Figure 19 Methods used to fabricate a flexible mold for R2R and R2P NIL compiled from various studies. Figure 20 Roller mold fabrication using imprint lithography technique by Hwang and the team [26] . Most of the other studies, however, use a simpler approach for AZD5363 fabrication of flexible molds for the R2R and R2P NIL processes, where a replica of a master mold is used as the flexible mold for the roller imprint process. In general, the desired structures are first patterned onto a silicon or quartz substrate using conventional nanolithography techniques

such as EBL and followed by the RIE process, similar to its P2P variant. The replication of the master mold can then be conducted using several methods. One of the common techniques involves deposition of an anti-stick layer onto the master mold, followed by a layer AZD6244 of metal such as check details nickel directly onto the master mold, where it will then be peeled off to be used as a flexible mold in the roller

nanoimprint process as observed in [32, 43, 46]. In some cases such as in [30], an imprint replica of the master mold is first obtained using nanoimprint lithography (step-and-repeat technique) onto a resist-coated wafer, where a nickel layer is then deposited onto the imprint and peeled off to be used as the flexible mold in the imprint process published in [42]. Alternatively, the imprint replica of the master mold may also be produced via the polymer cast molding technique using non-sticking polymers such as PDMS or ETFE to be used as the flexible soft mold for the imprint process as observed Tangeritin in the work of a few research groups [7, 15, 35]. It is highlighted in the work of Ye et al. [59] that polymer cast molds (typically made of PDMS) are usually more preferable in the UV-based roller imprinting process due to their advantages of being low cost, low surface energy (fewer sticking issues), chemically inert, elastic, and simpler to produce as compared to metal molds. One of the important challenges of producing roller molds is the surface planarity of the attached flexible mold

[51]. A similar uniformity is needed to achieve imprint rollers in order to prevent transmission of low-frequency and long-range surface waviness onto the replicated pattern. Conclusions Since its introduction back in 1995, the rapid development of the nanoimprint lithography process has resulted in a number of variants in the process, which can be categorized based on its two main operation features: resist curing and type of imprint contact. To date, in terms of resist curing, there are two fundamental types of processes: thermal NIL and ultraviolet (UV) NIL. As for the types of imprint contact, the process can be categorized into three common types: plate-to-plate (P2P) NIL, roll-to-plate (R2P) NIL, and roll-to-roll (R2R) NIL.