Prototyping of Masks, Masters, and Stamps/Molds for Soft Lithography Using an Office Printer and Photographic Reduction Tao Deng, Hongkai Wu, Scott T. Brittain, and George M. Whitesides* Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138 This paper describes a practical method for the fabrication of photomasks, masters, and stamps/ molds used in soft lithography that minimizes the need for specialized equip- ment. In this method, CAD files are first printed onto paper using an office printer with resolution of 6 0 0 dots/ in. Photographic reduction of these printed patterns transfers the images onto 35-mm film or microfiche. These photographic films can be used, after development, as photomasks in 1 :1 contact photolithography. With the resulting photoresist masters, it is straightforward to fabricate poly(dimethylsiloxane) (PDMS) stamps/ molds for soft lithography. This process can generate micro- structures as small as 15 μm; the overall time to go from CAD file to PDMS stamp is 4 -24 h. Although access to equipment sspin coater and ultraviolet exposure tools normally found in the clean room is still required, the cost of the photomask itself is small, and the time required to go from concept to device is short. A comparison between this method and all other methods that generate film-type photomasks has been performed using test patterns of lines, squares, and circles. Three microstructures have also been fabricated to demonstrate the utility of this method in practical applications. This paper describes a method for patterning photoresist that uses desktop printing and photographic reduction to make photomasks that can be used in 1:1 contact photolithography to fabricate the masters and stamps/ molds used in soft lithography. This method allows the generation of features with lateral dimensions as small as 15 μm, and with an edge roughness 1 of ∼1.5 μm. It offers a route to microstructures having dimensions useful in microfluidics, 2 microelectromechanical systems (MEMS), 3,4 and microanalytical systems. 5 It is especially appropriate for use in chemical and biochemical laboratories that do not have access to the facilities used to make photomasks to the standard of microelectronics, 6 because it bypasses the requirement for chrome masks. It also obviates the need for more readily available but still specialized devices such as high-resolution printers. 7 The work reported here does not represent new science: it intentionally focused on the exploitation of the simplest and most broadly available techniques that we could identify for forming patterns with features useful in functional microstructures. These straight- forward methods, when combined with soft lithography, 8 extend the capability for microfabrication of laboratories that have no (or limited) access to the facilities required to fabricate chrome masks or to carry out high-resolution printing. The objective of this work is to develop and compare methods for generating microstructures using facilities readily and inex- pensively available to chemistry and biology laboratories. We focused on conventional 35-mm cameras and commercial micro- fiche makers, with the objective of defining the minimum feature sizes that could be demonstrated by combining images generated using these systems with soft lithography. The conventional method for making photomasks for microfabrication is to design the pattern of interest using a CAD system, use this design to generate a chrome mask using specialized photolithographic or e-beam tools, and then proceed with photolithography. 6 This procedure works well and is the basis for the microelectronics industry. Its drawback is that the generation of chrome masks requires special facilities and is generally slow and expensive. We and others have demonstrated that a high-resolution printer (3387 dots/ in. (dpi); Linotype-Hell Co.) can quickly and inexpensively generate 20- μm patterns with tolerable edge roughness and 50- μm patterns with good quality. 7,9 Although this capability is adequate for many applications, there are circumstances in which even this high-resolution printing, while readily accessible com- mercially, may be unavailable or inconvenient or in which the ability to fabricate features smaller than 50 μm would be useful. We have shown that the combination of high-resolution printing and photographic reduction onto microfiche can generate masks, masters, and stamps/ molds for soft lithography with feature sizes as small as 10 μm. 10 In this work, we started with routine desktop printing, instead of high-resolution printing, to * To whom correspondence should be addressed; (e-mail) gwhitesides@ gmwgroup.harvard.edu. (1) The edge roughness of the lines is defined as the maximal variation of the lateral dimensions of the lines. (2) Kopp, M. U.; Mello, A. J. D.; Manz, A. Science 1998 , 280, 1046. (3) Kovacs, G.; Petersen, K.; Albin, M. Anal. Chem. 1996 , 68, 407. (4) Macdonald, N. Microelectron. Eng. 1996 , 32, 49. (5) Qin, D.; Xia, Y.; Rogers, J. A.; Jackman, R. J.; Zhao, X.-M.; Whitesides, G. M. In Microfabrication, Microstructures and Microsystems; Manz, A., Becker, H., Eds.; Springer-Verlag: Berlin, 1998; Vol. 194. (6) Elliott, D. J. Integrated Circuit Mask Technology; McGraw-Hill: New York, 1985. (7) Qin, D.; Xia, Y.; Whitesides, G. M. Adv. Mater. 1996 , 8, 917. (8) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998 , 37, 550. (9) We have also tried printers with 5000 dpi resolution, but the resolution we obtained was indistinguishable from that generated using a 3387 dpi printer. (10) Deng, T.; Tien, J.; Xu, B.; Whitesides, G. M. Langmuir 1999 , 15, 6575. Anal. Chem. 2000, 72, 3176-3180 3176 Analytical Chemistry, Vol. 72, No. 14, July 15, 2000 10.1021/ac991343m CCC: $19.00 © 2000 American Chemical Society Published on Web 06/20/2000