Fabrication of TiNi thin film stents
This dissertation introduced the fabrication and characterization of structured cylindrical TiNi thin films by magnetron sputtering, lithography and wet etching based on previous developments of shape memory alloy films. The results presented in this dissertation were never achieved before for any other research group. The TiNi thin films devices fabricated in this work have high potential for application as vascular implants, e.g. stents. The deposition of TiNi thin films on a cylindrical substrate employs a specific device which allows an in situ rotation of the substrate during the sputtering process and the use of a slit aperture to control the deposition angle. The introduction of this slit aperture resolved the film growth issue. However it reduced the deposition rate by a factor of 15. The mechanical properties of the deposited cylindrical TiNi films were tested by tensile testing. Axial and radial directions of the film were tested and both film directions showed a comparable mechanical properties with TiNi planar films. Superelastic plateau, closed loop hysteresis and a strain up to 6 % were observed. The crystallization mechanisms as a function of film thickness, annealing time and substrate were analyzed in cylindrical deposited TiNi thin film. XRD and TEM analysis have shown that 5 minutes annealing time at 650 °C first step and 450 °C second step is enough to crystallize the cylindrical TiNi thin films, which is essential for the fabrication of hybrid devices (thin film stents and conventional TiNi devices). A rotational UV lithography device based on the standard photolithography process to pattern cylindrical substrates of different diameters and lengths with a feature resolution down to 5 µm and an axial resolution (Δy) of 3 µm was developed and characterized. The developed rotational UV lithography device allows the patterning of complex geometries on the surface of a cylindrical substrate and was successfully employed to pattern cylindrical TiNi thin film. Wet etching technology was used to structure cylindrical TiNi films. The influence of etching parameters: movement of the etchant and etchant composition on the etching rate and lateral ratio was also analyzed and optimized during this research. Thus, this wet etching analysis allowed us to optimize the etchant solution thereby achieving a lateral ratio of 0.76. A deposition method was developed and patented to fabricate "thick" films with a very small mesh, based on UV lithography and wet etching technology. This method is a promising substitute for the solid state laser technology, as the aspect ratio of this method is comparable to the 10:1 ratio achieved by laser cut technology. In addition, it does not produce small burrs, as well as a heat affected zone or micro cracks. This method further avoids a possible contamination of the TiNi material with carbides.