edfas.org 7 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 3 the wafer center to the edge. Applying Eq 1 to the data from Figs. 3a and b, created the resistivity map of the ITO films, as shown in Fig. 3c. For thick ITO films with a thickness dITO of 89 nm or greater, the lowest resistivity values are approximately 2 x 10-4 Ω·cm, which is close to the bulk resistivity of the ITO film. For thin ITO films with a thickness dITO of 50 nm or less, the resistivity values range between 5 x 10-4 Ω·cm to 1.25 x 10-3 Ω·cm. This data illustrates the spatial distribution of resistivity ρ for all the ITO samples. Figure 3d depicts a schematic cross-section of a circular magnetron sputtering head. This configuration includes permanent magnets positioned behind the target, generating a high-density, torus-shaped plasma, with the substrate placed parallel to the target. The magnetic field increases the probability of electron collisions with neutral gas atoms, significantly boosting plasma density and, consequently, the sputtering rate. This setup results in a distinct erosion track on the target surface, adjacent to the high-density plasma torus. Studies have shown that when depositing metal oxide films using RF sputtering, the high flux of positive ions impacting the target at the erosion track releases lowenergy secondary electrons. These electrons can easily attach to oxygen atoms, forming negative oxygen ions, which are then accelerated toward the substrate by the cathode sheath. This indicates that the spatial distribution of resistivity in relation to the erosion track on the target surface is influenced by energetic negative oxygen ions. The findings suggest that the primary factor affecting the resistivity and band gap energy profiles of the deposited films is the depletion of metal due to re-sputtering by these energetic negative oxygen ions.[3,4] For these RF-sputtered ITO films, the resistivity shows significant spatial distribution, which is consistent with other studies on metal oxide films such as aluminumdoped zinc oxide.[3,4] The results suggest that the resistivity distribution is highly impacted by the growth conditions. Therefore, the uniformity of electronic properties needs to be monitored during the film deposition process. RESIDUAL STRESS OF ITO Figures 4a and b depict two distinct bow shapes observed from ITO films with average thickness of 14 and 559 nm. For the 14 nm ITO film, the wafer center appears lower than the edges, while the opposite trend is observed for the 559 nm ITO film. Figures 4c and d are the stress map calculated from Figs. 4a and b using Eq 2. The stress versus ITO thickness is plotted in Fig. 4e. The thinnest (14 nm) ITO film sample exhibits a tensile stress of 1.90 GPa. As the ITO film thickness increases, the tensile stress decreases and eventually transitions to compressive stress, stabilizing at approximately -0.4 GPa. The stress error was indicated with an error bar, which was carefully estimated considering the film thickness measurement error as well as the curvature measurement error. In the early stages of ITO film growth, crystals nucleate with different orientations, forming equiaxed grains Fig. 4 (a) 3D bow of ITO (d = 14 nm)/glass; (b) 3D bow of ITO (d = 559 nm)/glass; (c) 3D stress of ITO (d = 14 nm)/glass; (d) 3D stress of ITO (d = 559 nm)/glass; and (e) ITO film stress versus film thickness.[5] (a) (b) (c) (d) (e)
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