Use of MIR-FTIR and k-value Measurements to Assess Potential Solutions to Reduce Damage During Porous Low-k Integration

Multiple Internal Reflection (MIR) FTIR is used to assess the impact of potentially damaging BEOL integration process steps such as Chemical Mechanical Polishing (CMP) and NH3-based plasmas on a k=2.55 porous interlayer dielectric. The NH3 plasma is found to be very damaging, and a He plasma prior to the same NH3 plasma leads to a damage reduction. The impact of CMP is also confirmed and leads to an equivalent k-value degradation as a He + NH3 plasma. In order to reduce the integration damage, the addition of a low-k protective cap layer (k=3.3) is proposed. The required thickness to avoid impact from CMP and NH3-based plasmas is found to be 10 nm after CMP (Kstack=2.60 with 10 nm cap). As a result, those process steps are no longer a concern with this integration route.