Reciprocating-Disc-type Magnetorheological Finishing with Long-term Removal-function Stability

<title>Abstract</title> <p>The well known Disc-type magnetorheological finishing (DMRF) has damage-free advantage due to “surface contact” during polishing process. It has got more attention from integrated circuit (IC) industry to obtain wafer-level ultra-smooth planarization and other fields requiring ultra-precise surface processing. However, conventional DMRF configurations exhibiting unstable material removal rate (MRR) which constrain their application, one reason is the variation of the linear speed of the disc polishing surface, the another is evaporation of water in magnetorheological polishing fluid (MRPF) during polishing. To obtain stable material removal rate (MRR), a kind of reciprocating-disc-type magnetorheological finishing (RDMRF) equipment is designed and practiced in our experiment, and the effect factors contributing to drift in removal-function accuracy are identified and discussed, including influence of time. MRPF with sodium-hexametaphosphate (SHAMP) and vapor-deposited silica was used in polishing, and the results showed that the shear stress rises by 47% and sedimentation rate falls by 57% compared with pure magnetorheological fluid. Using this MRPF can achieve a surface roughness of Sa 1.562 nm on workpieces. A Preston-type model integrating workpiece elastic modulus and magnetic-field gradient is established. In order to avoid instability caused by temperature and evaporation, a magnetically actuated recovery loop was constructed to provide on-the-fly replenishment and continuous agitation of the MR fluid. With this loop engaged, the material-removal-rate drift was suppressed to &lt; 8% over 180 min and the areal roughness Sa converged to 1.519 nm. Deviation is caused by minute fluctuations in the composition of the slurry during the polishing process. This reveals the root cause limiting the stability of removal and further improvement in surface quality.</p>