Flexible Tag Gas Sensing Systems for Food Logistics Applications

In the framework of the GoodFood Integrated Project, a flexible tag gas sensing system with RFID communication capabilities for food logistics is being proposed. The final device will be a flexible label hosting sensing capabilities (temperature, humidity, chemical sensors) and an RFID interface for wireless data exchange within an Ambient Intelligence (AmI) infrastructure. This visionary application requires the development and optimization of several Micro System Technologies, like Flexible Circuits Technologies, micromachining of ultralow power consumption sensor substrates, and addressing specific integration issues. To allow the use of Metal OXide Semiconductor (MOX) gas sensor technology in a flexible tag application, several constraints must be fulfilled. MOX sensors to be used within a thin-film battery powered or completely passive flexible tag must feature extremely low power consumption, which can be reached through optimized pulsed temperature operation and specifically designed ultra-low power hotplate arrays. Another challenging issue is the reliable encapsulation of the hotplates inside the flexible polymeric substrate, which requires very small dimensions of the hotplate dies and specific packaging solutions in order to ensure mechanical robustness as well as proper electrical and fluidic connections. Hotplates with a power consumption lower than 5 mW at constant operating temperature and chip dimensions as low as 1.0 x 0.5 mm for a complete sensor array are being developed. Design studies and FEM simulation results will be shown. For the fabrication of a multilayer high density flexible circuit, a procedure based on the following main steps is being implemented, starting from a Kapton substrate: 1) Deposition of the dielectric interlayer; 2) Patterning of the dielectric layer to open the vias; 3) Deposition of the metallic layer by Physical Vapor Deposition or electrodeposition; 4) Photolithographic patterning of the metallic layer (lift-off process or chemical etching). These steps can be repeated until obtaining the desired number of layers, followed by the final curing and passivation. The most suitable RFID standard for the specific application was found to be the ISO15693-2 at 13.56 MHz. The RFID communication hardware is based on a custom front-end controlled by a MSP430 ultra-lowpower microcontroller (Texas Instruments), running a firmware which implements the ISO15693 standard. The antenna geometry is being studied through simulations and prototype characterisations. Design studies will be presented together with preliminary system prototypes.