(614c) Negative Tone Photoresists Based On Cationic Polymerization for High Resolution Lithography

As critical feature sizes in photoresists (also referred to as resists) have shrunk to 32 nm and below, a major cause of failure is pattern collapse caused by the very high capillary forces in the nano-scale features. Higher molecular weight resists tend to show superior pattern collapse behavior because they are more mechanically robust than low molecular resists, but generally have poorer resolution than lower molecular weight resists for several reasons, one of the major of which is swelling of the resist during development. This amounts to a trade-off between resolution (low molecular weight) and pattern collapse (high molecular weight). To combine benefits of both high and low molecular weight resists, we have developed a new family negative tone resists based small molecule glasses (molecular glass resists).

The molecular glasses are at the extreme low end of molecular weight and can be thought of almost as multi-functional monomers. In the exposed areas, photoacids are generated that cross-link the molecular resists during a post-exposure bake to create an incredibly high molecular weight polymer network that remains during development. This design approach shows superior resolution to negative tone resists with higher molecular weight polymers such as SU-8 because the unexposed areas are very low molecular weight with an enormously high dissolution rate. The exposed areas have very high cross-link density which improves swelling since each initial resist molecule has low functionality (2-4 per resist) relative to higher molecular weight polymers. The design shows superior pattern collapse to positive tone polymer resists which do not undergo an increase in molecular weight because the remaining resist is a very high molecular weight networked polymer. Pattern collapse is probed and quantified using a specially designed pattern layout structure that allows for the control and modulation of the capillary forces experienced by the photoresist line pairs. Several generations of this class of materials will be discussed along with a novel method of in-situ controlling the cationic polymerization reactions that allow for even higher resolution. These materials have shown dense resolution down to 15 nm at a sensitivity that is one-to-two orders of magnitude better than other resists with comparable resolution.