The electromagnetic effects of opaque mold materials in exposure, and complementary mask inspection in the innovative nanoimprint technique of White and Wood are explored through simulation. For a wavelength of 365 nm and 40 nm features in an opaque layer,with 10% transmission, an extinction coefficient of more than 3 is important for a high transmission in the open features and a refractive index larger than 2 is also useful. For complementary mask inspection the leakage is largest for the polarization perpendicular to the edge and depends on edge length. The leakage is not a strong function of misalignment and rises less than linearly, with the separation between the facing surfaces of the two masks. Defects one-third of a feature size are one-twentieth of a wavelength in size, and with filtering of the radiated spectrum can contribute up to 10% of the signal associated with leakage from a complete feature. (C) 2003 American Vacuum Society.