Real-time evolution of the indium tin oxide film properties and structure during annealing in vacuum
Applied Physics Letters
Indium tin oxide (ITO) is widely applied as a transparent conductive oxide coating. A standard and successful industrial route of production is its deposition by magnetron sputtering from a compound (oxide) target  . To increase cost efficiency, it would be preferable to sputter reactively from a metal target at sufficiently high partial pressure of oxygen. However, under this condition, a satisfactorily low resistivity of the films cannot readily be obtained,  so that a deposition on
... a deposition on heated substrates or post-deposition annealing is necessary. So far, the annealing processes for reactively sputtered ITO [3, 4] have only been studied for metal-rich films, in contrast to comprehensive studies after magnetron sputtering from ceramic targets       . Moreover, mainly isothermal heat treatment is considered in the literature, although annealing using a temperature ramp is of more relevance for practical application. Several investigations report on real-time in situ monitoring of the ITO film resistivity and reflectivity [9,10], which is used for an indirect characterization of the crystalline structure of the films. This approach requires simplifying assumptions on the linear dependence of the resistivity or reflectivity on the crystalline fraction, and the stability of the film roughness during annealing. Direct investigations of the influence of heat treatment on the ITO film structure are so far limited to post-annealing studies by X-ray diffraction (XRD) and scanning or transmission electron microscopy    7, 8, 11] . Annealing of ITO is known to be very efficient in increasing the carrier concentration. It can be quite reasonably explained by the Frank-Köstlin model  which accounts for tin donor activation at elevated temperatures. However, this model is valid only for crystalline ITO. The amorphous-to-crystalline transition in ITO during annealing is often assumed as the reason for this activation, but the physics behind the experimental observation is not clear. In the present letter we report the results of a real-time in situ investigation of the film properties and the structure evolution during annealing in vacuum. The films are produced by reactive pulsed middle frequency magnetron sputtering using the facility and procedure described in Ref. 13. The chamber was pumped to a base pressure of 4 × 10 -4 Pa before deposition. The deposition runs were carried out for 2.5 min at an Ar flow of 30 sccm (partial pressure 1.2 Pa) and O 2 flow of 64 sccm (0.3 Pa). The films are grown on Si (100) substrates (24 × 12 × 0.3 mm 3 ) covered with SiO 2 (510 nm), which were not heated externally during deposition. The average thickness of the deposited films is 130 nm. The as-deposited films show no crystalline peaks in the XRD patterns and are considered as amorphous. The post-deposition annealing of identical ITO samples was carried out at two different experimental setups, the ROssendorf Beam Line (ROBL) at the European Synchrotron Radiation Facility in Grenoble,  and the ITO deposition facility at Forschungszentrum Rossendorf  . In both cases the total pressure of the residual gas was below 6 × 10 -4 Pa. During annealing the sample was placed on a boralectric heater (Tectra, Germany). The sample temperature was controlled by a standard K-type thermocouple. In contrast to previous investigations [3-10] on isothermal annealing, in present work the annealing temperature T a was gradually increased from 20 to 330°C at a constant rate of approximately 5 K/min. In both experimental setups, the resistivity of the ITO films was monitored in situ by the four point probe technique. In order to study the evolution of the ITO film structure in real time, a UHV annealing chamber, equipped with an X-ray transparent beryllium dome, was mounted on a six-circle goniometer. The X-ray diffraction experiment was performed in Bragg-Brentano geometry within the range of scattering angles of 27-37°. The incident X-ray beam was monochromatized to 8.048 keV (λ = 0.154 nm). A multi-channel position sensitive detector allowed a fast scan acquisition time of 100 s. The XRD data were evaluated by the PeakFit software (Jandel Scientific).