A HFIPS-based polymer approach for 157-nm single layer photoresist

Resist materials for 157nm lithography is believed to be one of the key technology for producing patterns below 70nm. Many different types of fluorine-containing polymer platforms have been energetically pursued by a number of researchers, and some of them appear to be promising in giving a high transparency that has been the essential challenge in realizing this technology. While such highly transparent polymers are the premise in achieving a good imaging, how to get sufficient etch resistance of the polymers can be of another challenge. Actually it is often reported that the etch resistance and the transparency are in trade-off relationship in many cases as a function of fluorine atom content in the polymers. Therefore how to design an etch-resistant polymer while maintaining the good transparency is still a big challenge in developing a practically usable 157nm polymer platform. One of the polymer platforms that the authors believe useful for 157nm is the polymers having hexafluoroisopropanolstyrene (HFIPS) monomer unite in their backbones. The HFIPS unit is attractive because the styrene group provides good etch resistance and hexafluoroisopropanol group (HFIP) provides an acidic molecule while implementing a transparency into the molecule. The lithographic potential of the HFIPS-based polymer system was demonstrated with the fact that a prototype resist from this system was able to print a 75nm line and space 1:1 pairs with an attenuated PSM under 0.60NA stepper exposure. A relatively thin resist thickness, 100nm, was applied due to the limited transparency of the polymer. The patterned exhibited very smooth line edge and a clear pattern definition although a slight T-topping was observed. The results imply that we should be able to achieve a similar lithographic performance with a thicker film (150~200nm), if we can further increase the transparency of the HFIPS-based polymer. The authors are pursuing the approach further aiming at this direction and are getting several new polymers that are more transparent. The paper will present some of the results from later work with such an attempt. The paper will also discuss etch resistance of the HFIPS-based polymer. The etch rates measured for the HFIPS-based polymers were only around 10% faster than standard 248nm resist, which we believe fairly good among various fluorine-containing polymers so far proposed. This was convincing that this polymer system could provide a competitive platform in the practical use. It is generally thought that the etch rate of resist films are mainly affected by their polymer compositions or structures but there are few reported on the influence of the other components in resist formulation. The authors found that the concentration of PAG and quencher influenced both etch rate and resist surface roughness after the etch in this materials system, which implied there are some more room for further etch resistance improvement.