ICOPS agarwal 2007 v6

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RECIPES FOR PLASMA ATOMIC LAYER ETCHING* Ankur Agarwala) and Mark J. Kushnerb) a)Department of Chemical and Biomolecular Engineering University of Illinois, Urbana, IL 61801, USA aagarwl3@uiuc.edu b)Department of Electrical and Computer Engineering Iowa State University, Ames, IA 50011, USA mjk@iastate.edu http://uigelz.ece.iastate.edu 34th IEEE ICOPS, June 2007 *Work supported by the SRC and NSF

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Iowa State University Optical and Discharge Physics AGENDA Atomic Layer Processing Plasma Atomic Layer Etching (PALE) Non-sinusoidal Bias Waveforms Tailored Bias PALE Recipes SiO2 using Ar/c-C4F8 Self-aligned contacts Concluding Remarks ANKUR_ICOPS07_Agenda

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Iowa State University Optical and Discharge Physics ATOMIC LAYER PROCESSING Advanced microelectronics structures require extreme selectivity in etching materials with nm resolution. Atomic layer plasma processing may allow for this level of control. Current techniques employ specialized ion beam equipment. The high cost of atomic layer processing challenges its use. Plasma Atomic Layer Etching (PALE) is potentially an economic alternative. ANKUR_ICOPS07_01 Double Gate MOSFET Tri-gate MOSFET Refs: AIST, Japan; Intel Corporation

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Iowa State University Optical and Discharge Physics PLASMA ATOMIC LAYER ETCHING (PALE) In PALE etching proceeds monolayer by monolayer in a cyclic, self limiting process. First step: Top monolayer is passivated in non-etching plasma. Passivation makes top layer more easily etched compared to sub-layers. Second step: Remove top layer (self limiting). Exceeding threshold energy results in etching beyond top layer. ANKUR_ICOPS07_02

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Iowa State University Optical and Discharge Physics PLASMA ATOMIC LAYER ETCHING (PALE) PALE has been computationally and experimentally investigated using conventional plasma equipment. Inductively coupled plasma (ICP) Capacitively coupled plasma (CCP) Since the equipment is already in fabrication facilities, no additional integration costs are incurred. The low speed of PALE processes hinder its integration into production line. Speed can be increased but only at the cost of losing control of CD (critical dimensions) or damaging material interfaces. ANKUR_ICOPS07_03

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Iowa State University Optical and Discharge Physics INCREASING SPEED OF PALE … HOW? ANKUR_ICOPS07_04 Conventional PALE Tailored Bias PALE Conventional PALE Different gas mixtures for each step. Although self-limiting, purge steps increase process time. Tailored bias PALE Create nearly mono-energetic ion distribution. Control ion energies via changes in voltage amplitude. Single gas mixture for both steps eliminates purge and reduces time.

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Iowa State University Optical and Discharge Physics NON-SINUSOIDAL BIAS WAVEFORMS: IEADs ANKUR_ICOPS07_05 Custom waveform produces nearly constant sheath potential resulting in narrow IEAD. Peak energy of IEAD is controlled by amplitude. IED broadens at higher biases due to thickening of sheath and longer transit times.  = 10%; Vp-p = 200 V Ref: A. Agarwal and M.J. Kushner, J. Vac. Sci. Technol. A, 23, 1440 (2005)

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Iowa State University Optical and Discharge Physics HYBRID PLASMA EQUIPMENT MODEL (HPEM) ANKUR_ICOPS07_06 Electromagnetics Module: Antenna generated electric and magnetic fields Electron Energy Transport Module: Beam and bulk generated sources and transport coefficients. Fluid Kinetics Module: Electron and Heavy Particle Transport, Poisson’s equation Plasma Chemistry Monte Carlo Module: Ion and Neutral Energy and Angular Distributions Fluxes for feature profile model

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Iowa State University Optical and Discharge Physics MONTE CARLO FEATURE PROFILE MODEL Monte Carlo techniques address plasma surface interactions and evolution of surface morphology and profiles. Inputs: Initial material mesh Surface reaction mechanism Ion and neutral energy and angular distributions Fluxes at selected wafer locations. Fluxes and distributions from equipment scale model (HPEM) ANKUR_ICOPS07_07

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FLUOROCARBON PLASMA ETCHING OF SiO2/Si CFx radicals produce polymeric passivation layers which regulate delivery of precursors and activation energy. Chemisorption of CFx produces a complex at the oxide-polymer interface Low energy ion activation of the complex produces polymer. Polymer complex sputtered by energetic ions  etching. As SiO2 consumes the polymer, thicker layers on Si slow etch rates enabling selectivity. ANKUR_ICOPS07_08 Iowa State University Optical and Discharge Physics

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MAIN ETCH-PALE FOR VERY HIGH ASPECT RATIO FEATURES ANKUR_ICOPS07_09 Iowa State University Optical and Discharge Physics 10:1 Trench PALE will always be slow compared to conventional etching. Selectivity of PALE is only needed at end of etch at material interface. Combine: Rapid “main etch” to reach material interface PALE to clear feature with high selectivity. Feature to be investigated is SiO2-over-Si trench with an aspect ratio of 1:10.

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Iowa State University Optical and Discharge Physics Ar/c-C4F8 ICP FOR SiO2 ETCHING Test system is inductively coupled plasma with 5 MHz biased substrate. Ar/C4F8 = 75/25, 100 sccm, 15 mTorr, 500 W ICP Main etch is conventional sinusoidal waveform. PALE uses tailored bias waveform: Passivate: 50 V (peak-to-peak) Etch: 100 V (peak-to-peak) ANKUR_ICOPS07_10

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Iowa State University Optical and Discharge Physics MAIN ETCH OF SiO2-over-Si ANKUR_ICOPS07_11 Main etch performed using a sinusoidal bias waveform. Micro-trenching at sides of feature due to specular reflection off walls. Central SiO2 remains when underlying Si is exposed. Significant etching into Si during over-etch to clear feature. Ar/C4F8 = 75/25, 100 sccm, 15 mTorr, 500 W, 100 V at 5 MHz Aspect Ratio = 1:10 ANIMATION SLIDE-GIF

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Iowa State University Optical and Discharge Physics Ar/c-C4F8 TAILORED BIAS PALE: IEADs ANKUR_ICOPS07_12 PALE of SiO2 using ICP Ar/C4F8 with variable bias. Step 1 Vp-p = 50 V Passivate single layer with SiO2CxFy Low ion energies to reduce etching. Step 2 Vp-p = 100 V Etch/Sputter SiO2CxFy layer. Above threshold ion energies. Narrow IEADs enable discrimination between threshold energies of undelying SiO2 and polymer complex. Ar/C4F8 = 75/25, 100 sccm, 15 mTorr, 500 W

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Iowa State University Optical and Discharge Physics SiO2-over-Si: PALE vs CONVENTIONAL ETCH ANKUR_ICOPS07_13 Narrow IEAD enables etching of rough initial profile at bottom. Redeposition of etched products and polymer cover exposed Si and sidewall; avoids notching and damage. High speeds (~ 4 ML/cycle) with high etch selectivity. 5 cycles of PALE Si SiO2 ANIMATION SLIDE-GIF  1 cell = 3 Å Conventional Etching

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Iowa State University Optical and Discharge Physics PALE: ROUGHNESS vs STEP 2 ION ENERGY ANKUR_ICOPS07_14 Speed of PALE can be increased via change in ion energies. At high ion energies, distinction between threshold energies is lost. Final etch profile is rough. Already exposed underlying Si vulnerable at high ion energy. Surface roughness scales linearly with ion energies.

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Iowa State University Optical and Discharge Physics PALE: ETCH RATE vs STEP 2 ION ENERGY ANKUR_ICOPS07_15 Number of PALE cycles required to clear feature decrease with increasing ion energy. Etch rate saturates at high ion energies due to the rough initial feature profile. Trade-off between high etching rates and selectivity. Etching of already exposed underlying Si leads to roughness.

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Iowa State University Optical and Discharge Physics PALE: CONVENTIONAL vs TAILORED BIAS ANKUR_ICOPS07_16 ANIMATION SLIDE-GIF Conventional PALE scheme utilizes 20 cycles. High speeds (~ 3-4 ML/cycle) and extreme selectivity of PALE enable fast etching of self-aligned contacts. Final etch profile is smooth even at high etching rates. Tailored: 5 cycles Si SiO2 Plasma  1 cell = 3 Å Conventional: 20 cycles

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Iowa State University Optical and Discharge Physics CONCLUDING REMARKS Atomic layer control of etch processes will be critical for 32 nm node devices. PALE using conventional plasma equipment makes for an more economic processes. Slow etching rates of conventional PALE need to be optimized: trade-off between high selectivity and etch rate PALE of SiO2 in Ar/c-C4F8 plasma investigated using custom bias waveforms, Non-sinusoidal bias waveforms enable: Precision control of IEADs Elimination of purge step to increase process speeds High selectivity at high etching rates (~ 4 ML/cycle) ANKUR_ICOPS07_17