Titanium nitride thin-film bias resistors for AC coupled segmented silicon detectors

Silicon detectors with spatial resolution, such as strip and pixel detectors, are essential for high-energy physics experiments, where they are exposed to very high luminosities and particle fluences. Radiation causes damage in silicon and deteriorates detector performance. The main consequences of radiation damage are the increase in depletion voltage, the increase in detector leakage current leading to a worse signal-to-noise ratio, and the decrease in charge collection length and efficiency. One solution for mitigating the effect of increased leakage current is the use of capacitive coupling, which permits the separation of a signal from the leakage current flowing in the DC biasing circuit. This requires a bias resistor for each detector segment, with a suitable value of resistance in order to allow biasing of the detector while isolating the individual segments from each other. In this thesis, titanium nitride (TiN) is studied as potential bias resistor material in the form of very thin films with thicknesses of around 20 nm. Its resistivity at smaller resistor dimensions is expected to be sufficiently high to permit the fabrication of very small segments, for example pixels with an area of 25 × 25 µm2. This would be compatible with the very short charge collection length and caused by radiation damage. Furthermore, TiN is expected to be far less sensitive to radiation damage than the polycrystalline silicon commonly used as bias resistor, and it can be deposited at lower temperatures , which is beneficial by many aspects of semiconductor processing. In order to deposit suitable TiN thin films with good accuracy and repeatability, a new process of plasma-enhanced atomic layer deposition (PEALD) was developed. Crystallinity, conformality, surface morphology, chemical composition and resistivity of the films obtained this way were studied. The new PEALD process allowed for the deposition of good-quality TiN thin films at 300 °C from the routinely used and easily available precursors TiCl4 and NH3. TiN films with two different thicknesses were processed into bias resistor test structures, which were then tested for radiation hardness by irradiation with protons and photons of different energies. As expected, the radiation hardness of the resistors was excellent and none of the performed irradiations had any effect on the resistors' performance. It was discovered that relatively short annealing treatments, which are used routinely for sintering of aluminium, increased the resistance of the test structures significantly. This presents a very simple and straightforward way to tune the resistance of the TiN resistor structures to a desired value. It was concluded that TiN resistors fabricated in the way presented in this thesis have great potential for use in segmented silicon particle detectors.