We present the analyses of surface plasmon polaritons (SPPs) coupling induced interference in metal/dielectric (M/D) multilayer metamaterials and techniques to improve the performance of sub-wavelength plasmonic lithography. Expressions of beam spreading angles and interference patterns are derived from analyses of numerical simulations and the coupled mode theory. The new understandings provide useful guidelines and design criteria for plasmonic lithography. With proper layer structure design, sub-wavelength uniform periodic patterns with feature size of 1/12 of the mask's period can be realized. High pattern contrast of 0.8 and large field depth of 80 nm are also demonstrated numerically by considering the SPPs coupling in the photoresist. Both high contrast and large image depth are crucial for practical application of plasmonic lithography.