Effect of Wetting Time on CD Uniformity in Immersion Lithography

IBM Systems and Technology Group 

2 nd International Symposium on Immersion Lithography 

<strong>Effect</strong> <strong>of</strong> <strong>Wetting</strong> <strong>Time</strong> on CD Uniformity in 

Immersion Lithography 

Kaushal Patel, A. P. Mahorowala, T. A. Brunner, M. C. Lawson, 

J. M. Fullam, K. E. Petrillo, C. F. Robinson, D. A. Corliss 

September 13, 2005 

© 2005 IBM Corporation

Extracted Photoacid (ppb) 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 


Photoresist Component Extraction by Water 

� Extraction <strong>of</strong> resist 

components into water 

occurs at a very short timescale 

(1-5 seconds) 

y = y s (1-e -t/τ ) 

Measured 

Fit 

0 20 40 60 80 

Water Rinse <strong>Time</strong> (sec) 

� Photoacid extraction 

increases with exposure 

2 <strong>Effect</strong> <strong>of</strong> <strong>Wetting</strong> <strong>Time</strong> on CDU - Kaushal Patel September 13, 2005 

Extracted Photoacid (ppb) 

20 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

Data from Greg Wallraff, Carl Larson 

Unexposed 

Exposed 

Resist 1 Resist 2 



Photoresist Component Extraction by Water 

� Extraction rate depends on: 

– Type <strong>of</strong> component 

– Initial distribution in resist 

– Water contact time 

– Presence <strong>of</strong> a topcoat 

– Water penetration into resist 


Extracted Photoacid (nmol/ml) 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0 

Data from Greg Wallraff, Carl Larson 

PAG1 

PAG2 

PAG3 

6 60 300 

Water Extraction <strong>Time</strong> (sec) 

� Extraction <strong>of</strong> photoacid and quencher alters the distribution in the 

resist which could affect pr<strong>of</strong>iles 



Implication for CD Variation in Immersion Lithography 

� Spatial distribution <strong>of</strong> wetting (elution) time across the wafer 

during immersion exposure is non-uniform 

� Impact <strong>of</strong> short duration wetting on resist CD and pr<strong>of</strong>ile is 

not very well characterized 

� Non-uniform wetting could contribute to CD variation 

� To quantify such an effect, detailed analysis <strong>of</strong> the immersion 

process as well as precision metrology is necessary 

Reference: I. Pollentier et al, “Resist Pr<strong>of</strong>ile Control in Immersion Lithography using Scatterometry 

Measurements”, Proc. SPIE, Vol. 5754, p11 (2005) 



Objective 


1. Characterize the spatial distribution <strong>of</strong> wetting time 

2. Determine impact <strong>of</strong> short duration wetting on resist pr<strong>of</strong>iles 

– Correlate change in CD with wetting time 

3. Assess contribution <strong>of</strong> wetting time on CD variation 



Approach 


Collect stage traces 

Generate wetting maps 

Design <strong>of</strong> experiments 

Expose wafers 

Data analysis 


Resist Height 

ULT 

Build scatterometer library 

Measure CD 

Top CD 

Mid CD 

Side wall 

angle 

Bottom CD 



Experimental Conditions 

� Exposure 

– ASML AT:1150i / TEL Lithius track 

– Attenuated PSM mask 

– NA = 0.75 

– Annular Illumination, σ i /σ o = 0.75/0.5 

� Materials 

– Resists: (A) High activation (B) Low activation 

– Topcoat: Aqueous base soluble topcoat TC1 

– BARC AR40 




<strong>Wetting</strong> <strong>Time</strong> Maps 

� <strong>Wetting</strong> time maps were 

generated by analyzing the 

wafer stage trace 

� <strong>Wetting</strong> times are affected by: 

– Die size 

– Die layout and expose order 

– Stage velocity 

– Pre-exposure alignment 

sequence 

– Exposure dose 




<strong>Wetting</strong> <strong>Time</strong> Maps 

� Individual wetting 

time components 

were deciphered 




Scatterometry – Precision Metrology 

Resist Height 

ULT 


Mid CD 

Side wall 

angle 


Trapezoidal Model 

(6 parameters) 


� KLA-Tencor Spectra FX100 CD 

� New library for each stack 

� Target feature 

– 120nm line / 400nm pitch 

� 26 x 26 mm 2 field 

� 16 sites per field, 86 fields 

-13 -6.5 0 6.5 13 

Site location 

13 

6.5 

0 

-6.5 

-13 

-150 -75 0 75 150 

Field location 

150 

75 

0 

-75 

-150 



Contributions to CD Variation 

� Intrafield (ACLV) 

– Photomask CD error 

– Feature orientation relative to scan direction 

– Intensity variation across slit 

� Interfield (AWLV) 

– Scanning direction 

– Process (hotplate, develop process, wetting) 

� Residuals 

Reference: A. Wong, A. Molless, T. Brunner et al.,”Linewidth variation characterization by spatial 

decomposition”, JMMM, 1 pp. 106-115(2002). 




Analysis <strong>of</strong> CD Variation 

- 

- 

Raw Data 

Standardize 

Intrafield (ACLV) 

contributions 

Interfield (AWLV) 


Scan direction 

<strong>Wetting</strong> time 

Residual 

Illustration: Resist A / Topcoat TC1, 250 mm/s 

Bin 

200 

180 

160 

140 

120 

100 

80 

60 

40 

20 

σ z =1.0000 

0 

-4 -3 -2 -1 0 1 2 

Standardized Variation in Top CD (nm) 

z i = y i - μμ 

σσ 

Standardized Variation in Top CD 

σ z = 1.000 





- 

Raw Data 

Standardize 



- 

350 


σ =0.6849 

z 

300 σz = 0.685 




Residual 



250 

200 

150 

100 

50 

zACLV ACLV (site) = μz, , all fields, site 

Intrafield (ACLV) Contribution 

0 

-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 






- 

Raw Data 

Standardize 



- 

250 


σ =0.7316 

z 

σ 200 z = 0.732 




Residual 


σ z 


150 

100 

50 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

Intrafield (ACLV) Contribution Removed 

0 

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 



1.00 

0.69 


Total ACLV Scan <strong>Wetting</strong> Residual 




- 

Raw Data 

Standardize 



- 

700 


σ =0.0178 

z 

600 σz = 0.018 




Residual 



500 

400 

300 

200 

100 

zSCAN SCAN (dir) = μz, , all fields, dir 

Scan Direction Contribution 

0 

-0.02 -0.015 -0.01 -0.005 0 0.005 0.01 0.015 0.02 






- 

Raw Data 

Standardize 



- 

250 


σ =0.7314 

z 

σ 200 z = 0.731 




Residual 


σ z 


150 

100 

50 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

Scan Direction Contribution Removed 

0 

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 



1.00 

0.69 


0.02 





- 

- 

Raw Data 

Standardize 







Residual 


zWET WET = f(wetting time)? time) 

Total Wet <strong>Time</strong> Variation Signature 





- 

- Interfield (AWLV) 

Raw Data 

Standardize 






Residual 


Plot CD distribution versus wetting times 

Top Top CD 

-1 

-2 

0 1 2 

Pre-Wet <strong>Time</strong> (s) 


3 

2 

1 

0 

3 

3 

Z = (Z 0 – Zs)e )e-t/ t/τ + Z 


2 

1 

0 

-1 

-2 

0 0.2 0.4 0.6 0.8 

Post-Wet <strong>Time</strong> (s) 

+ Z s 

τ = 0.88s τ = 0.17s τ = 0.54s 


2 

1 

0 

-1 

-2 

0 1 2 

Total-Wet <strong>Time</strong> (s) 

Pre-Wet <strong>Time</strong> Post-Wet <strong>Time</strong> Total-Wet <strong>Time</strong> 

Fit 




- 

- Interfield (AWLV) 

Raw Data 

Standardize 






Residual 


Bottom CD CD 

0 1 2 

Pre-Wet <strong>Time</strong> (s) 


2 

1 

0 

-1 

-2 

-3 

Z = (Z 0 – Zs)e )e-t/ t/τ + Z s 


2 

1 

0 

-1 

-2 

τ = 0.79s τ = 0.13s τ = 2.38s 

-3 

0 0.2 0.4 0.6 0.8 

Post-Wet <strong>Time</strong> (s) 


2 

1 

0 

-1 

-2 

-3 

0 1 2 

Total-Wet <strong>Time</strong> (s) 

Pre-Wet <strong>Time</strong> Post-Wet <strong>Time</strong> Total-Wet <strong>Time</strong> 




- 

Raw Data 

Standardize 



- 

350 


σ =0.3508 

z 

300 σz = 0.351 




Residual 



250 

200 

150 

100 

50 

zWET WET = f(total wetting time) 

<strong>Wetting</strong> <strong>Time</strong> Contribution 

0 

-1 -0.5 0 0.5 1 1.5 2 






- 

- 

Raw Data 

Standardize 







Residual 


σ z 


160 

140 

120 

100 

80 

60 

40 

20 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

<strong>Wetting</strong> <strong>Time</strong> Contribution Removed = Residual 

σ z =0.6254 

σ z = 0.624 

0 

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 



1.00 

0.69 


0.02 

0.35 

0.62 




Impact <strong>of</strong> <strong>Wetting</strong> <strong>Time</strong> on Resist Pr<strong>of</strong>iles 

σ z 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

Resist A / Topcoat TC1 / 250 mm/s 


Middle CD 


Side Wall Angle 

Raw Intrafield Scan Direction <strong>Wetting</strong> <strong>Time</strong> Residual 


Side wall angle is is 

most sensitive to to 

total wetting time 



Impact <strong>of</strong> Scan Speed on CD Variation (<strong>Wetting</strong>) 

σ z (wet) 

0.5 

0.45 

0.4 

0.35 

0.3 

0.25 

0.2 

0.15 

0.1 

0.05 

0 

Resist A / Topcoat TC1 

0.08 

330 mm/s 

250 mm/s 

50 mm/s 

Impact <strong>of</strong> <strong>of</strong> wetting 

time is is reduced at at 

slow scan speed 

due to to saturation 

0.00 0.00 

Top CD Middle CD Bottom CD Side Wall Angle 


0.34 



Impact <strong>of</strong> Resist on CD Variation (<strong>Wetting</strong>) 

σ z (wet) 

0.5 

0.45 

0.4 

0.35 

0.3 

0.25 

0.2 

0.15 

0.1 

0.05 

0 

Topcoat TC1, 250 mm/s 

Resist A (High Ea) 

Resist B (Low Ea) 

Impact <strong>of</strong> <strong>of</strong> 

wetting time is is 

resist 

dependent 

Top CD Middle CD Bottom CD Side Wall Angle 


A 

B 


Summary 


� Presented a methodology to analyze the impact <strong>of</strong> wetting time 

on resist pr<strong>of</strong>iles 

� A quantifiable, non-linear CD response to wetting time was 

demonstrated 

– Response was an exponential decay to a saturation level 

– Side wall angle was most sensitive parameter to wetting time 

� The impact <strong>of</strong> wetting time on CD variation was reduced at slow 

scan speeds due to saturation 

� Impact <strong>of</strong> wetting time on CD variation was resist dependent 




Acknowledgment 

� IBM 

– Pushkara Rao Varanasi 

– Kurt Kimmel 

– George Gomba 

� ASML 

– Robert Watso 

– Bill Pierson 

� Albany Nanotech Operations Team

Effect of Wetting Time on CD Uniformity in Immersion Lithography

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