Nanometer-scale SiGe lateral recess metrology in CFET structures using micro-spot X-ray fluorescence

Abstract

Complementary FET (CFET) architectures stack nMOS and pMOS devices vertically, creating buried features that challenge conventional metrology. A critical module is the selective SiGe lateral recess, which defines the gate length and inner-spacer width and strongly impacts device performance. TEM and OCD provide valuable insight but face limitations in throughput, scalability, or sensitivity for recessed structures. We demonstrate micro-spot X-ray fluorescence (µXRF) as a non-destructive, wafer-scale method to quantify SiGe recess depth in device-relevant CFETs. Normalizing the Ge Kα signal using an on-wafer Si/SiGe reference stack isolates the lateral geometry; the normalized signal correlates strongly with top-view SEM dimensions and TEM-derived SiGe lengths (R2 ≈ 0.996). Applying this calibration across 495 dies per wafer reveals clear radial non-uniformity of the lateral recess. Repeatability and error propagation show a calibration-limited 3σ depth-resolution floor of ∼1.4 nm, with intrinsic µXRF sensitivity < 1 nm at dwell times of about one minute—indicating strong potential for future inline monitoring.