wetting of deep hydrophilic nanoholes by aqueous solutions · 9/1/2020 · public wetting of deep...
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PUBLIC
WETTING OF DEEP HYDROPHILIC NANOHOLES BY
AQUEOUS SOLUTIONS
GUY VEREECKE1*, AUDREY DARCOS2, HIDEAKI IINO3, FRANK HOLSTEYNS1,
AND EFRAIN ALTAMIRANO SANCHEZ1
1IMEC, KAPELDREEF 75, 3001 LEUVEN, BELGIUM *[email protected], 26 AVENUE DE LESPINET, 31400 TOULOUSE, FRANCE
3KURITA WATER INDUSTRIES LTD., 1-1, KAWADA, NOGI-MACHI, SHIMOTSUGA-GUN, TOCHIGI, 329-0105,
JAPAN
PUBLIC
NANOCONFINEMENT IN SEMICONDUCTOR MANUFACTURING
2
Source: LAM Research
Source: V. Vega-Gonzalez, IEDM Tech. Digest (2019)
3D-NAND memoryLogic Fin & Nano Sheets FET Logic BEOL Supervias
Supervia from M3 to M1
at 3 nm node
Source: S. S.-W. Wang, Semicond. Eng. (2018)
Hole CD 45-13 nm
Height = 95 nm
• Post-etch cleans
• STI oxide recess
• Selective semiconductor
etches
• RMG back etches
Hole CD 65-100 nm
AR ≥ 60
1D nano-confinement
2D nano-
confin’t
Courtesy: Y. Oniki, imec
PUBLIC
WETTING AT THE NANOSCALE
▪ Fluid transport is diffusion-limited in nano-
structures
▪ No impact of convection at wafer surface
3
SIMULATIONS PREDICT FAST WETTING
▪ Short wetting times from fast diffusion
▪ Diffusivity not expected to be an issue
▪ Diffusion rates calculated from the average
squared net diffusion distance: 𝑥 = 2𝐷𝑡
Source: M.T. Fuller & D.W. Hess, JECS (2003)
Wetting time of a 1 mm via
CD = 10 nm
100 nm
SoluteD
(1E-9 m2/s)
Diffusion
rate
(mm/s)
H+ 9.31 136
Cu2+ 0.71 38
Cl- 2.03 64
SO42- 1.07 46 T = 25°C
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WETTING CHARACTERIZATION
▪ Microscope inspection – nanofluidics
→Apparent viscosity increased by up to 4
▪ Not applicable to nanoelectronic
structures
4
▪ High-frequency acoustic reflectometry
→ Detection of partial wetting in Deep Trench Isolation structures – AR ~ 20,CD = 200 nm
Moving meniscus in a 11 nm
deep, 20 mm wide, nanochannel
Sources: V.N. Phan, Langmuir (2010), K. Mawatari, Anal. Chem. (2014)
Source: C. Virgilio et al., Solid State Phenom. (2016)
CHARACTERIZATION OF WETTING
BY ATR-FTIR
Nicolet 6700 FT-IR spectrometer
MCT detector cooled by liquid N2
Customized flow cell on ATR accessory
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CHARACTERIZATION BY IN-SITU ATR-FTIR
Nanoholes in
SiO2
Heating mattress (RT – 90°C)
Liquid cell
Solution injection port
Gas inlet
• N2 for in-situ drying
• CO2 to characterize diffusivity & permittivity
Si ATR crystal
• with blanket films
• with nano-channels/holes
q = 2aa
90
L*
L
t
a
≈≈
≈≈
Nanochannels in Si
▪ Backside heating
▪ DT BS / surface solution = 10°C at 90 °C
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CHARACTERIZATION OF WETTING BY IN-SITU ATR-FTIR
7
DETERMINATION OF THE OH STRETCHING / BENDING RATIO
OH
stretching
OH
bending
Penetration depth of evanescent wave at Si / H2O – q = 60°
Source: N. Vrancken et al., Langmuir (2016)
OH
stretching
peak ...
... is more sensitive to wetting
of nanostructures vs. ...
Wetting of nanopillars by UPW
I stretching / bending upon wetting
... OH bending peak
that is more
sensitive to bulk
PUBLIC
▪ OH stretching peak
▪ Water in nanoholes presents a
band at much lower frequency vs.
ice
CHARACTERIZATION OF WATER STRUCTURING
8
▪ Interpretation based on number of H-bonds
▪ But most H2O molecules are engaged in 4 H-
bonds (Chaplin, http://www1.lsbu.ac.uk/water)
→ Interpretation based on stiffness of H2O
network ?
Bulk water
Ice
Water in
nanoholes Free
OH
Complete
tetrahedral
coordination
Incomplete
tetrahedral
coordination
“ice-like”
water
Yalamanchili et al.,
Langmuir (1996)
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WATER STRUCTURING BY DISSOLVED IONS
▪ Bulk solutions
9
▪ Salt solutions in 32-nm nanochannels
▪ Trends afo salts in coherence with ions in solutions
▪ Interpretation as D H-bonds is doubtful over large distances
Structure-breaking salts Structure-making salts
Difference ATR-FTIR spectra 1 M salt solution - water
DGHB : average change in the number of
hydrogen bonds per water molecule
Source: Y. Marcus, Chem. Rev. (2009)
Ions Category
I-
Structure
breaking
ions
Br-
Cl-
SO42-
Na+ Borderline ions
Ca2+
Structure
making
ions
PO43-, Co2+
Fe2+
DGHB
-0.1
0.1
0.7
1.1
-1.1
-0.9
-0.5
WETTING OF
DEEP
NANOHOLES
Holes in PEALD SiO2 on Si
Depth ~ 300 nm
CD ~ 20 nm
Volume ~7%
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▪ Water structuring when wetting nanoholes
▪ Heating accelerates diffusion & modifies
water structure,
but does not suppress water structuring
EVIDENCE FOR UNCOMPLETE WETTING
11
▪ OH stretching / bending ratio vs. T & time
▪ Hysteresis is proof of uncomplete wetting
▪ Generation of gas pockets / nanobubbles with
unexpected long lifetime1
Wetting
of gas
pockets
Bulk water on top of holes
30°C
90°C90°C
30°C
Water structuring
in nanoholes
Source: Y.S. Ljunggren et al., Colloids Surf. A (1997).
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CO2 DISSOLUTION IN NANOHOLES FILLED WITH UPW
▪ No equilibrium after 12 hrs
▪ About 10 min needed to achieve
equilibrium in bulk UPW
▪ Decreased diffusivity likely due to
water structuring1
12
▪ High solubility
▪ About 50 X level of bulk UPW
▪ Likely due to decrease of permittivity from
water structuring
Bulk UPW
K. Morikawa et al., Anal. Chem. (2015)1 T. Tsukahara et al., J. Phys. Chem. B (2009) P. Fogg, Solubility Of Gases In Liquids (1991)
pCO2 = 1 atm
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WETTING KINETICS
▪ Characterization
13
▪ Ion effect at RT – 30°C
▪ Large variability likely from random
formation of gas pockets
▪ Long wetting times
▪ Not practical for manufacturing
▪ No significant effect of salt
addition
Temperature effect
Large variability but significant
effect: faster wetting at higher T
Heating needed for
manufacturability
Only backside heating used
99 % wetting
Nanoholes
Bulk solution
on top of nanoholes
90 % wetting
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EFFECT OF DISSOLVED SALTS ON WATER STRUCTURING AT RT
▪ OH stretching peaks
▪ Slight decrease of structuring by dissolved salts
14
▪ Difference spectra salt – UPW
▪ Both salts showed structure breaking
charateristics
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▪ Lower solubility in NaI solution vs.
UPW, indicating some restoration of
water properties using a structure
breaking salt – but slower diffusion
▪ Slower diffusion in CoCl2 solution,
with global structure making properties
– but structure breaking characteristic
in FTIR
▪ No simple relation btw structure
breaking / making properties and
characterisation by FTIR
EFFECT OF DISSOLVED SALTS ON DISSOLUTION OF CO2
▪ Dissolution of CO2 at RT & 1 atm PCO2
▪ Equilibrium achieved in ~ 2 hrs with NaI
Equilibrium level in bulk
obtained in ~10 min
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SUMMARY
▪ Wetting of 20 nm nanoholes in an oxide matrix was accompanied by the formation of
gas pockets / nanobubbles,
likely stabilized by slow gas diffusion from water structuring
▪ Slower wetting and diffusivity & lower permittivity from water structuring
▪ Higher solubility of CO2 (gases) (and lower solubility of salts) from decreased permittivity
▪ Partial restoration of water permittivity by the dissolution of salts , but not of diffusivity and
wetting rate
▪ More salts to be tested
▪ Wetting rate increased by backside heating
▪ Solution heating needed for manufacturing
depending on hole depth
▪ But nanobubbles issue not solved
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CONFIDENTIAL17