Single asperity tribochemical wear of silicon by atomic force microscopy
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Measurements of single asperity wear on oxidized silicon surface in aqueous potassium hydroxide (KOH) using atomic force microscopy (AFM), where the single crystal silicon tip was used both to tribologically load and image the surface, is presented. AFM was also operating in the lateral (frictional) force mode to investigate the pH dependence of kinetic friction between the tip and the SiO2 surface. It was shown that the Si tip wear amount strongly depended on the solution pH value and was at a maximum at around pH 10. It was also found that the Si removal volume in mol was approximately equal to that of SiO2 irrespective of the solution pH value. This equality implies that the formation of the Si–O–Si bridge between one Si atom of the tip and one SiO2 molecule of the specimen at the wear interface. The surface of the Si tip is then oxidized. Finally, the bond rupture by the tip movement will occur, the dimeric silica (OH)3Si–O–Si(OH)3, including the Si–O–Si bridge, is dissolved in the KOH solution. The frictional signal is also sensitive to the pH values of the solution and peaked at around pH 10. These results indicate that the removal behavior of the Si tip and SiO2 surface would be affected by the frictional force between the Si and the SiO2, because of an increased liquid temperature and a compressive stress in Si and SiO2 networks. Strong influence is observed by the pH of the ambient solution confirming the important role of the OH in the wear mechanism. Pressure dependence of the microwear behavior under aqueous electrolyte solutions has also been investigated. A microscopic removal mechanism, which is determined by interplay of the diffusion of water in Si and SiO2, is presented.
I. INTRODUCTION
Chemical/mechanical polishing (CMP) of Silicon is widely accepted as the planarization process of choice for technologies to produce an atomically flat and defect-free surface for further electronic device manufacturing.1 The process of CMP consists of rotation of a soft polishing pad on a Si wafer with alkaline silica (SiO2) slurry. This has been known to be a complicated process, in which mechanical wear by the SiO2 particles and chemical corrosion by the slurry solution occur simultaneously, and ambient fluid promote controlled material removal.2 Water in the slurry is certainly the most abundant chemical with which Si and SiO2 surfaces come into contact. Because of the acceleration of the H2O diffusion by the mechanical stress, silica is reactive with water at the interface of contacts.3,4 Cook5 has summarized diffusion of water into SiO2, which takes place at the leading edge of the contact surface, because of an increased liquid temperature and compressive stress a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0024 J. Mater. Res., Vol. 24, No. 1, Jan 2009
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in the glass network. Pietsch et al.6,7 have also proposed the Si removal process in which OH adsorption on the topmost Si atoms cataly
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