Sigurd Thienhaus

Development of multifunctional thin films using high-throughput experimentation methods

Abstract This paper describes the use of thin film high-throughput experimentation methods for the efficient development of multifunctional materials, using Ni – Ti – X and ferromagnetic shape memory alloys as examples. The thin films were fabricated in the form of binary, ternary, and quaternary materials libraries by special magnetron sputter deposition processes. These materials libraries were subsequently processed and characterized by high-throughput experimentation methods in order to relate compositional information with structural and functional properties. For this, appropriate visualization of the data is necessary. Results show that the martensitically transforming regions in ternary thin films are generally larger than was known from literature. Within these regions, the variation of the functional properties can be mapped with respect to the composition and microstructure, and thus the most suitable materials for applications can be effectively selected.

Modular high-throughput test stand for versatile screening of thin-film materials libraries

Abstract Versatile high-throughput characterization tools are required for the development of new materials using combinatorial techniques. Here, we describe a modular, high-throughput test stand for the screening of thin-film materials libraries, which can carry out automated electrical, magnetic and magnetoresistance measurements in the temperature range of −40 to 300 °C. As a proof of concept, we measured the temperature-dependent resistance of Fe–Pd–Mn ferromagnetic shape-memory alloy materials libraries, revealing reversible martensitic transformations and the associated transformation temperatures. Magneto-optical screening measurements of a materials library identify ferromagnetic samples, whereas resistivity maps support the discovery of new phases. A distance sensor in the same setup allows stress measurements in materials libraries deposited on cantilever arrays. A combination of these methods offers a fast and reliable high-throughput characterization technology for searching for new materials. Using this approach, a composition region has been identified in the Fe–Pd–Mn system that combines ferromagnetism and martensitic transformation.

Rapid Identification of Areas of Interest in Thin Film Materials Libraries by Combining Electrical, Optical, X-ray Diffraction, and Mechanical High-Throughput Measurements: A Case Study for the System Ni–Al

The efficient identification of compositional areas of interest in thin film materials systems fabricated by combinatorial deposition methods is essential in combinatorial materials science. We use a combination of compositional screening by EDX together with high-throughput measurements of electrical and optical properties of thin film libraries to determine efficiently the areas of interest in a materials system. Areas of interest are compositions which show distinctive properties. The crystallinity of the thus determined areas is identified by X-ray diffraction. Additionally, by using automated nanoindentation across the materials library, mechanical data of the thin films can be obtained which complements the identification of areas of interest. The feasibility of this approach is demonstrated by using a Ni-Al thin film library as a reference system. The obtained results promise that this approach can be used for the case of ternary and higher order systems.

High-throughput characterization of film thickness in thin film materials libraries by digital holographic microscopy

Abstract A high-throughput characterization technique based on digital holography for mapping film thickness in thin-film materials libraries was developed. Digital holographic microscopy is used for fully automatic measurements of the thickness of patterned films with nanometer resolution. The method has several significant advantages over conventional stylus profilometry: it is contactless and fast, substrate bending is compensated, and the experimental setup is simple. Patterned films prepared by different combinatorial thin-film approaches were characterized to investigate and demonstrate this method. The results show that this technique is valuable for the quick, reliable and high-throughput determination of the film thickness distribution in combinatorial materials research. Importantly, it can also be applied to thin films that have been structured by shadow masking.

Small-Scale Deposition of Thin Films and Nanoparticles by Microevaporation Sources

This paper reports on a novel miniaturized deposition technique based on micro-hotplates which are used as microevaporation sources (MES) for a localized deposition of thin films and nanoparticles. The feasibility of this small-scale deposition technique and its general properties are shown for depositions of Ag on unpatterned and microstructured substrates. The deposited films are rotationally symmetric and show a distinct lateral thickness change. We take advantage of this latter effect, as, e.g., all stages of film condensation can be observed within one experiment on one sample, in a size suitable for transmission electron microscopy investigations. For realizing the most laterally confined depositions, a micro-Knudsen cell was used. It is shown that the use of MES is also very suitable for the fabrication and deposition of nanoparticles.

Micro- to Nanostructured Devices for the Characterization of Scaling Effects in Shape-Memory Thin Films

Two microfabricated devices designed as test platforms for the investigation of scaling effects in micro- to nanosized substrate-attached shape-memory alloy (SMA) thin films as well as freestanding (suspended) thin-film microbridges are presented. These micromachined test platforms allow for simultaneous nanomechanical, electrical, and thermal tests on thin-film microbridges and can be seen as a basis for nanoscale SMA thin-film applications. The functionality of these devices is demonstrated for Ti52Ni32Cu16 thin films as active material. The martensitic phase-transition temperatures for the thin films as substrate-attached or suspended microstructures as well as the dependence on the lateral dimensions were examined. It was found that decreasing the bridge width from 4 to 1 μm leads to a substantial and asymmetrical decrease of the phase-transition temperatures: 20% [austenite finish temperature (Af) and martensite start temperature (Ms)] and 80% [austenite start temperature (As) and martensite finish temperature (Mf)]. Furthermore, it was found that detaching the thin film from its substrate also leads to a decrease of the transition temperatures. Finally, it is shown that shape-memory thin-film nanowires can be fabricated and characterized using the proposed devices.

Preparation of 24 ternary thin film materials libraries on a single substrate in one experiment for irreversible high-throughput studies.

For different areas of combinatorial materials science, it is desirable to have multiple materials libraries: especially for irreversible high-throughput studies, like, for example, corrosion resistance testing in different media or annealing of complete materials libraries at different temperatures. Therefore a new combinatorial sputter-deposition process was developed which yields 24 materials libraries in one experiment on a single substrate. It is discussed with the example of 24 Ti-Ni-Ag materials libraries. They are divided based on the composition coverage and orientation of composition gradient into two sets of 12 nearly identical materials libraries. Each materials library covers at least 30-40% of the complete ternary composition range. An acid etch test in buffered-HF solution was performed, illustrating the feasibility of our approach for destructive materials characterization. The results revealed that within the composition range of Ni < 30 at.%, the films were severely etched. The composition range which shows reversible martensitic transformations was confirmed to be outside this region. The high output of the present method makes it attractive for combinatorial studies requiring multiple materials libraries.

High-Throughput Characterization of Shape Memory Thin Films using Automated Temperature-Dependent Resistance Measurements

Shape memory alloy (SMA) thin films are used as actuator materials in MEMS due to their unique properties. Binary thin films with a composition close to Ni 50 Ti 50 are well-established materials, whereas ternaries like NiTiCu, NiTiPd, NiTiHf are less studied. Furthermore, new alloys are being developed which show a magnetic shape memory effect, e.g. Ni 2 MnGa. For the optimization of known, and the development of new, SMA thin films, a fast and reliable characterization technology is needed, which rapidly identifies the transformation temperatures (i.e. martensite and austenite start and finish temperatures) for a range of material compositions deposited on a whole wafer. In this paper, automated temperature-dependent resistance measurements are discussed as a means which yields the thermal hysteresis of the investigated thin films. Results of monitoring the uniformity of shape memory film depositions on the wafer level, as well as results on the use of this method as a tool for screening for new SMA films by characterization of materials libraries are reported.

Microgradient-heaters as tools for high-throughput experimentation.

A microgradient-heater (MGH) was developed, and its feasibility as a tool for high-throughput materials science experimentation was tested. The MGH is derived from microhot plate (MHP) systems and allows combinatorial thermal processing on the micronano scale. The temperature gradient is adjustable by the substrate material. For an Au-coated MGH membrane a temperature drop from 605 to 100 °C was measured over a distance of 965 μm, resulting in an average temperature change of 0.52 K/μm. As a proof of principle, we demonstrate the feasibility of MGHs on the example of a chemical vapor deposition (CVD) process. The achieved results show discontinuous changes in surface morphology within a continuous TiO2 film. Furthermore the MGH can be used to get insights into the energetic relations of film growth processes, giving it the potential for microcalorimetry measurements.

Ab initio calculations of the ferromagnetic shape memory alloy Ni–Mn–Al

Results of the first-principles calculations of the magnetic shape-memory alloy NiMnAl are presented. The properties of a series of alloys in the composition range Ni50Mn x Al50− x with 0 ≤ x ≤ 50 were deduced from the ab initio simulations. One essential result is that the alloy is ferromagnetic in the range from 14 to 31 at.% Mn. Furthermore, the martensitic phases 2 M, 10 M, and 14 M with long-periodic structure were calculated. They are metastable in the stoichiometric Ni2MnAl alloy due to additional bonding between specific atomic sites. Their properties are discussed in terms of the density of states and their charge distribution.