Novel plasma etching chemistries are needed to pattern high dielectric constant materials, such as transition metal oxides, to enable their integration in sub-0.13µm complementary metal oxide semiconductor (CMOS) devices. In the work, we aim to study the reaction kinetics of etching zirconium oxide thin films in a high-density chlorine discharge. An Electron Cyclotron Resonance (ECR) microwave reactor is used to generate a chlorine discharge for etching ZrO@sub 2@. The plasma properties, such as the electron density, temperature and distribution, are determined by a Langmuir probe. A gridded ion energy analyzer is built to monitor the ion flux, impinging energy, and distribution. Optical emission spectroscopy (OES) and quadrupole mass spectroscopy (QMS) are employed to identify the gas phase reactive species, including the reactants (Cl@sub 2@, Cl, Cl@super +@, Cl@super -@, ...) and the reaction products (ZrCl@sub x@, ZrO@sub x@, ...), and quantify their concentrations as a function of the chlorine pressure, substrate temperature, substrate bias, and the plasma source power. Surface reaction chemistry and the etching rate are determined by in-situ transmission infrared spectroscopy and laser interferometry. The reactant neutral to ion flux ratio, a strong function of the processing pressure and the input microwave power, is a key factor affecting the surface reaction chemistry and the etching anisotropy. The concentrations of various ZrCl@sub x@ with different x values are measured and showed a strong dependency on both the reactant neutral to ion flux ratio and the ion incident energy. The surface roughness of the silicon substrate after etching is measured by AFM and compared to that of a pristine silicon surface. The results indicate that the roughness is preserved with lower ion energy, and suggest that the substrate bias should be minimized near the end point.