1. PLASMA ETCHING OF HIGH ASPECT RATIO OXIDE-NITRIDE-OXIDE STACKS*
Shuo Huang, Chad Huard and Mark J. Kushner
University of Michigan, Ann Arbor, MI USA
Sang Ki Nam, Seungbo Shim and Wonyup Ko
Samsung Electronics Co., Republic of Korea
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

2. 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

3. 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)
University of Michigan
Institute for Plasma Science & Engr.
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4. 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
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5. 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, (2015).
blog.lamresearch.com/tech-brief-memory-grows-up-with-3d-nand
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6. 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

7. 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.
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8. 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

9. 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

10. 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

11. 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: 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.
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12. 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.
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13. 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.
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14. 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.

15. 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.

16. 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.

17. 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

18. 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.

19. 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.

20. University of Michigan Institute for Plasma Science & Engr.
Increasing etch probability
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.

21. University of Michigan Institute for Plasma Science & Engr.
Increasing etch probability
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.

22. University of Michigan Institute for Plasma Science & Engr.
Increasing etch probability
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.

23. University of Michigan Institute for Plasma Science & Engr.
Increasing etch probability
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.

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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/ kW
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30. University of Michigan Institute for Plasma Science & Engr.
Increasing 5 MHz Power: 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.

31. University of Michigan Institute for Plasma Science & Engr.
Increasing 5 MHz Power: 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.

32. University of Michigan Institute for Plasma Science & Engr.
Increasing 5 MHz Power: 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.

33. 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

34. 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