The ability to pattern materials into small structures is essential to modern science and technology. There are many opportunities that might be realized by making new types of small structures, or by simply downsizing existing structures. Soft lithography—a collection of novel patterning techniques based on printing, molding, and embossing with a transparent elastomeric stamp—represents a new conceptual approach to the fabrication and manufacturing of small structures or systems for applications. This approach exceeds the scope defined by classic photolithography. It has emerged as a new technology platform for a variety of applications that include MEMS, biochips, microfluidics, and flexible microelectronics. It is astonishing that more than 800 papers have been published reporting various extensions of this technology since the term of soft lithography was coined in 1998 (see Y. Xia and G. M. Whitesides, "Soft lithography," Angew. Chem. Int. Ed. 37:550-575, 1998). The recognition of soft lithography by the research community can be attributed to the following technical merits: i) it is simple, convenient, inexpensive and thus accessible to the full spectrum of users; ii) it is applicable to directly pattern a broad range of functional molecules and materials; iii) it generates structures at very small scales, on non-planar surfaces, and with three-dimensional architecture; and iv) it is readily adapted for rapid prototyping.

Small structures are traditionally fabricated using photolithography. However, this technology has a number of limitations that can hardly contend demands from communities other than the semiconductor industry. For example, photolithography requires major capital investment; it is poorly suited for patterning curved surfaces; it provides very little control over the surface chemistries; and it is directly applicable only to a limited set of polymers known as photoresists. To address a strong desire in the community for alternative approaches to microfabrication, the Whitesides Laboratory at Harvard University developed soft lithography. The development of soft lithography can be understood in light of a "push and pull" model. In this case, the "pull" mainly originates from new applications in areas outside of microelectronics, which rely on miniaturization and/or integration. Notable examples include microelectromechanical systems (MEMS), microoptics, and biotechnology and healthcare (biosurfaces, biochips, microfluidics, microreactors, lab-on-chip systems, and sensing devices). Most of these applications demand fabrication tools that are accessible at low costs, and which can provide tight control over the surface functionalities and be directly applicable to a broad range of materials. The "push" may just come from the curiosity or desire to solve scientific or engineering problems, to understand how nature works, and to push the performance of currently existing materials/devices/tools to higher levels and new limits. I was fortunate enough to join the Whitesides group at the right time and had the opportunity to work with many other group members, developing soft lithography into a major microfabrication tool.

Soft lithography represents a conceptually new approach to the fabrication and manufacturing of high-resolution structures that exceeds the capacity defined by conventional photolithography. The techniques of soft lithography complement those of photolithography and extend patterning into dimensions, materials, and geometries to which photolithography clearly falters or fails. For example, no access to the clean room facility is necessary once the elastomeric stamp or mold has been fabricated. Soft lithography enables the routine fabrication of high quality micro- and nanostructures in the setting of a typical chemical or biological laboratory. It also allows for rapid prototyping of various types of microfluidic and MEMS devices.

There is no doubt that research efforts on soft lithography will continue to develop strongly, with contributions from chemists, physicists, material scientists, and engineers, who will keep pushing this technology a step further towards higher levels of success. Soft lithography may become competitive with photolithography for patterning tasks where the capital cost of the equipment is the major concern, or where functional materials other than photoresists are the primary focus. It is expected that some commercial applications outside of microelectronics may emerge in the near future, which will further power the development of this technology. Someday, with new developments in both materials and techniques, soft lithography may even compete with photolithography in some of its core applications.

Younan Xia, Ph.D.
University of Washington
Seattle, WA, USA