PLASMA ETCHING OF HIGH ASPECT RATIO OXIDE-NITRIDE-OXIDE STACKS* Shuo Huang, Chad Huard and Mark J. Kushner University of Michigan, Ann Arbor, MI 48109-2122 USA shuoh@umich.edu, chuard@umich.edu, mjkush@umich.edu Sang Ki Nam, Seungbo Shim and Wonyup Ko Samsung Electronics Co., Republic of Korea sangki.j.nam@samsung.com, seungb.shim@samsung.com, wonyup.ko@samsung.com AVS 65th International Symposium, Long Beach, California, USA 23 October 2018 Work supported by Samsung Electronics, DOE Office of Fusion Energy Science and National Science Foundation. 1 1

University of Michigan Institute for Plasma Science & Engr. AGENDA High aspect ratio oxide-nitride-oxide (ONO) stacks Surface reaction mechanism of SiO2 and Si3N4 etching using fluorocarbon plasmas Profile simulation of plasma etching of ONO stacks Polymer etching, sputtering probabilities Bias power Via vs trench Concluding Remarks University of Michigan Institute for Plasma Science & Engr. AVS2018 2 2

HAR 3D-NAND ARCHITECTURE Large memory capacity is now being met by 3-D vertical structures initially consisting of hundreds of alternately deposited silicon nitride and silicon oxide layers (ONO) https://www.electronicsweekly.com/news/business/tosiba-sampling-64-layer-3d-nand-2016-07 http://semimd.com/blog/2014/01/29/3d-nand-to-10-nm-and-beyond/ University of Michigan Institute for Plasma Science & Engr. AVS2018 3 3

University of Michigan Institute for Plasma Science & Engr. HAR 3D-NAND ETCHING Fabrication of 3D-NAND memory requires at least a dozen steps (plus lithography). Arguably, the most critical is etching a high aspect ratio (HAR) via through alternating (256?) layers of ONO. Aspect ratio (AR) approaches (exceeds) 100 University of Michigan Institute for Plasma Science & Engr. iliputing.com/2016/08/micron-unveils-3d-nand-flash-storage-smartphones.html www.theregister.co.uk/2015/04/09/3d_nand_gigantically_expensive_to_make/ AVS2018 4 4

CHALLENGES IN HAR ETCHING OF ONO Etching of HAR vias through ONO shares the same challenges as conventional HAR etching Aspect ratio dependent etching (ARDE) and statistical etch depth Bowing, twisting, contact edge roughness Pattern distortion. Additional challenges result from the alternating material layers. T. Iwase et al., JJAP 55, 06HB02 (2016) J. K. Kim et al., JVSTA 33, 021303 (2015). blog.lamresearch.com/tech-brief-memory-grows-up-with-3d-nand http://www.reatiss.com/Gallery/Samsung_VNAND_K9HQGY8S5M_K9LPGY8S1M_K9ADGD8S0A AVS2018 5 5

POLYMER MEDIATED SELECTIVITY ONO via combines polymer mediated etching and physical sputtering. Etching in low-bias fluorocarbon plasmas occurs by reactions of the oxide/nitride with a thin polymer layer. Ion energy is delivered to surface through polymer while radicals react with the surface after diffusing through the polymer. Removing one Si atom removes 2 carbon atoms in SiO2, 1 in Si3N4 , none with Si producing polymer thickness: Si, Si3N4, SiO2 With high-bias, all polymer layers are thin and physical sputtering occurs, thereby decreasing selectivity. Oxide: Nitride:  Si pictures University of Michigan Institute for Plasma Science & Engr. AVS2018

HAR ETCHING OF ONO STACKS In this talk, discussion of computational investigation of plasma etching of HAR vias though oxide-nitride-oxide (ONO) stacks in fluorocarbon capacitively coupled plasmas. Emphasis is on polymer deposition and removal. Sputtering of polymers by energetic ions Isotropic removal of polymers by thermal neutrals (O and F) Consequences of reactor scale parameters (powers, fluxes and IEADs) will also be discussed. University of Michigan Institute for Plasma Science & Engr. AVS2018 7 7

MONTE CARLO FEATURE PROFILE MODEL Profile is defined on a 3D “voxel” mesh, with cells representing a material. Gas phase pseudo-particles are launched with fluxes and energy and angular distributions from HPEM. Trajectories are tracked until striking solid where surface mechanism occurs. Chemical reaction (material change) Etching (removing cells) Deposition (adding cells) Diffusion on and through materials Implantation and mixing Specular and diffusive scattering Charging addressed through solution of Poisson's equation producing in-feature electric fields for ion trajectories. pictures University of Michigan Institute for Plasma Science & Engr. AVS2018

Si / SiO2 / Si3N4 CONTINUOUS ETCHING Etching SiO2 consumes polymer by forming CO, CO2, COF products. Si3N4 consumes polymer forming FCN. Due to stoichiometry, less polymer consumed than SiO2. Thinner steady state polymer layer on SiO2 than Si3N4 or Si. Polymer layer on Si3N4 is more similar to Si. Etch rate of Si3N4 increases when polymer thickness becomes less than ion penetration depth (1-2 nm) pictures ICP Power: 1200 W at 10 MHz Feedstock: Ar/C4F8 = 95/5 University of Michigan Institute for Plasma Science & Engr. AVS2018

University of Michigan Institute for Plasma Science & Engr. ALE OF SiO2 AND Si3N4 Si3N4 etching does not consume as much polymer as SiO2 leading to gradual increase in thickness. Etch stop on Si3N4 enables selectivity. R. Gottscho, ALE 2017 pictures Si3N4 SiO2 Animation Slide University of Michigan Institute for Plasma Science & Engr. AVS2018

MULTI-FREQUENCY CCP ETCH TOOL ONO via etching performed for multi-frequency capacitively coupled plasma (CCP) reactor. Ar/C4F8/O2 = 0.75/0.15/0.1, 500 sccm, 25 mTorr. Top Electrode: 80 MHz, Biased Substrate: 10 MHz + 5 MHz. Base case: 80/10/5 MHz = 400 / 2500 / 5000 W = 125 / 1030 / 2450 V, DC bias = -390 V. University of Michigan Institute for Plasma Science & Engr. AVS2018 11

BASE CASE: NEUTRAL, ION FLUXES TO WAFER With significant dissociation, radical fluxes to wafer dominated by CFx, O, F. Reactive neutral fluxes exceed ion fluxes by 1-2 orders of magnitudes. Ion fluxes dominated by Ar+ due to large mole fraction of parent gas. Large CxFy+ fluxes due to their relatively low ionization potential. Ar/C4F8/O2 = 0.75/0.15/0.1, 25 mTorr, 500 sccm, 80/10/5 MHz = 0.4/2.5/5 kW = 125/1030/2450 V, Vdc=-1760 V University of Michigan Institute for Plasma Science & Engr. AVS2018 12

BASE CASE: IEADs TO WAFER Total Ions (2 dec) Sheath at wafer has components of both low and high frequencies. Combination of multi-frequencies and large range of ion masses (12 – 180 amu) results in broad ion energy distributions. Narrow ion angular distribution (< ±4 deg) results in more “anisotropic” etch profile. MIN MAX Ar/C4F8/O2 = 0.75/0.15/0.1, 25 mTorr, 500 sccm, 80/10/5 MHz = 0.4/2.5/5 kW = 125/1030/2450 V, Vdc=-1760 V University of Michigan Institute for Plasma Science & Engr. AVS2018 13

University of Michigan Institute for Plasma Science & Engr. Time ONO ETCHING “2D” trench simulated in 3D with finite depth to feature. ONO stack : Si3N4: 30 nm, SiO2: 20 nm, 80 coupled O-N layers. Mesh resolution: 5 nm/cell. Hard stopping layer. Mask erosion, non-specular ion/hot neutral scattering produce bowing mostly limited to top SiO2 layer. Etch front quickly tapers due to sidewall polymer deposition. Animation Slide University of Michigan Institute for Plasma Science & Engr.

University of Michigan Institute for Plasma Science & Engr. ONO Profile ONO Polymer Bulk Oxide Bulk Nitride ONO, BULK SiO2, Si3N4 3D simulation of “2D” trench. Thin, nearly monolayer polymer layer at sidewalls due to high energy ions. Less polymer deposition at high ARs due to limited transport of polymerizing CFx species. Greater CD loss (bowing) in pure oxide than pure nitride etching due to thinner polymer layer. For chemical sputtering, 1 silicon atom removed requires 2 carbon atoms for SiO2, 1 carbon atom removed for SiN For these conditions, physical sputtering also contributes. Animation Slide University of Michigan Institute for Plasma Science & Engr.

University of Michigan Institute for Plasma Science & Engr. ONO Profile ONO Polymer Bulk Oxide Bulk Nitride ONO, BULK SiO2, Si3N4 3D simulation of “2D” trench. Thin, nearly monolayer polymer layer at sidewalls due to high energy ions. Less polymer deposition at high ARs due to limited transport of polymerizing CFx species. Greater CD loss (bowing) in pure oxide than pure nitride etching due to thinner polymer layer. For chemical sputtering, 1 silicon atom removed requires 2 carbon atoms for SiO2, 1 carbon atom removed for SiN For these conditions, physical sputtering also contributes. University of Michigan Institute for Plasma Science & Engr.

POLYMER REMOVAL PROCESSES Control of fluorocarbon polymer thickness through chemical and physical removal Thermal etching by F and O atoms CFx(s) + F(g)  CF4(g) CFx(s) + O(g)  COF(g) Small atoms (e.g., F, O) diffuse through polymer layer, producing volumetric losses. Physical sputtering by ions CFx(s) + M+(g)  CFx(g) + M# Ion-assisted surface passivation at polymer/solid interface SiO2(s) + CFx(s) + M+  SiO2CxFy(s) + M# SiN(s) + CFx(s) + M+  SiNCxFy(s) + M# Etch reaction SiO2CxFy(s) + M+  SiFx(s) + COFy + M# SiNCxFy(s) + M+  SiFx(s) + CNF + M# pictures University of Michigan Institute for Plasma Science & Engr. AVS2018

University of Michigan Institute for Plasma Science & Engr. Increasing sputter probability POLYMER SPUTTERING 3D simulation of “2D” trench. For low polymer sputtering probability, ps, twisting sets in due to polymer build-up. As ps increases, less polymer deposition at sidewalls and twisting is reduced. As ps approaches unity sidewall is exposed, and CD loss (bowing) increases. Animation Slide Sputtering Probability Ion/Hot Atom Energy (eV) University of Michigan Institute for Plasma Science & Engr.

University of Michigan Institute for Plasma Science & Engr. Increasing sputter probability POLYMER SPUTTERING 3D simulation of “2D” trench. For low polymer sputtering probability, ps, twisting sets in due to polymer build-up. As ps increases, less polymer deposition at sidewalls and twisting is reduced. As ps approaches unity sidewall is exposed, and CD loss (bowing) increases. Sputtering Probability Ion/Hot Atom Energy (eV) University of Michigan Institute for Plasma Science & Engr.

University of Michigan Institute for Plasma Science & Engr. Increasing etch probability 0.005 0.01 0.02 0.05 POLYMER ETCH: O ATOMS 3D simulation of “2D” trench. Polymer is isotropically etched by thermal O atoms (temperature dependent) CFx(s) + O(g)  COF(g) Scaling parameter is OpO (oxygen flux  etch probability). Varied pO here though expect equivalence with flux. For low pO, (or low O) Initial tapering at low AR broadened by effective “over-etch” as AR increases. Onset of “waviness” not observed with bulk SiO2, Si3N4 – likely exacerbated in trench calculation. For high pO (or low O) more bowing at low ARs, less waviness, faster etch rate enabled by more physical sputtering. Animation Slide University of Michigan Institute for Plasma Science & Engr.

University of Michigan Institute for Plasma Science & Engr. Increasing etch probability 0.005 0.01 0.02 0.05 POLYMER ETCH: O ATOMS 3D simulation of “2D” trench. Polymer is isotropically etched by thermal O atoms (temperature dependent) CFx(s) + O(g)  COF(g) Scaling parameter is OpO (oxygen flux  etch probability). Varied pO here though expect equivalence with flux. For low pO, (or low O) Initial tapering at low AR broadened by effective “over-etch” as AR increases. Onset of “waviness” not observed with bulk SiO2, Si3N4 – likely exacerbated in trench calculation. For high pO (or low O) more bowing at low ARs, less waviness, faster etch rate enabled by more physical sputtering. University of Michigan Institute for Plasma Science & Engr.

University of Michigan Institute for Plasma Science & Engr. Increasing etch probability 0.002 0.004 0.008 0.02 POLYMER ETCH: F ATOMS 3D simulation of “2D” trench. F atoms etch polymer through surface and bulk reactions CFx(s) + F(g)  CFy(g) Production of F atoms is more coupled with production of polymerizing species. Low etch probability, pF, produce nearly unrecoverable taper due to thicker polymer. Lower etch rate  longer “overetch”  more mask erosion. Lower F atom flux F would likely also produce lower polymerizing flows. High pF or F produces flatter etch front as polymer is rapidly removed from sidewall. Animation Slide University of Michigan Institute for Plasma Science & Engr.

University of Michigan Institute for Plasma Science & Engr. Increasing etch probability 0.002 0.004 0.008 0.02 POLYMER ETCH: F ATOMS 3D simulation of “2D” trench. F atoms etch polymer through surface and bulk reactions CFx(s) + F(g)  CFy(g) Production of F atoms is more coupled with production of polymerizing species. Low etch probability, pF, produce nearly unrecoverable taper due to thicker polymer. Lower etch rate  longer “overetch”  more mask erosion. Lower F atom flux F would likely also produce lower polymerizing flows. High pF or F produces flatter etch front as polymer is rapidly removed from sidewall. University of Michigan Institute for Plasma Science & Engr.

ONO STACK – FLUXES TO ETCH FRONT Thermal Neutrals Ions, Hot Neutrals Fluxes of ions and neutrals to the etch front decrease with increasing depth. Neutral fluxes are conduction limited, though aided by specular conversion of ions scattering from side walls into hot neutrals. Decrease in ion fluxes is due to reduction in solid angle viewing plasma. Hot atom flux is less sensitive to AR. 3D simulation of “2D” trench. University of Michigan Institute for Plasma Science & Engr. AVS2018

University of Michigan Institute for Plasma Science & Engr. ETCH PRODUCTS 3D simulation of “2D” trench. Chemical sputtering produces COF and CNF fluxes larger than physical sputter products SiO2 and SiN by 2 orders of magnitudes. Etch produces COF exceeds CNF due contribution from polymer etching. Oscillation as successive layers are penetrated. University of Michigan Institute for Plasma Science & Engr. AVS2018

RATES: TRENCH AND PRE-ETCHED PROFILE ARDE in full trench etch. Oscillation in rate as etch proceeds through nitride and oxide – oscillation dampens as taper begins to sample both oxide and nitride. Higher etch rate for nitride than oxide due to conditions that keep polymer layer thin. University of Michigan Institute for Plasma Science & Engr. Animation Slide

University of Michigan Institute for Plasma Science & Engr. Time ONO VIA Etch of via through ONO stack (AR = 40) Polymer deposition occurs at surface of PR, oxide and low ARs of ONO stack. At high ARs, almost lower polymer deposition due to limited CFx conductance. Slower etch rates as AR increases (ARDE). Same trends as trench but more sensitive to polymer transport (larger surface-to- volume ratio). University of Michigan Institute for Plasma Science & Engr. Animation Slide 27

University of Michigan Institute for Plasma Science & Engr. Time ONO VIA Surface of etch profile with polymers blanked out. Etch of via through ONO stack (AR = 40) Polymer deposition occurs at surface of PR, oxide and low ARs of ONO stack. At high ARs, almost lower polymer deposition due to limited CFx conductance. Slower etch rates as AR increases (ARDE). Same trends as trench but more sensitive to polymer transport (larger surface-to-volume ratio). University of Michigan Institute for Plasma Science & Engr. AVS2018 Animation Slide 28

University of Michigan Institute for Plasma Science & Engr. IEADs vs 5 MHz POWER: 2.5 – 10 kW Increased ion energies and narrower IADs with increased 5 MHz power. Average ion energy increases from 1,600 to 2,600 eV with 5 MHz power 2.5 kW to 10 kW. Fluxes of ions and neutrals have small increase with 5 MHz power as majority of power is dissipated in ion acceleration. MIN MAX University of Michigan Institute for Plasma Science & Engr. Ar/C4F8/O2 = 0.75/0.15/0.1, 25 mTorr, 500 sccm 80/10/5 MHz=0.4/2.5/2.5-10 kW AVS2018 29

University of Michigan Institute for Plasma Science & Engr. Increasing 5 MHz Power: 2.5 5 7.5 10 kW ONO VIA: 5 MHz POWER Higher etch rate with power not only due to higher energy ion fluxes as chemical sputtering saturates above a few keV. Higher rates due largely to energetic hot neutrals able to reach deep into features – thermal neutrals cannot Waviness at low power (thick polymer) Higher ion fluxes sputter the sidewall at grazing incidence Removing the necking at low ARs. Increasing at higher ARs. Polymer thinning at higher power further reduces selectivity between SiO2/Si3N4 Animation Slide University of Michigan Institute for Plasma Science & Engr.

University of Michigan Institute for Plasma Science & Engr. Increasing 5 MHz Power: 2.5 5 7.5 10 kW ONO VIA: 5 MHz POWER Higher etch rate with power not only due to higher energy ion fluxes as chemical sputtering saturates above a few keV. Higher rates due largely to energetic hot neutrals able to reach deep into features – thermal neutrals cannot Waviness at low power (thick polymer) Higher ion fluxes sputter the sidewall at grazing incidence Removing the necking at low ARs. Increasing at higher ARs. Polymer thinning at higher power further reduces selectivity between SiO2/Si3N4 University of Michigan Institute for Plasma Science & Engr.

University of Michigan Institute for Plasma Science & Engr. Increasing 5 MHz Power: 2.5 5 7.5 10 kW ONO VIA: 5 MHz POWER Higher etch rate with power not only due to higher energy ion fluxes as chemical sputtering saturates above a few keV. Higher rates due largely to energetic hot neutrals able to reach deep into features – thermal neutrals cannot Waviness at low power (thick polymer) Higher ion fluxes sputter the sidewall at grazing incidence Removing the necking at low ARs. Increasing at higher ARs. Polymer thinning at higher power further reduces selectivity between SiO2/Si3N4 University of Michigan Institute for Plasma Science & Engr.

ONO VIA: RATE vs 5 MHz POWER ARDE roughly same for all powers. Larger surface to volume ratio in via vs trench results in more exposed surface area – more severe ARDE. Higher etch rates for higher power, with fluctuations when etching alternating oxide and nitride layers. Etch rate saturates at high power Chemical sputtering rates scale as 1/2 Re-deposition of etch product due to conductance limit Higher voltage delivers more neutral radicals to etch front. Etch rate eventually limited by neutral transport to etch front since radical flux into feature is constant. University of Michigan Institute for Plasma Science & Engr. AVS2018

University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS Computational investigation of high aspect ratio ONO stack etching using fluorocarbon plasmas. Effect of polymer deposition/removal on etch profile: Increasing physical sputtering of polymer by ions reduces twisting and increases etch rate. Increasing thermal etching of polymers by F and O alleviates tapering of etch front at the cost of more bowing at low ARs. Etch rate (ER) generally decreases with increasing AR (ARDE), with oscillating ER when proceeding through ONO layers. Fluxes of thermal neutrals to the etch front decrease with increasing AR due to conduction limit. Hot atom/radical flux maintained by specular reflection of ions. Due to larger surface-to-volume ratio, vias more sensitive to polymerization processes than trenches. University of Michigan Institute for Plasma Science & Engr. AVS2018