Advanced Metrology for Backside Metallization Using Picosecond Laser Ultrasonics

Abstract

Picosecond Ultrasonics (PULSE™) Technology has emerged as a leading metrology solution for characterizing single-layer and multilayer metal films in advanced semiconductor manufacturing [1]. As a noncontact, non-destructive technique, PULSE Technology has become the tool-of-record across multiple device segments, including logic, radio frequency (RF), memory, microelectromechanical systems (MEMS), and flash memory. Its ability to measure both film thickness and elastic modulus [2,3] in-line makes it indispensable for process control and material characterization.
Backside metallization (BSM) is essential in semiconductor fabrication, particularly for power electronics, logic ICs, and advanced memory devices. It involves depositing metal layers on the wafer’s reverse side to improve electrical conductivity, thermal dissipation, and mechanical stability. As device architecture becomes more complex, precise control and measurement of BSM layers are critical for ensuring performance and reliability. Driven by demand from semiconductors, photovoltaic, MEMS, and LED sectors, the BSM market is expanding rapidly [4]. Technologies such as PVD, CVD, electroplating, and sputtering enable tailored metal stacks, but they also pose metrology challenges—especially for non-destructive, multilayer characterization. These metal layers typically range from 50 nm to 3 µm in thickness, with total stack thicknesses reaching up to 5 µm.
Accurate characterization of BSM layers is essential for ensuring device reliability, performance, and manufacturing yield. Traditional metrology techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), four-point probe, X-ray reflectometry (XRR), and X-ray fluorescence (XRF) often face limitations in throughput, destructiveness, thickness limitations, or sensitivity to surface roughness. In contrast, Picosecond Ultrasonics offers a compelling alternative, providing high-precision measurements across a wide thickness range with micron-scale spatial resolution. Its ability to simultaneously resolve multiple layers in a single measurement, even on rough or nonplanar surfaces, makes it particularly well-suited for BSM applications.
In this paper, we demonstrate the application of Picosecond Ultrasonics to backside metallization metrology. We present data showing its capability to measure both single-layer and multilayer metal stacks with excellent repeatability, long-term stability, and high throughput. The technology’s small spot size—on the order of microns—enables measurements on small pads as small as 15 µm, improving spatial resolution and enabling in-line monitoring on product wafers. Furthermore, advanced features such as dual modulation, crossed polarization, and adaptive measurement controls enhance performance on challenging surfaces, including aluminum and complex BSM stacks.