edfas.org 5 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 3 identical growth conditions, achieving a growth rate of 1.43 nm/s. These consistent growth conditions ensured uniform stress evolution across the samples. SURFACE METROLOGY TECHNIQUES The resistivity of a conductive film can be calculated using the equation: (Eq 1) where ρ is the resistivity, RS is the sheet resistance, and tf is film thickness. The residual stress in the thin-film layer can be calculated using the Stoney equation:[1] (Eq 2) where σ is the stress, κ is the change of curvature, E/(1-ν) is the biaxial elastic modulus of the substrate (E represents Young’s modulus of the substrate, and ν represents the Poisson’s ratio), ts is substrate thickness, tf is film thickness. Figure 1 shows the surface metrology techniques used in the electrical resistivity and residual stress measurement. To evaluate the residual stress and resistivity of ITO films deposited on a glass substrate, it is necessary to measure the sheet resistance, film thickness, elastic modulus, and wafer curvature. These parameters can be measured using a four-point probe (4PP), a reflectometer, a nanoindenter, and a stylus profiler, respectively. The sheet resistance of the ITO films was determined using a four-point-probe system, specifically the Filmetrics R50 from KLA Instruments, equipped with a 4PP Type B probe featuring a 0.1 mm tip radius and 1.0 mm tip spacing. The thickness of the ITO films was measured using a Filmetrics F50-UVX reflectometer from KLA Instruments, which utilizes a light source covering wavelengths from 190 to 1700 nm. This setup enables thickness measurements ranging from 5 nm to 250 µm with an accuracy of ± 1 nm. The change in wafer curvature due to ITO film deposition can be measured using a stylus profiler both before and after the deposition process. For this measurement, the HRP-260 automated stylus profiler from KLA Instruments was used. The scan length was set to 80 mm, which is 80% of the wafer’s diameter. After each scan, the system rotates the sample by 15° and performs another scan. This process is repeated to create a 3D wafer curvature map with 12 traces for each sample. The change in curvature is determined by comparing the measurements taken before and after the ITO film deposition. The elastic modulus of ITO films was measured using a G200X Nanoindenter from KLA Instruments, equipped with a wafer chuck and a diamond Berkovich indenter tip. A standard method was used to correct for the influence of the substrate on the measurement. Arrays of at least 20 indentations were made at the center of the wafer, and the results were reported for 25% of the film thickness. A reference wafer made of fused silica, with a known modulus of 73.0 GPa and a Poisson’s ratio of 0.16, was used to calibrate the indenter tip. This reference wafer was measured to have an elastic modulus of 72.9 ± 0.4 GPa using the same method as for the ITO films. The elastic moduli of ITO films with thicknesses of 45, 91, 451, and 559 nm were evaluated in the experiment. In addition, atomic force microscopy (AFM) was used to perform grain analysis on the ITO films. Studies have shown that grain size measurements from surface techniques such as AFM are consistent with transmission measurements for the ITO material.[2] Fig. 1 Surface metrology techniques used in electrical resistivity and residual stress measurement.
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