**4.1 Sintering and Degree of Oxygen Deficiency (DOD)**

The degree of oxygen deficiency (DOD) is referred to oxygen deficient in relation to oxygen concentration of the fully oxidized powder (Schlott, et al., 1995). In (Weigert, et al., 1992) the degree of oxygen deficiency is defined as follows:

$$\text{DOD} = \text{(a-b)} \;/\text{(a-c)}\tag{15}$$

Here a is the oxygen concentration of stoiciometric oxidic compound, b is the oxygen concentration of the partially reduced oxidic compound and c is the oxygen concentration of the completely reduced metallic compound. The degree of oxygen deficiency is closely related to the oxygen vacancies contributing in parts to the electrical conductivity of ITO. Oxygen release is caused by thermolysis of the ITO powder and oxygen uptake is observed during cooling. The arrangement of oxygen vacancies is likely to be closely connected to the powder synthesis.

Thermal treatment in air leads to light-green colouring of the powder and thermal treatment in reducing atmospheres results in black colouring of the powder. Sputtering is causing a yellow colour of the target (Otsuka-Matsua-Yao, et al., 1997). After sintering the powder can also change colour to dark green or black(Udawatte, et al., 2000). The thermal history of the powder is of significant importance since the powder characteristics are closely correlated with the target quality having a profound impact on the characteristics of the sputtered thin layers (Weigert, et al., 1992).

Evidence proves that oxygen release and uptake of ITO follows a chemical hysteresis (Otsuka-Matsua-Yao, et al., 1997). In repeated heating and cooling cycles different quantities of oxygen are incorporated or released. According to the findings of experimental sintering investigations (Otsuka-Matsua-Yao, et al., 1997) the quantities of released oxygen below 773 K was marginal. A preliminar thermolysis at 1073 K was needed to release carbon monoxide absorbed on the surface. The authors came to the result, that the thermodynamic oxygen characteristics of ITO samples were changed during heating and cooling cycles. Hereby the total oxygen concentration is reduced during each cycle and the kinetics of oxygen release is slowed down cycle by cycle. The initiation temperaure of oxygen release was observed to increase on every iteration of heating and cooling cycle. It was assumed by the authors that there are several phases and several phase transformations inbetween these cycles causing

Sintering of Transparent Conductive Oxides:

partially reduction (Schlott, et al., 2001).

**4.2 Specific TCO sintering techniques** 

assisted sintering techniques (Schlott, et al., 2001).

suitable degrees of oxygen deficiency (see Figure 8).

**4.2.2 Pre-sintering** 

**4.2.1 Thermal vacuum degassing** 

10 W/mK.

From Oxide Ceramic Powders to Advanced Optoelectronic Materials 599

Improved thermal shock resistance characteristics also allow increased sputtering rates and shortened processing and operation times and consequently reduced production cost. At comparable sputtering rate increased thermal conductivity causes decreased target surface temperature and the decreased temperature gradient ensures a reduction of thermal stresses as well as increased reliability and target life time. This is especially true for target materials working as cathodes under pulsed sputtering operation conditions (Schlott, et al., 2001). In the state-of-the-art the thermal conductivity of ITO target materials is in the range of 7 –

In principle a high densification of the target materials is desirable, that means the density should exceed 95 % of the theoretical density in order to guarantee exceptable thermal conductivity characteristics. Usually the thermal conductivity could also be increased by

The theoretical melting point of indium-tin-oxide is 2223 °C. However above the temperature of 1600 °C ITO is evaporated due to a critical vapour pressure regime of the indium and tin metal phases (Vojnovich & Bratton, 1975). At a temperature of 1000 °C the

Water absorbed in the green body has to be thermally desorbed in order to avoid crack formation and specimen desintegration after sintering and subsequent cooling. This is especially true for capsuled and hot isostatically pressed samples. The gas formation during sintering has to be prevented by preheating of green bodies within controlled degassing atmospheres as for example N2 or Ar (Lippens, 1996). The thermal degassing process purifies the grain boundaries and is resulting in a reduction of specific surface area. Hereby the sintering activity is reduced and the densification has to be achieved by pressure

As referred to the previous section, thermal vacuum degassing exerts a strong influ-ence on the dgree of oxygen deficiency. Other studies (Falk, 2008) came to the result that vauum degassing and combined hot isostatic pressure sintering of capsuled ITO resulted in optimized sintering densities if the vacuum degassing time and temperature is correlated with a defect free micro structure, adopted concentration of free metal species as well as

Based on the findings of Son (Son & Kim, 1998) it was shown that pressure assisted presintering of In2O3 at 5 MPa could increase the densification rate significantly. Dilatometer experiments have proven that maximum densification rates were achieved at 1130 °C and the pressure was increased for another 5 MPa at this temperature. Sintering in air (1 atm) at

In (Son & Kim, 1998) it was found, that the agglomerated green structure was transformed in a homogeneous polycrystalline microstructure at temperatures above 1070 °C and the agglomerates showes increased densification rates compared to the surrounding matrix leading to pore formation, so called interagglomerate pores, inbetween the matrix and the

1500 °C, however, only leads to a desification of 76 % of the theoretical density.

vapour pressure of indium is 6.6⋅10-5 bar and that of tin is 6.6⋅10-7 bar (Nichols, 1982).

the discontinuous oxygen release. The related phases exhibit similar chemical composition and resemble the CaF2 structure. The phases are devided in coherent domaines and the surface energy could have a major impact on the *Gibb's free energy.* This means that chemical hysteresis appears and the oxygen concentration can not be determined explicitly as a function of the temperature and the oxygen partial pressure (Otsuka-Matsua-Yao, et al., 1997).

But there is evidence by experimental investigations that several microphases could act as barriers thereby preventing the oxygen release. These microstructures could originate from extended defects, these are sheared structures or domaines, which diverge from each other in therms of chemical compositions on a very small scale. However the exact analysis and qualitiative and quantitative identification of these structures by X-ray diffraction is not possible since they are charcterized by the structure of the rare earth elements, as for example In4SnO8, In4Sn2O10, In2O3 etc. The oxygen release could therefore depend on the interfacial energy between these microphases. In sputtering processes partially reduced indium-tin-oxide target materials are preferred in order to achieve thin film characteristics of minimum specific electric resistance and maximum sputtering efficiencies. Partially reduced targets reveal improved electrical and thermal characteristics as well as optimized densification.

The reduction of the powder or sintered body can be achieved by several methods, i.e. sintering in vacuum or reducing atmospheres (H2, CO, H2-Ar oder H2-N2). Alternatively the specimen can be hot pressed in graphite moulds or carbon releasing materials or the carbon releasing materials can be doped with the specimen (Weigert, et al., 1992). However under these reducing conditions the caculated oxygenn stoichiometry can not be accurately controlled. Target materials with reproducible and uniform degree of oxygen deficiency are preferred, since they guarantee superior sputtering efficiencies and reduced operation and coating times (Weigert, et al., 1992).

Furthermore the adding of reducing agents and ingredients causes normally the formation of metal particles of different sizes considerably larger than 50 µm (Schlott, et al., 1995). The microstructure appears spotty and targets processed by the powder qualities reveal lower fracture toughness characteristics. From this reason the target specimen shall be reduced below 1000 °C and preferably even below 800 °C. The subsequent densification of the powder and/or the sintered body is realized usually by hot isostatic pressing at temperatures above the melting point of the metal indium-tin-phases (Schlott, et al., 1995).

The hydrophilic characteristics of In2O3 should be noted. The water uptake from the surrounding atmosphere leads to the transformation of In2O3 to In(OH)3. The water absorption capacity is increasing with increased specific surface area (Lippens, 1996).

Ceramic ITO target materials show low thermal conductivities compared to the metallic target materials and are therefore very susceptible to thermal stresses resulting form nonuniform heating during the sputtering process. It is therefore preferred to increase the thermal conductivity within certain limits in order to guarantee improved thermal shock resistance characteristics (Schlott, et al., 2001).

Based on those optimized target materials the sputtering process is able to be performed at higher specific powder densities without resulting in target desintegration due to formation of thermal gradients.

Improved thermal shock resistance characteristics also allow increased sputtering rates and shortened processing and operation times and consequently reduced production cost. At comparable sputtering rate increased thermal conductivity causes decreased target surface temperature and the decreased temperature gradient ensures a reduction of thermal stresses as well as increased reliability and target life time. This is especially true for target materials working as cathodes under pulsed sputtering operation conditions (Schlott, et al., 2001). In the state-of-the-art the thermal conductivity of ITO target materials is in the range of 7 – 10 W/mK.

In principle a high densification of the target materials is desirable, that means the density should exceed 95 % of the theoretical density in order to guarantee exceptable thermal conductivity characteristics. Usually the thermal conductivity could also be increased by partially reduction (Schlott, et al., 2001).

The theoretical melting point of indium-tin-oxide is 2223 °C. However above the temperature of 1600 °C ITO is evaporated due to a critical vapour pressure regime of the indium and tin metal phases (Vojnovich & Bratton, 1975). At a temperature of 1000 °C the vapour pressure of indium is 6.6⋅10-5 bar and that of tin is 6.6⋅10-7 bar (Nichols, 1982).
