Reactive sputter deposition of piezoelectric zinc oxide thin films: Processing, structure and property correlation

Described in this study are the deposition and characterization of zinc oxide (ZnO) thin films by reactive RF magnetron sputtering with potential application in thin film resonator (TFR) bandpass filters with 3.5 GHz center frequency. Processing variables include substrate temperature, nucleation layer, sputtering power, reactive gas flow rates. The films have been characterized using X-ray diffraction, spectroscopic ellipsometry, field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Rutherford backscattering spectroscopy, particle induced X-ray emission, and Raman spectroscopy. Electrical resistivity, piezoelectric, and TFR performance have also been determined.; X-ray diffraction spectra have shown that the films possess different degree of crystallinity and are all c-axis oriented normal to the substrate. The film crystallinity and orientation were found to increase with substrate temperature during deposition with the best texture (rocking curve 1.42° FWHM) obtained at a temperature of 700°C. Images acquired by FESEM have shown that all the films are dense and relatively smooth with columnar structure of column diameters in the range 40 nor to 300 nm. Strong dependence of film microstructure on substrate temperature and substrate type has been revealed. A textured film of platinum (Pt) improves significantly crystallinity and texture in sputtered ZnO films. The TEM analysis revealed high concentration of stacking faults in the film deposited at room temperature and high density of dislocations in film deposited at 600°C. A mechanism for morphology evolution of ZnO thin films based on the competition between the surface diffusion and the impinging rate of adatoms is discussed.; Film resistivity has been correlated with the substrate temperature during deposition, such that resistivities on the order of 1010 Ω.cm can be obtained at temperatures below 500°C. At above 500°C, the concentration of charge carriers increases, which is highly deleterious. Sputtered ZnO films deposited on unheated substrates, despite poor crystallinity and texture, show high electrical resistivity and surprisingly good piezoelectric properties. This suggests that good piezoelectric response can be obtained even in nanocrystalline films, if the film has a high resistivity and good polarity alignment for adjacent crystallites. Measurements of Performance of TFR device using ZnO films have given an excellent bandwidth of 3.01% for films deposited at 600°C.