Electrical Properties of Al2O3 Incorporated CeO2 Thin Films Deposited by RF Magnetron Sputtering
Authors: Konishi Junya , Ohsawa Takashi , Suzuki Setsu , Ishibashi Keiji , Ri Sung-Gi , Takahashi Kenichiro , Yamamoto Yasuhiro 
Published: February 2020

High-k materials have been widely investigated as a new gate stack in metal-oxide-semiconductor (MOS) devices1,2) because leakage current through the conventional SiO2 gate increases as scaling down of MOS devices. Among them, CeO2 is one of the most noteworthy materials3) because of its high dielectric constant of 26, chemical stability and the promising interfacial properties owing to the compatibility with Si in terms of the crystal structure and the lattice constant: the cubic fluoric structure, lattice mismatch as small as 0.35%.

Cerium dioxide is, however, likely to crystalize even just after RF magnetron sputtering deposition at room temperature, leading to the increase of the leakage current along with grain boundaries4). In our preliminary study, we found that the thin films of Al2O3 doped CeO2 (Al2O3 as 10% composition) was kept amorphous after annealing up to 500 °C in a N2 atmosphere.

In this work, we will report the electrical properties of the Al2O3 incorporated CeO2 to suppress the poly-crystallization of the films as a function of the wide Al2O3 composition range. The oxide composite of CeO2 and Al2O3 was prepared by the RF magnetron sputtering equipped with the combinatorial mask system5). The deposition was carried out using 2 targets of CeO2 and Al2O3 with the applied RF powers of 150 and 200 W, respectively, in an Ar atmosphere with an O2 introduction (10%) at room temperature at a pressure of 0.5 Pa. We prepared the combinatorial composition spread thin films, with the total thickness of 32 nm, in which Al2O3 composition ratio was changed in the range of 0 ~ 60% against CeO2 in a sample. In the deposition process, we repeated the following procedures 80 times; first, 0.16 nm thick CeO2 layer deposition: second, a wedge shaped CeO2 layer deposition with a thickness of 0 to 0.24 nm by the moving mask: finally, a wedge shaped Al2O3 layer with a thickness of 0 to 0.24 nm by the mask moving to the counter direction. After the deposition, we prepared 2 kinds of samples; one was annealed in a N2 atmosphere, the other in an O2 atmosphere at 500­­ °C for 30 minutes. Then, Pt dot electrodes were formed using the metal mask with the 100 µm in diameter openings on the surface of the annealed samples by sputtering. The electrical properties were characterized by I-V and C-V measurements.

For the sample annealed in a N2 atmosphere, the leakage current density obtained from I-V characteristics at the electric field of -1 MV/cm decreased from around 1.0 × 10-6 to the minimum value of 1.0 × 10-8 A/cm2 with increasing Al2O3 composition from 0 to 20%. When the Al2O3 composition increased above 20% up to 60%, however, the leakage current density approached back to the value without Al2O3 doping. For the O2 annealed sample, the annealing effect was limited; the minimum leakage current density was around 1.0 × 10-7 A/cm2 at the Al2O3 composition of 10%. The leakage current density was greater than that without Al2O3 doping at the doping level excess 20%. From the result of C-V measurement at 1 MHz, the estimated dielectric constant of the films, assuming the interfacial SiO2 layer with a thickness of 3.5 nm, was rapidly decreased from 18 to 8 with the increase of the Al2O3 composition from 0 to 15%. For the higher Al2O3 composition than 15%, the estimated dielectric constant represented a constant value of 8. Judging from both view points of the leakage current and the dielectric constant, the optimum Al2O3 composition was considered to lie around 8%. Compared with the annealing in N2, the O2 annealing was not effective in improving electrical properties.

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