During e-beam lithography applications on isolating substrates, in particular on quartz-masks or glass, the impact of energy-rich electrons often give rises to strong surface charges. The charges cannot be dissipated fast enough and electrical surface potentials result. Consequently, the electron writing beam is deflected which causes a distortion of the desired structure. A conductive film applied on or under the e-beam resist layer is able to dissipate these charges.
The problem is solved if a thin (a few nm) metal layer of gold, silver or if needed chromium is evaporated onto the substrate. During exposure, the charges can now be quickly discharged. If the metal layer on the surface is contacted and thus grounded, this process becomes even more pronounced. This procedure is however time-consuming; at first, the metal layer has to be deposited which subsequently has to be removed again prior to the development step. The procedure can be simplified if the user is able to vapor-deposit an aluminum layer on the substrate and if e-beam resist with aqueous-alkaline developers can be used (e.g. AR-N 7400 – 7700). Aqueous-alkaline developers immediately attack the aluminum and dissolve the thin layer within seconds, before a “normal” development step is performed.
Due to the rather time-consuming metallization, the demand of users for conductive films which can be applied by spin coating is quite understandable. Unfortunately however are conducting polymers (e.g. polyaniline) almost always basically insoluble in organic solvents and thus not suitable to be used for the production of resists.
Meanwhile, a small number of conductive protective films are commercially available, but these are either very expensive or associated with technical difficulties (complete removal, durability). The SX AR-PC 5000/90.1 is water-soluble. During exposure of this resist, also here a certain amount of cross-linking occurs which makes the resist more insoluble. In most cases a mixture of water and isopropanol helps to remove residual resist. We currently continue in our effort to develop alternative conductive films.