Proximity Printing

Proximity printing is another optical lithography technique. As its name implies, it involves no contact between the mask and the wafer, which is why masks used with this technique have longer useful lives than those used in contact printing. During proximity printing, the mask is usually only 20-50 microns away from the wafer.

The resolution achieved by proximity printing is not as good as that of contact printing. This is due to the diffraction of light caused by its passing through slits that make up the pattern in the mask, and traversal across the gap between the mask and the wafer.

This type of diffraction is known as Fresnel diffraction, or near-field diffraction, since it results from a small gap between the mask and the wafer. Proximity printing resolution may be improved by diminishing the gap between the mask and the wafer and by using light of shorter wavelengths.