Wireless photoelectrochemical mechanical polishing for inert compound semiconductor wafers

Chemical mechanical polishing (CMP) remains the necessary polishing technology in chip manufacturing, but its applications on inert n-type gallium nitride (GaN) and carborundum (SiC) semiconductors are inefficient. Based on the mechanism of bipolar photoelectrochemistry, an efficient wireless photoelectrochemical mechanical polishing (WPECMP) method was proposed. The method applies a wireless electric field to the ultraviolet (UV) light-irradiated semiconductor wafer to separate photogenerated electron-hole pairs during polishing, rather than the traditional wire-connecting wafer to a power source. WPECMP is thus a universal polishing method for freestanding and semiconductor-on-insulator substrate wafers. The material removal rate (MRR) may reach similar to mu m h(-1) level because the unpaired holes oxidize the wafer to accelerate the material removal in the mechanical polishing process. An apparatus was devised to allow the wafer to alternate between oxidation and mechanical polishing, and the method was validated by finishing GaN wafers that are inert in CMP processing. The best MRR achieved 1.2 mu m h(-1), one order of magnetite higher than that of the CMP technology. The WPECMP method enables wafers to achieve a damage-free surface/subsurface, a surface roughness (Sa) of less than 0.082 nm in 5 x 5 mu m(2), and a surface flatness of less than 3.2 nm over 45 mm. The study opens a way to develop ultra-precision processing technologies with wireless electric field assistance.