A New Permanganate-Free Slurry for GaN-SiC CMP Applications

Abstract

Single crystal SiC wafers are known to be extremely difficult to polish by conventional CMP slurries because of their high hardness and chemical resistance. Previously only those manganese-containing CMP slurries are capable of producing measurable and useful polishing rates with this versatile wide band-gap substrate. A new permanganate-free SiC polishing slurry containing a generic formula of MXO2 etchant, where M is an alkali metal, X is a halogen, O is oxygen is disclosed. When mixed with an abrasive powder in an aqueous slurry form, the tribochemical reactant that activates under pressure, etches SiC effectively, rendering an enhanced Material Removal Rate (MRR) when processing CMP SiC wafers. The MRR can sometimes go up to a few order of magnitudes, as compared to the abrasive slurry without these chemical etchants. The series of MXO2 compounds that can activate SiC polishing belong to the chemical family of halites. Sodium chlorite, NaClO2, the simplest and most available member of the halites family, is a good example. The accelerated polishing rates offer increased throughput of the slow SiC CMP process. The new slurry is particularly useful for non-oxide wafer polishing, which includes SiC, GaN and AlN wafers. An outstanding character of the new halite-based polishing formulation that is different from the current permanganate-based slurries is that the waste stream produced from the CMP process can be easily treated in the waste water treatment facilities because they do not contain toxic heavy metal ions such as manganese and permanganate in the polishing formulations. Continuous exhaustive CMP polishing test with 32 4” 4H-N SiC wafers using a production CMP tool containing 32L of the alumina-chlorite slurry has demonstrated an MRR of 1.7um/hr (Si-face) when the slurry is fresh, and a final MRR of 1.0um/hr after 16 hours polishing at 800mL/min slurry flow rate with pH buffer control without fresh oxidant addition. The resulting 32 polished 4H-SiC test wafers show overall excellent smooth surface roughness with the best Ra of 0.05nm by AFM after fine CMP polishing.