Dry etching processes for ferroelectric capacitors

Dry etching processes for patterning of ferroelectric thin films using environmental benign etching gases was developed. The PbZr_xTi_1-xO3 (PZT) and RuO₂ films were patterned using CHCIFCF₃ and O₂. Selective etching between PZT and RuO₂ was achieved by changing the gas composition. The etched profile and surface were anisotropic and smooth. Surface residues were found after etching and were removed by a post-etch heat treatment. In addition, the reactive ion etching damage to PZT ferroelectric capacitors was first defined and studied using Ar and CHCIFCF₃ etch gases. It was found that the internal field developed in the capacitors during etching was responsible for causing the shift in hysteresis loop and the reduction of switchable remnant polarization. The increase in leakage current after etching can be attributed to the electrical properties change at the interface of Pt/PZT and the roughness along the side wall of a PZT capacitor after RIE. Furthermore, the etching damage effect to PZT capacitors was substantially recovered by post-etching annealing at 400 °C for 30min.

The dry etching processes for new layered structure SrBi₂(Ta_xNb_1-x)₂O₉ ferroelectric films were developed in this research using CHCIFCF₃and SF₆ as etching gases. Physical sputter etching was the dominant etching mechanism in RIE of SBT and SBN films. Sr-enrichment on the etched surfaces was observed for both SBT and SBN films and was removed by post etching cleaning solution developed in this research.

From a technological point of view, a practical dry etch process for etching of RuO2 films was first developed using O₂/CF₃CFH_{2 (R-134) etching gases. The maximum etch rate, 1625 Ȧ/min, of RuO2 was obtained at pressure of 75 mTorr and rf power of 200W in Or2-2.5% CF3CFH2 discharge. RuO2 films were successfully patterned under these conditions using SiO2 as the etch mask. The etching mechanism was investigated from a scientific aspect. Several impurity gases, such as N2, SF6, and H2, were added into the oxygen discharge for this investigation. Atomic oxygen is the dominant active species reacting with RuO2 films to form RuO4 volatile products. The addition of impurity gases in oxygen discharge increased the generation rate and/or the life-time of atomic oxygen in the reactor thereby enhancing the etch rate. CF3CFH2 gas had the strongest effect in increasing the generation rate (G), and accordingly, addition of a small amount of CF3CFH2 in oxygen discharge yielded the highest etch rate. The decrease in the etch rate with increasing mole fraction of R-134 in the feed (>5%) was mainly due to the F atoms reacting with RuO2 to form a surface residue layer that occupied surface reaction sites impeding the reaction between O and RuO2.}