Photon imaging using post-processed CMOS chips


Melai, Joost (2010) Photon imaging using post-processed CMOS chips. thesis.

Abstract:This thesis presents our work on an integrated photon detector made by post-processing of CMOS sensor arrays. The aim of the post-processing is to combine all elements of the detector into a single monolithic device. These elements include a photocathode to convert photon radiation into electronic signals (in the extreme case this conversion is from a single photon into a single electron), an electron multiplication structure to increase the magnitude of the signal pulse and a position sensitive recording element to register the place, time and size of the electron charge pulse.

The recording element is a CMOS imaging IC called Timepix. The rest of the detector structure is processed directly onto the CMOS substrate. It is essential that the post-processing steps are compatible with the underlying CMOS devices.

The multiplication structure that we use is InGrid, an integrated version of a socalled Micromegas detector structure. The device consists of a thin metal grid that is elevated 50-80 µm above the chip surface with dielectric pillars. These are made of SU-8, a photoimagable polymer. We have investigated the dielectric strength of SU-8. The value of remarkably high for a polymer: 4.4 MV/cm.

A large potential is applied between the grid and the anode, the structure is filled with a gas mixture. When electrons enter the space below the grid they are accelerated and upon collision with a gas atom secondary electrons are generated. This process is repeated a number of times before tht total charge cloud arrives to the anode plane (the chip) where it is detected.

The InGrid structure is coupled to a CsI photocathode to make it sensitive for UV photons. The system is capable of imaging UV photons with high selectivity and with a high resolution of 0.8 lp/pixel.
Item Type:Thesis
Electrical Engineering, Mathematics and Computer Science (EEMCS)
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