Driven by the exploding popularity of the Internet-of-Things (IoT), the demand for thin, flexible, lightweight intelligent sensingdevices is growing rapidly. Two attractive examples are intelligent packaging and wearable healthcare monitoring devices, which help to connect and track / monitor everything / everybody at any time and in any place. The remarkably swift development of flexible and printed electronics is promoting new possibilities for cost-effective manufacturing of such devices. However, compared with silicon-based circuits, state-of-the-art all-printed circuits are encountering low integration density, long switching time and corresponding high cost per function. Therefore, a heterogeneous platform is in great demand, which employs a cost-effective, large-area manufacturing technique while keeping the same complex functionality and processing capability as silicon-based systems. Due to temperature and mechanical reasons, traditional silicon integration methods, such as solder bonding and wire bonding, are not suitable for flexible printed electronics. This thesis aims to develop a generally applicable hetero-geneous integration platform for the realization of intelligent sensing devices on flexible substrates.
First, inkjet printing technique is introduced and studied. As the basic and key element, inkjet printing technology is employed to fabricate interconnections as well as electrodes of the printed sensors. Novel flexible media, plastic and paper, are evaluated as the substrates of printed electronic systems from two aspects: the electrical characteristics and performance reliability. In addition to widely used inkjet/photo paper, packaging paper is presented as a promising candidate for intelligent packaging applications due to the advantages in terms of lower price, higher temperature endurance and better reliability against 85◦/85% RH aging.
Second, the heterogeneous integration platform enabled byinkjet printing is presented. Benefiting from the non-contact, accurate alignment and fine resolution features, this integration technique has the advantages of simplified fabrication process and multi-substrate compatibility. The design rules have been studied and the integration process is optimized for silicon chips with/without packaging.
Finally, to verify the suitability, the heterogeneous integration platform is applied to two representative applications, each with unique emphasis and requirements.
For intelligent packaging, low-cost is one crucial requirement. Paper substrate is selected because it is cost-effective, recyclableand a commonly-used packaging material in industry. In order to fit into non-regular shape pack-ages, the intelligent packaging needs to be bent or folded, which brings about reliability concern for paper electronics. Therefore, bending and folding tests are applied to reveal the capability and the limitation of paper electronics in terms of flexibility. For applications such as fresh food tracking, humidity is an important physical quantity to monitor during transportation and storage. Therefore, a resistive humidity sensor based on multi-walled carbon nanotubes is fabricated and integrated. A commercial packaged microcontroller is used to sense and store the resistance of the sensor and control the LEDs to indicate the ambient humidity level. By integratingthe microcon-troller, LEDs and a switch with the printed sensor and battery, a prototype of a paper-based humidity sensor card is implemented.
For the healthcare application, user comfort is an essential element. Future long-term healthcare devices require a bio-sensing system which is small, thin, lightweight and wearable, has a long-battery life, and is easy to customize. The heterogeneous platform offers a promising solution for such systems from three aspects. 1) A fully integrated system-on-chip (SoC) is embedded to detect and process the bio-signal. The SoC solution features tiny size and low-power consumption, which contribute to system miniaturization and long battery lifetime. 2) Inkjet printing offers a cost-effective approach to fabricate personalized electrodes. 3) Inkjet printed interconnections enable the direct integration of the bare die instead of the packaged chip. This significantly reduces the physical size of the system, simplifies the manufacture process and lowers the cost. The concept is demonstrated by aminiaturized wearable Bio-Patch with the size of 4.5 cm×2.5 cm.
Stockholm: KTH Royal Institute of Technology, 2014. , v, 61 p.