The Study of Modified Layers in SiCOH Dielectrics using Spectroscopic Ellipsometry
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The Study of Modified Layers in SiCOH Dielectrics using Spectroscopic Ellipsometry Marcus A. Worsley*, Stacey F. Bent*, Stephen M. Gates‡, Kaushik Kumar‡, Timothy Dalton‡ and Jeffrey C. Hedrick‡ *Dept. of Chemical Engineering, Stanford University, Stanford, CA 94305-5025 ‡ T.J. Watson Research Center, IBM, Yorktown Heights, NY 10598 ABSTRACT The current challenge in designing new low-k dielectrics is realizing sufficient mechanical and chemical stability such that the material can be integrated into current damascene schemes. The material of interest in this study is a nonporous SiCOH composite (carbon-doped silicon oxide, also known as organosilicate glass “OSG”) for use as an intermetal dielectric (IMD). During integration of this IMD, processing steps such as etch, resist strip and chemicalmechanical polishing for planarization may chemically alter the outer layer of the dielectric. Here, spectroscopic ellipsometry is used to characterize the modified layer of SiCOH films after exposure to different resist strip plasmas. The data are analyzed based on a 2-layer model, consisting of a carbon-deficient layer on the surface of the low-k SiCOH dielectric. This model is supported by XPS and FTIR data. The effects of two types of plasma etch chemistry on the formation of this modified layer were studied, and differences between the two chemistries were found. The 2-layer model accurately describes the modifications produced by the oxidizing plasma, but its description of the modified layer formed by the plasma involving nitrogen is not complete. A 3-layer model with an additional nitrogen-doped layer is suggested. INTRODUCTION New materials are necessary to continue the existing trend toward smaller feature sizes in tomorrow’s computer chips. In the past, chip speed was limited by the size of a transistor, but in the near future the limitation will be defined by the RC delay in global interconnects [1,2]. As a first step to reduce the RC delay, many chip manufacturers have replaced aluminum wiring with lower resistance copper. In an effort to reduce the capacitance between lines, the next step the industry is taking is to find a material with a lower dielectric constant, k, to replace silicon oxide as an interlayer dielectric (IMD). There is a wide variety of new low-k materials [3-8] being researched and many issues have arisen involving the integration of these materials [2,9]. The current challenge has thus become one of designing low-k materials with mechanical and chemical properties such that they can be successfully used in current integration schemes. These films must have sufficient mechanical strength to withstand reliability stresses and chemical robustness to remain unaltered by integration processes such as photoresist strips. For the upcoming 90nm technology node, much of the industry is focused on incorporating organic content into a Si-O network to reduce the k-value to less than three [2,9]. A class of films that meets that criterion is the organosilicate glass (OSG), also referred to as SiCOH
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