engleski

MALTA: a CMOS pixel sensor with asynchronous readout for the ATLAS High-Luminosity upgrade

Radiation hard silicon sensors are required for the upgrade of the ATLAS tracking detector for the High- Luminosity Large Hadron Collider (HL-LHC) at CERN. A process modification in a standard 0.18 μm CMOS imaging technology combines small, low-capacitance electrodes (∼2 fF for the sensor) with a fully depleted active sensor volume. This results in a radiation hardness promising to meet the requirements of the ATLAS ITk outer pixel layers (1.5 × 1015 neq /cm2 ), and allows to achieve a high signal-to-noise ratio and fast signal response, as required by the HL-LHC 25 ns bunch crossing structure. The radiation hardness of the charge collection to Non-Ionizing Energy Loss (NIEL) has been previously characterised on pro- totypes for different pixel sensor cell designs. The encouraging results enabled the design of full-size monolithic CMOS sensors for the ATLAS ITk outermost pixel layer, which comprises ∼1.8 m2 of pixel sensor active area. In the MALTA sensor, we implement a fast, low-power analogue front-end together with a novel high-speed matrix readout architecture capable of meeting the challenging hit- rate requirements of up to 2 MHz/mm2 in the outer layers of the ITk pixel tracker. The front- end was optimized for the low sensor capacitance to achieve low noise (ENC < 20 e-) and low power operation (< 1 μW/pixel), with timing that meets the 25 ns requirement. The small size (∼2 μm) of the collection electrode also allows better shielding to prevent crosstalk from the full swing digital signals in the 36.4×36.4 μm2 pixel. MALTA features a 512×512 pixel matrix with a fully asynchronous readout architecture, without clock distribution over the matrix. This approach combines low digital power consumption with fast signal response and high hit-rate capability. This paper describes the implementation of this novel depleted monolithic sensor based on a low-capacitance analogue design with asynchronous readout, together with first test results from lab tests, radioactive source tests and X-ray measurements.

Active pixel sensors ; CMOS integrated circuits ; position sensitive particle detectors ; radiation effects ; radiation hardening (electronics) ; semiconductor detectors ; solid state circuit design

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