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Thinning of N-face GaN (0001) samples by inductively coupled plasma etching and chemomechanical polishing

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7 Author(s)
Rizzi, F. ; Institute of Photonics, SUPA, University of Strathclyde, Wolfson Centre, 106 Rottenrow, Glasgow G4 0NW, United Kingdom and Department of Physics, SUPA, University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG, United Kingdom ; Gu, E. ; Dawson, M.D. ; Watson, I.M.
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The processing of N-polar GaN (0001) samples has been studied, motivated by applications in which extensive back side thinning of freestanding GaN (FS-GaN) substrates is required. Experiments were conducted on FS-GaN from two commercial sources, in addition to epitaxial GaN with the N-face exposed by a laser lift-off process. The different types of samples produced equivalent results. Surface morphologies were examined over relatively large areas, using scanning electron microscopy and stylus profiling. The main focus of this study was on inductively coupled plasma (ICP) etch processes, employing Cl2/Ar or Cl2/BCl3Ar gas mixtures. Application of a standard etch recipe, optimized for feature etching of Ga-polar GaN (0001) surfaces, caused severe roughening of N-polar samples and confirmed the necessity for specific optimization of etch conditions for N-face material. A series of recipes with a reduced physical (sputter-based) contribution to etching allowed average surface roughness values to be consistently reduced to below 3 nm. Maximum N-face etch rates of 370–390 nm/min have been obtained in recipes examined to date. These are typically faster than etch rates obtained on Ga-face samples under the same conditions and adequate for the process flows of interest. Mechanistic aspects of the ICP etch process and possible factors contributing to residual surface roughness are discussed. This study also included work on chemomechanical polishing (CMP). The optimized CMP process had stock removal rates of ∼500 nm/h on the GaN N face. This was much slower than the ICP etching but showed the important capabi- - lity of recovering smooth surfaces on samples roughened in previous processing. In one example, a surface roughened by nonoptimized ICP etching was smoothed to give an average surface roughness of ∼2 nm.

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Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films  (Volume:25 ,  Issue: 2 )