Speaker
Dr
Jiaguo Zhang
(DESY)
Description
Recent progress in active-edge technology of silicon sensors enables the development of large area tiled silicon detectors with small dead space between modules by utilizing edgeless sensors. Such technology has been proven in successful productions of ATLAS and Medipix-based silicon pixel sensors by a few foundries. However, the drawbacks of edgeless sensors are non-uniform charge collection by edge pixels and poor radiation hardness for ionizing radiation. In this work, the charges, produced by X-rays and other ionizing particles, collected by edge pixels of edgeless sensors with different thicknesses and polarities, have been calculated using a developed model. The model takes into account the electric field distribution inside pixel sensor, the absorption of X-rays or other ionizing particles, drift and diffusion of electrons and holes, charge sharing, and threshold settings in ASICs. It is found that the non-uniform charge collection by edge pixels is caused by the strong bending of electric field and the non-uniformity depends on bias voltage, sensor thickness and distance from active edge to the last pixel (also called “edge space” for short). In addition, the last few pixels close to the active edge of the sensor with a commercial design are not sensitive to low-energy X-rays and other ionizing particles for sensors with thicker Si and smaller edge space. The results from model calculation have been compared to measurements and good agreement was obtained, which gives us confidence that the model can be used to optimize the edge design. From the edge optimization, it is found that in order to guarantee the sensitivity of the last few pixels to low-energy X-rays and other ionizing particles, the edge space should be kept at least 50% of the sensor thickness.
In addition, the radiation hardness of edgeless sensors with different polarities, i.e. p$^{+}$n, n$^{+}$p and n$^{+}$n with p-spray or p-stop, has been investigated using SYNOPSYS TCAD with radiation damage parameters implemented, especially for surface damage produced by ionizing radiation with extremely high doses. Results show that if no conventional guard ring is present, none of the commercial designs are able to achieve a high breakdown voltage (typically < 30 V) after irradiation to a dose of ~10 MGy. Finally, a radiation-hard pixel and edge design for edgeless sensors will be discussed according to the optimization results from TCAD.
Author
Dr
Jiaguo Zhang
(DESY)
Co-authors
David Pennicard
(DESY)
Prof.
Heinz Graafsma
(DESY)
Dr
Milija Sarajlic
(DESY)
