ARTICLE pubs.acs.org/Langmuir
Mold Design Rules for Residual Layer-Free Patterning in Thermal Imprint Lithography Hyunsik Yoon,† Sung Hoon Lee,‡ Seung Hyun Sung,† Kahp Y. Suh,*,‡ and Kookheon Char*,† †
National Creative Research Initiative Center for Intelligent Hybrids, School of Chemical and Biological Engineering, The WCU Program of Chemical Convergence for Energy and Environment, and ‡School of Mechanical and Aerospace Engineering, The WCU Program on Multiscale Mechanical Design, Seoul National University, Seoul, 151-744, Korea ABSTRACT: We present the mold design rules for assuring residual layer-free patterning in thermal imprint processes. Using simple relations for mass balance, structural stability, and work of adhesion, we derive the conditions with respect to the given single or multigeometrical feature of the mold, which are compared with simple thermal imprint experiments using soft imprint molds. Our analysis could serve as a guideline for designing the optimum mold geometry and selecting mold material in residual layer-free thermal imprint processes.
1. INTRODUCTION Thermal imprint lithography14 has been used extensively for fabricating optical, electronic, and organic devices. Typically, a polymer layer to be patterned is coated on a substrate and a mold with desired pattern is placed on the layer. The polymer is then heated above its glass transition temperature and pressure is applied on the mold to produce the negative of the mold pattern on the polymer layer. One inherent problem with this thermal imprint is the residual layer that remains after the imprint. The residual layer is typically removed by oxygen reactive ion etching (RIE) to expose the substrate surface. There have been attempts at developing methods for leaving no residual layer after the imprint, without resorting to oxygen RIE. In one, a rigiflex mold5 is used, in which the rubbery nature of the mold is utilized that essentially causes the underlying residual layer to dewet,6,7 exposing the substrate surface in the process. In another, the recessed parts of the mold are filled and this filled material is transferred to substrate after removal of the material remaining on the protruded parts.8 A prerequisite for leaving no residual after imprint is that the void space of the mold should be larger than the amount of the polymer that is in physical contact with the mold when it is placed on the polymer layer. If the polymer layer is thin enough to meet the prerequisite and a rubbery mold is used, then thermal imprint should lead to a patterned polymer layer in which there is no residual layer remaining on opened windows, that is, exposure of substrate surface with which the mold was in contact. Meeting the prerequisite often gives rise to a situation when the void space of the mold is not completely filled and this unfilled mass in the void has a meniscus simply because of the rise of the polymer r 2011 American Chemical Society
melt along the walls of the void. This relatively thin polymer structure with meniscus is not necessarily stable,9,10 and it becomes fragmented unless proper methods, such as utilization of roof collapse via deformation of an elastomeric mold,11 are taken. In this work, we present design rules that are necessary for producing stable polymer structure in nonresidual layer conditions. These rules are constraints on mold dimensions and are derived from dewetting considerations rather than polymer layer thickness, although there are also restrictions on polymer thickness for meeting the prerequisite. With the mold thus designed, no residual layer will remain after thermal imprint and the patterned polymer structure remains unchanged.
2. EXPERIMENTAL SECTION 2.1. Materials. For polymer resists, polystyrene (PS) resins of low molecular weight (MW = 2700, Aldrich) were used to ensure high mobility and thus clean exposure of the substrate. For mold materials, poly(urethane acrylate) (PUA, MINS 311 RM, Minuta Tech, Young’s modulus ∼20 MPa), and poly(dimethylsiloxane) (PDMS, Sylgard 184, Dow Corning, Young’s modulus ∼2 MPa) were used.5 2.2. Mold Preparation. PUA molds were fabricated by dispensing the PUA precursor onto the silicon master, and a poly(ethylene terephthalate) (PET) film (50 μm) was slightly pressed against the liquid drop for it to be used as a supporting backplane. After preparation of a polymer replica by UV exposure and mold removal, the PUA replica Received: October 8, 2010 Revised: May 13, 2011 Published: May 27, 2011 7944
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Figure 1. (a) Schematic illustration of imprinting with a PDMS mold having a flat bottom. Polymer can be trapped in the middle of the mold by concentrated pressure at the mold edges. (b) SEM image of trapped polymer during imprinting and (c) its enlarged view. (d) Schematic illustration of imprinting with a PDMS mold having a rounded bottom. (e) SEM image demonstrating the utility of the rounded bottom and (f) its enlarged view. was additionally exposed to UV for several hours for complete curing. To reduce the surface energy for easy release from the master, the PUA molds were treated with a fluorinated-SAM solution [(tridecafluoro1,1,2,2-tetrahydrooctyl)trichlorosilane (FOTCS), Gelest Corp.] diluted to 0.03 M in anhydrous heptane (Samchon Corp.) in an argon chamber. PDMS molds were fabricated by casting the PDMS prepolymer against a complementary relief structure on the silicon master prepared by photolithography. We used the PDMS precursor with mixing ratio of 10:1 (precursor:curing agent) and cured it at 60 °C for 10 h. The cured PDMS molds were manually removed and cut prior to use. 2.3. Soft Thermal Imprint Lithography. The prepared mold with microscale patterns was placed on a spin-coated PS thin film onto silicon substrate and then pressed with a constant pressure (∼4 bar with PUA mold and < 1 bar with PDMS mold). Such a low pressure turned out to prevent the roof collapse deformation that is typically generated by overpressure. The thickness of the polymer film ranged from 150 to 600 nm. While the mold was pressed, temperature was raised above Tg of the polymer (97 °C)12 and the sample was left undisturbed for a period of time. After cooling down to room temperature, the mold was removed and the imprinted patterns were analyzed with scanning electron microscopy (SEM). 2.4. SEM Measurements. The patterns generated were examined by SEM (Philips, XL30FEG) at an acceleration voltage higher than 10 kV. Samples were coated with a 4 nm Pt layer prior to analysis to prevent electron charging.
3. RESULTS AND DISCUSSION 3.1. Mold Edge Design. For residual layer-free thermal imprint processes, the mold material needs to be rubbery or soft. There are also advantages in using soft molds for thermal imprint. The soft nature assures conforming contact of the mold with the underlying layer, which in turn leads to a reduction in the applied pressure.13 This feature also helps prevent substrate breakage, unlike hard molds. This use of a soft mold is not without problems. One directly related to surface exposure is illustrated in Figure 1a. With the use of a soft PDMS mold having a flat bottom surface, polymer can be trapped in the middle of the mold, because the applied force is felt most strongly at the mold edges. Our speculation can be judged by scanning electron microscope (SEM) images, as shown in Figure 1b,c. This problem can be circumvented by making the bottom surface of the mold rounded, as illustrated schematically in Figure 1d. The SEM images in Figure 1e,f clearly demonstrate the utility of the rounded bottom. Alternatively, equal pressure distribution was
Figure 2. (a) Schematic illustration of feature geometry of the mold (L = diameter of holes, S = space between holes, x = thickness of polymer layer). (b) When the polymer layer is thin with respect to the height of the mold cavity, the substrate can be exposed, whereas (c) a residual polymer remains when the polymer film is thick enough to fill the cavity of the mold.
achieved by inserting a convex-shaped supporting block between the mold and a contacting substrate.13 3.2. Lower Limit of Aspect Ratio. The prerequisite on polymer layer thickness for ensuring surface exposure can be quantified given the geometry of the mold feature. For this purpose, we choose as a general case a mold pattern of an array of cylindrical holes. Another general case is an array of lines and spaces between lines. However, the stability of thin polymer strips is not yet securely established in equilibrium conditions.10 The feature geometry of the master from which the corresponding mold would be prepared is shown in Figure 2a, where L is the diameter of holes, S is the space between holes, and x is the thickness of polymer layer. Assuming that the polymer material above Tg is an incompressible liquid,11,14 we can calculate the condition that the entire polymer in the contact area should be collected into the holes on the basis of mass conservation, which is given by x 2L 3 θ 8 cos ð1 þ yÞ
Figure 5. (a) Tilted SEM image when the diameter is 3.2 μm and the height is 2.5 μm (aspect ratio = 0.78). The patterns are stable without dewetting in the center. (b) When the aspect ratio is decreased to 0.08 (11 μm in diameter, 0.9 μm in height), there are dewetting-induced voids inside the disk.
If we combine eq 1 with eq 4, we have R2 π H 2L < x < ð1 þ yÞ 4ð1 þ yÞ2
ð5Þ
It follows from eq 5 that L
L π
ð6Þ
which is a constraint on the hole height H for a given value of L. The condition z < H leads to the following inequality: R1 L < H
H=L > R1
ð7Þ
where eq 3 has been used. Figure 4 shows that, for all acute contact angles, eq 7 is a more constrained condition than eq 6. Therefore, the constraint on the mold dimensions is given by eq 7, indicating that H/L should be larger than a critical value for residual layer-free patterning. Shown in Figure 5 are the polymer resist patterns after soft thermal imprint9 using PUA as a mold and PS as a resist. Figure 5a shows a tilted SEM image of the polymer resist having a stable shape without dewetting when the mold height (H) is 2.5 μm (H/L = 0.78) and the diameter of the disk pattern (L) is 3.2 μm. When H/L is decreased to 0.08 (11 μm in diameter, 0.9 μm in height), however, dewetting occurred inside the diskshape resist pattern, which results in donut-shaped pillars with 7946
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Figure 7. Structural phase diagram for residual layer-free patterning without detachment and dewetting. Figure 6. Schematic illustrations of three cases. (a) The pattern height is less than R1LM (H < R1LM); there are dewetting-induced voids in the center of the disk. (b) The pattern height satisfies the condition of eq 13; all disks and pillars are stable. (c) The height is too high to meet the condition of eq 12 [H > 0.25(Wp-s/Wp-m 1)Lm]; the pillars are detached upon removal of the mold.
voids as shown in Figure 5b. The pattern symmetry has been broken, presumably because of the slip of the residual layer through uneven distribution of the contact pressure. These results are all in good agreement with the above criterion, as R1 is approximated to 0.0880.18 for θ = 50°∼70°.18 3.3. Upper Limit of Aspect Ratio. In order to avoid dewetting in the middle of the pattern, as eq 7 shows, the aspect ratio must be high. When we prepare a sample with high aspect ratio, however, we have to be concerned about lateral collapse between mold features. In the present case, the mold has a negative type, so there are holes instead of pillars. There are many reports about using PDMS molds with high aspect ratio hole patterns to obtain high aspect-ratio polymer pillar replicas.1921 Therefore, we can assume that there should be no problem in producing a mold with high aspect-ratio holes. Instead, when the target polymer resist has a low modulus, it is well-known that the patterned structures are prone to collapse due to mechanical instability. The relationship is shown below:22,23 H=L
R1
ð11Þ
Moreover, there is a constraint on Lm that comes from the relationships of eqs 9 and 10: H=Lm < β=Lm 1=6
or
H=Lm < 0:25ðWp-s =Wp-m 1Þ ð12Þ
Combining eqs 11 and 12 yields ð8Þ
where Ep is Young’s modulus and ν is Poisson’s ratio of the polymer resist. Rearranging the equation yields H=L < βS1=2 =L1=6
where Wp-m and Wp-s are the works of adhesion at the polymer/ mold and polymer/substrate interfaces, respectively. The equation means that the aspect ratio has an upper limit imposed by mechanical and adhesion stabilities; a patterned material needs to be sufficiently rigid and the work of adhesion between polymer and substrate needs to be sufficiently high to ensure clean release of the mold. In the current system as well as other polymer materials, the detachment condition shown in eq 10 is overwhelmingly more constrained than the mechanical stability, which should be taken into consideration for the mold design. 3.4. Mold with Various Feature Sizes but Same Pattern Height. For the preparation of a silicon master, either photolithography or electron-beam lithography is used. Therefore, the height H is typically the same for all features in the mold as shown in Figure 6. For the maximum hole diameter LM and the minimum Lm, it is sufficient that the condition of eq 7 be satisfied for the largest feature, since then the condition is automatically satisfied for the smallest size Lm. We have, therefore,
1 1 πLH þ πL2 Wp-m < πL2 Wp-s H=L < 0:25ðWp-s =Wp-m 1Þ 4 4
ð10Þ
R1 LM < H < βLm 5=6
or
R1 LM < H < 0:25ðWp-s =Wp-m 1ÞLm
ð13Þ When this condition is met or when H is chosen to satisfy eq 13, the patterning will lead to stable polymer structure without dewetting-induced voids or detachment of polymer resists, as schematically illustrated in Figure 6. Here, the conditions obtained above are exemplified with PS as a polymer material. As mentioned earlier, the mechanical stability can be neglected because of the high modulus of PS (Ep = 2.8 GPa),12 suggesting that detachment stability is a dominant factor for patterning failure, as shown in Figure 6c. Figure 7 shows a structural phase diagram for residual layer-free patterning without detachment and dewetting. In this diagram, the contact angle and the work of adhesion ratio (Wp-s/Wp-m) were assumed to be 60° and 2, respectively. The diagram reveals that Lm and LM can be adjusted within the stable windows to meet eq 13 for a given pattern height. It is worthwhile noting 7947
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4. CONCLUSIONS We have presented the conditions for mold dimensions that are necessary to ensure no residual layer after thermal imprint and compared the results with simple soft imprint experiments by using soft PUA or PDMS molds and patterned resists of low MW PS (MW = 2700). These conditions were derived with respect to the aspect ratio for a single feature of the mold or the pattern height for multiple features of the mold. When there is only one type of feature in the mold, the condition for residual layer-free patterning is given by eqs 7 and 10. For the case where there are multiple features in the mold pattern, it is given by eq 13. Despite extensive efforts directed toward thermal imprint in the past decade, little attention has been paid to the design aspect of the mold in resolving the issues critical to thermal imprint. This work could be an impetus for bringing attention to mold design and mold material, which have not been dealt with seriously in spite of their importance in thermal imprint.
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’ AUTHOR INFORMATION Corresponding Author
*(K.C.) Tel þ82-2-880-7431, fax þ82-2-873-1548, e-mail:
[email protected]. (K.Y.S.) Tel þ82-2-880-9103, fax þ82-2-883-1597, e-mail
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Creative Research Initiative Center for Intelligent Hybrids (2010-0018290), the WCU Programs (R31-10013, R31-2008-000-10083-0), the Center for Biomimetic Mechanical Systems (KRF-J03003), the Basic Science Research Program (2010-0027955), and the BK 21 Program through the National Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST). ’ REFERENCES (1) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85. (2) Guo, L. J. Adv. Mater. 2007, 19, 495. (3) Lalo, H.; Vieu, C. Langmuir 2009, 25, 7752. (4) Shift, H. J. Vac. Sci. Technol., B 2008, 26, 458. (5) Choi, S. J.; Yoo, P. J.; Baek, S. J.; Kim, T. W.; Lee, H. H. J. Am. Chem. Soc. 2004, 126, 7744. (6) Kim, Y. S.; Lee, H. H. Adv. Mater. 2003, 15, 332. (7) Yoon, H.; Lee, K. M.; Khang, D.; Choi., S. J.; Lee, H. H. Appl. Phys. Lett. 2004, 85, 1793. (8) Park, H.; Cheng, X. Nanotechnology 2009, 20, No. 245308. (9) Suh, K. Y.; Kim, Y. S.; Lee, H. H. Adv. Mater. 2001, 13, 1386. (10) Suh, K. Y.; Lee, H. H. J. Chem. Phys. 2001, 17, 8204. (11) Yoon, H.; Choi, M. K.; Suh, K. Y.; Char, K. J. Colloid Interface Sci. 2010, 346, 476. (12) Mark, J. E. Physical Properties of Polymers Handbook; AIP Press: Woodbury, NY, 1996. (13) Khang, D.; Lee, H. H. Adv. Mater. 2004, 16, 176. (14) Zhang, Y.; Lin, C. T.; Yang, S. Small 2010, 6, 768. (15) Bruinink, C. M.; Peter, M.; Maury, P. A.; de Boer, M.; Kuipers, L.; Huskens, J.; Reinhoudt, D. M. Adv. Funct. Mater. 2006, 16, 1555. (16) Lee, S. K.; Jung, J. M.; Lee, J. S.; Jung, H. T. Langmuir 2010, 26, 14359. 7948
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