Plasma Etching of Copper Thin Film over a Dielectric Step and Electromigration Failure Mechanism
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Plasma Etching of Copper Thin Film over a Dielectric Step and Electromigration Failure Mechanism Chi-Chou Lin and Yue Kuo Thin Film Nano & Microelectronics Research Laboratory, Texas A&M University, College Station, TX 77843-3122, U.S.A.
ABSTRACT Process and electromigration issues of the copper line over a dielectric step etched with a new plasma-based process have been studied. The N2 and CF4 additive gas effects on the line profile, undercut, and “neck” formation at the cusp area were investigated with respect to changes of the plasma phase chemistry and ion bombardment energy. The sidewall passivation layer hindered the excessive attack of the cusp region. The undercut of the photoresist pattern caused the residue formation. The lifetime of the etched copper was related to the line shape and the film topography, which directly affected the local current density and stress. With the proper control the plasma phase chemistry and ion bombardment energy, the Cu film over a topographic surface can be etched into fine lines with a long electromigration lifetime. INTRODUCTION Copper (Cu) has recently been a popular interconnect material for advanced electronic and optical devices [1]. However, it is difficult to prepare Cu into fine lines by the conventional plasma etching process due to the non-volatile plasma reaction product, i.e., Cu halides, below 400oC [1]. Currently, the chemical mechanical polishing (CMP), i.e., the single or dual damascene method, is the most popular Cu fine line definition process in ultra large scale integrated circuit (ULSIC) fabrication. However, the CMP method requires many environmental unfriendly chemicals such as strong oxidants and surfactants [2]. The process is complicated and difficult to control. Kuo and Lee reported a novel plasma based Cu etch process [3-7]. Instead of vaporizing the plasma reaction product, the Cu film was converted into a chloride or bromide compound, which was subsequently dissolved in a dilute hydrochloric acid (HCl) solution. Submicrometer wide Cu lines were successfully fabricated with this method [7]. This process was used in the fabrication of 15-inch thin film transistor liquid crystal displays (TFT-LCDs) [8] and etching of source, drain and gate electrodes of TFTs [9]. It has also been used to prepare interconnect lines in BiCMOS chips [1]. In practical applications, the metal film is often deposited on a topographic surface, e.g., above a vertical or near-vertical dielectric step by sputtering. The cusp structure is formed at the bottom of the step due to the shadow effect [10]. The film at the cusp region usually is thinner than that at the flat section and has high local stress, which is easily attacked under the dry or wet etching condition. In the plasma etching process, conductive residues are also commonly left at the cusp region, i.e., the stringer formation, which can shorten the adjacent lines [11]. Although there are many studies on the plasma etching of aluminum (Al) and molybdenum (Mo) films on the stepped surface [12,13], there is no rep
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