Middle-of-line plasma dry etch challenges for CFET integration

In this paper, middle-of-line (MOL) plasma etch development results for the monolithic CFET integration with nanosheet devices using scaling-relevant test vehicle (CPP48nm) are presented. Several critical MOL patterning steps are addressed, with the focus on the patterning of the trenches (M0) for contacting to the bottom and top devices. The patterning of M0A consists of SiO2 dielectric and thin SiN liner etch landing on epitaxial source drain (S/D). The critical M0 etch requirement is preserving the SiN gate spacer to avoid shorting between S/D and gate. Due to no-gate plug implementation in the process flow, the etch development must rely on very challenging, patterning the small critical dimension (CD) contacts to create enough dielectric barrier between the metal contact and the gate, and preferably, also very challenging, self-alignment to the thin gate spacer. The dependance of the M0 CD and the etch depth is accessed by using the range of the EUV lithography conditions and evaluating the maximum etch depth of the trench as a function of the printed CD. The minimum trench CD achieved on the bottom of the trench is ~ 13nm, and the minimum top CD in the range of ~ 16nm, with the evident etch non-uniformity observed in the etch depth. The trend of larger contact CD resulting in the deeper etch and process uniformity improvement is observed. Etch depth larger than 100nm is achieved when top M0 CD is >20nm. The option with the SiN liner deposition followed by SiN liner etch (spacer formation) post- M0 SiO2 is developed. This patterning sequence consists of SiO2 etch stopping on the thin SiN (over S/D) followed by additional SiN deposition and finally etching of the deposited SiN liner as well as SiN liner covering S/D. The option with SiN spacer formation minimizes the risk of short to the gate, due to extra SiN dielectric film protecting the gate. In addition, we present the results for another critical MOL patterning step, i.e., HAR metal recess post M0 metallization (AR~11)