Monolithic active pixel sensor for low energy X-ray applications

Over the last three decades, Monolithic Active Pixel Sensors (MAPS) have been developed to be used in several high-energy physics experiments [1-3]. The fact that no additional detector is required minimizes the material budget, reduces the fabrication cost and simplifies the integration procedure. Since the MAPS are designed in a standard CMOS (Complementary Metal Oxide Semi-conductor) technology, the design flow is also simplified. In the first MAPS, the charge generated in the undepleted sensitive volume diffused in all directions and was sensed on the collecting diodes. Spread of the charge over several pixels and slow charge collection due to the diffusion are the limiting factors for applications where the fast collection is necessary or charge sharing is not acceptable. However, few years ago, the trend in MAPS production moved to the fabrication of sensors with high-resistivity epitaxial layers. Several applications in high-energy physics were at the core of this development. Thanks to the fact that the epitaxial layer is highly resistive it can be depleted with a moderate voltage. This fact makes this new kind of sensors capable of detecting the low energy X-rays (<8 keV) with reasonable detection efficiency [4]. For instance 50 pm thick silicon detector absorbs 52.9 % of the 8 keV photons [5].