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Register a new userCharged particle detection performances of CMOS pixel sensors produced in a 0.18 um process with a high resistivity epitaxial layer
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The apparatus of the ALICE experiment at CERN will be upgraded in 2017/18 during the second long shutdown of the LHC (LS2). A major motivation for this upgrade is to extend the physics reach for charmed and beauty particles down to low transverse momenta. This requires a substantial improvement of the spatial resolution and the data rate capability of the ALICE Inner Tracking System (ITS). To achieve this goal, the new ITS will be equipped with 50 um thin CMOS Pixel Sensors (CPS) covering either the 3 innermost layers or all the 7 layers of the detector. The CPS being developed for the ITS upgrade at IPHC (Strasbourg) is derived from the MIMOSA 28 sensor realised for the STAR-PXL at RHIC in a 0.35 um CMOS process. In order to satisfy the ITS upgrade requirements in terms of readout speed and radiation tolerance, a CMOS process with a reduced feature size and a high resistivity epitaxial layer should be exploited. In this respect, the charged particle detection performance and radiation hardness of the TowerJazz 0.18 um CMOS process were studied with the help of the first prototype chip MIMOSA 32. The beam tests performed with negative pions of 120 GeV/c at the CERN-SPS allowed to measure a signal-to-noise ratio (SNR) for the non-irradiated chip in the range between 22 and 32 depending on the pixel design. The chip irradiated with the combined dose of 1 MRad and 10^13 n_eq/cm^2 was observed to yield a SNR ranging between 11 and 23 for coolant temperatures varying from 15 C to 30 C. These SNR values were measured to result in particle detection efficiencies above 99.5% and 98% before and after irradiation respectively. These satisfactory results allow to validate the TowerJazz 0.18 um CMOS process for the ALICE ITS upgrade.
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Swift, high resolution CMOS pixel sensors are being developed for the ILC vertex detector, aiming to allow approaching the interaction point very closely. A major issue is the time resolution of the sensors needed to deal with the high occupancy generated by the beam related background. A 128x576 pixel sensor providing digitised outputs at a read-out time of 92.5 us, was fabricated in 2008 within the EU project EUDET, and tested with charged particles at the CERN-SPS. Its prominent performances in terms of noise, detection efficiency versus fake hit rate, spatial resolution and radiation tolerance are overviewed. They validate the sensor architecture.
CMOS Pixel Sensors tend to become relevant for a growing spectrum of charged particle detection instruments. This comes mainly from their high granularity and low material budget. However, several potential applications require a higher read-out speed and radiation tolerance than those achieved with available devices based on a 0.35 micrometers feature size technology. This paper shows preliminary test results of new prototype sensors manufactured in a 0.18 micrometers process based on a high resistivity epitaxial layer of sizeable thickness. Grounded on these observed performances, we discuss a development strategy over the coming years to reach a full scale sensor matching the specifications of the upgraded version of the Inner Tracking System (ITS) of the ALICE experiment at CERN, for which a sensitive area of up to about 10 square meters may be equipped with pixel sensors.
Pixel sensors using 8 CMOS processing technology have been designed and characterized offering the benefits of industrial sensor fabrication, including large wafers, high throughput and yield, as well as low cost. The pixel sensors are produced using a 150 nm CMOS technology offered by LFoundry in Avezzano. The technology provides multiple metal and polysilicon layers, as well as metal-insulator-metal capacitors that can be employed for AC-coupling and redistribution layers. Several prototypes were fabricated and are characterized with minimum ionizing particles before and after irradiation to fluences up to 1.1 $times$ 10$^{15}$ n$_{rm eq}$ cm$^{-2}$. The CMOS-fabricated sensors perform equally well as standard pixel sensors in terms of noise and hit detection efficiency. AC-coupled sensors even reach 100% hit efficiency in a 3.2 GeV electron beam before irradiation.
The use of CMOS Pixel Sensors (CPS) for high resolution and low material vertex detectors has been validated with the 2014 and 2015 physics runs of the STAR-PXL detector at RHIC/BNL. This opens the door to the use of CPS for inner tracking devices, with 10-100 times larger sensitive area, which require therefore a sensor design privileging power saving, response uniformity and robustness. The 350 nm CMOS technology used for the STAR-PXL sensors was considered as too poorly suited to upcoming applications like the upgraded ALICE Inner Tracking System (ITS), which requires sensors with one order of magnitude improvement on readout speed and improved radiation tolerance. This triggered the exploration of a deeper sub-micron CMOS technology, Tower-Jazz 180 nm, for the design of a CPS well adapted for the new ALICE-ITS running conditions. This paper reports the R&D results for the conception of a CPS well adapted for the ALICE-ITS.
CMOS pixel sensors (CPS) represent a novel technological approach to building charged particle detectors. CMOS processes allow to integrate a sensing volume and readout electronics in a single silicon die allowing to build sensors with a small pixel pitch ($sim 20 mu m$) and low material budget ($sim 0.2-0.3% X_0$) per layer. These characteristics make CPS an attractive option for vertexing and tracking systems of high energy physics experiments. Moreover, thanks to the mass production industrial CMOS processes used for the manufacturing of CPS the fabrication construction cost can be significantly reduced in comparison to more standard semiconductor technologies. However, the attainable performance level of the CPS in terms of radiation hardness and readout speed is mostly determined by the fabrication parameters of the CMOS processes available on the market rather than by the CPS intrinsic potential. The permanent evolution of commercial CMOS processes towards smaller feature sizes and high resistivity epitaxial layers leads to the better radiation hardness and allows the implementation of accelerated readout circuits. The TowerJazz $0.18 mu m$ CMOS process being one of the most relevant examples recently became of interest for several future detector projects. The most imminent of these project is an upgrade of the Inner Tracking System (ITS) of the ALICE detector at LHC. It will be followed by the Micro-Vertex Detector (MVD) of the CBM experiment at FAIR. Other experiments like ILD consider CPS as one of the viable options for flavour tagging and tracking sub-systems.
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