Monolithic active pixel sensor

MAPS technology was initially developed in the 1990s as an alternative to traditional charge-coupled devices (CCDs) and hybrid pixel sensors. Advances in CMOS fabrication allowed for the integration of particle detection and signal processing on a single chip, reducing complexity and power consumption, while significantly increasing the readout speed with respect to CCDs. The introduction of MAPS into particle physics was driven by the need for lightweight, high-resolution sensors capable of operating in high-radiation environments. This marked a series of MIMOSA chips, of which MIMOSA28, alias ULTIMATE chip, was used in STAR Heavy Flavor Tracker. A significant advancement was made by the Rutherford Appleton Laboratory (RAL) in collaboration with TowerJazz semiconductor. This involved integrating complete CMOS circuitry within the pixel, which is crucial for more sophisticated readout methods. This process was instrumental in numerous subsequent developments, including the ALPIDE sensor used in the ALICE experiment, the first MAPS sensor with a sparse readout akin to hybrid sensors. == Operating principle ==

Referring to the cross-sectional view of a conventional monolithic active pixel sensor, the following layers are discernible, commencing from the bottom: • a highly doped (p++) silicon substrate, serving as a mechanical support. • a thin (several tens of micrometers, typically high-resistivity) p-type epitaxial layer, utilized as the sensitive volume. • n-type and p-type implants on top of the epitaxial layer. • metal layers employed for signal routing. N-type implants in contact with p-type epitaxial layer form p–n junctions that function as diodes for collecting current induced by the passage of an ionizing particle, while p-wells host the CMOS circuitry. A small voltage is applied to the diode, resulting in a depleted region. By applying a negative (reverse) bias voltage to the diode through the substrate, the depleted region can be further increased. Depending on the epitaxial layer thickness and resistivity, n-type implant doping, and bias applied, the epitaxial layer can be partially or fully depleted. Electrons originating from the epitaxial layer are confined within this region due to the disparity in doping levels, which imposes a potential barrier. Subsequently, these electrons diffuse thermally within the epitaxial layer until they encounter a depleted region, where, under the influence of the electric field, they drift towards the collection diode. Electrons generated within the p-wells or the substrate, which diffuse into the epitaxial layer, undergo the same process. Signal formation in MAPS depends on the junction capacitance of the collection diode and the input capacitance of the in-pixel readout circuit. A charge collection discharges the capacitance thus causing a voltage drop {{tmath|1= \textstyle \Delta V = \frac{Q}{C} }}, where Q is the amount of collected charge and C the total capacitance. To optimize the signal-to-noise ratio, the collection diode tends to be as small as possible to decrease the junction capacitance. To enhance readout speed and decrease data throughput, MAPS readout circuitry typically incorporates in-pixel amplification and discrimination. This approach retains only the information about the particle's hit position, effectively discarding the energy (charge) information. For instance, the ALPIDE chip, featuring 1024 × 512 pixel matrix with hit/no-hit readout, was designed to operate at 100 kHz readout rate. == High-energy physics applications ==

Given their very thin sensitive volume, MAPS wafers can be thinned down to a total thickness of less than 50 μm. Their low power dissipation and, consequently, light cooling requirements enable the use of lightweight support mechanics. This makes them an appealing detector for experiments that prioritize low material costs and high granularity, such as nuclear physics experiments. ALICE ITS2 The ALICE experiment at CERN has undergone a major upgrade during the Long Shutdown 2 of the LHC during 2019–2021, replacing its Inner Tracking System with a detector fully based on MAPS. The ITS2 state-of-the-art MAPS sensor, ALPIDE, is fabricated in the TowerJazz 180 nm CMOS Imaging Process. ALPIDE features in-pixel amplification, shaping and discrimination as well multiple-event buffers. A priority-encoder circuit is employed to read only the addresses of hit-pixels and achieve in-matrix data-sparsification. ITS2 achieves 5 μm spatial resolution with pixel pitch of 29.24 μm × 26.88 μm at power dissipation below 40 mW/cm2. A total of about ALPIDE chips arranged in 7 cylindrical layers cover a total detector surface is about 10 m2 counting 12.5 gigapixels. ALICE ITS3 A further upgrade of ALICE Inner Tracking System is planned for the Long Shutdown 3, in which the three innermost layers of the ITS2 will be fully replaced by wafer-scale, bent MAPS, produced in TPSCo 65 nm CMOS Imaging Sensor process. == Space-based applications ==

Pixel sensors for space-based tracking detectors necessitate low power consumption and improved heat dissipation to manage the satellite's limited power and cooling resources. MAPS presents a unique opportunity to address this requirement for the first time. == References ==