CMOS monolithic pixel sensors based on the column-drain architecture for the HL-LHC upgrade

Abstract

Depleted Monolithic Active Pixel Sensors (DMAPS) constitute a promising low cost alternative for the outer layers of the ATLAS experiment Inner Tracker (ITk). Realizations in modern, high resistivity CMOS technologies enhance their radiation tolerance by achieving substantial depletion of the sensing volume. Two DMAPS prototypes that use the same "column-drain" readout architecture and are based on different sensor implementation concepts named LF-Monopix and TJ-Monopix have been developed for the High Luminosity upgrade of the Large Hardon Collider (HL-LHC) . LF-Monopix was fabricated in the LFoundry 150 nm technology and features pixel size of 50x250 µm 2 and large collection electrode opted for high radiation tolerance. Detection efficiency up to 99% has been measured after irradiation to 1 · 10 15 n eq /cm 2 . TJ-Monopix is a large scale (1x2 cm 2 ) prototype featuring pixels of 36x40 µm 2 size. It was fabricated in a novel TowerJazz 180 nm modified process that enables full depletion of the sensitive layer, while employing a small collection electrode that is less sensitive to crosstalk. The resulting small sensor capacitance (<= 3 f F) is exploited by a compact, low power front end optimized to meet the 25ns timing requirement. Measurement results demonstrate the sensor performance in terms of Equivalent Noise Charge (ENC) ≈ 11e − , threshold ≈ 300 e − , threshold dispersion ≈ 30 e − and total power consumption lower than 120 mW/cm 2 .

Development of the monolithic "MALTA" CMOS sensor for the ATLAS ITK outer pixel layer

“…In a 5 × 10 14 n eq /cm 2 neutron irradiated sample the threshold increases to 500 e − ± 70 e − with an ENC of 12 e − . This is comparable to the values reported for TJ Monopix [9]: 271 e − ± 31 e − (ENC 11 e − ) before irradiation and 470 e − ± 50 e − (ENC 20 e − ) after neutron irradiation. Both chips also show a non-Gaussian noise spectrum with entries at higher noise values, caused by random telegraph noise in the transistors.…”

The Monopix chips: depleted monolithic active pixel sensors with a column-drain read-out architecture for the ATLAS Inner Tracker upgrade

“…TJ-Monopix is a DMAPS prototype with pixels using a small electrode for charge collection [7,8]. The chip was designed in a modified 180nm CMOS sensor process from Towerjazz, where a 1 kΩ cm epi-layer can be fully depleted through the addition of a low dose n-type implant [9].…”

Depleted CMOS sensors for HL-LHC

“…• The "LF-Monopix" sensor [5,10] has been produced in the LFoundry 150nm process and features a pixel matrix of 129 × 36 pixels of 50×250 µm 2 pixel size on a >2kΩcm p-type substrate. Each pixel contains a charge sensitive pre-amplifier, discriminator, threshold adjustment and a 8-bit Time-Over-Threshold / Time-Of-Arrival digital logic, which is readout through a column-drain readout architecture.…”

“…• Synchronous readout scheme ("TJ-Monopix" sensor with 36×40µm 2 pixel pitch and 224×448 pixel matrix) [10] comparable to the LF-Monopix described above.…”

“…The MALTA and TJ-Monopix sensors implement a front-end circuit [20,10] which is optimised for low-power operation of 75mW/cm 2 while still sufficiently fast to be compatible with the 25ns bunch structure of LHC. The small input capacitance of < 2fF allows one to achieve low noise operation (ENC≈15e − ) at a constant front-end current of 500nA per pixel.…”