Study on Properties and Their Applications of La2O3 and CeO2 Films Grown by Atomic Layer Deposition
- Study on Properties and Their Applications of La2O3 and CeO2 Films Grown by Atomic Layer Deposition
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- Over the last four decades, the aggressive scaling of complementary metal oxide semiconductor field effect transistor (CMOSFET) devices has led to the limitations of conventionally used SiO2-based gate dielectric since its very thin thickness resulted in significant problems such as large leakage current. From this point of view, it is necessary to replace the SiO2 with high-k dielectrics enabling physically thicker gate oxides. Additionally, for the requirements of thickness controllability down to few nm range, large area uniformity and excellent conformality, high-k thin film should be grown by atomic layer deposition (ALD) method.
Among several high-k dielectric materials satisfying the considerations for their introduction, lanthanum oxide (La2O3) and cerium oxide (CeO2) are very interesting and promising candidates. They themselves have received a great deal of attention as the high-k dielectric materials in dynamic random access memory (DRAM) capacitors as well as CMOS devices due to their superior properties such as the high dielectric constant, high dielectric breakdown strength and large band gap and conduction band offset with Si as well as thermodynamical stability in contact with Si. In addition, their particular applications into Hf-based dielectric can be the promising solutions for the challenging issues such as work function engineering and further equivalent oxide thickness (EOT) scaling required in the advanced gate stack technologies. Nevertheless, current researches on ALD of La2O3 and CeO2 are still in their infancy. Therefore, in this thesis, I will describe the overall studies on ALD of the La2O3 and CeO2 from their ALD processes and properties to potential applications into Hf-based dielectrics.
In the first and second sections (chapter III and IV), I investigated thermal and plasma-enhanced atomic layer deposition (T-ALD and PE-ALD, respectively) of La2O3 thin films by using tris(isopropyl-cyclopentadienyl)lanthanum [La(iPrCp)3] precursor with H2O and O2 plasma. The growth characteristics, film properties and electrical properties were discussed by several analysis techniques, mainly focusing on the comparative studies. It revealed that PE-ALD La2O3 exhibited the higher growth rate, better film quality and superior electrical properties that those of T-ALD.
Then, I investigated the flat band voltage (VFB) modulation by insertion of the PE-ALD La2O3 capping layer into HfO2 gate dielectrics for effective work function engineering. The location of La2O3 insertion layer in the HfO2 were precisely modulated at bottom, middle or top to clarify which location plays the dominant role for effective work function modulation by the interface dipole layer. Thereby, I proposed that the main mechanism of VFB modulation by La2O3 capping layer is dipole moment formation at the interfacial layer between high-k dielectric and Si substrate.
In the following sections (chapter V and VI), I investigated PE-ALD of CeO2 thin films by using newly synthesized tris(isopropyl-cyclopentadienyl)cerium [Ce(iPrCp)3] precursor with O2 plasma for the first time. After the evaporation characteristics of the Ce(iPrCp)3 precursor by simultaneous thermogravimetric-differential thermal analysis (TG-DTA) measurements, the growth characteristics, film properties and electrical properties were evaluated by several analysis techniques. It suggested that PE-ALD CeO2 has great benefits as the high-k gate dielectric.
Then, systematic investigations were conducted to comprehensively demonstrate the energy band diagram including the electron carrier transports for Al/PE-ALD CeO2/p-Si structure by combining ellipsometry and photoemission spectroscopy techniques with Fowler-Nordheim (F-N) tunneling and Pool-Frenkel (P-F) conduction. Thereby, I proposed that the increased interfacial layer and reduced trapped oxide densities result in the increased effective barrier height and decreased trap energy levels with increasing O2 annealing temperature.
In the final section (chapter VII), T-ALD of CeO2 thin film was investigated by using tetrakis(1-methoxy-2-methyl-2-propanolate)cerium [Ce(mmp)4] with H2O. After establishing the T-ALD CeO2 process, the effects of Ce doping into HfO2 gate dielectric were systematically investigated for significant enhancement of dielectric constant. For the realization of CeO2 doping into HfO2, ALD supercycles process was carried out. The growth characteristics, film properties and electrical properties were evaluated by several analysis techniques with a variety of Ce/(Ce+Hf) compositions. Thereby, I proposed that the maximum dielectric constant value was found to be ~39 for the Ce0.11Hf0.89O2 film with dominant tetragonal phases.
Therefore, my research in this thesis will not only provide many valuable information and technical methods on ALD La2O3 and CeO2 gate dielectrics and their applications but will also extend essential and fundamental degree of freedom for their practical implementation toward the advanced gate stack technologies in the future microelectronic industry.
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