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

The theory of interference mapping lithography (IML) is presented for low k1 (k1<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 0° 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 (k1=0.39) contacts was created and exposed on an ASML /1100 ArF scanner using NA of 0.75 and Quasar illumination (σin=0.72, σout=0.92, span angle=20°). Measurements on the exposed wafers show that IML CPL results in printing 100nm contacts through pitch (200nm minimum pitch to isolated) with 0.45μm DOF at 10% EL.