Ultra-thin relative humidity sensors for hybrid system-in-foil applications

This paper presents relative humidity sensors based on electrochemical and capacitive principles. The sensors are fabricated on flexible polymer substrates, which are used for ultra-thin chip embedding. The electrochemical sensor consists of spin-coated nanosheets, which shown huge range (about 5 orders of magnitude) and fast response (<; 2 s). The sensor film thickness as well as ionic conductivity change upon moisture absorption. The measured sensor relative conductance change on the flexible substrate is 368%. In addition, a 3-pm thick conventional capacitive sensor is fabricated on the polymer substrate, together with a humidity-insensitive reference capacitor. The measured sensor relative sensitivity is about 75% with a response time of about 6 s. Finally, a sensor system-in-foil is demonstrated using discrete components of the ultra-thin capacitive sensor and a 30-pm thick capacitance-to-digital readout chip.