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Langmuir 2007, 23, 1166-1170
Experimental and Theoretical Investigation on Surfactant Segregation in Imprint Lithography K. Wu,† X. Wang, E. K. Kim,† C. G. Willson, and J. G. Ekerdt* Department of Chemical Engineering, The UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed June 16, 2006 The effects of template surface composition on fluorinated surfactant segregation were investigated for imprint lithography with photopolymerizable vinyl ether formulations. Heptadecafluoro-1,1,2,2-tetrahydrodecyl vinyloxymethyloxy dimethylsilane, containing a vinyl ether group, was employed as the surfactant, and blanket templates were pressed onto the liquid and illuminated with UV radiation from below. The extent of surfactant segregation to the vinyl ether-template interface before polymerization was characterized using contact angle measurements and angleresolved X-ray photoelectron spectroscopy after removing the template from the cured vinyl ether polymer. Blanket surfaces consisting of bare quartz, high-density polyethylene, and quartz treated with tridecafluoro-1,1,2,2,tetrahydrooctyltrichlorosilane afforded templates with different surface energy and polarity. The highest degree of surfactant segregation was found with tridecafluoro-1,1,2,2,-tetrahydrooctyltrichlorosilane-treated quartz, whereas little surfactant segregation was found for bare quartz. A thermodynamic model is developed to predict the surface segregation profiles.
I. Introduction A high demand exists for the development of new nanofabrication techniques with the projected dimensional scaling in microelectronics device feature size. These techniques include nanoimprint lithography (NIL),1,2 step-and-flash imprint lithography (SFIL),3,4 and scanning probe lithography (SPL),5,6 each of which has the potential to achieve high-throughput and lowcost sub-100 nm resolution patterning. SFIL has the added advantage that the imprint process is performed at low pressure and room temperature, which minimizes magnification and distortion errors.7 In addition, the imprint template is transparent, which allows for conventional overlay error measurement techniques. Other promising applications for SFIL include microelectromechanical devices (MEMS), micro-optics, and patterned storage media. A major challenge for imprint lithography is to minimize the tendency for the resist polymer to adhere to the template, which leads to imprint defects. Large adhesive forces between the template and the imprinted material can result in cohesive failure of the etch barrier material and distort the alignment for imprint.8 One approach to reducing such interfacial forces is to form a fluorinated self-assembled monolayer (FSAM) from tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane on the template as a * To whom correspondence should be addressed. E-mail: ekerdt@ che.utexas.edu. Tel: +1-512- 471-4689. Fax: +1-512-471-7060. † Current address: Applied Materials, 3320 Scott Ave., Santa Clara, CA 95054. (1) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85. (2) Chou, S. Y.; Krauss, P. R.; Zhang, W.; Guo, L.; Zhuang, L. J. Vac. Sci. Technol., B 1997, 15, 2897. (3) Colburn, M.; Johnson, S.; Stewart, M.; Damle, S.; Bailey, T.; Choi, B.; Wedlake, M.; Michaelson, T.; Sreenivasan, S. V.; Ekerdt, J. G.; Willson, C. G. Proc. SPIE 1999, 3676(I), 379. (4) Colburn, M. E. Ph.D. Thesis, The University of Texas at Austin, Austin, TX, 2001. (5) Kraemer, S.; Fuierer, R. R.; Gorman, C. B. Chem. ReV. 2003, 103, 4367. (6) Ginger, D. S.; Zhang, H.; Mirkin, C. A. Angew. Chem., Int. Ed. 2003, 43, 30. (7) Colburn, M.; Grot, A.; Amistoso, M.; Choi, B. J.; Bailey, T.; Ekerdt, J.; Sreenivasan, S. V.; Hollenhorst, J.; Willson, C. G. Proc. SPIE 2000, 3997, 453. (8) Bailey, T.; Choi, B. J.; Colburn, M.; Grot, A.; Meissl, M.; Shaya, S.; Ekerdt, J. G.; Sreenivasan, S. V.; Willson, C. G. J. Vac. Sci. Technol., B 2000, 18, 3572.
release layer.3,9 However, the durability of these FSAM layers for thousands of imprints remains unresolved. One potential solution to limited FSAM durability is to incorporate an additive into the SFIL etch barrier liquid that lowers the interfacial forces.10,11 Surfactants, even at low concentrations, have the characteristic feature of lowering the surface tension of the liquid phase efficiently and thus have many applications in different fields. Introduction of fluorinated compounds into a photopolymerizable monomer formulation can lower the surface energy by forming a concentrated fluorocarbon layer on the surface. Bender et al. used 1H,1H,2H,2H-perfluorooctyltriethoxysilane as a release agent to reduce the surface energy of UV-cured imprint materials, and this enabled multiple imprints (50 times) without mold cleaning.11 Bongiovanni et al. studied both acrylate and vinyl ether systems and found adding fluorinated monomer to the photopolymerizable resin can significantly increase the water contact angle because of surfactant segregation to the air/resin interface;12-14 further, the surface or interfacial tension varied in response to the amount of surfactant added to the liquid phase. Others have reported the segregation of fluorocarbon-terminated polystyrene from a polystyrene/fluorocarbon-terminated polystyrene mixture, which is heated above the glass transition temperature, lowers the surface tension and leads to a surface enrichment in the fluorocarbon-terminated polystyrene.15,16 Our study examines monomer segregation in a photopolymerizable (9) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y. Langmuir 1999, 15, 4321. (10) Kim, E. K.; Stewart, M. D.; Wu, K.; Palmieri, F. L.; Dickey, M. D.; Ekerdt, J. G.; Willson, C. G. J. Vac. Sci. Technol., B 2005, 23, 2967. (11) Bender, M.; Otto, M.; Hadam, B.; Spangenberg, B.; Kurz, H. Microelectron. Eng. 2002, 61-62, 407. (12) Bongiovanni, R.; Malucelli, G.; Pollicino, A.; Priola, A. J. Appl. Polym. Sci. 1997, 63, 979. (13) Bongiovanni, R.; Beamson, G.; Mamo, A.; Priola, A.; Recca, A.; Tonelli, C. Polymer 2000, 41, 409. (14) Bongiovanni, R.; Sangermano, M.; Malucelli, G.; Priola, A.; Leonardi, A.; Ameduri, B.; Pollicino, A.; Recca, A. J. Polym. Sci. A 2003, 41, 2890-2897. (15) Yuan, C. O. M.; Koberstein, J. T. Macromolecules 1999, 32, 2329. (16) Mason, R.; Jalbert, C. A.; O’Rourke, Muisener, P. A. V.; Koberstein, J. T.; Elman, J. F.; Long, T. E.; Gunesin, B. Z. AdV. Colloid Interface Sci. 2001, 94, 1.
10.1021/la061736y CCC: $37.00 © 2007 American Chemical Society Published on Web 12/05/2006
Surfactant Segregation in Imprint Lithography
Langmuir, Vol. 23, No. 3, 2007 1167
Chart 1. Structure of the Vinyl Ether Monomers
Table 1. Monomer Formulations fluorinated VE surfactant (wt %)
CHVE (wt %)
EGDVE (wt %)
PAG (wt %)
0.0 1.0 1.5 2.0 2.5
69.9 68.9 68.4 67.9 67.4
30.0 30.0 30.0 30.0 30.0
0.1 0.1 0.1 0.1 0.1
resin in contact with a solid to establish how the nature of the solid surface affects surfactant segregation. Thermodynamic analysis of the adsorption of surfactant at the interface between the template and the resin can be done using the Gibbs adsorption equation:
Γ ) -(C/RT)(∂γ/∂C)
(1)
where, Γ is the surface (interface) excess concentration of the surfactant, C is the molar concentration of the surfactant in the solution, γ is the surface or interfacial tension, and R and T are the molar gas constant and temperature, respectively. In general, γ can be readily obtained from contact angle measurements, either directly at the air-polymer interface or after removal of the solid substrate.12,16 The surface excess concentration can also be measured directly using techniques such as static secondary ion mass spectroscopy and nuclear reaction analysis.17,18 Alternately, concentration profiles in the near-surface region can be measured in the near-surface region using angle resolved X-ray photoelectron spectroscopy (ARXPS).12,14 The present study employs a surfactant that can be incorporated into the polymer backbone and thereby frozen in place. After polymerization and removal of the template, the surfactant concentration profile is established using ARXPS, and the surface excess concentration is predicted from a thermodynamic model fit to the profile data. II. Experimental Section A. Materials. Cyclohexyl vinyl ether (CHVE), ethylene glycol divinyl ether (EGDVE), diiodomethane, and glycerol, available from Aldrich, were used as received. The photoacid generator (PAG) (4,4′-bis-(t-butylphenyl) iodonium tris(trifluoromethanesulfonyl)methide) was provided by 3M. The fluorinated vinyl ether surfactant (heptadecafluoro-1,1,2,2-tetrahydrodecyl) vinyloxymethyloxy dimethylsilane was synthesized according to the procedures reported elsewhere.19 The chemical structures of CHVE, EGDVE, and the surfactant are presented in Chart 1. B. Film Preparation. The monomer formulations were prepared as shown in Table 1. The monomer solution was dispensed onto clean glass slides and imprinted by a 1 in. × 1 in. template. Quartz mask blanks, FSAM-treated quartz,20 and a high-density polyethylene (17) Affrossman, S; Hartshorne, M.; Kiff, T.; Pethrick, R. A.; Richards, R. W. Macromolecules 1994, 27, 1588. (18) Hopkinson, I.; Kiff, F. T.; Ruchards, R. W.; Bucknall, D. G.; Clough, A. S. Polymer 1997,38, 87. (19) Kim, E. K. Ph.D. Dissertation, The University of Texas at Austin, Austin, 2005, p 174. (20) Wu, K.; Bailey, T. C.; Willson, C. G.; Ekerdt, J. G. Langmuir 2005, 21, 11795.
Figure 1. Water contact angle versus the surfactant concentration. The points represent the average value, and the bars designate the range of measured values for each sample. (HDPE) plate were employed as templates. The vapor-phase FSAM treatment was done under the conditions that yielded the highest packing density of tridecafluoro-1,1,2,2,-tetrahydrooctyltrichlorosilane on the quartz surface. Briefly, the tridecafluoro-1,1,2,2,tetrahydrooctyltrichlorosilane-based films have a water contact angle >110° and a thickness of ∼1 nm yet do not present a surface that is as densely packed with CF3 terminal groups as is theoretically possible, so the films are disordered to some extent.20 The time between dispensing the monomer solution and template imprinting was ∼5 s, and the time between template imprinting and initiating the curing reaction was ∼240 s. The curing reaction was performed using a JBA LS 65 UV lamp with a dose of 2.2 J/cm2 for 200 s. Polymer films were at least 2 µm thick. After curing, the template was separated and the imprinted film was characterized by measuring the liquid contact angles, and the concentration profile of the surfactant with ARXPS. To eliminate the sample errors, each imprinted film was cut into two halves; one was used for contact angle measurement, and one was used for XPS measurement. C. Measurement of Contact Angles. The static contact angles of deionized water, diiodomethane, and glycerol on the imprinted films were measured at ambient temperature using a Rame Hart Model 100 contact angle goniometer. Static, sessile drops (10 µL) were delivered from a micrometer syringe with a minimum division of 2 µL. Contact angles were measured on at least three different spots and averaged. D. X-ray Photoelectron Spectroscopy (XPS) Analysis. The XPS measurements for the surfactant concentration in the polymer film were performed on a Physical Electronics PHI5700 ESCA system equipped with a monochromatic source (Al KR at 1486.6 eV). At least three points were scanned for each sample, and the average value was used. The Ag 3d5/2 XPS peak at 368.3 eV from a sputtercleaned Ag foil was used to calibrate the system. Depth profile information was obtained from measurements at takeoff angles of 15° and 75° from the surface plane. The penetration depth, d, is calculated with d ) 3β sin θ
(2)
where β is the electron effective attenuation length and was calculated using the NIST Electron Effective-Attenuation-Length Database. The values of β were calculated for 799 eV (F 1s) electrons in either pure surfactant or a 70:30 CHVE/EGDVE mixture. This leads to d ) 3.08 and 3.18 nm for the surfactant and mixture, respectively, at a 15° takeoff angle and d ) 6.57 and 7.21 nm for the surfactant and mixture, respectively, at a 75° takeoff angle. The values of d ) 3 and 7 nm are used when fitting the data.
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Wu et al.
model is simplified by assuming Φ(xs) is a linear function of xs
Φ(xs) ) Kxs
(5)
and the molar free energy of mixing of the fluid is22
f/RT ) [x ln x + (1 - x) ln(1 - x)]
(6)
Then
∆f/RT ) [x ln x + (1 - x) ln(1 - x)] - [x0 ln x0 + (1 - x0) ln(1 - x0)] - [ln x0 - ln(1 - x0)](x - x0) (7) Defining x ) x0 + ∆x
Figure 2. Diiodomethane contact angle versus the surfactant concentration. The points represent the average value, and the bars designate the range of measured values for each sample.
∆f/RT ) [(x0 + ∆x) ln(x0 + ∆x) + (1 - x0 - ∆x) ln(1 - x0 - ∆x)] - [x0 ln x0 + (1 - x0) ln(1 - x0)] [ln x0 - ln(1 - x0)] ∆x (8) Using a Taylor expansion, eq 8 becomes
∆f/RT )
[
]
1 ∆x2 1 ∆x3 - 2 + ‚‚‚ (9) + 2 6 2x0(1 - x0) (1 - x0) x0
and omitting terms O(∆x3) yields
∆f/RT ≈
( )
x0 x ∆x2 -1 ) 2x0(1 - x0) 2(1 - x0) x0
2
(10)
Using the result by Cahn for ∆f,21 by standard methods of variational calculus at infinity boundary conditions, one obtains
(dxdz)
(11)
(dxdz) ) 2kK
(12)
∆f ) k
2
and the boundary condition Figure 3. Glycerol contact angle versus the surfactant concentration. The points represent the average value, and the bars designate the range of measured values for each sample.
s
Combining eqs 10 and 11, and sorting
III. Thermodynamic Model The excess free energy of a unit area of interfacial surface, ∆F, is based on the gradient theory of inhomogeneous systems21 and is calculated with
∆F ) Φ(xs) +
∫0∞ [∆f + k(dx/dz)2] dz
(4)
and k(dx/dz)2 is the contribution a gradient makes to the free energy. The maximum bulk surfactant concentration is 3.82 × 10-2 M, and because of this low concentration (of order 10-2 M), the (21) Cahn, J. W. J. Chem. Phys. 1977, 66, 3667.
x
RTx0
( )
x -1 2k(1 - x0) x0
(13)
Equation 13 is integrated with the boundary condition in eq 11 to give the surfactant composition profile x(z)
(3)
where Φ(xs) represents the contribution to the free energy of a unit area of template solid surface, xs is the limiting composition (molar fraction) of the fluid at z ) 0 (i.e., interface), x(∞) ) x0 is the bulk composition of the fluid, ∆f is the free energy needed to create a unit volume of uniform fluid of composition x from a large reservoir at composition x0 and is calculated with
∆f ) f(x) - f(x0) - (x - x0)(∂f/∂x)x0
dx )dz
x ) x0 -
Kx0x2k(1 - x0) 2kxRTx0
(x
exp -
RTx0
z 2k(1 - x0) x0
)
(14)
Finally, eq 14 can be integrated over the appropriate XPS sampling depth to find an average mole fraction, such as
∫03 x(z) dz xj(0 - 3nm) ) ∫03 dz
(15)
against which the XPS data (see below) are fit to determine the value of k, and the values of K for each template surface. (22) Guggenheim, E. A. Mixtures. The Theory of the Equilibrium Properties of Some Simple Cases of Mixtures, Solutions and Alloys; Clarendon Press: Oxford, 1952; p 25
Surfactant Segregation in Imprint Lithography
Figure 4. Surface tensions predicted by the model (lines) and calculated from the contact angle measurements (points) versus surfactant concentration. The bars represent the range of possible values calculated from the contact angles.
IV. Results The contact angle measurements were used to calculate the surface tension of imprinted films, and the XPS measurements were carried out to obtain the surfactant composition profile. The water contact angle of the pure VE formulation was ∼57°, which is typical for a surface with medium polarity containing both hydrophilic and hydrophobic groups. As shown in Figure 1, when the fluorinated surfactant was added to the formulation, the water contact angle increased and the rate of increase with weight percent surfactant was dependent on the solid surface. The FSAM-treated surface had the greatest effect; the water contact angle increased to 93° for a polymer formulation containing 2.5 wt % surfactant. The water contact angle increased for the HDPE-imprinted surface, changing to 84° for 2.5 wt % surfactant, whereas the untreated quartz surface did not appreciably influence surfactant segregation and the water contact angle increased to only 63°. Diiodomethane and glycerol contact angles were also measured. The same trends are seen in Figures 2 and 3, respectively. A FSAM-treated surface caused the most surfactant segregation, the HDPE surface was less effective than the FSAM-treated surface and untreated quartz had a very modest effect. The static water, diiodomethane, and glycerol contact angle measurements allow calculation of the surface tension of the cured polymer film using the acid-base theory;23 Figure 4 shows the calculated surface tensions. The template surface tensions were determined in a similar manner and are 12.9, 35.2, and 65.5 mN/m for FSAMtreated quartz, HDPE, and untreated quartz, respectively. The results clearly indicate that the interface property is modified by the fluorinated additive when a template with low polarity is used, and this is a consequence of surfactant segregation to the imprint template-monomer interface. To measure the degree of surfactant segregation after photopolymerization, XPS analysis was performed for different bulk surfactant compositions for the three different templating surfaces. Table 2 presents the F 1s-to-C 1s atomic ratios for two different takeoff angles. Published sensitivity factors are used to estimate the F 1s/C 1s ratio for the bulk compositions. The 15° takeoff angle data represent an average value over approximately the top 3 nm of the polymer, while the 75° values are an average of the top 7 nm. A higher F 1s/C 1s ratio means there is more (23) van Oss, C. J.; Good, R. J.; Chaudhury, M. K. Langmuir 1988, 4, 884.
Langmuir, Vol. 23, No. 3, 2007 1169
surfactant in the sampled region. In all cases there is more surfactant in the top 3 nm of the film than in the top 7 nm of the film. In general, the FSAM-treated surface caused the greatest degree of segregation, as measured by the F 1s/C 1s ratio. The F 1s/C 1s ratios are transformed into a surfactant mole fraction by assuming the relative mole fractions for CHVE, EGDVE, and the PAG remain constant and independent of thickness for a given formulation; these components are lumped together into an “effective compound”. This effective compound has a fixed F 1s/C 1s intensity ratio, and this approach leads to a pseudo-binary system of surfactant and effective compound. The F 1s signal intensity is then apportioned, by atomic percent, to either the surfactant or the PAG content of the effective compound. Similarly, the C 1s signal intensity is apportioned to either the surfactant or the effective compound [at the proper fixed composition (Table 1)]. A nonlinear least-squares method was used to fit the parameter K for each template, while forcing the fitted value of k to be the same for all templates. A common value of k was imposed since this is meant to measure the effects of the surfactant concentration gradient on the free energy and should essentially be template independent. The appropriate integral (eq 15) for 0-3 or 0-7 nm was fit to the average mole fractions for each template as the concentration of surfactant in the bulk increased from 1 to 2.5 wt % and for each template when the surfactant concentration was zero. The parameter values are summarized in Table 3. Interestingly, the K value for the untreated quartz is approximately 1 order of magnitude smaller than the values for HDPE and the FSAM-treated quartz. Table 2 lists the predicted F 1s/C 1s atomic ratios that are based on the average concentrations calculated with eq 15 and the fitted parameter values. Equation 14, along with the fitted values for the parameters, can be used to compute the surfactant composition at the interface xs(z ) 0)
xs ) x0 -
Kxx0(1 - x0)
x2kRT
(16)
The Gibbs adsorption equation (eq 1) can be reformatted using the bulk surfactant concentration, c0
dγ/dc0 ) -RTΓ/c0
(17)
Γ can be considered the actual surface concentration without significant error,24 which is expressed as
Γ ) (cs × N)2/3/N
(18)
where N is Avogadro’s number, and the surface concentration cs(z ) 0) is calculated from xs. Thus, γ(c0) can be obtained by numerically solving eq 17. The calculated surface tension as a function of surfactant concentration is plotted in Figure 4 for each template. The inset shows that the model is converging to a common surface tension value in the limit of very low bulk surfactant concentrations. The ranges of calculated surface tension values that are presented in Figure 4 result from the range of measured water, diiodomethane and glycerol contact angles (Figures 1-3). The fit between model prediction and measurement falls outside the range of values for the most dilute concentration FSAM-treated point, if the point is centered on the range of values; the source (24) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; John Wiley and Sons, Inc.: Hoboken, NJ, 2004.
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Table 2. XPS Results at Different Take-Off Angles: a Comparison of Experimental and Predicted Values FSAM F 1s/C 1s experimental (atom ratio)
HDPE
F 1s/C 1s model predicted (atom ratio)
F 1s/C 1s experimental (atom ratio)
untreated quartz
F 1s/C 1s model predicted (atom ratio)
F 1s/C 1s experimental (atom ratio)
F 1s/C 1s model predicted (atom ratio)
bulk surfactant concn (wt %)
15°
75°
15°
75°
15°
75°
15°
75°
15°
75°
15°
75°
bulk F 1s/C 1s
1.0 1.5 2.0 2.5
0.165 0.211 0.245 0.309
0.086 0.118 0.146 0.155
0.150 0.192 0.234 0.279
0.079 0.110 0.140 0.169
0.090 0.105 0.121 0.140
0.047 0.060 0.066 0.079
0.083 0.103 0.123 0.143
0.050 0.067 0.077 0.087
0.013 0.018 0.022 0.023
0.008 0.010 0.012 0.013
0.012 0.017 0.023 0.024
0.009 0.011 0.014 0.015
0.00496 0.00739 0.00982 0.01230
Table 3. Calculated Parameters K and k for Different Templates K (J/m2) FSAM-treated template HDPE plate untreated template
k (J)
-2.82 × 10 -1.10 × 10-4 -2.81 × 10-5 -4
1.89 × 10-12 1.89 × 10-12 1.89 × 10-12
of this discrepancy remains unresolved. Possible model effects are discussed below.
V. Discussion Bongiovanni et al. studied segregation of fluorinated polymerizable surfactants to both the air-polymer interface and the polymer-glass interface for polymerizations conducted on glass slides.14 They found considerable segregation to the air-polymer interface and negligible segregation to the glass-polymer interface, based on water contact angle measurements. Consistent with these results, we find negligible segregation to the templatepolymer interface when an untreated quartz template is employed. Ameduri et al. reported segregation of fluoromonoacrylates, which were added at concentrations less that 1 wt %, to the air-polymer interface;25 the driving force for segregation was reasoned to be the high surface activity of the monomers. In general, the low polarity of the air drives the segregation of surfactants to the air-liquid interface.24 The system investigated herein has two solid interfaces, a glass slide and a template, in contact with the monomer containing a polymerizable, fluorinated vinyl ether. The driving force for surfactant segregation to the template-polymer interface is related to the polarity of the surface. The lowest polarity surface, the FSAM-treated quartz, had the greatest effect on surfactant segregation as measured by contact angle, and F 1s/C 1s XPS ratios. Stated differently, the FSAM-treated template presents the most compatible environment on which the fluorinated surfactant can adsorb. When a template with a particular surface energy is brought in contact with the monomer solution, surfactant segregation adjusts to minimize the free energy of the system. The model developed in this work predicts both the distribution of surfactant in the cured polymer (eq 14) and the surface tension of the polymer after imprint as a function of surfactant concentration. The model parameter K is clearly related to the polarity and surface energy of the template. FSAM-treated quartz had the lowest surface tension, 12.9 mN/m, and is thereby expected to have the lowest polarity. Consistent with the template surface tension values, the model predicts the most negative K value for an FSAM-treated template. The model-predicted surface tension values agree reasonably well with the experimental results (Figure 4). Some of the (25) Ameduri, B.; Bongiovanni, R.; Lombardi, V.; Pollicino, A.; Priola, A.; Recca, A. J. Polym. Sci. A 2001, 39, 4227.
disagreement could be related to the model assumptions. An ideal solution assumption was used for the free energy, and this may not be valid since the surfactant concentration is slightly greater than 10-2 M. The model assumes the interactions between the template surface and fluid are short-range, and the free energy due to the template surface effect, Φ(xs), is linearly related to the surfactant surface composition, xs. Finally, the application of a Taylor expansion to obtain the analytical solution for x(z) may lower the accuracy of the model. Additional study is required to more accurately model the effect of the template surface on surfactant migration. It is instructive to explore how these results impact strategies for affecting release of a templating surface such as found in imprint lithography processes that have relied on several strategies to facilitate release of the patterned template from the cured etch barrier, including treating the template with fluorinated chlorosilanes to lower the template surface energy or adding surfactant to the monomer.10 The results show that the properties both of the surfactant and the template must be considered. FSAM durability on the imprint template remains unresolved, and this study shows that it may not be necessary to maintain a fully dense, defect-free FSAM layer when the treated template is used in conjunction with surfactant in the monomer. As long as the surface energy of the treated template remained sufficiently low as the FSAM layer degrades with multiple imprints, this surface energy could still serve as a driving force for surfactant segregation. Beyond some level of FSAM layer degradation, the segregation driving force may not be sufficient. The model developed herein provides a means to optimize the surfactant concentration and template properties necessary to affect template release.
VI. Conclusion The effect of template surface on the segregation of fluorinated surfactant in the etch barrier formulation was investigated. A template with low surface energy and polarity is beneficial to the adsorption of the surfactant at the template-polymer barrier interface, which efficiently reduces the surface tension of the cured etch barrier. This phenomenon was confirmed by contact angle and XPS analysis. A model developed for the prediction of the surfactant concentration profile and the surface tension of the etch barrier after imprint as a function of surfactant concentration is presented. The model predictions in surface tension are shown to be in qualitative agreement with experimental results. Acknowledgment. The authors would like to thank Dr. Roger Bonnecaze for helpful discussion and suggestions. LA061736Y
