Microscale additive manufacturing and modeling of interdigitated capacitive touch sensors

• Published in: Sensors and actuators. A. Physical. 2016-09, Vol.248, p.94-103
• Main Authors: Rahman, Md Taibur, Rahimi, Arya, Gupta, Subhanshu, Panat, Rahul
• Format: Article
• Language: English
• Subjects: 3-D printing; Aerosol Jet printing; Electrostatic field simulations; Interdigitated capacitor; Printed electronics; RC time constant; Touch sensor; Wearable devices
• ISSN: 0924-4247; 1873-3069
• DOI: 10.1016/j.sna.2016.07.014

•Demonstration of capacitive touch sensors by Aerosol Jet based additive printing with feature sizes down to 45 μm.•Manufacturing innovation allowing highly sensitive touch sensors with native capacitance of a 1–5 pF and capacitance gradation in hundred fF range.•Characterization of manufacturing variability and its effect on sensor capacitance.•3D electromagnetic simulations that establish electric field distribution to enable predictive sensor design.•Results provide a generic simulation tool of touch sensor design in IoT devices and systems. Touch sensors have created a paradigm shift in the human-machine interaction in modern electronic devices. Several emerging applications require that the sensors conform to curved 3-D surfaces and provide an improved spatial resolution through miniaturized dimensions. The proliferation of sensor applications also requires that the environmental impact from their manufacturing be minimized. This paper demonstrates and characterizes interdigitated capacitive touch sensors manufactured using an Aerosol Jet based additive technique that reduces waste and minimizes the use of harmful chemicals. The sensors are manufactured with the capacitive elements at an in-plane length scale of about 50μm by 1.5–5mm, a thickness of 0.5μm, and a native capacitance of a 1–5pF. The sensor capacitance variation is within 8% over multiple samples, establishing the repeatability of the Aerosol Jet method. Scanning electron microscopy and atomic force microscopy are used to characterize the sensor electrodes. 3-D electromagnetic simulations are carried out to predict the capacitance of the printed sensors and the electric field distribution. The simulations show a reasonable agreement with the experimental values of the sensors’ native capacitance (within 12.5%). The model shows the native capacitance to be relatively insensitive to the thickness of the sensor electrodes, allowing touch sensors to be fabricated with reduced material usage and cost. The model is further used to establish the important sensor dimensions governing its electrical performance. Lastly, electromagnetic field distribution predicted by the model is used to establish the physical range of the touch action to be about a millimeter out of the plane of the sensors for the geometries considered in the current work.