To shift the light phase, an extra layer of transmissive material with a different refractive index is necessary on the mask. Thus, one extra pattern is necessary (where a phase-shift effect must occur). For each layername the layout must support masks with two or more different transmission attributes.
As this technique is not supported by conventional layout
editors, we extended PED with the capabilities for phase-shift
masks edition. This is accomplished by specifying a value for the modulus
and phase attributes of the transmittance, for the background and
every polygon of any layer. Typically, one will use 
(unity
modulus and zero phase) and
for non-shifting and shifting patterns, respectively. For
normal binary masks, the phase value is discarded and depending on
the modulus value (1 or 0), the masks are considered positive or negative. In
Figure 3.10 the phase-shift mask widget is presented.
