The effects of ultra-smooth surface atomic step morphology on CMP performances of sapphire and SiC wafers
Whether sapphire or SiC wafer, clear and regular atomic step morphology could be observed all over the ultra-smooth wafer surface via atomic force microscopy (AFM) using our CMP technology. However, towards sapphire and SiC wafers, the variations of atomic step widths and step directions on the whole of wafer surface are different. The step widths and step directions on the different positions of sapphire wafer are uniform, while that on SiC wafer are distinct. Thus, the effects of atomic step width on CMP removal rate of sapphire and SiC wafers were studied. On the other side, the CMP removal model of super-hard hexagonal crystalline wafer to realize atomically ultra-smooth surface is proposed. The variations of atomic step morphology towards different defects on sapphire and SiC wafers surface are analyzed, and the formation mechanism of the defects is discussed.
Ultraprecision CMP for sapphire, GaN, and SiC for advanced optoelectronics materials
Abstract Chemical mechanical polishing (CMP) of sapphire, GaN, and SiC substrates, which are categorized as hard-to-process materials, is demonstrated with a colloidal silica slurry under acidic and alkaline slurry pH conditions. Atomic level surface flatness was achieved by CMP and was confirmed to be equivalent to an almost ideally minimized surface roughness. By comparing the Preston coefficients under different slurry conditions, differences in the CMP properties among the three substrate materials and difficulties in the CMP of the GaN and SiC substrates are presented. The difference in CMP properties between the (0001) and (000-1) planes of GaN and SiC due to their non-revers crystallographical symmetry is also presented. Oxidation processes that occur during CMP of GaN and SiC are also discussed. By comparing the removal rate among GaN, SiC, and their oxides, it was found that the rate-limiting step in the total CMP process for GaN and SiC was surface oxidation reaction of GaN and SiC.
Effects of process parameter variations on the removal rate in chemical mechanical polishing of 4H-SiC
The effects of temperature, slurry pH, applied pressure, and polishing rotation rate on the material removal rate during chemical mechanical polishing (CMP) of 4H-silicon carbide wafers using colloidal silica slurry and polyurethane/polyester fiber polishing pads have been studied. Measured removal rates varied from around 100 Å/hr to nearly 2500 Å/hr depending on the values of the various parameters. The amount of material removed was determined by measuring the wafer mass before and after polishing. Variations in temperature and slurry pH did not produce significant changes in the measured removal rates. Higher polishing pressures resulted in increased material removal rates from 200 to 500 Å/hr but also produced excessive polishing pad damage. Variations in pad rotational speeds produced the largest changes in material removal rates, from around 200 to around 2000 Å/hr for rotational speeds between 60 and 180 rpm, but the variations were non-linear and somewhat inconsistent. This CMP formula is shown to consistently produce damage free surfaces but the optimum removal rate is slow.