Various approaches for the preparation of nanostructures with dimension on macroscopic areas are known. In contrast to cost-intensive top-down lithographic techniques, various bottom-up methods based on ion beam technologies to form large arrays of nanostructured surfaces are well established. In principle, it can be distinguished between two routes at the preparation of nanostructures by low-energy ion bombardment sputtering.
The destructive route is characterized that under certain conditions, given by the self-organization processes, the ion beam induced erosion process can lead to the formation of e.g. well-ordered Si nanostructures like dots or ripples on the surface. Using a constructive route, i.e. glancing angle deposition by ion beam sputtering, sculptured thin films consisting of various nanostructures of several shapes, such as inclined and vertical columns, screws, and spirals, were deposited on Si substrates. It will be shown that morphology, shape, and diameter of the structures are influenced and can thus be controlled by adjusting various deposition parameters, including substrate temperature and ratio of substrate rotational speed to film deposition rate.
Surface-enhanced Raman scattering (SERS) from silver nanosculptured thin films (STF) was studied in detail for biosensing. The influences of the nanostructures' sizes, topology, the substrate features, and the preparation conditions on the enhancement were examined. Enhancement factors on the order of 10 7 were obtained from silver nanorods deposited on bare silicon substrates with respect to their dense counterparts, using 4-aminotheophenol (4-ATP) for the Raman emission. The low detection limit that can be achieved with STFs is below 1 μg/lit of the probe molecule 4-ATP in Ethanol solution. Theoretical modeling based on a single small spheroidal nanoparticle helped in explaining the main properties of SERS from STFs. Stability of the films was noticed over a period of one year without significant degradation.
Nanosculptured thin films (STF) are prepared by the oblique angle deposition technique and take different forms of nano columnar structures. Varieties of STFs were investigated to find the optimum structure for biosensing based on the surface enhanced fluorescence. A comparative study was carried out with STFs containing the nanocolumnar structures that differ in their shape, height (h), and tilt angle with respect to the surface (α), thickness (d), and arrangement. The greatest enhancement of the fluorescent signal was found for Ag-based STFs on Si(100), giving an enhancement factor of ×71, where h = 400 nm, d = 75 nm, and α = 23° relative to Ag closed film using fluorescent dye Rhodamine 123. We immobilized the fluorescent receptor to the thiol self-assembly monolayer on Ag-based STF and Ag dense film to demonstrate the applications of STFs for specific biosensing. Upon excitation of the fluorophore by an Hg light source, a CCD camera with controlled exposure time would detect the pattern of fluorescent receptor Anti-Rabbit IgG on the surfaces. A specially designed optical fiber housing attached to the microscope allowed quantitative measurement of the fluorescence spectrum on a microspot parallel to the image grab.
Nano-sculptured thin films (STF) are prepared by the glancing angle deposition technique and take different forms of
nano columnar structures. Varieties of STFs are investigated to find the optimum structure for biosensing based on the
surface enhanced fluourescence (SEF). The highest amplification of fluorescent signal is found for Ag based STFs on
fused silica giving an enhancement factor of x23°, where h=400nm, d=75nm, á=23o relative to Ag dense film using
fluorescent dye Rhodamine 123. Based on this, a demonstration of monitoring of antibodies and even confirmation of
successful immobilization of the receptors presented. Bound antibody to the thiol self assembly monolayer on sample
surface is then quantified by means of the fluorescent signal. Upon excitation of the fluorophore by Hg source light, a
CCD camera with a controlled exposure time detects the pattern of fluorescent antibody/E-coli bacteria colonies on the
STF surface. A fiber optic holder attached to the microscope allowed quantitative measurement of the fluorescence
spectrum on a microspot.
When an electromagnetic wave interacts with a nano structured metallic surface or a nanoparticle, the electromagnetic
fields near the surface are greatly enhanced by factors up to 1000. This phenomenon is due to two processes: (i) the
'lightning rod' effect, conventionally described as the crowding of the electric field lines at a sharp metallic tip and (ii)
the excitation of localized surface plasmons at the metal surface. Both are responsible for the enhancement of
fluorescence, second-harmonic generation and Raman scattering. For metal nanoparticles often both processes are
involved in creating the localized enhanced near field. Since sculptured thin films (STFs) can have a rod like structure
and an overall large porosity, it is expected that these structures will exhibit enhanced fluorescence and Raman signals.
Results of comparative study of surface enhanced fluorescence are presented from STFs containing metal nano
structures. The highest enhancement is found for Ag based STFs giving an enhancement factor of x14.
The etching of transparent materials with high precision and high quality is still a challenge for laser processing. Laser backside etching allows the processing of transparent materials with pulsed UV-lasers. The laser-etched structures in fused silica are characterized by a high fidelity and a low surface roughness. Different machining techniques were applied for laser etching of binary and three-dimensional microstructures with micron and sub-micron sizes. Applying contour mask technique micro sized cylindrical lens and prism array were fabricated. Using small spot laser written gratings with uniform or variable depth was machined with nanometer depth resolution and the etching of free-form surfaces with a size of 1 mm2 and a P-V-value of less than 1 micron by means of laser scanning is demonstrated. Additionally, graded multilevel elements and submicron gratings were engraved with nanometer depth accuracy applying mask projection techniques.
We develop a Plasma Jet Chemical Etching (PJCE) technique for high rate precision machining of optical materials aiming in a technology mature for precision asphere and free-form surface topology fabrication. The present contribution summarizes the achievements after about twelve months experience with a prototype production tool facility. PJCE is performed with the help of a microwave driven reactive plasma-jet working in a broad pressure range (10-600 mbar). We developed a moveable lightweight microwave plasma jet source for dwell time techniques performed in a roughly pumped process chamber equipped with a six axis system for precision workpiece and plasma source movement. Volume etch rates of some 10 mm3/min have been achieved for fused silica and silicon, respectively, using reactive (CF4,SF6,O2) and inert (Ar,He) gas mixtures and applying a microwave (2.45 GHz) power in the 100-200 W range. Large quartz plates (80-160 mm) have been figured using dwell time methods to achieve aspheric deformations of some 10 micrometers . The figured surfaces show shape errors of 1-2 micrometers and a microroughness of 50-100 nm RMS but no sub-surface damage enabling a small tool shape conserving post polishing up to the sub-nanometer roughness level. Thus, surface shaping to the nanometer error range can be done by ion beam finishing.
Atomic force microscopy (AFM) is usually the instrument of choice for the investigation of the surface roughness of thin films. Often a detailed image and roughness analysis is hindered by tip artifacts. Many of these artifacts arise from a spatial convolution or dilation of the actual tip and the shape of the surface features imaged. Therefore a careful tip evaluation and calibration is important for a reliable roughness evaluation. In this study about a process for the fabrication of self- assembled nanometer-sized surface structures using low- energy ion sputtering of semiconductor surfaces is reported. The dimension of these structures (typically between 10 and 100 nm), the distance between them and their shape can be tuned by the parameters of the sputter process. With the help of surfaces prepared by this way the influence of the actual AFM tip quality on the measured surface topography was evaluated. Furthermore, it is shown that the tip quality has a strong influence on the parameters extracted from first- and second-order statistics of the surface roughness. This applies particularly with regard to surfaces characterized by a low surface roughness (approximately 1 nm) as generally obtained by means of thin film technologies.
Ion beam figuring (IBF) using inert gas (e.g. Ar) and (Reactive) ion beam etching [(R)IBE] gain growing interest in precision optical surface processing, RIBE mainly for proportional transfer of 3D-resist masks structures in hard optical materials and IBF for finishing and nanometer precision surface figuring in high performance optics technology. Ion beam and plasma jet etching techniques related to different optical surface figuring requirements have been developed at IOM during the last decade. Some of these techniques have been proven to be mature for application in industrial production. The developmental work include material related process tuning with respect to enhance the processing speed and to improve surface roughness and waviness, further various processing algorithms related to different surface figure requirements and processing related equipment modification. Plasma jet assisted chemical etching is under development with respect to efficient machining techniques for precision asphere fabrication. The paper gives an overview of precision engineering techniques for optical surface processing focusing on the status of ion beam and plasma techniques. The status of the proportional transfer of 3D micro-optical resist structures (e.g. micro-lens arrays, blazed fresnel lens structures) into hard optical and optoelectronic materials by (reactive) ion beam etching will be summarized.
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