As projection optics have drastically improved over time, the effects of illumination non-uniformity have a more significant impact on the pattern fidelity. For instance elliptical uniformity results in a difference in sigma for horizontal and vertical features and radial non-uniformity results in a difference in through pitch patterning.
This paper specifically explores the effects of radially non-uniform illumination on through pitch imaging. Explaining how annular illumination works and showing an example of selection of annular apertures for pitch tuning will allow us to understand the effects of illumination non-uniformity on imaging through pitch for standard disk profiles. A proof that this pitch tuning, which is the overlaying of diffracted orders at a given pitch, is mathematically equivalent to strong phase shift condition at small angles will be demonstrated. Simulations with Intel Photolithography Simulation tool (iPhotoTM), using measured illumination profiles collected with a Litel Source Metrology InstrumentTM (SMI) from two different back end metal lithography tools will demonstrate that the model data fits actual wafer data on a metal interconnect process for tools with different illumination profiles. This practical example illustrates the need for lithography engineers to do sensitivity studies of layers for their process on illumination non-uniformity & to set appropriate limits on the allowable range of illumination uniformity.