Optimization of a Cl₂-H₂ inductively coupled plasma etching process adapted to nonthermalized InP wafers for the realization of deep ridge heterostructures

Inductively coupled plasma etching using Cl₂-H₂ chemistry with no additive gas (CH₄, Ar, or N₂) is studied to realize deep (>5 μm) ridges with smooth and vertical sidewalls. The process is optimized for nonthermalized InP wafers to avoid the use of thermal grease. Cleaning of the rear side of the wafer after etching is avoided, which is suitable for an industrial process or for critical subsequent steps such as epitaxial regrowth. The influence of the Cl₂/H₂ ratio on the etching mechanism is investigated for both InP bulk layers and InGaAs/InP or InGaAlAs/InP heterostructures. The authors show that this ratio is the main parameter controlling the ridge profile, in a similar way for both bulk InP and InGa(Al)As/InP samples. Smooth and vertical sidewalls with neither undercuts nor notches can be obtained in the 0.5-1 mT pressure range for a hydrogen percentage of 35%-45% in the gas mixture. Etching rates from 900 to 1300 nm/min together with a selectivity over SiN_x dielectric mask as high as 24:1-29:1 are measured for the InP bulk layers under these conditions. Etching does not affect the optical quality of the heterostructures as evidenced from micro-photoluminescence measurements performed on 1.6- to 0.85-μm-wide deep etched ridge waveguides. The process is well adapted to the realization of low loss deep ridge waveguides or buried heterostructures.