SiO 2 ETCH RATE AND PROFILE CONTROL USING PULSE POWER IN 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 September 21 st, 2011 * Work supported by DOE Plasma Science Center and Semiconductor Research Corp.

AGENDA  Motivation for controlling f(  )  Description of the model  Typical Ar/CF 4 /O 2 pulsed plasma properties  Etch property with different PRF  Constant Power with DC Bias  Constant Voltage with DC Bias  Without DC Bias  Concluding remarks University of Michigan Institute for Plasma Science & Engr. SHS_MJK_ISPC

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 + CF 4 CF 3 + F + e k SHS_MJK_ISPC

ETCH RATE vs. FLUX RATIOS University of Michigan Institute for Plasma Science & Engr. Ref: D. C. Gray, J. Butterbaugh, and H. H. Sawin, J. Vac. Sci. Technol. A 9, 779 (1991) Flux Ratio (F/Ar + )Flux Ratio (CF 2 /Ar + ) Etching Yield (Si/Ar + )  Large fluorine to ion flux ratio enhance etching yield of Si.  Large fluorocarbon to ion flux ratio reduce etching yield of Si. SHS_MJK_ISPC

Ref: K. Ono, M. Tuda, H. Ootera, and T. Oomori, Pure and Appl. Chem. Vol 66 No 6, 1327 (1994)  Large chlorine radical to ion flux ratio makes undercut in etch profile due to too much chemical reactions.  Etch profile result in ECR Cl 2 plasma after 200% over etch with different flux ratios p-Si University of Michigan Institute for Plasma Science & Engr. ETCH PROFILE vs. FLUX RATIOS  Flux Ratio (Cl / Ion) = 0.3  Flux Ratio (Cl / Ion) = 0.8 SHS_MJK_ISPC

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 Fluid Kinetics Module Fluid equations (continuity, momentum, energy) Poisson’s equation T e, S b, S eb, k Electron Monte Carlo Simulation University of Michigan Institute for Plasma Science & Engr. SHS_MJK_ISPC

MONTE CARLO FEATURE PROFILE MODEL (MCFPM)  The MCFPM resolves the surface topology on a 2D Cartesian mesh.  Each cell has a material identity. Gas phase species are represented by Monte Carlo pseuodoparticles.  Pseuodoparticles are launched with energies and angles sampled from the distributions obtained from the HPEM  Cells identities changed, removed, added for reactions, etching deposition. PCMCM Energy and angular distributions for ions and neutrals MCFPM Etch rates and profile University of Michigan Institute for Plasma Science & Engr.  Poisson’s equation solved for charging HPEM SHS_MJK_ISPC

REACTOR GEOMETRY: 2 FREQUENCY CCP  2D, cylindrically symmetric  Ar/CF 4 /O 2 = 75/20/5, 40 mTorr, 200 sccm  Base conditions  Lower electrode: LF = 10 MHz, 500 W, CW  Upper electrode: HF = 40 MHz, 500 W, Pulsed University of Michigan Institute for Plasma Science & Engr. SHS_MJK_ISPC

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 ON period – ionization only needs to equal electron losses averaged over the pulse period.  Pulse power for high frequency.  Duty-cycle = 25%, PRF = 50, 100, 200, 415, 625 kHz  Average Power = 500 W SHS_MJK_ISPC

Typical Plasma Properties SHS_MJK_ISPC

PULSED CCP: n e, T e, f(  ) 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(  ).  [e] TeTe MIN MAX f(  ) ANIMATION SLIDE-GIF  40 mTorr, Ar/CF 4 /O 2 =75/20/5  LF = 10 MHz, 500 W  HF = 40 MHz, pulsed 500 W  PRF = 100 kHz, Duty-cycle = 25% SHS_MJK_ISPC

ELECTRON DENSITY  CW University of Michigan Institute for Plasma Science & Engr.  Duty = 50%  Duty = 25% MIN MAX ANIMATION SLIDE-GIF  At 50% duty, the electron density is not significantly modulated by pulsing, so the plasma is quasi-CW.  At 25% duty, modulation in [e] occurs due to electron losses during the longer inter-pulse period.  The lower duty cycle is more likely to reach higher value of electron density.  40 mTorr, Ar/CF 4 /O 2 =75/20/5  LF = 10 MHz, 500 W  HF = 40 MHz, 500 W (CW or pulse) SHS_MJK_ISPC

ELECTRON SOURCES BY BULK ELECTRONS University of Michigan Institute for Plasma Science & Engr.  CW  Duty = 50%  Duty = 25%  The electrons have two groups: bulk low energy electrons and beam-like secondary electrons.  The bulk electron source is negative due to electron attachment and dissociative recombination.  At the start of the pulse-on cycle, is there a impulsive positive electron source due to the overshoot of E/N. MIN MAX  40 mTorr, Ar/CF 4 /O 2 =75/20/5  LF 500 W, HF 500 W ANIMATION SLIDE-GIF SHS_MJK_ISPC

ELECTRON SOURCES BY BEAM ELECTRONS University of Michigan Institute for Plasma Science & Engr.  CW  Duty = 50%  Duty = 25% MIN MAX  40 mTorr, Ar/CF 4 /O 2 =75/20/5  LF = 10 MHz, 500 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_ISPC

Etch Properties SHS_MJK_ISPC

F / POLY FLUX RATIO: CONSTANT POWER · F to polymerizing flux ratio is largest at 200 kHz of PRF.  40 mTorr, Ar/CF 4 /O 2 =75/20/5, 200 sccm  LF 10 MHz 500 W, Pulsed HF 40 MHz 500 W University of Michigan Institute for Plasma Science & Engr.

ETCH PROFILE IN SiO 2 & IEAD: CONST. POWER University of Michigan Institute for Plasma Science & Engr.  40 mTorr, Ar/CF 4 /O 2 =75/20/5, 200 sccm  LF 10 MHz 500 W, Pulsed HF 40 MHz 500 W Angle (degree) Energy (eV) · Etch rate is fastest at 200 kHz PRF with larger ion energy and F to polymerizing flux ratio.  Cycle Average IEAD Height (  m) Width (  m) 100 kHz CW CW ANIMATION SLIDE-GIF  Etch Profile (300 sec) CD 70 nm -64 V -92 V -107 V-134 VBias: SHS_MJK_ISPC

F / POLY FLUX RATIO: CONSTANT VOLTAGE · F to polymerizing flux ratio is controlled not only by PRF, but also by DC bias. · DC bias is manipulated by the blocking capacitor on the substrate.  Without DC Bias  With DC Bias University of Michigan Institute for Plasma Science & Engr.  40 mTorr, Ar/CF 4 /O 2 =75/20/5, 200 sccm  LF 10 MHz 250 V, Pulsed HF 40 MHz 500 W

ETCH PROFILE IN SiO 2 & IEAD: CONST. VOLTAGE · Etch rate is fastest at 415 kHz having larger fluorine flux.  Cycle Average IEAD 100 kHz CW CW  Etch Profile (300 sec) CD 70 nm -88 V -103 V -116 V-129 VBias: Angle (degree) Width (  m) ANIMATION SLIDE-GIF University of Michigan Institute for Plasma Science & Engr.  40 mTorr, Ar/CF 4 /O 2 =75/20/5, 200 sccm  LF 10 MHz 250 V, Pulsed HF 40 MHz 500 W

ETCH PROFILE IN SiO 2 & IEAD: NO BIAS · Etch rate is fastest at CW excitation due to continuously delivered power.  Cycle Average IEAD 100 kHz CW CW  Etch Profile (300 sec) CD 70 nm  40 mTorr, Ar/CF 4 /O 2 =75/20/5, 200 sccm  LF 10 MHz 250 V, Pulsed HF 40 MHz 500 W University of Michigan Institute for Plasma Science & Engr.

POWER NORMALIZED ETCH RATE · Power normalized etch rate is dependant on the pulse repetition frequency and DC bias of the substrate. University of Michigan Institute for Plasma Science & Engr.  40 mTorr, Ar/CF 4 /O 2 =75/20/5, 200 sccm  LF 10 MHz, Pulsed HF 40 MHz, Duty 25%

CONCLUDING REMARKS  Extension of tail of f(  ) beyond that obtained with CW excitation produces a different mix of fluxes to substrate.  Ratios of fluxes and IEADs are tunable using pulsed excitation.  Ratios of fluxes are IEADs are tunable using blocking capacitor.  Consequently, etch rate can be controlled by pulsed power with different blocking capacitors.  With constant power operation, fastest etch rate is achieved at 200 kHz having larger F to polymerizing flux ratio.  With constant voltage operation, fastest etch rate is achieved at 415 kHz having larger fluorine flux.  Without DC bias, the etch rate decrease as pulse repetition frequency decreases. University of Michigan Institute for Plasma Science & Engr. SHS_MJK_ISPC