1. Field of the Invention
The present invention relates to a method for etching an electrode material for a capacitor using a ferroelectric film, and more particularly to a method for etching platinum (Pt) film.
2. Description of the Related Art
In recent years, ferroelectric non-volatile memory devices, using as a capacitor a ferroelectric thin film with spontaneous polarization, have been extensively developed. Examples of a material for the ferroelectric thin film include oxides such as PZT (PbZr.sub.x Ti.sub.1-x O.sub.3, lead titanate zirconate), PbTiO.sub.3 (lead titanate), BaTiO.sub.3 (barium titanate), and PLZT (Pb.sub.x La.sub.1-x Zr.sub.y Ti.sub.1-y O.sub.3, lanthanum lead titanate zirconate).
In a case where a ferroelectric thin film is utilized for a ferroelectric non-volatile memory device, a Pt film is generally used as an electrode material because of its suitability for crystal growth of the ferroelectric film thereon. In the ferroelectric non-volatile memory device, memory cells are highly integrated, which necessitates microprocessing of the Pt film.
Conventionally, the Pt film has been processed by wet etching with aqua regia, ion milling with argon, etc. However, the microprocessing as described above is difficult to conduct by wet etching. As for ion milling using an inert gas (e.g., argon), there are problems such as a low rate of etching, contamination of the Pt film caused by re-adhering of etched material to the surface of the Pt film, and a pattern size shift caused by re-adhering of etched material to the side walls of the remaining portions of the Pt film after etching. In order to avoid such re-adhering of etched material, the amount of ions which are vertically incident upon a ferroelectric film is decreased by tapering the side walls of the remaining portions of the ferroelectric film after etching (e.g., see Japanese Laid-Open Patent Publication No. 5-109668). In this case, because of the tapered side walls, each line width of memories made of the ferroelectric film cannot be controlled with good precision. As means for overcoming such conventional problems involved in microprocessing, dry etching techniques have been studied (Nishikawa et al., Jpn. J. Appl. Phys. Vol. 32, 6102-6108 (1993); The Japan Society of Applied Physics 30a-ZE-3, Spring Meeting in 1993). Specifically, dry etching techniques for etching a Pt film with chlorine (Cl.sub.2) gas or hydrogen bromide (HBr) gas using a resist have been studied. However, according to such dry etching using a resist, a selection ratio between a resist and Pt is low, so that the resist is also partially etched together with the Pt to be tapered, causing a pattern size shift.
The problems involved in the prior art will be described with reference to FIGS. 4A through 4C. In these figures, the reference numeral 21 denotes a silicon substrate, 22 an NSG film, 23 a Ti film, 24 a TiN film, 25 a Pt/Ti film, 26 a Pt film, 27 a PZT film, 28 an etching mask, 29 remaining Pt, 30 side wall deposited films, and 31 a residue. For example, as shown in FIG. 4A, the PZT film 27 is patterned, and then, the Pt film 26 and the Pt/Ti film 25 are dry-etched with Cl.sub.2 gas. In this case, the selection ratio between the resist and Pt is low, so that part of the Pt remains (remaining Pt 29) as shown in FIG. 4B. When etching is further conducted, the remaining Pt 29 works as an etching mask to cause a residue 31 as shown in FIG. 4C.
As described above, conventional microprocessing techniques of the Pt film respectively have problems.