As device geometries shrink, new illumination sources will be needed to obtain practical resolution. The krypton fluoride (KrF) excimer laser (248nm) is capable of 0.30 μm resolution with conventional illumination methods. A weak quadrupole off-axis illumination technique combines the advantages of two-beam imaging (off-axis illumination), and three-beam imaging (conventional illumination). This allows for the advantage of increased Depth-of-Focus (DOF) without the degradation of isolated patterns. This method is suitable for use with an attenuated phase shift mask (APSM) which also enhances resolution and DOF.
"Super Resolution Techniques for DUV Optical Lithography,"
Journal of the Microelectronic Engineering Conference: Vol. 7
, Article 16.
Available at: http://scholarworks.rit.edu/ritamec/vol7/iss1/16