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SiO2 ETCH RATE AND PROFILE CONTROL USING PULSE POWER IN CAPACITIVELY COUPLED PLASMAS* Sang-Heon Songa) and Mark J. Kushnerb) a)Department of Nuclear Engineering and Radiological Sciences University of Michigan, Ann Arbor, MI 48109, USA [email protected] b)Department of Electrical Engineering and Computer Science University of Michigan, Ann Arbor, MI 48109, USA [email protected] http://uigelz.eecs.umich.edu September 21st, 2011 * Work supported by DOE Plasma Science Center and Semiconductor Research Corp. AGENDA Motivation for controlling f(e) Description of the model Typical Ar/CF4/O2 pulsed plasma properties Etch property with different PRF Constant Power with DC Bias Constant Voltage with DC Bias Without DC Bias Concluding remarks SHS_MJK_ISPC University of Michigan Institute for Plasma Science & Engr. CONTROL OF ELECTRON KINETICS – f(e) Controlling the generation of reactive species for technological devices benefits from customizing the electron energy (velocity) distribution function. k CF3 + F + e e + CF4 dN k r , t dt nekij r , t N j i, j 12 2e kij r , t f e , r , t e d e 0 me df v , r , t dt SHS_MJK_ISPC v x f r , v qE r , t me f v , r , t v f v , r , t t c University of Michigan Institute for Plasma Science & Engr. ETCH RATE vs. FLUX RATIOS Large fluorine to ion flux ratio enhance etching yield of Si. Etching Yield (Si/Ar+) Etching Yield (Si/Ar+) Large fluorocarbon to ion flux ratio reduce etching yield of Si. Flux Ratio (F/Ar+) Flux Ratio (CF2/Ar+) Ref: D. C. Gray, J. Butterbaugh, and H. H. Sawin, J. Vac. Sci. Technol. A 9, 779 (1991) SHS_MJK_ISPC University of Michigan Institute for Plasma Science & Engr. ETCH PROFILE vs. FLUX RATIOS Large chlorine radical to ion flux ratio makes undercut in etch profile due to too much chemical reactions. Etch profile result in ECR Cl2 plasma after 200% over etch with different flux ratios Flux Ratio (Cl / Ion) = 0.3 p-Si Ref: K. Ono, M. Tuda, H. Ootera, and T. Oomori, Pure and Appl. Chem. Vol 66 No 6, 1327 (1994) SHS_MJK_ISPC Flux Ratio (Cl / Ion) = 0.8 p-Si University of Michigan Institute for Plasma Science & Engr. HYBRID PLASMA EQUIPMENT MODEL (HPEM) Electron Monte Carlo Simulation Te, Sb, Seb, k E, Ni, ne Fluid Kinetics Module Fluid equations (continuity, momentum, energy) Poisson’s equation 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 SHS_MJK_ISPC University of Michigan Institute for Plasma Science & Engr. MONTE CARLO FEATURE PROFILE MODEL (MCFPM) HPEM PCMCM Energy and angular distributions for ions and neutrals MCFPM Etch rates and profile SHS_MJK_ISPC 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. Poisson’s equation solved for charging University of Michigan Institute for Plasma Science & Engr. REACTOR GEOMETRY: 2 FREQUENCY CCP 2D, cylindrically symmetric Ar/CF4/O2 = 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 SHS_MJK_ISPC University of Michigan Institute for Plasma Science & Engr. PULSE POWER Use of pulse power provides a means for controlling f(e). 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. Pmax Power(t) Pave Duty Cycle Pt dt 1 0 Pmin = 1/PRF Time Pulse power for high frequency. Duty-cycle = 25%, PRF = 50, 100, 200, 415, 625 kHz Average Power = 500 W SHS_MJK_ISPC University of Michigan Institute for Plasma Science & Engr. Typical Plasma Properties SHS_MJK_ISPC PULSED CCP: ne, Te, f(e) Pulsing with a PRF and moderate duty cycle produces nominal intra-cycles changes [e] but does modulate f(e). ANIMATION SLIDE-GIF 40 mTorr, Ar/CF4/O2=75/20/5 LF = 10 MHz, 500 W HF = 40 MHz, pulsed 500 W PRF = 100 kHz, Duty-cycle = 25% [e] f(e) MIN MAX Te SHS_MJK_ISPC University of Michigan Institute for Plasma Science & Engr. CW ELECTRON DENSITY At 50% duty, the electron density is not significantly modulated by pulsing, so the plasma is quasi-CW. Duty = 50% Duty = 25% 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/CF4/O2=75/20/5 LF = 10 MHz, 500 W HF = 40 MHz, 500 W (CW or pulse) MIN SHS_MJK_ISPC ANIMATION SLIDE-GIF MAX University of Michigan Institute for Plasma Science & Engr. CW Duty = 50% ELECTRON SOURCES BY BULK ELECTRONS 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. Duty = 25% At the start of the pulse-on cycle, is there a impulsive positive electron source due to the overshoot of E/N. 40 mTorr, Ar/CF4/O2=75/20/5 LF 500 W, HF 500 W MIN SHS_MJK_ISPC ANIMATION SLIDE-GIF MAX University of Michigan Institute for Plasma Science & Engr. CW Duty = 50% ELECTRON SOURCES BY BEAM ELECTRONS The beam electrons result from secondary emission from electrodes and acceleration in sheaths. The electron source by beam electron is always positive. Duty = 25% The electron source by beam electrons compensates the electron losses and sustains the plasma. 40 mTorr, Ar/CF4/O2=75/20/5 LF = 10 MHz, 500 W HF = 40 MHz, 500 W (CW or pulse) MIN SHS_MJK_ISPC ANIMATION SLIDE-GIF MAX University of Michigan Institute for Plasma Science & Engr. Etch Properties SHS_MJK_ISPC F / POLY FLUX RATIO: CONSTANT POWER F to polymerizing flux ratio is largest at 200 kHz of PRF. 5.0 4.0 3.0 2.0 1.0 0.0 625 415 200 40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm LF 10 MHz 500 W, Pulsed HF 40 MHz 500 W 100 kHz University of Michigan Institute for Plasma Science & Engr. ETCH PROFILE IN SiO2 & IEAD: CONST. POWER Etch rate is fastest at 200 kHz PRF with larger ion energy and F to polymerizing flux ratio. Cycle Average IEAD Etch Profile (300 sec) 200 CW 100 kHz 415 CW 415 200 100 CD Height (mm) Energy (eV) 70 nm Bias: -64 V SHS_MJK_ISPC Width (mm) ANIMATION SLIDE-GIF 40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm LF 10 MHz 500 W, Pulsed HF 40 MHz 500 W -92 V -107 V -134 V Angle (degree) University of Michigan Institute for Plasma Science & Engr. 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. With DC Bias Without DC Bias 5.0 5.0 4.0 4.0 3.0 3.0 2.0 2.0 1.0 1.0 0.0 0.0 CW 625 415 200 100 kHz 40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm LF 10 MHz 250 V, Pulsed HF 40 MHz 500 W CW 625 415 200 50 kHz University of Michigan Institute for Plasma Science & Engr. ETCH PROFILE IN SiO2 & IEAD: CONST. VOLTAGE Etch rate is fastest at 415 kHz having larger fluorine flux. Etch Profile (300 sec) 200 CW 100 kHz 415 CD Cycle Average IEAD CW 415 200 100 -103 V -116 V -129 V 70 nm Bias: -88 V Width (mm) ANIMATION SLIDE-GIF 40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm LF 10 MHz 250 V, Pulsed HF 40 MHz 500 W Angle (degree) University of Michigan Institute for Plasma Science & Engr. ETCH PROFILE IN SiO2 & IEAD: NO BIAS Etch rate is fastest at CW excitation due to continuously delivered power. Cycle Average IEAD Etch Profile (300 sec) 200 CW 100 kHz 415 CW 415 200 100 CD 70 nm 40 mTorr, Ar/CF4/O2=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. 40 mTorr, Ar/CF4/O2=75/20/5, 200 sccm LF 10 MHz, Pulsed HF 40 MHz, Duty 25% University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS Extension of tail of f(e) 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. SHS_MJK_ISPC University of Michigan Institute for Plasma Science & Engr.