Pressure: The pressure of the sputtering gas is very important, not only does it provide the inert gas ions needed for the sputtering process of the target, but it also acts as a moderator for the ejected

atoms from the target. By careful control of the pressure, one can influence both the structural and magnetic properties of the film. At low pressures, the sputtered and reflected neutral atoms have much higher energies than the plasma gas between the target and substrate and arrive at the substrate with super-thermal energies. The sputtered atoms in this situation have a high surface mobility at the substrate. At higher sputtering pressures, the sputtered and reflected neutral atoms, are thermalised [Somekh (1984)] by the plasma gas, due to the increased number of collisions before arriving at the substrate. As a consequence, the sputtered atoms will have a lower surface mobility at the substrate. This will have a significant effect in the growth kinetics of the depositing film and is reflected in the magnetic and structural properties. It is therefore essential that the sputtering pressure is carefully investigated to obtain the optimum magnetic and structural properties.

There have been numerous studies exploring the effects of pressure on sputter deposited films, and it is well established that the pressure has a significant effect on the magnetic properties, mainly through stress [Ref. List (4.2)] and columnar growth [Shimada et al (1981), Leamy et al (1979)]. The as- deposited films tend to suffer from residual stresses if the effects of pressure have not been accounted

2 4 6 8 10 12 14 16 18 20 22 -40 -30 -20 -10 0 10 20 30 40 50

Compressive

Tensile

Figure 4.1: Stress dependence as function of pressure for Co-based films obtained by RF sputter deposition. [Data obtained from Materne et al (1988)].

for. This generally changes from being a compressive to a tensile stress as the sputtering pressure is increased. An example of such an effect is shown in Figure 4.1, where an investigation of stress in sputter deposited low magnetostrictive Co-based films was performed by Materne et al (1988) as a function of pressure. At low argon pressures, the films were found to be under compressive stress, whilst at higher pressures the stress was found to be tensile. A transition region at approximately 9mTorr existed where the stress changes sign, and the as-deposited films were found to be in a ‘stress free’ state; this also corresponded to the lowest values of the coercive field (15-30 A/m) as a function of pressure. It is the general consensus that at low pressures, the high surface mobility of the sputtered atoms promotes the formation of dense films which are under compressive stress. The compressive stress is inferred to be due to argon entrapment and lattice distortion caused by energetic incident particles. At higher pressures, the sputtered atoms are less energetic because of the increased scattering. This lowers the surface mobility of the sputtered atoms; it also causes the sputtered atoms to arrive at the substrate at more oblique angles, because of the increased scattering. This can result in the films having a columnar morphology, which induces a tensile stress in the film. The columnar growth is a result of the self-shadowing of the incident atoms by those already incorporated into the growing film [Leamy et al (1979)]. This is obviously controlled by the surface mobility of the sputtered atoms, which in turn is related to the incident energy of the sputtered atoms. As the pressure is further increased to even higher pressures, there is a reduction in tensile stress (Fig. 4.1) in the films, because of the more distinct columnar structure. The more open, columnar morphology is said to prevent the ease with which the stress can be transmitted between the columns [Hudson et al (1996)]. It has also been suggested that the tensile stress at higher pressures may be due to oxygen contamination, since there seems to be a strong correlation between oxygen incorporation and columnar growth morphology developed at high pressures [Leamy et al (1978,1979), Materne et al (1988)]. However, the effect of oxygen on stress is not clearly understood, since there have also been reports that at high pressures, oxygen incorporation leads to compressive stresses [Hoffman (1976), Hudson et al (1996)]. Similar trends due to pressure also occur on other magnetic systems such as FeCoB [Shimada et al (1981)], and FeSiB [Naoe et al (1979)] thin films which have been sputtered deposited. It should be noted that intrinsic stress from sputtering is not only confined to magnetic films and can occur in any sputter

Figure 4.2: FeCoB films obtained by RF sputter deposition. Scanning electron microscopy of fractured edges of films deposited at (a) 3mTorr, (b) 50mTorr, and (c) 150mTorr indicating the development of columnar morphology at high pressures. [Data obtained from Shimada et al (1981)].

deposited systems, for example copper [Craig et al (1981)] or tungsten [Vink et al (1993)] films. The morphology of thin films has been summarised by Thornton et al (1974) and Craig et al (1981), where they describe various structure zones, depending on the sputtering pressure and substrate temperature. They infer that the stress and structure of the film is related to the energy of the bombarding particles at the substrate during the sputter deposition. Dense non-columnar films are obtained when the impinging particles (sputtered and reflected neutrals) have high kinetic energies (low pressures), and a distinct columnar texture is developed at low particle energies (high pressures). This is shown in Figure 4.2, where FeCoB thin films were sputter deposited by Shimada et al (1981) at various argon pressures to investigate the magnetic and structural properties. It was found that the columnar texture only appeared at high sputtering pressures (>50mTorr), and the films deposited at low argon pressures (<10mTorr) showed no characteristic structure. These results are in agreement with other studies [Naoe et al (1979), Materne et al (1988)] which also have found that films deposited at low pressures do not have any characteristic columnar morphology. It is well accepted that to obtain good magnetic amorphous films with a high saturation magnetisation and a low coercive field, the sputtering conditions need to be carefully chosen to suppress the formation of columnar growth, which is found to be unfavourable for the soft magnetic properties.

In most sputtering systems, the sputtering gas, usually argon, is allowed to flow continuously through the chamber at a constant pressure; this helps with the removal of any out-gassing impurities which could otherwise build up within the chamber and be incorporated into the depositing film. It is important that the gas flow-rate is not to excessive, since this can lead to the existence of pressure gradients within the chamber and cause non-uniform sputtering of the target, which can be reflected in a non-uniform film thickness.

4.2.2 Target-substrate separation: The separation between the target-substrate has a similar effect,