Printed electronics is a process fabricating flexible electronic devices by printing functional ink on a flexible substrate. The process has the advantages in terms of cost, productivity, and eco-friendliness. In addition, its flexibility is also an advantage. Therefore, printed electronics is known to have a potential to replace lithography-based process. In the lithography-based process, etching is used for fabrication of air-gap based sensors such as pressure sensors and accelerometer. Although the printed electronics has many advantages, it does not have etching. Therefore, it is difficult to fabricate air-gap based electronic devices in printed electronics.
In this study, we developed the printed electronics method for fabrication of capacitive air-gap touch sensor. The method is similar to wet etching in the lithography-based process. In the process, sacrificial layer was formed with a material that can be chemically or thermally decomposed. Then an air-gap was formed by removing the sacrificial layer. The capacitive air-gap touch sensor is consists of bottom electrode, air-gap, and top electrode. The sensor samples were fabricated by following sequence. First, bottom electrode was formed by roll-to-roll gravure printing silver ink on a flexible PET substrate. Then the sacrificial layer was formed on the bottom electrode by spin coating. In this study, polydimethylsiloxane (PDMS) was used as a material for the sacrificial layer. PDMS can be decomposed easily by tetrabutylammonium (TBAF) fluoride solution. On the sacrificial layer a stretchable silver ink was printed to form a top electrode. Then the samples were placed into a TBAF fluoride solution bath and heated on a hot plate to remove PDMS and to form an air-gap. The samples were taken out of the bath and rinsed with isopropanol.
Scanning electron microscope (SEM) was used to measure the size and the shape of the air-gap. The width and thickness of the air-gap was 1500 μm and 100 μm, respectively. The size and shape of the air-gap was same as that of the sacrificial layer. The electrical performance was measured with a push-pull gauge. Ten samples of air-gap touch sensor were prepared for the measurement. As a result, applying force from 0 to 2.5 N increased the capacitance from 0.300 nF to 0.338 nF proportionally.