Optical flatness and alignment mark contrast in highly planar technologies

The authors discuss an interlevel alignment problem encountered during the implementation of a highly planarized microelectronics manufacturing technology. The problem is caused by alignment marks with poor bright field image contrast and affects the ability of alignment and registration systems to properly detect and resolve mark positions. The bright field image contrast of the marks, which are formed by recessing polysilicon through a patterned silicon nitride layer, is found to depend strongly on both the depth of the mark as well as the thickness of the overlying nitride. Computer simulation is used to explore a correlation between poor contrast and small optical step height, highlighting a distinction between physical flatness and optical flatness. A simple optical path length calculation which takes into account process variations serves as a check for optical flatness and provides a set of guidelines for ensuring reliable detection of alignment marks.