Current Voltage Characteristics through Grains and Grain Boundaries Of HfSixOy Thin Films Measured By Tunneling Atomic Force Microscopy

High-k dielectric films are of interest as alternative gate insulators for metal-oxide semiconductor devices. Inour previous study, surface morphology and leakage current distribution of high-k dielectric films(HfSixOy andZrO2) have been simultaneously investigated at the nanoscale by tunneling atomic force microscopy (TUNA) [1]. Itwas found that the dominant leakage current paths of high-k dielectric films were located at the grain boundaries.Probable reasons for the larger leakage current through grain boundaries are different current conductionmechanisms for grains and grain boundaries or the minor thickness of grain boundaries compared to that of grains.Figure 1 shows the typical topography and corresponding current map of an 8.0 nm thickHfSixOy film measured byTUNA. Crystalline grains can clearly be observed in the topography map. In the TUNA current map, theconductive structures (dark color), corresponding to the grain boundaries in the topography map, are clearly visiblewhich indicate larger currents. Figure 2 shows local I-V curves for different grains and grain boundaries of theHfSixOy film. The difference between I-V curves through grains and grain boundaries can clearly be observed. Thedistribution of the local I-V curves through grains has a wide spread, while that through grain boundaries showstwo groups (i.e., group A and B) with rather narrow spread. The reason for the two I-V groups for the grainboundaries will be discussed. Most important, leakage currents through grain boundaries are found to increaseabove the noise level at lower voltages for group A. The I-V curves will be evaluated by conventional and advancedevaluation methods (e.g., Fowler-Nordheim, Poole-Frenkel and α-V plot [2]) and discussed in detail to identify thephysical reasons for the differences. In order to analyze the relationship between local I-V curves and the thicknessof grains and grain boundaries transmission electron microscopy characterization of the films will be performed. REFERENCESFIGURE 1. (a) TUNA topography map and (b) corresponding TUNAcurrent map of the HfSixOy film. Scan size: 2x1 μm, applied biasvoltage: -9.3 V. FIGURE 2. Local I-V curves through grainsand grain boundaries of the HfSixOy filmmeasured by TUNA.1. V. Yanev, et. al., Appl. Phys. Letters 92, 252910 (2008).2. V. Mikhaelashvili, et al., J. Appl. Phys. 84, 6747 (1998). Keyword: TUNA, high-k, grain and grain boundary