Residual stress reduction in piezoelectric Sc0.4Al0.6N films by variable-pressure sputtering from 0.4 to 1.0 Pa

Highlights: • C-axis-oriented Sc_0.4Al_0.6N films with the wurtzite structure on silicon (100). • Two-step pressure sputter deposition technique from 0.4 to 1.0 Pa. • Adequate quality of piezoelectric Sc_0.4Al_0.6N underlayer formation at 0.4 Pa. • Stress-free deposition at 1.0 Pa. • Balancing high piezoelectric response and low biaxial stress. -- Abstract: Scandium aluminum nitride (ScAlN) films are promising piezoelectric materials for various microelectromechanical systems. However, residual stress in ScAlN films is a serious problem. In this study, Sc_0.4Al_0.6N films were deposited on silicon (100) substrates by using a reactive magnetron sputtering system. The total gas pressure during the latter stage of film deposition was increased to a high pressure (1.0 Pa) without quenching the plasma and was initiated subsequent to the deposition of an adequate-quality piezoelectric underlayer at a lower pressure (0.4 Pa). The final film is 2.3 μm thick and exhibits a piezoelectric response of 20.1 pC⋅N⁻¹, and its residual compressive stress is about 25% that of a film deposited solely at 0.4 Pa. The result is due to two effects: 1) good crystalline quality of the Sc_0.4Al_0.6N underlayer due to the atomic peening effect at the low pressure and 2) inherited (002)_w growth that is attributed to layer-by-layer selective adsorption of atoms which had lower energies at the higher gas pressure.