The atomic scale removal mechanism during chemomechanical polishing of Silicon: An atomic force microscopy study
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The atomic scale removal mechanism during chemomechanical polishing of Silicon: An atomic force microscopy study Futoshi Katsuki and Junji Watanabe1 Corporate Research and Development Laboratories, Sumitomo Metal Industries Limited, 1-8 Fuso-cho, Amagasaki 660-0891, Japan 1 Department of Mechanical Engineering and Materials Science, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan ABSTRACT The pressure dependence of the microwear of an oxidized Si surface under aqueous electrolyte solutions has been investigated using an atomic force microscope with a single crystal Si tip. The removal ratio of Si tip to SiO2 surface is found to be highly sensitive to the contact pressure. We present a microscopic removal mechanism which is determined by an interplay of the diffusion of H2O in Si and SiO2. INTRODUCTION The polishing technology used for manufacturing ultraflat and atomically smooth Si surfaces on a large scale is the chemomechanical polishing (CMP) technique [1]. Shrinking device dimensions and increasing wafer diameters demand extremely tight specifications with respect to flatness and surface uniformity. Thus the thickness variation in the sub-µm range as well as roughness RMS (root mean square) values in the sub-nm regime is a prerequisite for further electronic device manufacturing [2]. The process of CMP consists of rotation of a soft polishing pad on a Si wafer with alkaline silica (SiO2) slurry, and has been known as a complicated process, where mechanical wear by the SiO2 particles and chemical corrosion by the slurry solution occur simultaneously. Water in the slurry is certainly the most abundant chemical with which a Si wafer surface continuously comes into contact. Thus the microscopic model for the mechanism of removal during CMP is explained by a nucleophilic attack of OH- to silicon atoms catalyzing the corrosive reaction of water resulting in cleavage of silicon backbonds. Then, a silicic acid species, such as Si(OH)4, (OH)3Si-O-Si(OH) 3,... is formed at the wear interface and removed by the mechanical action of the SiO2 particles [3,4]. Those mechanical action causes frictional forces which in turn lead to an increase of the silicon surface temperature. Karaki et al. have reported the temperature of the polishing slurry is strongly influencing the silicon removal rate due to the decrease of the activation energy of the corrosive reaction [5]. Despite the combination of chemical and mechanical action during CMP is the key for obtaining superior surface quality, little is known about the influence of the mechanical action, such as the polishing pressure, on the chemical corrosive reaction. In the present study, the influence of the polishing pressure on the corrosive reaction are discussed as well as the atomic scale removal mechanism involved. We use an atomic force microscope (AFM), which is a powerful tool for atomic scale wear analysis [6]. The wear test is performed by scratching a thermally oxidized SiO2 film on a Si wafer with a Si AFM tip while applying various contact pr
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