Abstract: The high quality ridge-waveguide,an optical modulator,was fabricated,with 450 nm thick LiNbO₃ coatings on Si-substrate,by Ar⁺-ion etching and reactive ion etching(RIE),respectively.The influence of the RIE conditions,including the pressure,CHF₃/Ar flow-rate ratio and ICP/RIE power,on the etching rate,etching selectivity ratio,roughness/steepness//straightness of sidewalls,was investigated with scanning electron microscopy and atomic force microscopy.The results show that the reactive ion-etching outperforms Ar⁺ ion-etching,because of much higher etching-selectivity ratio,fairly smoother/steeper/straighter side-walls,in spite of its lower etching-rate.Specifically,etched under the optimized conditions and measured in the end-coupling method,the transmission loss of the ridge waveguide(10 mm-long,4 μm-wide,370 nm-high and dipping at 72°) was found to be ~5.2 dB/cm.