Design, Fabrication and Optimization of Thermal Radiation Detectors Based on Thin Polymer Membranes

Abstract: The number of applications in which infrared radiation sensors are used is increasing. In some applications, the cost of the sensor itself is an issue, and simple solutions are thus required. In this thesis, the investigations have related to the use of thin polymer membranes in thermal infrared sensors, such as bolometers and thermopiles.Infrared sensors are usually subcategorized into photonic sensors and thermal sensors. For detection of infrared radiation using a photodetector, there is a requirement for low band-gap material. The need of cooling makes infrared photodetectors rather expensive, and not an alternative for low-cost applications. In thermal sensors, the heat generated from the incident infrared radiation is converted into an electrical output by means of a heat sensitive element. Thermal sensors operate at room temperature, which makes them a low-cost alternative. The basic structure of thermal sensors consists of a temperature sensitive element connected to a heat sink through a structure with low thermal conductance. It is common to use thin membranes of Silicon or Silicon Nitride as thermal insulation between the heat sink and the sensitive element. In comparison, polymers have a thermal conductance that is lower than in these materials, and this increases the generated temperature in the sensitive element. A polymer such as SU-8 has a low thermal conductivity and is applied using a spin coater. This reduces the number of complex processing steps. This thesis presents a new application of SU-8 as a closed membrane in a thermal sensor.The concept was initially demonstrated by fabricating a nickel bolometer and titanium/nickel thermopile structure with a 5 µm SU-8 / SiO2 membrane. However, for the sensor responsivity to be able to compete with commercial thermal sensors the structures, some optimization was required. Since the thermopile generates its own voltage output and requires no external bias, the optimizations were focused on this structure. There exist a number available software tools for thermal simulation of components. However, to the author’s best knowledge, there exist no tool for design optimization of thermopiles with closed membranes. An optimization tool using iterative thermal simulations was developed and evaluated. A new thermopile structure, based on the optimization results, was both fabricated and characterized. Using an infrared laser with a small spot, the measured responsivity of the manufactured thermopile was higher than that of a commercial sensor. In the case of a defocused spot and for longer wavelengths, the infrared absorption in the absorption layer reduces and degrades the responsivity.The thermopile was further evaluated as a sensor in a carbon dioxide meter application based on the NDIR principle. An increase in the CO2 concentration demonstrated a clear decrease in the thermopile voltage response, as was expected. By normalizing the voltage response and comparing it with a commercial sensor, this showed that the SU-8 based thermopile is relatively more sensitive to changes in the CO2 concentration.