Mechanical and Aerospace Engineering, University of Texas at Arlington
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Chakraborty, A., Liu, X., Luo, C. Micropunching Lithography for Generating Micro- and Submicron-patterns on Polymer Substrates. J. Vis. Exp. (65), e3725, doi:10.3791/3725 (2012).
Conducting polymers have attracted great attention since the discovery of high conductivity in doped polyacetylene in 19771. They offer the advantages of low weight, easy tailoring of properties and a wide spectrum of applications2,3. Due to sensitivity of conducting polymers to environmental conditions (e.g., air, oxygen, moisture, high temperature and chemical solutions), lithographic techniques present significant technical challenges when working with these materials4. For example, current photolithographic methods, such as ultra-violet (UV), are unsuitable for patterning the conducting polymers due to the involvement of wet and/or dry etching processes in these methods. In addition, current micro/nanosystems mainly have a planar form5,6. One layer of structures is built on the top surfaces of another layer of fabricated features. Multiple layers of these structures are stacked together to form numerous devices on a common substrate. The sidewall surfaces of the microstructures have not been used in constructing devices. On the other hand, sidewall patterns could be used, for example, to build 3-D circuits, modify fluidic channels and direct horizontal growth of nanowires and nanotubes.
A macropunching method has been applied in the manufacturing industry to create macropatterns in a sheet metal for over a hundred years. Motivated by this approach, we have developed a micropunching lithography method (MPL) to overcome the obstacles of patterning conducting polymers and generating sidewall patterns. Like the macropunching method, the MPL also includes two operations (Fig. 1): (i) cutting; and (ii) drawing. The "cutting" operation was applied to pattern three conducting polymers4, polypyrrole (PPy), Poly(3,4-ethylenedioxythiophen)-poly(4-styrenesulphonate) (PEDOT) and polyaniline (PANI). It was also employed to create Al microstructures7. The fabricated microstructures of conducting polymers have been used as humidity8, chemical8, and glucose sensors9. Combined microstructures of Al and conducting polymers have been employed to fabricate capacitors and various heterojunctions9,10,11. The "cutting" operation was also applied to generate submicron-patterns, such as 100- and 500-nm-wide PPy lines as well as 100-nm-wide Au wires. The "drawing" operation was employed for two applications: (i) produce Au sidewall patterns on high density polyethylene (HDPE) channels which could be used for building 3D microsystems12,13,14, and (ii) fabricate polydimethylsiloxane (PDMS) micropillars on HDPE substrates to increase the contact angle of the channel15.
A. Schematics of the MPL
The macropunching method includes "cutting" and "drawing" operations. The "cutting" operation adopts molds of sharp-edged convex structures and includes three basic steps (Fig. 1(a1-a3)). First, place a sheet metal on a rigid substrate (Fig. 1(a1)). Second, bring a Si mold and the substrate into physical contact by a high force. During this second step, the part of the metal directly underneath convex mold structures is first cut off from the neighboring metal by the convex mold structures, and then pushed down to the bottom of the concave patterns in the substrate (Fig. 1(a2)). Finally, separate the mold and the substrate, completing the patterning of the sheet metal (Fig. 1(a3)). The "drawing" operation uses a similar fabrication process. However, it adopts molds of round-edged convex structures (Fig. 1(b1)). Furthermore, the applied insertion force and speed are much smaller and lower than their counterparts in the "cutting" operation. These differences lower the stresses present in the part of the sheet metal under convex structures. Consequently, this part of the sheet metal is just pushed down but not cut off in the "drawing" operation (Fig. 1(b2-b3)).
In the "cutting" operation of the MPL (Fig. 1(c1-c3)), (i) a Si substrate coated with a layer of an intermediate polymer and a layer of a material to be printed are heated above glass transition temperature (Tg: softening temperature) of the intermediate polymer and below Tm (melting temperature) or Tg of the targeted material (Fig. 1(c1)), (ii) the mold and the substrate are brought into physical contact by high pressure, followed by subsequent cooling (Fig. 1(c2)), and (iii) they are separated when their temperature is below Tg of the intermediate polymer, completing the pattern transfer from the mold to the targeted layer (Fig. 1(c3)). The "drawing" operation of the MPL (Fig. 1(d1-d3)) has fabrication steps similar to the "cutting." Nevertheless, the "drawing" uses soft PDMS molds. It also involves a smaller insertion force, a lower insertion speed, and a higher printing temperature (which lowers the viscosity of the intermediate polymer and thus increases its mobility). Accordingly, the features at the top surface of the substrate curve up due to the surface tension and the high mobility of intermediate polymer. The Si mold may be cleaned and re-used for successive embossing steps. The mold may be cleaned with acetone and DI water; and dried thoroughly with N2 before each use. In case residues remain in the microfeatures of the mold, it may be cleaned with Nanostrip solution and DI water; and dried with N2.
B. Cutting Operation in MPL for Generating Metal and Conducting Polymer Micropatterns
C. Cutting Operation of the MPL for Generating Sub-micron Ppatterns of Metal and Conducting Polymer
Based on the procedure illustrated in Fig. 1(c1-c3), Si molds with sub-micron features are used to generate desired patterns of metal and conducting polymers. The fabrication is detailed below.
D. Drawing Operation of the MPL for Generating Micropatterns on Sidewalls of Polymer and Si Substrates.
Following the procedure in Fig. 1(d1-d3), the "drawing" operation is used to generate Au and PDMS micropatterns on the sidewalls of HDPE microchannels. The corresponding material on the HDPE substrate is Au or PDMS, which follows the surface profile of the intermediate-layer polymer during imprinting. The fabrication is detailed below.
E. Representative Results
In summary, the results of MPL are listed below:
Figure 1. The "cutting" process in creation of convex macropatterns in a sheet metal (cross-section schematics): (a1) place a sheet metal on the top of the substrate, (a2) insert the mold into the substrate, and (a3) separate the mold and the substrate. The "drawing" process in fabrication of concave macropatterns: (b1) place a sheet metal on the substrate, (b2) insert the mold into the substrate, and (b3) separate the mold and the substrate. The "cutting" operation of the MPL method in the fabrication of convex structures (cross-section schematics): (c1) heat the substrate, (c2) insert the mold into the substrate, and (c3) separate the mold and the substrate. The "drawing" operation of the MPL approach in the fabrication of concave structures: (d1) heat the substrate, (d2) insert the mold into the substrate, and (d3) separate the mold and the substrate.
Figure 2. Designs of Si molds (top view): (a1) straight lines; (a2) square dots; (a3) truss structures; and (a4) serpentine lines. (b) The hot embossing machine. SEM images of generated Al structures: (c1) 10-μm-wide lines; (c2) 20×20 μm2 dots; and (c3) truss structures. (d1) Schematic of microstructures consisting of multiple structures; (d2) 300-μm-wide straight; (d3) 50-μm-wide serpentine microwire patterns of PPy, PEDOT, and SPANI fabricated simultaneously using the "cutting" operation of the MPL. (e) The humidity sensing experimental setup; and (f) humidity sensing results with PPy film and microwire sensor 4, 7, 8. Click here to view larger figure.
Figure 3. Layouts of: (a1) two- and (a2) three-layer devices; (b) layout of a Si mold (top view) used to fabricate multi-layer devices; (c) SEM image of a 300-μm-wide, microline-shaped PPY-PEDOT heterojunction; and close-up SEM views of cross-sections of: (d) PPy-PEDOT heterojunction; (e) Al-PEDOT diode; (f) PEDOT-PMMA-PEDOT capacitor; heterojunction characterization results: (g1) PPy/PEDOT; (g2) Al/PEDOT; and (g3) PEDOT/PMMA/PEDOT 9,11.
Figure 4. (a) AFM scan of the embossed 500-nm-wide PPy wires; SEM images of (b) embossed 100-nm-wide PPy lines and (c) 100-nm-wide Au wires. Click here to view larger figure.
Figure 5. Fabrication of a HDPE substrate with Au patterns: (a-b) using a mask of desired features, expose and develop the S1813 layer; (c-d) deposit Au and remove the S1813 layer; (e-f) imprinting the substrates using a Si reinforced PDMS mold; and (g) after demolding, a substrate with sidewall patterns consisting of Au features 12.
Figure 6. Fabrication of a PDMS film with micropillars: (a) fabricate an SU-8 mold; (b) spin-coat and cure a PDMS layer; (c) remove the PDMS layer from the SU-8 mold; (d) imprinting the substrate using an Al mold; and (e-f) after demolding, a substrate with sidewall patterns consisting of PDMS micropillars, are obtained 15.
Figure 7. (a) layout of the Au dots; SEM images of: (b) 10 x 10 μm2 dots; and (c) 110-μm-wide-lines. The dimensions of the channels generated in HDPE are 1 cm x 300 μm x 42 μm (length x width x depth); PDMS micropillars generated on the top, bottom and sidewall surfaces1-mm-wide HDPE channels: (d) cross-section view of the channel; SEM images of (e) top; (f) bottom corner of the channel; and (g-h) contact angle measurement results on PDMS pillars12,15. The PDMS pillars have the dimensions 10 μm x 10 μm x 27 μm. The dimensions of the channels in HDPE are 20 mm x 1 mm x 1 mm (length x width x height).
Troubleshooting information: Critical points regarding generation of single- and multiple-layer micropatterns of conducting polymers and metals using the "cutting" operation: (1) Temperature of embossing ensures fluidity of the intermediate PMMA layer which generates optimum results. It is advisable to start at the lower limit of the range and increase temperature gradually if desired results are not achieved. Too high temperature may cause the conducting polymer layer to change its chemical and/or electrical properties. (2) If imprinting force is too high, it may cause the Si mold to crack during embossing; whereas low force may result in improper mold filling and the polymer layers may not be cut-off. (3) The embossing machine should be programmed to start imprinting only after the substrate has attained the set temperature value. (4) The profile of the imprint depends upon feature size, imprinting force, temperature and mold insert time; and may vary between one with sharp edges to a more rounded one. As the number of polymer layers increase, the profile tends to be rounded at the edges. (5) Si molds with sharp edges are preferred to ensure that the conducting polymer/metal layers are cut-off as desired. Use of a Si mold with sloping sidewalls is not recommended. (6) Use of very deep Si mold may cause stiction between the mold and polymer layer(s). If the mold is too shallow it may not be able to achieve "cutting" of the top layer(s). (7) It is recommended that an anti-stiction film (e.g., teflon) should be coated on the Si mold for easily separating it from the substrate during the demolding process. (8) Low mold insert time may not result in "cutting" and the mold profile may be rounded. If the feature size is small, longer insert times are required and vice versa. (9) Thicker conducting polymer/metal films are more mechanically stronger compared to thinner ones. However, the combined thickness of the top layer should not be more than the thickness of the intermediate PMMA layer. (10) Demolding temperature should be below 105 °C (Tg of PMMA). A high value may result in the substrate being curved-up after demolding and a low value may cause the Si mold to stick to the substrate and break apart.
Critical points regarding the generation of the sub-micron PPy and Au wires: (1) Before using the Si molds for the first time, no AFM and SEM scans of the mold should be performed. This is necessary to maintain the pristine surface of the silicon mold. (2) Aggressive cleaning procedures such as using NanoStrip solution or oxygen plasma should be avoided, as there is a possibility of increasing the surface roughness of the silicon mold. This may induce stiction between the silicon mold and the conducting polymer layer. (3) The thickness of the top layer (conducting polymer or metal) should be less than the depth of the mold for cutting-off of the layer at the edges. There is no such limitation on the thickness of the insulating polymer layer. (4) The surface roughness of the silicon mold should be minimum. In case the surface of the silicon mold is rough due to processing or contamination, there may not be perfect contact between the mold and the substrate, resulting in improper pattern transfer.
Critical points regarding generation of Au micropatterns on HDPE channels: (1) At higher embossing temperatures (≥136 °C), the Au lines do not curve up to follow the surface as the HDPE is softened. (2) At higher mold depths (≥42 μm), the Au lines may break at the edges of the HDPE channels due to stress 12.
Critical points regarding the generation of HDPE channels with PDMS micropillars: (1) If the height of the PDMS micropillars is large, it may fall down after release from the SU-8 mold. (2) High aspect ratio of PDMS pillars may be damaged during the "drawing" step 15.
No conflicts of interest declared.
This work was supported in part through NSFDMI-0508454, NSF/LEQSF(2006)-Pfund-53, NSF-CMMI-0811888, and NSF-CMMI-0900595 grants.
|PMMA||Sigma-Aldrich||495C9||The solvent is cholorobenzene. Handle PMMA solution under a fume hood with adequate ventilation. Do not breathe the vapor. Refer to MSDS for safe handling instructions.|
|PPy||Sigma-Aldrich||--||5% by weight in water. Used as received.|
|PEDOT-PSS||H. C. Starck Co.||Baytron P HC V4||Proprietary solvent. Used as received.|
|SPANI||Sigma-Aldrich||--||Water soluble form. Used as received.|
|Hot embossing machine||JenoptikMikrotechnik Co.||HEX 01/LT|
|Sputter machine||Cressington Co.||208HR|
|FIB machine||Carl Zeiss, Inc.||FIB Crossbeam 1540 XB|
|Spin coater||Headway Research Inc.||PWM32-PS-R790 Spinner System|
|RIE machine||Technics MicroRIE Co.||--|
|PDMS||Dow Corning||Sylgard 184 Silicone elastomer kit|
|HDPE sheet||US Plastic Corp.||--|
|PMMA sheet||Cyro Co.||--|
|Double-sided adhesive tape||Scotch Co.||--|
|Single-sided tape||Delphon Co.||Ultratape # 1310|
|Glass micropipettes||FHC, Inc.||30-30-1|
|Clip||Office Depot||Bulldog clip|
|Humidifier||Vicks Co.||Filter free humidifier|