COUPLING OF SCALES IN MODELING OF SEMICONDUCTOR MANUFACTURING* Yiting Zhang** and Mark J. Kushner University of Michigan Department of Electrical and Computer Engineering, Ann Arbor, MI 48109-2122, USA e-mail: mjkush@umich.edu http://uigelz.eecs.umich.edu Quantemol Workshop 11 September 2015 (Part 1) * Work supported by Semiconductor Research Corp., National Science Foundation and DOE Office of Fusion Energy Science. ** Present address: Lam Research, Fremont, CA USA QMW2015

University of Michigan Institute for Plasma Science & Engr. AGENDA The coupled scales in semiconductor processing Progress in 3D profile evolution Why this will always be difficult – what we are not controlling (or don’t know how to control)? Concluding Remarks University of Michigan Institute for Plasma Science & Engr. QMW2015

PLASMAS FOR NANO-ELECTRONICS The optimization of plasmas for micro-electronics (now, nano-electronics) fabrication now require resolution of essentially a monolayer in surface modification. The quest for control of these processes has resulted in a large variety of systems. Plasmas are dominantly: Low pressure (< 10s – 100s mTorr) Inductively, capacitively or microwave excited (or 2 of the three, or all three) Single or multiple frequencies (now up to 4 ) Remote and direct plasma sources Development of these processes may have lost sight of a few important premises….  Intel 22 nm FinFET University of Michigan Institute for Plasma Science & Engr. QMW2015 3

REACTOR SCALE: RADICAL FORMATION AND fe() Selectivity of etching and quality of deposition depends on the identity and ratio of fluxes of radicals and ions. These fluxes depend on the rates of electron impact dissociation of feedstock gases and their fragments. These rates depend on the distribution of electron energies, f(). Ultimately plasma nano-fabrication depends on controlling f() of electrons. University of Michigan Institute for Plasma Science & Engr. QMW2015

DEVELOPMENT OF fion(,) SHEATH SCALE: DEVELOPMENT OF fion(,) The delivery of fluxes to the sheath edge by reactor scale processes enables customizing fion(,) through control of sheath physics. In high plasma density systems, (nominal) separation of plasma sources and bias enable nearly (but not quite) control fion(,). Example: Pulsed ICP (low energy portion) and pulse bias (high energy portion) in Ar/Cl2. Ultimately plasma nano-fabrication depends on controlling f(,) of ions. ANIMATION SLIDE University of Michigan Institute for Plasma Science & Engr. QMW2015

FEATURE SCALE DEPENDS ON fe(), fion(,) The control of fe(), fion(,) provides desired fluxes to the substrate. The production of desired features then depends on symbiotic surface chemistry. Much of this chemistry is at best poorly known. Challenges in embracing the value of modeling in semiconductor fabrication is dominantly a consequence of poor feature scale chemistry. Oxide etch mechanism. Ultimately plasma nano-fabrication depends on symbiotic surface chemistry University of Michigan Institute for Plasma Science & Engr. QMW2015

Electron, Ion Cross Section Database Monte Carlo Simulation HYBRID PLASMA EQUIPMENT MODEL Electron, Ion Cross Section Database Plasma Chemistry Monte Carlo Simulation Surface Chemistry Module The Hybrid Plasma Equipment Model (HPEM) is a modular simulator that combines fluid and kinetic approaches. University of Michigan Institute for Plasma Science & Engr. QMW2015

Energy and angular distributions for ions and neutrals MONTE CARLO FEATURE PROFILE MODEL (MCFPM) The MCFPM resolves the surface topology on a 2D or 3D 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. HPEM PCMCM Energy and angular distributions for ions and neutrals MCFPM Poisson’s equation solved for charging Etch rates and profile University of Michigan Institute for Plasma Science & Engr. QMW2015

3D PROFILE SIMULATION QMW2015 9

EXPERIMENTAL VALIDATION: TRENCH ETCH  Post mask open ICP chamber: 10 mTorr, Cl2/He= 100/ 50 sccm. Coil: 15 MHz, 500 W ; Bias: 15 MHz, 500 V. Feature line/ pitch= 50/ 100 nm, Mask: 120 nm  IEADs on wafer  Model Mask 50 120 100 Si 400 SiO2  Exp: S. Sriraman, Lam Research Unit: nm QMW2015

EXPERIMENTAL VALIDATION: TRENCH ETCH Post STI: 26 sec  Model Validation will always be good – lack of fundamental plasma- surface coefficients requires calibration of surface mechanism. With mask erosion, ion reflection is enhanced and contribute to bowing and necking effect. Mismatch – more calibration of diffusive vs specular reflection. Exp: S. Sriraman, Lam Research Post STI: 80 sec He/Cl2 = 50/100 sccm, 10 mTorr ANIMATION SLIDE University of Michigan Institute for Plasma Science & Engr. QMW2015

DIE ORIENTATION TO RADIALLY DIVERGING IONS Dies on the wafer are typically in a Cartesian grid. Angular asymmetries in ion energy distributions are typically in the radial direction. There is then a location-centric asymmetry that may be produced in 3D structures Ion Flux Die orientation on wafer University of Michigan Institute for Plasma Science & Engr. QMW2015

SYMMETRIC IEAD vs ORIENTATION OF FEATURE back Ar/Cl2 = 80/20, 20 mTorr; Hardmask, Silicon Original side With azimuthally symmetric IEAD, profile is almost independent of its orientation in a die. Top view, Mask removed Rotate Rotate Original University of Michigan Institute for Plasma Science & Engr.

ASSYMMETRIC IEAD vs ORIENTATION OF FEATURE Asymmetry in IEAD reflected in orientation of feature on die. back side (+3o y axis) Rotate Original side back (-3o y axis) Ar/Cl2 = 80/20, 20 mTorr; Hardmask, Silicon

CONTACT EDGE ROUGHNESS (CER) Contact (CER) and line edge roughness (LER) are continuing lithographic and etch challenges. The origins of CER/LER are in part in the randomness of lithography as feature sizes shrink and partly in mechanical stresses of PR. Plasma tool solutions, such as DC augmented dual-frequency CCP have been proposed. Once CER/LER is in the mask, what is the likelihood for transfer of the roughness to the etched feature? K. Ban et al., Proc. SPIE Vol. 8322, paper 83221A, 2012. Y. Feurprier et al., Proc. SPIE Vol. 9428, paper 94280F, 2015 S-M. Kim et al., Proc. SPIE vol. 9048, paper 90480A, 2014. University of Michigan Institute for Plasma Science & Engr. QMW2015

CER AND ETCH PATTERN TRANSFER Investigate etch pattern transfer of CER using HPEM/MCFPM-3D. Test system is Ar/Cl2 plasma etching of Si or SiO2 with erodeable photoresist mask. Ar/Cl2 = 80/20, 20 mTorr, ICP with 10 MHz substrate bias. Ion energy distributions from HPEM Cl flux = 1 x 1017 cm-2s-1 University of Michigan Institute for Plasma Science & Engr. QMW2015

CER: PHOTORESIST PATTERN Compare circular PR opening with vias having serrated edges. Note: large features here – the important parameter is aspect ratio. Absolute size scales out. Horizontal slice through feature. Top view of mask. University of Michigan Institute for Plasma Science & Engr. QMW2015

CER: ETCH CHARACTERISTICS Ar/Cl2 = 80/20, 20 mTorr, ICP with 10 MHz substrate bias. Note onset of bowing as the mask erodes. Redeposition (aqua color) occurs throughout feature and mask. Erosion of mask flattens cerration. Significant grooves only with most severe serration. University of Michigan Institute for Plasma Science & Engr. QMW2015

CER: EARLY ETCH CHARACTERISTICS Serration of the via walls is only significant at the top of the feature. There is sufficient scattering from the PR with advantagous large angles to begin to blur the serration. 100% vertical anisotropic ions would preserve the serration. Horizontal slices from bottom of feature to top of PR. Ar/Cl2 = 80/20, 20 mTorr, ICP with 10 MHz substrate bias. University of Michigan Institute for Plasma Science & Engr. QMW2015

CER: LATE-ETCH CHARACTERISTICS Serration of "middle case" is nearly fully smoothed over late in etch. Severe serration survives only in the vicinity of the mask. Slight ARDE effect – non-CER appears a bit larger. More "ion – driven" processes (e.g., dielectric etch) will be more sensitive to CER. Horizontal slices from bottom of feature to top of PR. Ar/Cl2 = 80/20, 20 mTorr, ICP with 10 MHz substrate bias. University of Michigan Institute for Plasma Science & Engr. QMW2015

ARDE – ASPECT RATIO DEPENDENT ETCHING As the dynamic range of features increases, ARDE (or RIE lag) becomes more problematic. The causes of ARDE are poorly understood – attributed to ion angular distributions, side-wall scattering, transport in feature. R. Bates et al. JVSTA 32, 051302 (2014) https://cmi.epfl.ch/etch/AMS200.php University of Michigan Institute for Plasma Science & Engr. QMW2015

ARDE – ASPECT RATIO DEPENDENT ETCHING Investigate ARDE in Si etching over SiO2 with erodeable hard mask. Ar/Cl2 = 80/20, 20 mTorr, ICP with 10 MHz substrate bias. Note: Important scaling parameter is aspect ratio, not absolute dimension. University of Michigan Institute for Plasma Science & Engr. QMW2015

ARDE – ASPECT RATIO DEPENDENT ETCHING Investigate ARDE in Si etching over SiO2 with erodeable hard mask. Redposition is signficant in all features. Ar/Cl2 = 80/20, 20 mTorr, ICP with 10 MHz substrate bias. Note: Scaling parameter is aspect ratio. University of Michigan Institute for Plasma Science & Engr. QMW2015

University of Michigan Institute for Plasma Science & Engr. ARDE – DEPTHS, RATES ARDE produces a x2 change in etch rate over a x3 change in feature size. Etch rates decrease for all feature sizes as aspect ratio increases. For a range of parameterization of -50V to +100V on the bias, there is little relative change in the ARDE. The etch rate scales with voltage, but relative rates between feature sizes is nearly the same. This result implies a transport limit in ARDE. Ar/Cl2 = 80/20, 20 mTorr, ICP, 10 MHz bias. University of Michigan Institute for Plasma Science & Engr. QMW2015

ALE – ATOMIC LAYER ETCHING Extreme etch selectivity and high rate are difficult to simultaneously achieve. ALE – atomic layer etching removes a single layer of atoms/groups at a time using self limiting methods. ALE is typically follows conventional rapid etch to near the interface. There will always be a tradeoff in continuing to etch side walls and ability to clear corners with low ion (or moderate) energies used during ALE portion of cycle. K. J. Kanarik et al., JVSTA 33, 020802 (2015) University of Michigan Institute for Plasma Science & Engr. QMW2015

ALE – FLUXES, ION ENERGY DISTRIBUTIONS Ar/Cl2 ICP etching of Si over SiO2. Main etch followed by alternating 1 second cycles of Cl dominate fluxes with "as low energy ions as you can get“ Ar+ dominated fluxes with "low but not zero energy ions" Main Etch ALE-Passivation ALE-Etch University of Michigan Institute for Plasma Science & Engr. QMW2015

University of Michigan Institute for Plasma Science & Engr. ALE – CLEARING CORNERS Ar/Cl2 ICP etching of Si over SiO2 of square via Main etch followed by alternating 1 second cycles of passivation and chemical-sputtering 3D square via – figures 2D slice through center. Observations: Clearing corners requires prolonged etch and high selectivity to underlying SiO2. Low energy ions during chemical-sputtering have broad angular distribution. Some unavoidable bowing of the feature occurs during over-etch, even with ALE. University of Michigan Institute for Plasma Science & Engr. QMW2015

University of Michigan Institute for Plasma Science & Engr. ALE – SENSITIVITY Ar/Cl2 ICP etching of Si over SiO2. High sensitivity to process parameters. Increase of 30 V in dc bias during ALE etch step loses ability to retain selectivity while clearing corners. University of Michigan Institute for Plasma Science & Engr. QMW2015

WHAT ARE WE NOT CONTROLLING? PHOTONS... QMW2015 29

PHOTON STIMULATED PROCESSES VUV chain scission and cross linking of polymers, producing change in secondary electron emission coefficients. VUV sustained etching in Cl and HBr plasmas below accepted ion energy threshold. Secondary electron emission [Kishimoto, J. Appl. Poly. Sci. 21, 2721 (1977)] Shin et al., JVSTA 30, Mar/Apr 2012 University of Michigan Institute for Plasma Science & Engr. QMW2015

LOW-k PROCESS INTEGRATION Typical porous SiO2 has CH3 lining pores with Si-C bonding – referred to as SiOCH. Ave pore radius: 0.8-1.1 nm Porosity: up to 50% Etching and sealing SiOCH is an integrated, multistep process Etch Ar/C4F8/O2 CCP Clean Ar/O2 or He/H2 ICP Activate He/H2 ICP Seal Ar/NH3 ICP Mask Porous Low-k SiCOH Si University of Michigan Institute for Plasma Science & Engr. QMW2015

LOW-k DAMAGE: O2 AND H2 PLASMAS O atoms can abstract H from –CH3 groups and remove -CH3: O + Si-CH3 (s)  Si-CH2(s) + OH(g) O + Si-CH2(s)  Si(s) + CH2O(s) O + CH2O(s)  CO(g) + H2O(g). O atoms can cause Si-C bond scission and remove –CH3 groups: O + Si-CH3 (s)  -CH3(s) + Si(s) + O(g) O + -CH3 (s)  -CH2O(s) + H(g) O + CH2O(s)  CO(g) + H2O(g).  H removes -CH3 as CH4(g) and abstracts H forming Si-CHx-1 groups: H + Si-CH3 (s)  -Si(s) + CH4 (g) H + Si-CHx(s)  Si-CHx-1(s) + H2(g). Ref: M.F.A.M. van Hest et al., Thin Solid Films 449 40 (2004) O. V. Braginsky et al., Journal of Aplied Physics 108 073303 (2010) A. M. Urbanowicz et al., Journal of The Electrochemical Society, 157 5 H565-H573 (2010). University of Michigan Institute for Plasma Science & Engr. QMW2015 32

PHOTON GENERATION AND DAMAGE: Ar/O2, He/H2 ICP Photons penetrate into the porous SiCOH, are adsorbed by SiO2 and break Si-CH3 bonds producing adsorbed •CH3(ads) which enhances demethylation rate (-CH3 removal): hv + Si-CH3(s)  -Si•(s) + •CH3(ads) hv + SiO2(s)  SiO2*(s). Ar/O2 Plasmas: e + O  O(1D), O(3s), O(5s), O(5p) + e O(3s)  O + hν (130 nm) O(3s)  O(1D) + hν (164 nm) O(5p)  O(5s) + hν (777 nm) O(5s)  O + hν (136 nm) He/H2 Plasmas: e + He  He* + e e + He  He** + e e + He*  He* + e He**  He + hν (~100 nm) University of Michigan Institute for Plasma Science & Engr. QMW2015 Ref: J. Lee and D. B. Graves, J. Phys. D 43, 425201 (2010). 33

DAMAGE: Ar/O2 AND He/H2 (PHOTON FLUX) Photons form O2 plasmas penetrate ~100 nm but for He/H2 plasmas ~20 nm . Overall O2 plasmas cause ~3 times more damage. Ar/O2 Clean Model He/H2 Clean Experiment Ref: M. A. Worsley, S. F. Bent, S. M. Gates, N. C. M. Fuller, W. Volksen, M. Steen and T. Dalton, J. Vac. Sci. Technol. B 23, 395 (2005). Animation Slide University of Michigan Institute for Plasma Science & Engr. QMW2015 34

DAMAGE: Ar/O2 AND He/H2 (INTERCONNECTIVITY) A higher interconnectivity enables more damage.  Ar/O2 Clean Interconnectivity 40% Model Interconnectivity 100% Experiment Animation Slide Ref: M. A. Worsley, S. F. Bent, S. M. Gates, N. C. M. Fuller, W. Volksen, M. Steen and T. Dalton, J. Vac. Sci. Technol. B 23, 395 (2005). QMW2015 35

WHAT ARE WE NOT CONTROLLING? SEQUENCING OF REACTANTS... QMW2015 36

University of Michigan Institute for Plasma Science & Engr. HiPIMS-LIKE PVD High power impulse magnetron sputtering (HiPIMS) provides many of the advantages of pulsed processing. Additional parameter space for optimizing "overlap" of species incident onto substrate. Investigate HiPIMS-like pulsed PVD of Cu. Ar, 10 mTorr 8.33 kHz, 13.5 s voltage pulse (12% duty cycle), -400 V HPEM with kinetic sputtered atoms and ions into target. Ar, Ar(1s2,3,4,5),Ar(4p),Ar(4d), Ar+ Cu, Cu(2D),Cu(2P),Cu+, e University of Michigan Institute for Plasma Science & Engr. QMW2015

HiPIMS-PLASMA DENSITY AND SOURCES Plasma density modulated by 10 during pulse. (Note: on axis peak.) Ionization provided by thermal electrons and secondary electrons trapped in cusp of magnetic field. Ar, 10 mTorr, Cu target, 8.33 kHz (120 s period), -400 V (12% duty cycle) University of Michigan Institute for Plasma Science & Engr. Animation Slide QMW2015