Intrinsic compressive film stress

Thin films can exhibit intrinsic compressive or tensile stress. A thin film under compressive stress tries to expand a thin film under tensile stress tries to contract. Once the optimum values for stress and stress gradients are found, it is crucial to maintain a stable process. Several possible techniques for measurement and in-process control of stress and stress gradients are discussed below. 

In addition, based on their ion impact approach, Mirkarimi et al. [17] established a quantitative model of defect generation during ion-assisted film growth. They found that the maximum height of the defect production distribution exhibits the same (mion ion) dependence as cBN formation. Moreover, as the authors emphasize, this is the very same dependence as that found by Windischmann [51] for the formation of intrinsic compressive stress in thin films deposited under ion bombardment. 

The stress of oxidized PS layers is always compressive. For porous oxides, values below 108 N nT2 are reported [Ba5], which is nearly one order of magnitude smaller than values of intrinsic stress generated by low-temperature thermal oxidation of bulk silicon. The compressive stress in OPS has successfully been used to lift up released mesoporous films and thereby fabricate 3D microstructures [La9],... 

To all these intrinsic reasons, one would have to add the expected modifications in the electronic structure of the growing film as it thickens, due to the decreasing influence of the substrate. This can be better judged for a system that is not pseudomorphic, such as Ag/Cu(lll). The large (12%) mismatch between Ag and Cu would provoke such a tremendous compressive stress for a pseudomorphic layer that the Ag layers keep their own lattice parameter from the first monolayer on. For 1 ML of Ag/Cu(lll), the surface state has been found to be 120 meV lower in energy than for bulk Ag(lll) [79], and shifts with increasing Ag coverage to the bulk value. 

The internal stress in the a-Si H films is a further manifestation that the silicon network is rigid and overcoordinated. Typical films grown under CVD conditions have a compressive stress of about 2kbar, which is observed as a visible curvature of the substrate. The different thermal contractions of the film and substrate during cooling contribute only a fraction of the stress, most of which is an intrinsic property of the film. Such a stress is common in deposited films and is a... 

As already mentioned, in general, evaporated films have low density, open columnar microstruature and tensile intrinsic stress while sputter deposited or ion plated films have higher density, close-packed microstructure and compressive stress. It is further known, that evaporated films prepared under continuous bombardment with ions of increasing kinetic energy are densified and show stress changes from tensile to compressive [288,289]. 

Heat treatment of dense coatings obtained by RLVIP-deposition with arc-currents between 30 and 70 A causes microstructural relaxation phenomena and oxidation resulting in a decrease of the film density and of the film extinction coefficient. Consequently the initially high compressive intrinsic stress is lowered considerably and the physical film thickness is increased [291, 293]. The optical, mechanical and environmental stability of the annealed films remains excellent. 

The observation of viscous flow in SiOz films was first reported by EerNisse(23,24). Essentially, a compressive intrinsic stress was found to exist in Si02 films grown below 1000°C and this stress was relieved at higher temperatures. The densification of SiO films was reported(20,21) and a unified model that explains both the occurrence of stress in Si0 and the higher density was published(25). This model utilized the concept of viscous relaxation in a Maxwell solid. The main idea is depicted in Fig.7 where the molar volume change is seen to cause the stress and density increase which are both relieved via viscous flow at sufficiently high temperatures. These ideas were recently incorporated into a revised oxidation mode1(26). Part of this revision modifies the oxidation expression to include the stress driven viscous relaxation. From a consideration of SiC as a simple Maxwell solid the expression for F becomes ... 

V, B, or O, tailored film properties can be attained. High intrinsic hardness and compressive stresses inhibiting the crack growth are among the beneficial properties of PVD. 

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