Soft lithography creates three-dimensional patterns and structures at the microscale and nanoscale levels. Unlike photolithography, a fabrication technique used by the semiconductor industry, soft lithography produces microfluidic devices, microarrays, and microelectromechanical systems (MEMS). The “soft” in soft lithography refers to its use of mechanically soft materials such as polydimethylsiloxane (PDMS) to produce a stamp or mold. Lithography is a printing process that renders an image on a flat surface.
How Soft Lithography Works
During soft lithography, a master mold with the desired microscale or nanoscale pattern is fabricated. Computer-aided design (CAD) software is used to produce this pattern, and traditional microfabrication techniques are used to produce the master mold. These fabrication techniques include photolithography, which uses light to etch features, and electron beam lithography, which patterns features with an accelerated beam of electrons. To achieve nanoscale features, nanoimprint lithography can be used.
It’s important to note that soft lithography’s master mold is not made of PDMS. Rather, it’s made of a material such as chrome-coated glass, which is light-sensitive. When this photosensitive material is exposed to light during photolithography, it loses its resistance to etching. To produce the desired pattern, a photomask with transparent areas allows light to shine through. In other words, the photomask determines where the light shines onto the chrome-coated glass and, in turn, where patterns are etched.
Next, a liquid polymer such as PDMS is poured into the etched areas or channels of the master mold and allowed to cure, typically at an elevated temperature of ∼70°C overnight. After the PDMS hardens, it is peeled away from the mold. This PDMS stamp, as it’s known, carries the reverse pattern of the master mold and is pressed onto the target substrate. Through physical contact, capillary action, bonding, or other surface actions, the pattern is applied. Various substrate materials, including glass slides, can be used.
How Soft Lithography Compares to 3D Printing
Like soft lithography, 3D printing can create structures at the microscale and nanoscale levels; however, many 3D printers are limited to the macroscale. In addition, most (but not all) 3D printers can’t match soft lithography in terms of resolution. 3D printing also has some things in common with photolithography, which is commonly used with soft lithography. Both photolithography and resin 3D printing, a group of 3DP technologies, use a light source and a polymer. Yet that’s where the similarities end.
During soft lithography, material is removed to create features such as microchannels. In a subsequent step, a liquid polymer such as PDMS is poured into these channels and allowed to cure, typically with the application of heat. By contrast, resin 3D printing adds rather than subtracts material. The material that is added is a polymer, but this polymer cures or hardens with the application of light rather than heat. In addition, this photosensitive polymer is added one layer at a time rather than poured all at once.
There isn’t just one type of resin 3D printing, however. Members of this technology family include stereolithography (SLA), digital light processing (DLP), and projection micro stereolithography (PµSL). SLA and DLP are generally used for macroscale rather than microscale and nanoscale applications. By contrast, PµSL technology from Boston Microfabrication (BMF) is designed for microscale 3D printing. For applications such as microfluidics, BMF’s microArch® series of 3D printers can replace soft lithography.
Soft Lithography vs. PµSL 3D Printing
Unlike soft lithography, PµSL 3D Printing uses a machine rather than a master mold and a press for a PDMS stamp. Even more importantly, this form of resin 3D printing can create complex 3D channels that are difficult if not impossible to achieve with soft lithography. In addition to achieving high accuracy, precision, and resolution, PµSL 3D printing eliminates the challenge of using soft lithography to cut microscale features into layers and then bond these layers together to create one functioning device.