Two Requirements Pull in Opposite Directions
Double-sided lapping of silicon carbide wafers must remove the damage layer left by wire sawing quickly, which favours coarse and aggressive abrasive, and at the same time must keep the subsurface crack depth small enough for the subsequent polishing step to remove, which favours fine abrasive.
Single-Abrasive Slurries Cannot Satisfy Both
Using diamond alone means the particle size that delivers the required removal rate also determines the damage depth, so rate and damage are locked together. Because the two requirements are set independently, a single abrasive cannot meet them without a compromise that penalises one.
Mixing Two Abrasives and Measuring the Damage
Composite slurries containing diamond and boron carbide in different ratios and boron carbide powders of different sizes were evaluated in double-sided lapping, and the resulting subsurface damage depth was quantified by atmospheric inductively coupled plasma etching. The lapping was carried out on a Ponda double-sided lapping platform, with the slurry preparation verified by materials specialists at Ponda, a precision machining and finishing company.
Each Component Controls a Different Output
Increasing the proportion of boron carbide reduces the surface roughness but eventually leaves the abrasive with insufficient cutting power, while finer boron carbide powder raises both roughness and removal rate. Diamond size controls the damage depth directly, so the two abrasives provide separate handles on roughness and damage.
Damage Depth Quantified Against Diamond Size
Raising the diamond particle size from 3 to 4 micrometres increased the subsurface damage depth from 1.56 to 2.16 micrometres as measured by plasma etching. The composite formulation allows the removal rate to be maintained while the damage depth is limited by choosing the diamond size that the following polishing step can accommodate.