High Resolution aGe-Thermistor for IR-Detection

This project deals with thermal infrared detectors (bolometers) based on the technology of thin-film amorphous germanium (aGe) thermistors and micromachined carriers existing at the research site. Bolometers with different thermistor design will be realized in the course of the project. A thorough characterisation of the thermistor samples will be carried out to clarify the sensitivity limiting fluctuation processes. Efforts to minimise their effect will be developed like new design rules and dimensioning rules for proper operating conditions, for example. Major enhancements of the bolometer sensitivity will be achieved through technological measures. A technology for floating backside contacts will be developed in order to reduce the sample impedance at low temperatures to moderate values. The thermal parameters of the thermistor will be optimised by a proper design of the device and careful selection of the construction materials used for the membrane suspension. A screening of commercial available silicon nitride wafer passivating layers with respect to their mechanical properties built-in stress as well as new modifications of the low temperature CVD process for the thermistor passivation will help to decrease the supporting layer thickness to the lowest practical value. The realization and comparison of different types of resistance bolometer samples based on aGe is one main goal of the project. The infrared detection properties of the samples will be investigated at room temperature in ambient air as well as below room temperature down to 77 K in high vacuum environment. An evaluation of the sensitivity limits for different thermistor constructions and their dependence on the operating conditions (temperature, thermal boundary conditions, electrical bias) is planned. By these measures IR detectors which may exceed the state of the art can be expected as a result of the project.

The project was dedicated to the development of an infrared radiation sensor, called resistance bolometer. The transduction of radiation energy is here performed in two steps. First radiation is absorbed by the structure which leads to a slight raise of the detector temperature. This temperature variation is converted into an electrical signal using a thermistor, i.e., a temperature dependent electrical resistor. This device is decisive for the achievable sensitivity, which depend mainly on three main noise contributions. These comprise the thermal noise caused by the "Brownian motion" of electrons in matter, temperature fluctuations of tiny bodies, and random fluctuations of the electrical conductivity of the resistor material. Only the last property can be minimized by advances of technology. Within the frame of the project very low noise thermistors are created. The developed sensor device is based on a thin layer of the semiconducting element germanium, which is evaporated onto a substrate in high vacuum under carefully controlled deposition conditions. The second challenge for the developer is an efficient conversion of irradiated power into distinct temperature changes. As a rule of thumb the radiation absorber has to have as little as possible heat capacitance. This goal is achieved using techniques known from modern microelectronics in conjunction with high tech micromachining of single crystal silicon chips. The achieved specifications of the devices developed in the frame of the project are state of the art.