Fluorinated ethylene-propylene: a complementary alternative to PDMS for nanoimprint stamps

Polydimethylsiloxane (PDMS) is used by many for nanoimprint applications due to its affordability, ease of preparation, mechanical flexibility, compatibility with imprint resists and transparency to UV light. However PDMS is notoriously flexible, tacky and permeable to air. Here fluorinated ethylene-propylene (FEP) is considered as a viable and versatile alternative material for nanoimprint stamps. FEP possesses many of the desirable nanoimprint attributes associated with PDMS but crucially also features a range of complementary characteristics, including an order of magnitude more mechanical strength allowing it to handle higher loads than PDMS, an intrinsically non-stick surface and is compatible with oxygen sensitive resists. Unlike elastomeric polymers, FEP is glassy so patterning may be realised via hot embossing. Not only is this a facile and rapid means of physical structuring but it also facilitates combinatorial patterning, providing a versatility beyond that of traditional casting materials. Due to the intrinsically slow creep of FEP both micro- and nanopatterning are successfully performed sequentially. Feature sizes from 45 nm were successfully realised via the hot-embossing method. To further demonstrate the potential of the material, a modified computer numerical control machine is used. It is capable of photo-, nanoimprint- and laser lithography in conjunction with patterned FEP foils. The tool is used to perform pattern transfer into a developmental nanoimprint resist from Micro Resist Technology, mr-NIL210 XP, and Nano SU-8 3005 negative tone photo resist from MicroChem. Ultimately three-tier lithography is performed in unison and advantageous step-and-repeat performance is achieved with fabricated FEP imprint stamps as they demould more compliantly and resist pressure and contamination better than PDMS.