A photoresist is a light-sensitive polymer. When exposed to ultraviolet light, it turns to a soluble material. Those exposed areas can then be dissolved by using a solvent, leaving behind a pattern.
While this has been done for years in semiconductor lithography, the problem is that the 193nm ArF light beam is too thick for sub-22nm designs to be done using a single pass of a laser. Extreme ultraviolet, however, has run into repeated delays because the power source is insufficient to make it economically viable. While work continues on the power source, researchers also are trying to make the photoresist more sensitive--basically tackling the problem from two sides instead of just one.
Most of the resists in use today are chemically amplified resists. In those, incoming light does not directly cause the backbone polymer to become soluble. Rather, the light is absorbed by photoacid generators (PAGs), which release multiple photoacid molecules per photon. The photoacid reacts with protective groups on the polymer backbone, “de-protecting” it and causing it to become soluble in developer. With EUV resists, the incoming light can also excite photoelectrons, which in turn can drive the photoacid generation reaction.
Thus, at the very heart of chemically amplified resist chemistry we find a tradeoff between resolution and sensitivity. To maximize resolution, resist designers would like to have small backbone molecules, and for the photoacid de-protection reaction to take place close to the location of the PAG. Both the photoacid diffusion distance and the amount of resist de-protected by each photoacid molecule should be small.
To maximize sensitivity, on the other hand, designers look for nearly opposite characteristics. The photon capture cross-section should be large, to make use of as many photons as are available. Each photon should generate many photoacid molecules, which should rapidly diffuse into the resist layer to de-protect as many polymer chains as possible. Unfortunately, the large diffusion distance this behavior implies contributes to blur, the uncertainty in the dimensions of an exposed feature.