Piezoelectric Abrasives for Self-Powered Electrochemical Polishing of Silicon

Silicon wafers need a very flat, smooth surface for semiconductor manufacturing. Chemical mechanical polishing produces this surface by chemically changing a thin layer of silicon and then removing it with abrasive particles. The process works best when this layer forms evenly and can be removed without leaving scratches or defects. Electrochemical mechanical polishing improves this process by using an electric field to oxidize the silicon surface, creating a softer layer that is easier to remove. Conventional systems generate this field with external power supplies, electrodes, and electrical circuits, which can require substantial changes to standard polishing equipment. A simpler approach would generate the electric field within the polishing slurry itself while retaining the basic structure of a conventional chemical mechanical polishing process. In a recently published research paper in Materials Science in Semiconductor Processing, Dr. Jiangfan Yi, Dr. Baoxiu Wang, and Dr. Weidong Yang, led by Professor Tao Sun from Shanghai University of Engineering Science, developed a silicon-wafer polishing slurry in which tetragonal BaTiO₃ particles function simultaneously as abrasives and mechanically activated electric-field generators. Downward pressure and rotational shear polarize the particles during polishing, allowing electrochemical oxidation to occur without an external voltage supply or modified electrical circuitry.

Briefly, in their experiments, both powders were phase-pure, although the tetragonal particles were smaller and more irregular than the rounded cubic particles. Under the same applied load, only the tetragonal powder generated a measurable voltage and the researchers introduced alkaline polishing slurries, tetragonal BaTiO₃ produced a material removal rate roughly three times that obtained with cubic BaTiO₃ and also yielded a smoother surface. Comparing two crystal phases of the same compound therefore helped distinguish the effect of piezoelectric polarization from ordinary abrasive action.

The authors performed surface chemical analysis to understand the link of performance difference to enhanced silicon oxidation and found that the SiO₂-related signal increased several-fold after polishing with the piezoelectric abrasive, accompanied by greater oxygen content and a lower Si–Si contribution. When they applied pressure, the tetragonal particles generate an electric field that helps oxidize the silicon surface. This forms a very thin silica layer, which is weakened by hydroxyl reactions and then removed by the moving abrasive particles. The piezoelectric abrasives therefore use the polishing force itself to create a surface layer that is easier to remove. Polishing performance depended on the relative speeds of the platen and wafer carrier. The team obtained the highest material removal and lowest surface roughness when both rotated at the same speed. When their speeds differed, each part of the wafer remained under the particle-generated electric field for less time, limiting surface oxidation and reducing material removal. Synchronized rotation also produced sufficient shear to activate the piezoelectric particles effectively.

Pressure produced a related two-stage response. Material removal increased rapidly at first, while surface roughness declined. Beyond a moderate pressure, the rate of improvement slowed and the surface finish changed relatively little. The authors cautiously attribute this transition to the piezoelectric response approaching saturation, after which additional pressure contributes mainly through stronger mechanical abrasion.

Electrolyte concentration had a clear effect on polishing and the authors found small additions of NaCl or KOH helped carry the electric field through the slurry and increased material removal. When too much electrolyte was added, the abrasive particles began to cluster, and the wafer surface became rougher. KOH gave the best balance between removal rate and surface smoothness. Ethylamine behaved differently: it removed material more slowly, but it produced the smoothest surface. The new approach proposed by Professor Sun and colleagues is applicable to silicon-wafer planarization using standard chemical mechanical polishing equipment. Because the electric field is generated inside the slurry by mechanically stressed tetragonal BaTiO₃ particles, manufacturers could introduce an electrochemical contribution without installing external power supplies, electrode assemblies, or specialized control circuits. This makes the method particularly relevant to polishing lines where replacing or extensively modifying existing tools would be impractical.

A second important application is high-throughput wafer finishing. The increased material removal rate could shorten polishing cycles while maintaining nanometre-scale surface roughness. Its engineering advantage comes from forming a thin, more easily removed oxidized surface layer during polishing, instead of relying only on stronger mechanical abrasion. This may allow material removal to be increased without simply raising abrasive severity, which can damage the wafer surface.

The findings also provide practical guidance for slurry formulation and process control. Electrolyte conductivity can be adjusted to improve transmission of the particle-generated field, while ion concentration must remain low enough to preserve abrasive dispersion. The identified response around 0.1 M KOH offers a useful formulation point where conductivity, removal rate, and surface finish were favorably combined. Pressure and rotational synchronization can likewise be selected to activate the piezoelectric particles efficiently while maintaining sufficient reaction time at the wafer interface. For process engineers, the method introduces a controllable polishing variable based on abrasive crystal phase rather than abrasive hardness alone.