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Removal mechanism of nano-bubble with AFM for immersion lithography
Microelectronic Engineering 83 (2006) 655–658 www.elsevier.com/locate/mee
Removal mechanism of nano-bubble with AFM for immersion lithography Akira Kawai *, Kenta Suzuki Department of Electrical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan Available online 25 January 2006
Abstract We discuss adhesion and removal mechanisms of nano-bubble adhered on a resist surface for immersion lithography. We employed the AFM (atomic force microscope) for the observation of nano-bubbles on a resist surface. In addition, by the thermodynamic analysis, it can be considered that the nano-bubble adhered on the resist surface tends to be a flat shape and spread on the resist surface. It is difficult to adhere the bubbles on the resist surface. We also observed the nano-bubble adhered on both ArF and F2 excimer resist surfaces with the AFM. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Nano-scale bubble; Atomic force microscope; Immersion lithography; ArF and F2 excimer resists; Thermodynamics; Adhesion; Removal
1. Introduction Recently, adhesion and removal control of nano-bubble has become important technique for the immersion lithography. The nano-bubbles adhered on a resist film surface affect strongly to the quality of the pattern exposure and development processes [1,2]. In this paper, we discuss the adhesion and removal properties of nano-bubbles on an ArF excimer resist surfaces using the AFM. In an immersion medium, it is effective to employ the AFM for the observation of nano-bubbles [3–6].
lever was 0.05 N/m and the curvature radius of tip apex was 8 nm. The measurement system of nano-bubbles is shown in Fig. 2. The bubble angle / can be obtained in the following formula, / = 180 h, where h is the contact angle of the DI-water on the resist film surface. By approaching the AFM tip to the nano-bubble surface, a certain repulsive force should act to the tip/water/air system [4]. The topological image due to the repulsive force, the nano-bubble can be imaged clearly in the immersion liquid. 3. Results and discussion
2. Experiment The ArF excimer resist consisting of acrylic resin was used for the investigation. In Fig. 1a, the ArF resists was formed on a Si(1 0 0) substrate by spinning method. The film thickness of the resists was 300 nm. As the immersion liquid, deionised water (air saturated, 9 mg/l of dissolved oxygen gas concentration) was used. Fig. 1b shows an AFM tip made of Si3N4. The spring constant of the canti-
*
Corresponding author. E-mail address: [email protected] (A. Kawai).
0167-9317/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.12.029
Fig. 3 shows the component map of the surface energy of each material evaluated by the sessile drop method. In Fig. 3, the symbols, cdl and cpl , are dispersion and polar components of surface free energy [mJ/m2]. The cp of ArF excimer resist is relatively low. Fig. 4a shows the AFM images of the nano-bubbles observed on the ArF resist surface. The nano-bubbles of 39–46 nm diameter were observed clearly and the dense image of the polymer aggregates on the resist surface was also confirmed. It is clearly observed that three bubbles condense and shrink spontaneously with the lapse of time. Fig. 4b shows a schematic diagram of the bubble condensation.
656
A. Kawai, K. Suzuki / Microelectronic Engineering 83 (2006) 655–658
50nm Bubble 2
Bubble 1
A
A 2~3 hours later
200nm
100nm
Bubble 2
a
Nano-bubbles
a Bubble 1
b
Fig. 1. Resist surface and AFM tip images: (a) ArF excimer resist film surface. (b) Tip apex of AFM.
b Fig. 4. Deformation and condensation of nano-bubbles on ArF excimer resist surface: (a) AFM images of nano-bubbles formed on the ArF excimer resist surface (b) Schematic diagram of condensation of nanobubbles.
DI-water AFM tip Nano-bubble SBR
Height of bubble H [nm]
10
Height H
Contact line L φ
SLR
SBL
Resist
Diameter D Fig. 2. Schematic diagram of bubble imaging with AFM.
6 4
A
2
0
DI-water
A
Lapse of time 50
100
150
200
250
Position on resist surface [nm]
Si substrate
6
Fig. 5. Cross sectional profile of nano-bubble adhered on ArF resist surface.
p s
F2 excimer resist
4
60
ArF excimer resist
2
0 0
2
4
Dispersion components
6 p s
8
(mJ/m2)1/2
Fig. 3. Component map of surface free energy.
We can observe the deformation and condensation of nano-bubbles in immersion liquid as a characteristic nature. Fig. 5 shows the cross sectional profiles of the nanobubble adhered on the ArF resist surface at a line A-A 0 in Fig. 4a. Moreover, Fig. 6 shows the height and diameter variation of the nano-bubble with the lapse of time. It is clearly observed that the bubble height decreases gradually and slightly spreads with the lapse of time. Fig. 7 shows the change of bubble angle / with the lapse of time. The shape of nano-bubbles is mostly convex semilens. This result explains the following simulation based on
H eight H , D iameter D [n m ]
(mJ/m2)1/2
Bubble2
0
8
Polar components
Bubble1
8
50
Diameter D
Bubble 1
40 30
Bubble 2 20
Height H Bubble 1
Bubble 2
20
40
10 0 0
10
30
50
60
70
Lapse of time [min] Fig. 6. Height and diameter change of nano-bubble with the lapse of time.
the free energy change. Fig. 8 shows the volume change of nano-bubble with the lapse of time. It is clearly observed that the result shows no change of total volume of the nano-bubbles in this experiment. In order to analyze the adhesion and removal mechanisms of the nano-scale bubble, the force balance between buoyancy and contact line
A. Kawai, K. Suzuki / Microelectronic Engineering 83 (2006) 655–658
657 Volume is constant 10-23[m3]
25
20
Total energy E of system [fJ]
Bubble Angle
[degree]
4
Bubble 2
15
10
Bubble 1
5
2 0
Unstable
-2 -4 -6
Stable -8 -10
0 0
10
20
30
40
50
60
70
Lapse of time [min]
-12 0
12
50
100
180
150
Bubble angle φ [degree]
Fig. 7. Bubble angle / change of nano-bubble with the lapse of time.
Fig. 10. Total free energy E of the system depending on the bubble angle /.
10 -24
itational acceleration [m/s2], V is volume of nano-bubble [m3]. Fig. 9 shows the forces acting on nano-bubble depending on the bubble angle /. In Fig. 9, the buoyancy is considerably small, therefore, it can be neglected compared with the line tension. Hence, we estimate the total free energy E of the system as the following equation.
Bubble volume [m3]
25 20
Total 15
Bubble 1
10
E ¼ cBL S BL þ cBR S BR cRL S RL ;
5
Bubble2 0 0
10
20
30
40
50
60
70
Lapse of time [min] Fig. 8. Volume change of nano-bubbles with the lapse of time.
tension which act to nano-bubble should be taken into consideration. The force F [nN] acting on bubbles can be given in the following equation: F ¼ F L F B ¼ cl L qgV ;
ð1Þ
where FL is contact line tension [N], FB is buoyancy [N], L is contact line [m], q is density of liquid [g/cm3], g is grav-
Line tension FL and buoyancy FB [nN]
105
Vol ume V=10-25[m3]
10-26
ð2Þ 2
10-27
where cBL is bubble/liquid interface energy [mJ/m ], SBL is bubble/liquid interface area [m2], cBR is bubble/resist interface energy [mJ/m2], SBR is bubble/liquid interface area [m2], cRL is resist/liquid interface energy [mJ/m2], SRL is resist/liquid interface area [m2]. As mentioned previously, Fig. 2 shows the model system in this simulation. Fig. 10 shows the simulation result of total energy E of the system depending on the bubble angle /. In this simulation result, it can be explained well that the nano-bubble formed on the resist surface become flat in thermodynamics. In this regard, as shown in Fig. 7, the nano-bubble observed on the ArF resist surface decrease the angle / with the lapse of time as shown in Fig. 4a. Fig. 11 shows the schematic diagram of adhesion nature of nano-bubble formed on the resist surface. The nanobubble formed on the resist surface will become flat and spread. Fig. 12 shows the AFM image of nano-bubble
100 Line tension FL
10-5 10-10
10-27
10-26
In liquid
10-25
Nano-bubble
Possible Decrease of
10-15 Buoyancy FB
Impossible Increase of
Excimer resist
(Less than 12 )
0
0
50
Bubble angle
100
150
180
[degree]
Fig. 9. Line tension and buoyancy depending on the bubble angle /.
Fig. 11. Schematic diagram of adhesion nature of nano-bubble formed on the resist surface.
658
A. Kawai, K. Suzuki / Microelectronic Engineering 83 (2006) 655–658 Two Nano-bubbles
Two Nano-bubbles
analysis. The adhesion nature of nano-bubble can be explained thermodynamically.
Single Nano-bubble
Acknowledgements 50 nm
adhered on the F2 excimer resist surface. The height and diameter of nano-bubbles were 40 nm and 100 200 nm, respectively. Two nano-bubbles condense spontaneously with the lapse of time. The same tendency of bubble condensation can be observed.
This present work was partially supported by Grantin-Aid for Scientific Research from Japan Society for the Promotion of Science (Scientific Research (B) 16360171). The present work was also supported by Grant-in-Aid for Scientific Research, Ministry of Education, Culture, Sports, Science and Technology (Exploratory research, 16656105). This work was also partially supported by Tokyo Ohka Foundation for the Promotion of Science and Technology.
4. Conclusion
References
We could observe the nano-bubble adhered on the ArF and F2 excimer resist surfaces with the AFM. It is effective to employ the AFM for observation of nature of the nano-bubble adhered on the resist surface in DIwater. During the observation, the volume of the nanobubble was constant and the nano-bubble became flat. Therefore, the buoyancy is considerably small as compared with line tension and it can be neglected in this
[1] H. Endo, A. Kawai, J. Photopolym. Sci. Technol. 17 (2004) 105. [2] S. Donders, R. Moermon, H. Boom, Proceedings of International Symposium on Immersion & 157 nm Lithography, (2004). [3] A. Kawai, A. Ishikawa, T. Niiyama, M. Harumoto, O. Tamada, M. Sanada, SPIE ML 5753 (2005) 94. [4] T. Niiyama, A. Kawai, J. Photopolym. Sci. Technol. 18 (2005) 373. [5] A. Kawai, J. Photopolym. Sci. Technol. 18 (2005) 348. [6] B. Budhlall, X. He, I. Hyder, S. Mehta, G. Parris, Proceedings of International Symposium on Immersion & 157 nm Lithography, (2004).
a
b
c
Fig. 12. AFM images of nano-bubbles adhered on F2 excimer resist surface: (a) 136 min later, (b) 262 min later and (c) 282 min later.
Removal mechanism of nano-bubble with AFM for immersion lithography Akira Kawai *, Kenta Suzuki Department of Electrical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan Available online 25 January 2006
Abstract We discuss adhesion and removal mechanisms of nano-bubble adhered on a resist surface for immersion lithography. We employed the AFM (atomic force microscope) for the observation of nano-bubbles on a resist surface. In addition, by the thermodynamic analysis, it can be considered that the nano-bubble adhered on the resist surface tends to be a flat shape and spread on the resist surface. It is difficult to adhere the bubbles on the resist surface. We also observed the nano-bubble adhered on both ArF and F2 excimer resist surfaces with the AFM. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Nano-scale bubble; Atomic force microscope; Immersion lithography; ArF and F2 excimer resists; Thermodynamics; Adhesion; Removal
1. Introduction Recently, adhesion and removal control of nano-bubble has become important technique for the immersion lithography. The nano-bubbles adhered on a resist film surface affect strongly to the quality of the pattern exposure and development processes [1,2]. In this paper, we discuss the adhesion and removal properties of nano-bubbles on an ArF excimer resist surfaces using the AFM. In an immersion medium, it is effective to employ the AFM for the observation of nano-bubbles [3–6].
lever was 0.05 N/m and the curvature radius of tip apex was 8 nm. The measurement system of nano-bubbles is shown in Fig. 2. The bubble angle / can be obtained in the following formula, / = 180 h, where h is the contact angle of the DI-water on the resist film surface. By approaching the AFM tip to the nano-bubble surface, a certain repulsive force should act to the tip/water/air system [4]. The topological image due to the repulsive force, the nano-bubble can be imaged clearly in the immersion liquid. 3. Results and discussion
2. Experiment The ArF excimer resist consisting of acrylic resin was used for the investigation. In Fig. 1a, the ArF resists was formed on a Si(1 0 0) substrate by spinning method. The film thickness of the resists was 300 nm. As the immersion liquid, deionised water (air saturated, 9 mg/l of dissolved oxygen gas concentration) was used. Fig. 1b shows an AFM tip made of Si3N4. The spring constant of the canti-
*
Corresponding author. E-mail address: [email protected] (A. Kawai).
0167-9317/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.12.029
Fig. 3 shows the component map of the surface energy of each material evaluated by the sessile drop method. In Fig. 3, the symbols, cdl and cpl , are dispersion and polar components of surface free energy [mJ/m2]. The cp of ArF excimer resist is relatively low. Fig. 4a shows the AFM images of the nano-bubbles observed on the ArF resist surface. The nano-bubbles of 39–46 nm diameter were observed clearly and the dense image of the polymer aggregates on the resist surface was also confirmed. It is clearly observed that three bubbles condense and shrink spontaneously with the lapse of time. Fig. 4b shows a schematic diagram of the bubble condensation.
656
A. Kawai, K. Suzuki / Microelectronic Engineering 83 (2006) 655–658
50nm Bubble 2
Bubble 1
A
A 2~3 hours later
200nm
100nm
Bubble 2
a
Nano-bubbles
a Bubble 1
b
Fig. 1. Resist surface and AFM tip images: (a) ArF excimer resist film surface. (b) Tip apex of AFM.
b Fig. 4. Deformation and condensation of nano-bubbles on ArF excimer resist surface: (a) AFM images of nano-bubbles formed on the ArF excimer resist surface (b) Schematic diagram of condensation of nanobubbles.
DI-water AFM tip Nano-bubble SBR
Height of bubble H [nm]
10
Height H
Contact line L φ
SLR
SBL
Resist
Diameter D Fig. 2. Schematic diagram of bubble imaging with AFM.
6 4
A
2
0
DI-water
A
Lapse of time 50
100
150
200
250
Position on resist surface [nm]
Si substrate
6
Fig. 5. Cross sectional profile of nano-bubble adhered on ArF resist surface.
p s
F2 excimer resist
4
60
ArF excimer resist
2
0 0
2
4
Dispersion components
6 p s
8
(mJ/m2)1/2
Fig. 3. Component map of surface free energy.
We can observe the deformation and condensation of nano-bubbles in immersion liquid as a characteristic nature. Fig. 5 shows the cross sectional profiles of the nanobubble adhered on the ArF resist surface at a line A-A 0 in Fig. 4a. Moreover, Fig. 6 shows the height and diameter variation of the nano-bubble with the lapse of time. It is clearly observed that the bubble height decreases gradually and slightly spreads with the lapse of time. Fig. 7 shows the change of bubble angle / with the lapse of time. The shape of nano-bubbles is mostly convex semilens. This result explains the following simulation based on
H eight H , D iameter D [n m ]
(mJ/m2)1/2
Bubble2
0
8
Polar components
Bubble1
8
50
Diameter D
Bubble 1
40 30
Bubble 2 20
Height H Bubble 1
Bubble 2
20
40
10 0 0
10
30
50
60
70
Lapse of time [min] Fig. 6. Height and diameter change of nano-bubble with the lapse of time.
the free energy change. Fig. 8 shows the volume change of nano-bubble with the lapse of time. It is clearly observed that the result shows no change of total volume of the nano-bubbles in this experiment. In order to analyze the adhesion and removal mechanisms of the nano-scale bubble, the force balance between buoyancy and contact line
A. Kawai, K. Suzuki / Microelectronic Engineering 83 (2006) 655–658
657 Volume is constant 10-23[m3]
25
20
Total energy E of system [fJ]
Bubble Angle
[degree]
4
Bubble 2
15
10
Bubble 1
5
2 0
Unstable
-2 -4 -6
Stable -8 -10
0 0
10
20
30
40
50
60
70
Lapse of time [min]
-12 0
12
50
100
180
150
Bubble angle φ [degree]
Fig. 7. Bubble angle / change of nano-bubble with the lapse of time.
Fig. 10. Total free energy E of the system depending on the bubble angle /.
10 -24
itational acceleration [m/s2], V is volume of nano-bubble [m3]. Fig. 9 shows the forces acting on nano-bubble depending on the bubble angle /. In Fig. 9, the buoyancy is considerably small, therefore, it can be neglected compared with the line tension. Hence, we estimate the total free energy E of the system as the following equation.
Bubble volume [m3]
25 20
Total 15
Bubble 1
10
E ¼ cBL S BL þ cBR S BR cRL S RL ;
5
Bubble2 0 0
10
20
30
40
50
60
70
Lapse of time [min] Fig. 8. Volume change of nano-bubbles with the lapse of time.
tension which act to nano-bubble should be taken into consideration. The force F [nN] acting on bubbles can be given in the following equation: F ¼ F L F B ¼ cl L qgV ;
ð1Þ
where FL is contact line tension [N], FB is buoyancy [N], L is contact line [m], q is density of liquid [g/cm3], g is grav-
Line tension FL and buoyancy FB [nN]
105
Vol ume V=10-25[m3]
10-26
ð2Þ 2
10-27
where cBL is bubble/liquid interface energy [mJ/m ], SBL is bubble/liquid interface area [m2], cBR is bubble/resist interface energy [mJ/m2], SBR is bubble/liquid interface area [m2], cRL is resist/liquid interface energy [mJ/m2], SRL is resist/liquid interface area [m2]. As mentioned previously, Fig. 2 shows the model system in this simulation. Fig. 10 shows the simulation result of total energy E of the system depending on the bubble angle /. In this simulation result, it can be explained well that the nano-bubble formed on the resist surface become flat in thermodynamics. In this regard, as shown in Fig. 7, the nano-bubble observed on the ArF resist surface decrease the angle / with the lapse of time as shown in Fig. 4a. Fig. 11 shows the schematic diagram of adhesion nature of nano-bubble formed on the resist surface. The nanobubble formed on the resist surface will become flat and spread. Fig. 12 shows the AFM image of nano-bubble
100 Line tension FL
10-5 10-10
10-27
10-26
In liquid
10-25
Nano-bubble
Possible Decrease of
10-15 Buoyancy FB
Impossible Increase of
Excimer resist
(Less than 12 )
0
0
50
Bubble angle
100
150
180
[degree]
Fig. 9. Line tension and buoyancy depending on the bubble angle /.
Fig. 11. Schematic diagram of adhesion nature of nano-bubble formed on the resist surface.
658
A. Kawai, K. Suzuki / Microelectronic Engineering 83 (2006) 655–658 Two Nano-bubbles
Two Nano-bubbles
analysis. The adhesion nature of nano-bubble can be explained thermodynamically.
Single Nano-bubble
Acknowledgements 50 nm
adhered on the F2 excimer resist surface. The height and diameter of nano-bubbles were 40 nm and 100 200 nm, respectively. Two nano-bubbles condense spontaneously with the lapse of time. The same tendency of bubble condensation can be observed.
This present work was partially supported by Grantin-Aid for Scientific Research from Japan Society for the Promotion of Science (Scientific Research (B) 16360171). The present work was also supported by Grant-in-Aid for Scientific Research, Ministry of Education, Culture, Sports, Science and Technology (Exploratory research, 16656105). This work was also partially supported by Tokyo Ohka Foundation for the Promotion of Science and Technology.
4. Conclusion
References
We could observe the nano-bubble adhered on the ArF and F2 excimer resist surfaces with the AFM. It is effective to employ the AFM for observation of nature of the nano-bubble adhered on the resist surface in DIwater. During the observation, the volume of the nanobubble was constant and the nano-bubble became flat. Therefore, the buoyancy is considerably small as compared with line tension and it can be neglected in this
[1] H. Endo, A. Kawai, J. Photopolym. Sci. Technol. 17 (2004) 105. [2] S. Donders, R. Moermon, H. Boom, Proceedings of International Symposium on Immersion & 157 nm Lithography, (2004). [3] A. Kawai, A. Ishikawa, T. Niiyama, M. Harumoto, O. Tamada, M. Sanada, SPIE ML 5753 (2005) 94. [4] T. Niiyama, A. Kawai, J. Photopolym. Sci. Technol. 18 (2005) 373. [5] A. Kawai, J. Photopolym. Sci. Technol. 18 (2005) 348. [6] B. Budhlall, X. He, I. Hyder, S. Mehta, G. Parris, Proceedings of International Symposium on Immersion & 157 nm Lithography, (2004).
a
b
c
Fig. 12. AFM images of nano-bubbles adhered on F2 excimer resist surface: (a) 136 min later, (b) 262 min later and (c) 282 min later.