Hong Young Chang Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Republic of Korea

Hong Young Chang Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Republic of Korea

Index 1. Introduction 2. Some plasma sources 3. Related issues 4. Summary -2

Why is Large Area Plasma Source important? Flat Panel Display Process glass size increases continuously. (8 G : 2200 mm X 2500 mm) Si thin film Solar Cell Microcrystalline Si deposition using Large Area Very High Frequency(VHF) plasma Semiconductor Dual Frequency CCP (using VHF), wafer size increases continuously. Near future, 450 mm wafer will be used. -3

Wafer size change and expected key issues -200 mm / 1990-300 mm / 2001-450 mm / 2012 (?) Expected key issues 1. Uniformity over wafer (Plasma and Radical Distributions) 2. Discharge stability at various pressures 3. Low process damages (Emissive, physical and electrical) 4. High process performance (ER, Sel., DR, etc.)

Why is VHF Plasma needed? Plasma frequency dependence of electron density and electron temperature -argon plasma power density: 0.13 W/cm2, pressure : 50 & 20 mtorr. M. Takai et al. J. Non-Cryst. Solids 266-269 (2000) 90 PIC calculation, at 2.67 Pa S. J. You, et al. J. Appl. Phys. vol. 94 no. 12 7422 Driving frequency P i P e Ion bombardment energy Electron density Electron temperature -5

Standing Wave Effect -Standing wave : Incident wave + reflected wave x node -Velocity of light = 300000 km/s -RF frequency = 13.56 MHz -Half wave length = 11 m 13.56 MHz -Strong capacitive E-field at electrode center 2.3 m -6

Standing wave effect Strong Capacitive E- field at the electrode center Skin effect - Strong Induced E-field at the electrode edge Capacitive electric field Induced electric field B-field A. Perret, et al Appl. Phys. Lett. 83, 243 (2003) -7

-Single Electrode Source

Multi-feeding source 1,500 1,000 13.56 MHz 27.12 MHz 40 MHz E-field (A.U) 500 Non-Uniformity(%) 0 0 100 200 300 400 500 10 1 Length single feeding multi feeding -Simulation result -View of multi feeding port 10 15 20 25 30 35 40 45 Frequency(MHz) -There is little uniformity difference between single feeding and multi feeding concept. -Another concept is needed.

Multi electrode source -Conventional single electrode -Electrode -y -x -Newly designed Multi-electrode -Power feeding x-direction : split electrode to reduce the interference of EM wave y-direction : multi-feeding to reject the non-uniformity of RF field distribution

-13.56 MHz -40.68 MHz -60 MHz -80 MHz

-Diagnostic results Plasma uniformity -< 8x8 Probe array> 1.0 1.0 Measured the spatial profile of the J i /J i max 0.9 0.8 0.7 0.6 0.5 J i /J i max 0.9 0.8 0.7 0.6 0.5 ion saturation current with the homemade probe array 0.4-20 -10 X (cm) 0 10 20 0.4-10.3% 10-20 0-10 -10 20-20 Y (cm) X (cm) 0 10-12.6% 20-20 -10 0 20 10 Y (cm) -< Example of measured result with probe array in Normal CCP>

Multi electrode source Non - uniformity 100 min max 2 100 - Multi electrode -4.03% -Non-uniformity - 13.56 MHz - 40 MHz -13

-VHF Source Simulation -Single electrode -Multi electrode -Concept of single feeding line -Multi feeding electrode -14

-VHF Source Installation -450mm plasma equipment -Power input port -Installed 450mm CCP sources -15

-2D Probe Array Installation -Structure -2D probe -Sensor Arrays -- SiO2 coated PCB -Al2O3 anodized -Chamber -Shielding case -ID 700 mm -2D Probe -Powered electrode - ~ 500 mm -Ground -Electrode -Data cable -450 mm -diameter(<23mm) -Vacuum feedthrough -- NW25 -Vacuum port (NW25) -buttress -& -Data cable -Vacuum feedthrough

-2D Probe Array Controller -450mm electrode size -Program menu -Captured data -17

Measurement of Large area CCP (Ion flux, μa/cm 2 ) 200 0.07000 0.07250 200 0.07000 0.07250 150 0.07500 0.07750 150 0.07500 0.07750 100 0.08000 0.08250 100 0.08000 0.08250 50 0.08500 0.08750 50 0.08500 0.08750 Y Axis 0-50 0.09000 Y Axis 0-50 0.09000-100 -150-200 -100 5.4% 15.3% -150-200 200-200 -150-100 -50 0 50 100 150 200 X Axis -Center 50W, Side 200W 0.07000 0.07875 200-200 -150-100 -50 0 50 100 150 200 X Axis -Center 50W, Side 300W 0.07000 0.07875 150 0.08750 0.09625 150 0.08750 0.09625 100 0.1050 0.1138 100 0.1050 0.1138 50 0.1225 0.1313 50 0.1225 0.1313 Y Axis 0-50 0.1400 Y Axis 0-50 0.1400-100 -150-200 29.5% -100-150 27.17% -200-200 -150-100 -50 0 50 100 150 200 X Axis -Center 75W, Side 200W -200-150 -100-50 0 50 100 150 200 X Axis -Center 75W, side 300W -18

-Measurement of Large area CCP (Ion flux, μa/cm 2 ) Y Axis 200 150 100 50 0-50 0.03000 0.03625 0.04250 0.04875 0.05500 0.06125 0.06750 0.07375 0.08000 0.08625 0.09250 0.09875 0.1050 0.1113 0.1175 0.1238 0.1300 Y Axis 200 150 100 50 0-50 0.03000 0.03625 0.04250 0.04875 0.05500 0.06125 0.06750 0.07375 0.08000 0.08625 0.09250 0.09875 0.1050 0.1113 0.1175 0.1238 0.1300-100 -100-150 -200-150 9.4% 4.7% -200-200 -150-100 -50 0 50 100 150 200 X Axis -Center 50W, Side 125W Y Axis 200 150 100 50 0-50 -100-150 -200-200 -150-100 -50 0 50 100 150 200 8.3% 0.03000 0.03625 0.04250 0.04875 0.05500 0.06125 0.06750 0.07375 0.08000 0.08625 0.09250 0.09875 0.1050 0.1113 0.1175 0.1238 0.1300 X Axis -Center 125W, Side 125W -200-150 -100-50 0 50 100 150 200 X Axis -Center 200W, Side 125W -19

Dual Power Phase Shift Control 54 MHz Power Phase Shift of Upper electrode & Lower electrode Bera K, Rauf S and Collins K 2008 IEEE Trans. Plasma Sci. 36 1366-20

Uniformity control by Phase Modulation - Solar cell K. Kawamura, et al. Thin Solid Films 506-507 (2006) 22-26 -21

Multiple ICP and Helicon Source -Flat Panel Display High Density 1E12 cm -3 Auto-balancing equal power absorption Low Pressure Operation about 10 mtorr Permanent Magnet Helicon tubes -22

Pulse modulation -Subramonium, APL 85, 2004 -Shimatani, et. al., Vacuum 66, 2002 -Samukawa, et. al., JVST A 14, 199 -These results suggest the possibilities of improvement of plasma uniformity by pulsing the discharge. -But, there s insufficient information regarding this improvement especially in electronegative plasmas.

Pulse Modulation of ICP Source Power: Spatial profiles Electron density (cm -3 ) -Temporal evolution at various positions 7E10 6E10 5E10 4E10 3E10 2E10 1E10 13.56MHz 900W 10kHzPulse mode Duty cycle 50% Ar 20mTorr 000mm 050mm 100mm 150mm 200mm 250mm 300mm 350mm Electron density (cm -3 ) -Spatial profiles at various moments 7E10 6E10 5E10 4E10 3E10 2E10 1E10 11usec 31usec 49usec 71usec 91usec 0 25 50 75 100 time (usec) -Pulse Off -Pulse On 0 50 100 150 200 250 300 350 Distance from chamber wall (mm) Uniformity (within 150~350mm area) 11 usec 11.8 % N e profile could be get better during RF off period. 31 usec 7.7 % 49 usec 5.5 % 71 usec 7.3 % 91 usec 6.6 % Max Min Uniformity(%) 100 2 Mean

Pulse modulation -Rising -Decay Electron density [10 10 /cm 3 ] 8 7 6 5 4 3 2 1 0-1 0 5 10 15 20 25 Position [cm] 0 ms 0.1 ms 0.2 ms 0.3 ms 0.4 ms 0.5 ms 1.0 ms 1.5 ms 2.0 ms Electron density [10 10 /cm 3 ] 7 6 5 4 3 2 1 0 0 5 10 15 20 25 Position [cm] 2.1 ms 2.2 ms 2.3 ms 2.4 ms 2.5 ms 2.6 ms 2.7 ms 2.8 ms 2.9 ms 3.0 ms 3.5 ms 4.0 ms 4.5 ms 5.0 ms 5.5 ms 6.0 ms

Parallel Stray Capacitance - CCP Plasma Source Atmosphere -Large Gap for small stray C Matching Box Power electrode Blocking Capacitor Ground electrode Strong electric field in Plasma Generation Region Plasma Generation Region Stray C This is needed to be minimized -26

Parallel Stray Capacitance Ground electrode Wafer Matching Box Plasma Blocking Capacitor generation region -Strong E-field Power electrode Plasma Generation Region Stray C This is needed to be minimized Stray Capacitance The use of Thick Dielectric for small parallel Stray Capacitance -27

Series Stray Capacitance floating Plasma generation Matching Box Blocking Capacitor ground Plasma Generation Region Stray Capacitance Small stray capacitance High voltage is applied Arcing, undesirable discharge -28

Effect of Chamber Wall Asymmetry in CCP Plasma Source Electromagnetic wave Surface current Slot valve distorts field uniformity as well as wall surface current flow Ground strap Slot valve Glass in/out -29

CCP source s heating Electron heating in the CCP The electron heating in the rf discharges corresponds to how the electrons gain their energy from the rf electric field energy. e E e e n e-n collsion nonlinear sheath interaction Ohmic Heating Stochastic Heating V. A. Godyak Electron in RF electric field collective interaction with sheath Pressure Heating M. M. Turner

EEDF of the CCP CCP source s heating Origion of Bi Maxwellian Electron Distribution Ramsauer gas effect Stochastic heating - Godyak. Pressure heating e - Turner. e Electron non-local property L - Tsendin, Kaganovich. sheath e e sheath

CCP source s heating Plasma parameter control with Discharge gap length The bi-maxwellian EEPF approaches to the Maxwellian EEPF, because of enhancement of electric field in the bulk with decreasing gap length. ( Gap length Ne & Te ) 60 mtorr, 1A, Gap-changed J E rf ne rf E rf J / n d 1/ 2 [ S. J. You. et.al, Appl. Phys. Letts, 2005] d 1/ 2 2 E rf 2 D low e T e

CCP source s heating Plasma parameter control with Driving frequency Driving frequency Druyvestein to bi-maxwellian distribution (at constant Voltage conditions) Ne & Te Driving frequency bi-maxwellian to Druyvesteyn distribution (at constant Current conditions) 200 mtorr, 1A (fixed current) [ Abdel.et.al, Appl. Phys. Letts, 2003 ] [ S. J. You. et.al, Appl. Phys. Letts, 2006]

CCP source s heating Effect of driving frequency on the plasma parameters Effect of the driving frequency at constant discharge power Driving frequency Te & Ne ~ constant or decrease ( at constant power conditions) Enhancement of collisional heating in the bulk plasma with the driving frequency. [ S. K. Ahn. et.al, Appl. Phys. Letts, 2006]

CCP source control knob discharge gap length driving frequency gas pressure discharge power

-ICP sources -Electron temperature dependence on RF frequency

-ICP sources - Problems in high frequency -E-M field structure (f=13.56 MHz) -Antenna current vector -Electric field amplitude

-ICP sources - Problems in high frequency -E-M field structure (f=60.00 MHz) -Antenna current vector -Electric field amplitude

-ICP sources - C.D.A -Structure of PCB Capacitor Distributed Antenna (40 MHz) -Diameter = 450 mm -Capacitor -Antenna ground -Top plate -Base plate -Cooling fan -Power port -PCB - C.D.A model LX10 base plate -Convection cooling system -Capacitance uncertainty is -Antenna operation 40 MHz 1.6 KW RF -100 pf ± 1 pf for 12 capacitors -Antenna operation 40 MHz 1.6 KW RF

1 Antenna coil properties Asymmetry Factors Asymmetry Factors and Uniformity Coil plasma capacitive coupling coil termination impedance azimuth coil current asymmetric E-field Coil E-field asymmetry (1-turn coil 5-turn coil symmetric) Asymmetric E-field ion flux uniformity 2 Reactor Structure Wafer clamp support load-lock bay asymmetry (check) 3 Gas Pumping One-side pumping asymmetric pumping asymmetry Neutral species (ion flux ambipolar transport) 4 Gas Injection Nozzle neutral asymmetry (showerhead radial jet) 5 Wall Reactive Wall reactivity radical wall loss uniformity (gas species) 6 Gas Pressure n D 2 a n iz n t Da 1/2 ~( ) ~5cm iz Neutral species pressure dependence Ion flux pressure Da Da n~ n iz ~ Da Scale length 5cm T 3~5eV P 5mTorr e

Summary 1. Standing wave effect and skin effect should be considered in large area plasma source 2. RF phase shift or modulation control uniformity. 3. Multi-feeding is also a method for uniformity control. 4. Multi-plasma sources(multiple helicon or multi-electrode CCP) can be used to increase plasma uniformity. 5. Stray capacitance should be considered in plasma system design. 6. Asymmetry factors be taken into account plasma uniformity. -41