This invention relates generally to ferroelectric films. More particularly, the present invention relates to a ferroelectric film having improved electrical characteristics and the corresponding fabrication method for producing the ferroelectric film.
Ferroelectric films are typically used as the dielectric material for a ferroelectric capacitor in a ferroelectric memory cell. For proper memory operation, it is critical that the ferroelectric film achieves desirable electrical performance evidenced by the ability to liberate a detectable charge in response to the application of an externally applied electrical field. It has been shown in the laboratory that crystal growth characteristics and film orientation are critical to achieving this desirable electrical performance. Typically, the ferroelectric film is deposited on a conducting electrode surface, such as the bottom electrode of the ferroelectric capacitor. The bottom electrode can be fabricated from a variety of films such as: platinum, iridium, iridium oxide, ruthenium oxide, palladium, as well as other noble metals and their oxides, or other suitable conductive materials known in the art. Each of these electrodes has a unique surface with changing roughness, and/or conductivity, which dramatically impacts the sticking coefficients for the various components of a subsequently deposited ferroelectric film. As the sticking coefficients vary, the composition of the ferroelectric film varies. As the electrode surface begins to accept a layer of ferroelectric film, the sticking coefficients for the constituents change and dictate the composition of the bulk film. Therefore, the composition required for optimum electrical performance cannot be obtained using a single fixed set of sputtering conditions or solution chemistry.
For subsequent comparison to the improved ferroelectric film of the present invention, a typical prior art ferroelectric PZT film (lead zirconate titanate) having a thickness of about 2400Angstroms and deposited with a single step deposition has roughly 75% of the crystal domains oriented in the &lt;111&gt; crystal orientation. The "switched charge" liberated using a three-volt pulse ["Qsw(3 v)"] was measured at about 21 micro-Coulombs per centimeter squared. The applied voltage at which the switched charge of the ferroelectric capacitor is 90% saturated ["V(90%)"] was measured at about 4.5 volts.
What is desired is a ferroelectric film having optimized electrical performance in which specific adjustments in deposition conditions are made to compensate for the electrode variables described above and to dictate a desired film composition profile.