Projection Printing

Projection printing is the third technique used in optical lithography. It also involves no contact between the mask and the wafer. In fact, this technique employs a large gap between the mask and the wafer, such that Fresnel diffraction is no longer involved. Instead, far-field diffraction is in effect under this technique, which is also known as Fraunhofer diffraction.

Projection printing is the technique employed by most modern optical lithography equipment. Projection printers use a well-designed objective lens between the mask and the wafer, which collects diffracted light from the mask and projects it onto the wafer. The capability of a lens to collect diffracted light and project this onto the wafer is measured by its numerical aperture (NA). The NA values of lenses used in projection printers typically range from 0.16 to 0.40.

The resolution achieved by projection printers depends on the wavelength and coherence of the incident light and the NA of the lens. The resolution achievable by a lens is governed by Rayleigh's criterion, which defines the minimum distance between two images for them to be resolvable. Thus, for any given value of NA, there exists a minimum resolvable dimension.

Using a lens with a higher NA will result in better resolution of the image, but this advantage has a price. The depth of focus of a lens is inversely proportional to the square of the NA, so improving the resolution by increasing the NA reduces the depth of focus of the system. Poor depth of focus will cause some points of the wafer to be out of focus, since no wafer surface is perfectly flat. Thus, proper design of any aligner used in projection printing considers the compromise between resolution and depth of focus.