3.4.1 Phase-Shift Masks

One important enhancement in lithographic techniques is the use of phase-shift masks [30]. They take advantage of the interference effect in a coherent or partially coherent imaging system to reduce the spatial frequency of a given object or to enhance its edge contrast [31]. This way, it is possible to optimize both the resolution and DOF.

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 $1 \angle 0$ (unity modulus and zero phase) and $1 \angle 180$ 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.

Figure 3.10: Phase-shift mask edition.