2009 Volume 73 Issue 12 Pages 943-947
In general, when thin films of copper (Cu), as a wiring material, and platinum (Pt) are formed on a ceramic material, a thin metal film, such as titanium (Ti), is used as the basal layer. Thus, effective strategy to lower the resistance of the wiring material is to increase the size of crystal grains. However, after annealing, the surface roughness of the wiring increases, and the adhesive strength deteriorates, which has the undesired result that the wiring becomes easily exfoliated. There are many reports in which these effects are attributed to the recrystallization of Pt and the diffusion of Ti. However, there are few reports in which observations of this phenomenon in real time are described. In the present study, we observed the cross-sectional structure of a Pt/Ti-containing thin film and performed dynamic high-temperature in-situ field emission electron gun type-scanning electron microscope (FE-SEM) analyses. A mechanism to explain the above effects is proposed.
The Pt/Ti thin film was formed by a magnetron sputtering technique onto silicon oxide (SiO2) substrate. Ti and Pt thin films were generated continuously in a single chamber. The resulting film thicknesses were 50 nm and 500 nm for Ti and Pt, respectively. The cross-sectional structure of each thin film was studied with a dual-beam focused ion beam (FIB)-SEM and ion milling, while high-angle annular dark-field (HAADF) imaging was carried out with a field emission electron gun type-transmission electron microscope (FE-TEM), and the cross-sectional microstructure was evaluated by electron backscatter diffraction (EBSD). In addition, high-temperature dynamic in-situ FE-SEM (Quanta-FEG) was used to observe the surface of the Pt/Ti thin film. After the temperature was increased from room temperature to 1000°C, the temperature of the observed sample was maintained at 1000°C for 30 min. During this time period, we performed reflection electronic imaging (environmental scanning electron microscope mode (ESEM mode)) to obtain dynamic images with a 25 μm×25 μm area.
The real-time measurement using dynamic high-temperature in-situ FE-SEM indicates that diffusion of Ti occurs along grain boundaries in the Pt thin film. We observed this event as the recrystallization of Pt, which precipitated onto the surface of the Pt thin film. Regarding the action of Ti, when heat treatment was applied to the surface of the Pt/Ti thin film in an oxygen atmosphere, the following events were observed: the Ti thin film under the Pt thin film formed spherical nanoparticles and diffused through the grain boundaries of the Pt thin film. Thereafter, Ti cohered on the surface of the Pt thin film. However, when Pt recrystallized and the size of the crystal grains increased, the grains did not diffuse in the horizontal direction of the film, but began to cover the top of the Pt thin film. The model proposed by Hanzawa et al., in which the adhesive strength of a Pt/Ti thin film on a ceramic substrate deteriorates as the surface roughness of the Pt thin film increases, was proved to be valid through this study.
