Ferroelectric based capacitors are increasing in demand as integrated circuit elements. Capacitors having the lead titanium zirconate (PZT) family of dielectrics offer large dielectric constants which, in turn, make the construction of small capacitors with relatively large capacitances possible. A ferroelectric capacitor consists of a PZT layer sandwiched between two planar electrodes. Capacitors utilizing platinum electrodes are particularly advantageous, since such capacitors exhibit excellent crystallinity compared to capacitors utilizing other electrode materials.
Ferroelectric based field-effect transistors are also known to the art. These transistors have a structure which may be viewed as a capacitor in which the top electrode has been replaced by a semi-conductor layer having two separated contacts corresponding to the source and drain of the transistor. The bottom electrode and ferroelectric layer are constructed in essentially the same manner as the bottom electrode and ferroelectric layer of a ferroelectric capacitor.
An integrated circuit utilizing ferroelectric capacitor-like structures is typically constructed in two phases. First, the conventional CMOS circuits which connect to the ferroelectric devices are constructed in the silicon substrate. A protective layer of SiO.sub.2 is then placed over the CMOS devices and the ferroelectric devices constructed on the protective layer or on a second protective layer deposited over the SiO.sub.2 layer. The ferroelectric devices are connected to the underlying CMOS devices by etching vias in the protective layer.
The bottom electrode and ferroelectric layer of a ferroelectric structure are typically constructed by depositing a patterned bottom electrode on the protective layer and then covering the surface with the ferroelectric layer. The top electrodes are then deposited and the electrode/ferroelectric layer is stack etched back to the protective layer.
For example, U.S. Pat. No. 5,242,534 describes a construction method in which a titanium oxide layer is formed over the SiO.sub.2 layer. A layer of titanium is then deposited, and the layer is masked and etched in those regions that are to become the bottom electrode leaving a trench in the titanium layer. The bottom electrode, typically platinum or a titanium/platinum stack, is then deposited in the trench. The mask is then removed and the exposed titanium is oxidized. This leaves the regions between the bottom electrodes covered with titanium oxide and the bottom electrode recessed in the titanium oxide layer. The PZT dielectric layer is then deposited over the wafer. The top electrodes are typically deposited as a uniform layer. The top electrodes and dielectric layer are then stack etched back to the titanium oxide layer in the regions between the capacitor-like structures.
For the purposes of the present discussion, the regions between the capacitors will be referred to as the "field". The above described process requires the PZT dielectric layer to crystallize both over the platinum electrodes and in the field. The PZT compositions typically utilized for the dielectric layer can form two types of crystals, perovskite and pyrochlore. The PZT compositions and deposition conditions are chosen such that the portion of the layer that crystallizes over the platinum electrode is all perovskite. Unfortunately, these compositions and conditions do not lead to a pure perovskite layer over the titanium oxide in the field. Typically, the field contains a mixture of perovskite and pyrochlore.
The respective etch rates for perovskite and pyrochlore structures differ significantly, with pyrochlore structures etching at a much slower rate than perovskite structures. As a result, the stack etch leaves pyrochlore islands in the field. These structures make it difficult to etch the vias used to connect the ferroelectric structures to the underlying CMOS circuits in the field.
Broadly, it is the object of the present invention to provide an improved method for constructing ferroelectric based devices.
It is a further object of the present invention to provide a method for constructing ferroelectric based devices that do not lead to pyrochlore islands in the field region.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.