Novel compensation chemical metal polishing for low dishing and high global planarity for ultra-planar die applications in micro-optics and micro-electro-mechanical systems

Abstract

Some recent micro-optical applications require die planarity on the scale of a fraction of wavelength of light. A novel chemical mechanical polishing (CMP) process on a damascene structure is described for such requirements. We present an approach that decouples obtaining patterning and planarity, into separate steps, not only to better control metal dishing but also to establish a high degree of global planarity in the metal/dielectric matrix. A tungsten-on-oxide damascene structure is used for demonstration. On a damascene structure, tungsten is polished to form a pattern on the entire wafer. A controlled, uniform, partial oxide removal step, commensurate to the dishing, is then done to expose tungsten pillars. This step provides compensation for initial overpolish-dependent dishing. The exposed tungsten is finally polished to the compensated oxide level to obtain a highly planar wafer surface. Local and global planarity data measured across the wafer are presented and compared to conventional CMP. The local planarity, measured as root mean square (RMS) of deviation from local surface mean height, is measured to be 5.5±1.5 nm. This gives an eight-fold improvement and enhanced uniformity over conventional damascene on comparable structures. Global planarity is controlled to a 12 nm standard deviation with respect to the wafer surface.