Study On Magnetorheological Polishing With Large Polishing Tool Using Electromagnet Excitation Device

In recent years,with the rapid development of integrated circuit(IC),lightemitting diode(LED)lighting and optoelectronic products,the demand for single crystal silicon wafe,sapphire substrate,optical glass and other Ultra-smooth flat surface has become more and more increased.The surfaces of these devices are required high planarity and processing efficiency,and must be finished to nano-level roughness with no surface or sub-surface damage.Conventional nano-polishing technologies were not suitable for mass production of ultra-smooth surface due to the problems of low efficiency,high cost and easy sub-surface damage.Magnetorheological finishing(MRF)has been considered as an advanced nanopolishing technology for fabricate mirror surface without surface or sub-surface damage.Based on this,a magnetorheological polishing method using electromagnet excitation device has been proposed.In order to improve the polishing efficiency and obtain a sub-nano level high quality surface,a large polishing tool has been obtained by optimizing the magnetic field.The magnetic field and MR planarization polishing experiments using electromagnet has been optimizited and discussed respectivety,and the main conclusions have been summarized as the following aspects:(1)In order to enhance the intensity of magnetic and the area of polishing tool,an electromagnetic excitation device was designed.Maxwell simulation software has been used to simulate the magnetic field.The effects of different size parameters on the magnetic field intensity and effective magnetic field area of the linear pole and the teeth shape pole have been analyzed,and the optimized size parameters of the magnetic pole structure has been obtained.The distribution characteristics of magnetic induction intensity and the effect of maximum magnetic induction intensity on excitation current were discussed.(2)An experimental device of magnetorheological polishing with large polishing tool excited by electromagnet was built,and the verification experiment of magnetorheological polishing was carried out.The experimental results show that a large area polishing marks can be obtained on the glass workpiece by using the electromagnet with sharp tooth array pole,and the profile shape of polishing marks is similar to the magnetic field obtained by simulation.It is proved that the simulation results are effective.The effects of different polishing gap and excitation current on the polishing area has been discussed.The final MR polishing experiment demonstrated that the glasssurface could be improved to a roughness value of 1 nm in Ra after polishing,Experiments show that compared with permanent magnet excitation device,this excitation device with teeth shape pole can increase the area of polishing tool and achieve high efficiency and high precision polishing method.(3)The effects of polishing gap,rotational speed of polishing trough and workpiece and excitation current on material removal rate(MRR)and surface roughness of K9 glass were investigated.It was found that increasing the excitation current can effectively improve the polishing efficiency and obtain better surface roughness.Finally,the silicon wafes were polished by MR planarization under the condition of polishing gap 1.5 mm,polishing trough speed 15 r/min,workpiece speed 160 r/min and excitation current 12 A,The MRR 0.67mg/min in average was obtained,and the surface roughness could be decreased from initial Ra 373.03 nm to lowest Ra 0.38 nm after polishing for 180 min.