OPTIMIZINGUNIFORMITY IN PLASMA ETCHING OF HIGH ASPECT RATIO FEATURES BYENGINEERING THE FOCUSRING*ShuoHuang andMark J. KushnerUniversity of Michigan, Ann Arbor, MI 48109, [email protected], [email protected] and Sang Ki NamSamsung Electronics Co., Republic of [email protected], [email protected] InternationalConference on Plasma Science,Denver, Colorado, USA24-28 June 2018
Work supported by SamsungElectronics Co.,DOEFusionEnergy Science and National Science Foundation.
University of MichiganInstitute for Plasma Science & Engr.
AGENDA
Across-wafer uniformityEffect of focus ring (FR)Modeling platformsMulti-frequency CCPsIon energy and angular distributionsEtch profilesOptimizing uniformity through dielectricsunderFRConcluding remarks
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University of MichiganInstitute for Plasma Science & Engr.
ACROSS-WAFER UNIFORMITY
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Bending of sheath at wafer edge.
As feature sizes shrink, etch profiles become increasingly sensitive to ion energy and angular distributions (IEADs).Especially for high aspect ratio (HAR) features which requirehigh bias power and high energy(~ a fewkeV) ions.Plasma sheath/E-field bending at wafer edge, resulting in incident ions with tiltedangles and tilted feature profiles.Slightnon-uniformity in reactant production and ion transportcan persistdown to the feature level.
Hwang andKanarik, Solid State Technology, p16, July 2016.Gambino et al.,Microelectron. Eng. 135, 73 (2015).
Vertical (center) and tilted(edge) profile.
University of MichiganInstitute for Plasma Science & Engr.
WAFER EDGE SURROUNDED BY FOCUS RING
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The wafer edge is surrounded by focusring (FR), which is mainly used to maintainuniformfluxesand IEADs towafer.Subtleties in the change of FR can have significant effect on across wafer uniformity.As the FR is eroded during its lifetime, E-field at wafer edge and feature profiles change from outer tilting to inner tilting.
Haren et al., Proc. SPIE 10589, 105890D (2018).Wise, J. Micro/Nanolith. MEMS MOEMS 12, 041311 (2013).
Feature profile at edge evolving from outer to inner tilting with FR erosion.
Wafer edge surrounded by FR.
University of MichiganInstitute for Plasma Science & Engr.
RESEARCH OBJECTIVES
Modeling of multi-frequency CCPs: ion energy and angular distributions and consequence on etch profilesHeight of FRWafer-FR gapOptimizing uniformity by engineering FR using underlying dielectrics
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Plasma ChemistryMonte Carlo Simulation
SurfaceKineticsModule
HybridPlasma Equipment Model (HPEM): A 2D modularsimulatorfor reactor scale processes that combinesfluid and kinetic approaches.Electron Energy Transport Module(EETM): Electron energy equation solved for updating electron properties.Fluid-Kinetics Poisson Module(FKPM): Momentum equations and Poisson’s equations solved for updating densities of species and electric potentials.
Electron, Ion Cross Section Database
HYBRID PLASMA EQUIPMENT MODEL (HPEM)
EETM
FKPM
University of MichiganInstitute for Plasma Science & Engr.
EMM
Electron Energy Equation
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MONTE CARLO FEATURE PROFILE MODEL (MCFPM)
Multiscalemodel:Reactor scale (HPEM): cm,ps-ns;Sheath scale (PCMCM):μm,μs-ms;Feature scale (MCFPM): nm, s.MCFPM resolves surface topology on 3D Cartesian mesh.Eachcell has a material identity.Gas phase species are represented by Monte Carlopseudoparticles, which are launched with energy and angular distributions obtained from HPEM.Cell identities changed, removed or added for reactionssuch as etching and deposition.
University of MichiganInstitute for Plasma Science & Engr.
PCMCM
Ion energy and angular distributions
MCFPM
Etch rates and etch profile
HPEM
ne,Te, EEDF, ion and neutral densities
Reactor scale
Sheath scale
Feature scale
SiO2
Si
Resist
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University of MichiganInstitute for Plasma Science & Engr.
MULTI-FREQUENCY CCP REACTOR
Multi-frequency CCP reactor modeled using 2-dimensional HPEM.Top electrode: 80 MHz, 200 V.Bottom electrode: 10/5 MHz = 800/800 V.Gas mixtures:Ar/C4F8/O2=75/15/10, 500sccm, 25mTorr.100 mm wafer confined by Si focus ring with gap of 1 mm in between.Dielectric under focus ring:ε= 2 – 40. (4 for base case: quartz)Conductivity (1/Ω-cm): wafer: 10-2, FR: 10-5, dielectric: 10-15.
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MULTI-FREQ CCP – PLASMA PROPERTIES
Electron density peaks at the edge of the wafer due to field enhancement.Electrons are mainly produced through ionization by bulk electrons.Debye length at the wafer edge is ~ 0.5 mm, smaller than the gap size (1mm).Someplasmamoldingintothe wafer-FR gap.Ions acceleratedinto gap,resulting in erosion ofconsumable parts.
University of MichiganInstitute for Plasma Science & Engr.
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University of MichiganInstitute for Plasma Science & Engr.
MULTI-FREQ CCP – POTENTIAL, E-FIELD
E-field perpendicular to wafer surface resulting from conformal sheath except at edge where tilting occurs.At wafer edge,sheathbending downwards due to plasma molding into gap.Potential drop inside FR in the radial direction, leading to horizontal component for E-field around the wafer edge and so incident ions with tilted angles.
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FLUXES TO GAP SURFACE – IONS, RADICALS
Decreased electron and ion fluxes (instantaneous fluxes) to the gap surface due to partial molding of the plasma.CxFyradical fluxes into the gap decrease less significantly than ions, resulting inlargeCxFy/ion flux ratio and enhanced deposition inside gap.
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University of MichiganInstitute for Plasma Science & Engr.
IEADs – WAFER CENTER TO EDGE
Center
Mid-radius
High energy ions (400 to 1500 eV).Gradient in plasma sheath from center to edge.From center to edge, IEAD varies from symmetric to asymmetric (inward tilting).Symmetric IEAD at wafer center due to symmetric fluxes and perpendicular E-field to surface.IEAD at edge becomes inward skewed by about 2 – 4odue to sheath bending.
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Edge
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ETCH PROFILES – WAFER CENTER TO EDGE
Time evolution of etch profiles from center to edge.At wafer center, anisotropic profile with no tilting due to symmetric IEADs.At wafer edge, asymmetry in IEADs persists down into the features.Unequal mask erosion at different sides.Bowing in upper portion of feature due to reflected ions.Tilted etch front.
AnimationSlide
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POS. IONS, ELECTRIC POTENTIAL – FR HEIGHT
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University of MichiganInstitute for Plasma Science & Engr.
High FR
Medium FR
Low FR
When FR is high, shieldingof plasmaout of the wafer-FR gap.Less plasma molding into gap.Sheath at wafer edgeis upwardbending.When FR is low, electric potential is more skewed in the horizontal direction, resulting in ions with more tilted angles.
IEADs AT WAFER EDGE – FR HEIGHT
FR is eroded as reactor is used for processing hundreds of wafers.As height of FR decreases, ion angular distributions change from slightly outward tilting to inward tilting.Little variation for IEDs at wafer edge.
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IEAD collected
ETCH PROFILES AT WAFER EDGE – FR HEIGHT
IEAD collected
High FR
Medium FR
AnimationSlide
University of MichiganInstitute for Plasma Science & Engr.
Low FR
The transition of the profiles corresponds wellwith IEADs.WhenFR is new and high, feature is slightly outwardtilting.As FR is reused and height ofFRdecreased,profile becomes inwardtilting.
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POS. IONS, ELECTRIC POTENTIAL – GAP: 1 – 2 mm
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University of MichiganInstitute for Plasma Science & Engr.
Small gap
Medium gap
Large gap
As the gap size becomeslargercompared with the Debyelength (~ 0.5 mm),the sheath becomes more conformal to the gap surface.More plasma molding into the gap, resulting in enhanced erosion of the focus ring.
IEADs AT WAFER EDGE – GAP SIZE: 1 – 2 mm
IEAD
As FR is eroded, wafer-FR gap increases, allowing plasma penetrating into the gap and conformal to surface.As gap size increases, IADs at wafer edge becomes more tilted.IEDs at edge vary little with gap size.
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PROFILES AT WAFER EDGE – GAP SIZE: 1 – 2 mm
IEAD
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Small gap
AnimationSlide
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Medium gap
Large gap
As gap size increases, features become slightly more tilted, which corresponds well with the IEADs.Feature profiles at edge sensitive to FR geometries.
IEADs AT FR SURFACES
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FR is mainly eroded by physical sputtering and chemically enhanced etching.Slight ion fluxintogap, resulting in sparse IEAD.On all surfaces, incident ions have broadand asymmetric IEADs.
Wafer-FR gap
FR top surface
FR right surface
Reducing the ion energy or the ion flux is a possible solution to protect the focusring(maintain high FR and small gap).
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University of MichiganInstitute for Plasma Science & Engr.
FOCUS RINGKIT – UNDERLYING DIELECTRICS
Capacitance of plasma sheath above FR is inseries withcap. of FR and underlying dielectrics (FR kit).Opportunities to tunesheathpotential andIEADs through underlying dielectrics.Minimizing the effect on other parameters.Parameterization:ε= 2, 4, 10, 20, 30, 40.
FR
Dielectric
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Plasma sheath
University of MichiganInstitute for Plasma Science & Engr.
ELECTRIC POTENTIAL, POS. IONS –ε= 2, 40
Low-k,ε=2
High-k,ε=40
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For low-k dielectric, electric potential in FR is more skewed, resulting in lower sheath potential (thinner sheath) at FR surface.Lowerεresults in lower capacitance, which shares more voltage drop inside dielectric, so smaller sheath potential.
ε= 2 – 40
IEDs incident into wafer-FR gap almost not affected by varying dielectric.For top and right surface of FR, IEDs decrease by about 200 eV.Increased potential drop in dielectric and decreased sheath potential at focus ring surface.Significantdecrease of ion energy alleviates the consumption of FR by plasma.
Wafer-FR gap
FR top surface
FR right surface
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University of MichiganInstitute for Plasma Science & Engr.
IEDs AT FR SURFACES –ε= 2 – 40
University of MichiganInstitute for Plasma Science & Engr.
CONCLUDING REMARKS
Computationalinvestigationona multi-frequencycapacitivelycoupled plasmas (MF-CCP) sustainedinAr/C4F8/O2mixtureswhile varying the properties of theFR kit.Feature profiles at wafer edge are sensitive to focus ring geometries.As FR height decreases, IADs at wafer edge varies from outward to inward tilting due to modulated sheath profile at wafer edge.As wafer-FR gap size increases, IADs at wafer edge becomes more tilted due to more plasma molding into the gap.Maintaining geometry of FR (height and small wafer-FR gap) is critical in maintaining across-wafer uniformity.Varying the underlying dielectric from high-k to low-k, the sheath potential at FR surface decreases, resulting in better protection of FR and less tilted profiles.
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