Extending Reflectometry Range, A Zero-Crossing Algorithm for Thick Film Metrology

TL;DR

This paper introduces the LRZ algorithm, a fast, model-free reflectometry method that extends measurable film thickness to 50 um, enabling high-throughput, accurate thick film metrology for manufacturing.

Contribution

The paper presents the LRZ algorithm, which improves thick film measurement range and efficiency without relying on complex models or iterative fitting.

Findings

LRZ achieves at least 50 um thickness measurement range.

Validation shows high correlation (r=0.97) with white light interferometry.

LRZ is approximately 7 times faster than WLI.

Abstract

Accurate and high-efficiency film metrology remains a key challenge in High-Volume Manufacturing (HVM), where conventional spectroscopic reflectometry and white light interferometry (WLI) are either limited by model dependence or throughput. In this work, we extend the measurable film-thickness range of reflectometry to at least 50 um through a new model-free algorithm, the Linearized Reflectance Zero-Crossing (LRZ) method. The approach builds upon the previously reported Linearized Reflectance Extrema (LRE) technique but eliminates the sensitivity to spectral sampling and fringe attenuation that degrade performance in the thick-film regime. By linearizing phase response and extracting zero-crossing positions in wavenumber space, LRZ provides robust and repeatable thickness estimation without iterative fitting, achieving comparable accuracy with much higher computational efficiency than conventional model-based methods. Validation using more than 80 measurements on alumina films over NiFe substrates shows excellent correlation with WLI (r = 0.97) and low gauge repeatability and reproducibility (GR&R < 3%). Moreover, LRZ achieves an average Move-Acquire-Measure (MAM) time of approximately 2 s, which is about 7 times faster than WLI. The proposed method enables fast, accurate, and model-independent optical metrology for thick films, offering a practical solution for advanced HVM process control.