Sol-gel processing provides an interesting alternative method for the fabrication of thin ferroelectric layers which is useful in nonvolatile memory devices applications.
Much of the recent sol-gel ferroelectric thin film research has been directed at the PbO based perovskite compounds such as $PbTiO_3$, PZT. The microstructure and crystallization and C-V characteristics of lead titanite thin films an a silicon substrate were highly dependent on the sol-gel processing methods, the substrate materials, film thickness, heat heating conditions. So, in this study, we found optinum conditions to fabricate high quality dried gel films. When dried gel films were heat treated, microstructural and crystallizational aspects of lead titanate thickness and solution catalyst were also considered.
The solution formed should have a high concentration of the necessary components and be stable under ambient conditions. With these considerations, lead acetate trihydrate and titanium isopropoxide in methoxyethanol were used as precursor materials. The lead acetate solution was dehydrated by successive distillations, until the new distillate was anhydrous. Stock solutions were formed by combining the lead and titanium precursor solutions. Thin films were cast on polished (100) single crystal silicon wafer at 3000r.p.m. using a photoresist spinner. Coating solutions were syrigned through in-line filters directly onto the substrates in clean-room conditions(25℃, 50% relative humidity, class 100)
Lead acetate trihydrates were partly methoxyethylated and trihydrates were partly nemoved (about 70%). Titanium isopropoxide underwent complete transesterifications by methoxyethanol. Pb. Ti complex alkoxide were formed by mixing Pb precursor and Ti precursor, and stable for coating in 1 molar concentration.
Using coating solution(0.5 molar concentration, $R_w=2$, viscosity 1 cps), Crack free and uniform dried gel films with thickness of 0.2㎛ were formed by coating conditions (temp. 25℃, 50% relative humidity, class 100 clean bench, 3000 r.p.m.)and rapid drying (at 215℃, for 5min).
Dried gel films with thickness of 0.36㎛ could be heat treated(heating rate 0.5℃/min.) at 500℃ for 60min. to make crack free lead titanate films. But dried gel films with thickness of 0.68㎛ were cracked by same heat treatment. So, drying and heat treating conditions should be improved.
Uncatalyzed dried-gel films with thickness of 0.12㎛ were crystallized to perovskite structures by heat treatment above 600℃ Heat treated films were inhomogeneous in microstructures, with flat band voltage(in capacitance voltage plot) and dielectric constant of 0.10V and 8.5, respectively. But 0.1M nitric acid catalyzed dried-gel films were crystallized to pyrochlore structures by heat treatment above 500℃. Heat treated (at 600℃) films were homogeneous in microstructures, with flat band voltage of -0.40V. It means that films are stable in oxide semiconductor structures. Dielectric constant was decreased to 2.8 because of difference in crystal structures.
Dried gel films with thickness of 0.36㎛ were crystallized to anisotropic perovskit structures by heat treatment above 500℃ regardless of catalyzing. Uncatalyzed films heat treated at 600℃ for 60min. were inhomogeneous in microstructure, with c/a ratio in perovskite structure, flat band voltage, and dielectric constant of 1.046, 0.75V, and 33, respectively. But in case of 0.1M nitric acid catalyzed films, heat treated films were homogeneous in microstructure, with c/a ratio, flat band voltage, and dielectric constant of 1,054, -0.45V, and 46, respectively. By catalyzing, tetragonality was increased, and stable films were formed.
For films with thickness of 0.12㎛(heat treated at 500℃ for 60min), grain growth were constrained by substrate, and grain size distributions were bimnodal, and perovskite structures were not crystallized. But, for films with thickness of 0.36㎛, the degree of constraint of grain growth and bimodal grain size distributions were reduced, and perovskite structures were easily crystallized. So, it is desirable to apply to microelectronics that the thickness of lead titanate thin films is thicker than 1㎛.
