Soft Lithography is an umbrella term for a set of techniques that rely on printing and molding to make microstructures and nanostructures. It was originally developed in order to circumvent the limitations of photolithography, which has been the basic technology used for making all microelectronic systems. The invention of photolithography is arguably as important as that of the wheel, bronze, or movable type in terms of its impact on society. It is, however, a technology that is specialized for use in microelectronics.

For making other kinds of micro-systems, photolithography is not necessarily the right technology to use. It is not only limited in the materials it can use and in the geometries it can produce, but it is expensive and can only pattern a small area at any given time. In addition, the size of the features one can make with photolithography is limited by diffraction of light. As a result, to a first approximation, photolithography is confined to extremely flat silicon substrates; curved surfaces, for example, cannot evenly accommodate light beams moving in a straight line. One could not, for example, fabricate electronic circuits on a plastic sheet or a flexible display on a curved car dashboard. While Soft Lithography has different limitations, the physics-based constraints in these techniques are relatively minimal, especially relative to the broad range of capabilities this technique enables.

It is important to emphasize that while Soft Lithography clearly is of value for electronics, it is by no means limited to this field. In fact, Soft Lithography is finding application in a range of different fields from consumer products to industrial processes to life sciences, because the fundamental capability it enables is critical to so many development challenges: the exquisite control over an infinite range of structures and chemistries from the nano- to the meso-scale, and the integration of these into useful systems and devices.

The basic principle of the first phase for any Soft Lithographic technique is illustrated below. We start with the fabrication of a ‘Master’ using proven techniques, such as photolithography, e-beam, or micro-machining. A Master could also be an existing structure that doesn’t require processing like a human hair or some woven fabric. An elastomer, such as polyurethane or a silicone, is poured onto the Master, hardened using heat or ultraviolet light, and peeled off to yield a ‘mold’. The resulting mold is the exact structural inverse of the original Master - down to nanometer accuracy depending on the combination of materials used and the precision of the replication process.

Thanks to their distinct physical characteristics, such as softness, flexibility, elasticity and minimal stickiness, these polymer molds can be used as stamps for transferring the Master pattern to virtually any surface. While our techniques often begin with polymer stamps and molds, we could just as easily impart structures and chemistries onto a variety of non-plastic surfaces (such as metals, ceramics or oxides) of practically any shape or size. Furthermore, we can pattern these diverse materials with a broad range of materials, including silicon on glass or organic molecules on metal.

Note that a single Master can be used tens to hundreds of times, depending on application, to produce tens to hundreds of molds, and each mold can be used to transfer the pattern tens to a hundred times depending on application. And each mold can also act as a Master from which we can again accurately replicate tens to hundreds of molds. The result is a highly scalable and economical process. It is also important to note that molds can, in principle, be fabricated with meter-sized dimensions and surface feature sizes ranging in size from nanometers to millimeters.

The transfer of a pattern from a patterned stamp, created using Soft Lithography, to a surface requires an ‘ink’. We use conventional inks to create color effects on surfaces much like we would use a stamp to imprint a return address on an envelope. Industries are interested in the wide range of specialized ‘inks’ that we pattern on surfaces that modify the characteristics of the material, including its: water-repellency, interfacial energy, electrical conductance, heat conductance, optical properties, stiffness, strength and other physical properties. The chemical and physical properties of the ‘ink’ may be just as important for the performance of the integrated system or device as the design on the stamp.