Figure 9-7 shows the leakage current, measured at various temperatures, for the Cu-comb capacitors with SiC as well as SiCO dielectric cap-barriers. Notably, the dependence of leakage current on the cap-barrier dielectric material implies that the dominant leakage paths are not in the bulk OSG but in the bulk cap-barrier and along the cap-barrier/OSG interface (CMP-surface) [24,25]. The fact that the leakage current of the SiCO sample at a field of 1.6 MV/cm between 25 and 250oC is at least three orders of magnitude smaller than that of the SiC sample, can be attributed to the semiconductor nature of SiC and the better electrical property of the SiCO film, as studied in the previous paragraph using the MIS capacitors. Moreover, the CMP-surface leak-path
may be efficiently eliminated in the SiCO sample since both the SiCO cap-barrier and the OSG layer are similar materials of Si-C-O compounds, while the SiC cap-barrier is an oxygen-absent Si-C compound. Similar presumption was reported for the CMP-surface at the α-SiCN(cap-barrier)/α-SiCN(hard-mask) interface of the same material in the Cu damascene interconnect [24]. Two different electric-field-dependent conduction mechanisms were also observed in the Cu-comb capacitors. First, all Cu-comb capacitors exhibit ohmic conduction at low electric fields (<0.3 and <0.4 MV/cm for SiC and SiCO cap-barriers, respectively) since the leakage current (I) is linearly correlated with the electric field (E) [19,20]. Nevertheless, at high electric fields (>0.3 and >0.4 MV/cm for SiC and SiCO cap-barriers, respectively), the Cu-comb capacitors with SiC and SiCO cap-barriers exhibit quite different nonlinear I-E relation, particularly at temperatures above 200oC. This implies that the SiC and SiCO samples have different conduction mechanisms at high electric fields. From the best fitting of the known conduction mechanisms in dielectrics [19,20], we found that the SiC sample exhibits F-P emission, while the SiCO sample reveals SE conduction, as shown in Fig. 9-8. It is interesting to observe that the Cu-comb capacitors and the Cu-MIS capacitors exhibit the same electric-field-dependent conduction mechanisms. This further confirms that the dominant leakage paths are not only along the cap-barrier/OSG interface (CMP-surface) but also in the bulk cap-barrier. Furthermore, it was found in our previous work that the more leaked
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and SiCO dielectric cap-barriers measured at 200oC. Notably, the breakdown field of the SiCO sample is about 25% higher than that of the SiC sample. This increase in breakdown field is presumably due to the better electrical property of bulk SiCO and the diminution of CMP-surface leak-path. In addition, the lower k-value of the SiCO film would also reduce the surface-electric field at the cap-barrier/OSG interface (CMP-surface), as illustrated in Fig. 9-10 [25,26]. The lower surface-electric field at the CMP-surface would result in a lower leakage current and higher breakdown field for the SiCO sample.
Figure 9-11 shows the surface-electric field obtained from Raphael simulation for the Cu-comb capacitors biased with an electric voltage of 24 V, which would produce an electric field of 2 MV/cm in the dielectric layer of the Cu-comb capacitor. Notably, the surface-electric field near the Cu-line of the SiCO sample is about 6% lower than that of the SiC sample.
9-5 Summary
The leakage current and breakdown field of the Cu-MIS and Cu-comb capacitors are dependent on the dielectrics (α-SiC and α-SiCO) used as insulator and dielectric cap-barrier, respectively. The Cu-MIS and Cu-comb capacitors with an α-SiCO (k=3.7) dielectric barrier film exhibit a leakage current at least three orders of magnitude smaller than those with an α-SiC (k=4.4) dielectric film at an applied electric field of 1.6 MV/cm between 25 and 250oC. Moreover, the breakdown field of the Cu-MIS and Cu-comb capacitors with an α-SiCO dielectric barrier, measured at 200oC, are 60% and 25% respectively, higher than that of the capacitors with an α-SiC dielectric barrier. The decreased leakage current and increased breakdown field of the Cu-MIS and Cu-comb capacitors with an α-SiCO dielectric barrier are attributed to the higher density, oxygen-improved film’s property, non-semiconductor behavior, and lower fringe- or surface-electric field of the α-SiCO dielectric film.
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Table 9-1 Basic film properties for 3MS-based α-SiC and α-SiCO dielectric barrier films with a thickness of 50 nm used in this study.
Sample ID SiC SiCO
Structure α-SiC α-SiCO Chemical composition SiC0.92O0.04 SiC0.97O0.66
Deposition rate (nm/min) 100.4 102.6 Film stress (MPa) -200 -210
Film density (g/cm3) 1.24 1.80 Refractive index @ 633 nm 2.04 1.72
Dielectric constant @ 1 MHz 4.41 3.73