Electrical Properties of Rare-Earth Oxides and La2O3 Stacked Gate Dielectrics

The electrical properties of La2O3 gate dielectric stacked with other rare-earth oxides were investigated. A reduction of fixed charged defects in the dielectric film was observed in CeOx/La2O3/NdOx compared to that of La2O3/NdOx. Obtained equivalent oxide thickness (EOT) was smaller for CeOx/La2O3/NdOx gate stack compare to that of CeOx/La2O3 gate stack while the estimated fixed charged defects in the dielectric film were almost same. Our results show that combination of several rare-earth oxides is useful in improving electrical properties of highk gate stacks. Introduction Rare earth oxide La2O3 is regarded as one of the potential candidates of high-k gate dielectric in the future high-k gate stack technology because a ‘direct contact’ high-k/Si gate stack structure can be easily formed using La2O3 gate dielectric. Stacking with other rare-earth layers may provide a way in improving the electrical properties of the La2O3 gate dielectric. Previous studies showed that stacking with Ce-oxide (CeOx) has the effect of charged defects reduction [1]. On the other hand, stacking with Nd-oxide (NdOx) can suppress the EOT increment during the fabrication process [2]. In this work, we studied the electrical properties of La2O3 gate dielectric stacked with other rare-earth oxides such as NdOx, CeOx, etc. Experiment MOS capacitors were fabricated on a n-type Si(100) wafers with a resistivity of 1~10 Ω-cm. After chemical cleanings with H2SO4+H2O2 followed by HF dipping, rare earth oxides (La2O3, CeO2, Nd2O3, etc.) were deposited by electron-beam deposition in an ultra-high vacuum chamber at a substrate temperature of 300oC. Thicknesses of film were controlled by moving a mechanical shutter during the deposition. After depositing stack structures such as CeOx/La2O3, NdOx/La2O3 and CeOx/La2O3/NdOx, A tungsten (W) film was in situ deposited by RF sputtering as a metal gate electrode Postmetallization annealing (PMA) was conducted in a forming gas ambient (H2:3%, N2:97%) at 500 C for 30 min. Result and discussion Figure 1 shows the obtained relations of the flatband voltages (Vfb) and EOT for the samples with La2O3, CeOx/La2O3, NdOx/La2O3 and CeOx/La2O3/NdOx gate dielectrics. Charged defect densities in the films were estimated from the slope of the fitted lines [3]. The obtained fixed charged defects in the CeOx/La2O3/NdOx dielectric film were smaller compared to that of La2O3/NdOx. Figure 2 shows the capacitance-voltage (C-V) characteristics of the fabricated MOS capacitors with the almost same physical thicknesses. A smaller EOT value of 1.1 nm was obtained for CeOx/La2O3/NdOx compare to the value of 1.3nm obtained for CeOx/La2O3 layered device. The reduction can be understood as the generation of radical oxygen atoms was suppressed by using NdOx. Conclusion Electrical properties of La2O3 gate dielectric stacked with other rare-earth oxides such as CeOx/La2O3, NdOx/La2O3 and CeOx/La2O3/NdOx were studied. Obtained results show that fixed charge in film can be reduced by introducing CeOx layer, EOT increase can be suppressed by introducing NdOx layer into the La2O3 gate stack. It was obtained that for the CeOx/La2O3/NdOx gate stack, estimated fixed charged defects was smaller compared to that of La2O3/NdOx gate stack and the calculated EOT was smaller compare to that of CeOx/La2O3 gate stack. Acknowledgment This work is supported by NEDO. Reference [1] M. Kouda et al., VLSI Symp. Tech., 10B-3, p. 200-201 (2009). VL [2]M. Kouda et al., IWDTF-11, S2-2, P.11-12 (2011). [3] S. C. Song et al., IEDM, 2007, pp. 337-341 (2007). Figure 2 C-V characteristics of MOS capacitors with several types of gate dielectrics. The physical thicknesses of the dielectrics were adjusted to around 1nm for all samples shown in the figure. Figure 1 Obtained EOT-Vfb relations of the investigated MOS capacitors. Charged defect densities were estimated from slop of the fitted lines. 1.3 1.4 1.5 1.6 1.7 1.8 0.1 0.2