Nanopatterning highly curved surfaces using hybrid nanoimprint lithography

High-resolution, cost-effective patterning of curved surfaces is essential for many applications, such as microelectromechanical systems (MEMS), electronic devices, and optics.¹Nanoimprint lithography (NIL) has been demonstrated as a high-throughput, low-cost lithographic technique with sub-10nm resolution.² However, it can generally only be used for flat surfaces because the mold is often fabricated with a rigid material (such as silicon, nickel, silicon dioxide, or quartz) which enables high spatial resolution. When printing on highly-curved, non-flat surfaces, soft lithography is preferred over NIL. Using a flexible polymer such as polydimethylsiloxane (PDMS) allows for intimate physical contact between the soft lithography mold and the substrate without applying high levels of pressure.³ However, using conventional soft lithography to produce sub-100nm scale features has been limited by the low elastic modulus of commercial PDMS used for fabricating stamps. A patterning technique that combines the spatial resolution of NIL and the conformality of soft lithography would be ideal.

To allow for nanoscale imprinting of curved surfaces, we developed a photo-curable silicon-containing polymer for NIL molds where the individual polymer chains are bonded (crosslinked) to each other. Our new mold made from this crosslinked polymer can imprint features smaller than 30nm.⁴ Based on this material, we developed a mold using a photo-cured rigid polymer as the patterning template on an elastic PDMS support. Combining the rigid polymer and the elastic support provided sub-15nm spatial resolution free from cracks and fractures during conformal contact and mold release.⁵

We fused the thin (100–200nm) rigid feature layer with the thick (∼2mm) elastic PDMS support by using a gradient interpenetrating polymer network, where the polymer networks were graded into each other to prevent the formation of a sharp interface. This process allowed us to create a mold that combined the advantages of a high-resolution nanoimprint mold and a conformal soft lithography stamp (see Figure 1).

Figure 1. Schematic of the hybrid mold structure.

Figure 2. Scanning electron microscopy (SEM) images of the imprinted patterns on flat substrates using the hybrid mold: (a) dot array at 100nm pitch with 15nm diameter features and (b) ring array with an outer diameter of 300nm, a ring width of 50nm, and a periodicity of 400nm.

Figure 3. Imprinted 200nm pitch gratings on curved substrates: (a) atomic force microscopy (AFM) image of a micro-fiber with a 10μm diameter and (b) SEM image of a micro-bead with a 20μm diameter.

Figure 4. SEM images of the imprinted 500nm pitch gratings on non-flat substrates: (a) a dust particle with an in-plane size of 18μm and (b) micropost array with a size of 4μm, a pitch of 8μm, and a height of 500nm.

The complementary mechanical properties of the rigid crosslinked patterning layer and elastic support of the hybrid mold are crucial factors to achieve ideal imprinting results. The great mechanical strength of the rigid patterning layer is key to achieving fine lithographic resolution, and the soft, elastic support with high flexibility allows us to conformally contact the mold and substrate without applying large imprint pressure to create sub-15nm features (see Figure 2).

The flexibility of the hybrid mold makes it suitable for imprinting curved and non-flat substrates. As a demonstration, we patterned 200nm pitch gratings on the surface of a cylindrical micro-fiber and a spherical micro-bead. We spin-coated or dispensed the photocurable imprint resist (patternable polymer) onto the curved surface, imprinted the resist using the hybrid mold with 200nm pitch gratings, and then removed the mold and hardened the resist using UV light. The hybrid mold successfully duplicated the mold features because it could locally deform and bend to the shape of the curved substrate, due to its high elasticity (see Figure 3).

Besides the need to pattern on curved substrates, particle-related defects are one of the key concerns for nanoimprint lithography. Any particles present can amplify imprint defects to sizes much larger than the size of the particle itself. Our mold dramatically reduced the severity of particle-induced defects by the local deformation of the hybrid mold around the particle with a slight external pressure: see Figure 4(a). In addition, 500nm pitch grating structures were successfully imprinted on the surfaces and trenches of a micropost array: see Figure 4(b). This indicated that the hybrid mold could tolerate dense particles and could pattern highly wavy surfaces.⁶

We created a high-resolution, highly flexible mold by combining a thin, rigid feature layer and a thick, elastic support layer. Our mold can be used to pattern both flat surfaces and wavy, irregular surfaces, which allowed us to imprint micropost arrays that are potentially useful for creating artificial compound eyes and advanced optics. In the future, we will be working on using imprinted patterns to assist in the etching of surface-relief gratings into silica fibers using reactive ion etching (RIE), for use in high-temperature sensors.