) (b) Department of Physics, University of California, Los Angeles, CA 90095 (moore@physics.ucla.edu , gekelman@physics.ucla.edu ) November 16, 2011 * 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 2 MHz Ar/O2 • Uniformity and Edge Effect • O2 Percentage • Pressure • Plasma properties for dual-frequency Ar/O2 • Concluding Remarks YZHANG_GEC2011_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 of microelectronic industry. • High driving frequencies produce higher electron densities at moderate sheath voltage and higher ion fluxes with moderate ion energies. • A low frequency contributes 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. • Tegal 6500 series systems high-density plasma etch tools featuring the HRe–™ capacitively coupled plasma etch reactor and dual-frequency RF power technology.   A. Perret, Appl. Phys.Lett 86 (2005) YZHANG_GEC2011_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_GEC2011_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. • Assessment of O2 addition to Ar plasmas, pressure of operation, dual-frequency effects. 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_GEC2011_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: 2mTorr • Ar plasmas: • Ar , Ar*, Ar+, e • Ar/O2 plasmas: • Ar , Ar*, Ar+, e • O2 ,O2*, O2+, O, O*,O+, O- YZHANG_GEC2011_06

  8. University of Michigan Institute for Plasma Science & Engr. PULSED LASER-INDUCED FLUORESCENCE (LIF) • A non-invasive optical technique for measuring the ion velocity distribution function. • Ions moving along the direction of laser propagation will have the absorption wavelengths Doppler-shifted from λ0, • Ion velocity parallel to the laser obtained from Δλ=λ0-λL=v//λ0/c • B. Jacobs, PRL 105, 075001(2010) YZHANG_GEC2011_07

  9. University of Michigan Institute for Plasma Science & Engr. PLASMA PROPERTIES • Majority of power deposition that produces ions comes from inductively coupled coils. • Te is fairly uniform in the reactor due to high thermal conductivity - peaking near coils where E-field is largest. • Small amount of electro- negativity [O2-] /[M+] =0.0175, with ions pooling at the peak of the plasma potential. • Ar/O2=80/20, 2mTorr, 50 SCCM • Freq=2 MHz, 500 V • DC Bias=-400 V YZHANG_GEC2011_08

  10. University of Michigan Institute for Plasma Science & Engr. Ar+ IEAD FROM BULK TO SHEATH • In the bulk plasma and pre-sheath, the IEAD is essentially thermal and broad in angle. Boundaries of the pre-sheath are hard to determine. • In the sheath, ions are accelerated by the E-field in vertical direction and the angular distribution narrows. • Note: Discontinuities with energy increase caused by mesh resolution in collecting statistics. • Ar/O2=80/20, 2mTorr, 50 SCCM • Freq=2 MHz, 500 V • DC Bias=-400 V YZHANG_GEC2011_09

  11. University of Michigan Institute for Plasma Science & Engr. IEAD NEAR EDGE OF WAFER • IEADs are separately collected over wafer middle, edge and chuck regions. • Non-uniformity near the wafer edge and chuck region - IEAD has broader angular distribution. • Focus ring helps improve uniformity. • Maximum energy consistent regardless of wafer radius. 0.5 mm above wafer • Ar/O2=0.8/0.2, 2mTorr, 50 SCCM • Freq=2 MHz VRFM=500 Volt • DC Bias=-400 Volt YZHANG_GEC2011_10

  12. University of Michigan Institute for Plasma Science & Engr. IEAD vs RF PHASE: PRESHEATH • IEADs near presheath boundary are independent of phase, and slowly drifting. • In the pre-sheath, small ion drifts cause the IEAD to slightly change vs phase. • Experimental result shows the same trend. Phase • B. Jacobs (2010) • Ar/O2 = 0.8/0.2, • 0.5 mTorr, 50 SCCM • LF= 600kHz, 425W • HF=2 MHz, 1.5kW • Sheath ~3.6 mm • LIF measured 4.2 mm above wafer • Phase regard to HF • Ar/O2 =0.8/0.2, 2mTorr, 50 SCCM • Freq=2 MHz, 500 V • DC Bias =-400 V • IEAD 4.2 mm above wafer YZHANG_GEC2011_11

  13. University of Michigan Institute for Plasma Science & Engr. IEAD UNDER DIFFERENT RF PHASES • Due to periodic acceleration in sheath, IEAD depends on phase. • During low acceleration phases, IEAD drifts in sheath. • During high acceleration phase, IEAD narrows as perpendicular component of velocity distribution increases. Phase • B. Jacobs (2010) • Ar/O2 = 0.8/0.2, • 0.5 mTorr, 50 SCCM • LF= 600kHz, 425W • HF=2 MHz, 1.5kW • Sheath ~3.6 mm • LIF measured 4.2 mm above wafer • Phase regard to HF • Ar/O2 =0.8/0.2, 2mTorr, 50 SCCM • Freq=2 MHz, 500 V • DC Bias =-400 V • IEAD 0.5 mm above wafer YZHANG_GEC2011_12

  14. IEAD vs PHASES FROM BULK TO SHEATH University of Michigan Institute for Plasma Science & Engr. Phase 3.3 mm 2.6 mm 1.9 mm 1.2 mm • Ar/O2 =0.8/0.2, 2mTorr, 50 SCCM,Freq=2 MHz, 500 V • DC Bias =-400 V ,IEAD 0.5 mm above wafer 0.5 mm YZHANG_GEC2011_13

  15. University of Michigan Institute for Plasma Science & Engr. O2 ADDITION TO Ar • With increasing O2 in Ar/O2, negative ion ( O-) formation decreases fluxes to substrate for fixed power. • Sheath potential only moderately increases - for up to 20% O2, IEADs are only nominally affected since negative ions are limited to core of plasma. • Ar+ IEAD on wafer • 2 mTorr, 300 SCCM. • Freq=2 MHz, 300 W. YZHANG_GEC2011_14

  16. University of Michigan Institute for Plasma Science & Engr. IEADs vs PRESSURE • With decreasing pressure and increasing mean free path, trajectories are more ballistic - ions still drift into wafer at low energy during anodic part of cycle. • With higher pressure, lower plasma density increases thickness of sheath . Thicker sheath, more collisions, longer transit time – more distributed ion trajectories through sheath. • Ar+ IEAD on wafer • 5/10/20mTorr, 75/150/300 SCCM. • Freq=2 MHz, 500 V • DC Bias =-400 V YZHANG_GEC2011_15

  17. University of Michigan Institute for Plasma Science & Engr. IEADs vs HIGH FREQUENCY • If high frequency (10 MHz) is close to low frequency (2 MHz), they will interfere each other and contribute to multiple peaks in IEADs. • When high frequency is largely separated from the low frequency (2 MHz) since they changes so fast that ion fail to response, 30 MHz and 60 MHz show similar properties for ion distribution function. • Ar/O2=0.8/0.2, 2mTorr, 50 SCCM • HF = 10/30/60 MHz, 100 V • LF = 2 MHz 400 V • DC BIAS = -100 V, IEAD on wafer YZHANG_GEC2011_16

  18. University of Michigan Institute for Plasma Science & Engr. DUAL-FREQ IEAD vs PHASES • High frequency produces additional peaks in IEADs compared to single low frequency – structure is phase dependent. • Experiments show similar trend. • 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 HF • 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_GEC2011_17

  19. University of Michigan Institute for Plasma Science & Engr. SHEATH vs HIGH FREQUENCY • The sheath and pre-sheath thickness are nearly independent of HF on substrate (for fixed voltage). • Higher frequencies add modulation onto IEADs as a function of phase. • Ar/O2=0.8/0.2, 2mTorr, 50 SCCM • HF = 10/60 MHz, 100 V LF = 2 MHz, 400 V • DC BIAS = -100 V, Phase refers to LF • IEAD 0.5mm above wafer YZHANG_GEC2011_18

  20. University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS • In the pre-sheath, 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. • Multiple peaks in IEADs come from IEADs alternately accelerated by rf field during the whole RF period. • Sheath and Pre-sheath thicknesses are both increased with the pressure. On the other hand, higher pressure bring more collisions and ions reach low energy and broad angular distribution. • Dual Frequency enhance electron and ion densities, provide flexibility of control of ion distribution while adding modulation to IEAD. YZHANG_GEC2011_19

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