Magnetron Sputtering of ZnO

As outlined in Sect. 5.2, many attempts have been made to deposit ZnO-based films by sputtering. The compilation of papers in Table 5.1 may serve as a reference to identify useful papers. The classification criteria are plasma excitation used (DC/MF/RF) reactive or ceramic deposition material deposited and film properties relevant for TCO applications. 

The non-continuous change of reactive gas partial pressure is the primary problem in reactive sputtering of ZnO-based TCO s. The hysteresis characteristics of the reactive process can be modeled within the framework of the Berg model, where the condensation of reactive gas atoms on the surfaces 

This simple approach allows the balance equation for the reactive gas partial pressure and deposition rate to be formulated. An example is given in Fig. 5.7 for the stability analysis of process control loops. 

The second problem is the decrease of the deposition rate in the oxide mode, which is most significant for highly reactive oxides such as MgO and TiC 2, where the deposition rate changes by more than one order of magnitude [72,73]. For ZnO on the other hand, the deposition rate decreases only by a factor of 2, as shown in Sect. 5.3.4.3. 

The third problem is process stability at high power levels during longterm operation In DC operation, the oxidized area of the target is subject to continuous ion bombardment. The oxidized surface exhibits some conductivity, which allows for charge neutralization to a certain extent. When the power is increased further, the dielectrics charge up until the electrical field 

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The following sections review the work on magnetron sputtering of ZnO films focusing on TCO properties. Section 5.2 outlines the history of ZnO... 

Table 5.1. Literature survey on magnetron sputtering of ZnO-based TCO films...

Table 5.2. Process parameters for reactive MF magnetron sputtering of ZnO Al...

Fig. 5.10. Influence of oxygen flow g(C>2) on target voltage U (a) and on deposition rate a and resistivity p, respectively, (b) for reactive MF magnetron sputtering of ZnO Al at Ts = 200°C. Segmented targets were used (Zn 0.9-1.5 wt% Al). Results shown are for Zn 1.5wt% Al (reprinted from [51])...

This section is on the reactive MF magnetron sputtering of ZnO films in the transition mode. The first part is on the deposition of dielectric ZnO films. The reactive sputtering of ZnO Al is discussed in subsequent sections. 

Fig. 5.13. (a) Process characteristic for reactive MF magnetron sputtering of ZnO showing the dependence of reactive gas partial pressure p(O2) on discharge power P and process set points chosen for deposition, (b) dependence of growth rate a on reactive gas partial pressure p(O2) for different substrate temperatures (reprinted from [90])... 

The transition mode process described here differs fundamentally from conventional reactive sputtering processes for ZnO coatings which do not permit optimization of the film characteristics since they are hysteresis-based. An example of this is the work of Jacobson et al. [96] on reactive DC-Magnetron sputtering of ZnO coatings. 

The transition mode process control described above is the key to reactive magnetron sputtering of ZnO Al films. Several approaches have proven to be useful, either adjusting the reactive gas flow or the discharge power as a function of appropriate process variables. 

Fig. 5.24. Sheet resistance and film thickness distribution for static deposition DC magnetron sputtering of ZnO Al on glass from sintered ZnO A I2O3 targets using a new or an eroded sputtering target (reprinted from [117])...

Fig. 5.28. Operating principal of the symmetry control for reactive MF magnetron sputtering of ZnO Al. Three A-sensors are used to monitor oxygen partial pressure at different positions along the target...

At first we have investigated the stmcture of ZnO layers grown by dc-magnetron sputtering of ZnO Ga ceramic target, as a function of the distance between the target and the substrate. 

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