Patterning via self-organization and self-folding: Beyond conventional lithography
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nventional lithography Photolithography is the most prevalent micro- and nanoscale patterning technique due to its effectiveness and efficiency. The process is illustrated in Figure 1. The term photolithography has a literal translation of “to write in stone using light,” based on the Greek roots of the term. Typically, the “stone” is a silicon wafer coated with a light-sensitive polymer. Patterns of light (or electrons/ions) focused onto this coating change the solubility of the polymer in localized regions. The coating that remains after dissolving (i.e., developing) the soluble regions protects the substrate from subsequent processing, such as etching. The coating is therefore called a “photoresist,” because it is designed to be sensitive to light and to resist etching. A pattern emerges on the substrate after etching the exposed regions and removing the photoresist. Conventional photolithography is the cornerstone technology that enables fabrication in modern electronics. However, it also has many limitations, including (1) the need for a mask, focusing optics, light-sensitive materials, and expensive specialized equipment; (2) resolution limits defined by optics; and (3) poor compatibility with unconventional materials that may be soft, nonplanar, or difficult to process. Other “beam-based”
lithography techniques, such as electron-beam lithography, can provide higher-resolution patterns than photolithography without the need for a mask, but otherwise, suffer similar limitations.
Approaches to address the challenges of conventional lithography To address these issues, there have been numerous alternative (or “unconventional”) approaches,1–3 including, for example, soft lithography,4 contact printing,5 dip-pen lithography,6 imprint lithography,7 and three-dimensional (3D) printing.8 No single method addresses all of the limitations of photolithography, and therefore, choosing a patterning method depends on the application, the materials, and the resources available. Among these alternate methods, self-organization—the formation of patterns by harnessing forces across a range of scales—is an attractive option as it (1) forms complex patterns with minimal process monitoring and control, (2) can make structures beyond the resolution limit of photolithography, and (3) often provides more sustainable routes for fabrication by using ambient energy or minimizing environmental hazards. The contributions in this issue summarize key
Sung Hoon Kang, Department of Mechanical Engineering and Hopkins Extreme Materials Institute, Johns Hopkins University, USA; [email protected] Michael D. Dickey, Department of Chemical and Biomolecular Engineering, North Carolina State University, USA; [email protected] DOI: 10.1557/mrs.2016.3
© 2016 Materials Research Society
MRS BULLETIN • VOLUME 41 • FEBRUARY 2016 • www.mrs.org/bulletin
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PATTERNING VIA SELF-ORGANIZATION AND SELF-FOLDING: BEYOND CONVENTIONAL LITHOGRAPHY
Examples and applications
Figure 1. Conventional photolithography (and other “beambased” lithographies) focuses pattern
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