Invited Paper PS+EM-WeA7
Innovative Approaches for Future Challenges in MOL/BEOL Etch

Wednesday, October 24, 2018, 4:20 pm, Room 104C

Critical dimensions (CD) continue to shrink driven by the quest for cheaper, faster and less power-consuming devices. If simple shrink was not enough, all of the back end, middle and front end of line (BEOL, MOL and MOL) also have introduced structural complexity and stringent topographic dimension, material property integrity and fundamental integration yield requirements. Self-aligned contact (SAC), high aspect ratio contact (HARC) and damascene structures in the MOL and BEOL typify challenging integrations. SAC structures are formed by oxide being etched from a nitride encasement. The oxide must be etched both beside and over thin (few nm) nitride films with near infinite selectivity as horizontal nitride layers can be exposed far before the deepest oxide in contact vias are removed. These structures are subject to “plugging” if the films get to thick, loss of nitride if the films get too thin, and etch rate or profile integrity loss elsewhere due to imbalances in ion energy flux or radical loss due to shadowing in a deep via. Obtaining the perfect balance of radical flux, ion flux and ion energy for these structures over a single die, let alone an entire wafer full of dies, is nearly impossible. Put in more general terms, fabrication challenges for plasma etch related to controlling local CD Uniformity (LCDU) and mitigating depth loading and CD loading are ever present due to difference in aspect ratio dependence (ARD) of transport of radicals and ions (and their energy) in features.

Atomic layer etching (ALE) has gained favor as an approach to extract more control over the fabrication of small CD complex topographic structures. The idea is that alternating steps of self-limiting processes (e.g., passivation layer formation) and desorption (e.g., the removal of a passivation layer) mitigate aspect ratio dependence effects that lead to the aforementioned problems. The problem is that not all passivation processes are self-limiting. Fluorocarbon based processes are not self-limiting rendering them quasi-atomic layer etch. Without special consideration, quasi-ALE has the same problems that continuous processes possess.

We have demonstrated the use of a new method of rapid advanced cyclic etch (RACE) comprising an isotropic CD trim step, mixed mode CVD, ALD and anisotropic bombardment to perform aspect ratio independent deposition and thereby eliminate CD bias effects. We show that X-Y CD control by ALD, CVD and trim can also be influenced by line-of-sight re-deposition from feature bottom. The ability to manage CD will be discussed as a means of enabling advanced patterning processes for both logic, interconnect and memory at advanced technology node.