Impulsive control for fast nanopositioning

In this paper we present a non-linear control scheme for high-speed nanopositioning based on impulsive control. Unlike in the case of a linear feedback controller, the controller states are altered in a discontinuous manner at specific instances in time. Using this technique, it is possible to simultaneously achieve good tracking performance, disturbance rejection and tolerance to measurement noise. Impulsive control is demonstrated experimentally on an atomic force microscope. A significant improvement in tracking performance is demonstrated.     

Scanning probe microscopy based on magnetoresistive sensing

Integrated sensors are essential for scanning probe microscopy (SPM) based systems that employ arrays of microcantilevers for high throughput. Common integrated sensors, such as piezoresistive, piezoelectric, capacitive and thermoelectric sensors, suffer from low bandwidth and/or low resolution. In this paper, a novel magnetoresistive-sensor-based scanning probe microscopy (MR-SPM) technique is presented. The principle of MR-SPM is first demonstrated using experiments with magnetic cantilevers and commercial MR sensors. A new cantilever design tailored to MR-SPM is then presented and micromagnetic simulations are employed to evaluate the achievable resolution. A remarkable resolution of 0.84?? over a bandwidth of 1?MHz is estimated, which would significantly outperform state-of-the-art optical deflection sensors. Due to its combination of high resolution at high bandwidth, and its amenability to integration in probe arrays, MR-SPM holds great promise for low-cost, high-throughput SPM.     

Cleaved-edge-overgrowth nanogap electrodes

We present a method to fabricate multiple metal nanogap electrodes of tailored width and distance in parallel, on the cleaved plane of a GaAs/AlGaAs heterostructure. The three-dimensional patterned structures are obtained by a combination of molecular-beam-epitaxial regrowth on a crystal facet, using the cleaved-edge-overgrowth (CEO) method, and subsequent wet selective etching and metallization steps. SEM and AFM studies reveal smooth and co-planar electrodes of width and distance of the order of 10 nm. Preliminary electrical characterization indicates electrical gap insulation in the 100 MΩ range with kΩ lead resistance. We propose our methodology to realize multiple electrode geometries that would allow investigation of the electrical conductivity of complex nanoscale objects such as branched organic molecules.     

Extending acoustic microscopy for comprehensive failure analysis applications

In manufacturing of microelectronic components, non-destructive failure analysis methods are important for quality control. These non-destructive methods enable rapid defect localization which then guides micro-structural investigations involving destructive sample preparation. Scanning acoustic microscopy (SAM) is a powerful tool for the inspection of internal structures in optically opaque materials. Depth-specific information can be extracted and applied to create two- and three-dimensional images without the need for time consuming tomographic scan procedures. While traditional SAM imaging of the signal intensity is very valuable, it leaves most of the potential of acoustic microscopy unused. The aim of the current work was to develop comprehensive analysis algorithms to utilize the full potential of SAM and thus to extend the range of its applications. Examples representing different application fields were investigated in the current study. The examples include advanced flip-chip devices, bonded wafer pairs, solder tape connectors of a photovoltaic solar panel and high density chip-to-chip interconnects relevant for 3D integration. Progress achieved during this work can be divided into four categories: Signal Analysis and Parametric Imaging, Signal Analysis and Defect Evaluation, Image Processing and Resolution Enhancement and acoustic GHz microscopy (GHz-SAM). For the first three categories, data acquisition was performed using a commercially available scanning acoustic microscope equipped with several ultrasonic transducers covering the frequency range from 15 to 175 MHz. In the fourth category, data acquisition was performed employing a prototype of a novel acoustic GHz microscopy tool which is currently under development into a commercial system. In the first three categories, recorded acoustic data were subjected to sophisticated algorithms operating in time, frequency and spatial domains for performing signal and image analysis. Acoustic microscopy, combined with such advanced signal and image processing algorithms, proved to be a powerful tool for non-destructive inspection.     

Technological and economical aspects on the influence of reduced Cu(In,Ga)Se2 thickness and Ga grading for co-evaporated Cu(In,Ga)Se2 modules

Reducing the Cu(In,Ga)Se₂ (CIGS) thickness is one way of improving the throughput and capacity in existing production, provided that the efficiency can be kept at a high level. Our experimental results from an in-line co-evaporation process show that it is possible to produce CIGS solar cells with good efficiency at a CIGS thickness of less than 1 μm. An efficiency of 14.4% was obtained for an evaporation time of 8 min and a resulting CIGS thickness of only 0.8 μm. The quantum efficiency measurements show only a minor reduction of the collection in the infrared region that can be related to losses caused by reduced absorption. Passivation of the back contact has been found to be important for thin devices and one way of obtaining good back contact properties, or to reduce the impact of back contact recombination is to use an increased Ga content near the back contact. We have found that Ga grading is feasible also in the three stage process, i.e. a Ga-rich layer near the back contact from stage one is to a high degree retained also after stages two and three. In this paper we discuss the implication of efficiency reduction for the economy of the production and how high efficiency loss that can be tolerated, provided that the output is doubled at equal production cost for the CIGS layer.     

Influence of the ion bombardment of O2 plasmas on low-k materials

In this study, special tests were devised in order to investigate the influence of ion bombardment on the damage induced in low-k dielectrics by oxygen plasmas. By placing a sample that suffered a lot of ion bombardment and one which suffered little ion bombardment simultaneously in the same plasma, it was possible to verify that ion bombardment in fact helped to protect the low-k film against oxygen plasma induced damage. Exhaustive analyses (ellipsometry, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, porosimetry, capacitance-voltage (C-V) measurements, water contact angle analysis) show that ion bombardment induced the formation of a denser top layer in the film, which then hampered further penetration of active oxygen species deeper into the bulk. This was further confirmed by other tests combining capacitively and inductively coupled plasmas. Therefore, it was possible to conclude that, at least for these plasmas, ion bombardment may help to reduce plasma induced damage to low-k materials.     

Time-dependent mechanical behavior of lime-mortar masonry

An extensive experimental program has been set up to characterize the time-dependent deformation behavior of masonry, subjected to the creep failure mode. Different types of short-term creep tests were performed on small masonry specimens, which were constructed with air-hardening lime mortar. To assess the influence of the carbonation process on creep behavior, several specimens were subjected to accelerated carbonation. The time-dependent deformations are modeled using a viscoelastic, rheological model which includes damage effects. The applicability of the model is validated by comparing theoretical and experimental results and extending the time frame to long-term predictions. Good agreement was found between experimental and simulated time-dependent deformations. The accuracy of the proposed model is estimated by including the scatter on the most important material parameters in the analysis.     

Energy transfer from CdSe/CdS nanorods to amorphous carbon

Semiconductor nanocrystals placed nearby a metal film significantly change their optical properties. In this work, we examine the change in fluorescence intensity, lifetime, and blinking behavior of individual CdSe/CdS nanorods close to a 9 nm thick amorphous carbon film. Energy transfer between the donor and acceptor was investigated in detail yielding a R(-4) distance dependence for the nanorod-carbon system. The F?rster critical distance was determined to be R0=24.9 nm, which is nearly identical with the theoretical value of 24.8 nm predicted by the classical approach. Additionally, antibunching measurements were performed in order to prove the presence of single isolated emitters.     

Oxygen dissociation by concerted action of di-iron centers in metal-organic coordination networks at surfaces: modeling non-heme iron enzymes

The high chemical reactivity of unsaturated metal sites is a key factor for the development of novel devices with applications in sensor engineering and catalysis. It is also central in the research for sustainable energy concepts, e.g., the efficient production and conversion of chemical fuels. Here, we study the process of oxygen dissociation by a surface-supported metal-organic network that displays close structural and functional analogies with the cofactors of non-heme enzymes. We synthesize a two-dimensional array of chemically active di-iron sites on a Cu(001) surface where molecular oxygen readily dissociates at room temperature. We provide an atomic-level structural and electronic characterization before and after reaction by combining scanning tunneling microscopy, X-ray absorption spectroscopy, and density functional theory. The latter identifies a novel mechanism for O2 dissociation controlled by the cooperative catalytic action of two Fe2+ ions. The high structural flexibility of the organic ligands, the mobility of the metal centers, and the hydrogen bonding formation are shown to be essential for the functionality of these active centers allowing to mimick biologically relevant reactions in a confined environment.     

CdSe thin films as solar control coatings

CdSe thin films deposited by a physical vapour deposition method were investigated as solar control coatings on architectural glazings. The optical transmittance and the near-normal specular reflectance in the range 0.40−2.40 μm and spectral distribution of reflected and transmitted intesities in the same range showed that CdSe thin films have solar control characteristics comparable to commercially available metallic coatings and other materials such as PbS and Cu_ξS films. The solar control characteristics of CdSe films were found to be dependent on film parameters, including deposition rate and deposition temperature.