High-k material is becoming important to manufacture of integrated circuits [1-3]. Especially, Al2O3 is expected for the gate dielectric film of power devices [4] and Metal Insulator Metal (MIM) capacitor [3]. Atomic Layer Deposition (ALD) is an effective method for forming conformal thin dielectric films. In a recent study, using Al2O3 film formed by ALD at low temperature, less variation in the electrical characteristics could be obtained compared with that formed at high temperature [3]. However, large flat-band voltage shift was shown because of low oxidation ability by H2O at low temperature. In order to solve this problem, it is shown that Post Deposition Process (PDP) is an effective way [3]. It is considered that the microwave excited plasma oxidation is candidate for PDP because it has the strong oxidizing capability [5]. However, the electrical characteristics were degraded because the Al2O3/substrate interface was oxidized by the excess strong plasma oxidation in former study [6].
In this paper, we report the effects on the microwave excited Ar/O2 plasma treatment to the electrical characteristics of Al2O3MIS capacitor. ALD and PDP were carried out continuously in the same chamber at low temperature to suppress the oxidation.
Experiment
MIS capacitors were fabricated on n-type Si(100) wafers with resistivity of 8-12 Ωcm. The active region was formed by thermal oxidation and etching using the buffered HF with surfactant. After the native oxide on the active region was removed by 0.5% diluted HF, 10nm Al2O3 was formed by Trimethylaluminium (TMA)/H2O ALD process at stage temperature of 75oC. The microwave excited Ar/O2 plasma treatment was carried out as PDP just after the ALD-Al2O3 formation in the same chamber. The PDP conditions are as follows: stage temperature was 75oC, process time was 1min, Ar flow rate was 200sccm and 2.45GHz microwave power was 800W. The pressure and O2flow rate at PDP were varied as experimental parameter. We choose the weaker oxidation capability of all PDP conditions than former one [6] in this experiment. Finally, aluminum films were deposited as top and bottom electrode by evaporation.
Results and Discussion
Figs.1(a) and (b) show C-V characteristics for the case of bias swept negative to positive and positive to negative, respectively. From Fig.1(a), flat-band voltage without PDP is -2.23V. It indicates the existence of positive fixed oxide charges. In all samples with PDP, flat-band voltages are improved (-1.71~-0.95V) and capacitance densities are increased slightly. It is considered that Ar/O2 treatment is effective to improve the quality of Al2O3 film. However, from Fig.1(b), the hysteresis exists regardless of PDP or not. Therefore, the trap density in Al2O3film cannot be completely vanished by these PDP conditions.
Figs.2(a) and (b) show I-V characteristics and band diagram when the positive bias is applied to top electrode. Similarly, Figs.3(a) and (b) show ones when the negative bias is applied. Figs.2(a) and 3(a) show that PDP is effective to decrease the leakage current in both bias case. Especially, the leakage current in the negative bias case is lower than positive bias case. These results imply that the Al electrode side of Al2O3 film is intensively improved as shown dark region of Figs.2(b) and 3(b) by Ar/O2 treatment, however, the quality near the Si/Al2O3 interface could not be improved perfectly. It is considered that the PDP is insufficient to improve the interface, and the process condition or the process step should be changed to improve all region of Al2O3film.
Conclusion
We investigated effects on the microwave excited Ar/O2 plasma treatment to electrical characteristics of Al2O3 MIS capacitor. As a result, using PDP conditions having relatively lower oxidizing ability than the former one [6], we found that the electrical characteristics of Al2O3 film can be improved such as the flat-band voltage (that is the fixed oxide charge), the capacitance density and the leakage current. However, the treatments could not improve the whole Al2O3 film, especially the interface. Therefore, Ar/O2 treatment is effective to improve the quality of Al2O3film, however, more improvement is required.
Acknowledgement
This work was carried out at fluctuation free facility of New Industry Creation Hatchery Center, Tohoku University.
Reference
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