| Ultra-thin stainless steel 304 is the ideal material for the flexible display, and the difficulty to improve the surface roughness of stainless steel substrate is in the bottleneck of applying ultra-thin stainless steel substrate to the next generation of flexible display technology. Chemical Mechanical Polishing (CMP) has been widely used in leveling of workpiece surface because of combining mechanical action and chemical action, and possessing good processing effect, such as the ultra-precision and no damage on the surface.The swinging gravity type polishing machine adopted by CMP process is a kind of equipment relatively simple in structure but precise in grinding and polishing. It is able to make the pressure steadily distributed on the ultra-thin stainless steel substrate, and achieving complex friction track. However, the clamping mode of ultra-thin stainless steel substrate is to use the material such as glue or soluble wax to paste artifacts on the bottom of press block, which will directly affect the global planarization of the polished artifacts. In addition, the temperature of the polishing interface influences the activity of the polishing slurry, the reunion of abrasive and fiber properties of polishing pad surface, etc., which influence the removal rate and surface quality of materials.To address the aforementioned issues, this paper designed a new type of polishing head inheriting the advantages of stable pressure of the gravity type polishing machine. This polishing head replaced the traditional paste method by vacuum suction method to achieve global planarization of ultra-thin stainless steel substrate. Then, integrating an on-line temperature detection device in the polishing head. The temperature detection device can accurately detect the temperature of polishing interface without affecting the polishing process. Secondly, the inflow and outflow of the heat of ultra-thin stainless steel substrate in the process of CMP was analyzed, and the temperature change of stainless steel substrate in the polishing process was also simulated and calculated by ANSYS software. Finally the polishing head, integrated with an on-line temperature detection device was used for polishing experiments. The temperature on polishing interface of detection results and theoretical simulation results was compared to verify effectiveness of temperature detection device. Experiments were also carried out to analyze the effect law of the different polishing pressures, speeds and flow rates of polishing slurry on the temperature rise, material removal rate and surface roughness after polishing.The main research methods and conclusions of this paper are as follows:(1)On the basis of comparing various compression and clamping methods of CMP equipment, an improved scheme of the polishing head was proposed. The scheme used the metal vacuum suction cup with small holes to adsorb ultra-thin stainless steel substrate, and used gravity to put pressure. The deformation of ultra-thin stainless steel substrate and the contact pressure between the substrate and the polishing pad during the polishing process were analyzed by ABAQUS software, to determine the sucker diameter as 1mm. By calculating that the minimum vacuum adsorption stability of stainless steel substrate is 0.19 × 105 Pa, pumping speed of adsorption is 0.5L/s for 0.05 seconds, vacuum generator was selected to provide negative pressure for the gas circuit of polishing head.(2) The temperature of the back surface is only 0.0886℃ lower than that of the polished surface by calculation when the workpiece is 304 stainless steel plate with the thickness of 0.1mm, therefore, the back surface of the ultra-thin stainless steel substrate can be chosen as the object of temperature detection. Under the consideration of all aspects of accuracy of temperature detection, installation and signal transmission, the STT-F model surface thermal resistance temperature sensor of three-wire system, SBWV type temperature transmitter, ZYX-T012 type via hole conductive ring and PCI-9118DG data acquisition card were determined to build the online polishing interface temperature detection device. This device can obtain the real-time temperature of polishing interface in high precision and the system error is only about 0.335% without affecting the CMP process.(3) Analyzing the inflow and outflow heat of each part of the stainless steel substrate according to the principle of temperature rise, and simulating the changes of temperature by the software ANSYS, results can be seen that when the polishing pad and polishing head are in the same speed, heat inflow into ultra-thin stainless steel substrate is uniform and the swing of polishing head will not affect the temperature distribution uniformity; temperature rise of stainless steel substrate has tended to be stable, and temperature rise increases with polishing pressure and rotating speed of polishing head and polishing disk.(4) Adopting the polishing head with an on-line temperature detection device to conduct CMP experiments of ultra-thin stainless steel substrate. From the experimental results, it can be seen that temperature changes of ultra-thin stainless steel substrate can be detected by the on-line temperature detection device. Besides, the experimental results by the detection device are very close to theoretical calculation results of temperature tendency or values, thus validating the effectiveness of the on-line temperature detection device.(5) The vacuum suction polishing head improved the polishing efficiency, reduced the cost and avoided uneven global planarization due to viscous material thickness, so the roughness of the substrate was almost unanimously. |
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