Figure 7-2 shows the leakage current of Cu-comb capacitors with an α-SiCN/α-SiC bilayer barrier of different thickness ratios measured at various temperatures. The dominant current conduction mechanisms in the Cu-comb capacitors were identified by fitting slopes for various conduction mechanisms, as
shown in Fig. 7-3. Evidently, the fitting slopes vary with the electric field. All comb capacitors exhibit ionic conduction at electric fields below 0.5 MV/cm at all measurement temperatures employed in this study. The ionic current shows a hysteresis effect (Fig. 7-3a), as confirmed by repeatedly sweeping the electric field from –1.25 to +1.25 MV/cm and back down [8,9]. The ionic conduction of Cu-comb capacitors becomes more apparent at low temperatures (e.g. 25oC), whereas a conduction mechanism like ohmic conduction prevails at temperatures above 200oC.
Notably, the comb capacitors with an α-SiCN/α-SiC bilayer barrier of 40 nm/10 nm or 30 nm/20 nm thickness exhibit ohmic conduction at electric fileds between 0.25 and 0.5 MV/cm and at temperatures of 200 to 250oC; in this region, the leakage current (I) is linearly correlated with the electric field (E) (Fig. 7-3b), and the current can be expressed by Eq. (7-1) [8]. The comb-capacitor with an α-SiCN/α-SiC bilayer barrier of 50 nm/2 nm thickness exhibits Schottky emission (SE) at electric fields above 0.5 MV/cm, particularly at temperatures above 200oC. The SE conduction shows Ln(I/T2) linearly correlated with E1/2 (Fig. 3c), and the current can be expressed by Eq. (7-2) [8]. Nevertheless, the comb-capacitor with an α-SiCN/α-SiC bilayer barrier of 45 nm/5 nm thickness exhibits SE conduction only at electric fields of 0.5 to 0.8 MV/cm, and Frenkel-Poole (F-P) emission appears after the kink at 0.8 MV/cm, in particular at temperatures above 200oC. The F-P conduction shows Ln(I/E) linearly correlated with E1/2 (Fig. 7-3d), and the current can be expressed by Eq. (7-3) [8].
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where ∆Eae is the activation energy of electron, β represents (q3/4πε)1/2 and ε is the dielectric dynamic permittivity, k is the Boltzmann constant, q is an electron-charge, ΦSE is the barrier height of metal/dielectric interface, and ΦFP is the barrier height of trap potential well. Table 7-1 summarizes the leakage mechanisms at various electric fields for the Cu-comb capacitors measured at temperatures of 200 to 250oC.
The leakage mechanism between the Cu lines is dependent on the thickness ratio of the α-SiCN/α-SiC bilayer barrier. In contrast to the α-SiCN(50 nm)/α-SiC(2 nm) sample, there is a transition of SE to F-P conduction at 0.8 MV/cm electric field in the α-SiCN(45 nm)/α-SiC(5 nm) sample, while the α-SiCN(40 nm)/α-SiC(10 nm) and α-SiCN(30 nm)/α-SiC(20 nm) samples exhibit the same leakage mechanisms at all electric fields employed in this study. Figure 7-4 shows the leakage current of various Cu-comb capacitors versus measurement temperature. Notably, the leakage current behavior at the low electric field of 0.65 MV/cm can be divided into two groups, with α-SiCN(50 nm)/α-SiC(2 nm) and α-SiCN(45 nm)/α-SiC(5 nm) samples in one group (Fig. 7-4a); at the high electric field of 1.25 MV/cm, however, the leakage current behavior of the α-SiCN(45 nm)/α-SiC(5 nm) sample deviates from that of the α-SiCN(50 nm)/α-SiC(2 nm) sample (Fig. 7-4b). This is due to the transition of SE to F-P conduction in the α-SiCN(45 nm)/α-SiC(5 nm) sample at 0.8 MV/cm electric field at elevated temperatures.
The leakage current density of the bulk OSG is at least 20 times larger than that of the bulk α-SiCN, α-SiC, α-SiCO (ESL) and PECVD oxide (ILD) films studied using MIS capacitors; thus the effective leakage current component through the bulk OSG (250 nm) is expected to be at least two orders of magnitude larger than that through the α-SiCN (≤50 nm), α-SiC (≤20 nm), ESL (40 nm) and ILD (≤10 nm below ESL) dielectric films in the Cu-comb capacitors. However, it has been reported that the
localized surface defects at the α-SiC/OSG interface (CMP-surface) can degrade the leakage current and TDDB reliability [10,11]. Since there is no CMP-induced damage at the OSG/ESL and ESL/ILD interfaces, we did not observe the pseudo-breakdown phenomenon, which is supposedly arisen from the defects at the OSG/ESL and ESL/ILD interfaces [12]. This will be further discussed later and illustrated in Fig. 7-8. Therefore, the dominant leakage path in the Cu-comb capacitor could be the electronic current through the bulk of OSG and/or the α-SiC/OSG interface.
Possible determining factors for the dominant leakage path in the Cu-comb capacitor may include the electric field and/or physical stress at the α-SiC/OSG interface and/or in the bulk of OSG. Figure 7-5 shows the electric field at the α-SiC/OSG interface and in the bulk of OSG obtained from the Raphael simulation for various Cu-comb capacitors biased with an electric voltage of 24 V. The higher electric field at the α-SiC/OSG interface than that in the bulk of OSG may be due to a number of factors, such as shorter distance at the top of Cu lines, angular shape at the corner of Cu lines, and higher dielectric constant of the α-SiC layer [10]. Since the behavior of the simulation obtained electric field is contrary to the magnitude of leakage current with respect to the bilayer thickness ratio of the Cu-comb capacitors, we presume that the increased leakage current between Cu lines for the α-SiCN(40 nm)/α-SiC(10 nm) and α-SiCN(30 nm)/α-SiC(20 nm) samples is attributed to the
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bonds at the α-SiC/OSG interface, as illustrated in Fig. 7-6. For the Cu-comb capacitors with α-SiCN(40 nm)/α-SiC(10 nm) and α-SiCN(30 nm)/α-SiC(20 nm) bilayer thicknesses, which exhibit a larger leakage current at temperatures above 200oC, the ohmic conduction dominates at an electric field between 0.25~0.5 MV/cm (Table 7-1) and the current is carried by thermally excited electrons hopping from one isolated trap to the next [8]. At an electric field above 0.5 MV/cm, the current is dominated by the F-P emission, which is due to field-enhanced thermal excitation of trapped electrons into conduction band [8]. Both of the ohmic conduction and the F-P emission mechanisms result from a large number of interfacial defects at the α-SiC/OSG interface.
7-4 Breakdown Mechanism
Figure 7-7 shows the breakdown field measured at 200oC for various comb capacitors with the data obtained from 10 randomly chosen samples in each case.
All samples exhibit a comparable breakdown field and the breakdown is presumably due to dielectric breakdown in the bulk of OSG rather than that at the α-SiC/OSG interface; the large variation of the breakdown field is possibly resulting from the discordant force of manual probing and/or unfavorable samples at the wafer edge. Figure 7-8 shows the TDDB lifetime of various comb capacitors under different BTS conditions with the data obtained from 6 randomly chosen samples. It is found that all the comb capacitors reveal a comparable TDDB lifetime under a given BTS condition. The fact that the breakdown field (Fig. 7-7) and the TDDB lifetime (Fig. 7-8) of the Cu-comb capacitors show little dependence on the thickness ratio of the α-SiCN/α-SiC bilayer barrier implies that the breakdown is very likely due to dielectric breakdown in the bulk of OSG. Figure 7-9 illustrates the proposed leakage paths of the Cu-comb capacitor studied in this chapter. We
may conclude that the leakage and breakdown mechanisms in the Cu-comb capacitor with an α-SiCN/α-SiC bilayer barrier is closely correlated with the quality of the α-SiC/OSG interface and the OSG layer.
7-5 Summary
It is found that the leakage mechanism between Cu lines is dependent on the thickness ratio of the α-SiCN/α-SiC bilayer barrier in the Cu-comb capacitor. Using an α-SiCN(40 nm)/α-SiC(10 nm) or α-SiCN(30 nm)/α-SiC(20 nm) bilayer barrier, the increased leakage (Frenkel-Poole emission) between Cu lines is attributed to the large number of interfacial defects, such as cracks, voids, traps or dangling bonds at the α-SiC/OSG interface, which are generated by the larger tensile force of the thicker α-SiC film. The Cu-comb capacitor with an α-SiCN(50 nm)/α-SiC(2 nm) bilayer barrier exhibits a much smaller leakage current. On the other hand, the breakdown field and TDDB lifetime of the Cu-comb capacitor reveal little dependence on the thickness ratio of the α-SiCN/α-SiC bilayer barrier, and the observed breakdown of the Cu-comb capacitor is presumably due to dielectric breakdown of the bulk OSG layer.
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Table 7-1 Leakage mechanisms at various electric fields measured at 200 to 250oC for Cu-comb capacitors with a bilayer-structured Cu cap-barrier of various α-SiCN/α-SiC bilayer thicknesses.
α-SiCN/α-SiC Bilayer Thickness (nm/nm) Electric Field
(MV/cm)
50/2 45/5 40/10 30/20
0~0.25 Ionic Ionic Ionic Ionic 0.25~0.5 Ionic Ionic Ohmic Ohmic
0.5~0.8 SE SE F-P F-P
0.8~1.25 SE F-P F-P F-P