Study On Chemical Mechanical Polishing Mechanism And Process Of MgO Single Crystal Substrate

MgO single crystal has the properties such as lower dielectric constant (ε=9.6), lower dielectric dissipation in high frequency (77K, 10GHz, tg8=4×10-5), similar lattice matching with high temperature superconductor (HTS) material. At present, it is used as the most popular substrate for the HTS in manufacturing high frequency microwave device taking into consideration its lattice matching, coefficient of thermal expansion, chemical stability, microwave property, growing process, cost and good commercialization application.MgO single crystal has a face-centered cubic ionic structure with hard and brittle properties. The ions within MgO single crystal are close-packed along {100} crystal face and the bonding force between {100} crystal faces is weak so that MgO single crystal is easily cleaved along {100} crystal face. So it is a typical difficult-to-machine material. At present, the technique of ultra-precision manufaturing for MgO single crystal has not been systematically studied, and the mechanism of material removal and surface defect during ultra-precision machining procedure has not been fully understood. There is not a mature theory and technique of ultra-precision machining to guide the production of MgO single crystal substrate. For manufacturing scale of MgO single crystal substrate, it more depends on a worker's experience to guarantee the quality of the product. As the development of HTS film device proceeds, the MgO single crystal substrate has to meet more strict requirement, such as higher surface quality, higher consistency, etc., which suggests the empiristic manufacturing model is not good enough for the next generation product. For improving the manufacturing level of MgO single crystal substrate, it is very important to perform fundamental study on the mechanism and technique of ultra-precision machining for MgO single crystal substrate.At present, chemical mechanical polishing (CMP) is a popular ultra-precision machining method for hard-and-brittle crystal material and IC chip fabrication. The CMP technique was introduced and employed to polish MgO single crystal substrate by this thesis. Through studying the material removal, surface formation and defect generation mechanism, plus defect removal and thin substrate warping reducing process, the physical and chemical behaviour of MgO single crystal substrate during CMP was disclosed. The main research conclusions are as follows: (1) The systematic CMP experiments for MgO single crystal were conducted with different sizes of SiO2 abrasives and different chemical reagents in the polishing slurry. The results showed that no chemical and mechanical interaction happened during CMP process using the slurry containing HCl, HNO3 or H2SO4 reagent. The material removal rate (MRR) was low to 50 nm/min. Weak chemical and mechanical interaction happened during CMP process using the slurry containing oxalic acid, malonic acid or citric acid reagent. Complexing reaction occurred between the reagent and the MgO material. The MRR was affected by the concentration of the complex ions in the slurry and the MRR was increased obviously when the concentration of the complex ions was high. Notable chemical and mechanical interaction happened during CMP process using the slurry containing H3PO4 reagent. The MRR was higher than 300 nm/min. High quality of polished MgO single crystal substrate could be achieved efficiently with the slurry containing 73 nm SiO2 and H3PO4 reagent.(2) The material removal mechanism of MgO single crystal substrate in CMP process was studied. Through the XPS and XRD analysis on the MgO single crystal substrate before and after CMP, a passivating film (magnesium phosphate) was found which was generated during CMP process. The material removal was realized by the circulation of the passivating film generating, the passivating film removal, and the passivating film regenerating.(3) The influence of different polishing slurries on the surface topography of MgO single crystal substrate during CMP was analyzed. The results showed that the efficient slurry for achieving high quality of MgO single crystal substrate should have the following properties:a) MRR should be high enough; b) static etch rate should be low enough; c) the chemical and mechanical interaction should be balanced during CMP process.(4) The four stages of the pit defect-generating, enlarging, reducing and disappearing-were observed through examining the topography of the pit defect during CMP process. Based on the change of the pit topography and taking into consideration the static etch experiment and abrasive tracking simulation during CMP process, the generation mechanism of the pit defect was explored. Based on the generation mechanism of the pit defect during CMP, the pit removal process was suggested and proved.(5) Through studying the variation of the scratch on MgO single crystal substrate surface during CMP and static etch process, the removal mechanism of the scratch was discussed. Based on the experiment results, two important indexes - scratch removal rate SRR and scratch removal ratioηs- for the scratch removal was developed to evaluate the scratch removal efficiency. The greater SRR is and the closerηS approaches 1, the better the scratch removal effect of polishing process and polishing slurry are. Contrariwise, SRR and ηS approach 0, polishing process and polishing slurry do not have the ability to remove the surface scratch.(6) The warping mechanism of thin MgO single crystal substrate during single-side lapping and polishing process was analyzed. The bonding method, lapping and polishing process for avoiding the releasement of the uneven residual stress left on the two sides of the MgO single crystal substrate was developed, which could reduce the warping after polishing remarkabley.