Elliptical Ultrasonic Assisted Fixed-Abrasive Chemical Mechanical Polishing Of Advanced Hard Brittle Materials

Benefit from its excellent optical,electrical and magnetic properties,hard and brittle materials represented by advanced ceramics,optical glass and new semiconductor materials are widely used in the optoelectronic industry.However,the internal atoms are mainly connected by covalent bonds or ionic bonds,and the material exhibits extremely high hardness and low fracture toughness,which brings great difficulty to the processing of abrasive grains.Whether traditional diamond grinding or chemical mechanical polishing based on free abrasive particles is impossible to balance the processing efficiency and surface quality.In recent years,with the continuous development of 3D packaging technology,the integration of devices has become higher and higher,and the demand for ultra-thin silicon wafers and high thermal conductivity aluminum nitride ceramics substrate has also increased rapidly.In this context,a new,more efficient and environmentally-friendly planarization method for these two typical hard and brittle materials is strongly required to replace diamond grinding and chemical mechanical polishing.In this work,a bonded abrasive pellet containing soft grains was employed to realize the fixed-abrasive dry CMP.With this method,higher material removal rate can be obtained and the environmental pollution of alkaline slurry can be effectively avoided.The use of soft abrasives also greatly reduced the subsurface damage under the assistance of chemical reaction compared to the traditional fixed abrasive method like diamond grinding.However,the chip discharge problem still exist which limits the further improvement of processing efficiency.Consequently,an elliptic ultrasonic vibration was applied to the abrasive pellet during dry CMP.In order to perform the proposed novel method,we designed and fabricated an elliptical ultrasonic vibrator based on the inverse piezoelectric effect of piezoelectric ceramic,and built a four-axis dedicated polishing platform.In addition,we also pointed out that the adhesion and tribochemical reaction between soft abrasive particles and workpiece materials under tribological energy was the main material removal mechanism.The existence of elliptical ultrasonic vibration can make the abrasive trajectory of the abrasive grain a regular spatial spiral,which further contributed to the material removal.In this paper,single crystal silicon and aluminum nitride ceramics were selected as processing targets,and the detailed fixed abrasive pellet formulation was determined for these two materials from the perspective of facilitating the occurrence of tribochemical reaction.The processing results showed that compared with the diamond abrasive grinding,the soft abrasive pellet can greatly improve the surface quality of the workpiece,but the material removal rate was lower and the wear of the grinding pellet was also more serious.The presence of elliptical ultrasonic vibration significantly improved the surface roughness and material removal rate.It also improved the chip removal efficiency and reduced the wear of the grinding pellet.