edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 3 6 ELECTRICAL RESISTIVITY OF ITO Figure 2a illustrates the thickness map of the thickest ITO film, measured using a 115-point map across the entire 100 mm diameter wafer with a 5 mm edge exclusion. The center thickness is 615 nm, while the edge thickness is 510 nm. The thickness gradually decreases from the center to the edge, forming a circular distribution. Figure 2b shows the sheet resistance map of the same wafer. It reveals that both the center and the edge exhibit high sheet resistance, approximately 4.6 Ω/sq, while the area in between has the lowest sheet resistance, around 3.3 Ω/sq. According to Eq 1, if the ITO film has uniform resistivity across the wafer, RS should be inversely proportional to the film thickness. However, no monotonic change in RS is observed from the center to the edge, as seen with the film thickness. This discrepancy between the ITO thickness map and the RS map suggests a variation in ITO resistivity from the wafer center to the edge. Figures 2c and 2d depict the ITO thickness map and sheet resistance map from a different sample. The ITO thickness ranges from 75 to 89 nm, decreasing from the center to the edge. The sheet resistance at the wafer edge is high, at 116 Ω/sq, while it decreases to 25 Ω/sq toward the center. The center of the wafer shows an RS of approximately 50 Ω/sq. To better understand the spatial distribution of the ITO thickness dITO and the sheet resistance RS, and to calculate the point-to-point resistivity ρ, line scans of dITO and RS across the wafer were conducted for all nine ITO samples. The line scan was performed from X = –45 mm to X = 45 mm at 1 mm intervals. Figure 3a shows the line scan of the ITO thickness for all nine samples. The ITO thickness at the wafer center ranges from 15 to 615 nm, with all samples exhibiting a gradual decrease in thickness from the center to the edge. Figure 3b presents the line scan of the ITO sheet resistance for all nine samples. For thin ITO films, the RS at the center is generally lower than at the edge. However, for thicker ITO films, the sheet resistance does not show a monotonic change from Fig. 2 (a) Thickness map and (b) sheet resistance map of ITO film with thickness of 615 nm in the center. (c) Thickness map and (d) sheet resistance map of ITO film with thickness of 89 nm in the center. The samples are ITO films deposited on 100 mm glass wafers. Fig. 3 (a) Thickness line profiles and (b) sheet resistance line profiles of ITO films with thickness ranging from 15 to 615 nm in the center. (c) Resistivity line profiles of ITO films calculated from the thickness and sheet resistance results. (d) Schematic cross-section of a circular magnetron sputtering head. (a) (b) (c) (d) (a) (b) (c) (d)
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