The suggestion of novel attenuated phase shift mask structure in
Extreme Ultraviolet Lithography
Hanyang University.
Chang Young Jeong1, Sangsul Lee1, Jonggul Doh1, Jae Uk Lee1, Inhwan Lee2, Seung-yu Rah3, and Jinho Ahn1*
Department
p
of Materials Science and Engineering,
g
g Hanyang
y g University,
y 2 Department
p
of Nanoscale Semiconductor Engineering,
g
g Hanyang
y g University,
y
3 Pohang Accelerator Laboratory,
The condition of aerial image simulation
Introduction
The phase shift mask in EUVL
What is EUV Lithography ?
Extreme ultra violet lithography (EUVL) using 13.5nm wavelength is expected to be the mainstream
of production process for 22nm half pitch and below. Mask shadowing is a unique phenomenon
caused by using of multilayer mirror-based mask with oblique incident angle of light. Reducing the
absorber thickness is the most effective method to minimize the mask shadowing effect. A phase
shift concept is a potential solution to improve the image contrast with a thinner absorber stack.
The concept of a phase shift mask for EUVL has been studied for a number of years. However,
there are many fabricating issues to be solved before it can be applied to manufacturing.
What is the mask shadowing effect?
EUVL could be more easily extended to 22nm node and below by applying PSM.
 However, it is very difficult to make a reliable and manufacturable PSM.
 What is the main factor to control the phase shift in EUVL mask?
 The structure of capping layer is one of the main factor to influence the phase shift.
 The optimization of capping structure is needed.

 Simulator: EM-SuiteTM (Panoramic Technology Inc.)
 Simulation condition
EUV Light
Angle of incidence = 6˚
The illumination beam is shadowed by the
edge of the absorber.
 The effective mask CD is changed.
 Printed pattern shifted and biased.
 Correction for shadowing effect should be
considered.
 Reducing the absorber thickness is the
key issue.
 How about using phase shift concept
to improve image contrast with thinner
absorber stack?

R2, Φ2
Parameter
Value
Wavelength
NA
Magnification factor
sigma
Incident angle
Pattern width
13.5nm
0.25, 0.32
4
0.6
6°
22nm 1:1 L/S
TaN
(Δr = propagation distance)
Absorber
Absorber
Phase
Shift layer
Phase
Shift layer
 Mask structure
TaN
TaN/capping structure/ML
Substrate
40.5nm
Schematic diagram of EUV PSM
Mo/Si Multilayer

Material
δ
β
TaN
0.0730
0.0436
40 pairs of
Mo/Si ML
Absorber stack
thickness (nm)
Structure
Phase shift
: |Φ1-Φ2| = 180˚
Phase shift (ΔΦ) = (2πδ/λ)*Δr
Shadowing Region
Height
70nm
R1, Φ1
Si
0.0010
0.0018
Mo
0.0761
0.0064
Ru
0.1137
0.0171
Refractive index (n)= 1-δ + ίβ
Mo shows the similar δ value with TaN absorber, where as, Mo shows the lower β value than
TaN
T N absorber.
b b
Simulation results of proposed PSM structure
Phase difference as a function of TaN absorber and various phase
shift layer thickness
Ru phase shift layer
Reflectivity on the absorber stack and image contrast
with Mo phase shifter thickness
Near field intensity profile with TaN, Mo thickness
Si phase shift layer
TaN 40.5, Mo 0(nm)
TaN 32.5, Mo 8(nm)
Mo phase shift layer

As the Mo phase shifter thickness increases, reflectivity from
the absorber stack increases.

However, image contrast does not decrease.

Reflectivity loss can be compensated with out of phase
condition.
0.25NA

The out of phase condition was achieved at the ~40nm absorber stack
(absorber + phase shifter) thickness.

As the Mo thickness increased, TaN absorber thickness latitude increased.
MEEF value for 22nm L/S 1:1 pattern pitch
0.32NA
0.25NA

0.32NA
Results of manufactured PSM stack
Horizo ntal
V ertic al
0nm Mo, 40.5nm TaN
11.1% EL, 2 27nm DoF
2 4nm Mo, 16 .5nm TaN
Level
1 6nm Mo, 24 .5nm TaN

The influence of phase shift on the imaging property including
image contrast, mask error enhancement factor (MEEF), and
process window was calculated using EM-SUITE simulation tool.

In addition, we manufactured the proposed PSM stack and
measured reflectivity.

The lithographic performances of manufactured attenuated PSM
will be investigated using coherent scattering microscopy (CSM)
installed at Pohang Accelerator Laboratory.

TEM image(left) and measured reflectivity(right) of manufactured
PSM consists of 16.5nm thick TaN absorber, 24nm thick Mo phase
shift layer on Mo/Si ML.

Measured reflectivity on the absorber stack was 16.8% at 13.3nm
peak wavelength.
2 1.4% EL , 167nm D oF
24nm M o , 16.5nm TaN
22.1% EL, 16 7nm DoF
defocus

16nm M o , 24.5nm TaN
13 .1% EL, 233nm D oF
d efocus
 As the Mo phase shifter thickness increases, PW increases
It can be anticipated that the asymmetry could be
reduced by the decrease of MEEF value.
In this paper, we suggest that one of the best ways to minimize
shadowing effect by applying the thin absorber stack using
phase shift concept.
concept
0nm Mo, 40.5nm TaN
20.4% E L, 17 0nm DoF

Future work
8 nm Mo, 32.5nm TaN
10.7% EL, 2 10nm DoF
As the Mo thickness increased, further increase of
asymmetry has not shown at vertical pattern.

TEM image and measured reflectivity of manufactured PSM
19.3% EL , 175nm D oF
8nm Mo, 32.5nm TaN

Summary and Future work
0.32NA
0.25NA
The Asymmetry of EM field could be still observed at
vertical pattern.
Summary
H-V overlapping PW for 22nm L/S patterns
depending on Mo phase shifter thickness
9.9% EL, 215n m D oF

Image contrast increases according to the increase of
reflectivity at 0.25NA illumination condition.
As the Mo phase shifter thickness increases, MEEF value
decreases for 0.25NA and 0.32NA condition.
H orizont al
Ve rtical
TaN 24.5, Mo 16(nm)
TaN 16.5, Mo 24(nm)
At TaN absorber + Mo phase shift layer = 40.5nm

Level
1
if you want to get further information about CSM, see these posters
→ Mask (MA) / MA
MA-P20
P20 / The study on influence of carbon
contamination on imaging performance of EUV mask using
CSM/ICS
→ Mask (MA) / MA-P21 / Application of the coherent
scattering microscopy on the EUV mask fabrication
→ Reticle Contamination (RC) / RC-P01 / Analysis of compound
on EUV mask surface and chamber circumstance using EUV
CSM/ICS and RGA
October 17~20, 2010 International Symposium on Extreme Ultraviolet Lithography, Kobe, Japan