Single Asperity Wear and Stress-Assisted Dissolution of Copper
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1025-B16-04
Single Asperity Wear and Stress-Assisted Dissolution of Copper Bun H Chua, Abhijit Chandra, and Pranav Shrotriya Mechanical Engineering Department, Iowa State University, 2025 Black Engineering Building, Ames, IA, 50011 ABSTRACT The focus of this paper is to investigate the synergistic influence of contact loads, surface stress state and chemical environment on material removal during mechanical stimulation of copper surface. A unique setup is used to generate well characterized stress states on a polished copper specimen. Stressed surface of copper specimen is stimulated using tip of the atomic force microscope (AFM) and material removed during stimulation is measured as a function of contact loads, surface stress state and chemical environment. Measured material removal rates display a complex dependence on contact pressures and in-plane stress state which changes as the pH of chemical environment changes from acidic to basic. A surface material removal mechanism based on a single asperity wear and stress-assisted dissolution is proposed to explain the experimental observations. INTRODUCTION Copper is becoming a commonly used material in integrated circuit devices because of its high electric and heat conduction. Copper based devices are manufactured using additive patterning and subsequently undergo chemical mechanical planarization (CMP) to ensure reliable interconnection [1]. During CMP, material removal is accomplished through synergistic combination of chemical effects and mechanical abrasion [2]. Empirical models such as Preston’s equation [2] are used to describe the material removal during CMP but a mechanism based understanding of the synergistic interactions between chemical environment and mechanical loading is still lacking. Chemical environment determines the surface chemical reactions and plays an important part in determining material removal mechanism during mechanical stimulation. For example, Pourbaix diagram [3] shows that in alkaline solution, copper surfaces are passivated with formation of a hydrated oxide layer. When the sample surface is mechanically stimulated, the oxide layers are abraded away and exposed bare copper surfaces are rapidly repassivated. The process of abrasion and repassivation leads to material removal. However, in acidic solution, copper surfaces undergo rapid etching as the formation of oxide passivation layer is not preferred thermodynamically. The surface is rapidly etched therefore, corrosive inhibitors such as benzotriazole (BTA) are used in acidic slurries to form a passivating layer and to suppress the etching of copper surface. Hence, material removal in acidic slurries containing BTA proceeds through abrasion of passivating layer by mechanical abrasion and rapid etching and passivation of the exposed bare copper surfaces. Previous investigations have shown that on surfaces undergoing chemical etching alone, rate of chemical reactions is influenced by surface stress state [4]. Hence, it is important to identify the influence of contact loads and surfa
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