Transcript PPT
CONTROLLING ION AND UV/VUV PHOTON FLUXES IN PULSED LOW PRESSURE PLASMAS FOR MATERIALS PROCESSING* Peng Tian and Mark J. Kushner University of Michigan, Ann Arbor, MI 48109 USA [email protected], [email protected] 65th Gaseous Electronics Conference 2012, Austin, Texas, USA * Work supported by Semiconductor Research Corporation, DOE Office of Fusion Energy Science and National Science Foundation GEC 2012 P.T. AGENDA UV/VUV photons during plasma material processing Description of model Pulsed Plasma Photon/Ion flux ratios onto wafer surface in ICP Duty cycle Aspect ratio of the reactor Gas pressure Photon/Ion flux ratios onto wafer surface in CCP Concluding Remarks GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. PHOTONS IN PLASMA PROCESSING UV/VUV photon fluxes are ubiquitous in low pressure plasma material processing. The consequences of UV/VUV fluxes and the possible synergies with other reactive species are just now becoming apparent. Controlling UV/VUV fluxes, or their relative values compared to other reaction fluxes, may be important in controlling these synergistic interactions. In this talk, we discuss methods to control relative fluxes of UV/VUV photons using pulsed plasmas. GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. SYNERGISTIC EFFECTS ON PR ROUGHNESS The roughening of photo-resist (PR) is important to maintaining critical dimensions. PR roughening is weakly dependent on substrate temperature with only ion bombardment. When adding VUV fluxes of 1-10% of the ion fluxes, roughening was quite sensitive to temperature. D. Nest, D. B. Graves, S. Engelmann, R. L. Bruce, F. Weilnboech et al. Appl. Phys. Lett. 92, 153113 (2008) GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. PHOTON PROMOTED ETCHING Recent observations of VUV sustained etching in Cl plasmas below accepted ion energy threshold. Need to control ion and photon fluxes – perhaps independently. University of Michigan Institute for Plasma Science & Engr. PULSED PLASMAS Carrier frequency is modulated by a pulse, with controllable duty cycle, pulse repetition frequency and fixed average power over a period. The fast rising edge can “over-shoot” the self sustaining E/N, raising the “hot tail” in EEDF f(). Controllable rate coefficients compared to CW excitation 1/ 2 2 kCW f CW , t d 0 0 me 1 2 f , t dt 0 Pulsed m d k ave pulsed e Pmax Power(t) 1/ 2 Duty Cycle Pave Pt dt 1 0 Pmin = 1/ GEC 2012 P.T. Time University of Michigan Institute for Plasma Science & Engr. HYBRID PLASMA EQUIPMENT MODEL r E r , , Br , z r , je r , k r , Te r S r Er , z r , N i r , ne r , Ti r Anatural Atrapped Surface Chemistry Module The Hybrid Plasma Equipment Model (HPEM) is a modular simulator that combines fluid and kinetic approaches. Radiation transport is addressed using a spectrally resolved Monte Carlo simulation. GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. RADIATION TRANSPORT MODEL IN HPEM Frequency resolved radiation transport in HPEM is modeled using a Monte Carlo simulation that accounts for radiation trapping GEC 2012 P.T. ATOMIC MODELS FOR Ar/Cl2 Threshold energy for excitation and ionization has a notable difference. With customized EEDF f(), separate control over ion and photon fluxes is possible. e + Ar(3s) e + Ar(1s2,3,4,5) (11.55~11.83 eV) e + Ar(3s) e + e + Ar+ (16.0 eV) e + Cl(3s23p5) e + Cl(4s,4p,3d) e + Cl(3s23p5) e + e + Cl+ GEC 2012 P.T. (8.90~10.90 eV) (12.99 eV) University of Michigan Institute for Plasma Science & Engr. ICP ICP-f(), Te, PHOTON FLUX– TIME VARIATION The tail of EEDF f() has been raised during pulse-on period. f() Te VUV Photon = cycle average Ar/Cl2 = 80/20, 20 mTorr, 150 Wave, 50 kHz, duty cycle = 15% MIN GEC 2012 P.T. MAX Animation Slide University of Michigan Institute for Plasma Science & Engr. ICP-AVERAGED DENSITIES AND FLUXES Excited state density and photon flux decay faster than ions. Even with radiation trapping, excited states still have a shorter lifetime than ions. Neutral and Ion Density Total Ion and Photon Flux (Smoothed data) Ar/Cl2 = 80/20, 20 mTorr, 200 sccm, 10 MHz, 150 W, 15% DC, 50 kHz PRF GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. ICP-DUTY CYCLE ICP-DUTY CYCLE – CUSTOMIZED f() Animation Slide = cycle average f() Te With smaller duty cycle, a fairly hotter tail of EEDF can be generated. Ar/Cl2 = 80/20, 20 mTorr, 150 Wave, 50 kHz, 15%, 35%, 55% DC GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. ICP-ION & PHOTON FLUX vs DUTY CYCLE Longer lifetime of ions and lowered ambipolar diffusion during after glow enables ion flux to stay at a relatively constant value. With lower “over-shoot” E/N in larger duty cycle pulses, maximum photon flux decreases with increasing duty cycle. The flux decays fast due to shorter lifetime of excited states. Ar/Cl2 = 80/20, 20 mTorr, 150 Wave, 50 kHz GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. ICP-PHOTON/ION FLUX vs DUTY CYCLE Varying duty cycle provides control over coincidence of photon and ion fluxes, and cycle averaged fluxes to substrate. 10 MHz, 150 W, Ar/Cl2 = 80/20, 20 mTorr, 200 sccm, 50 kHz PRF GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. ICP-ASPECT RATIO OF REACTOR ICP-ASPECT RATIO – TRANSPORT OF IONS vs PHOTONS Although moderately trapped, the transport of Ar resonance radiation is significantly less diffusive than ions. Animation Slide Since sources are near the coils, aspect ratio can be used to discriminate fluxes. Ar/Cl2 = 80/20, 20 mTorr, 150 W, 50 kHz, duty cycle = 15% GEC 2012 P.T. MIN MAX University of Michigan Institute for Plasma Science & Engr. ICP-ION & PHOTON FLUX vs ASPECT RATIO The ion fluxes are more sensitive to aspect ratio. The more diffuse nature cause it decrease with greater height. Photon fluxes traverse the reactor in a more “ballistic” way and are less sensitive to height. Ar/Cl2 = 80/20, 20 mTorr, 150 Wave, 50 kHz, 15% DC GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. ICP-PHOTON/ION FLUX vs ASPECT RATIO Photon flux ratio can be controlled by varying aspect ratio (height). The ratio is more sensitive in pulsed plasmas. Ar/Cl2 = 80/20, 20 mTorr, 150 Wave, 50 kHz, 15% DC GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. ICP-PRESSURE ICP-PRESSURE – f() AND TRANSPORT OF IONS = cycle average Te f() Keeping power constant, higher pressure will decrease E/N value, thus a “cooler” EEDF f(). Ar/Cl2 = 80/20, 20 mTorr, 150 Wave, 50 kHz, 15%, 35%, 55% DC Animation Slide GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. ICP-ION & PHOTON FLUX vs PRESSURE Lower rate of diffusion combined with more volumetric losses due to ion-ion and e-ion recombination produces a decrease in flux. A “cooler” EEDF f() leads to a decrease in photon flux. Ar/Cl2 = 80/20, 150 Wave, 50 kHz, 15% DC GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. ICP-PHOTON/ION FLUX vs PRESSURE Varying pressure provides another means of controlling the ratio of photon to ion flux. Ar/Cl2 = 80/20, 150 Wave, 50 kHz, 15% DC GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. CCP CCP-f(), Te, PHOTON FLUX– TIME VARIATION f() 1.6 cm Te VUV Photon Ar/Cl2 = 80/20, 500 Wave, 100 kHz, 15% DC GEC 2012 P.T. Stochastic heating provide higher Te near wafer (sheath). University of Michigan Institute for Plasma Science & Engr. CCP-PHOTON/ION FLUX vs DUTY CYCLE Ion Flux Photon Flux During after-glow, with thin sheath and low plasma potential, photon flux is higher than ion flux. Ar/Cl2 = 80/20, 500 Wave, 100 kHz, 15% DC Ratio GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. CCP-PHOTON/ION FLUX vs DUTY CYCLE Similar to ICP, the flux ratio increases as duty cycle decreases. However, the peak shifts with the falling edge of pulse. Ar/Cl2 = 80/20, 500 Wave, 100kHz, Varying DC GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. CCP-PHOTON/ION FLUX vs FREQUENCY Flux ratio can also be controlled by RF frequency. However, the difference is not large. Ar/Cl2 = 80/20, 500 Wave, 100kHz, Varying RF GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS A possible method to separately control photon/ion fluxes by pulsed RF power in inductively coupled plasmas and capacitively coupled plasmas has been investigated by computational approach. Photon/ion fluxes ratio to wafer surface in RIE reactors can be controlled as they have different reaction rate to pulsed power deposition. In pulsed ICPs, photon/ion fluxes ratio can be controlled by varying duty cycle, gas pressure and aspect ratio of the reactor. In pulsed CCPs, photon/ion fluxes ratio can be controlled by varying duty cycle and RF frequency. GEC 2012 P.T. University of Michigan Institute for Plasma Science & Engr.