by Debra Vogler, Senior Technical Editor, Solid State Technology
Data presented by IMEC at the SEMATECH Immersion Workshop (Keystone, CO) shows patterning of through-pitch contact layers without assist features using combined illumination sources (Soft Annular and Soft QUASAR) and ASML immersion scanners, both an XT:1700Ei and an XT:1900Gi.
IMEC researcher Joost Bekaert, told WaferNEWS that the use of these specific source shapes [presented in the group’s paper] has the additional benefit of a low mask error enhancement factor (MEEF) towards dense contact pitches, while providing satisfactory and balanced process latitude through pitch.
As chipmakers continue the downscaling of devices, the pitches and targets on the wafer also become smaller. “Using single-patterning ArF lithography, downscaling requires increasing the NA at an aggressive pace, and for the patterning of contacts at half-pitches below 90nm, hyper-NA immersion imaging is required,” Bekaert explained. “However, the increase of NA unavoidably comes together with a general decrease in process latitude. In particular, for the contact layer, obtaining sufficient process margin through pitch has become a very difficult task.”
Traditional illumination sources for contact patterning are conventional illumination or off-axis illumination (OAI) conditions, such as annular or QUASAR. Denser pitches require more aggressive OAI, but that comes at a cost of losing patterning performance of more isolated contacts, which requires adding assist features to the photomask. The IMEC team reported that utilizing ASML’s Soft Annular and Soft QUASAR provided a more balanced trade-off than the conventional choices.
As the patterning of contact layers with minimal half-pitch below 90nm becomes more difficult due to low depth-of-focus for isolated contacts and/or a higher MEEF for denser contact arrays, IMEC’s development is expected to be of growing significance going forward. — D.V.
Figure 1. SEM image of a contact layer for a logic design, printed to 65nm contacts in resist. The pitch ranges from 122nm to isolated; the exposure was performed on an ASML XT:1900Gi with NA=1.35, using a Soft Quasar illumination source shape (see inset). No assist features were used.