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PLASMA ETCHING OF EXTREMELY HIGH ASPECT RATIO FEATURES: TWISTING EFFECTS*

PLASMA ETCHING OF EXTREMELY HIGH ASPECT RATIO FEATURES: TWISTING EFFECTS* Mingmei Wang a) , Ankur Agarwal b) , Yang Yang a) and Mark J. Kushner a) a) Iowa State University, Ames, IA 50011, USA mmwang@iastate.edu mjk@iastate.edu b) University of Illinois, Urbana, IL 61801, USA

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PLASMA ETCHING OF EXTREMELY HIGH ASPECT RATIO FEATURES: TWISTING EFFECTS*

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  1. PLASMA ETCHING OF EXTREMELY HIGH ASPECT RATIO FEATURES: TWISTING EFFECTS* Mingmei Wanga), Ankur Agarwalb), Yang Yanga) and Mark J. Kushnera) a)Iowa State University, Ames, IA 50011, USA mmwang@iastate.edu mjk@iastate.edu b)University of Illinois, Urbana, IL 61801, USA http://uigelz.ece.iastate.edu 60th Gaseous Electronics Conference, October 2007 *Work supported by Micron Technology Inc., SRC and NSF

  2. AGENDA • High Aspect Ratio Contact (HARC) Etching • Approach and Methodology • Charging of features • Fluorocarbon etching of HARC • SiO2-over-Si etching • Potential • Effect of open field • Concluding Remarks Iowa State University Optical and Discharge Physics MINGMEI_GEC07_AGENDA

  3. HARC ETCHING: ISSUES • As aspect ratio (AR) of features increases, complexity of plasma etching increases. • Aspect Ratio Dependent Etching • Etch rate decreases with increasing AR. • Charging of features due to ion and electron bombardment. • Electric field variations affect ion trajectories; deviation from ideal profile. • Non-uniform ion flux despite uniform bulk plasma. • As AR increases, the cross-sectional area of each via is smaller. • Increasingly random nature of incident ions and radicals. Ref: Micron Technology, Inc. Iowa State University Optical and Discharge Physics MINGMEI_GEC07_01

  4. OBJECTIVES AND APPROACH • Objectives • Computationally investigate consequences of charging of high aspect ratio features in SiO2. • Approach • Reactor scale: Hybrid Plasma Equipment Model. • Feature scale: Monte Carlo Feature Profile Model. • Poisson’s equation is solved for electric potentials. • Acceleration of ions and electrons due to electric fields in feature. • Dissipation of charge through material conductivity. Iowa State University Optical and Discharge Physics MINGMEI_GEC07_02

  5. HYBRID PLASMA EQUIPMENT MODEL (HPEM) • 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 Iowa State University Optical and Discharge Physics MINGMEI_GEC07_04

  6. 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. • Ion and radical fluxes at selected wafer locations. • Maxwellian electron fluxes with Lambertian distribution • Fluxes and distributions from HPEM. Iowa State University Optical and Discharge Physics MINGMEI_GEC07_05

  7. - - - - - - - - - + + + + + + + + + + MCFPM: CHARGING ALGORITHMS • The electric potential is solved using the method of Successive Over Relaxation (SOR). • Large mesh sizes pose computational challenges to solve for potential after launch of each particle. • Electric field is being updated after the launch of every 30 charged particles. • Particles are a few nm on a side. • Total particles launched (ions and radicals): 150,000-300,000. • The charge of pseudo-particles mesh is adjusted to account for finite sized particles. Charged particle Mask SiO2 Si Iowa State University Optical and Discharge Physics MINGMEI_GEC07_03

  8. 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. Iowa State University Optical and Discharge Physics MINGMEI_GEC07_06

  9. FLUOROCARBON ETCH OF HARC • Dual frequency capacitively-coupled (CCP) reactor geometry. • Base case conditions: • Ar/C4F8/O2 = 80/15/5, 300 sccm • 40 mTorr • 500 W at 25 MHz • 4000 W at 10 MHz • Low frequency: Substrate High Frequency: Showerhead Iowa State University Optical and Discharge Physics MINGMEI_GEC07_07

  10. REACTANT FLUXES • 10 mTorr, HF 500 W, LF 4 kW, Ar/C4F8/O2 = 80/15/5, 300 sccm • Dominant Ions: Ar+, CF2+, C2F4+, CF+ • Dominant Neutrals: CF2, C2F4, CF, CF3, F • Polymer clearing fluxes • O = 3  1016 cm-2.s-1 • O+ = 3  1014 cm-2.s-1 Iowa State University Optical and Discharge Physics MINGMEI_GEC07_08

  11. ION ENERGY ANGULAR DISTRIBUTIONS (IEADs) • IEADs for sum of all ions. • Peak in ion energy increase with increasing bias power. • High ion energies required for etching of HAR features. • Narrow angular distribution reduce sidewall impacts. • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, LF 10 MHz, HF 500 W. Iowa State University Optical and Discharge Physics MINGMEI_GEC07_09

  12. SiO2-over-Si HARC ETCH: NO CHARGING • Etch profile evolution without charging. • Etch rate higher at higher bias powers owing to high ion energies. • No charging: • Generally straight profiles. • High ion energies  low polymer coverages. • Some evidence of randomness due to small contact area • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, 10 MHz, HF 500 W. Iowa State University Optical and Discharge Physics Aspect Ratio = 1:10 MINGMEI_GEC07_10

  13. SiO2-over-Si HARC ETCH: EFFECT OF CHARGING • Charging effects are considered: • Charge buildup on polymer affects plasma potential. • Ion trajectories influenced by electric-field. • Electrons neutralize charge deep in trench. • Lower ion energies (due to buildup of charge) • Lower etch rates. • Deviation from “ideal” anisotropic etch profiles. • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, 10 MHz, HF 500 W. Animation Slide Iowa State University Optical and Discharge Physics Aspect Ratio = 1:10 MINGMEI_GEC07_11a

  14. SiO2-over-Si HARC ETCH: EFFECT OF CHARGING • Charging effects are considered: • Charge buildup on polymer affects plasma potential. • Ion trajectories influenced by electric-field. • Electrons neutralize charge deep in trench. • Lower ion energies (due to buildup of charge) • Lower etch rates. • Deviation from “ideal” anisotropic etch profiles. • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, 10 MHz, HF 500 W. Iowa State University Optical and Discharge Physics Aspect Ratio = 1:10 MINGMEI_GEC07_11b

  15. 213 V 116 V 151 V 110 V Max -5 -3 -6 -4 Min 0 -6 213 SiO2/Si HARC ETCH: PLASMA POTENTIAL • Charge deposition on polymer affects plasma potential. • Small depths: • Electrons effectively neutralize charge buildup. • Potential essentially maintained at zero. • Large depths: • Trapping of charge in polymer perturbs ion trajectories. • Electrons are “pulled” into bottom of trench by large positive potential and neutralizes. AR = 1:10 Iowa State University Optical and Discharge Physics Increasing Power Animation Slide MINGMEI_GEC07_12a

  16. 0 -6 213 SiO2/Si HARC ETCH: PLASMA POTENTIAL 213 V 116 V 151 V 110 V • Charge deposition on polymer affects plasma potential. • Small depths: • Electrons effectively neutralize charge buildup. • Potential essentially maintained at zero. • Large depths: • Trapping of charge in polymer perturbs ion trajectories. • Electrons are “pulled” into bottom of trench by large positive potential and neutralizes. Max -5 -3 -6 -4 Min AR = 1:10 Iowa State University Optical and Discharge Physics Increasing Power MINGMEI_GEC07_12b

  17. SiO2-over-Si HARC ETCH: RANDOMNESS? • Monte Carlo modeling utilizes random number generator to simulate a physical process. • Different seed numbers • All other conditions are same. • Is it reproducible? • No charging effects: • Etch profiles vary little • Anisotropic etch • No anomalies observed • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, LF 4 kW, HF 500 W. Different seed numbers Iowa State University Optical and Discharge Physics Aspect Ratio = 1:10 MINGMEI_GEC07_13

  18. SiO2/Si HARC ETCH: RANDOMNESS OF CHARGING? • Different seed numbers • All other conditions are same. • Is it reproducible? • Charging effects: • Stochastic nature of incident ion fluxes reflected in profiles. • Twisting observed • Etch direction shifts which reinforces anomoly. • Some unphysical behavior also observed (last trench) • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, LF 4 kW, HF 500 W. Different seed numbers Iowa State University Optical and Discharge Physics Aspect Ratio = 1:10 MINGMEI_GEC07_14

  19. SiO2/Si HARC ETCH: RANDOMNESS OF CHARGING? • 6 Trenches receiving “same fluxes. • Stochastic nature of fluxes produces random twisting. • Similar behavior observed experimentally. • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, LF 4 kW, HF 500 W. Ref: Micron Technology, Inc. Aspect Ratio = 1:10 Iowa State University Optical and Discharge Physics MINGMEI_GEC07_15

  20. EFFECT OF OPEN FIELD: NO CHARGING • 4 trenches followed by a “plasma-only” region with hard mask. • Open field has large sidewall polymerization. • Charging not considered • Trenches have some randomness in profiles owing to non-uniform ion fluxes. • No effect due to plasma-only region. Aspect Ratio = 1:10 Iowa State University Optical and Discharge Physics Animation Slide MINGMEI_GEC07_16a

  21. I II III IV EFFECT OF OPEN FIELD: NO CHARGING • 4 trenches followed by a “plasma-only” region with hard mask. • Open field has large sidewall polymerization. • Charging not considered • Trenches have some randomness in profiles owing to non-uniform ion fluxes. • No effect due to plasma-only region. Aspect Ratio = 1:10 Iowa State University Optical and Discharge Physics MINGMEI_GEC07_16b

  22. EFFECT OF OPEN FIELD: EFFECT OF CHARGING • Open field can impact etch of adjacent trenches by trapping of charge in polymer. • Transverse electric fields from external charge significantly affects adjacent trenches. • Inner trenches less affected by charging. • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, LF 4 kW, HF 500 W. Animation Slide Iowa State University Optical and Discharge Physics Aspect Ratio = 1:10 MINGMEI_GEC07_17a

  23. EFFECT OF OPEN FIELD: EFFECT OF CHARGING • Open field can impact etch of adjacent trenches by trapping of charge in polymer. • Transverse electric fields from external charge significantly affects adjacent trenches. • Inner trenches less affected by charging. • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, LF 4 kW, HF 500 W. I II III IV Aspect Ratio = 1:10 Iowa State University Optical and Discharge Physics MINGMEI_GEC07_17b

  24. I II OPEN FIELD EFFECT ON CHARGING • Open field impacts adjacent trenches by transverse electric field from trapped charged in polymer. • Isolating open field by making spacer of SiO2 thicker reduces transverse fields and perturbation of etch profiles. • Smaller deviation for the adjacent trench. • Note effect of stochastic ion fluxes in second trench. • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, LF 4 kW, HF 500 W. Iowa State University Optical and Discharge Physics Aspect Ratio = 1:10 MINGMEI_GEC07_18

  25. COMPUTATIONAL ASPECTS: DISSIPATION OF CHARGE • Dissipation of charge accounted for through material conductivity • I: Static charge • II: Only electron charges move • III: Both ion and electron charges move • Positive charges inside materials leads to high potentials inside the trench • Lower ion energies  polymer deposition • Etch stop observed • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, LF 4 kW, HF 500 W. Iowa State University Optical and Discharge Physics Aspect Ratio = 1:10 MINGMEI_GEC07_20

  26. ELECTRIC FIELDS: BOUNDARY CONDITIONS • Boundary conditions for Poisson’s equation: Zero potential at mesh boundaries. • Both electron and ion charges move • Small mesh: • Unphysical high gradients in fields • Leads to etch stops • Wide mesh: • Gradients in fields relaxed • Etch progresses to completion • Higher conductivity less effect of charging • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, LF 4 kW, HF 500 W. Iowa State University Optical and Discharge Physics Aspect Ratio = 1:10 MINGMEI_GEC07_21

  27. CONCLUDING REMARKS • Etching of high aspect ratio contacts (HARC) has been computationally investigated in fluorocarbon plasma. • Charging of features has been included to investigate anomalies such as twisting observed during etching of HARCs. • Charge buildup in/on polymer layer decreases etch rates and deviates the etching profile. • Ultimately a stochastic process for small features. • Various factors affect etching profiles: • Special structures like open field. • High energy ions may mitigate the effect of charging. • Charge dissipation due to material conductivity. Iowa State University Optical and Discharge Physics MINGMEI_GEC07_22

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