Novel Materials For Nanoimprint Lithography

Nanoimprint lithography has been a promising technique to fabricate nanostructures with high throughput and low cost. Metallic nanostructures are an essential element in a variety of nanoscale devices ranging from optical, electrical, magnetic devices and bio-sensors. It is of a great challenge to fabricate patterns on curved or non-planar surfaces by traditional photolithography using a rigid mask. A hybrid UV-curing imprint technique in combination with a double transfer coating method is able to form nanopattened film of UV-cured resist with a thin and uniform residual layer on highly curved surfaces, which is critical to the subsequent high-fidelity pattern transfer process. A thermoplastic polymer film is still required for fabricating metal structures through the lift off process, which is impossible to be uniformly coated onto a curved surface. If a degradable UV-resist is used in the fabrication, the thermoplastic layer for lift-off can be omitted, which can largely simplify the processes and decrease defects. Thus a UV-cured resist that can be removed under mild conditions is highly desirable for UV-curing nanoimprint lithography.In this paper, we design and develop a series of elastomeric materials and degradable resists to meet the requirement of fabricating nanostructures on curved surfaces:Degradable polycyclic cross-linker for UV-curing nanoimprint lithography. We design and synthesize a degradable polycyclic diacrylate cross-linker, 2,10-diacryloyloxymethyl-1,4,9,12-tetraoxaspiro[4.2.4.2] tetradecane (DAMTT), from easily available starting materials by bis-ketalization of 1,4-cyclohexanedione with glycerol, followed by esterification with acryloylchoride. The chemical structure of the degradable cross-linker is characterized by FTIR,1H NMR and GC-MS. UV-NIL resist based on DAMTT has a very low shrinkage (about 2.5%) and high Young’s modulus of 1.89 GPa after UV-curing, and soluble in acidic media. Patterns with sub-10-nm resolution are faithfully imprinted into the DAMTT film. The degradable cross-linker is incorporated with a multifunctional acrylated siloxane to increase O2 RIE resistance as an etch barrier for high aspect ratio pattern transfer. The cross-linked film of the UV-curable resist remains soluble in acidic media even its formulation contains acrylated siloxane up to 50% (wt/wt). Moreover, in contrary to most UV-NIL resists which are insoluble solvents aftter curing, DAMTT resist can independently acheive a lift-off process without the assistance of a thermoplastic polymer layer. That made it possible to fabricate metal nanostructures on highly curved surface via a hybrid UV-curing nanoimprint technique. For example, metal gratings were successfully fabricated on the cylindrical surface of an optical fiber via a lift off process.An oxygen insensitive degradable resist for fabricating metallic patterns on highly curved surfaces by UV nanoimprint lithography. We design an oxygen insensitive degradable resist for UV-nanoimprint comprising the aforementioned polycyclic degradable acrylate monomer, DAMTT and a multifunctional thiol monomer, pentaerythritol tetra(3-mercaptopropionate) (PETMP). The resist can be quickly UV-cured in the air atmosphere and achieve a high monomer conversion of over 98%, which greatly reduce the adhesion force between the resist and the soft mold. High conversion, in company with an adequate Young’s modulus (about 1 GPa) and an extremely low shrinkage (1.34%), promise high nanoimprint resolution of sub-50nm. The cross-linked resist is able to break into linear molecules in a hot acid solvent. As a result, metallic patterns are fabricated on highly curved surfaces via the lift off process without the assistance of a thermoplastic polymer layer.High resolution soft mold for UV-curing nanoimprint lithography based on an oxygen insensitive degradable resist. Soft nanoimprint lithography has been developed to overcome the disadvantages of conventional nanoimprint based on rigid mold. Hybrid nanoimprint-soft lithography (HNSL) mold is an efficient strategy to improve nanoimprint resolution using a rigid UV-curable resist as the structural layer. In this paper, we design a novel UV-curable resist for hybrid soft mold fabrication, which is degradable under mild acidic conditions and insensitive to oxygen. The resist system comprises the acid-degradable cross-linker, DAMTT, and an acyrlated polysiloxane, poly[(mercaptopropyl)methylsiloxane] (PMMS). Its cross-linking mechanism is based on thiol-ene chemistry, so that oxygen sensitivity of acrylate group can be avoided. The cured resist can be decomposed into linear chains through the cleavage of acid-labile ketal links and dissolved in organic solvent when heated in an acidic solution. The resist possesses necessary properties for mold fabrication and imprinting, including low shrinkage (about 5%), high modulus, high thermal stability, high UV transparency and stability in normal environment. Furthermore, since the cured resist is degradable in acidic media, the stained master can be renewed in a mild condition. Various nanoscaled patterns are fabricated on planar and curved substrates via UV-nanoimprint when the same resist is used as a patterning layer.Fabrication of metallic patterns by nanoimprint associated with chemical etching. Lift off process is the most widely used technique to fabricate metallic patterns on substrates. The curved surfaces may cause difference in thickness of the micropatterns. For uniform patterning by evaporation, rotation of the curved substrate is required during metal evaporation, and it makes the more complex and more difficult to handle. Inspired by micro contact lithography (μCP), we develop a new method to fabricate metallic patterns on planar and curved surfaces with high resolution. First, metal was deposited on the target substrate and the surface was patterned by HNSL mold using double transfer method. When the residual layer was removed by reactive ion etching, the patterned substrate was immersed in an etchantand the resist served as a mask and Cr and Au patterns were consequently fabricated. Additionally, Cr grating is patterned on the surface of optical fibers and serves as a mask during reactive ion etching so that surface relief fiber Bragg grating with a depth of 280nm is obtained. Further more, patterns with different line widths can be obtained using the same mold when the samples are chemical etched for different time.New elastomeric materials for the elastic support of nanoimprint soft mold. Elastomers are used to fabricate soft molds because they can make conformal contact with surfaces over relatively large areas. PDMS is the most widely used material because of its high transparency and high stability. Unfortunately, PDMS also presents a number of technical problems for soft lithography, and the most serious one is that its low modulus is not high enough to pattern high resolution patterns without deformation. In this paper, we develop a high performance transparent ethylene propylene diene terpolymer (EPDM) which is cross-linked by a thiol-ene system. The material has a high tensile strength of over 18 MPa and is highly transparent down to 250nm. The transparent EPDM was used as the elastomatic supporting material to fabricate hybrid soft mold for UV nanoimprint lithography. Various patterns with high resolution were obtain on planar and curved surfaces.We also report high efficient reinforcement of dynamic cross-linked silane-modified phenol formaldehyde resin (SMPF) in ethylene-propylene diene terpolymer rubber (EPDM). Only 5phr of SMPF is able to greatly improve the tensile strength of EPDM from 5 MPa to 28 MPa, which is 55% higher than that of EPDM filled with 30phr of carbon black N330, and it still remains a high elongation of more than 600%. SEM and XRD results reveal the reinforcement mechanism of EPDM. SMPF is uniformly dispersed in the EPDM matrix by a conventional Banbury mixer and dynamic cross-linked to form a greatly expanded hard phase comprising interpenetrating polymer networks of EPDM and SMPF during the mixing process, which leads to a high modulus. High tensile strength is attributed to strain-induced crystallization of EPDM at high strain according to X-ray diffraciton patterns of stretched EPDM/SMPF composites. These results reveal high efficient reinforcement of thermoset resin in rubber, demonstrate a new mechanism of rubber reinforcement and suggest a new direction to rubber reinforcement.