J. Seekamp

Nanoimprint lithography: challenges and prospects

We review the salient aspects of nanoimprint lithography and consider the challenges it faces in becoming a standard fabrication technique, such as costs and throughput. We discuss material issues such as visco-elasticity and functionality of the printed material. By way of an illustration, we present printing results of 50?nm features over a 2?2?cm2 area which are reproducible with high fidelity. Data of printing 15?nm features in PMMA using a Cr stamp was obtained.

Nanoimprint lithography: an alternative nanofabrication approach

A status report of nanoimprint lithography is given in the context of alternative nanofabrication methods. Since the ultimate resolution of nanoimprint appears to be determined by the stamp, this is discussed in detail, particularly the recent developments on polymer stamps. The scope of the technique is illustrated with applications in passive optical structures and organic devices. Throughout the report, critical dimensions are discussed, as well as other challenges facing nanoimprint lithography.

Nanoimprint lithography for organic electronics

Thin films made of organic semiconductors (α-sexithiophene, PDAS and PBAS) have been printed and the impact on morphology studied by optical, atomic force and electron microscopy. Surfaces in contact with the stamp during printing undergo a change towards smoother and more ordered material at the macromolecular scale. Interdigitated nanoelectrodes to be used as source and drain in TFTs have been made and printed down to 100 nm. PDAS and PBAS can be printed at room temperature and preserve their printed feature provided they are cross-linked afterwards.

Nanoimprinted passive optical devices

We report on the feasibility and process parameters of nanoimprint lithography to fabricate low refractive index passive optical devices. Diffraction gratings printed in polymethylmethacrylate (PMMA) exhibit a sharp dispersion with a full width at half maximum of about 20 nm. Waveguides were printed in polystyrene (PS) on silicon oxide and had losses between 8–20 dB cm−1 at wavelengths between 650–400 nm, respectively. Finally, one-dimensional photonic structures were also printed in PS and their transmission and morphology characterized. The expected Bragg peak was observed in transmission and atomic force microscopy images have shown a good pattern transfer. A square lattice was printed in PMMA and more than 40 print cycles were obtained, i.e., potentially more than 1000 imprints from one master stamp.

Structuring of self-assembled three-dimensional photonic crystals by direct electron-beam lithography

An electron-beam lithography technique is described capable of structuring three-dimensional self-assembled photonic crystals. It is shown that the control of the writing depth can be achieved by varying the electron acceleration voltage. Microscopic structures with a depth from 0.4 up to 2 μm are fabricated with a typical lateral resolution of 0.4 μm. The relevance of this technique for the fabrication of deterministic defects sites in opal photonic crystals is discussed and its extension towards buried structures is suggested.

Polymer stamps for nanoimprinting

Stamp fabrication for nanoimprinting can be significantly simplified, when specialized crosslinking polymers are applied to pattern definition. The polymer patterns can be used as stamps themselves. Two possibilities are reported: (1) An e-beam sensitive resist was developed, which enables the fabrication of polymer-on-silicon stamps. Patterns with a feature size of 70 nm could be created. (2) Full plastic stamps were obtained by a casting-moulding technique, which enable pattern transfer from any conventional mould. The quality of the two stamp variants were proved by imprinting experiments.

A comparison of thermally and photochemically cross-linked polymers for nanoimprinting

The characteristics and benefits of two types of cross-linking prepolymers with low glass transition temperature (Tg) for nanoimprinting are reported. They are soluble in organic solvents and their solutions can be processed like those of common thermoplastics. The imprinted patterns receive high thermal and mechanical stability through cross-linking polymerization. The course of the polymerization was investigated to determine the appropriate conditions for the imprint process. In thermally cross-linked polymers mr-I 9000, the cross-linking occurs during imprinting. Process time and temperature depend on the polymerization rate. Volume shrinkage during the polymerization does not adversely affect imprinting. Photochemically crosslinked polymers mr-L 6000 make possible imprint temperatures below 100 °C and short imprint times. The Tg of the prepolymer determines the imprint temperature. The cross-linking reaction and structural stabilization is performed after imprinting. 50-nm trenches and sub-50-nm dots confirm the successful application of the polymers.

Fabrication of semiconductor-and polymer-based photonic crystals using nanoimprint lithography

The technology of fabricating photonic crystals with the use of nanoimprint lithography is described. One-and two-dimensional photonic crystals are produced by direct extrusion of polymethyl methacrylate by Si moulds obtained via interference lithography and reactive ion etching. The period of 2D photonic crystals, which present a square array of holes, ranges from 270 to 700 nm; the aperture diameter amounts to the half-period of the structure. The holes are round-shaped with even edges. One-dimensional GaAs-based photonic crystals are fabricated by reactive ion etching of GaAs to a depth of 1 μm through a mask formed using nanoimprint lithography. The resulting crystals have a period of 800 nm, a ridge width of 200 nm, and smooth nearly vertical side walls.

Fluorescence microscopy for quality control in nanoimprint lithography

Fluorescence microscopy is introduced as a low cost quality control process for nanoimprint lithography. To depict imprinted structures down to 1 µm lateral size and to detect residues down to 100 nm lateral size, the standard printable polymer mr-I8000 is labelled with less than 0.1 wt.% fluorescent dye. Three different types of stamps are used to determine the dependence of the shape and size of stamp features in a series of imprints. The quality of a stamp is given by the sticking polymer residues per unit area. Fluorescence light images as well as visible light images are analysed. Changes in the area of the stamp covered with polymer as a function of the number of imprints is summarised in a statistical process chart. Adhesion was artificially induced in order to observe self cleaning of virgin stamps. They were detected and monitored, suggesting that this method is a suitable technique for quality control and that it could be easily adapted to the nanoimprint process.

Pressure Induced Anisotropic Electrical Conductivity in Vanadium (V) Oxide-Based Tubular Structures

The hydrothermal treatment of vanadium pentoxide xerogel previously intercalated with long-chain primary amines leads to micro and nano-tubular products whose walls are conformed by amine bilayers sandwiched between V2O5lamellae. The microcrystalline powder product may be partially ordered at a macroscopic scale by pressure. Effects of the pressure are observed in the morphology, the intensity of the reflections in the X-ray diffractograms as well as in the electrical conductivity. The anisotropy degree estimated as the ratio of the electrical conductivities perpendicular and parallel to the direction of the applied pressure is about 102at room temperature.