Soft lithography refers to a family of techniques used for fabricating/replicating structures using elastomeric stamps, molds and conformable photomasks. Ideally, it is a prominent technique for rapidly fabricating miniaturized devices with elastomeric materials. Technically, fabrication of these devices conventionally requires casting of precured elastomers on rigid silicon–photoresist (Si–Pr) composite master molds in order to replicate the microfeatures on the Si–Pr master mold. This strategy then is frequently used with polydimethylsiloxane (PDMS) for implementing different microfabrication techniques such as replica molding and microcontact printing. This technique has enjoyed success over the past two decades; unfortunately, shortfalls such as the development of novel microtechnological innovations, associated with master mold fabrication has limited continued advancement. Various researchers have attempted to address this issue by proposing various techniques to copy the existing master molds instead of using time consuming, expensive, and cumbersome master mold fabrication methods to refabricate the damaged master molds or to scale up the fabrication. Regardless of these advances, alternate approaches that could help circumvent inherent drawbacks would be highly welcome.
In general, soft lithography enables rapid microfabrication of many types of microsystems by replica molding elastomers into master molds. However, master molds can be very costly, hard to fabricate, vulnerable to damage, and have limited casting life. In light of this, researchers from the Department of Mechanical Engineering at Carnegie Mellon University: Utku Sonmez (PhD candidate), Stephen Coyle (PhD candidate), Professor Rebecca E. Taylor and Professor Philip R. LeDuc, proposed a new approach to clone master molds for soft lithography. Specifically, their approach would enable the multiplication of master molds into monolithic thermoplastic sheets for further soft lithographic fabrication. Their work is currently published in the research journal, Small.
The proposed technique encompassed the use of polycarbonate thermoplastic sheets as a starting material and melted them on PDMS molds by heating them above the glass transition temperature without applying any external force. The team then applied the polycarbonate heat molding (PCH molding) technique to copy various types of master molds that were fabricated through photolithography, mechanical micromilling, and 3D printing to obtain a variety of microstructures at multiple scales in monolithic PC copy molds. Further, the technique was used to combine master molds fabricated through different methods into a single monolithic PC copy mold for the fabrication of multilayered complex microdevices.
The research team reported that when tested with master molds fabricated through photolithography, mechanical micromilling as well as 3D printing, their technique successfully copied the microstructures with submicron feature sizes and high aspect ratios. The technique presented was fast, reliable, and economical for copying molds without the need of sophisticated equipment or specialized facilities; attributes that could allow researchers to more easily scale up their fabrication and share their master mold geometries with each other fostering interdisciplinary collaborations.
In summary, the study presented a new PCH molding technique to fabricate alternative master molds for soft lithographic applications. Remarkably, the presented technique did not require any hazardous or cytotoxic solvents, costly polymers, or any specialized equipment, which made it useful to almost any laboratory that has a PC sheet and an oven, which are easy to obtain. In a statement to Advances in Engineering, they highlighted that their microfabrication technique can be performed outside the cleanroom without using any sophisticated equipment, suggesting a simple way for high throughput rigid monolithic mold fabrication that can be used in analytical chemistry studies, biomedical research, and microelectromechanical systems.
Utku M. Sonmez, Stephen Coyle, Rebecca E. Taylor, and Philip R. LeDuc. Polycarbonate Heat Molding for Soft Lithography. Small 2020, volume 16, 2000241.