Andrea Edit Pap

Thermal oxidation of porous silicon: Study on structure

The structural changes of porous silicon (PS) samples during oxidation are investigated and analyzed using various microscopy techniques and x-ray diffraction. It is found that the surface roughness of oxidized PS layers increases with the oxidation at 200–400°C and decreased at 600–800°C. At 800°C a partially fused surface is observed. The oxide formed on the wall of porous silicon skeleton is amorphous. The shifts of Si(400) peaks are observed in the x-ray diffraction patterns, which are correlated to the lattice deformation induced by thermal expansion coefficient mismatch between the grown SiO2 and the residual Si, and to the intrinsic stress caused by the Si–O bonds at the Si–SiO2 interface. These explanations are supported by thermomechanical modeling using three-dimensional finite element method.

Optical properties of porous silicon.: Part I: Fabrication and investigation of single layers

The wavelength dependent optical parameters such as refractive index n and absorption coefficient a of porous silicon (PS) have been calculated from optical transmission spectra of plan-parallel films having different porosities. Since the effects of manufacturing conditions on film porosity and thickness have been investigated as well, a complete set of data is obtained enabling the design and easy realization of PS layers with certain structural and optical properties in advanced applications for optoelectronics and sensors.

Optical properties of porous silicon. Part II: Fabrication and investigation of multilayer structures

Abstract By knowing the wavelength dependent optical parameters (refractive index n and absorption coefficient α ) and the obtained thickness of porous silicon (PS) layers when anodizing boron-doped Si wafers, one can design and realize periodic structures by alternating the current density during anodization. Such periodicity in the structure––and thus in optical properties within a film––results in a superlattice, which acts as a Bragg grating. By considering the proper Bragg conditions, the fabrication of optical filter/reflector components can be performed in a simple manner. In this paper, both theoretical and experimental aspects of the fabrication procedure are investigated.

Large area self-assembled masking for photonic applications

Ordered porous structures for photonic application were fabricated on p- and n-type silicon by means of masking against ion implantation with Langmuir-Blodgett (LB) films. LB films from Stober silica spheres [J. Colloid Interface Sci. 26, 62 (1968)] of 350nm diameter were applied in the boron and phosphorus ion-implantation step, thereby offering a laterally periodic doping pattern. Ordered porous silicon structures were obtained after performing an anodic etch and were then removed by alkaline etching resulting in the required two-dimensional photonic arrangement. The LB silica masks and the resulting silicon structures were studied by field emission scanning electron microscope analysis.

Laser-induced surface activation of LTCC materials for chemical metallization

Low-temperature cofired ceramics (LTCCs) are mainly applied in hybrid microelectronics packaging technology, where the fabrication of metallic conductors on LTCC materials is done using various printing technologies. The conventional process is fast and cost-effective in the case of mass production, but too slow and difficult when repair and/or modifications of the circuitry are needed. Printing also fails when deposition of thin metal films on LTCC is required. Here, a simple laser-assisted process is presented, by which the surface of LTCCs can be activated for consecutive electroless chemical metal plating. The method enables the realization of thick high-conductance metallic Cu micropatterns and thin seed layers of Ag and Au, with a lateral resolution of a few tens of micrometers. The process is also suitable for 3-D-MEMS applications. A study of LTCC surfaces treated with Nd:YAG pulses is carried out using field emission scanning electron microscope (FESEM), SEM, x-ray diffraction (XRD) and Raman measurements.

Reference-free quantification of particle-like surface contaminations by grazing incidence X-ray fluorescence analysis

The analysis of the elemental composition of aerosol particles by non-destructive grazing incidence X-ray fluorescence analysis (GIXRF) is possible if the particles are deposited on a flat substrate. If those particles exhibit surface areas parallel to the substrate surface, under certain experimental conditions, total reflection of incident X-rays might arise also at those sites thereby preventing X-rays from penetrating the particles. For a reliable quantitative analysis, this effect and the interaction with the X-ray standing wave field (XSW) has to be further investigated in detail. To study the effects occurring when nanoscaled objects are probed with GIXRF, artificial nanostructures of known size, shape and composition have been manufactured on flat silicon wafer surfaces, with the intention to simulate deposited nanoscaled aerosol particles. A reference-free quantification of the deposited mass was performed employing a simple model for the propagation of the XSW through the sample material. Depending on the quality of the manufactured structures, good agreement between nominal masses and measured values could be stated. Only moderate agreement was found for samples that were more difficult to manufacture. GIXRF measurements yield information on the physical dimensions of the structures which are well in line with results obtained by a combination of scanning electron microscopy and energy-dispersive X-ray spectrometry (SEM/EDX). The presented quantification model, which is based on existing software for XSW calculations, can be transferred to environmental nanoparticles sampled directly from the aerosol phase. All measurements were performed in the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at BESSY II using well-characterized monochromatic synchrotron radiation and calibrated instrumentation.

Thermo-mechanical design and characterization of low dissipation micro-hotplates operated above 500C

This work describes the results of a systematic investigation of micro-hotplates capable of operating up to 600?C both in static and dynamic modes. The goal of development is to form a reduced power consumption micro-pellistor for portable devices. For the selection of optimum device geometry and the membrane layer structure, alternatives FEM analysis was applied. The materials considered were Si3N4, SiO2, TiO2/Pt, Al2O3 and their combination in various multilayer structures. To reduce the chip size DRIE was selected for the release of the membrane. Experimental characterization of the hotplates was carried out by various techniques; the average hotplate temperature was deduced from the resistance of the applied Pt heater and verified by micro-melting point measurements. Buckling of the membranes was tested by means of optical methods and the cumulative stress of the multilayer structure was quantified by Makyoh-topography. Pulsed mode cyclic heating revealed the dynamic properties and also served for accelerated stability tests. For demonstration, micro-heaters with heat dissipation up to 23?C/mW and t90%<3ms were constructed. The hotplates were coated with Pt catalyst to form a combustive type gas sensor operated at elevated temperature.

Fractal kinetic behavior of plasmin on the surface of fibrin meshwork.

Intravascular fibrin clots are resolved by plasmin acting at the interface of gel phasesubstrate and fluid-borne enzyme. The classic Michaelis.Menten kinetic scheme cannot describe satisfactorily this heterogeneous-phase proteolysis because it assumes homogeneous well-mixed conditions. A more suitable model for these spatial constraints,known as fractal kinetics, includes a time-dependence of the Michaelis coefficient Km(F) = Km0F (1+ t)h, where h is a fractal exponent of time, t. The aim of the present study was to build up and experimentally validate a mathematical model for surface-acting plasmin that can contribute to a better understanding of the factors that influence fibrinolytic rates. The kinetic model was fitted to turbidimetric data for fibrinolysis under various conditions. The model predicted Km0(F) = 1.98 μM and h = 0.25 for fibrin composed of thin fibers and Km0(F) = 5.01 μM and h = 0.16 for thick fibers in line with a slower macroscale lytic rate (due to a stronger clustering trend reflected in the h value) despite faster cleavage of individual thin fibers (seen as lower Km0(F) ). ε-Aminocaproic acid at 1 mM or 8 U/mL carboxypeptidase-B eliminated the time-dependence of Km F and increased the lysis rate suggesting a role of C-terminal lysines in the progressive clustering of plasmin. This fractal kinetic concept gained structural support from imaging techniques. Atomic force microscopy revealed significant changes in plasmin distribution on a patterned fibrinogen surface in line with the time-dependent clustering of fluorescent plasminogen in confocal laser microscopy. These data from complementary approaches support a mechanism for loss of plasmin activity resulting from C-terminal lysine-dependent redistribution of enzyme molecules on the fibrin surface.

Copper plating on and electrical investigation of a low-permittivity cycloolefin-copolymer

Abstract Adhesive (>5 MPa) and conductive (ρ~2 μΩ cm) copper films with a thickness of ~200 nm were deposited by chemical means on a recently developed low- k cycloolefin-copolymer (COC). The steps of the metallization procedure (chemical etching, metal seeding and copper plating) were optimized. Raman spectroscopy, SEM and AFM were used in the investigation of surface structures formed in the course of processing. Since this polymer is a good candidate for high-frequency microelectronics applications, electrical measurements up to 3 GHz were performed as well.

Laser-enhanced selective TiO2 deposition on Si

A novel additive deposition method for the fabrication of TiO2 micro-patterns is demonstrated in this paper. Thin (<1 μm) and narrow (∼3 μm) titanium oxide/hydroxide and TiO2 deposits on Si wafers were realized using laser direct writing in gel films and solutions, respectively. By post-annealing the laser-written patterns (derived from gel) in a furnace, the deposits turned into stochiometric TiO2. The obtained micro-structures were characterized by using SEM/EDX and AFM.