Glass substrates are emerging as an important platform for advanced packaging and redistribution layers, driven by their combination of low coefficient of thermal expansion, excellent surface flatness, and low dielectric loss. These properties enable finer RDL pitch than conventional organic substrates, better lithographic overlay across large panel areas, and lower signal loss at high frequencies. However, the transition to glass introduces new metrology challenges. Dielectric films on glass require simultaneous characterization of multiple semi-transparent layers. TGV fill quality demands non-contact depth and profile measurement. Furthermore, panel-level flatness must be monitored inline across large areas at nanometere sensitivity. lineWLI addresses all three directly. Its signal processing handles multilayer transparent stacks natively. Its single-shot line sensor delivers the throughput needed for panel-scale inspection. Finally, as an optical technology lineWLI does not need to touch thin and fragile glass substrates.
The first test measurements were done on 1mm thick float glass. For illustrative purposes a scratch was induced with a silicon carbide glass cutter. The scale is in nanometers.
The silicon carbide tool left a clear scratch in the tool. Even though the scratch is clearly visible with the naked eye, lineWLI shows it to be 25 µm wide and 140 nm deep.
The elevation map show a diagonal texture which is assumed to be an artifact from the production process. A profile along the y-dimensions without the scratch, shows them to be 3 … 5 nm high and 10 … 15 µm wide.
