Hybrid Scatterometry Measurement for BEOL Process Control
Scaling of interconnect design rules in advanced nodes has been accompanied by a reducing metrology budget for BEOL
process control. Traditional inline optical metrology measurements of BEOL processes rely on 1-dimensional (1D) film
pads to characterize film thickness. Such pads are designed on the assumption that solid copper blocks from previous
metallization layers prevent any light from penetrating through the copper, thus simplifying the effective film stack for
the 1D optical model. However, the reduction of the copper thickness in each metallization layer and CMP dishing
effects within the pad, have introduced undesired noise in the measurement. To resolve this challenge and to measure
structures that are more representative of product, scatterometry has been proposed as an alternative measurement.
Scatterometry is a diffraction based optical measurement technique using Rigorous Coupled Wave Analysis (RCWA),
where light diffracted from a periodic structure is used to characterize the profile.
Scatterometry measurements on 3D structures have been shown to demonstrate strong correlation to electrical resistance
parameters for BEOL Etch and CMP processes. However, there is significant modeling complexity in such 3D
scatterometry models, in particlar due to complexity of front-end-of-line (FEOL) and middle-of-line (MOL) structures.
The accompanying measurement noise associated with such structures can contribute significant measurement error. To
address the measurement noise of the 3D structures and the impact of incoming process variation, a hybrid scatterometry
technique is proposed that utilizes key information from the structure to significantly reduce the measurement
uncertainty of the scatterometry measurement. Hybrid metrology combines measurements from two or more metrology
techniques to enable or improve the measurement of a critical parameter.
In this work, the hybrid scatterometry technique is evaluated for 7nm and 14nm node BEOL measurements of interlayer
dielectric (ILD) thickness, hard mask thickness and dielectric trench etch in complex 3D structures. The data obtained
from the hybrid scatterometry technique demonstrates stable measurement precision, improved within wafer and wafer
to wafer range, robustness in cases where 3D scatterometry measurements incur undesired shifts in the measurements,
accuracy as compared to TEM and correlation to process deposition time. Process capability indicator comparisons also
demonstrate improvement as compared to conventional scatterometry measurements. The results validate the suitability
of the method for monitoring of production BEOL processes.
Keywords: OCD, scatterometry, hybrid metrology, BEOL, process control, variability, dielectric