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Large Area Submicrometer Contact Printing Using a Contact Aligner Timothy Burgin,* Vi-En Choong, and George Maracas Physical Sciences Research Laboratory, Motorola, 2100 East Elliot Road, MD: EL508, Tempe, Arizona 85284 Received January 13, 2000. In Final Form: March 29, 2000 Submicrometer patterns were produced in a thin layer of gold over a 3 in. diameter substrate with an accuracy of g40 nm and a runout (feature to feature misalignment between the template and the stamped pattern) of approximately 1 µm, using microcontact printing (µ-CP). Successful pattern reproduction required careful control of the forces exerted on the substrate during the µ-CP process, as well as the encompassing pressure, which was achieved using a custom-built stamp aligner. The use of a thin film stamp bonded to a rigid glass support in conjunction with the aligner significantly improved the runout and eliminated contact of recessed regions of the stamp with the substrate.
Introduction There is a renewed interest in using contact lithographies for micrometer and submicrometer patterning of surfaces. Microcontact printing (µ-CP), in particular, has potential advantages over conventional fabrication methods in low cost lithography, in platforms for hierarchial self-assembly, and in directed placement of inherently sensitive materials.1 These applications will benefit from the ability to pattern large areas simultaneously in one step. Unlike photo, e-beam, and scanning probe lithographies, µ-CP allows simultaneous patterning over the entire substrate, eliminating the need for an expensive stepper or writing tool. µ-CP also eliminates the stitching errors (misalignment of adjacent patterned areas or die) associated with photolithography. Furthermore, µ-CP bypasses the photoresist processing steps required with most of the other patterning schemes and is capable of rapid throughput (∼30 s/substrate). These advantages could lead to significant cost savings over conventional photo and e-beam lithographies. µ-CP serves as a complementary method that offers certain advantages over other contact lithography techniques.2 Because the stamp and the support structures can be made transparent, µ-CP enables optical alignment of the stamp with the substrate. High fidelity, arbitrary pattern reproduction for structures with 0.55 µm minimum feature size and large feature dimensions greater than (1) (a) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188. (b) Kumar, A.; Biebuyck, H, A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (c) Whitesides, G. M.; Xia, Y. Langmuir 1997, 13, 2059. (d) Wilbur, J. L.; Kumar, A., Beibuyck, H. A.; Kim, E.; Whitesides, G. M. Nanotechnology 1996, 7, 452. (e) Biebuyck, H. A.; Larsen, N. B.; Delemarche, E.; Michel, B. IBM Res. Dev. 1997, 41, 1. (f) Jackman, R. J.; Brittain, S. T., Whitesides, G. M. J. MEMS 1998, 7, 261. (g) Mrksich, M.; Dike, L. E.; Whitesides, G. M. Exp. Cell Res. 1997, 235, 305. (h) Mrksich, M.; Whitesides, G. M. Trends Biotechnol. 1995, 13, 228. (i) Mrksich, M.; Grunwell, J. R.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 12009. (j) Rao, J.; Yan, L.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 2629. (k) Lahiri, J.; Ostuni, E.; Whitesides, G. M. Langmuir 1999, 15, 2055. (l) Ostuni, E.; Yan, L.; Whitesides, G. M. Colloids Surf., B 1999, 15, 1. (2) (a) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 551 and references therein. (b) Krauss, P. R.; Chou, S. Y. Appl. Phys. Lett. 1997, 71, 3174. (c) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. J. Vac. Sci. Technol., B 1996, 14, 4129. (d) Rogers, J. A.; Paul, K. E.; Whitesides, G. M. J. Vac. Sci. Technol., B 1998, 16, 59. (e) Aizenberg, J.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Appl. Opt. 1998, 37, 2145. (f) Qin, D.; Xia, Y.; Black, A. J.; Whitesides, G. M. J. Vac. Sci. Technol., B 1998, 16, 98.
10 000 µm2 has been achieved in these laboratories. The ultimate resolution of µ-CP is not known, but diffusive spreading of the ink during the stamping process limits the resolution to ∼80 nm when using alkanethiols on gold.3 Experimental Section Stamp Preparation. The stamp was molded from a 450 nm SiO2/50 nm TaSi/Si substrate pattern using standard lithographic methods. Selective reactive ion etching (RIE) of the SiO2 layer produced 450 nm deep features with nearly vertical side walls and a smooth surface at the bottom of the etched features. The substrate was passivated with 1,1,2,2-(tetrahydro)tridecafluorooctyltrichlorosilane before use. Sylguard 184 (Dow Corning, 10:1 base/curing agent) poly(dimethylsiloxane) (PDMS) elastomer was degassed in vacuo and carefully poured onto the template. The template was pressed onto a glass support plate (25.5 cm diameter), pretreated with allyltrimethoxysilane, using a 3 kg weight. The thickness of the stamp (100 µm) was controlled by placing precision shim stock between the stamp and the glass support plate. The template was held in the center of the glass plate using small blocks of cured PDMS. The elastomer was cured at a range of temperatures between room temperature and 70 °C. The cure time ranged from 7 days at room temperature to overnight at 70 °C. The template was removed by mechanically prying the template from the glass support plate. The edges of the stamp were trimmed with a razor blade, being careful to avoid contamination of the stamp. The stamp was then exposed to low particulate air overnight. Residual low molecular weight polymer was removed by rinsing with acetone. Particulate contamination could be removed from the stamp with cellophane tape followed by thorough rinsing with acetone. The stamp was inked by soaking in a 1 mM solution of hexadecanethiol or eicosanethiol in acetone for 1 h. Upon removal from the inking solution the stamp was rinsed with ethanol and dried in vacuo for 15 min to remove adsorbed solvent from the stamp. Microcontact Printing with Stamp Aligner. Three inch 200 Å Au/50 Å Ti/Si substrates were patterned using a stamp aligner (Figure 1), built in house, within a few days of preparation and were rinsed with semiconductor grade acetone and methanol before use. No noticeable degradation in the ability of the monolayer to perform as a resist was evident relative to freshly prepared substrates. The substrate was placed on the center of a chuck mounted on an air-actuated piston. Around the edges of the chuck are located mechanical stops. The stamping chamber and the chamber lying above the glass stamping plate were both (3) Delemarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324.
10.1021/la000038p CCC: $19.00 © 2000 American Chemical Society Published on Web 05/16/2000
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Figure 1. Schematic of custom built stamp aligner. evacuated to ∼50 Torr, and the mechanical stops on the chuck were brought into contact with the stamping plate, resulting in the substrate lying in close proximity (∼100 µm) to the stamp without coming into contact. After alignment, an elastic bladder lying on top of the chuck was back filled with air until the substrate was brought into contact with the stamp and complete adhesion had been achieved. After a contact time of 10 s the bladder was deflated, the chuck lowered, and the substrate removed from the stamp using an extractor pin. The aligner chambers were returned to atmospheric pressure, and the substrate was immediately etched in an aqeous solution of 0.1 M KCN/0.001 M K2Fe(CN)6 for 4 min. The aligner used to perform the µ-CP operation was built in house. Analysis. Runout and line width measurements were performed using a Nikon 3i laser metrology tool. The 3i has a minimum resolution of 10 µm in their shortest dimension) in the pattern results in monolayer or partial monolayer formation in regions which should be free of ink. Consequently, the gold is not completely removed from these regions during the wet etch development. The causes of sagging are manyfold. The template used to prepare the stamp must be thoroughly passivated or irreversible localized distortions in the PDMS will occur during stamp removal. The low shear modulus5 of the PDMS also makes the stamp highly susceptible to mechanical distortions during the stamping process. These mechanical stresses are caused by entrapped air, particulate contamination, nonuniform pressure across the substrate, or a pressure differential between the cavities within the stamp and the opposing surface of the substrate. The effects of sagging can be mitigated to some extent by minimizing the “open areas” in the template design, but not eliminated. When the stamp is applied as a thin film (∼100 µm) on a rigid glass support, the long-range mechanical stability of the stamp is improved.6 Figure 3 shows the white light constructive interference bands for the recessed area (320 µm × 240 µm) of a supported and free-standing PDMS stamp adhered to a silicon substrate at ∼50 Torr. The stamps were prepared from the same template. In both cases, the PDMS was curved away from the surface of the substrate. This result suggested that sagging was due to mechanical stresses induced during the stamping opera(4) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. Adv. Mater. 1994, 6, 600. (5) Delemarche, E.; Schmid, H.; Biebuyck, H.; Michel, B. Adv. Mater. 1997, 9, 741. (6) (a) Rogers, J. A.; Paul, K. E.; Whitesides, G. M. J. Vac. Sci. Technol., B 1998, 16, 88. (b) Folch, A.; Schmidt, M. A. IEEE J. Micromech. Syst. 1999, 8, 85.
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Figure 2. A 3 in. Au/Cr/Si substrate patterned by µ-CP. This particular substrate was patterned with an array of 180 MHz SAW devices with minimum feature sizes of ∼3 µm. A wafer containing submicrometer features is not shown as it appears featureless at this scale. Table 1. Stamping Issues issues
initial status
technique
air pockets
always present
defects
line bridging and line breaks common (∼3/device)
reduced pressure stamping (aligner) cleaning with cellophane tape, followed by rinsing with acetone
sagging
pervasive problem for features >0.1 mm ∼40 µm/cm line widths of stamped features were 10% larger than those of the template line widths of stamped features varied by 13% across a 7.5 cm substrate
glass-backed stamp/improved template preparation method glass-backed stamp improved template preparation method/pressure control (aligner) improved template preparation method/pressure control (aligner)
runout fidelity of stamped pattern pattern uniformity
Figure 3. White light interference bands observed with normal incidence for recessed areas (320 µm × 240 µm) of a PDMS stamp adhered to silicon substrate. A typical image obtained with a ∼100 µm thick PDMS film bonded to glass (a) and freestanding PDMS stamp (b) are shown.
tion and not to structural relaxation of the recessed features onto the surface during adhesion. Further, the radius of curvature within the cavity was much higher for the free-standing PDMS stamp than that for the glassbacked stamp. This marked increase in long-range (>10
current status essentially eliminated bridging and line breaks drastically reduced (∼1/150 devices) 7.5 cm diameter substrates have been prepared without sagging ∼0.3 µm/cm line widths of stamped features were the same as those of the template to within 40 nm line widths of stamped features had an average deviation of 20 nm
µm) rigidity for the glass-backed stamp resulted in a significant reduction in sagging. Localized mechanical stresses in the stamp were further reduced by minimizing and equilibrating the pressure exerted on the substrate while initiating adhesion of the stamp to the substrate. Control of the pressure was attained using the stamp aligner. The substrate was placed on a chuck suspended on spring-loaded pins, which allow self-parallelization of the substrate to the stamp and aid in pressure equalization. By use of an air-actuated piston, the chuck was brought into contact with the glass stamp support, bringing the substrate into close proximity (∼100 µm) to the stamp. A uniform pressure (typically
