The aim of this paper is to present an integrated process flow for a smart tag with integrated sensors and RFID
communication, a Flexible Tag Microlab (FTM). The heart of the designed container tracing system is an RFID system
(Reader + Tag) with gas sensing capabilities on board. In the former prototypes, the chemical sensors were integrated on
the reader, whereas the tags where addressed like conventional RFID-tags containing also physical (temperature,
humidity and light) sensors. However, this paper will show how the gas sensing reader functionalities are being
transferred to the tag, reaching a flexible tag microlab, which represents a real innovation in the field of flexible labels.
Key issues for the realisation of the FTM, such us flexible substrates and gas sensor integration technologies will be
The process flow employed for the two metal levels interconnect fabrication will be described in detail. The material
used is the DuPontTM Pyralux(R) AP 8525R double-sided copper-clad laminate, formed by a Kapton foil with a copper
layer on each side. The vias and windows openings are performed by femtosecond laser ablation. The copper
interconnections are realized by photolithography and wet chemical etching.
The MOX sensors hotplates specially developed to fulfil the FTM constrains in terms of low power consumption has
been used to prove two integration technologies into the flexible substrates: Chip on Flex (COF) wire bonding and
Anisotropic Conductive Adhesive (ACA) flip chip bonding. Both technologies will be compared and benchmarked for
future product developments.
The objective of this ongoing work is the development of a microlab on flexible tag, capable to monitor the quality of the food, during transport, storage and vending. The idea is to bring together different sensor technologies that will be integrated into a data communication environment for online food monitoring during the logistics chain.
The proposed solution is the concept of silicon chips and microcomponents assembled and integrated on top of a flexible substrate acting mainly as a passive interconnect structure.
Three technologies have been identified as necessary to get the final integration:
a) Substrate technology. This technology refers to the realisation of the flexible substrate with the metallic interconnections.
b) Assembly technology to integrate the discrete components on the flexible substrate. The conventional processes are wire bonding, flip chip, and adhesive bonding.
c) Encapsulation technology and windows opening over the gas sensitive areas.
The first flexible tag prototype integrates two different metal oxide sensor arrays with a commercial microprocessor. The dimensions are 43 mm long, 22 mm wide and about 2 mm thick and two metal levels are necessary for the interconnect. The strategy undertaken by the groups involved in this work, consists in the evaluation of different approaches, that combine diverse process sequences and materials, with the final aim of identifying the best solution.
Regarding the substrate technology, the approach realized using Pyralux copper-clad laminated composites, constructed of DuPont Kapton polyimide film with copper foil on both sides, as flexible substrate will be described in this paper. The cupper interconnections are generated by standard photolithography and wet etching and the vias definition in Kapton is performed by femtosecond laser ablation. On the other hand, the assembly technology based on the use of anisotropically conductive adhesives will be also illustrated.
An emerging non-photolithographic technology, soft lithography, is applied with the aim of patterning ferroelectric thin film oxides, in particular sol-gel deposited Lead Zirconate Titanate (PZT). Soft lithography relies on the replication of a patterned master using an elastomeric material and then, using this as a stamp to create micro- and nanometer scale patterns and structures. Following this procedure, a set of masters were fabricated both using photolithographic and Focused Ion Beam (FIB) methods. Masters were replicated using a commercial polymer (PDMS), satisfactorily reproducing the negative of the master’s features down to a limit of 500 nm. The obtained stamps were used to produce patterned self-assembled monolayers (SAMs) of alkanethiols over surfaces of gold, using the technique of microcontact printing (mCP). The patterned SAMs were then employed as a molecular thin resist to create microstructures of gold by wet etching. This is a key step for establishing a new route for the fabrication of ferroelectric thin film capacitors made of Pt/PZT/Au by Reactive Ion Etching (RIE) using the patterned gold features as both mask and top electrode, thus avoiding alignment stages in the fabrication procedure.