Paper PS2-FrM1
Silicon Oxide Sidewall Passivation during HBr Inductively Coupled Plasma (ICP) Etching of InP and GaAs Materials for the Fabrication of Photonic Devices

Friday, October 24, 2008, 8:20 am, Room 306

The ICP etching technique is now widely used for the anisotropic etching of III-V heterostructures, a key building-block for photonic devices. Chlorinated atmospheres are generally used for both InP and GaAs materials, with few studies devoted to HBr. In any case, very few studies exist on the understanding of the sidewall passivation mechanisms occurring during the etching of III-Vs. Using EDX-TEM ex-situ analysis, we have shown for the Cl2-H2 chemistry [JVSTB 26, 666 (2008)] that a silicon oxide layer acting as a lateral etch-inhibitor can build-up on the etched sidewalls of InP-based heterostructures, when a Si wafer is used as the sample tray. This configuration corresponds to most commercial ICP etch systems having an electrode diameter of 4-in or more, used to etch III-V samples of 2-in or less size. In this work, we have analyzed by ex-situ EDX-TEM the passivation layer deposited on the sidewalls of InP and GaAs pillars etched with HBr using a Si tray. A Si-rich layer can build-up on the etched sidewalls under low pressure and high ICP power conditions, leading to anisotropic profiles. The passivation mechanism resembles that identified in Si gate etching using Cl2-HBr-O2 plasmas, and we suggest that a minimum amount of oxygen should exist in the plasma for the passivation layer to build-up. OES measurements indeed showed that oxygen is present in the gas phase, even w/o intentional O2 addition. In our conditions (0.5 mT–1 mT pressure range and ~1000 W ICP power), high values of plasma potential (> 20V) and positive ion current (> 3 mA/cm2) are measured, and oxygen could come from the sputtering of either the Al2O3 ceramic inner parts or the passivated walls of the reactor. We identified that the walls state greatly influences the sidewall passivation process, indicating that the species desorbed from the conditioned walls play an important role. Moreover, we show that adding less than 10 % of O2 to the gas mixture can modify the passivation mechanism: it is strongly enhanced in the case of GaAs material, and the layer is changed from a Si-rich layer to a more stoechiometric SiO2 in any case. The InP and GaAs planar etch rate is also increased, to the benefit of selectivity against dielectric mask, indicating that the concentration of reactive radicals is modified by the addition of a small amount of O2 in HBr. Low loss laser ridge waveguides on InP(311)B substrate and AlGaAs/GaAs microcavities are demonstrated with the optimized process.