Development of vacuum ultraviolet multichannel ellipsometry and its application to the characterization of ultrathin zirconium oxide films

Vacuum UV spectroscopic ellipsometry (VUV SE) was developed with a multichannel detection system. This system can cover up to 8.5 eV with brief nitrogen purge through optical path. We applied this technique to the characterization of zirconium oxide (ZrO2) films for which the conventional spectroscopic ellipsometer fails due to the poor sensitivity. ZrO2 is one of the high-k dielectrics which can be used for the storage capacitor in dynamic random access memory devices. However, due to the large leakage current of ZrO2, thin layer of aluminum oxide (Al2O3) is sandwiched between two ZrO2 layers forming 'ZrO2(top)/Al2O3/ZrO2(bottom)'. This structure also prevents the reaction between Al2O3 and electrodes during deposition. As overall thickness of this structure is less than 10 nm, the thickness and the properties of each layer need to be controlled precisely in order to fabricate a well-defined capacitor. However, the optical properties of ZrO2 films are highly dependent on the preparation process and conditions. Thus, many considerations are required for the analysis of VUV SE data. Through the complicated analysis, we find that the optical properties of the bottom ZrO2 film are dependent on its own thickness as well as the deposition temperature for the subsequent Al2O3 layer. Moreover, those of the top ZrO2 layer showed the dependence on the crystalline structure of the bottom ZrO2 along with the thickness of interfacial Al2O3 layer.