Structure of Pr Oxide Films on Si Deposited by Reactive Sputtering
Authors: Kumagai Kenta , Yamaguchi Kouta , Hara Kenta , Suzuki Setsu , Ishibashi Keiji , Yamamoto Yasuhiro 
Published: February 2020

Down scaling of metal-oxide-semiconductor (MOS) devices has led to the drastic increase in leakage current through the gate oxides. This problem has forced us to replace the SiO2 gate dielectric by that with higher dielectric constant (high-k) 1,2). In the device application, the interface with Si plays a key role and in most cases is the dominant factor in determining the overall electrical properties1). For most actively investigated high-k materials typified by Hf oxide, however, the SiO2interfacial layer can be unavoidably formed due to the interface reaction, leading to the increase of the EOT. Praseodymium oxide forms an interfacial silicate layer which has a higher dielectric constant than that of SiO2
3,4). In this work, we will report the structure of Pr oxide films deposited by the sputtering method as well as their chemical states of the elements composing the film based on the analyses by cross-sectional transmission electron microscope (XTEM), X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS).
After organic removal cleaning, p-type Si (100) substrates were dipped in the diluted HF solution (4 %) for 2 min to remove the native oxide and leave the Si surface hydrogen-terminated.
Praseodymium oxide thin films were deposited by reactive RF magnetron sputtering at room temperature in an ambient of Ar and O2 with the O2 flow rate fraction of 10 % at 1.5 Pa with the RF-power of 50 W using the 2” Pr metal target. The Si substrate was set at the off-centered position with respect to the sputtering cathode to avoid re-sputtering by the O- ions from the target surface5). Deposited film thickness evaluated by XTEM was typically 35 nm. Deposited films were post-annealed in an ambient of nitrogen (N2) for 30 minutes at temperatures of 300, 500 and 800 °C.
In the XRD patterns obtained with the fixed grazing incident angle of 2 °, the dominant diffraction line of cubic PrO2 was observed in the course of the annealing up to 500 °C. After annealing at 800 °C, rearrangement of the crystalline structure occurred; the new diffraction line corresponding to hexagonal Pr2O3appeared. In fact, the phase transition was observed from cubic to hexagonal structure. In the XPS spectra observed at the interface between Pr oxides and Si substrates, the peak position of Pr3d5/2 consistently shifted to lower binding energies from 934.4 to 933 eV with increasing annealing temperature up to 800 °C. The Si2p spectrum was observed at lower binding energies than 103.9 eV corresponding to the SiO2. The O1s spectrum was found to be composed by two components at 530.3 eV of Pr2O3 and 531.7 eV of the Pr silicate6). The Pr silicate spectrum faintly observed in the as-deposited film became dominant with increasing annealing temperature. The XPS results revealed that the formation of the Pr silicate progressed as increasing annealing temperature. The XTEM image of deposited films revealed that the SiO2 layer was formed at the interface between the as-deposited film and the Si substrate, although the interfacial SiO2 layer was not detected in the in-depth profile by XPS analysis. The reason for this is probably the surface roughing during in-depth profiling by Ar sputter etching. After annealing at 800 °C, the film structure was completely changed from those after lower temperature annealing; the interfacial SiO2 layer was lost, and the Pr oxide film had three-layered structure composed by the amorphous silicate layer, the Pr2O3layer having poly-crystal islands located with equal spacing and the continuous columnar crystalline layer from the interface to the surface.
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