Mask Cleaning Mechanisms and Techniques - Sematech

EUV mask blanks 

& 

Advanced cleaning techniques 

Abbas Rastegar 

EUV Mask Blank Development 

EUV Mask Technology Workshop 

February 2004

Outline 

• EUV mask blanks 

• Cleaning requirements of EUV mask substrates 

• Current quality of EUV mask substrates and defects 

• Particle adhesion mechanism in liquids 

• Advanced mask cleaning techniques 

• Conclusion 

A. Rastegar International SeMaTech 

2

EUV Mask Terminology 

• EUV Bulk Material: Initial bulk Low Thermal Expansion Material (LTEM) 

having intrinsic properties of CTE, homogeneity, stiffness, etc. 

• EUV Substrate: The finished 152mm x 152mm x 6.32mm (6” x 6” x 0.25”) 

LTEM before any reflective or absorber films are deposited 

• EUV Mask Blank: The finished 152mm x 152mm x 6.32mm (6” x 6” x 0.25”) 

blank with all reflective, buffer, absorber films completed 

• EUV Mask: Completed patterned reticle that is scanned exp. system 


3

EUV Masks 

‰ Thermally stable substrate materials (LTEM; 

ULE ® , Zerodur ® ) 

‰ Surface planarity (flatness) < 50nm peak to 

valley 

‰ Multilayer reflector is similar to optics 

elements, but 

‰ Requires zero defects in or on multilayer 

‰ Buffer and absorber layer are etched to 

create the mask pattern 

‰ No pellicle 

‰ Mask requires ultra clean storage and 

handling 

Patterned EUV mask 

Absorber 

Patterned Absorbers 

~ 100 nm thick 

(e.g. Al, Cr, TaN, W) 

Buffer Layer 

~ 50 nm thick 

(e.g. SiO 2 ) 

Reflective Multilayers 

~ 300 nm thick 

(Mo - Si = 13.5nm) 

40 - 50 Pairs 

φ 1 

6 

o 

LTEM Sub. 

Low thermal expansion substrate 

Buffer 

Multilayer 

Low thermal expansion 

substrate 

Mask cross section 


4

EUV Substrate specifications 

6.35 ± 0.1 mm 

152.0 ± 0.1 mm 

142 mm 

Backside flatness quality area: 

(500 nm) 

50 nm P-V flatness 

HSFR 1 µm 

Critical 

142 mm 

Defect quality area: 

0 defects >50 nm PSL 

equivalent 

Edge region flatness: 

152 mm 

HSFR λ spatial > 400 nm 

Substrate material: 

CTE < 30 ppb/ ºK at 22±3ºC 

Critical 

Critical 


5

Cleaning aspects of EUV lithography 

Conventional Photolithography 

EUV lithography 

UV 

EUV 

•Transmission 

•Pellicle 

•Reflection 

• NO Pellicle 

Mask cleaning in fab is required 

A. Rastegar International SeMaTech

Outline 








7

EUV Mask Blank Development Center 

Development substrates 

Super polished quartz substrate with Cr in the back 

side 

Proposed process flow 

Substrate clean Æ defect inspectionÆ ML deposition 

ÆML clean Æ defect inspection 


8

Cleaning requirements 

ITRS Roadmap: Defect density = 0.003 @ 32nm 

We have to develop cleaning processes that are capable 

of cleaning sub-100 nm particle on non-patterned 

1. quartz surface with Chromium back side 

2. Si cap layer on top of MoSi ML film on quartz substrate 

with Chromium back side 

Surface roughness: No measurable change G =0 

Surface flatness: No measurable change G’ =0 

*This presentation only discuss quartz surface and MoSi Ml will be presented separately 


9

Outline 








10

Quartz substrate 

Typical roughness:(1.5-1.9 nm RMS ) 


11

Natural defects: maps 

Natural defects: any defect that is detected by our inspection tool when we get them from suppliers 

Sample1-Cr backside 

Sample 2-Cr backside 

Sample 3- No Cr 


12

Natural defects: size distribution 

400 

350 

300 

Inspection capability 

Sample 1 

Sample 2 

250 

Defect counts 

200 

150 

100 

50 

0 

-50 

Source: A. Rastegar 

0 50 100 150 200 250 300 350 

defect size (nm) 


13

Natural defect: removability 


14

Natural defect: Compositions 

Mostly Carbon, SiO2, and traces of Cu, Al, N 

Thanks to Dan Cochran-Sematech 


15

Outline 








16

Particles & surface 

Adhesion 

Deposition 

Removal 

• Brownian motion 

• Gravity 

• Inertial motions 

• External fields 

Will not be discussed here 

Mask 

• Electrostatic force 

• van der Waals force 

• Born repulsion 

• Dissolve/decompose 

• Detach by under 

etching 

• Detach by collision 

• Shear forces 


17

Particle Removal 

Particle attached to 

the surface 

• Breaking the 

vdW forces 

• Lift-off from the 

surface 

• Transport away 

from surface 

• under etching 

• Repulsive forces 

• Diffusion 

• Zeta potential 

• Convection 


18

Adhesion Forces in the liquid 

diffusion 

Liquid flow 

• Van der Waals interaction :(3nm) 

V 

vdW 

= 

A 

6 

H 

⎛ d 

p 

(2x 

+ d 

p 

) 

⎜ 

⎝ 2x( 

x + d 

p 

) 

⎛ ( x + d 

+ ln 

⎜ 

⎝ x 

p 

) ⎞⎞ 

⎟ 

⎟ 

⎠ 

⎠ 

Wafer surface 

• Electrostatic interaction : (N -1 )Debye-Huckle length 

(thickness of the electrostatic double-layer) 

Diffusion layer 

V 

el 

Ψ : 

ε d 

8 

⎛ 

⎜ 

⎝ 

2 2 

( Ψ + Ψ ) 

2 

⎛ e 

ln 

⎜ 

⎝ e 

electrostatic surface potential (particle , 

−1⎞ 

⎟ + 2Ψ Ψ 

x 

p 

⎠ 

wafer) 

⎛ e 

ln 

⎜ 

⎝ e 

κ x 

κ x 

p 

= ⎜ 

p w 

2κ 

w κ x 

2 

2000e 

N 

εk 

T 

+ 1⎞⎞ 

⎟ 

1 

⎟ 

− 

⎠⎠ 

• Short range interactions : (1-3 nm) solvation and other 

types of steric forces (i.g. attractive hydrophobic forces, chemical bond) 

κ = 

B 

A 

I 

d p 

= 2R 

ε w 

Particle 

x 

r 

Notation info 

last page 


19

Particle-Surface interactions in the liquids 

origin of ] potential 

Shear 

plane 

Double Diffusion 

layer layer 

+ 

+ + + 

+ - - 

+ 

+ + 

+ 

+ + 

+ + - + 

+ + 

+ + 

+ - 

+ + 

+ 

+ 

+ + 

- + + 

- 

+ 

- 

- 

- 

+ 

+ 

+ 

+ 

+ 

- 

+ 

Bulk 

liquid 

- + 

+ 

- 

+ 

+ + - 

- 

+ 

- + + 

- 

+ + + 

- - - + 

+ 

- 

+ 

- 

- -- - +++ + + 

+ - + + - 

- + 

- 

+ 

- 

+ 

- - 

- 

+ 

- - + 

- - - - - 

- + + + 

- - 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ + 

+ 

+ 

+ 

+ 

+ 

+ 

- 

- 

- 

+ 

+ 

+ 

- 

+ 

Potential Energy 

Ψ w 

Ψ S 

ζ w 

x 

Notation info 

last page 


Ψ p 

Ψ S 

ζ p 

x 


20

Particle-Surface interactions in the liquids 

Use of ] potential for lift off 

Particle close to surface but not attached 

U = V + V + V 

interactio n 

vdW 

w 

el 

p 

el 

U 

interactio n 

> 0 

Uinteractio n 

< 0 

repulsion 

attraction 

x 

V >> V ⎯→U 

interactio n 

≈ V + V 

el 

vdW 

w 

el 

p 

el 

ζ 

ζ 

ζ 

ζ 

p 

p 

p 

p 

< 0 , ζ 

> 0 , ζ 

< 0 , ζ 

> 0 , ζ 

w 

w 

w 

w 

< 0 

> 0 

> 0 

< 0 

repulsion 

deposition 


Ψ w 

Ψ S 

ζ w 

x 


Ψ p 

Ψ S 

ζ p 

Notation info 

last page 


x 

21

Adhesion in liquids (summary) 

• Ionic strength of liquid(I) :Range of electrostatic interaction 

I V el 

V vdW 

> V el deposition 

• Zeta potential(]) : 

1 

= 

κ 

εk 

T 

2000e 

N 

B 

1 

∝ 

2 

AI 

I 

ζ 

p 

,ζ w 

ζ 

p 

,ζ w 

the 

same 

opposite 

sign 

sign 

• Particle size(d p ) :(figure) 

Diffusion constant (D) 

1 

D ∝ 

d p 

repulsion 

attraction 

Notation info 

last page 

Particle adhesion rate 

10 -2 Log/log 

Particle diameter 

(Pm) 

10 


22

Controlling of the particle adhesion in liquids 

• Deposition Removal process can be controlled by 

controlling the Zeta (])potentials 

• Zeta potential(]) :can be controlled by adding 

surfactants to acidic solutions 

Hydrophobic 

] potential (mV) (HCL, pH 3.3) 

Surfactant No Anionic Cationic 

• Surfactants 

part 

Hydrophilic 

Anionic (R-SO3 - ) 

Cationic (R-NH3 + ) 

Nonionic 

Si -23 -32 +63 

Si 3 N 4 +43 -52 +45 

SiO 2 +7 -7 +55 

Polystyrene +39 -67 +78 

particles 

Si-PSL d R R 

Source: M. Itano et.al. Daikin Industries 

part 


23

Outline 








24

Advanced mask cleans 

• Single mask wet cleaning 

– Megasonic effect 

– Marangoni dry 

• Super critical fluid cleaning 

• Cryogenic aerosol cleaning 

• Shock wave cleaning ( Next speaker) 

• Remote plasma cleans 

• Plasmax 


25

Typical Wet Cleans 

Object Chemical Popular 

name 

Particles APM(NH 4 OH/H 2 O 2 /H 2 O)(1:1:5) RCA (SC-1) 

Organics SPM(H 2 SO 4 /H 2 O 2 ) (5 or 8:1) Piranha 

APM(NH 4 OH/H 2 O 2 /H 2 O) 

Metalics HPM(HCl/H 2 O 2 /H 2 O) (1:1: >5) 

SPM(H 2 SO 4 /H 2 O 2 ) 

DHF (HF/H 2 O) 

Native Oxides DHF (HF/H 2 O) (1:>50) 

BHF (NH 4 F /HF/H 2 O) 

RCA (SC-1) 

RCA (SC-2) 

Piranha 

Diluted HF 

Buffered HF 

UV 

SPM 

SC-1 

O3 rinse 

Dry 


26

Megasonic & Ultrasonic Cleans 

Ultrasound 

100 kHz 0.8 1.2 MHz 

Cleaning 

Megasound 

• By reducing the boundary layer 

increases transport of chemicals 

• Physically transfer energy to the 

particle and helps in removal 

• Cavitation 

Boundary layer 

Acoustic wave 

Shear 

velocity 

• Higher frequency leads to better removal of 

small particles and less cavitation 

• Less effective to sub 100 nm particles 


27

Drying 

IPA drying :IPA evaporates and vapor remove 

water droplets form surface 

Marangoni drying 

• IPA diffuse into water close to surface (A) than 

far from surface (B) 

• IPA concentration gradient leads to surface 

tension gradient(J B > J A ) 

• Marangoni flow goes from low J A towards high 

J B 

• Marangoni flow can also remove particles 

IPA +H 2 O 

IPA 

IPA vapor 

Marangoni flow 

water B A 

coolers 


28

Super critical fluid cleans 

SC CO 2 :T C =31.06 C, P C =73.8 Bar 

No surface tension :Excellent wet-ability 

to all surfaces- (deep vias) 

Co-solvent(1%) :required for cleaning 

P 

Solid 

Sublimation 

Gas 

Melting 

Boiling 

Fluid 

Super 

critical 

P c 

Compatibility :compatible with low-k 

T 

T c 


29

Cryogenic Aerosol Cleaning ( Ar, CO 2 ) 

Aerosol :Created by cooling a gas and rapid 

expansion 

Cleaning mechanism 

Cleaning mechanism 

Solid Ar 

Particle 

Solid Ar shell 

Pressure 

Solid 

Gas 

Sublimation 

Melting 

Temperature 

Fluid 

Boiling 

Super 

critical 

T c 

P c 

Liquid 

Collision 

Liquid flow 

Sublimation/Evaporation 

Thermal stress 

Source: M. Okada et.al. J.Vac. Sci. Technol. B20, 71(2002) 


30

Remote Plasma Cleans 

Log Active Species Concentration 

Interesting Operation Area 

O* H* 

Oxidizing 

Reducing 

Plasma flow 

Sapphire disk 

Wafer 

Exhaust 

Metal window 

Quartz window 

H/O Atomic Ratio in Plasma 

• Oxidation: CHNSO +O* Æ CO 2 + H 2 O+ NO 2 + SO 2 

• Reduction: CHNSO +H* Æ CO 2 + (-C-H X )+ CN + H 2 S 

After Etch After dry plasma After rinse 


31

PLASMAX 

Working mechanism: 

• Mechanical excitation 

• Plasma properties 

• Surface chemistry 

gas feed 

Plasma 

• gas flow 

vibration 

issues: 

• acceleration uniformity across the 

wafer 

vibrator 

• mounting conditions 

Source: W. H. Semke et.al. J.Vac. Sci. Technol. B18, 3221(2000) 


32

Summary & Conclusions 

Substrate 

• Quartz plates with Cr in backside show higher defects than normal quartz surface 

• Most of substrate defects are particles 

• Larger particles composed of C, SiO2, Al,Cu 

• Surface roughness and flatness should not be changed by cleaning 

Removal 

Wet clean still is favored 

• Easier to remove metals (high solubility) and particles (] control) 

• Energy gain in separation of particles in liquid compared to gas 

• Possibility of megasonication Æ better energy transfer 

• Single chemistry (Ozone based) + Marangoni dry is promising 


33

Other Cleaning Techniques 

Remote plasma 

+ Etch residues, organics 

- Plasma damage, particles 

• Supercritical fluids + HAR structures 

- Require co-solvent 

• Cryogenic Aerosols + Good particle removal, 

- Substrate damage 

• PLASMAX + Particle removal 

- Uniformity 

• Liquid Assisted Laser + No aggressive chemistry 

- Substrate damage 


34

Backup 


35

] potential and pH of solution 

• Iso-Electric Point(IEP) (Point of zero charge) 

positive and negative charges are equal 

IEP 

IEP 

IEP 

SiO 

3 

2 

2 

Si N 

4 

Al O 

⎯→ 

3 

⎯→ 

⎯→ 

pH ≈ 2 − 3 

pH ≈ 3 − 4 

pH ≈ 8 

• ] w depends on surface electrostatic potential 

and ionic strength 

Surface Energy 


Ψ w 

Ψ w 

Ψ S 

ζ 

w 

pH 

x 

IEP 

pH below IEP 

pH above IEP 

pH Ψ w 

ζ w 

pH Ι ζ w 

pH Ψ w 

ζ w 

Ionic strength 

I 

HCl 

IEP 7 ( H 2 O ) 

NaOH 

pH 

( H 2 O ) 

pH Ι ζ w 

Notation info 

last page 


36

37 


velocity 

particle 

P 

velocity 

fluid 

f 

coefficient 

diffusion 

mutual 

component 

of 

fractions 

mole 

mass 

particle 

mass 

s 

component 

gas 

v 

v 

D 

m 

m 

P 

: 

: 

12 

’ 

: 

: 

: 

: 

1,2 

1,2 

γ 

Notations 

Notations 

es 

ch 

of 

number 

Z 

diameter 

particle 

density 

particle 

gas 

coefficient 

Thermophoretic 

K 

density 

particle 

factor 

correction 

Cunningham 

velocity 

particle 

p 

p 

r 

p 

c 

x 

d 

C 

v 

arg 

: 

viscosity 

: 

: 

: 

: 

: 

: 

: 

ρ 

µ 

ρ 

wafer) 

(particle, 

potential 

surface 

atic 

relectrost 

area 

contact 

of 

radius 

constant 

Hamaker 

wafer 

of 

constant 

: 

, 

: 

: 

: 

w 

P 

H 

w 

r 

A 

dielectric 

Ψ 

Ψ 

ε 

field 

diffusophoretic 

in 

velocity 

particle 

df 

field 

thermophoretic 

in 

velocity 

particle 

th 

field 

electric 

external 

in 

velocity 

particle 

e 

field 

gravitational 

in 

velocity 

particle 

g 

Bultzmann 

v 

v 

v 

v 

k B : 

: 

: 

: 

constant 

: 

Zeta potentials 

Ionic strength 

number 

Avagadro’s 

: 

, 

: 

: 

ζ p ζ w 

I 

N A

Mask Cleaning Mechanisms and Techniques - Sematech

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