Determining Free Volume Changes During the PEB and PAB of a Chemically Amplified Resist

Chemically amplified resists provide some trade-off between resolution and amplification. While it is necessary for a single photogenerated acid to be mobile enough to cause several deprotection reactions, this inevitably leads to some linewidth spread. An acid molecule mobile enough to travel to several reaction sites is also mobile enough to move into unexposed regions of the resist. In order to optimize this trade off and design better photoresists, it is necessary to gain a complete understanding of the mechanism of acid transport during the post exposure bake. Some proposed mechanisms for acid transport consider the transport to be reaction enhanced in some way. The reaction front propagation model suggests that the deprotection reaction enhances local acid transport by either the generation of free volume or the plasticization of the resist by the reaction by-products. As the volatile reaction byproducts leave the resist, the free (or plasticized) volume collapses, providing a mechanism to halt acid transport. However, direct experimental evidence has not yet been provided to establish that free volume is indeed generated during the post exposure bake (PEB).