==Exposure tool selection==
Contact lithography places the
glass mask in direct contact with the sample. Because it is in direct contact with the photoresist , it is subject to picking up particles and photoresist then transferring them to subsequent wafers. The better the contact the more often the mask will need to be cleaned. However it is capable of patterning an entire wafer with a single exposure. The minimum feature size will be larger than with projection lithography, and is governed by the wavelength and how much gap is between the mask and photoresist. At the LNF we only recommend contact lithography for 2µm and larger features and 2µm or larger alignment tolerance.
With project lithography light shines through the mask, goes through a reduction lens and projects onto the substrate. Since the mask never comes into contact with the sample it stays cleaner. At the LNF 500nm gratings will reliably print on [[SPR 955]], however the resolution will depend on the feature type and photoresist thickness. As feature size decreases the depth of focus also decreases so thinner resist must be used. The [[GCA AS200 AutoStep]] has a max die size of 14.7mm x 14.7mm and a minimum alignment tolerance of 200nm for wafers, if running pieces the tolerance will be larger. This also strongly depends on the accuracy of the artwork on the mask; mask plates produced with the [[Heidelberg µPG 501 Mask Maker]] will require larger tolerances because of this.