The development of a miniaturised spectroscopic reflectance system for thin-film multilayer thickness measurements

The in-process inspection of multilayer thin-film thickness across the width of a substrate is a requirement in roll-to-roll (R2R) manufacturing. However, the current inspection systems present limitations related to the nature of the techniques, in addition to the low scalability, modularity, high cost and complexity of implementation on the manufacturing floor. This work introduces the in-house development of a dimension-reduced and modular, single-sensor reflectometer instrument, enabling precise measurement of individual layers within multi-layer coated samples. This work includes the development of a new hardware design integrating a microprocessor, a light source, a logic converter and a spectrometer sensor; additionally, a unique software platform was developed to perform near real-time multilayer coating thickness measurements. Seven electrodeposited samples were created for testing and validation purposes. The results of more than one hundred and thirty measurements per sample show that the thickness of an Indium Tin Oxide (ITO) layer ranged between 125.48 to 138.82 ± 2.87 nm, achieving less than 7% error compared to vendor specifications (specification: 130 nm). Additionally, the thickness measurements from the sensor revealed a linear response of the Poly(3,4-ethylenedioxythiophene):Poly sodium 4-styrene sulfonate (PEDOT:PSS) layer with increasing electrodeposition charge, resulting in thicknesses ranging from 201.47 to 506.03 ± 2.73 nm. The successful thickness single-point measurements lay the foundations for the development multi-sensor array to allow thickness measurements along the full width of coated substrates in R2R manufacturing.