Benchtop Nanoscale Patterning Using Soft Lithography. V. Meenakshi, Y. Babayan, and T. W. Odom, Department of Chemistry, Northwestern University

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1 V. Meenakshi, Y. Babayan, and T. W. Odom, Department of Chemistry, Northwestern University INTRODUCTORY INFORMATION FOR STUDENTS AND INSTRUCTORS INTRODUCTION AND BACKGROUND Recent advances in nanoscience and the discovery of size and shape-dependent properties of nanomaterials have generated interest in developing new methods to create nanoscale (sub- 100 nm) structures. Specifically, the fields of microelectronics and optoelectronics have been a driving force for miniaturization and the development of new micro- and nanofabrication techniques. In order to obtain patterns and structures at nanometer length-scales, research has focused on two complementary approaches: top-down and bottom-up. Top-down (a version of miniaturization) refers to reducing sizes from the macroscale to the microscale into the nanoscale, while bottom-up approaches involve building or assembling larger structures from nanoscale building blocks (e.g. molecules). Top-down approaches typically use lithographic techniques to generate nanoscale patterns on substrates. The most common top-down methods include photolithography, electron-beam lithography, and focused ion beam milling.(1-3) Conventional fabrication techniques require commercial equipment, expensive clean room facilities and high operational costs. Because of these requirements, access to patterning techniques is typically limited to a select group of users. In contrast, soft lithography is not subject to these limitations. Soft lithography consists of a suite of techniques that use only a patterned elastomeric material, such as polydimethylsiloxane (PDMS), as a soft mask to replicate and transfer patterns from a master onto a surface. This parallel process consists of three steps: (i) the fabrication of a master; (ii) the generation of a PDMS mold or stamp from the master; and (iii) the creation of a replica of the master from the patterned PDMS stamp. A pattern can be generated over an entire area of a substrate that is in contact with the stamp. Hence, soft lithography is suitable for forming patterns over large areas (> 50 cm 2 ) and has the ability to prototype structures and pattern non-planar surfaces using ordinary lab facilities. A set of soft lithographic techniques,(4) including microcontact printing (µcp), micromolding in capillaries (MIMIC), and replica molding (RM), will be covered in the experiments outlined in this paper. We will show how all of these techniques can be used to generate nanoscale patterns on the benchtop.

2 FABRICATION OF MASTERS AND PREPARATION OF PDMS STAMPS Fabrication of Masters The fabrication of masters usually involves expensive instrumentation and facilities. An inexpensive and readily available master with sub-500 nm features can be obtained from data storage devices such as compact discs (CDs) and digital versatile discs (DVDs). Conventional discs store digital data in the form of bumps and flat areas arranged in a long spiral track starting at the center of the disc. The discs have a pattern imprinted into a Figure 1: Cross section of a CD. polycarbonate (PC) layer covered with a reflective aluminum (Al) coating, which in turn is covered by a transparent protective acrylic layer. Figure 1 shows a cross section of a CD. The PC layer is comprised of 1.2-µm wide and 110-nm tall bumps that are spaced by 690 nm; these features are pre-molded onto the CD during manufacturing. The Al recording layer that is in contact with the PC substrate has the inverse features. Two different types of masters can be made from a CD, either from the PC layer or the Al layer. In order for either layer to be used as a master, a commercially available CD (in this paper, Sony CD-R) is cut into pieces with scissors. Using tweezers, the Al layer can then be peeled off to reveal the PC surface. Both the PC surface and the Al layer, with complementary features, can now be used as masters. Fabrication of PDMS Molds An elastomeric mold patterned with a relief structure is the key element in soft lithography. PDMS is a silicone-based organic polymer that is commonly used as the mold material; it is optically transparent, non-toxic and generally inert. PDMS molds can be prepared by casting the pre-polymer [Sylgard 184 from Dow Corning available as a two component kit: (i) silicon rubber base and (ii) a curing agent] over the master, curing at 70 C for about an hour to cross-link the elastomer and then peeling the PDMS off the surface (Figure 2). Thin PDMS molds (~ 2 mm thick) are ideal, so that conformal contact can be achieved between the mold and the surface. Supplemental Material: Introductory Information 2

3 After curing, the mold can be removed from the master by cutting around the patterned area using a sharp razor blade or a scalpel. The low surface free energy and elasticity of the PDMS allows it to be peeled off easily from the master without damaging itself or the master. In these experiments, a protrusion in the master corresponds to a flat region in the PDMS mold (note: a PDMS mold replicated against the Al surface resembles the PC master). PDMS molds made from either the PC or Al surface of CD are now ready to be used for nanopatterning by soft lithography. Note that the same master can be used multiple times to produce multiple copies of the PDMS molds. SOFT LITHOGRAPHY Replica Molding (RM) Replica molding has the capability to generate multiple replicas of nanostructures starting from a single master. Both the simplicity, low cost and versatility confirm its potential use in patterning nanostructures. Advantages of RM include its ability to replicate three dimensional topologies in a single step and to provide high fidelity structure replication with accuracy down to the size of large molecules (~ 2 nm). Replica molding is a technique that involves casting organic polymers against a PDMS mold to produce a pattern (or replica) that is identical to the master. Feature sizes as small as 30-nm wide and 5-nm Figure 2: Scheme for making PDMS mold using PC layer. Figure 3: Scheme of RM. tall are possible with this technique.(5) This technique has found widespread application in the manufacturing of structures such as compact discs and diffraction gratings. Figure 3 illustrates Supplemental Material: Introductory Information 3

4 the general procedure behind RM. A drop of liquid pre-polymer, such as polyurethane (PU), is placed between a substrate (e.g., a microscope slide) and a PDMS mold that comes into conformal contact with the substrate. After the PU is cured under UV light, the PDMS can be carefully peeled off to reveal a PU-replica of the master on the glass surface. Micro-Molding in Capillaries (MIMIC) One type of RM is micro-molding in capillaries (MIMIC), a soft lithographic technique based on the spontaneous filling of a fluid in capillaries formed between two surfaces. Figure 4 illustrates the general procedure of MIMIC. A PDMS mold is placed on the surface of a substrate (e.g., a microscope slide) to form a network of empty channels. At the ends of the channels, a drop of low-viscosity photocurable fluid (such as PU) can be placed; the fluid will spontaneously be drawn into the channels by capillary forces. After the PU is cured under UV light, the PDMS mold can be removed to reveal patterned PU micro- or nanostructures on the substrate. Similar to RM, the polyurethane features very closely replicate the size and shape of the features of the master. The application of capillary molding is limited by the viscosity of the liquid precursors, and it remains a challenge to fill channels with dimensions less than 100 nm. Micro-contact Printing and Etching Figure 4: Scheme for MIMIC. Micro-contact printing (µcp) is a powerful technique for selectively patterning and functionalizing surfaces with organic molecules. This technique uses a patterned PDMS stamp (a stamp is same as the mold referred to earlier in the article) to deliver a chemical ink onto a noble metal surface. When the soft PDMS stamp conformally contacts the metal surface, the molecules are transferred directly from the stamp to the surface (in the areas of direct stamp/metal contact) to form self-assembled monolayers (SAMs) by chemisorption (Figure 5). The physical dimensions of the features on the stamp primarily determine the minimum feature sizes of the patterns that can be generated with this technique. Although µcp is extremely useful for a wide range of applications, its resolution encounters practical limits around nm, Supplemental Material: Introductory Information 4

5 because of the combined effects of: (i) surface diffusion of molecular inks; (ii) disorder at the edges of the printed SAMs; and (iii) the isotropic nature of many of the etching and deposition methods used to convert the patterned SAMs into patterns of functional materials.(2,6) Thin films of coinage metals such as Au, Ag, Cu, and Pd can be used for µcp, although the most widely used system involves patterning alkanethiol molecules on Au. In an earlier report, vinyl gold films (used in signage industry) were employed as substrates for the formation of SAMs of thiol molecules.(7) The procedure to obtain Au surfaces suitable for µcp, however, required many laborious steps. For our experiments, archival gold CD-Rs from Diversified Systems Group, Inc. are used. The CDs can be treated with nitric acid to remove the protective polymer coating and expose the patterned Au surface. The basic steps involved in µcp are outlined in Figure 5. A patterned stamp is coated with the ink solution and then brought into conformal contact with an Au film for several seconds. The stamp is then removed from the surface to leave behind a patterned SAM on Au. These patterned SAMs can also be used to generate metallic structures with the same pattern as the PDMS stamp because the SAMs can act like a mask or etch resist to protect the underlying metal surface (Figure 5, left). Figure 5: Procedure for µcp and etching of Au film (left) and Au-CD (right). Besides nanostructures of Au lines (or other, different patterns depending on the PDMS mask), more complex patterns can be produced by µcp on patterned Au surfaces. For example, after µcp using a PDMS stamp patterned with lines at an angle to the Au lines of the CD, the sample can be exposed to an Au etchant to remove the unprotected Au-areas and produce a checkerboard pattern (Figure 5, right). Thus, µcp combined with wet chemical etching can provide a way to generate complicated metallic nanostructures. Supplemental Material: Introductory Information 5

6 REFERENCES (1) Bratton, D.; Yang, D.; Dai, J.; Ober, C. K. Polym. Adv. Technol. 2006, 17, (2) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. Rev. 2005, 105, (3) Harriott, L. R.; Hull, R. Introduction to Nanoscale Science and Technology 2004, (4) Xia, Y.; Whitesides, G. M. Angew. Chem. In. Ed. 1998, 37, (5) Xia, Y.; McClelland, J. J.; Gupta, R.; Qin, D.; Zhao, X. M.; Sohn, L. L.; Celotta, R. J.; Whitesides, G. M. Adv. Mater. 1997, 9, (6) Biebuyck, H. A.; Larsen, N. B.; Delamarche, E.; Michel, B. IBM J. Res. Dev. 1997, 41, (7) McKenzie, L. C.; Huffman, L. M.; Parent, K. E.; Hutchison, J. E.; Thompson, J. E. J. Chem. Educ. 2004, 81, Supplemental Material: Introductory Information 6

7 V. Meenakshi, Y. Babayan, and T. W. Odom, Department of Chemistry, Northwestern University STUDENT HANDOUT CONCEPTS This handout provides a step-by-step demonstration of various nanoscale patterning experiments and explains the basic principles behind the techniques. You will learn (i) how to fabricate masters from compact discs (CDs) and (ii) how to pattern nanometer-sized features using soft lithography (microcontact printing, replica molding and micro-molding in capillaries). Note that these experiments are demonstrated in detail in the video lab manual, which can be found online at FABRICATION OF MASTERS Conventional masters for pattern replication are fabricated using photolithography. While photolithography is a versatile technique, it is very expensive and time consuming. A simple way to make masters is to use commercially available compact discs (CDs), which have patterns of nanoscale dimensions. Basic CD composition and feature sizes are shown in Figure 1. Figure 1: Schematic diagram of the layers of a CD. Procedure: 1. Obtain a CD-R and use the scissors to cut out a piece ~ 1x1 in 2.

8 2. Turn the CD upside down and use tweezers to peel off the Al layer; it should easily separate from the PC layer. When the layers are separated, place the patterned side facing up into petri dishes and label the containers. 3. If available, use an atomic force microscope (AFM) to image the patterns on the Al and PC layers. Compare the width and height of the patterns. 4. These PC (left) and Al (right) layers can now be used as masters for mold generation. PREPARATION OF PDMS MOLDS Poly(dimethyl)siloxane is a silicone-based organic polymer with the chemical formula of (CH 3 ) 3 SiO[SiO(CH 3 ) 2 ] n Si(CH 3 ) 3. PDMS is clear, inert and nontoxic. It is commonly used in soft lithographic techniques because of its low-surface energy, which allows the polymer to conform easily to the surface. Procedure: 1. Weigh out ~20 g of the Sylgard 184 pre-polymer into a plastic cup. To the same cup, add 2 g of curing agent (10:1 weight ratio of pre-polymer to curing agent). Supplemental Material: Student Handout 2

9 2. Mix the pre-polymer/curing agent mixture vigorously with a plastic fork until it is full of bubbles (1-2 min). 3. Place the cup with the polymer mixture into a desiccator for min to degas (remove the bubbles) the PDMS. Make sure all the bubbles are gone before removing. 4. Slowly pour PDMS over the PC and Al masters obtained from the CD. 5. Place the petri dish with the sample into an oven to cure at 70 C for 1 hour. 6. Using a scalpel, gently and evenly cut out a piece of the patterned PDMS from each sample. Supplemental Material: Student Handout 3

10 7. Remove the mold using tweezers and place it facedown on a clean glass slide. Make sure to label the molds so you know from which master they came. 8. Image the molds using AFM. Questions: 1. What is the relationship between the patterns of the Al and the PC masters? 2. What is the relationship between the patterns of the PDMS molds and the masters? SOFT LITHOGRAPHY: REPLICA MOLDING (RM) Procedure: 1. Clean a glass slide using soap and water. Rinse the slide with ethanol and dry with a stream of nitrogen. 2. Place couple of drops of liquid polyurethane on the glass slide. 3. Using tweezers pick up the PDMS mold (either from the Al or the PC master) and place it patterned side down on top of the polyurethane drop. Press down lightly on the mold with tweezers to ensure that the mold makes contact with the glass. Supplemental Material: Student Handout 4

11 4. Place the sample ~5 inches under UV lamp for 10 min to cure the polyurethane. 5. Remove the mold with tweezers and place the patterned side on a clean glass slide. 6. To visualize the polyurethane pattern look at it under an optical microscope or AFM. Questions: 1. Characterize your RM sample and compare it to your PDMS mold. How well did the pattern transfer? 2. In molding techniques, how does the pattern compare with the pattern of the mold? How could you obtain the exact pattern as the one on the mold? 3. What are the limitations of RM? Supplemental Material: Student Handout 5

12 Benchtop Nanoscale Patterning Using Soft Lithography SOFT LITHOGRAPHY: MICRO MOLDING IN CAPILLARIES (MIMIC) Procedure: 1. Clean a glass slide using soap and water. Rinse the slide with ethanol and dry with a stream of nitrogen. 2. Pick up one of the PDMS molds (if you used the PDMS from the Al master for the RM experiment, use the mold from the PC master for this experiment) and determine the direction of the lines on the mold. This can be accomplished by shining light from a laser pointer through a PDMS mold (patterned side facing down) held above a piece of white paper. Make sure you understand how the diffraction pattern relates to the direction of the lines on the mold. WARNING: Make sure not to shine the laser into your eyes. Do not place your head directly above the sample. 3. Place the mold facedown on the cleaned glass slide. 4. To create nanochannels, trim the ends of the PDMS mold by pressing a razor blade down perpendicular to the glass substrate and across (perpendicular to) the direction of the lines. Supplemental Material: Student Handout 6

13 5. Place a drop of liquid polyurethane at one of the open ends of the channels. Wait for ~10 min for the channels to fill with polyurethane by capillary action. 6. Place the sample under UV lamp and let the polyurethane cure for 10 min. 7. Remove the mold and place it facedown on a clean glass slide. As you hold the sample and rotate it through different angles, you should see a diffraction pattern on the slide indicating successful molding. 8. Look at the polyurethane sample under an optical microscope or AFM. Questions: 1. Characterize your MIMIC sample and compare it to your PDMS mold. How well did the pattern transfer? 2. Compare and contract the two molding techniques RM and MIMIC. What are their advantages and disadvantages? Supplemental Material: Student Handout 7

14 3. Which technique produced the best replication of the CD pattern from the PDMS mold? 4. In molding techniques, how does the pattern compare with the pattern of the mold? How could you obtain the exact pattern as the one on the mold? SOFT LITHOGRAPHY: MICRO-CONTACT PRINTING AND ETCHING This lab introduces a technique that takes advantage of the chemical interactions between the material (in this lab, archival grade Au CDs, Figure 2) that needs to be patterned and its surface functionality. Self-assembled monolayers (SAMs) can be used as the patterning material and etch resist to generate metallic nanostructures. Figure Figure 2: Scheme 2: Composition of layers of Au-CD. archival Au CD. The areas patterned with SAMs protect the Au from being etched when subject to Au etchant. Procedure: A. Preparing Au CD Sample 1. Using scissors cut out a piece of Au recordable archival CD. 2. Pour ~20 ml of concentrated HNO 3 into a small beaker. WARNING: HNO 3 is highly corrosive and can cause severe burns. Handle with care. Place the dull side of the CD facedown into a beaker, making sure the CD is completely immersed in the acid. This acid treatment of the CD will remove the protective layer from the CD. Supplemental Material: Student Handout 8

15 3. Pour off the acid into an appropriately labeled waste container and remove the CD from the beaker using tweezers and rinse it with copious amount of water. The protective polymer layer should peel off. 4. Dry the CD with nitrogen. The Au CD sample is ready to be used. 5. To visualize the Au lines you can look at it in the microscope or shine a laser pointer onto the sample and look at the diffraction pattern that is produced. The best way to see the diffraction pattern is by shining a laser pointer through the Au-CD held above a piece of white paper. B. Micro-Contact Printing Using PDMS Stamps (NOTE: The word stamp is used for these experiments because the PDMS mold is now used to print molecules onto gold surfaces.) 1. Make 5 ml of a 1 mm solution of octadecanethiol in ethanol. This ink solution will be used for µcp. Supplemental Material: Student Handout 9

16 2. Dip a cotton swab into the octadecanethiol solution and rub it back and forth across the patterned side of the PDMS stamp (generated from the either the PC or Al layers of the CD) for 10 s. 3. Dry the ink on the stamp under a stream of nitrogen for 30 sec. It is very important that you remove excess ink solution. 4. Place the stamp into conformal contact (you might have to slightly press on the stamp from the top) with the Au surface such that the lines on the stamp are perpendicular to the lines on the Au substrate. To determine which way the lines are patterned on the CD, you can use the laser diffraction method described earlier. Leave the stamp on the substrate for 10 sec and then remove it with tweezers. This process will transfer your thiol ink onto the Au surface. 5. To visualize the ink transfer from the stamp to the Au sample, breathe on the Au sample. You should see a diffraction pattern. The diffraction pattern indicates ink transfer and that your experiment was successful. Supplemental Material: Student Handout 10

17 C. SAMs as Etch Resists 1. Make 15 ml of Au etching solution by diluting the commercial etchant with deionized water (1:3 v/v). Stir the solution well with a stir bar. 2. Using plastic hemostats, place your patterned Au sample into the etching solution for sec. Remove the sample, rinse it with water and dry under a nitrogen stream. 3. To visualize the Au checkerboard pattern you can look at it in the microscope or shine a laser pointer onto the sample and look at the diffraction pattern that is produced. You might have to dim the lights to see the additional diffraction spots produced after etching. Questions: 1. Compare the etched and unetched patterns by looking at the diffraction pattern from the sample by shining a laser pointer onto the sample. What does the diffraction look like from each sample? 2. What are the limitations of micro-contact printing? How does diffusion of the ink play a role? 3. Is micro-contact printing followed by etching a good way to make micro- or nanostructures? Why or why not? Supplemental Material: Student Handout 11