Figure 7.1(a) indicates C-V curves of Al2O3 inter-poly capacitors with surface NH3 nitridation annealed at 800oC to 1000oC. The EOT is increased as raising annealing temperature, which can be ascribed to the thick interfacial layer (IL) growth.
Figure 7.1(b) compares the J-E characteristics of the Al2O3 inter-poly capacitors with
NH3 nitridation at various PDA temperatures under both polarities. It is found that the sample with 900oC annealing can effectively reduce the low-field leakage current than 800oC- and 1000oC-annealed samples, which is helpful to suppress charge loss from the floating gate.
Figure 7.2(a) presents the transient currents for the annealed Al2O3 IPDs under a low field of 2 MV/cm in order to suppress the creation of stress-induced traps. Filling of the pre-existing electron traps in the dielectric leads to the decrease of the current leakage magnitude over time for all samples [115], [125]. Moreover, the rate of leakage current reduction in either polarity is nearly identical, suggesting that the traps are distributed uniformly across the films. Therefore, we believe that the dependence of the IPD characteristic on annealing temperature is closely related to the bulk defects in the dielectric, i.e., point defects. Dielectric relaxation current (transient gate current) for the annealed Al2O3 IPDs was measured instantly after a gate voltage step, which is shown in Fig. 7.2(b). When gate voltage pulses are applied to inter-poly capacitors, the gate current amplitude reaches a high level instantly after the switch and then decays to a low constant level. The J-t curves are nearly a straight line in log-log scale with slope ~ -1 [126], [127]. Due to the asymmetry of the energy band structure, electron transport is easier in positive polarity than negative polarity. As a result, positive polarity shows larger relaxation current. Moreover, dielectric relaxation current is also depended on the PDA temperature. 900oC-annealing apparent reduce the relaxation current due to smaller trapping density and trapping rate, which is evidenced below.
Figure 7.3(a) depicts the curves gate voltage shifts of the Al2O3 inter-poly capacitors with surface nitridation annealed at 800oC to 1000oC under a constant current stress (CCS) of 5 mA/cm2 in both polarities. The increase in the absolute gate
voltage indicates that the primary mechanism responsible for the long-term wear-out in Al2O3 is the creation of electron traps. Moreover, Al2O3 inter-poly capacitors annealed at 900oC exhibits small electron trapping rate than annealed at 800oC or 1000oC. The Weibull distributions of QBD for samples with various PDA temperatures are shown in Fig. 7.3(b). The dependence of QBD on polarity due to NH3 nitridation has been discussed in previous chapter. For positive gate bias, IPD with NH3 surface nitridation can significantly suppress the formation of an additional layer with lower dielectric constant during post-annealing process and obtain smoother interface, compared to that without nitridation treatment. Furthermore, the presence of a thin Si-N layer can make PDA more effective in eliminating traps existing in the as-deposited films and improve dielectric characteristics under negative polarity. The results clearly reveal Al2O3 IPD with optimized 900oC PDA and surface NH3
nitridation can effectively reduce electron trapping rate as well as obtain high QBD, which can be ascribed to the less bulk defect density after high-temperature annealing.
The extracted dielectric constant (κ) and IL thickness of Al2O3 inter-poly capacitors with surface nitridation annealed at 800oC to 1000oC is seen in Fig. 7.4.
The result is consistent with EOT extracted from C-V curves and clearly reveals Al2O3
IPD with 900oC PDA exhibits better dielectric properties, i.e. higher κ-value and thinner IL, than 800oC and 1000oC annealing.
7.3.2 Conduction Mechanism of the RS Al2O3 IPD
Figure 7.5 illustrates the temperature dependence of gate current density at 6 MV/cm of Al2O3 inter-poly capacitors with surface NH3 nitridation annealed at 800oC to 1000oC in O2 ambient. In general, all samples exhibited considerably weak
measuring temperature dependence on the leakage current density in either polarity, implying that the tunneling mechanism for the Al2O3 IPD is Fowler Nordheim-like, rather than Frankel Poole-like. On the other hand, the magnitude of the leakage current was found to be strongly depended on the annealing temperature. The leakage current density of IPD subjected to 900°C annealing is less than 30 nA/cm2 at 6 MV/cm, which is one order of magnitude lower as compared to 800°C annealing.
Moreover, further increasing the annealing temperature up to 1000°C did not help improve the characteristic of the dielectric. The reason for this may be many-fold, and a more detailed description will be given later. According to the results shown in Fig.
7.1 and Fig. 7.5, we believes Al2O3 IPD with 900oC PDA can significantly reduce the charge loss from floating gate due to less than 30 nA/cm2 leakage current at 6 MV/cm even measured at 150oC. The extracted positive-biased and negative-biased effective barrier height (ϕB) of the 900oC-annealed Al2O3 IPD is 2.18eV and 2.24eV, respectively, by assuming the effective electron mass in Al2O3 to be 0.2m0 [119].
Al2O3 IPD with 900oC PDA exhibits the largest ϕB than 800oC- and 1000oC-annealed sample, which can reduce electron tunneling probability and lead to better charge retention characteristics.
The trapped charge centroid (Xt) and density (Qt) in the Al2O3 IPDs can be estimated according to the bi-directional I-V method [128], [129].
Poly-II is positively and negatively stressed by a constant current respectively. Gate voltage shifts are extracted from the linear portion of the J-V curves at a current level 100 times smaller (1 µA/cm2) than that of the injection current (100 µA/cm2) in order to reduce possible re-emission of the trapped electrons [128]. Figure 7.6(a) compares the calculated Xt of Al2O3 inter-poly capacitors annealed at various PDA temperatures under positive and negative CCS. Polarity dependent dielectric characteristics can be partially explained by variation of charge centroids. It is shown that the trapped charge centroid for negative CCS is relatively far away from the electron injection electrode, i.e. cathode, results in smaller charge trapping rate and higher QBD than positive CCS, which is consistent with previous results. Traps near electron injection cathode in positive polarity also enhance dielectric relaxation current. Furthermore, as PDA temperature increases, both centroids of positive and negative CCS move from Al2O3/Poly-I interface toward Poly-II/Al2O3 interface.
The trapped charge densities of Al2O3 inter-poly capacitors at various PDA temperatures under positive and negative CCS are shown in Fig. 7.6(b). Pronouncedly, negative CCS results in smaller trapped electron density than positive CCS, consistent with the lower trapping rate observed in Fig. 7.5(a). Moreover, IPD with 900°C annealing shows a nearly one order of magnitude reduction in the trapped charge density as compared to the 800°C-annealed sample. Al2O3 inter-poly capacitors with optimized 900oC PDA and surface NH3 nitridation can suppress electron trapping rate less than 1×10-4, which is relatively small as comparing to the 800°C- and 1000oC-annealed samples. The corresponding band diagrams including charge centroids and trapped charge densities of Al2O3 inter-poly capacitors with surface NH3 nitridation under both polarities are drawn in Fig. 7.7. As charge centroid moving toward electron injection cathode, large voltage shift during CCS is induced
and enhances electron trapping. As a result, polarity dependent dielectric reliabilities are observed in the Al2O3 IPDs regardless of annealing temperatures.