Enhanced imaging in Extreme UV lithography by optimising the Molybdenum/Silicon thickness ratio in 2-D phase shifting mask design

Enhanced imaging in Extreme UV lithography by optimising the

Molybdenum/Silicon thickness ratio in 2-D phase shifting mask design

A.M. Nugrowati, A.S. van de Nes, S.F. Pereira, J.J.M. Braat

Department of Imaging Science and Technology

Delft University of Technology

Lorentzweg 1, 2628 CJ, The Netherlands; e-mail: a.m.nugrowati@tnw.tudelft.nl

Phase shift mask (PSM)

k₁ = f(resist,mask,illumination)

ÆResolution enhancement with PSM of 180o_{phase shift}

Resolution :

Extreme UV

13.5nm

Complex refractive index

(absorbing & low optical contrast)

The optimisation of phase shift mask to enhance the image of mask patterns in extreme UV lithography is discussed.

Structure of PSM

•

Attenuated : with absorber for intensity modulation

•

Chromeless : no absorber Æ high intensity in both regions

•

Etch-stop layer : controls Si/Mo pairs to be removed for 180o _shift

Problems:

Methods

Results

2-D Fourier Modal Method

lateral shift & imbalanced reflectivity

θ is given by the system, height (h) can be optimised, refl.(R) & height(h) decrease for higher Γ(Mo absorption)

Chosen requirements: ∆Φ = 180o_{± 5}o

1-D calculation(planar structure)

Advanced PSM for extreme UV

ÆMaterials used at 13.5nm-wavelength:

Molybdenum (Mo): higher optical contrast but higher absorption Silicon (Si) : lower absorption but lower optical contrast ÆMultilayer Bragg-mirror mask structure in vacuum (n=1)

Γ = Mo thickness_{(Mo+Si) thickness}

Solution: OptimiseΓ1& Γ2(adequate R & minimum h)

>0.65 >0.65 Chromeless <0.15 >0.65 Attenuated non-etched etched R (on structure) PSM

Non-etched planar structure θ R1,φ1 Reference position θ R2,φ2 h (vacuum thickness)

Etched planar structure

2-D calculation (periodic structure; L = 40 λ) • Collimated point source illumination (TE&TM) • Near field on mask using Fourier Modal Method (evaluation: far field in image plane with NA=0.25)

Optimisation of lateral shift & reflectivity balance

[ ]

1

(

2

[ ]

1

)

TM 0 k

γ

− ₋ − ₌ -1 ε I α ε α H H

[ ]

(

2

)

TE 0 k ε −αα E=

γ

E TE-pol: TM-pol: 2 0sin m Lπm k

α

= +

θ

Find Γ1&Γ2to reduce h

Field calculation with varying permittivity ε(x) and periodicity L

Æangular Fourier frequency for mode m Æpermittivity Fourier expansion

Conclusion

= boundary between the structure and vacuum

• Comparison with Γ1=Γ2=0.4 (current EUV PSM structure)

• Analysis for θ=6o_{and TE polarisation}

• Reduced lateral shift for optimisedΓ1 andΓ2 in both designs

• More balanced reflectivity between averaged R₁and R₂; reduced peak intensity on the edge of the structure 0 5 10 15 20 25 30 35 40

0 0.5 1 1.5

x-axis direction (λ unit)

Refl ecti vi ty ( T E -pol )

Lateral shift vs. Incident angle (Attenuated PSM)

non-etched region etched region lateral shift θ = 9 o θ = 6 o θ = 3 o R efle ctiv ity (T E -p ol ) 0 5 10 15 20 25 30 35 40 0 0.5 1 1.5

x−axis direction (λ unit)

Re fle cti vi ty-(T E -p ol )

Trench (h) vs. Reflectivity balance (Chromeless PSM) h = 5λ

h = 10λ

non-etched region etched region θ = 0ο

avg R2

avg R1

( )

ε

x =

∑

Mm=−M

ε

mexp(i

α

mx) ; M = total number of modes

0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 5 10 15 20 25 30 35 40 0 5 10 15 20

non−etched region etched region

TE−pol intensity of the optimised Attenuated mask

z− ax is ( λ uni t) x−axis (λ unit) 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 x−axis (λ unit) z− ax is ( λ uni t)

TE−pol intensity of the optimised Chromeless mask

0 5 10 15 20 25 30 35 40 0

5

10

15

20 non−etched region etched region

It is shown that the PSM design optimisation by controlling the thickness ratio of the multilayer structure reduces lateral shift and imbalanced reflectivity of the imaged periodic pattern.

Res = k_1NAλ

MASK

E-field

Intensity

0O ₁₈₀O

Conventional binary mask Phase shift mask

Substrate

Non-etched region Etched region θ

θ R1,φ1 R2,φ2

2nm SiO2 capping and/or Cr absorber layer(s)

Top multilayer: p Si/Mo pairs,Γ1

Base multilayer: 30 Si/Mo pairs,Γ2

7nm SiO2 etch-stop layer

L (periodicity) X Y Reference position ∆φ=φ1−φ2 3o_<θ<9o h (trench height) 0 0.5 1 1.5 0 5 10 15 20 25 30 35 40 0 0.2 0.4 0.6 0.8 1 x−axis (λ unit) Intensi ty opt: (Γ1=0.54;Γ2=0.59) ; h=4.5λ initial: (Γ=0.4) ; h=8.9λ Image on mask

Far field in image plane

non−etched region etched region

TE polarisation intensity of Attenuated PSM

0 0.5 1 1.5 2 0 5 10 15 20 25 30 35 40 0 0.5 1 1.5 Image on mask

Far field in image plane

etched region non−etched region

TE polarisation intensity of Chromeless PSM

In te ns ity x−axis (λ unit) opt: (Γ1=0.55;Γ2=0.6) ; h = 6.44λ initial: (Γ1=Γ2==0.4) ; h = 11λ

In general, the optimisation method allows for a complete control of all design parameters to produce the required image pattern in the far field. Re( ) 1

Im( ) 0

n n >

∼

This work is done in the framework of the European Project More Moore (IST-1-507754-IP)