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 ssongs@umich.edu b) Department of Electrical Engineering and Computer Science University of Michigan, Ann Arbor, MI 48109, USA mjkush@umich.edu http://uigelz.eecs.umich.edu 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 100 415 kHz CW 100 415 CW 100 415 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 10 40 mTorr 10 40 10 40  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