Mittuniversitetet

This thesis involves a study the fabrication and characterization of photon radiation detectors. The focus has been to develop and improve the performance of optical measurement systems, but also to reduce their cost. The work is based on the study of two types of detectors, the position sensitive detector and the thermal detector.

Infrared detectors are usually subcategorized into photonic detectors and thermal detectors. In the thermal detectors, heat generated from the incident infrared radiation is converted into an electrical output by some sensitive element. The basic structure of these detectors consists of a temperature sensitive element connected to a heat sink through a thermally isolating structure. Thin membranes of Silicon and Silicon nitride have been commonly used as thermally insulation between the heat sink and the sensitive elements. However, these materials suffer from relatively high thermal conductivity, which lowers the response of the detector. The fabrication of these membranes also requires rather advanced processing techniques and equipment. SU-8 is an epoxy based photoresist, which has low thermal conductivity and requires only standard photolithography. A new application of SU-8 as a self-supported membrane in a thermal detector is presented. This application is demonstrated by the fabrication and characterization of both an infrared sensitive thermopile and a bolometer detector. The bolometer consists of nickel resistances connected in a Wheatstone bridge configuration, whereas the thermopile uses serially interconnected Ti/Ni thermocouple junctions.

The position sensitive detectors include the lateral effect photodiodes and the quadrant detectors. Typical applications for these detectors are distance measurements and as centering devices. In the quadrant detectors, the active region consists of four pn-junctions separated by a narrow gap. The size of the active region in these detectors depends on the size of the light spot. In outdoor application, this spot size dependence degrades the performance of the four-quadrant detectors. In this thesis, a modified four-quadrant detector having the pn-junctions separated by a larger distance has been fabricated and characterized. By separating the pn-junctions the horizontal electric filed in the active region is removed, making the detector spot size insensitive.

Linearity of the lateral effect photodiodes depends on the uniformity of the resistive layer in the active region. The introduction of mechanical stress in an LPSD results in a resistance change mainly due to resistivity changes, and this affects the linearity of the detector. Measurements and simulations, where mechanical stress is applied to LPSDs are presented, and support this conclusion.

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.