Electrical and Structural Properties of ZrO2/Y2O3/ZrO2 Dielectric Film for DRAM Capacitor

Abstract

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.