Christiane Zamponi

Low temperature aluminum nitride thin films for sensory applications

A low-temperature sputter deposition process for the synthesis of aluminum nitride (AlN) thin films that is attractive for applications with a limited temperature budget is presented. Influence of the reactive gas concentration, plasma treatment of the nucleation surface and film thickness on the microstructural, piezoelectric and dielectric properties of AlN is investigated. An improved crystal quality with respect to the increased film thickness was observed; where full width at half maximum (FWHM) of the AlN films decreased from 2.88 ± 0.16° down to 1.25 ± 0.07° and the effective longitudinal piezoelectric coefficient (d33,f) increased from 2.30 ± 0.32 pm/V up to 5.57 ± 0.34 pm/V for film thicknesses in the range of 30 nm to 2 μm. Dielectric loss angle (tan δ) decreased from 0.626% ± 0.005% to 0.025% ± 0.011% for the same thickness range. The average relative permittivity (er) was calculated as 10.4 ± 0.05. An almost constant transversal piezoelectric coefficient (|e31,f|) of 1.39 ± 0.01 C/m2 was measured for samples in the range of 0.5 μm to 2 μm. Transmission electron microscopy (TEM) investigations performed on thin (100 nm) and thick (1.6 μm) films revealed an (002) oriented AlN nucleation and growth starting directly from the AlN-Pt interface independent of the film thickness and exhibit comparable quality with the state-of-the-art AlN thin films sputtered at much higher substrate temperatures.

Artificial Single Variant Martensite in Freestanding Fe70Pd30 Films Obtained by Coherent Epitaxial Growth

2010 WILEY-VCH Verlag Gmb Microactuators and sensors based on magnetic shape-memory (MSM) alloys will benefit from the large strain close to 10% obtained in these materials. These strains exceed the values obtainable by magnetostriction or piezoelectricity by more than one order of magnitude. Thus, they can be used directly for most applications, avoiding additional complications of mechanical amplification. As the highest strains to-date are obtained in bulk single crystals, the use of epitaxial films is most promising for microsystems, owing to their single-crystal-like microstructure. With reduced actuator size, however, the influence of interfaces, and in particular of oxidation, becomes more important. Though the prototype Ni2MnGa system is relatively inert, an oxide surface layer may hinder the martensitic transformation in thin films. For the Fe70Pd30 system [8] oxidation is expected not to be critical due to the high content of a noble element. In Fe70Pd30 the martensitic transformation occurs around room temperature (RT). First reports indicate that epitaxial growth of thin Fe70Pd30 films can be obtained already at RT. For epitaxial growth of the NiMnGa system, a minimum temperature of 350 8C is required. Recently it was shown that epitaxial growth of Fe70Pd30 is possible on various metallic buffers. Due to coherent growth, huge tetragonal distortions were stabilized in 50 nm thick films, covering most of the Bain transformation path from face-centered cubic (fcc) to bodycentered cubic (bcc) structure. Here, we show how this approach can be extended to obtain freestanding films of micrometer thickness, thus fulfilling both key requirements for the integration into microsystems as well as prerequisites for MSM films, that is, martensitic, ferromagnetic at RT, freestanding, and single-crystalline-like. In addition, our experiments reveal that the transformation behavior in these films differs from in the bulk. While this topic has been an extensive playground for theory, experiments are rare, since a detailed analysis often requires epitaxial films. Recently, experiments on epitaxial films, for example, have revealed a variant selection by the rigid interface to the substrate or by reduction of the magnetic stray field energy in freestanding films. The present experiments are more fundamental since they show that circumventing the forward martensitic transformation by forming the martensitic structure directly at RT hinders the nucleation of the reverse transformation to austenite. This remarkable suppression of the transformation not only gives a better understanding of the martensitic transformation but also opens innovative routes for microsensors. Films of Fe70Pd30, 1.2mm thick, were deposited with a low deposition rate of 0.024 nms 1 at 30W sputtering power in a magnetron sputtering system on Au-buffered MgO(001) oriented, epi-polished substrates. The crystal structure of the Fe70Pd30 films was analyzed by four-circle and temperature-dependent two-circle X-ray diffraction (XRD), where x and w denote the tilt and rotational angles, respectively. The u–2u scans (see Fig. 1a) show the 200 Au reflection of the buffer layer (2u1⁄4 44.448) as well as the Fe70Pd30 002 reflection (57.388). Assuming a constant volume of the Fe70Pd30 unit cell compared to the cubic austenite, [15] the lattice parameters a and c were calculated to be 0.287 nm and 0.321 nm ( 0.001 nm), respectively, which constitutes a c/a ratio of 1.12. Temperature-dependent XRD measurements showed no change in the crystal structure in the accessible temperature range between 150 and 375K. The pole figure of the 101 reflection reveals a four-fold symmetry (Fig. 1b). The maximum intensity is obtained at an average of 47.278 at w1⁄4 458. The peak in the w direction is rather sharp with a small full width at half maximum (FWHM), indicating well-oriented growth of the body-centered tetragonal (bct) unit cell rotated by 458compared to the edges of the MgO cell. The larger FWHM in the x direction indicates relaxation of the lattice. There are no indications that twinning has occurred in the film. The sample for transmission electron microscopy (TEM) investigations was prepared by focused ion beam (FIB) lift-out

Measurements on cracktips in stainless steel AISI 321 by using a new positron microprobe

High resolution positron microscopy provides a new method for non-destructive investigations of plastic deformation with spatial resolution in the micron range. As positron annihilation is highly sensitive to lattice defects, low concentrations of dislocations are detectable, so that the plastic zone in front of a cracktip appears larger than in comparable metallographic methods. To demonstrate this, a plastic zone in the common stainless steel AISI 321 is imaged with the Bonn Positron Microprobe (BPM) with a spatial resolution of 20 μm.

Magnetron Sputtering a New Fabrication Method of Iron Based Biodegradable Implant Materials

It was shown in the previous decade that pure-iron has a large potential as a biodegradable medical implant material. It is necessary to tailor the material properties according to the intended use of the device. It is of great interest to investigate not only the influence of processing on the material properties but also alternative fabrication methods. In this work for the first time magnetron sputtering in combination with UV lithography was used to fabricate free standing, patterned pure-iron thick films. For the intended use as biodegradable implant material free standing thick films were characterized in terms of microstructure, degradation performance, and mechanical properties before and after various heat treatments. The influence of microstructural changes on the degradation behavior was determined by linear polarization measurements. The mechanical properties were characterized by tensile tests. Microstructure, surface, and composition were investigated by scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) measurements. The foils exhibited a preferential orientation in direction and a fine grained structure. Furthermore they showed a higher strength compared to cast iron and corrosion rates in the range of 0.1 mm/year. Their mechanical properties were tuned by grain coarsening resulting in a slight increase of the degradation rate.

Fe–Pd thin films as a model system for self-organized exchange coupled nanomagnets

In equilibrium the Fe–Pd system on the iron rich side of the phase diagram demixes into Fe and L10-ordered FePd. Here, we examine the suitability of the demixing process for self-organized formation of exchange coupled thin film magnets. In this way the benefit of the high magnetization of Fe is combined with the high magnetocrystalline anisotropy of FePd. By using combinatorial methods the influence of composition and thickness on structure, microstructure, and magnetic properties is analyzed. Experiments show the different thermodynamic and kinetic conditions required for demixing and ordering. In particular, for nanostructures the interface energy during demixing must be considered.

Thin magnesium layer confirmed as an antibacterial and biocompatible implant coating in a co‑culture model

Implant-associated infections commonly result from biofilm-forming bacteria and present severe complications in total joint arthroplasty. Therefore, there is a requirement for the development of biocompatible implant surfaces that prevent bacterial biofilm formation. The present study coated titanium samples with a thin, rapidly corroding layer of magnesium, which were subsequently investigated with respect to their antibacterial and cytotoxic surface properties using a Staphylococcus epidermidis (S. epidermidis) and human osteoblast (hOB) co-culture model. Primary hOBs and S. epidermidis were co-cultured on cylindrical titanium samples (Ti6Al4V) coated with pure magnesium via magnetron sputtering (5 µm thickness) for 7 days. Uncoated titanium test samples served as controls. Vital hOBs were identified by trypan blue staining at days 2 and 7. Planktonic S. epidermidis were quantified by counting the number of colony forming units (CFU). The quantification of biofilm-bound S. epidermidis on the surfaces of test samples was performed by ultrasonic treatment and CFU counting at days 2 and 7. The number of planktonic and biofilm-bound S. epidermidis on the magnesium-coated samples decreased by four orders of magnitude when compared with the titanium control following 7 days of co-culture. The number of vital hOBs on the magnesium-coated samples was observed to increase (40,000 cells/ml) when compared with the controls (20,000 cells/ml). The results of the present study indicate that rapidly corroding magnesium-coated titanium may be a viable coating material that possesses antibacterial and biocompatible properties. A co-culture test is more rigorous than a monoculture study, as it accounts for confounding effects and assesses additional interactions that are more representative of in vivo situations. These results provide a foundation for the future testing of this type of surface in animals.

Microstructures of magnetron sputtered Fe Au thin films

Abstract Freestanding films of highly pure iron and gold multilayers were fabricated and characterized for their intended use as biodegradable implant materials. These samples were deposited using magnetron sputtering on unheated substrates. This technology allows the combination of various non-compounding materials. After annealing for 2 h at 685 °C and 850 °C, respectively to homogenize the multilayer, the microstructures were investigated using X-ray diffraction, energy dispersive X-ray spectroscopy and scanning transmission electron microscopy. Due to the annealing, the multilayered microstructure converts into a new multiphase system consisting of an iron matrix and two different kinds of gold morphologies: segregations along grain boundaries and nanosized core–shell like precipitates.

Mechanical properties and corrosion behaviour of freestanding, precipitate-free magnesium WE43 thin films

Abstract Magnetron sputtered freestanding thin films of two modified WE43 alloys (Mg4Y3Nd and Mg4Y3Gd) consist of a supersaturated single phase microstructure with a strong texture in [0001] direction for a wide range of deposition conditions. While the deposition conditions have no significant influence on the corrosion behaviour of these samples, they strongly influence the mechanical properties, which can be tuned between extremely brittle behaviour for high sputtering pressures (2.0 × 10−2 mbar) and ductile behaviour with a maximum strain of about 18 % at room temperature for low sputtering pressures (8.0 × 10−4 mbar) for both investigated alloys.

Rotational UV lithography device for cylindrical substrate exposure.

Optical photolithograhy is a well developed technique, which is normally restricted to planar substrates used in microelectronics or microelectromechanical system fabrication. For other applications--e.g., patterning of stents--photolithography would be an attractive alternative to techniques such as laser structuring provided that the planar technique could be adapted to cylindrical geometries. This study presents the development of a three-dimensional UV photolithography exposure method using a synchronizing movement between a planar Cr mask and a circular substrate. This technique was successfully applied to tubes with outer diameters between 1 and 5 mm. A lateral resolution for a 5 microm feature size of 4.8 microm was achieved, which is close to the resolution of 4.6 microm for similar planar films.

Superelastic NiTi Thin Films for Medical Applications

Shape memory alloys are able to provide high work output when due to the martensitic transformation. Therefore, they are a promising candidate for actuation mechanisms in microsystems, e.g. in microvalves. Sputter deposited SMA thin films are already in use as free-standing films or as composites. Since it is also possible to deposit and structure the SMA composites on Si substrates by photolithographic techniques, the fabrication process is compatible to MEMS and therefore most favorable for an number of applications. Superelastic shape memory materials are of special interest in medical applications due to the large strains at constant stress and their biocompatibility. Superelastic NiTi thin films have been fabricated by magnetron sputtering using cast melted targets. Special heat treatment was performed to adjust superelastic properties and transformation temperatures. A superelastic strain of up to 6.5% at 37°C was obtained. Although NiTi shows an excellent biocompatibility enhanced antibacterial properties would significantly broaden its application range. Coatings containing Ag have already been used for this application. In order to apply this approach to TiNi-based alloys thin films of different TiNiAg compositions have been prepared by sputtering.