Recently, lead-based ferroelectric thin films have been actively studied for potential application to the charge storage capacitors of ultra large scale integration (ULSI) dynamic random access memories (DRAMs) and nonvolatile ferroelectric random access memories (NVFRAMs). It has been reported that (Pb,La)(Zr,Ti)$O_3$ (PLZT) films have a high dielectric constant, good leakage current characteristics and fatigue resistance. It is very important to control the composition of PLZT films to obtain single-phase perovskite and desirable electrical properties. However, it is very difficult to fabricate multicomponent films, such as perovskite PLZT film, by chemical vapor deposition(CVD) due mainly to the difficulty of composition control. Therefore, in this research, I studied on the fabrication and characterization of electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD)(Pb,La)(Zr,Ti)$O_3$ thin films for the charge storage capacitors of ULSI DRAMs. Lanthanum-modified lead zirconate titanate (PLZT) thin films were fabricated by ECR PECVD using MO sources.
The composition of the PLZT film could be controlled by varying the flow rate of each MO source. Perovskite PLZT thin films with smooth surface and fine grains were successfully fabricated on Pt/Ti/$SiO_2$/Si at 450℃. Deposition mechanism of the PLZT film in the ECR plasma was studied by examining the incorporation rate of each cation in the film at various MO source flow rates. Microstructure of the deposited films and Rapid thermal annealing (RTA) treatment effect were also evaluated.
The composition and the electrical properties of the films were investigated as a function of deposition temperature and Pb-source flow rate. As the deposition temperature was increased, the vapor pressure of lead oxide molecules increased and thus the PLZT film tended to be Pb-deficiency, resulting in the formation of non-perovskite phase and the degradation of electrical properties. This problem could be relieved by increasing Pb- source flow rate. RTA treatment greatly enhanced the electrical properties of the PLZT film as far as the as-deposited film had stoichiometric composition and pure perovskite structure.
The characteristics of PLZT films for various La concentrations were investigated for the application to the charge storage capacitor of a Gbit-scale DRAM. The La doped films showed paraelectric-like slim loop P-V hysteresis curves and almost no frequency dependence of permittivity. The doping of La made the film surface very smooth. After rapid thermal annealing at 650℃ for 1 min, the capacitance as well as the leakage current characteristics of the films were greatly improved. The 50nm thick PLZT(5.5/56/44) film had an effective capacitance of 139 fF/μ㎡ and an $SiO_2$ equivalent thickness of 0.25 nm. Perovskite PLZT films with excellent electrical properties were successfully fabricated at low temperature using ECR-PECVD method for the charge storage capacitor of a Gbit-scale DRAM.
PLZT (8/67/33) thin films were deposited on both Pt/$SiO_2$/Si and Pt/Ti/$SiO_2$/Si substrates for various deposition times. The nucleation and growth mechanism and the thickness dependence of the dielectric constant of PLZT films were investigated. For the Pt/$SiO_2$/Si substrate, nuclei of both non-perovskite and perovskite phases formed on the film surface and grew together. In this case, long incubation time for the nucleation was needed because suitable nucleation sites were not supplied. However, for the Pt/Ti/$SiO_2$/Si substrate, nuclei of a perovskite phase dominantly formed at the Pt grain boundary and grew. In this case, incubation time of the nucleation was scarcely needed because the nucleation sites were supplied by the out-diffused Ti through Pt grain boundary. The dielectric constant of the bulk of the PLZT film was 1420. Apparent dielectric constant decreased with decreasing film thickness due to the effect of the low dielectric interfacial layer.
The effects of seed layers on the characteristics of PLZT thin films were investigated. The seed layers used were lead oxide, titanate, lead titanate and La-doped lead titanate. The effects of the out-diffused Ti from the substrate were also investigated by using both a Pt/$SiO_2$/Si substrate without a Ti layer and a Pt/Ti/$SiO_2$/Si substrate with a Ti layer. By using titanate, lead titanate and La-doped lead titanate as seed layers, PLZT films with stoichiometric composition and perovskite single phase could be fabricated. As the out-diffused Ti from the Pt/Ti/$SiO_2$/Si substrate already acted as a seed, the growth rate, composition and crystal phase of the PLZT films deposited on the Pt/Ti/$SiO_2$/Si substrate were less affected by the seed layers than those of the PLZT films deposited on the Pt/$SiO_2$/Si substrate. La-doped lead titanate was the most effective seed layer in surface morphology, crystallinity and electrical properties of the PLZT films deposited on both substrates.
