E. Oesterschulze

Multipurpose sensor tips for scanning near‐field microscopy

The reproducible micromachining of hollow metal tips on Si cantilevers and their applicability to scanning probe microscopy techniques are described. Provided with apertures below 130 nm and hollow pyramidal tips proved to be highly suited probes for scanning near‐field optical microscopy (SNOM). First results of combined SFM/SNOM measurements together with scanning electron microscopy (SEM) photographs of the new sensors are presented. The SNOM images show a resolution of about 100 nm demonstrating the usefulness of these probes.

Thermal imaging and measurement techniques for electronic materials and devices

Abstract The temperature stress occurring during electrical operation plays an important part in optimizing the performance and reliability of electronic devices. Thermal stress results from short transient processes (that can lead to critical junction temperatures within the chip) as well as from long-term cyclic stresses in a real system environment. The thermal characterization of materials, electronic components and modules by experiment represents an important contribution to quantifying and minimizing temperature stresses within the scope of a comprehensive approach to thermal management. This overview article describes the principles, characteristics and applications of (noncontacting) optical techniques that measure the absolute or relative temperatures of electronic devices or detect the thermal properties of materials. The range of techniques extends from conventional thermography (thermal imaging) via scanning laser probing (beam reflection and deflection techniques) up to near-field thermal microscopy. The presentation focuses on passive techniques that investigate the device under test in electrical operation (self-heating), but also take a look at photothermal methods that heat the specimen with a laser beam and analyze the thermal response (active techniques).

Atomic force microscopy and lateral force microscopy using piezoresistive cantilevers

In this article a novel probe for atomic force microscopy will be introduced. It is based on a conventional micromachined silicon cantilever with an integrated electronic sensor which determines the cantilever deflection. The principle setup of this probe is described and a theoretical and an experimental investigation of the sensitivity of the probe will be presented. The probe is suitable for operation in the static as well as in the dynamic mode and was employed for imaging various materials. To enhance the sensitivity of the detection system and to obtain simultaneously a cantilever with a small spring constant, the geometry of the cantilever was modified. Finite element method calculations were performed to get the optimum sensor design. Furthermore, a different cantilever concept was designed to perform lateral force microscopy as well.

Evaluating probes for “electrical” atomic force microscopy

The availability of very sharp, wear-proof, electrically conductive probes is one crucial issue for conductive atomic force microscopy (AFM) techniques such as scanning capacitance microscopy, scanning spreading resistance microscopy, and nanopotentiometry. The purpose of this systematic study is to give an overview of the existing probes and to evaluate their performance for the electrical techniques with emphasis on applications on Si at high contact forces. The suitability of the characterized probes has been demonstrated by applying conductive AFM techniques to test structures and state-of-the-art semiconductor devices. Two classes of probes were examined geometrically and electrically: Si sensors with a conductive coating and integrated pyramidal tips made of metal or diamond. Structural information about the conductive materials was obtained by electron microscopy and other analytical tools. Swift and nondestructive procedures to characterize the geometrical and electrical properties of the probes p...

Determination of preferential binding sites for anti-dsRNA antibodies on double-stranded RNA by scanning force microscopy

The monoclonal anti-dsRNA antibody J2 binds double-stranded RNAs (dsRNA) in an apparently sequence-nonspecific way. The mAb only recognizes antigens with double-stranded regions of at least 40 bp and its affinity to poly(A) poly(U) and to dsRNAs with mixed base pair composition is about tenfold higher than to poly(I) poly(C). Because no specific binding site could be determined, the number, the exact dimensions, and other distinct features of the binding sites on a given antigen are difficult to evaluate by biochemical methods. We therefore employed scanning force microscopy (SFM) as a method to analyze antibody-dsRNA interaction and protein-RNA binding in general. Several in vitro-synthesized dsRNA substrates, generated from the Dictyostelium PSV-A gene, were used. In addition to the expected sequence-nonspecific binding, imaging of the complexes indicated preferential binding of antibodies to the ends of dsRNA molecules as well as to certain internal sites. Analysis of 2,000 bound antibodies suggested that the consensus sequence of a preferential internal binding site is A2N9A3N9A2, thus presenting A residues on one face of the helix. The site was verified by site-directed mutagenesis, which abolished preferential binding to this region. The data demonstrate that SFM can be efficiently used to identify and characterize binding sites for proteins with no or incomplete sequence specificity. This is especially the case for many proteins involved in RNA metabolism.

Fabrication of integrated diamond cantilevers with tips for SPM applications

Abstract The fabrication of diamond cantilevers with diamond tips integrated on silicon wafers for scanning probe microscopy (SPM) applications is reported. Hot filament CVD diamond deposition and standard techniques of silicon micro-machining are employed. The deposition of well-developed tips depends critically on the pretreatment applied to enhance nucleation density. With an optimized process, well-shaped tips with a radius of curvature in the order of 30 nm are obtained. According to micro-Raman investigations they consist of high quality diamond. Another critical step is the definition of the cantilever area. It can be solved by proper process design. Preliminary performance tests show the cantilevers to possess high resonance frequencies.

FABRICATION OF SMALL DIAMOND TIPS FOR SCANNING PROBE MICROSCOPY APPLICATION

A process for the fabrication of diamond cantilevers integrated on a silicon wafer is presented. At one end the cantilevers possess a tip with a small radius of curvature thus allowing their use in scanning probe microscopy applications. The influence of various procedures to enhance diamond nucleation of the properties of the tips is investigated. Ultrasonic pretreatment with 1 μm diamond paste and subsequent hot-filament chemical-vapor deposition turned out to yield the best results. Micro-Raman measurements show the tips to consist of stress-free diamond up to their very apex.

Thermal imaging of thin films by scanning thermal microscope

In macroscopic photothermal measurement techniques inhomogeneous thermal waves are used to probe thermal properties of materials on a macroscopic scale. The same principle has been adapted to a high‐resolution scanning thermal microscope. Heating the thermal probe periodically results in an amplitude and a phase signal which can be referred to the dynamical thermal behavior of the sample. This measurement mode allows the investigation of the thermal diffusivity of samples in contrast to experiments known from static scanning thermal microscopy which are related to the thermal conductivity. The photothermal scanning thermal microscope technique was used to investigate grains of thin polycrystalline diamond films. For topography measurements the thermal probes were additionally employed for scanning tunneling microscopy.

Influence of electrode size and geometry on electrochemical experiments with combined SECM–SFM probes

Gold electrodes integrated into silicon scanning force microscopy (SFM) probes allow the acquisition of spatially correlated data for sample morphology (via SFM) and local electrochemical reactivity via scanning electrochemical microscopy (SECM). The lateral resolution of both techniques is controlled by different properties of the integrated probes. The topographic tracking provided by the SFM mechanism allows the realization of very small working distances for the SECM measurements. Microfabrication technology was used in order to reduce the size of the active electrode area of the tip into the sub-100 nm regime. The functionality of the probes was tested using electrochemical methods. Experiments revealed that the response could be quantitatively compared to numerical simulation. The low working distance, in combination with the small size of the active electrode area, allows for high lateral resolution in the SECM images. This is illustrated with different model substrates that cover a range of different rate constants and illustrate the dependence of the SECM contrast on the local kinetics of the sample in the sub-micrometre size range.

Surface investigations by scanning thermal microscopy

A scanning thermal microscope has been developed which is capable of imaging thermal properties of materials with high spatial resolution. First results indicate a lateral resolution less than 200 nm. The microscope employs a miniaturized thermal probe whose tip is formed as a thermocouple. The probe is laser heated to generate a thermovoltage. A sample approaching the heated tip leads to a heat flow from the tip to the cooler sample surface and thus to a decrease of the measured voltage. In the initial experiments we scanned the tip above the sample surface with open feedback loop and mapped the thermovoltage at each location of the scan range. Furthermore, we closed the feedback loop keeping the thermovoltage constant and measured the z displacement of the piezoelectric tube carrying the probe. All these measurements yield topographical as well as thermal information of the sample surface.