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CONTROL OF ELECTRON ENERGY DISTRIBUTIONS AND FLUX RATIOS IN PULSED CAPACITIVELY COUPLED PLASMAS* Sang-Heon Song a) and Mark J. Kushner b) a) Department.

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Presentation on theme: "CONTROL OF ELECTRON ENERGY DISTRIBUTIONS AND FLUX RATIOS IN PULSED CAPACITIVELY COUPLED PLASMAS* Sang-Heon Song a) and Mark J. Kushner b) a) Department."— Presentation transcript:

1 CONTROL OF ELECTRON ENERGY DISTRIBUTIONS AND FLUX RATIOS IN PULSED CAPACITIVELY COUPLED PLASMAS* Sang-Heon Song a) and Mark J. Kushner b) a) Department of Nuclear Engineering and Radiological Sciences University of Michigan, Ann Arbor, MI 48109, USA b) Department of Electrical Engineering and Computer Science University of Michigan, Ann Arbor, MI 48109, USA Oct 2010 AVS * Work supported by DOE Plasma Science Center and Semiconductor Research Corp.

2 AGENDA  Motivation for controlling f(  )  Description of the model  Typical Ar pulsed plasma properties  Typical CF 4 /O 2 pulsed plasma properties  f(  ) and flux ratios with different  PRF  Duty Cycle  Pressure  Concluding Remarks University of Michigan Institute for Plasma Science & Engr. SHS_MJK_AVS2010_02

3 CONTROL OF ELECTRON KINETICS- f(  )  Controlling the generation of reactive species for technological devices benefits from customizing the electron energy (velocity) distribution function. University of Michigan Institute for Plasma Science & Engr. e + SiH 4 SiH 3 + H + e k  LCD  Solar Cell  Need SiH 3 radicals* * Ref: Tatsuya Ohira, Phys. Rev. B 52 (1995) SHS_MJK_AVS2010_03

4 HYBRID PLASMA EQUIPMENT MODEL (HPEM)  Fluid Kinetics Module:  Heavy particle and electron continuity, momentum, energy  Poisson’s equation  Electron Monte Carlo Simulation:  Includes secondary electron transport  Captures anomalous electron heating  Includes electron-electron collisions E, N i, n e, T i Fluid Kinetics Module Fluid equations (continuity, momentum, energy) Poisson’s equation T e, S, k Electron Monte Carlo Simulation University of Michigan Institute for Plasma Science & Engr. SHS_MJK_AVS2010_04

5 REACTOR GEOMETRY  2D, cylindrically symmetric  Ar, CF 4 /O 2, 10 – 40 mTorr, 200 sccm  Base conditions  Lower electrode: LF = 10 MHz, 300 W, CW  Upper electrode: HF = 40 MHz, 500 W, Pulsed University of Michigan Institute for Plasma Science & Engr. SHS_MJK_AVS2010_05

6 PULSE POWER Time  = 1/PRF Duty Cycle Power(t) P min P max University of Michigan Institute for Plasma Science & Engr.  Use of pulse power provides a means for controlling f(  ).  Pulsing enables ionization to exceed electron losses during a portion of the period – ionization only needs to equal electron losses averaged over the pulse period.  Pulse power for high frequency.  Duty-cycle = 25%, PRF = 100 kHz, 415 kHz  Average Power = 500 W SHS_MJK_AVS2010_06

7 Ar SHS_MJK_AVS2010_07

8 PULSED CCP: Ar, 40 mTorr University of Michigan Institute for Plasma Science & Engr.  Pulsing with a PRF and moderate duty cycle produces nominal intra-cycles changes [e] but does modulate f(  ).  LF = 10 MHz, 300 W  HF = 40 MHz, pulsed 500 W  PRF = 100 kHz, Duty-cycle = 25%  [e]  Te SHS_MJK_AVS2010_08 MIN MAX f(  ) V HF 226 V V LF 106 V ANIMATION SLIDE-GIF

9 PULSED CCP: Ar, DUTY CYCLE  Excursions of tail are more extreme with lower duty cycle – more likely to reach high thresholds. University of Michigan Institute for Plasma Science & Engr.  Duty cycle = 25%  Cycle Average  Duty cycle = 50% SHS_MJK_AVS2010_09  LF 10 MHz, pulsed HF 40 MHz  PRF = 100 kHz, Ar 40 mTorr V HF 128 V V LF 67 V V HF 226 V V LF 106 V ANIMATION SLIDE-GIF

10 PULSED CCP: Ar, PRESSURE  Pulsed systems are more sensitive to pressure due to differences in the rates of thermalization in the afterglow. University of Michigan Institute for Plasma Science & Engr.  10 mTorr  Cycle Average  40 mTorr SHS_MJK_AVS2010_10  LF 10 MHz, pulsed HF 40 MHz  PRF = 100 kHz V HF 226 V V LF 106 V V HF 274 V V LF 146 V ANIMATION SLIDE-GIF

11 CF 4 /O 2 SHS_MJK_AVS2010_11

12 ELECTRON DENSITY  CW  At 415 kHz, the electron density is not significantly modulated by pulsing, so the plasma is quasi-CW.  At 100 kHz, modulation in [e] occurs due to electron losses during the longer inter-pulse period.  The lower PRF is less uniform due to larger bulk electron losses during longer pulse-off cycle. University of Michigan Institute for Plasma Science & Engr.  PRF=415 kHz  PRF=100 kHz MIN MAX ANIMATION SLIDE-GIF  40 mTorr, CF 4 /O 2 =80/20, 200 sccm  LF = 10 MHz, 300 W  HF = 40 MHz, 500 W (CW or pulse) SHS_MJK_AVS2010_12

13 ELECTRON SOURCES BY BULK ELECTRONS University of Michigan Institute for Plasma Science & Engr.  CW  PRF=415 kHz  PRF=100 kHz  The electrons have two groups: bulk low energy electrons and beam-like secondary electrons.  The electron source by bulk electron is negative due to electron attachment and dissociative recombination.  Only at the start of the pulse- on cycle, is there a positive electron source due to the overshoot of E/N. MIN MAX  40 mTorr, CF 4 /O 2 =80/20, 200 sccm  LF 300 W, HF 500 W ANIMATION SLIDE-GIF SHS_MJK_AVS2010_13

14 ELECTRON SOURCES BY BEAM ELECTRONS University of Michigan Institute for Plasma Science & Engr.  CW  PRF=415 kHz  PRF=100 kHz MIN MAX  40 mTorr, CF 4 /O 2 =80/20, 200 sccm  LF = 10 MHz, 300 W  HF = 40 MHz, 500 W (CW or pulse)  The beam electrons result from secondary emission from electrodes and acceleration in sheaths.  The electron source by beam electron is always positive.  The electron source by beam electrons compensates the electron losses and sustains the plasma. ANIMATION SLIDE-GIF SHS_MJK_AVS2010_14

15 TYPICAL f(  ): CF 4 /O 2 vs. Ar  Less Maxwellian f(  ) with CF 4 /O 2 due to lower e-e collisions.  Enhanced sheath heating with CF 4 /O 2 due to lower plasma density.  Tail of f(  ) comes up to compensate for the attachment and recombination that occurs at lower energy.  CF 4 /O 2  Ar University of Michigan Institute for Plasma Science & Engr. SHS_MJK_AVS2010_15  40 mTorr, 200 sccm  LF = 10 MHz, 300 W  HF = 40 MHz, 500 W (25% dc) V HF 226 V V LF 106 V V HF 203 V V LF 168 V ANIMATION SLIDE-GIF

16  In etching of dielectrics in fluorocarbon gas mixtures, the polymer layer thickness depends on ratio of fluxes.  Ions – Activation of dielectric etch, sputtering of polymer  CF x radicals – Formation of polymer  O – Etching of polymer  F – Diffusion through polymer, etch of dielectric and polymer  Investigate flux ratios with varying  PRF  Duty cycle  Pressure RATIO OF FLUXES: CF 4 /O 2 University of Michigan Institute for Plasma Science & Engr.  Flux Ratios:  Poly = (CF 3 +CF 2 +CF+C) / Ions  O = O / Ions  F = F / Ions SHS_MJK_AVS2010_16

17 f(  ): CF 4 /O 2, PRF  Average  The time averaged f(  ) for pulsing is similar to CW excitation.  Extension of tail of f(  ) beyond CW excitation during pulsing produces different excitation and ionization rates, and different mix of fluxes to wafer. University of Michigan Institute for Plasma Science & Engr.  PRF = 100 kHz  40 mTorr, CF 4 /O 2 =80/20, 200 sccm  LF = 10 MHz, 300 W  HF = 40 MHz, 500 W (25% dc) ANIMATION SLIDE-GIF SHS_MJK_AVS2010_17 V HF 203 V V LF 168 V

18 CW kHz CW CW University of Michigan Institute for Plasma Science & Engr. RATIO OF FLUXES: CF 4 /O 2, PRF  Ratios of fluxes are tunable using pulsed excitation.  Polymer layer thickness may be reduced by pulsed excitation because poly to ion flux ratio decreases. F O Poly  40 mTorr, CF 4 /O 2 =80/20, 200 sccm, Duty-cycle = 25%  LF = 10 MHz, 300 W  HF = 40 MHz, 500 W SHS_MJK_AVS2010_18

19 f(  ): CF 4 /O 2, DUTY CYCLE  Control of average f(  ) over with changes in duty cycle is limited if keep power constant. ANIMATION SLIDE-GIF University of Michigan Institute for Plasma Science & Engr.  40 mTorr, CF 4 /O 2 =80/20, 200 sccm  LF 10 MHz, Pulsed HF 40 MHz, PRF = 100 kHz  Duty cycle = 25%  Cycle Average  Duty cycle = 50% SHS_MJK_AVS2010_19 V HF 191 V V LF 168 V V HF 203 V V LF 168 V

20 RATIO OF FLUXES: CF 4 /O 2, DUTY CYCLE  Flux ratio control is limited if keep power constant.  With smaller duty cycle, polymer flux ratio is more reduced compared to the others. University of Michigan Institute for Plasma Science & Engr. F O Poly 50% 25% 50% 25% 50% 25% SHS_MJK_AVS2010_20  LF 10 MHz, Pulsed HF 40 MHz, PRF = 100 kHz  40 mTorr, CF 4 /O 2 =80/20, 200 sccm CW

21 f(  ): CF 4 /O 2, PRESSURE  Pulsed systems are sensitive to pressure due to differences in the rates of thermalization in the afterglow. ANIMATION SLIDE-GIF University of Michigan Institute for Plasma Science & Engr.  CF 4 /O 2 =80/20, 200 sccm, PRF = 100 kHz  LF 10 MHz, Pulsed HF 40 MHz  10 mTorr  Cycle Average  40 mTorr SHS_MJK_AVS2010_21 V HF 191 V V LF 168 V V HF 233 V V LF 188 V

22 RATIO OF FLUXES: CF 4 /O 2, PRESSURE  Flux ratios decrease as pressure decreases.  Polymer layer thickness may be reduced with lower pressure in the pulsed CCP. University of Michigan Institute for Plasma Science & Engr. F O Poly mTorr  CF 4 /O 2 =80/20, 200 sccm  LF = 10 MHz, 300 W  HF = 40 MHz, 500 W SHS_MJK_AVS2010_22 CW P P P P P  PRF = 100 kHz, Duty-cycle = 25% P P: Pulsed excitation CW: CW excitation

23 CONCLUDING REMARKS  Extension of tail of f(  ) beyond CW excitation produces different mix of fluxes.  Ratios of fluxes are tunable using pulsed excitation.  Different PRF provide different flux ratios due to different relaxation time during pulse-off cycle.  Duty cycle is another knob to control f(  ) and flux ratios, but it is limited if keep power constant  Pressure provide another freedom for customizing f(  ) and flux ratios in pulsed CCPs. SHS_MJK_AVS2010_23


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