Paper PS2-FrM3
Characterisation of InP Ridge Sidewalls Patterned in Inductively Coupled Halogen Plasmas

Friday, October 24, 2008, 9:00 am, Room 306

High-aspect-ratio etching of InP-based heterostructures is a critical building block for photonic device fabrication. Indeed highly anisotropic profiles and smooth sidewalls free from undercuts or notches are required to minimize optical scattering losses. Recently it was shown that anisotropic etching can be obtained in Cl2-H2 and HBr inductively coupled plasmas (ICP), due to the passivation of the InP sidewalls by a Si-containing layer originating from the Si sample tray [JVSTB 26(2008)666]. This study is focused on the chemical characterisation by means of X-ray photoelectron spectroscopy of the bottom and sidewall surfaces of InP ridge patterns etched with Cl2-H2 and HBr chemistries. Anisotropic profiles are obtained for low pressure (<1mT), high ICP power (up to 1000W for HBr), H2 percentage (H2%) in the 35-45% range for the Cl2-H2 mixture, moderate dc bias (-140V), sample temperature of ~190°C. ICP etching results are compared to HCl wet etching, taken as reference. Surface chemistry at the pattern bottom can be summarized as follows. Etching in Cl2-H2 with a H2% ~36% gives a surface very close to HCl. Width of the P2p, In3d and In4d InP-bulk contributions are very close to the reference, this indicates that no other species than the oxide is present above the bulk material. The intensity ratios for In/P-bulk (0.9) In/P-oxide (0.6) point out that Cl2-H2 etching produces a slightly P-rich surface. In the case of HBr, the much larger width of the InP-bulk components suggests the occurrence of an additional species that could be amorphous InP. Moreover, an extra component is observed on the P2p spectrum at +0.8eV from that of InP-bulk. In the absence of Br from the surface, we suggest attribution to P-H species. Finally, the high In/P-bulk and In/P-oxide ratios, 4.3 and 3.9 respectively, clearly state that HBr produces an In-rich surface. Sidewall chemistry shows significant differences as compared to the bottom. For example in the case of Cl2-H2, the P2p, In3d and In4d InP-bulk contributions are about 1.5 times larger. Moreover the In/P ratio falls down to 0.4 and about 0.1 for In/P-bulk and In/P-oxide respectively. Opposite, the HCl etched sidewall is identical to the bottom (In/P-bulk = 0.9). For Cl2-H2 etching, this definitely points out a variation of composition in the top 10nm, with a decreasing In/P ratio from the “bulk” to the sidewall surface. Similar analysis are presently carried out on HBr and HBr-O2 etched samples.