Paper PS-MoA3
Optical Emission Study of an Inductively Coupled Cl₂/H₂ Plasma during InP Etching of Micro-nanostructures used for Photonic Applications

Monday, October 15, 2007, 2:40 pm, Room 607

Cl2/H2-based chemistry has proven to be very efficient for highly anisotropic ICP etching of InP-based heterostructures used in photonic devices. It was shown recently that the Cl2/H2 ratio is a key parameter to control the sidewall profile. At low pressure (0.5mT-1mT), the onset of anisotropic regime occurs at H2 = 35-45%, where the evolution of the etch rate with H2 percentage shows a maximum. A possible explanation, proposed in literature, is the decrease of the reactive atoms and ions (Cl, Cl+) because of the by-products (i.e. HCl) formation. However, deeper understanding of InP etching mechanism requires more detailed investigations. We have used OES combined with electron and positive ion density measurements, during the etching of InP ridge structures, to obtain insight into the etch mechanism of InP in Cl2/H2 ICP plasma. The pressure was 0.5 mT, the ICP power was 800W, the DC bias voltage was –150 V, the total gas flow was kept constant at 28 sccm, and the H2 concentration is varied from 0 to 100%. The main emission lines recorded during the etching process were Cl (725.7 and 754.7nm), H (656.3nm), In (325.6, 410.2, 451.1nm), InCl (350nm), PH (340nm). In order to estimate the relative atom concentrations, 10% of argon was added in the initial gas mixture. The etch rate and the In-line intensity have roughly the same behavior versus Cl2/H2 ratio, which can be divided into three regions. For H2 concentration between 0-25% (corresponding to strongly undercut profiles), the etch rate rapidly decreases with %H2 increase, so as the positive ion current and the reactive species concentration – the Cl density falls down continuously when H2 increase from 0 to 100%. For H2 concentration greater than 60%, the etch rate also decreases down to very low values < 100nm/min and the etched surface becomes grassy. Despite the H2 concentration increases, the concentration of H atoms decreases, probably due to the decrease in the electron density. For intermediate H2 concentration (the second region which lies between 35% and 45%), corresponding to the highly anisotropic region, the etch rate remains constant. This intermediate region corresponds to a maximum in H concentration. The etch rate could thus be the result of a balanced effect between the Cl density decrease and the H density increase, with a change in etching mechanisms of P-atoms; for high H density P-atoms leave the InP surface by PHx formation, as suggested by the increase of PH-line intensity.