(P.14) High-Speed Contactless Electrical Evaluation of Printed Electronics using Inductive Sensors Adam Lewis*, Chris Hunt, Owen Thomas and Martin Wickham National Physical Laboratory, Teddington, UK Abstract
The market growth of conductive inks for flexible and printed electronics is predicted to increase from $2.3 billion to $3.2 billion by 2025 1. It is important that we are able to verify the electrical properties of printed electronics to maintain output quality and reduce wastage. Therefore we are driven to find a suitable non-contact technique suitable to measure the electrical properties of the printed structures during manufacture in a roll-to-roll process. The emphasis of the work reported here is to use inductive sensors to measure electrical response. The impedance of an inductive sensor is affected by eddy currents induced in nearby conductors such that the electrical loss will be increased. The electrical response will be affected by a number of factors including material resistivity and thickness, temperature and sensor lift-off (distance between sample and sensor). By controlling temperature and lift-off we are able to use an inductive sensor to measure electrical impedance and relate it to the electrical properties of printed ink structures. By combining the electrical measuring system with another (such as optical) into a single metrology system for evaluating film properties, with extra thickness and lift-off data, it would be possible to extract the actual electrical resistivity of printed tracks and films. A range of samples with varying values of resistivity and height were fabricated by screen printing. Using a surface profiler the height of our samples were measured, typical examples can be seen in Table 1; additionally the resistance between two fixed-position probes was measured on a probe station. In the example shown here, sample 5 contains no silver whereas the remaining samples contain varying amounts of silver and carbon such that a range of values of resistivity could be investigated. The inductive sensor was based on a 100 µH inductor (7.5 mm diameter) which had its cap removed to expose the coil. Using an impedance analyser the frequency response of the inductor between 8.5 to 11 MHz (this range covers the resonant frequency of the inductor) was monitored with an air gap between the sensor and sample of ~20 µm. The maximum impedance measured within this frequency range is plotted in Figure 1 against the sheet resistivity. The red line in Figure 1 shows the value of the impedance the bare substrate. This data clearly shows that values of sheet resistivity below 8 Ω/square can be detected using this technique. Contactless inductive sensing using eddy currents can provide information on changes in the electrical properties of samples. This makes it feasible to measure the electrical properties which at present are not monitored in roll-to-roll printed electronics manufacture.
1
D. Savastano, “Conductive Inks Drive Growth in Flexible and Printed Electronics”, Printed Electronics Now, March 2015 108