Cationic graft lithography is a novel photoresist technology that seeks to overcome the limitations of current photoresists in the move to next-generation techniques for microchip production.  The low-absorbance requirement of current photoresists forces the semiconductor industry to use different materials for each exposure wavelength.  New materials must be invented for each new wavelength generation.  In addition, these resists are liquid-developed, which can result in feature collapse upon drying.  In cationic graft lithography, photochemistry needs to occur only at the top surface of the film, thus avoiding the absorbance requirement (see figure 1 below).  Development is done using a dry etch process so feature collapse is prevented.  In the grafting process, an inert polymer film containing a photoacid generator (PAG) is coated on to a wafer.  Exposing this film to ultraviolet radiation generates acid on its surface, to which a vapor-phase silicon-containing monomer is grafted.  The silicon provides a barrier to oxygen etching.

Figure 1. Process schematic for the Grafting process
A solution containing inert polymer and PAG is coated on the substrate to be patterned.  Selective areas of the resulting film are then irradiated, converting the PAG to acid.  The wafer is then placed in a vacuum chamber and exposed to pure monomer vapor, which polymerizes upon contact with acid in the surface of the film.  A subsequent oxygen reactive ion etch removes the base layer in unexposed areas, while the graft in the exposed areas resists the etch development.