Wide bandgap materials such as gallium nitride are currently used for light emitter devices [1]. Otherwise, GaN physical properties open new prospects in microelectronics manufacturing [2]. By combining a wide bandgap (3.4 eV), strong chemical bonds and high electronics mobility, GaN based devices should operate under higher temperature, higher power and higher frequency than typical silicon devices.

 

GaN etching is one of the first process steps in device structure developments. Due to inert chemical nature of GaN, wet etching is limited [3]. As a consequence, it is necessary to use dry etching method [4] to obtain a reliable MESA structures. Chlorine plasmas are commonly used because GaCl3 is the most volatile etching product. Due to the strong bond energy of III nitrides, GaN etching also requires high physical sputtering which is provided by heavy neutral gas like argon and high bias voltage.

 

In this study, the GaN etching was performed into an industrial Alcatel 601 E tool which consists of an Inductively Coupled Plasma source and a diffusion chamber [5]. This high density plasma system was initially dedicated to silicon deep etching and modified to use chlorine gases. Plasma is generated by a single-ring antenna coupled to a RF power supply operating at 13.56 MHz. The 6 inch chuck is independently biased and thermally regulated. The process gases including argon and chlorine (Cl2) are injected at the top of the source.

 

Cl2/Ar plasma etching was performed on GaN epitaxial layers (12µm) grown on sapphire by metalorganic chemical vapor deposition (MOCVD). After SiO2 deposition by Plasma Enhanced Chemical Vapor Deposition (PECVD), wafers were patterned using conventional photolithography. Samples were subsequently mounted on 6 inch coverplates made of different materials.

 

We have carried out a parameter screening to optimize the etch efficiency of GaN. The best results in term of profile quality are obtained with a silicon coverplate. An etch rate of 250 nm/min is reached with our current setup. However, defects like columns or pits are observed at the etched surface under some conditions. The origin of those defects is also investigated in this study. Moreover, diagnostics such as Langmuir probe, optical emission spectroscopy and mass spectrometry have been used to characterize the plasma and understand the etching mechanisms.

 

[1] H. Morkoc, Wide Band Gap Nitrides and Devices, Springer, Berlin, 1998.

[2] G.T. Dang et al., IEEE Trans. Elect. Dev., vol. 47 (2000) 692

[3] D. Zhuang et al., Mater. Sci. & Eng., R 48 (2005) 1–46

[4] S.J. Pearton et al., J. Appl. Phys., Vol. 86 (1999)

[5] M. Boufnichel et al., JVST B, 20 1508 (2002)