Abstract: A method is disclosed for forming dual damascene structures with a multi-purpose composite mask. The composite mask serves not only to prevent via poisoning, but also to improve the lithographic characteristics of forming a dual damascene structure. This is accomplished by using a mask comprising silicon-based as well as polymeric dielectric layers. Thus, one of the components of the composite mask, namely, the polymeric dielectric, makes it possible to protect the via openings by conformally covering the sidewalls of the via and, at the same time, by bringing controllability to the height of the protective dielectric in the via opening. In addition, because the polymeric dielectric also serves as the main plasma resisting layer during the trench etch, the required photoresist is much thinner; therefore, the lithography process window can be extended beneficially.