Gray Assist Bar OPC
N. V. Lafferty
Mentor Graphics, IMEC, RIT
G. Vandenberghe
IMEC
B. W. Smith
Rochester Institute of Technology
M. Lassiter, P. M. Martin
Photronics
© imec 2004
Motivation for Experiments
As the OPC effect of a scatter bar increases, so can its
printability.
Scatter Bars, or SRAFs, enhance a main feature’s
resolution when placed at harmonics of the main bar’s
pitch.
Reducing the transmission of a typical opaque scatter
bar can maintain the OPC effect while reducing its
printability.
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
2
Why Gray Bars?
With Gray Bar OPC, some of the printability of
conventional opaque bars can be avoided.
1.2
Relative Intensity
1
0.8
200nm gray bar
CD = 70nm
0.6
0.4
0.2
0
-600
200nm Gray Bar
60nm Opaque Bar
-400
-200
0
200
400
600
Position [nm]
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
3
Outline
Introduction
Mask making
Experimental data

Proximity Correction

DOF Improvement

Through pitch process window optimization
Conclusions
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
4
The Reticle - Design
 Main
features (Cr-lines) are 70nm (1x) at various pitches
 Cr bias and gray assist bar width are varied for each pitch
(up to 90 variations per pitch)
Design
Gray
Mask
Cr
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
5
The Reticle - Manufacturing
Manufactured by Photronics
2 lithography steps

Self-Aligned Process
Gray
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
Cr
Gray
6
The Reticle – Profile
Profilometer scans of gray assist bar and Cr
900nm pitch
 2 different assist bar cases

© imec 2004
N. Lafferty – Gray Bar OPC/EXT
7
Graybar Optical Analysis
Intrinsic graybar transmission at 193 = 44.2%
Fused silica blank transmission at 193 = 90.2%
Relative transmission of Graybar at 193 = 49.0%
1.0
Graybar Material Transmission
Transmission
0.8
0.6
Exp pT 0°
0.4
0.2
0
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
200
400
600
Wavelength (nm)
800
1000
8
The Reticle – CD metrology
Cr-features were chosen to be 280nm (4x) through
pitch
Graybar CDs were linear
Smallest printed bar was 100nm (4x)
Measured Gray bar CD [nm, 4X]

700
600
500
400
300
200
100
0
0
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
200
400
600
800
Designed Gray bar CD [nm, 4X]
1000
9
Outline
Introduction
Mask making
Experimental data

Proximity Correction

DOF Improvement

Through pitch process window optimization
Conclusions
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
10
Experimental Conditions
Exposures
ASML PAS5500/1100 ArF 193nm Scanner
 0.75NA, 0.89σo/0.65σi Quasar™ illumination
 Exposed modules: 280nm (4X) Cr CD
 150nm Sumitomo PAR817 on 77nm Brewer Science ARC©29A

Metrology
Litho target: 70nm
 KLA 8250XR (for wafer and reticle SEM measurements)
 KLA ProDATA 1.4.1
 KLA PROLITH 8.0.3

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N. Lafferty – Gray Bar OPC/EXT
11
Impact on Diffraction Orders
Diffraction orders of a binary 1-D grating:
s
Mag . 0th order =  
 p
s

Mag . 1st order =   sin c
 p

s
s

p 
s
 2s 
Mag . 2nd order =   sin c 
 p
 p
p
 b
 
Mag . 0th order = 1 −   1 − I b  ⋅ 
 s
 
(
s

Mag . 1st order =   sin c
 p

)
s

p 
b
b
s
 − 1 − I b   sin c 
p
 p
 p
(
)
s
 2s 
b
 2b 
Mag . 2nd order =   sin c  − 1 − I b   sin c 
 p
 p
 p
 p
(
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
)
Cr
Gray
Diffraction orders of a 1-D grating with frequencypreserving gray assist bars:
Cr
b
12
Diffraction Information
240 nm Pitch
|Magnitude|
0.8
0.6
0.4
0.2
-NA
+NA
Primary orders
1
0.8
0.2
1
0.4
0.2
-NA
+NA
Primary orders
N. Lafferty – Gray Bar OPC/EXT
0.2
Primary orders
+NA
0.7670
0.8
0.6
0.4
0.2
0
-NA
Cr
Primary orders
1
Gray
0.6
0
Cr
0.6212
0.8
0.4
-NA
|Magnitude|
0.4
+NA
0.6
1
0.6
-NA
0.8250
0.8
0
Cr
0.6876
Primary orders
|Magnitude|
Cr
|Magnitude|
0
0
© imec 2004
1
0.6818
Gray
|Magnitude|
Gray Bar Size
|Magnitude|
1
400 nm Pitch
+NA
0.6800
0.8
0.6
0.4
0.2
0
-NA
Primary orders
+NA
13
Aerial Image Slices
Aerial Image Through Focus
240 nm Pitch
Relative Intensity
240 nm
Best focus
0.2µm defocus
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
500nm Pitch
-100
-50
150
500nm Pitch
Optimized Gray Assist Bar
1
0.3
0.2
© imec 2004
100
1.1
0.7
0.6
0.5
0.4
-150
50
Position [nm]
No Bar
1.1
1
0.9
0.8
0
Relative Intensity
Relative Intensity
Aerial Image Through Focus
-150
500 nm Pitch
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
-100
-50
0
Position [nm]
N. Lafferty – Gray Bar OPC/EXT
50
100
150
-150
-100
-50
0
Position [nm]
50
100
150
14
Isofocal Slices Through Pitch
Relative Intensity
No GrayIsofocal
Assist
SliceBars
Comparrison (No Gray Assist Bars)
50
60
70
80
90
100
110
120
Position [nm]
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
15
Isofocal Slices Through Pitch
Relative Intensity
With Gray
Assist Bars and Cr Bias
Isofocal Slice Comparrison (with Gray Assist Bars)
50
60
70
80
90
100
110
120
Position [nm]
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
16
Outline
Introduction
Mask making
Experimental data

Proximity Correction

DOF Improvement

Through pitch process window optimization
Conclusions
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
17
Proximity Matching
CD tuning with gray bars

At a given dose and focus, addition of gray assist bars can be used
to select cases through pitch with the same CD
120
Pitch [nm]
Wafer CD [nm]
100
240
260
280
300
350
400
500
80
60
40
20
Experiment
0
0
50
100
150
200
Gray bar width [nm, 1x]
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
18
Proximity matching
Experiment
100
100
80
80
CD [nm]
CD [nm]
Simulation
60
40
No gray bar
With gray bar
20
0
100
60
40
20
No gray bar
With gray bar
0
200
300
400
500
600
100
200
Pitch [nm]
300
400
500
600
Pitch [nm]
By adding gray assist bars,
proximity effect can be
corrected.
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
19
Outline
Introduction
Mask making
Experimental data

Proximity Correction

DOF Improvement

Through pitch process window optimization
Conclusions
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
20
DOF and PW with Gray Bars
Before and at Forbidden Pitch:
2nd order lies outside of pupil
 Graybar reduces 1st and 0th order and hence the image contrast
No Bar
50nm Bar
70nm Bar
10
8
6
4
2
Simulated
0
0
0.1
0.2
0.3
DOF [µm]
Ref: E. Hendrickx [5377-30]
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
0.4
Exposure Latitude [%]
Exposure Latitude [%]

No Bar
50nm Bar
70nm Bar
10
8
6
4
2
Experiment
0
0
0.1
0.2
0.3
0.4
DOF [µm]
21
DOF and PW with Gray Bars
Solution:
Cr bias before/at forbidden pitch to match 0th order
 Small increase in +/- 1st order
 0th order decrease
10
0nm Bias
5nm Bias
8
6
4
2
Simulated
0
0
0.1
0.2
DOF [µm]
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
0.3
0.4
Exposure Latitude [%]
Exposure Latitude [%]

10
8
6
4
0nm Bias
5nm Bias
2
Experiment
0
0
0.1
0.2
0.3
0.4
DOF [µm]
22
DOF and PW with Gray Bars
At pitches > forbidden pitch:

2nd order begins to enter pupil
Exposure Latitude [%]
12
10
8
6
No Bar
90nm Bar
110nm Bar
150nm Bar
250nm Bar
4
2
0
0
© imec 2004
8% EL
0.1
N. Lafferty – Gray Bar OPC/EXT
500nm Pitch
70nm CD
Experiment
0.2
DOF [µm]
0.3
0.4
23
Outline
Introduction
Mask making
Experimental data

Proximity Correction

DOF Improvement

Through pitch process window optimization
Conclusions
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
24
Individual Process Window Increase
Without Gray Bar
With Gray Bar
Unassisted Cr
Larger Pitches < 8%EL

Optimized Assist Features
Significant EL gain
Not Acceptable
14
280nm
300nm
400nm
500nm
240nm
12
10
8
6
4
2
Experiment
0
0
© imec 2004
Exposure Latitude [%]
Exposure Latitude [%]
14
0.1
0.2
0.3
DOF [µm]
N. Lafferty – Gray Bar OPC/EXT
0.4
0.5
0.6
280nm
300nm
400nm
500nm
240nm
12
10
8
6
4
2
Experiment
0
0
0.2
0.4
0.6
DOF [µm]
25
Overlapping Process Window Increase
No Assist Features
With Gray Assist Features
No overlapping Process Window
5.6% Exposure Latitude
0.17µm DOF
Overlapping Process Window
70nm CDs Through Pitch
Gray Assist Bars Added
Overlapping Process Window
70 nm CD
Pitch 240nm to 900nm
Dose
Group1
Doc: Through Pitch without gray (Experimental)
Dose
22
Group1
Doc: Through Pitch Quad Solution (experiment) : U100-317
Experiment
Experiment
21
22
20
21
19
20
18
U100-023
U100-044
U100-072
U100-112
U100-163
U100-223
U100-296
U100-390
U100-467
F 0.02
E 20.41
17
19
16
18
15
-0.2
© imec 2004
-0.1
0.0
N. Lafferty – Gray Bar OPC/EXT
Focus
0.1
0.2
-0.2
-0.1
0.0
Focus
0.1
0.2
26
Outline
Introduction
Mask making
Experimental data

Proximity Correction

DOF Improvement

Through pitch process window optimization
Conclusions
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
27
Conclusions
We have experimentally proven that Gray Assist Bar
OPC can:
 Reduce
0th order diffraction information, while adding 2nd
order information
 Improve NILS of assisted vs. unassisted cases
 Be a viable approach for reducing proximity effects
 Increase usable DOF and exposure latitude for features
spaced > forbidden pitch
 Tune individual process windows for maximum overlap
through pitch
In Addition:
 The
graybar mask was manufactured successfully with good
control of the gray bars.
 The reticle is an excellent step forward in assist feature OPC
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
28
Acknowledgements
Stephen Hsu

Creating GDS file
Eric Hendrickx

Discussions on forbidden pitch treatment and illumination effects
Patrick Willems

Exposures/Measurements
This work is in follow-up to a previous SPIE 2001 paper
‘Mutually Optimizing Resolution Enhancement
Techniques: Illumination, APSM, Assist Feature OPC
and Gray Bars’, Dr. Bruce W. Smith [4348-48]
© imec 2004
N. Lafferty – Gray Bar OPC/EXT
29
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