Significance of plasma-surface interactions in the etch behavior of low-k materials

Low-k materials are an integral component in the advancement of semiconductor device performance by reducing parasitic capacitance and enabling faster device switching for a given thickness compared to traditional dielectric materials such as SiO2. With the advances in logic scaling, low-k materials are increasingly more prominent in the structures of advanced devices. For example, low-k materials are essential as the spacer material to provide both etch selectivity between dielectric materials and electrical isolation in field effect transistors. Consequently, the integration of low-k materials requires that the etch behavior of these materials be well understood so that the device structures can be reliably and reproducibly fabricated. In this study, the authors used a high-density plasma reactor with benchmark CF4- and NF3-based process chemistries to etch low-k materials including SiCN, SiOCN, and SiBCN in addition to Si, SiO2, and SiN reference materials. Numerous characterization techniques were utilized to understand the relationships between the plasma conditions, the evolution of the surface chemistry of the materials, and the resulting etch behavior. These techniques consisted of optical emission spectroscopy, spectroscopic ellipsometry, x-ray photoelectron spectroscopy, and attenuated total reflection Fourier transform infrared spectroscopy. The etch behavior of low-k materials under a given etch process is vital for establishing the etch selectivities in multilayer structures that are required to yield complex device geometries. For example, a directly proportional correlation was observed between the etch rate and intrinsic nitrogen concentration of the low-k materials. Potential mechanisms for the observed etch behaviors were explored using modeling and found that the intrinsic nitrogen composition in the low-k materials can result in energetically favorable reactions that result in the weakening and volatilization of the Si-N bond. Identifying the underlying mechanisms for the etch behaviors of low-k materials will provide key guidance into the development of etch processes that integrate these materials in current and future device structures.