Improved complementary phase shift mask (c:PSM) imaging techniques are described, including a method in which scattering bars are provided on the trim mask in order to allow better CD uniformity to be achieved in the double exposure process. The number, size and position of the scattering bars can be optimised to achieve a desired isofocal CD and/or a desired level of sensitivity of the CD to trim exposure energy used in the second exposure step of the c:PSM process. The trim exposure dose can be regulated, and/or the trim width used on the trim mask can be optimised, to compensate for iso-dense bias so as to achieve optical proximity correction.