Dry etching mechanism of lead zirconate titanate (PZT) films as capacitor materials in ferroelectric random access memory (FRAM) was studied in high density $CF_4$ and $Cl_2/CF_4$ inductively coupled plasmas (ICP) system where an independent control of the energy and the flux of bombarding ions are possible without disturbing the plasma. The concentrations of the atomic radicals as well as the flux and energy of bombarding ions were monitored as a function of various etching parameters such as etching gas ratio, substrate bias voltage ($V_s$), induction coil power and process pressure. The concentrations of possible of etching species, such as atomic fluorine and chlorine, were analyzed by optical emission spectroscopy (OES) using Ar actinometry methode. Ions in the plasma were identified and monitored with a quadruple mass spectroscopy (QMS). The compositions and the chemical bonding states of etched PZT films were examined by an x-ray photoelectron spectroscopy (XPS). The selectivity of PZT film to Pt electrode and the etching profile of PZT films, which are important parameters in the integrations of FRAM devices, were also studied.
The etch by-products in $CF_4$-based plasma are metal-fluorides (or oxyfluorides). PZT films are etched in $CF_4$-based plasma by chemically assisted sputter etching, and either the formation or the removal of metal-fluorides becomes the important step depending on the etching condition. At low sputtering rate, such as the case of low bombarding ion energy and flux, the removal of metal-fluorides becomes the important etching step. As the sputtering rate increases, the concentration of atomic F plays an important role and the formation of the etch by-products as well as the removal of the byproduct becomes the important etching step. The etched surface of PZT film is Pb-rich because the volatility of Pb fluoride is the lowest among the etching by-products.
In the etching of PZT films in $Cl_2$/$CF_4$ mixed plasma, the etch products are metal-fluoride (or oxyfluoride) and metal-chlorides. From the results of XPS and SEM, metal-chlorides are readily removed during the etch process, unlike the metal-fluorides. Therefore, the formation of metal-chlorides, which depends on the concentration of atomic Cl, is an important step in $Cl_2$-based plasma etching. The etch rate of PZT films shows a maximum at 90% $Cl_2$/$(Cl_2+CF_4)$, as does the Cl concentration. With increasing $Cl_2$/$(Cl_2+CF_4)$ ratio, the amount of Pb decreases in the etched PZT film surface. The etch selectivity of PZT to Pt is less than 1.3 in $CF_4$-based plasma, but more than 2 in $Cl_2$/$CF_4$ mixed plasma. The amount of sidewall residue of PZT films is much lower in $Cl_2$/$CF_4$ plasma than in $CF_4$ plasma, and the etching profile of PZT films is steeper in the conditions of low pressure and high substrate bias voltage