Investigation of Strong Metallic Ta Reduction in ZrO2/Ta2O5 Multi-Laminate Layer Growth

Introduction

  High permittivity (high-k) dielectrics such as HfO₂, ZrO₂, TiO₂, La₂O₃ and Ta₂O₅ have been extensively researched to apply for gate oxide and MIM capacitors [1-5]. Especially, the use of laminates or mixtures of upper mentioned dielectric materials allows enhanced capacitance density and lowered leakage currents. Jõgi et al. revealed tetragonal ZrO₂ become to be stabilized in Ta₂O₅-ZrO₂ laminate system when the laminate thickness exceeded 50nm and allows higher charge storage factors [6]. For the continuing down-scaling of integrated circuit dimensions, however, it is difficult to use such thick layer as gate oxide or capacitor dielectric.

  In this paper, thin ZrO₂-Ta₂O₅ multi-laminate layers under 10nm were grown by atomic layer deposition (ALD) method and it was concisely discussed about the crystallization and metallic Ta reduction at the interface between ZrO₂ and Ta₂O₅.

Experimental

  A 10nm-thick ZrO₂-Ta₂O₅ multi-laminate layer was fabricated on TiN (20 nm)/SiO₂ (100 nm)/Si by ALD method. The ZrO₂-Ta₂O₅ multi-laminate layer consisted of 12 times laminate process of (6 ZrO₂+2 Ta₂O₅). After the growth, rapid thermal process (RTP) was followed at 550 ºC during 120s. This sample was measured by high-resolution X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) for the analysis of crystallization and chemical bonding status.

Results and Discussion

 From the results of XRD analysis, thin ZrO₂-Ta₂O₅ multi-laminate layer was well crystallized after the RTP anneal of 550 ºC, and stabilized tetragonal ZrO₂ peak was clearly detected (not shown here).

A 10nm-thick ZrO₂-Ta₂O₅ multi-laminate sample with RTP was analyzed by XPS measurements and it was clearly confirmed that there existed metallic Ta 4f_7/2 peak as shown in Figure 1 (green). The samples of as-grown Ta₂O₅ layer (black), Ta₂O₅ layer with 550 ºC RTP anneal (red) and as-grown ZrO₂-Ta₂O₅ layer (blue) were also analyzed for the reference. In case of the samples of as-grown Ta₂O₅ layer and Ta₂O₅ layer with 550 ºC RTP anneal, the metallic Ta 4f_7/2 peak was not detected. On the other hand, metallic Ta was detected from the as-grown ZrO₂-Ta₂O₅ multi-laminate sample. From these results, it is thought that the metallic Ta was reduced by the reaction with as-coming Zr source (and/or ZrO₂), not by thermal decomposition.

  Next, dependency of Gibbs free energy on the growth temperature for the reactions between Zr atom and Ta₂O₅ was shown in Figure 2. From the results, it was clearly known that the delta Gibbs energy had negative values in case of the reaction between Zr and Ta₂O₅, which means to prefer the reduction of Ta₂O₅ to metallic Ta when Zr source came after the deposition of Ta₂O₅.

 In out thin ZrO₂-Ta₂O₅ multi-laminate process, its dielectric constant showed 70~100 (depend on the layer thickness), which is higher dielectric constant than the reported values of ZrO₂ and Ta₂O₅. It is thought that this significant high dielectric constant is related to the metallic Ta reduction and precise mechanism is under studying.

Conclusions

 A grown 10nm-thick ZrO₂-Ta₂O₅ multi-laminate layer was analyzed by XPS measurements, and metallic Ta reduction was confirmed. These results were supported by the Gibbs free energy calculations. Furthermore, the relation between high dielectric constant and metallic Ta reduction is under studying.