Relative Humidity Sensors for System-in-Foil Applications

This paper presents the design and fabrication of system-in-foil for relative humidity sensing. The sensors are based on electrochemical and capacitive principles. They are fabricated on flexible polymer substrates, which are used for ultrathin chip embedding. Microcontrollers and sensor readout ICs are embedded in polymer package using a foil assembly technology called ChipFilm Patch. A method for the verification of the chip embedding process is presented herein.The measured electrochemical sensor relative conductance sensitivity on the flexible substrate is about 368%. In addition, a 30-mm 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 6s. Finally, a sensor system-in-foil is demonstrated using discrete components of the ultra-thin capacitive sensor and a 30-mm thick capacitance-to-digital readout chip.