K. Dickmann

Nanostructure fabrication using laser field enhancement in the near field of a scanning tunneling microscope tip

The scanning tunneling microscope (STM) has been combined with laser excitation and was used for modification of metal surfaces in air. This technique enables processing of structures with a lateral resolution of approximately 10 nm. The form of the created features ranges from craters and ditches to hillocks. The process has been demonstrated on gold and gold/palladium substrates by utilization of tungsten, silver, and platin/iridium tips. Using pure silver tips or silver‐coated tungsten tips, a transfer of tip atoms to the substrate occurred. In the case of uncoated tungsten tips, we observed substrate evaporation and surface grain reorganization at low laser intensities, respectively. No distortion of the employed tips during the structuring process was observed. Several future oriented applications are conceivable, such as, for example, high density data storage and fresnel optics for x rays.

NEW ETCHING PROCEDURE FOR SILVER SCANNING TUNNELING MICROSCOPY TIPS

Due to its physical and chemical features silver is a promising material for scanning tunneling microscopy (STM) tips in the field of nanostructuring by a laser/STM combination. This nanostructuring is archived by field enhancement of optical radiation by a factor of up to 103 in the near field between a STM tip and a substrate. The magnitude of the field enchantment depends on the geometry and material of the utilized tip. State of the art procedures of processing silver tips cannot be applied to this application due to insufficient quality and reproducibility, respectively. We have developed a new simple single‐step etching procedure for silver tips with a high reproducibility and a tip radius of less than 100 nm. This was realized by electrochemical etching in an ammonia solution and subsequent electronic controlled movement of the tip out of the electrolyte.

Characterization of electrochemically etched tungsten tips for scanning tunneling microscopy

Shape and composition of electrochemically etched tungsten tips for use in scanning tunneling microscopy (STM) were investigated in a transmission electron microscope (TEM) with a Gathan imaging filter (GIP). The tips are prepared by a lamella drop-off technique. We observe typical tip radii of less than 10 nm. After a storage of some days under ambient conditions, an amorphous oxide film is detectable. The electron energy-loss spectroscopy confirms that the surface is contaminated by compounds that contain carbon, too.

Focusing of laser radiation in the near-field of a tip (FOLANT) for applications in nanostructuring

Current research work has shown that ‘focusing’ of laser radiation down to a few nanometres can be achieved by using near-field technology, e.g. scanning tunnelling microscopy (STM) or atomic force microscopy (AFM), in combination with a laser. Lateral external illumination of a probe tip with laser radiation can cause a tremendous intensity enhancement of up to 5×105 in the near-field underneath the probe tip. This effect can be explained by different electrostatic as well as electrodynamic effects known from surface-enhanced raman spectroscopy (SERS). This enhancement effect was utilized to concentrate laser radiation with high intensity between a tip and a substrate. This FOLANT (focusing of laser radiation in the near-field of a tip) technique can be applied for material processing on a nanometre scale. Using an STM/laser combination, hillocks, pits and grooves with lateral dimensions down to 10 nm have been obtained on gold substrates. The AFM/laser combination enabled nanostructures down to 20 nm to be established on dielectric materials such as polycarbonate.© 1997 John Wiley & Sons, Ltd.

Nano-material processing with laser radiation in the near field of a scanning probe tip

We report preliminary results of using a scanning probe microscope/laser combination to perform nanostructuring on insulator and metal surfaces in air. This technique enables processing of structures with a lateral resolution of approximately 10 nm. In this paper we present our last structuring results with both scanning tunnelling and scanning force microscopy. Some possible interaction mechanisms responsible for the structuring will be discussed.

Design and performance of an excimer-laser based optical system for high precision microstructuring

We report on the design of an excimer-laser based system for high precision micromachining of spinnerets using a mask imaging technique. Both the illumination and imaging unit are optimized for specified demagnification ratios of 5 and 15, respectively. Detailed investigations were performed to measure the resolution, depth of focus and the sensitivity for the position accuracy of the substrate depending on illumination parameters. A special test mask for measuring the resolution in combination with a new definition of measurement procedure is used. SEM views of ablation results with high machining quality are presented.

Wide bandwidth transimpedance preamplifier for a scanning tunneling microscope

A new two-stage transimpedance preamplifier in feedback picoammeter setup for use in scanning tunneling microscopy is represented. The first stage consisting of a wide band field effect transistor input operational amplifier with resistive feedback has a transimpedance of 20 kΩ and a 3 dB bandwidth of 35 MHz. This setup is built to investigate sub μs fluctuations in the tunneling current.

Calculation of laser-induced temperature field on moving thin metal foils in consideration of Stefan problem

Abstract A temperature field on a moving thin sheet heated by a high-quality laser beam (TEM00) is calculated. Effects of melting and solidification of sheet material is taken into account. Convective and radiation cooling is included into a mathematical model. Nonlinear effects connected with temperature dependencies of material properties are considered. An explicit finite-difference numerical method was used to calculate the non-steady-state 2D and 3D Stefan problems. A surface, on which the melting occur, is replaced by a zone of finite thickness. All physical properties in this zone are smoothed and considered as continuous values. Evaporation of liquid metal begins when temperature reaches some critical value. This effect is also included in the developed model. As a result, the geometry of the melting zone can be derived and is compared with a cross section of real laser welding seem.

Wide-band low-noise tunnel current measurements in laser assisted experiments

A composite current-to-voltage converter that shows a superior bandwidth and signal-to-noise ratio (bandwidth ∼50 MHz at 410 kΩ transimpedance) was designed. Two options for the input stage were tested: a high input impedance wide-band operational amplifier (OP) OPA655 (Burr–Brown) and a differential cascade which is based on the junction field effect transistors SST440 (Siliconics). By using a wideband current feedback OPA658 in the second stage the achieved bandwidth gain product is ∼10 GHz. A method for testing the performance of the preamplifier by using laser induced electron emission is described. The technique’s possibility for measurements of laser induced acoustic waves with high sensitivity and nanometer local resolution is demonstrated. For further applications the design of a wide-bandwidth, low-noise, single tunneling electron counting capable preamplifier is discussed.

Coaxial scanning near-field optical microscope tips: an alternative for conventional tips with high transmission efficiency?

Since the development of the first scanning near‐field optical microscope, great effort has been put into the development of near‐field optical probe tips with a good lateral resolution of the order of several tens of nanometres and a high power throughput. Several groups have proposed the use of scanning near‐field optical tips with a coaxial structure. From electromagnetic theory, it is known that such structures have no cut‐off frequency and a power transmission close to unity. In this paper we discuss some topics concerning the stimulation of optical coaxial waves and their propagation behaviour.