| As the advancement of information electronic technology, optic technology, and semiconductor illumination technology,the demand for ultra-smooth flat surfaces, such as IC substrate, sapphire substrate and LCD panel, has increased rapidly in recent years. Those surfaces need to be polished to a nano level roughness and several microns planarity without subsurface damage. Conventional ultra-precision polishing technologies have been proven to be inefficient in mass production of ultra-smooth flat surfaces when cost, effieciency and sub-surface damage were concerned simultaneously. Magnetorheological(MR) finishing technology has been recognized as a promising advanced finishing technique to fabricate mirror surfaces without causing surface or sub-surface damage. However, the process is less efficient and costly as the obtained finishing “spot” is very small.To address above issues, a high efficient MR planarization based on large polishing tool is proposed, where the working area is greatly expanded and the machining efficiency is significantly enhanced. The process optimization of the developed MR planarization is conducted and its machining mechanism is explored. A MR polishing machine tool is developed, on which the industrial application researchs are carried out. The industrial standard is also established and the certificate of scientific and technological achievements is obtained. The main research contents and results in this study are summarized as below:(1) A permanent magnetic(PM) yoke with a straight air gap is developed as the magnetic excitation unit, in order to enlarge the working area of the formed magnetorheological(MR) polishing tool. Finite element modelling is used to simulate the magnetic performance of the newly developed yoke, which is found to be in agreement with the experimental measurement. With the developed MR polishing apparatus, the polishing characteristics and performance of the newly developed yoke are examined.(2) A finite element analysis combining an orthogonal method is carried out to optimize the PM yoke, hence to further improve the efficiency of the MR planarization. To assess the optimization,the magnetic performances,the MR polishing tools and the polishing areas prior to and after optimization are compared. Final tests are conducted on optical glasses to evaluate the polishing performances of the modified yoke. To reduce ploughing strations, a yoke with “Gaussian” field distribution is designed and simulated.(3) To improve surface planarity, a reciprocate motion is added into the magnetorheoloigcal planarization process. A trajectory equation is established to describe how the workpiece moves in relation to the MR polishing tool. To analyze planarity, a mathematical model to calculate removal depth is constructed based on the established trajectory equation. To explore effects of some process parameters, including trajectory type, stroke and reciprocate velocity, on surface planarity, a simulation study is conducted on MATLAB software. Some finishing experiments are carried out as well.(4) The machining mechanism of MR planarization process is investigated. The effects of process conditions, including reciprocate translation, work gap, excitation gap and concentration of carbonyl iron particles, on polishing forces, surface roughness and volumetric removal rate are comprehensively explored and the sub-surface damage is also examined. Based on the investigation, mathmatic models are established for predicting polishing forces, surface roughness and volumetric removal rate.(5) A MR polishing machine tool is developed, on which industrial application researchs for sapphire wafers and isostatically pressed graphites are carried out.(6) Using the magnetorheological planarization process, a maximum volumetric removal rate of 0.794 mm3/min, a planarity of 1 μm in PV and a surface roughness of 0.6 nm in Ra are obtained on K9 glass substrates, and a maximum removal rate of 4.63 mg/h and the best surface roughness of 0.3 nm in Ra are obtained on sapphire wafers, and a maximum removal rate of 3.1 mg/min and the best surface roughness of 10 nm in Ra are obtained on isostatically pressed graphites. |
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