Oxidizing Species Dependence of the Interface Reaction during Atomic-Layer-Deposition Process and Post-Deposition-Anneal
Authors: Suwa Tomoyuki , Teramoto Akinobu , Koda Yasumasa , Saito Masaya , Sugita Hisaya , Hayashi Marie , Tsuchimoto Junichi , Ishii Hidekazu , Shiba Yoshinobu , Shirai Yasuyuki , Sugawa Shigetoshi 
Published: February 2020

Introduction
Al2O3 film is an attractive gate dielectric material for the power devices [1] and metal-insulator-metal capacitors [2]. For Al2O3 deposition, the atomic layer deposition (ALD) is one of the most promising methods [3]. It is considered that the oxidation is one of the important processes in ALD. In the case of using radical oxidation in ALD, it is suggested by XPS evaluation that the oxidizing reaction is occurred at the Al2O3/Si interface during ALD [4]. In the former study, H2O oxidation at stage temperature of 75 °C was shown to be effective because of not oxidizing Si [4]. However the oxidation ability of H2O becomes low at such low temperature. Therefore, it is required to introduce the post-deposition-annealing (PDA) without oxidizing the interface to improve Al2O3 film quality [4, 5].
In this paper, the characteristics of Al2O3/Si interface formed during ALD and PDA are evaluated by MIS capacitor, and its interface structure was also observed by Rutherford backscattering spectroscopy (RBS) for clarifying the mechanism of its degradation.
Experiment
MIS capacitors were fabricated on Cz-n Si(100) wafers (8-12 ohm cm). At first, the native oxide on Si surface was removed by diluted HF (0.5%), and after that Al2O3
films (20 nm) were deposited on Si by ALD [4]. Finally, Al-electrodes were formed by the evaporation. The Al2O3 films were prepared with and without PDA. Here, PDA-I and PDA-II were carried out by O2 annealing at 400 oC [5] and Ar/O2 plasma oxidation [6], respectively.
For RBS measurement, Co(10 nm)/SiN(5 nm)/Si(100)-substrate were used. SiN was formed by low-pressure thermal CVD at 725 oC on Si substrate, and after that Co was formed by sputtering. Here, Co was used because it is easy to detect the oxidizing reaction at Al2O3/Co interface. Al2O3
films were deposited on Co by 11-cycles-ALD. Here, by 11-cycles-ALD, Al2O3film (3 nm) was deposited on Si.
Results and Discussion
Figure 1 shows the capacitance-voltage (C-V) curves of MIS capacitors. Four samples for each were measured. In the case of Al2O3 without PDA, flat band voltage (Vfb) is negative value such as around -2.0 V. It is suggested that the positive charges (1.87×1012 cm-2 ~ 2.53×1012 cm-2) exist in Al2O3 film. After PDA-I, Vfb becomes almost same value such as 0.1 V and the negative charges (1.12×1012 cm-2 ~ 1.17×1012 cm-2) exist in Al2O3 film. In addition, Vfb variability drastically decreases. It is considered that O2 annealing at 400 oC is very effective to improve Al2O3 film quality [5]. On the other hand, after PDA-II, Vfb drastically shifts to the positive direction by the negative charges (2.96×1012 cm-2 ~ 8.61×1012 cm-2) and its variability is large. Moreover the slope of C-V curve with PDA-II is smaller than that without PDA because of increasing the interface trap. As a result, it is considered that the interfacial quality became poor by Ar/O2plasma oxidation in this experiment.
Figure 2 shows the energy spectra of 450 keV He+ ions backscattered from Al2O3/Co/SiN/Si. Al2O3films without PDA, with PDA-II 20sec, 60sec and 300sec were measured. Several observed peaks indicate the energy of He ions scattered by Co, Al, Si, O and N atoms, respectively. Figure 3 (a)-(c) show the narrow spectra related to O, Al and Co, respectively. Especially, in the case of PDA-II 300 sec, each spectrum in fig. 3(a)-(c) spreads more widely. It is suggested that Al and Co interdiffuse at the interfacial layer during PDA-II 300sec. As a result, it is suggested that the intermediate oxide states consisting of Al-Co-O are formed.
Conclusion
We investigated the influence of PDA to the Al2O3 interface. O2 annealing at 400 oC is very effective for improving the Al2O3 film quality. However, by Ar/O2 plasma oxidation, it is considered that Al and Co interdiffuse at the interface. It is expected that a similar phenomenon is also caused with Al2O3/Si interface, then it is considered that the intermediate oxide states consisting of Al-Si-O cause the electrical characteristic degradation such as increase of fixed charge, interface trap and variability.
Acknowledgments
This work was carried out at fluctuation free facility of New Industry Creation Hatchery Center, Tohoku University.
Reference
[1] P.D. Ye et al., Appl. Phys. Lett., 86(2005) 063501
[2] A. Farcy et al., Microelectronic Eng., 85(2008) 1940-1946
[3] R. L. Puurunen, J. Appl. Phys. 97(2005) 121301
[4] H. Sugita, et al., ECS Trans, 66(2015) 305-314
[5] Y. Koda, et al., ECS Trans., to be published at 229thECS meeting
[6] K. Sekine, et al., IEEE TRANS. ON ELECTRON DEVICE, 48 (8) (2001) 1550

Figure 1