The exposure via projection uses the so-called step-and-repeat technique. Thereby only one or a few dies were projected onto the wafer at a time. The entire wafer is exposed step by step - die by die.
The advantage of this method is that the structures on the reticle are enlarged 4-fold or 10-fold. If the structures are projected onto the wafer in reduced scale, also defects, like particles, are reduced. In contrast to other exposure methods the resolution is improved.
In addition a thin foil - pellicle - is attached to the mask, and therefore particles will be held off the mask and are out of focus during projection.
Besides the projection with lenses also a projection with a complex system of mirrors can be used (scale 1:1). Compared to lenses there are no color aberrations and thermal expansion of the photomask can be adjusted. However, mirror images can be distorted or warped. The resolution is limited due to the scale of 1:1.