2.4 Photomask

The photomask carries the pattern to be printed. The layout of this pattern is generated by transforming the circuit diagram with the help of CAD tools into geometrical shapes. The mask containing one chip pattern or a matrix of several chip patterns is also called reticle. The mask is drawn in 2x-10x actual size, with 5x most common due to die size requirements. It can either be clear-field (correct positive) or dark-field (correct negative), depending if the image or its mirror images appears opaque. The latter one is preferred because light scattering effects are reduced, and the probability that particles will fall on an opaque area, where they are not imaged, is increased. Airborne particle contamination can be further relaxed by putting a thin protective membrane over the mask. This pellicle captures particles and keeps them out of focus, where they have little effect on the image. Depending on their operation principle photomasks can be divided into two broad categories: conventional binary masks and advanced phase-shifting masks.