To utilize the potential of nanoimprint lithography (NIL) you need polymers, which give relief patterns with good thermal and etch resistance, a necessity for subsequent process steps. Thermoplastic polymers with high thermal stability require high imprint temperatures. Such temperatures can cause polymer degradation and problems with pattern transfer due to the different coefficients of thermal expansion of substrate, polymer and stamp. The characteristics and benefits of two types of cross-linking prepolymers with low glass transition temperature (Tg) for nanoimprinting are described. They are soluble in organic solvents and their solutions can be processed like those of poly (methyl methacrylate) (PMMA). The imprinted patterns receive high thermal and mechanical stability through cross-linking polymerization and exhibit high plasma etch resistance. The course of the polymerization was investigated to determine the appropriate conditions for the imprint process. In thermally cross-linking allyl polymers, 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 cross-linking epoxy-based polymers permit imprint temperatures below 100°C and short imprint times. Tg of the prepolymer determines the imprint temperature. The cross-linking reaction and structural stabilization is performed after imprinting. SEM images demonstrate the realization of the cross-linking polymer approach. Isolated lines down to 50 nm width confirm the successful application of the polymers.
Gold nanoelectrodes with gaps of less than 10 nm were formed by conventional E-beam lithography on silicon substrates covered by Al<sub>2</sub>O<sub>3</sub>. Molecular films were deposited on the electrodes by Langmuir-Shaefer technique. The I-V curves of such systems show a suppressed conductance indicating a correlated electron tunnelling through the system. All measurements were made at room temperature.
Nanoimprint lithography over 2 inch wafers with a patterned area of 40,000 micrometer squared consisting of interdigitated lines of 100 nm width with varying distance between the lines has been performed. By performing metal lift-off and subsequent UV-lithography for definition of contact regions and pads, complete metal arrays have been fabricated. The structure is electrically characterized by admittance spectroscopy. In this paper we describe the design and realization of a compact nanoimprint lithography system. Furthermore, various aspects of nanoimprint lithography are discussed, and nanoimprint lithography is compared with other nanostructuring technologies.
Quantum well wire structures in metalorganic vapor phase epitaxy (MOVPE) grown Ga<SUB>.53</SUB>In<SUB>.47</SUB>As/InP and in Ga<SUB>.85</SUB>In<SUB>.15</SUB>As/GaAs have been fabricated by electron beam lithography and subsequent metalorganic reactive ion etching (MORIE) and/or wet etching. The dry etching was optimized for low-damage conditions and for mask-to-wafer pattern transfer. In the wet etching process, an underetching was implemented in order to reduce the linewidth defined by the etching mask. A wet etching step has been used after the dry etching for removal of the partly damaged surface region and for smoothing of the sidewalls of the wires. Differently processed areas were excited selectively by low- temperature cathodoluminescence (CL) from which the optical quality of the wire material was evaluated and blue shifts for the wires as large as 10 meV were observed. Individual wires have also been imaged and effects of one-dimensional exciton diffusion have been probed.
The need for higher resolution in the study of materials has led to the development of sharper probes for imaging of the geometrical and surface structures. This development has led to the Scanning Tunnelling Microscope (STM), and variations thereof such as the atomic force microscope. Simultaneously there is a strong need for the possibility of making spectroscopic investigations of nm-structures. In this paper we describe results from spectral analysis of photons emitted as a consequence of tunnel-injection of minority carriers in semiconductor bulk or Quantum-Well (QW) samples. We denote this techtiique Scanning Tunneling Luminescence (STL), which is yet a new type of tunneling microscopy. Two different types of STL-experiments will be described, differing in the type of tip being used. In the first experiment we use a metallic tip to inject electrons into p-type JnP, where the charge carriers recombine, partly radiatve1y. In the second experiment we employ a tip of a large band-gap semiconductor, p-type GaP, from which electrons tunnel into an n-type InP/GaInAs/InP QW sample. We report on the first observation of such minority carrier injection where the sharply defined carrier (hole) energy distribution of the emitting tip is employed for almost monoenergetic injection and on the observation at higher bias levels of how electrons are also being injected from the InP surface into the tip where they recombine radiatively.