Photolithography comprises the formation of images with visible or U V radiation in a photoresist using contact proximity, or projection printing. Here we will discuss about these printing techniques.
1. Contact Printing
In this printing technique, the photomask is pressed against the resist coated wafer with a pressure typically in the range of 0.05 atm to 0.3 atm and exposure by light of wavelength near 400 micro meters. A resolution of less than 1 micro meter linewidth is possible, but it may vary across the wafer because of spatial non-uniformity of the contact. To provide better contact over the whole wafer, a thin (0.2 mm) flexible mask has been used.
2. Proximity Printing
In proximity or shadow printing, there exists a gap between mask and wafer in the range of 20 to 50 micro meters. This has the advantage of longer mask life because there is no contact between the mask and the wafer. In the proximity printing, the mask and wafer are both placed in an equipment called a projection aligner. Looking through a microscope, an operator brings the mask into close proximity [say 10 to 20 micro meters] to the wafer and properly aligns the wafer and mask using alignment mark on the mask and the wafer. UV light is then projected through the mask on to the entire resist coated wafer at one time. This mask that is used is a full wafer x 1 mask. The resolution of this process is a function of the wavelength of the light source and the distance between the mask and the wafer. Typically, the resolution of proximity printing is 2 to 4 micro meter and is therefore not suitable for a process requiring less than a 2 um minimum line width.
3. Projection Printing
In this case the image is actually projected with the help of a system of lenses, onto the wafer. The mask can be used a large number of times, substantially reducing the mask cost per wafer. Theoretically a mask can be used an unlimited-number of times, but actual usage is limited to about 100,000 times because the mask must be cleaned due to dust accumulation, and it is scratched at each cleaning. This is costliest of the conventional systems, however mask life is good, and resolution obtained is higher than proximity printing together with large separation between mask and wafer.