Development of high-performance flexible single-crystalline silicon device and its application to flexible sensor

Abstract

This dissertation focuses on the development of flexible single-crystalline Si (sc-Si) device which is a good candidate for high-performance flexible electronics. To make flexible sc-Si devices, advanced thinning and transfer methods were developed and a quantitative strain analysis of the transferred sc-Si membrane were performed. Also, the self-heating effect of the flexible sc-Si transistor on a polydimethylsiloxane (PDMS)/polyimide (PI) film were investigated, and the improvement of the reliability was accomplished by introducing a silver heat spreading layer between the device layer and polymer film. By utilizing the developed technologies for flexible sc-Si devices, high-performance integrated circuits (ICs) for flexible sensor applications were demonstrated, and the sensing/electrical properties were studied. For a quantitative strain analysis of a sc-Si membrane, two sc-Si membrane strain gauges, each with a different stack, were fabricated on a PDMS/ PI film using a silicon-on-insulator (SOI) wafer. One gauge contains 10-μm-thick handling Si layer, whereas the handling Si layer was completely removed for the other case, which membrane stacks have been commonly adopted for flexible sc-Si devices. Although the Si membrane with the 10-μm-thick handling Si layer was flexible, the bending strain applied to the active Si layer (0.127%) was three times higher than the strain applied to the Si membrane without the handling Si layer (0.037%) at a bending radius of 5 mm. This leads to the more reliable electrical and mechanical performance of the device fabricated on the Si membrane without the handling Si layer. The experimental results were verified through a finite element method simulation and analytical modeling. Also, the advanced flexible membrane stack was proposed for the realization of the ultra-flexible Si devices. As a result, the fabricated Si membrane was mechanically stable even at the bending radius of 1 mm because the neutral mechanical plan, where the strain is zero, was positioned at the active Si layer.

Despite the excellent device performance and mechanical flexibility of Si membrane transistors on polymer substrates, they still suffer from poor thermal dissipation, which causes reliability concerns and can lead to premature failure. In this work, under an operational condition of VG = 3 V and VD = 8 V, the temperature of a Si membrane transistor on polymer substrate soared up to about 64˚C instantly and remained consistently high. The excess heat generated from the active channel was found to significantly degrade the device performance. In contrast, the implementation of a silver heat spreading layer between the active channel and polymer substrate significantly alleviated the self-heating effect as the silver film rapidly spread out the generated heat. Also, the influence of the self-heating on the interface trap generation was further investigated by the bias stress test. The interface trap density increased significantly in the device without the HSL compared to the device with the HSL. The efficient heat spreading, monitored via a high-resolution infrared thermal microscope, was well correlated with the electrical characteristics of the devices. These results may provide a help to realize the high performance flexible devices using sc-Si membrane.

Finally, integrated circuits (ICs) which can be utilized for various flexible sensor applications, were demonstrated. The IC, which contains sensing resistor and 2-stage common source amplifier based on the sc-Si membrane, was designed. The sc-Si membrane can sense the parameters such as pressure, temperature and pH. The change in the resistance of the sc-Si membrane reflects the change in sensing parameters. The fractional change of the resistance changes an input voltage of the amplifier and the amplified output voltage can be obtained. The designed sensor IC was fabricated and transferred onto polypropylene (PP) rod having a bending radius of 1 mm. Flexible sensor IC on the PP rod successfully detected changes in pressure, temperature and pH of a liquid in the polyethylene terephthalate (PET) bottle. The developed flexible sensor IC can be utilized in a wide range of applications where the high performance and flexibility are required simultaneously.