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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 *guy.vereecke@imec.be2INSA, 26 AVENUE DE LESPINET, 31400 TOULOUSE, FRANCE

3KURITA WATER INDUSTRIES LTD., 1-1, KAWADA, NOGI-MACHI, SHIMOTSUGA-GUN, TOCHIGI, 329-0105,

JAPAN

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NANOCONFINEMENT IN SEMICONDUCTOR MANUFACTURING

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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

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WETTING AT THE NANOSCALE

▪ Fluid transport is diffusion-limited in nano-

structures

▪ No impact of convection at wafer surface

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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

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▪ 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

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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

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▪ OH stretching peak

▪ Water in nanoholes presents a

band at much lower frequency vs.

ice

CHARACTERIZATION OF WATER STRUCTURING

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▪ 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

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▪ 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

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▪ 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

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▪ 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

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▪ 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

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▪ 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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