## 2 Dielectrophoresis Polishing and Apparatus  
DEP is a physical phenomenon in which electrically neutral particles in a non-uniform electric field is subjected to an electric field force [21]. The existence of the force does not require the object being electrically charged, and the direction of the force points towards the electrode. This force is called the DEP force, and Figure 1 shows an electrically neutral particle in a non-uniform electric field. All neutral particles have the DEP effect in non-uniform electric fields, where the value of the DEP force depends on the electrical property, size and shape of the particles, and the change rate of the electric field intensity. The DEPP method utilizes the DEP effect of abrasives in a non-uniform electric field to assist polishing.  
The mechanism of DEPP is shown in Figure 2. The workpiece is a sapphire substrate, and its surface has a certain degree of surface roughness after grinding. When the polishing area does not form a non-uniform electric field (traditional CMP), abrasives will be randomly suspended in the polishing slurry, and only a small amount of abrasives will be brought in contact with the sapphire substrate and polishing pad that generate effective material removal, as shown in Figure 2a. The insulating layer and the electrode are added in the vertical direction of the polishing area to form a non-uniform electric field, and the abrasives are polarized subject to DEP force, as shown in Figure 2b. Under DEP force,the abrasives will move in the direction towards the electrode and congregate on the surface of the sapphire substrate to effectively increase the contact with the sapphire substrate and the polishing pad. In the same time, polishing slurry that follows the abrasives tends to move towards the sapphire substrate to increase the pressure between the sapphire substrate and polishing slurry, as shown in Figure 2c. According to Figure 2, we can see that the DEPP technology can drive the abrasives in polishing slurry to move and assemble on the surface of sapphire substrates, resulting in more abrasives to participate in the polishing process, to improve the material removal and to increase the efficiency of sapphire polishing.  
The schematic diagram of the DEPP apparatus is shown in Figure 3, which is obtained by retrofitting a traditional flat polishing machine. The lower plate is made of an insulating layer, an electrode and a polishing pad. The upper plate is made of an insulating layer and an electrode, and a sapphire substrate as the workpiece with paraffin is pasted on the upper plate. The sapphire substrate and the lower plate are driven by two different motors, and the rotating speed of the motors can be  
Figure 1: Dielectrophoresis (DEP) force on a neutral particle in a non-uniform electric field.  
Figure 2: Mechanism of the dielectrophoresis polishing (DEPP) process: (**a**) polishing area without a non-uniform electric field; (**b**) abrasives are polarized in the non-uniform electric field; and (**c**) abrasives move and assemble to the surface of the sapphire substrate by DEP force.  
adjusted independently. A high voltage power supply connects the electrodes through carbon brushes on the plates, forming a non-uniform electric field for the DEPP machine.