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  1. DEVELOPMENT OF ION ENERGY ANGULAR DISTRIBUTION THROUGH THE PRE-SHEATH AND SHEATH IN DUAL-FREQUENCY CAPACITIVELY COUPLED PLASMAS* Yiting Zhanga, Nathaniel Mooreb, Walter Gekelmanb 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 , gekelman@physics.ucla.edu ) September 2011 * Work supported by National Science Foundation and Semiconductor Research Corp.

  2. University of Michigan Institute for Plasma Science & Engr. AGENDA • Introduction to dual frequency capacitively coupled plasma (CCP) sources and Ion Energy Angular Distributions (IEAD) • Description of the model • Plasma properties for 2 MHz / 30 MHz • Ar Plasma properties • Ar/O2 Plasma Properties • Uniformity and Edge Effect • Concluding Remarks YZHANG_MIPSE2011_01

  3. University of Michigan Institute for Plasma Science & Engr. DUAL FREQUENCY CCP SOURCES • Capacitively coupled discharges (CCPs) are widely used for etching and deposition of microelectronic industry. • High driving frequency achieve 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. • However, the non-uniformity problems arise with increases of the driving frequency.  A. Perret, Appl. Phys.Lett 86 (2005) YZHANG_MIPSE2011_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) at the substrate provides the potential for improving plasma processes. • A narrow angular IEAD at the substrate with the majority ion flux perpendicular to the substrate is desired for anisotropic processing. • Edge effects produce slanted IEADs. • S.-B. Wang and A.E. Wendt, • J. Appl. Phys., Vol 88, No.2 • B. Jacobs, PhD Dissertation YZHANG_MIPSE2011_03

  5. University of Michigan Institute for Plasma Science & Engr. GOALS • Results from a computational investigation of ion transport through RF sheaths will be discussed. • Investigate the motion of ion species in the RF pre-sheath and sheath region of CCPs using sub-meshing technique to provide finer resolution at different 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. YZHANG_MIPSE2011_04

  6. University of Michigan Institute for Plasma Science & Engr. HYBRID PLASMA EQUIPMENT MODEL (HPEM) EETM FKM PCMCM Se(r) Monte Carlo Simulation f(ε) or Electron Energy Equation Continuity, Momentum, Energy, Poisson equation Monte Carlo Module N(r) Es(r) • 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 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_MIPSE2011_05

  7. University of Michigan Institute for Plasma Science & Engr. REACTOR GEOMETRY • Inductively coupled with 2-freq CCP on substrate • 2D, cylindrically symmetric. • Base conditions • ICP Power: 400kHz,300 Watt • High Freq RF: 10 MHz 300 Watt 300 Volt • Low Freq RF: 2MHz 100 Watt 150 Volt • Specify power, adjust voltage. • Main Species in Ar • Ar , Ar*, Ar+, e • Main Species in Ar/O2 • Ar , Ar*, Ar+, e • O2 ,O2*, O2+, O, O*,O+, O- YZHANG_MIPSE2011_06

  8. University of Michigan Institute for Plasma Science & Engr. PLASMA PROPERTIES • Majority of power deposition that produces ions comes from inductively coupled coils. • Ion acceleration is produced by capacitive coupling. • Plasma distribution determines local sheath thickness, potential and ion mixing ratio at wafer. • Te peaks near coil where E-field is largest. • Electro-static waves due to double layers. Ion Density (cm-3) • Ar/O2 =0.8/0.2, • 20mTorr, 300 SCCM • Freq=2 MHz, 300 Watt YZHANG_MIPSE2011_07

  9. 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 fromΔλ=λ0-λL=v//λ0/c • B. Jacobs, PRL 105, 075001(2010) YZHANG_MIPSE2011_08

  10. University of Michigan Institute for Plasma Science & Engr. Ar+ IEAD FROM BULK TO SHEATH • IEAD changes significantly through sheath from bulk plasma. • In the bulk plasma and pre-sheath, the IEAD is essentially thermal and broad in angle. • In the sheath, ions are accelerated by the E-field in z direction and the angle narrows. • Ar, 20mTorr, 300 SCCM • HF=30 MHz 100Watt • LF=2 MHz 300Watt YZHANG_MIPSE2011_09

  11. University of Michigan Institute for Plasma Science & Engr. IEAD NEAR EDGE OF WAFER • IEADs are separately collected over center, middle and edge regions. • Non-uniformity near the edge region - IEAD has broader angular distribution. • Maximum energy consistent regardless of radius. Center Middle Edge 0.5 mm above wafer • Ar, 20mTorr, 300 SCCM • HF=30MHz 100Watt • LF=2 MHz 300Watt YZHANG_MIPSE2011_10

  12. University of Michigan Institute for Plasma Science & Engr. PEAKS IN ION ENERGY DISTRIBUTION vs PHASE • IEAD properties differ during the RF period. • Argon ions are most energetic shortly after the maximum in accelerating field. • Experiments show similar trend. • B.Jacobs, W.Gekelman, PRL 105, 075001(2010) • Ar/O2=0.8/0.2, • 0.5 mTorr, 50 SCCM • LF600kHz, 425W • HF=2MHz, 1.5kW • Phase refers to HF • Ar, 20mTorr, 300 SCCM • HF=30MHz 100Watt • LF=2 MHz 300Watt • Phase refer LF YZHANG_MIPSE2011_11

  13. University of Michigan Institute for Plasma Science & Engr. IEAD UNDER DIFFERENT RF PHASES • B. Jacobs, PhD Dissertation (2010) • IEADs far above wafer are independent of phase, and slowly drifting. • In the pre-sheath, small ion drifts cause the IEAD to slightly change vs phase. • Ar/O2=0.8/0.2, • 0.5 mTorr, 50 SCCM • HF600kHz, 425W • LF=2 MHz, 1.5kW • Sheath ~3.6 mm • LIF measured 4.2 mm above wafer • Ar/O2 =0.8/0.2, • 20mTorr, 300 SCCM • Freq=2 MHz • IEAD 4 mm above wafer YZHANG_MIPSE2011_12

  14. University of Michigan Institute for Plasma Science & Engr. IEAD UNDER DIFFERENT RF PHASES • Due to periodic acceleration in sheath, development of 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. • B. Jacobs, W. Gekelman, PRL 105, 075001(2010) • Ar/O2=0.8/0.2, • 0.5 mTorr, 50 SCCM • HF600kHz, 425W • LF=2 MHz, 1.5kW • Sheath ~3.6 mm • LIF measured 1 mm above wafer • Ar/O2 =0.8/0.2, • 20mTorr, 300 SCCM • Freq=2 MHz • IEAD 0.5 mm above wafer YZHANG_MIPSE2011_13

  15. University of Michigan Institute for Plasma Science & Engr. O2 ADDITION TO AR • With increasing O2, negative ion ( O2-, O-) formation increases the sheath potential for fixed power. • IEAD for Ar+ extends in energy and narrows in angle. • Ar+ IEAD on wafer • 20 mTorr, 300 SCCM. • Freq=2 MHz, 300 W. YZHANG_MIPSE2011_14

  16. 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. • Edge Effect can be observed clearly by using the high resolution afforded by sub-meshing. Multiple peaks in IEADs come from IEADs alternately accelerated by rf field during the whole RF period. • Increasing O2 changes the sheath properties – a narrower IEAD achieved when percentage of O2 increase from 5% to 20%. YZHANG_MIPSE2011_15