Capability of Sputtered Micro-patterned NiTi Thick Films
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Today, most NiTi devices are manufactured by a combination of conventional metal fabrication steps, e.g., melting, extrusion, cold working, etc., and are subsequently structured by high accuracy laser cutting. This combination has been proven to be very successful; however, there are several limitations to this fabrication route, e.g., in respect to the fabrication of more complex device designs, device miniaturization or the combination of different materials for the integration of further functionality. These issues have to be addressed in order to develop new devices and applications. The fabrication of micro-patterned films using magnetron sputtering, UV lithography, and wet etching has great potential to overcome limitations of conventional device manufacturing. Due to its fabrication characteristics, this method allows the production of devices with complex designs, high structural accuracy, smooth edge profile, at layer thicknesses up to 75 µm. The aim of this study is to present recent developments in the field of NiTi thin film technology, its advantages and limitations, as well as new possible applications in the medical and in non-medical fields. These developments include among others NiTi scaffold structures covered with NiTi membranes for their potential use as filters, heart valve components or aneurysm treatments, as well as micro-actuators for consumable electronics or automotive applications.
KeywordsNiTi Shape memory Magnetron sputtering Lithography Medical applications Integration of functionality
NiTi is a well-established alloy in the medical industry. It is used in orthodontic applications, for a variety of medical instruments or for temporary or permanent vascular implants [1, 2, 3, 4]. Its superelastic properties have numerous advantages, among them constant force delivery, kink resistance, and different deployment methods . The latter allow for the use of smaller catheter diameters to deliver implants and instruments, which is in particular important for minimal invasive treatments. The increasing popularity of vascular disease treatments utilizing stents in the 1990s and the technological ability to fabricate and structure high-quality NiTi tubes was a main trigger for the rise of NiTi in medical applications . Stent markets have been growing since, and NiTi is still one of the main alloys in this field.
NiTi films that are deposited at room temperature are amorphous and require a subsequent heat treatment for crystallization . Hence, the microstructure of sputter-deposited NiTi differs to that of bulk materials which crystallize from the melt and often experience a high degree of cold work to reduce the dimension of the semi-finished product. Microstructural investigations of sputtered films have been carried out for a large variety of binary to quaternary compositions and heat treatments, e.g., [16, 17, 18, 19, 20, 21, 22], which demonstrates the large influence of the two parameters on microstructural features (grain size, distribution, type, and size of precipitates), transformation temperatures and consequently their functional properties. Using sputtering techniques, alloy optimization by varying the chemical composition can be achieved in numerous ways, ranging from different target compositions, changing sputtering power or target-substrate distances to complex combinatorial methods where a large area of the composition spread can be investigated simultaneously by high-throughput methods [22, 23, 24].
Standard UV lithography allows for structuring photo-sensitive resist with feature sizes in the submicron range . The pattern of the structured resist is then transferred to the NiTi film, e.g., using wet chemical etching. NiTi films with thicknesses of 75 µm have been deposited following the process described in . Since sputtered edges are not perfectly vertical, the minimum feature size increases with increasing film thickness, i.e., ~1 µm for very thin NiTi films, and ~10 µm for film with 75 µm thickness. Thus, aspect ratios of up to 7–8 can be achieved. This is in particular interesting for the fabrication of thin mesh structures where maximum pore or strut size can be in the range of a few tens of micrometers only.
The NiTi layer can be released from the substrate using a sacrificial layer technique. When flat substrates are used, the released, amorphous film is obviously also flat. The cylindrical shape of vascular implants can be achieved by a constraint heat treatment, during which the amorphous film crystallizes within a quartz tube of the desired diameter. For many applications, an open design (beginning and end of circumference are not joint) may be feasible, for others the joint circumference may be required. Pulsatile fatigue investigations on stent structures with laser-welded joints of the circumference showed that the fatigue resistance of the micro-welded joints is sufficiently high, and fractures were rather observed at stent struts than the welding tags . Also, the overall functional behavior of NiTi sheet metal was found to be mainly unaffected by welding, since no significant change in the stress–strain transformation plateau was detected . The necessity of jointing can be avoided when depositing NiTi directly on cylindrical substrates, e.g., using a hollow cathode setup  or a rotating substrate approach . The latter is economically less interesting due to the significant decrease in the effective sputtering rate.
Except for laser-welded joints, structured, sputtered NiTi films exhibit no heat-affected zones (HAZ), in contrast to laser-cut tubes. The microstructure is hence less affected which in combination with the lack of oxide and carbide inclusions promises higher fatigue life and improved mechanical properties. Investigations on the fatigue life of sputtered diamond-shaped samples reveal indeed a high fatigue endurance limit. Up to mean strains of 6 %, maximum alternating strains of ±1.5 % did not lead to a fracture of the specimen during 3 × 10E6 cycles, a significantly higher value compared to ±0.5 % maximum alternating strain for NiTi standard material .
Limitations of the NiTi sputtering technology are the limited film thickness and lateral dimension (the latter determined by the wafer size used, e.g., 6″, 8″, etc.,), the necessity to utilize the usable area effectively in order to be cost efficient (good if many parts fit into the wafer area), and the lack of cold work as parameter to influence microstructure and thus functional properties.
The unique features of NiTi sputtering, however, have high potential to realize some of the next generation medical implants or tools, where the current technology cannot provide, e.g., the freedom of design or the ability to combine and structure a series of additional materials. The capability of NiTi sputtering is discussed in the following chapter.
Results and Discussion
We have divided this chapter according to what we have named NiTi sputter technology platforms, namely 2D, 2.5-dimensional (2.5D), and multi-layered structures. For each platform, the fabrication process is adjusted in order to realize different features of a NiTi-based sample.
Figure 2a shows an 8-µm-thick mesh structure with 8 µm strut width and 75 × 20 µm2 diamond-shaped pore size. Strut width and pore size can be optimized according to application requirements, e.g., when used as embolic filters or as surfaces to engineer tissue growth. Figure 2c, d shows a multitude of parallel, fine lamellae connected to a supporting beam; Fig. 2e, f a connector structure; and Fig. 2g, h generic, shape set stent structures of different diameter.
The application potential of the 2.5D platform in the medical field is manifold: micro-patterned surfaces for drug eluting systems, tissue engineering or friction control, robust filters with scaffolds, artificial heart valves, or flow diverters for aneurysm treatments can be realized.
The NiTi structure can be completely or partly coated. In the former case, the thickness of the radiopaque material has to be sufficiently high to increase radiopacity but small enough not to hinder the stress-induced martensitic transformation of the underlying NiTi. Also, delamination of the radiopaque layer must be avoided during mechanical loading. In the second case, the radiopaque layer can be patterned so that heavily loaded areas are uncoated and thus free to transform, whereas less loaded areas are coated (Fig. 6c).
A novel field is the combination of flexible NiTi substrates with structured isolating and conducting layers. Circuit paths can be deposited on NiTi with a top and bottom isolating layer. The top isolation might be absent in certain areas so that electrodes for stimulation, sensing, or mapping can be realized.
The lack of oxide and carbide inclusions in sputtered films promises good fatigue life also for actuator applications.
Often Joule heating is used as a simple heating mechanism to increase the sample temperature above the austenite finish temperature. Using the combination with isolating and conducting layers, local heating elements can be created on the NiTi surface and the NiTi actuator structure can be activated in selected areas only, if required.
Summary and Conclusion
The present paper investigates the capability of magnetron sputtering, UV lithography, and wet chemical etching to fabricate different NiTi-based components.
The characteristic features of sputtered micro-patterned NiTi films (feature resolution, maximum thickness, thickness and compositional homogeneity, certain microstructural aspects, fatigue performance) have been discussed briefly. The main focus of this paper, however, is not on the functional properties or the microstructure of sputtered films, but the freedom of design that can be realized with the combination of the above-named microsystem technology processes. In particular, 2.5D structures offer in our view novel design opportunities in the medical field for drug eluting systems, filters, valves, tissue engineering, or flow diverters. Prototypes of 2D and 2.5D structures can be fabricated with only few adjustments (different lithography masks) required. The integration of additional functionality using radiopaque, electrically conducting or insulating, or magnetic layers is a promising approach to design and fabricate next generation medical devices.
The underlying process to fabricate NiTi films presented in this paper is a combination of three microsystem technology processes: UV lithography, physical vapor deposition, and wet etching as described in . In order to obtain more complex geometries such as 2.5D structures or structures that are combined with other materials, some of the above process steps are repeated or slightly altered. The NiTi films are deposited using magnetron sputtering devices at base pressures below 1 × 10−7 mbar, Ar gas flow, and deposition rates between 3.8 and 5.6 nm/s, depending on sputtering system.
Subsequently, the amorphous samples were crystallized in a high vacuum chamber in order to avoid oxidation during the annealing process. Heat treatment was carried out ex situ by means of a rapid thermal annealing system. The halogen-lamp driven heating chamber enables typical heating rates of 50 K/s in a vacuum environment of about 10−6–10−7 mbar. The annealing temperature was held constant for 10 min at the maximum temperature of 650 °C, which turned out to be sufficient for crystallization of amorphous NiTi. In a second step, films were annealed for further 10 min at 450 °C, which is a common procedure for Ni-rich Nitinol samples to induce the formation of Ni4Ti3 precipitates in order to adjust phase transformation temperatures.
We thank Dr. C. Zamponi for EDX measurements and Dr. Jens Trentmann at the department of Radiology and Neuroradiology at the UKSH Kiel for radiopacity investigations.
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