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In this study, ultrathin (4-40 nm) Cu-layer-based transparent heaters prepared on glass substrates were investigated for cost-effective applications. The Cu heaters were embedded between ZnO layers serving as anti-reflection and anti-corrosion layers. The Cu layer thicknesses varied in the range of 4-40 nm, and the corresponding structural, electrical, optical and thermal properties were evaluated. Cu was found to follow the Volmer-Weber 3D growth mode in the early deposition stage, where isolated islands grow and coalesce to form a continuous layer at ~12 nm. In the thickness regime of discontinuous Cu layers, a significant increase in sheet resistance was observed due to the reduced current paths and the high severity of electron scattering at the Cu/ZnO interfaces. Because of light absorption associated with the localized surface plasmon resonance (LSPR) in the presence of pores in the films, visible transparency peaks near the thickness of the complete film-closure, beyond which stronger light absorption decreases transparency. The sheet resistance of the transparent heaters was modulated in the range of 0.8-96.2 Ω/sq. The heating characteristics well follow Joule’s law which predicts a higher temperature for a lower-resistance heater at a given voltage. The measured temperature-power relation is linear, from which the important heater parameter of convective heat transfer coefficient is extracted.

In this study, we prepared highly thermostable semi-transparent heaters composed of W layers with thicknesses of 1-20 nm, on which a 30 nm-thick ZnO layer was deposited to serve as an anti-oxidation barrier. The optical transmittance and sheet resistance of the heaters could be greatly modulated by varying the W layer thickness. For layer thicknesses up to 10 nm, the initial Joule heating above 100 oC significantly reduced the sheet resistance, by 300% for a 6 nm-thick W layer at a fixed voltage for a duration of 400 s. During the test period, heater current and heating capability continuously increased. In subsequent heater operations, the heaters exhibited highly reproducible heating capability. In contrast, for films thicker than 10 nm, the Joule heating process resulted in only a marginal reduction in sheet resistance, i.e., by 4% for a 20 nm-thick W layer. In order to investigate the sharp dependence of heater characteristics on thickness, we performed x-ray diffraction analyses, which revealed that the films thinner than 10 nm were composed of both the equilibrium low-resistivity α-phase and metastable high-resistivity β-phase, and films thicker than 10 nm contained mostly α-phase. The Joule heating process for the thinner films was found to transform the β-phase into α-phase at temperatures above 100 oC, which resulted in significant improvement in the heating capability of the 6 nm-thick W layer. For films thicker than 10 nm, the W layers contained mostly α-phase and no such transformation-induced effects were observed. Finally, W heaters composed of α-phase exhibited highly thermostable and reproducible heater properties, which make the heaters suitable for applications with semi-transparent heaters.

In this study, we report a methodology that simultaneously improved the corrosion resistance and visible transparency of ZnO/Cu/ZnO transparent heaters with the addition of reactively sputtered Al2O3 layers. To assess corrosion resistance, ZnO and Al2O3 layers with thicknesses in the range of 0-20 nm were deposited onto 20 nm-thick Cu layer, and the corrosion behavior of the underlying Cu layers was investigated by evaluating the sheet resistance change in an 85 °C/85% humidity test. While the 20 nm-thick ZnO layer was not an effective moisture barrier, showing a sheet resistance increase of more than 50% after 10,000 m, the sheet resistance of the Cu layer below the 5 nm-thick Al2O3 layer did not show an observable increase for equal duration. Optical transmittance was also investigated by constructing glass/ZnO/Cu/ZnO/Al2O3 structures with varying thicknesses for the top ZnO and Al2O3 layers, in the range of 10-30 and 10-60 nm, respectively The thicknesses of the bottom ZnO and Cu layers were fixed at 30 nm and 10 nm. The results revealed that the average visible light transmittance of the ZnO/Cu/ZnO/Al2O3 structure increased by 2.5% over the optimized ZnO/Cu/ZnO structure.

In this study, the critical role of substrate bias during the sputter deposition of Al thin films is discussed. Two sets of Al thin films having a nominal thickness of 300 nm were deposited at sputtering pressures of 4.1 and 1.5 mTorr, respectively, with an applied negative substrate bias in the range of 0-200 V. It was found that the microstructure, surface roughness, film resistivity and grain size were greatly altered by the combination of bias magnitudes and sputtering pressures. The sputtering pressure of 4.1 mTorr resulted in greater changes in the film properties with the application of substrate bias, and a lesser but still significant degree was observed for the films deposited at 1.5 mTorr. The resistivity values for the films deposited at 1.5 mTorr were found to be significantly lower, with the lowest resistivity value of 3.1 µΩcm achieved at a substrate bias of 50 V. Based on grain size measured by the line intercept method and MayadasShatzkes grain boundary scattering model, the resistivity contribution of grain boundary scattering for the lowest-resistivity film was found to be 0.37 µΩcm, which indicates that the film resistivity in the optimized condition is close to the known bulk resistivity of 2.65 µΩcm.