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Monolithic Active Pixel Sensors for Charged Particle Tracking

Dernière mise à jour : jeudi 14 avril 2016, par Auguste BESSON , Mathieu Goffe

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Vertex detectors are of growing importance in particle physics experiments as the knowledge of the event flavour is becoming an issue for several aspects of the most fundamental questions to investigate in the coming decades. Existing technologies (i.e. CCDs and Hybrid Pixels) exhibit high performances, but those needed to take full advantage of future experiments may be too demanding for these techniques. This triggered the development of a new type of detector, Monolithic Active Pixel Sensors (MAPS), in our laboratory (in collaboration with LEPSI-Strasbourg).

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Figure : Principle of operation of a CMOS sensor designed for charged particle tracking

These devices were used previously for visible light detection (e.g. commercial camcorders). The principle of operation for charged particle detection is illustrated on the figure. The impinging particle produces excess carriers in the (low-resistivity) epitaxial layer at a rate of about 80 electron-hole pairs per micron. The electrons liberated diffuse thermally inside the layer, which lies inbetween two highly dopped zones : the substrate and the p-wells. The dopping levels of the latter are three orders of magnitude higher than the epitaxial one, translating in potential barriers at the region boundaries. As a consequence, the excess electrons remain inside the epitaxial layer. Regularly implanted n-wells collect the electrons passing in their neighbourhood. The density of the n-wells is the leading parameter for the sensor spatial resolution.