Comparison of thickness determination methods for physical-vapor-deposited aluminum coatings in packaging applications
Methods used to determine the aluminum coating thickness on polymer films may not measure the geometrical thickness directly but may instead measure the mass or other properties, thus leading to different thickness values. Common methods include the determination of evaporation rates using a quartz crystal microbalance (QCM) and the quantitative analysis of dissolved aluminum ions by inductively-coupled plasma mass spectrometry (ICP-MS), which provide mass thickness values. Alternatively, atomic force microscopy (AFM) and interference (INT) across the step of a partially removed aluminum layer yield geometrical values, and optical density (OD) and electrical resistance (ER) measure other properties. We compared the ability of these methods to determine the thickness of aluminum coatings applied to polyethylene terephthalate (PET) and paper by physical vapor deposition. We measured ER using four-point probes, five-point probes, and eddy currents. ER and OD achieved high precision but low accuracy, showing that the resistivity and absorption coefficients of thin aluminum layers can deviate from bulk constants. When the constant values were adjusted, both methods achieved higher accuracy. ICP-MS and QCM values were similar, when a geometrical model was applied, and in comparison AFM and INT showed low precision but high accuracy. When the aluminum was applied to paper instead of PET, only ICP-MS generated reliable results. In summary, the values derived using these different methods are only in agreement when method-specific constants such as absorption coefficients and resistivity are suitably modified.