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

Optical and Discharge Physics 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

Optical and Discharge Physics 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

OBJECTIVES AND APPROACH Optical and Discharge Physics 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

HYBRID PLASMA EQUIPMENT MODEL (HPEM) Optical and Discharge Physics 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

MONTE CARLO FEATURE PROFILE MODEL Optical and Discharge Physics 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

MCFPM: CHARGING ALGORITHMS Optical and Discharge Physics 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

FLUOROCARBON PLASMA ETCHING OF SiO2/Si Optical and Discharge Physics 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

FLUOROCARBON ETCH OF HARC Optical and Discharge Physics 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

Optical and Discharge Physics 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

ION ENERGY ANGULAR DISTRIBUTIONS (IEADs) Optical and Discharge Physics 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

SiO2-over-Si HARC ETCH: NO CHARGING Optical and Discharge Physics 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

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

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

SiO2/Si HARC ETCH: PLASMA POTENTIAL Optical and Discharge Physics 116 V 151 V -3 -6 -4 110 V Max Min 213 V -5 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. -6 213 AR = 1:10 Iowa State University Optical and Discharge Physics Increasing Power Animation Slide MINGMEI_GEC07_12a

SiO2/Si HARC ETCH: PLASMA POTENTIAL Optical and Discharge Physics Max 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. Min -5 -3 -6 -4 -6 213 AR = 1:10 Increasing Power Iowa State University Optical and Discharge Physics MINGMEI_GEC07_12b

SiO2-over-Si HARC ETCH: RANDOMNESS? Optical and Discharge Physics 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

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

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

EFFECT OF OPEN FIELD: NO CHARGING Optical and Discharge Physics 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

EFFECT OF OPEN FIELD: NO CHARGING Optical and Discharge Physics 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. I II III IV Aspect Ratio = 1:10 Iowa State University Optical and Discharge Physics MINGMEI_GEC07_16b

EFFECT OF OPEN FIELD: EFFECT OF CHARGING Optical and Discharge Physics 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

EFFECT OF OPEN FIELD: EFFECT OF CHARGING Optical and Discharge Physics 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

OPEN FIELD EFFECT ON CHARGING Optical and Discharge Physics II 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

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

ELECTRIC FIELDS: BOUNDARY CONDITIONS Optical and Discharge Physics 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

Optical and Discharge Physics 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