Contact hole reticle optimization by using interference mapping lithography (IML™)

The theory of interference mapping lithography (IML) is presented for low k(1) (k(1)<0.4) contact hole imaging. IML with a coherent source is shown to be analogous to methods used in creating a Fresnel lens. With IML for a partially coherent source, the interference map is calculated by using the first eigenfunction of the transmission cross coefficient (TCC). From this interference map, clear 0degrees AFs and for clear 180 AFs are placed in the optimal location. Thus, IML is a method to place AFs via a model. From the interference map, a method for creating a CPL mask is demonstrated. Using IML, techniques to optimize a binary mask or a CPL mask are presented for maximizing the exposure latitude (EL) or depth of focus (DOF). These techniques are verified with simulation. Using IML for maximum EL, a CPL mask with 100nm (k(1)=0.39) contacts was created and exposed on an ASML /1100 ArF scanner using NA of 0.75 and Quasar illumination (sigma(in)=0.72, sigma(out)=0.92, span angle=20degrees). Measurements on the exposed wafers show that IML CPL results in printing 100nm contacts through pitch (200nm minimum pitch to isolated) with 0.45mum DOF at 10% EL.