ZrO₂/Al₂O₃/ZrO₂ (ZAZ) structure is widely used as a capacitor dielectric film of Dynamic Random Access Memory (DRAM). This is a structure in which Al₂O₃ film is added as an interlayer to improve the leakage current in the ZrO₂ material which can meet the dielectric constant and leakage characteristics required by the device. Nonetheless, the added Al₂O₃ film decreases capacitance density compared with a capacitor composed of the single ZrO₂ layer, because the Al₂O₃ film itself has a low dielectric constant and the upper ZrO₂ film is not crystallized properly. Although this ZAZ structure has been extensively used to meet the device requirements of DRAMs down to the design rule of ~20nm, it is difficult to further use it for the DRAM with design rule lower than 20nm. Accordingly, there are active research and studies in progress to discover a new dielectric material to replace the ZAZ structure.

In this study, Y₂O₃ was used to replace Al₂O₃ to suggest a strategy to increase the total dielectric constant while maintaining the role of improving the leakage characteristics performed by the Al₂O₃ film. The dielectric constant and bandgap of Y₂O₃ are higher than that of Al₂O₃ and that of ZrO₂, respectively, and it is also known as a material stabilizing the phase of ZrO₂ into high-k phase.[1] Therefore, Y₂O₃ is considered a promising candidate to replace Al₂O₃.

The evaluation was carried out using Y(ⁱPrCp)₂(ⁱPr-amd) precursor, synthesized by Air Liquide Co., which is liquid at room temperature, and O₃ as the oxygen source. The saturated deposition rate of the Y₂O₃ thin film was ~0.14nm/cycle when it was grown by an atomic layer deposition (ALD) method. Cubic phase Y₂O₃ film was observed by X-ray diffraction.

For comparison, ZAZ and ZrO₂/Y₂O₃/ZrO₂(ZYZ) were deposited on TiN substrate by ALD method, and their electrical and physical properties were evaluated. As expected, the equivalent oxide thickness could be decreased by ~0.2nm at a physical thickness of ~10nm, while the leakage current level remained almost invariant (~9 X 10^-10A/cm² to ~4 X 10^-9A/cm² at 0.8V) when the Al₂O₃ was replaced with the Y₂O₃ layer. The effect of heat treatment on the crystal structure of ZrO₂ in ZAZ and ZYZ dielectric films and the electrical properties according to variations in their thickness were also studied. These results are promising as the newly suggested ZYZ structure may replace the present ZAZ without any major changes in electrode and integration strategy.

References [1] Bo-Eun Park et al., Journal of Alloys and Compounds 722 (2017)