(Invited) Characteristics of Charge Trap Flash Memory with Al2O3/(Ta/Nb)Ox/Al2O3 Multi-Layer

Introduction

Charge trap flash memory with high-k layers has attracted attention as a promising next-generation nonvolatile memory candidate. As a typical charge trap flash memory, the TaN/Al₂O₃/Si₃N₄/SiO₂/Si structure has been widely investigated [1]. However, according to scale down of the device size, a low operation voltage is required. All high-k stack structures, such as Al₂O₃/HfAlO_x/Al₂O₃ and Al₂O₃/La₂O₃/Al₂O₃ have been proposed to replace both Si₃N₄ charge trapping and SiO₂tunneling layers [2, 3].

It is easily understood that the conduction band offsets of charge trapping material should be lower than those of tunneling and blocking materials to make trap (program) and detrap (erase) occur in charge trapping layer during program and erase operations. The offsets of Al₂O₃ and Ta₂O₅ on Si were found to be 2.08 and 0.28 eV, respectively [4]. Hence, we should paid attention to a large difference of conduction band offset in the Al₂O₃/Ta₂O₅/Al₂O₃ stack structure. Furthermore, considering to reduce EOT, we employed (Ta/Nb)O_xfilms to increase k-value and keep low leakage current as charge trapping layer.

In this paper, we report on the electrical performance of Pt/Al₂O₃/(Ta/Nb)O_x/Al₂O₃/Si capacitors, which were fabricated at low temperature of 200 °C, by changing thicknesses of Al₂O₃ blocking and tunneling layers and (Ta/Nb)O_xcharge trapping layer.

Experimental

The native oxide on p-Si(100) substrate was removed by diluted HF acid rinse. Al₂O₃ tunneling layers (4-16 nm) were deposited by PE-ALD method at 200 °C using TMA precursor and plasma oxygen gas. The (Ta/Nb)O_x charge trapping layers (1-20 nm) were subsequently deposited by ALD at 200 °C using M(NtAm)(NMe₂)₃ [M=Ta:Nb(1:1mol)] cocktail precursor and H₂O gas. The Al₂O₃blocking layers (4-16 nm) were also deposited by PE-ALD at 200 °C. Finally, a 150-nm-thick Pt electrode was fabricated by sputtering.

Results and Discussion

A cross sectional TEM image of Pt/Al₂O₃/ (Ta/Nb)O_x/Al₂O₃/Si capacitor is shown in Fig. 1. We did not observe any severe chemical reaction in any layer, and all layers were found to be amorphous. In addition, the image reveals the presence of SiO₂ interfacial layer (~1.5 nm) at Al₂O₃/Si interface grown during PE-ALD process.

Figure 2 shows flatband voltage shift (ΔVfb) as a function of EOT in Pt/Al₂O₃/(Ta/Nb)O_x/Al₂O₃/Si capacitor. The ΔVfb and EOT values were estimated from C-V data using MIRAI-ACCEPT program [5]. Injected charge (Q_inj) was kept to be 1mC/cm². The thicknesses Al₂O₃ blocking layer were varied 4, 8 and 16nm, while the thickness of the Al₂O₃ tunneling layer was 16nm. Note that large ΔVfb value of 10 V appears because large amount of charge is trapped in the Al₂O₃/(Ta/Nb)O_x /Al₂O₃ structure. The ΔVfb value increases as thickness of the Al₂O₃ blocking layer increases. It suggests that the charge-trapping efficiency of (Ta/Nb)O_x layer may be lowering compared to Si₃N₄ layer. Therefore, although electrons captured in (Ta/Nb)O_x layer can be readily detrapped and transfer to the Al₂O₃ blocking layer, electrons can be trapped in the thick blocking layer. Furthermore, we also found that ΔVfb value decreases with increasing the thickness of (Ta/Nb)O_xlayer.

Figure 3 shows normalized ΔVfb behavior as a function of retention time for the same capacitor shown in Fig. 2. Electric filed was applied 5MV/cm or -5MV/cm. In case of 5MV/cm, the ΔVfb degradation of the Al₂O₃blocking layer (16 nm) sample was not observed. On the other hand, in case of -5MV/cm, the ΔVfb values decrease with increasing retention time for all samples, regardless of thickness of the blocking layer. This indicates that the electron detrapping occurs easily across the blocking layer rather than the tunneling layer.

Conclusions

We have investigated memory characteristics of Pt/Al₂O₃/(Ta/Nb)O_x/Al₂O₃/Si capacitors by changing each layer thickness. We found that large charge can be trapped in Al₂O₃/(Ta/Nb)O_x/Al₂O₃ multi-layer structure even in low temperature fabrication at 200 °C.

References

[1] J-S. Lee et al., Jpn. J. Appl. Phys.  45 (2009) 3213.

[2] H-Y. Gou et al., Thin Solid Films 529 (2013) 380.

[3] H-J. Kim et al., Mater. Sci. Semicond. Process. 13 (2010) 9.

[4] S. Miyazaki et al., Appl. Sur. Sci. 113/114 (1997) 585.

[5] N. Yasuda et al., Ext. Abst. SSDM.(2005) p.250.

Fig. 1 A cross sectional TEM image of  Pt/Al₂O₃/(Ta/Nb)O_x/Al₂O₃/Si capacitor.

Fig. 2 Flatband voltage shift (ΔVfb) as a function of EOT in Pt/Al₂O₃/(Ta/Nb)O_x/Al₂O₃/Si capacitor.

Fig. 3 Normalized ΔVfb behavior as a function of retention time for Pt/Al₂O₃/(Ta/Nb)O_x/Al₂O₃/Si capacitor.