Study Of Flexible Humidity, Temperature And Pressure Sensors Based On Polyimide Metallization Patterning

In recent years, the flexible devices are emerging as the important applications for future display, robotics, in vitro diagnostics, and so on. Polymer can be used as a versatile component which can act as both the flexible structural support and the functional material to generate, transmit, and process mechanical and electrical inputs in various ways. In our work, the novel selective metallization patterning process on polyimide was studied. Subsequently, based on this technology, humidity, temperature and pressure sensors were successfully fabricated. The performance of aforementioned sensors were analyzed and discussed. The main content of this thesis is as following:First, the selective metallization patterning technique on polyimide was studied by all-wet chemical process. By KOH surface treatment, the polyimide surface was modified into PAA and K+ ions were inserted into PI matrix. By soaking in silver ammonia solution, the ion exchange took place between K+ and Ag+ ions. Using inkjet-printed ink as mask, patterned silver layer was fabricated by either heat treatment or hydrogen peroxide rapid reduction method. After that, the as-received Ag-PI nanocomposites were characterized. The results showed that silver particles were of face-centered-cubic crystalline structure. And the silver layer composed of compact micro/nanoparticles was well conductive. It is also found that in the first method, the modified structure (PAA) will re-cycloimidize to the polyimide (PI) while in the latter one, the surface PAA will not be affected. Subsequently, the influences of KOH and silver ammonia processing time and reduction time on the electrical properties of silver were studied. And the optimal experimental parameters were found. At last, through tape peeling testing, it was found that the as-deposited Ag films have superior adhesion with PI substrate. And the bending testing indicated that the Ag films have the good robust properties which can bear more than 10,000 bending cycles and no obvious degradation was found.Secondly, the innovative humidity sensor based on "one-mask process" had been developed which can batch-fabricate the sensor arrays. PI and its derivative PAA were acted as both the substrate and the sensing element. The interdigitated electrodes (IDEs) were realized by our developed metallization patterning process. The humidity sensors based on PAA and PI as sensing elements were fabricated by simple "one-mask process". It was demonstrated that the performance of the PI-based sensor was superior to the PAA-based one. At last, in order to solve the slow response time, a mathematical model had been employed which can deduce the real input humidity level accurately and quickly from the sensor’s dynamic response.Thirdly, the temperature sensor based on "one-mask process" was investigated. Two resistive-type temperature sensors with different shapes, Snake and spiral, were designed here. The results indicated that the largest hysteresis was smaller than 1% and the response time was several tens second. For both sensors, the correlation factors of fitting curves were higher than 0.998. The largest resistance deflection between the experimental data and the fitting results was less than 4.8%. In -25℃ to 100℃ scale, the sensitivity is about 0.002/℃ for Snake-shaped resistor and Spiral-shaped one. It implies that the fabricated temperature sensors were of good performance and the sensitivity is close to the Pt RTD under 100℃.Fourthly, the pressure sensor with graphite/PDMS composite material as sensing element was studied. With PI as substrate, the pressure sensor was established by IDEs fabrication by "one-mask process" and graphite/PDMS composite piezoresistive coating by drop-casting. The primary results demonstrated that when the curvature changes from 0.03 mm-1 to 0.12 mm-1, the resistance of the sensor varies from 8 kΩ to 12 kΩ. The largest hysteresis was less than 3%. The repeatability was about 2% when bending curvature is under 0.10 mm-1. In addition, both the response and the recovery time was less than 1 s.At last, the integration of the humidity, temperature and pressure sensors on the same polyimide film was studied. And the potentials applied to detecting human comfort degree, robotics, intelligent skin and so on are demonstrated.