Ion beam sputtering, due to the adaptable deposition parameters, is becoming common in the area of thin films. Nano-sized thin films may exhibit new physical properties with potential applications in electrical, optical or mechanical devices. To this end, a fine control of the film microstructure is essential, and interfacial effects during growth are of prime importance, since they affect the physical properties as well as the mechanical strength of resulting nanoscale structures.
In situ substrate curvature measurements, using a multiple beam optical set-up (kSA-MOSS) can provide direct determination of stress evolution during magnetron sputtering deposition. The high measurement sensitivity (~0.5 GPa per deposited monolayer for a 100 µm-thick substrate) allows a detailed characterisation of the initial growth stages, where surface stress and grain boundaries play a key role. The combination with ex-situ analyses (XRD strain tensor measurements, HRTEM and AFM) allows an interpretation of stress data in terms of contributions due to microstructure, interfacial reaction, or defect incorporation at high adatom energy.
The development of a compressive stress during the initial growth stages of amorphous Si layers (a-Si) has been attributed to a “local transformation strain” related to the surface stress by Floro et al. [1], followed by the emergence of a tensile stress arising from coalescence processes. The interplay between kinetic effects and thermal effects, on both stress components, was addressed by varying the conditions, namely argon pressure and deposition temperature, during growth.
For Mo1-xSix films grown on a-Si, the various stages of stress evolution were correlated to change in surface stress, growth of an interfacial amorphous layer, and to amorphous-to-crystal phase transition occurring at a concentration-dependent critical thickness [2]. The phenomena can be explained using a thermodynamic approach based on the competition of surface and volume energy terms, while the observed tensile stress is linked to the volume change at the phase transition.
Nevertheless, the amorphous to crystalline transformation during the growth of Ta films on a-Si, shows a different stress signature. The crystalline Ta β-phase is favoured compared to the α-phase, and there is no volume change associated to the Ta crystallisation, so that the intrinsic growth stress can be tuned from compressive to tensile as a function of the deposition parameters.
Finally, on crystalline template layers, a distinctive stress behaviour is observed, attributed mainly to an initial epitaxial growth.
The examination of the different cases highlights the possible sources of stress during thin film growth, while real time monitoring of the stress evolution provides valuable information on interfacial reactions and morphology.
[1] Floro et al. PRL vol. 91, n° 9, 096101 (2003)
[2] Fillon et al. PRL 104, 096101 (2010)