Paper PS1-TuA3
Limitation of Surface Defects in GaN Deep Etching

Tuesday, October 20, 2015, 3:00 pm, Room 210A

Gallium nitride (GaN) is a III-V semiconductor with attractive physical properties for power microelectronics. It actually combines a wide and direct bandgap, a high electron mobility and strong chemical bonds. Therefore, GaN power components can operate under higher temperature, higher power and higher frequency than silicon devices.

For Schottky diodes with pseudo-vertical structure, GaN MESA features with a height between 6 and 10 µm are required. This is considered as deep etching, compared to the thickness typically etched for light emitter devices (a few hundred nm). Ion-enhanced plasma etching with chlorine-based chemistry is commonly used for GaN deep etching.

Previous studies have already shown that GaN can be etched in Cl₂/Ar inductively coupled plasmas (ICP) with etch rates as high as 1 µm.min^-1. However, after etching, the etched surface exhibits three types of defects such as columns, pits and a high roughness (“White GaN”). Columns and pits are related to nanopipes and dislocations created during epitaxial growth of GaN. In addition, oxygen based species, coming from either the SiO₂ coverplate and mask, or the alumina/quartz tube, play an important role in the columnar regime. They preferentially oxidize dislocations, leading to the observed columns. “White GaN” origin is a surface over-oxidation. Such defects must be suppressed in order to provide good electrical contact.

A comparative study of GaN etching has been performed on different reactors: an ICP reactor with a diffusion chamber, another ICP reactor with no diffusion chamber, a dual-frequency capacitive reactor and an Ion Beam Etching system. The etched surface state was subsequently analyzed by means of SEM, EDX, AFM and XPS. This study revealed a correlation between the etch rate, the surface defects density and the surface composition. Actually, with the IBE chamber, the etch rate was the lowest and no defects were observed on the surface. The dual-frequency capacitive reactor allows the highest etch rate and no defects were found on the surface. But, the Ga:N relative density was altered in both cases, resulting in poor electrical properties. Consequently, a trade-off should be made between process performances and electrical properties. ICP chambers met this compromise.

Moreover, it has been demonstrated that surface fluorination, by addition of a fluorine-containing gas, leads to a limitation of surface defects. Fluorine species are able to protect GaN surface with the formation of a Ga_xF_y -like “passivation layer”, detected by XPS. This result led to the development of a defect-free time-multiplexed etching process consisting in alternating etching and passivation steps.