Middle of Line (MoL) Cleaning Challenges in Sub-20nm Node Device Manufacturing

In advanced technology nodes (sub 20nm), the gate & active contact architecture has become very complex. This architecture not only introduced new materials but also integrated additional patterning mask layers. This necessitated a separate Middle of Line (MoL) zone whereas conventionally contact integration used to be a Front End of Line (FEoL) process. This paper discusses wet cleaning challenges in MoL that were unforeseen with conventional contact architecture. Typical chemistries such as Sulfuric Peroxide Mixture (SPM), dilute Hydrofloric Acid (dHF), Aqua Regia, Standard Clean 1 (SC1), etc. that were used for contact cleaning or in salicidization process are found to be too aggressive due to smaller process window, shrinking Critical Dimensions (CD), and other challenges arising from overall tighter tolerances. As a result of device scaling, most of the MoL mask patterning is done with immersion lithography and double patterning techniques such as Litho-Etch Litho-Etch (LELE) are also needed. Immersion lithography is very sensitive to pre-litho backside and frontside particles which make pre-litho cleaning in MoL very critical as well. Also due to lack of high aspect ratio features in MoL (mostly contact holes), physical particle removal techniques such as droplet spray and MegaSonic can be very effectively used to achieve higher Particle Removal Efficiency (PRE). This paper summarizes such different scenarios & related challenges.