MALTA: an asynchronous readout CMOS monolithic pixel detector for the ATLAS High-Luminosity upgrade

Abstract

The ATLAS collaboration is currently investigating CMOS monolithic pixel sensors for the outermost layer of the upgrade of its Inner Tracker (ITk). For this application, two large scale prototypes featuring small collection electrode have been produced in a radiation-hard process modification of a standard 0.18 μm CMOS imaging technology: the MALTA, with a novel asynchronous readout, and the TJ MONOPIX, based on the well established “column-drain” architecture. The MALTA chip is the first full-scale prototype suitable for the development of a monolithic module for the ITk. It features a fast and low-power front-end, an architecture designed to cope with an hit-rate up to 2 MHz/mm2 without clock distribution over the matrix, hence reducing total power consumption, and LVDS drivers. Laboratory tests confirmed the performance of the asynchronous architecture expected from simulations. Extensive testbeam measurements have proved an average detection efficiency of 96% before irradiation at a threshold of ∼230 e− with dispersion of ∼36 e− and ENC lower than 10 e−. A non fully functional pixel masking scheme, forces operation at relatively high thresholds, causing inefficiency. A severe degradation of efficiency has been measured after neutron irradiation at a fluence 1 × 1015 1 MeV neq/cm2. Consistent results have been produced with the TJ MONOPIX. A correlation with inefficiency plots and pixel layout has triggered TCAD simulations, ending up to two possible solutions, implemented in a new prototype, the MiniMALTA.

Increased radiation tolerance of CMOS sensors with small collection electrodes through accelerated charge collection

“…The reconstructed track is then extrapolated to the plane of the DUT; after the planes alignment procedure, tracks are required to have χ 2 /Ndof 4 smaller than 10, the unbiased residual distribution between the track intercept point and the cluster position on the DUT reaches a width of 13.5 µm in both x and y coordinates. 3 The choice of using only a subset of the telescope planes in the reconstruction stems from optimisation studies related to the impact of the multiple scattering on the position resolution 4 The number of degree of freedom of a track is 2*N-4 where N is the number of spacial points used in the fit…”

“…The prototype showed severe charge collection inefficiencies near the pixel edges after irradiation despite the process modification [3]. Detailed investigation including TCAD simulations [4] showed the low lateral electric field near the pixel borders significantly increases the collection time for signal charge generated in that area, especially for pixel pitches above 30µm.…”

“…Following these results, the MALTA [3] large-scale prototype was designed and fabricated; it consists of a matrix of 512x512 squared pixels with 36.4µm pitch. The small electrode diameter of 3µ leads to an electrode capacitance of 5 fF ensuring low noise operation and high Q/C ratio in the analog input circuit 1 .…”

Latest Developments and Results of Radiation Tolerance CMOS Sensors with Small Collection Electrodes

“…The first two prototypes designed and issued were the large scale MALTA sensor [6] in January 2018, with a matrix of 512x512 and Mini-MALTA [7] in January 2019, a reduced-size prototype with a 16x64 pixel matrix. The pixel sensors presented on this paper have a small pixel size (36.4 x 36.4 µm2) and are produced on high resistivity epitaxial p-type silicon.…”

Radiation Hard Monolithic Cmos Sensors With Small Electrode Size for the Atlas Experiment in the Hl-LHC

“…Following these tests, two large scale designs were fabricated. These designs, however, showed poor charge collection at the pixel borders [5,6]. Detailed TCAD simulations [7] showed that the low lateral electric field near the pixel borders significantly increases the collection time for the signal charge generated in that area, especially for larger pixel pitches.…”