**1. Introduction**

586 Sintering of Ceramics – New Emerging Techniques

[7] Kopchenova, Е.V., Karyukina, V.N. Estimation of the рН of mineral suspension and the

mineralogical study. М.: Nedra, 1969. P.148-155.

implication of this index in mineralogical studies // Modern methods for

Since Cadmium thin films have been sputtered and thermally oxidized for the first time in 1907 (Chopra, et al., 1983) the technological innovation in transparent conductive oxides (TCO) has developed rapidly and substantially during the last years. Indium-tin-oxide (In:90:Sn:10, ITO) is today the most important TCO material due to the potential combinations of high electrical conductivity in the range of 104 S/cm and high transparency in the visible range of 90 % with a layer thickness of 100 nm.

Today TCO thin films are processed by chemical or physical vapour deposition, especially vacuum based sputtering and evaporation processes. Due to these elaborate and complex manufacturing, especially for large-size TCO applications direct structuring of TCO layers by printing, sol-gel coating and other powder- and paste-based manufacturing processes have been the subject of many investigations (Hyatt, 1989, Straue, et al., 2009).

These as-processed TCO materials serve as transparent electrodes in liquid crystal displays, thin-film electroluminescence displays, electrochromic displays, transparent conductive coatings of highly sensitive radiation detectors, ferroelectric photoconductors and memory devices, transparent conductive oxidic films as gate-electrodes for injection and charge coupled devices and are used in products as flat panel displays, touchscreens, organic light emitting diodes, electroluminescence lamps as well as numerous components of solar technology.

Resulting from the increased indium consumption and the expected supply gap of indium raw materials within the next decades (Carlin, 2007) an increasing priority is attributed to the development of suitable substitute materials, whereas currently aluminium doped zinc oxide (ZnO:Al, AZO) have consistently attached the highest importance (Ellmer, et al., 2008). Beside the TCO applications mentioned above AZO is used in highly promising developments such as blue and ultraviolet lasers, components of improved light amplification of GaN-based LEDs, transparent thin film transistors, photo detectors, varistors, catalysts and optical current transformers.

Sintering of Transparent Conductive Oxides:

polished ITO surfaces (c).

(Kukla, et al., 1998).

thin film qualities (Schlott, et al., 1996).

From Oxide Ceramic Powders to Advanced Optoelectronic Materials 589

Nodules have to be avoided, since they modify the sputtering process and it is therefore necessary to interrupt the process and to clean or even to exchange the target material. Nodule formation is a severe problem since nodules show a reduced sputtering voltage compared to the surrounding material. Thereby the nodules prevent the sputtering of the material being covered by the nodule layers. Furthermore when the nodule formation is avoided, arc discharge is not needed and the sputtering process is excecuted at increased sputtering voltages enabling to operate the facility at higher efficiencies (Nadaud, et al., 1995). The nodule formation was observed for metallic indium-tin targets during the reactive sputtering process (Schlott, et al., 1996) and also for sputtering of oxide ceramic ITO targets (Schlott, et al., 1995). The elimination of inclusions and metallic phases is

predominant for the effective avaoidance of nodule formation (see Figure 2).

Fig. 2. Cold isostatically pressed ITO body (a, left) and sintered ITO body with a shrinkage of 15 % (a, right), SEM topography of fractured ITO surface after sintering with a mean grain size of 25 µm (b) and outbreak (dark) and segregations (bright) analysed by SEM at

A trouble-free microstructure is important to prevent the chipping (Schlott, et al., 1996). Impurties and metallic InSn eutectica have to be avoided in order to guarantee sufficient

Once being formed the nodules grow contineously during the sputtering process. They do not dissolve unless they explode due to thermal stresses or due to the power of micro arc discharge effects (Schlott, et al., 1996). If that is done the desintegrated nodule particles are scattered and form nucleus of new nodules or they form small holes, so called pinholes, in the growing layer contributing to a significant quality degradation of the sputtered thin film

Nodule formation is observed particularly frequently when the target material consists of several tiles being composed to enlarge the target surface. The split between the tiles are considered as collecting sites for impurites and dust. Furthermore the split could be coated

Sputtered Al:ZnO thin films have already been used in commercially available flat panel displays und thin layer solar cells with an electrical resistance of 1-3 . 10-4 Ω cm at aluminium doping concentrations between 1.6 to 3.2 at.-% (Anders, et al., 2010).

Among others current research is focussing on sputtering process stability based on ITO target materials, on AZO target materials with improved electrical conductivity, thermal and mechanical stability as well as highest transparency of TCO thin layers by enhanced optimization of chemical composition, microstructure tailoring and advanced sintering and densification methodologies of TCO target materials.
