3D Pixel Detectors Research Articles

Charge-sensitive amplifier design for high-speed interface readout front-end ASICs

The design of Charge-Sensitive Amplifiers (CSAs) and the peaking time impact on high-speed interface readout front-end ASICs for 3D pixel detectors with high timing precision, is addressed in this work. The performance of the CSAs is extracted for several architectures, implemented in 180 nm CMOS, 130 nm SiGe BiCMOS, 65 nm CMOS and 22 nm FD-SOI CMOS process nodes for comparison purposes versus technology scaling. The peaking time, the charge gain, the noise and the bandwidth are simulated for all implemented CSAs, while the best CSA candidate performance-wise for high-speed and high timing precision readout ASICs is designated. Furthermore, a full readout chain, capable of resolving particles with a time precision of <200 ps, is implemented and simulated. The designed readout ASIC exhibits a Time-Walk performance of ≤180 ps, for input charges of 6.3−10ke−, while inducing an equivalent noise charge of ≈570e−.

Performance of highly irradiated FBK planar and 3D pixel detectors

The High Luminosity upgrade of the CERN Large Hadron Collider (HL-LHC) requires new radiation tolerant silicon pixel sensors. In the case of the CMS experiment, the first layer of pixel detectors will be installed at about 3 cm from the beam line, where an integrated fluence of about 20×1015neqcm−2 (1 MeV equivalent neutrons) is expected (HL-LHC Runs 4 and 5). The first tracker layer will be substituted before the start of HL-LHC Run 6. The 3D concept for silicon pixel sensors presents several advantages with respect to traditional (planar) sensors. Thanks to short anode-to-cathode distances, 3D sensors are much more resistant to radiation damage, making them suitable for use in the inner layer of the future tracker. This paper describes results from beam tests with highly irradiated planar and 3D sensor and RD53A readout chip combinations. RD53A is the first prototype in 65 nm technology developed by the RD53 collaboration for use in HL-LHC pixel detectors. The sensors were made in FBK foundry in Trento, Italy, and their development was done in collaboration with INFN (Istituto Nazionale di Fisica Nucleare, Italy). Both planar and 3D sensors feature a pixel area of 2500 μm2 and an active thickness of 150 μm. The pixel detectors, irradiated to fluences up to 24×1015neqcm−2, were tested in the DESY test beam facility and the analysis of the data shows excellent performances even for the highest irradiation fluences. All results are obtained in the framework of the CMS R&D activities.

Wafer Level Packaging Technology Applied to Pixel Detectors

Pixel technology is commonly used in the tracking systems of High Energy Physics detectors with physical areas that have largely increased in the last decades. To ease the production of several square meters of sensitive area, the possibility of using the industrial Wafer Level Packaging to reassemble good single sensor tiles from multiple wafers into a reconstructed full wafer is investigated. This process reconstructs wafers by compression molding using silicon charged epoxy resin. We tested high glass transition temperature low-stress epoxy resins filled with silica particles to best match the thermal expansion of the silicon die. These resins are developed and characterized for industrial processes, designed specifically for fan-out wafer-level package and panel-level packaging. In order to be compatible with wafer processing during the hybridization of the pixel detectors, such as the bump-bonding, the reconstructed wafer must respect challenging technical requirements. Wafer planarity, tile positioning accuracy, and overall thickness are amongst the main ones. In this paper the description of the process is given and preliminary results on a few reconstructed wafers using dummy tiles are reported. Strategies for Wafer Level Packaging improvements are discussed together with future applications to 3D sensors or CMOS pixel detectors.

First characterisation of 3D pixel detectors irradiated at extreme fluences

Highly irradiated silicon sensors are currently being investigated within the scenario of the most powerful particle accelerator proposed for the post-LHC era, i.e the Future Circular Collider (FCC), for which radiation tolerance up to a level of 8 × 1017 neq/cm2 is required. In this paper, 3D strip and pad detectors irradiated with neutrons up to a fluence of 3 × 1017 neq/cm2 (1 MeV neutron equivalent) are characterised for the first time: the charge collection measurements with Transient Current Technique (TCT) are carried out at several voltages and the results are reported.

New test beam results of 3D and pad detectors constructed with poly-crystalline CVD diamond

Chemical Vapour Deposition (CVD) diamond is being considered as a material for particle detectors in a harsh radiation environment. This article presents beam test results of 3D pixel detectors fabricated with poly-crystalline CVD diamonds. The cells of the devices had a size of 50µm×50µm with columns 2.6µm in diameter. The cells were ganged in a 3×2 and 5×1 pattern to match the layouts of the pixel read-out electronics currently used in the CMS and ATLAS experiments at the Large Hadron Collider, respectively. In beam tests, using tracks reconstructed with a high precision tracking telescope, a tracking efficiency of 99.3% was achieved. The efficiency of both devices plateaus at a bias voltage of 30V. Also irradiated poly-crystalline CVD diamond pad detectors were investigated. In high rate beam tests with particle fluxes up to 20MHz/cm2 and irradiations up to 8 ⋅ 1015n/cm2 it was shown that the pulse height of irradiated poly-crystalline CVD diamonds does not depend on flux to the O2%.

Radiation hardness of small-pitch 3D pixel sensors up to a fluence of 3×1016 neq/cm2

Small-pitch 3D silicon pixel detectors have been investigated as radiation-hard candidates for the innermost layers of the HL-LHC pixel detector upgrades. Prototype 3D sensors with pixel sizes of 50 × 50 and 25 × 100 μm2 connected to the existing ATLAS FE-I4 readout chip have been produced by CNM Barcelona. Irradiations up to particle fluences of 3 × 1016 neq/cm2, beyond the full expected HL-LHC fluences at the end of lifetime, have been carried out at Karlsruhe and CERN. The performance of the 50 × 50 μm2 devices has been measured in the laboratory and beam tests at CERN SPS. A high charge collected and a high hit efficiency of 98% were found up to the highest fluence. The bias voltage to reach the target efficiency of 97% at perpendicular beam incidence was found to be about 100 V at 1.4 × 1016 neq/cm2 and 150 V at 2.8 × 1016 neq/cm2, significantly lower than for the previous IBL 3D generation with larger inter-electrode distance and than for planar sensors. The power dissipation at -25°C and 1.4 × 1016 neq/cm2 was found to be 13 mW/cm2. Hence, 3D pixel detectors demonstrated superior radiation hardness and were chosen as the baseline for the inner layer of the ATLAS HL-LHC pixel detector upgrade.

Study of sub-pixel position resolution with time-correlated transient signals in 3D pixelated CdZnTe detectors with varying pixel sizes

We evaluated the sub-pixel position resolution achievable in large-volume CdZnTe pixelated detectors with conventional pixel patterns and for several different pixel sizes: 2.8 mm, 1.72 mm, 1.4 mm and 0.8 mm. Achieving position resolution below the physical dimensions of pixels (sub-pixel resolution) is a practical path for making high-granularity position-sensitive detectors, <100 μm, using a limited number of pixels dictated by the mechanical constraints and multi-channel readout electronics. High position sensitivity is important for improving the imaging capability of CZT gamma cameras. It also allows for making more accurate corrections of response non-uniformities caused by crystal defects, thus enabling use of standard-grade (unselected) and less expensive CZT crystals for producing large-volume position-sensitive CZT detectors feasible for many practical applications. We analyzed the digitized charge signals from a representative 9 pixels and the cathode, generated using a pulsed-laser light beam focused down to 10 μm (650 nm) to scan over a selected 3×3 pixel area. We applied our digital pulse processing technique to the time-correlated signals captured from adjacent pixels to achieve and evaluate the capability for sub-pixel position resolution. As an example, we also demonstrated an application of 3D corrections to improve the energy resolution and positional information of the events for the tested detectors.

Using a pulsed laser beam to investigate the feasibility of sub-pixel position resolution with time-correlated transient signals in 3D pixelated CdZnTe detectors

We evaluated the X-Y position resolution achievable in 3D pixelated detectors by processing the signal waveforms readout from neighboring pixels. In these measurements we used a focused light beam, down to 10 μm, generated by a ∼1 mW pulsed laser (650 nm) to carry out raster scans over selected 3×3 pixel areas, while recording the charge signals from the 9 pixels and the cathode using two synchronized digital oscilloscopes.

The CT-PPS tracking system with 3D pixel detectors

The CMS-TOTEM Precision Proton Spectrometer (CT-PPS) detector will be installed in Roman pots (RP) positioned on either side of CMS, at about 210 m from the interaction point. This detector will measure leading protons, allowing detailed studies of diffractive physics and central exclusive production in standard LHC running conditions. An essential component of the CT-PPS apparatus is the tracking system, which consists of two detector stations per arm equipped with six 3D silicon pixel-sensor modules, each read out by six PSI46dig chips. The front-end electronics has been designed to fulfill the mechanical constraints of the RP and to be compatible as much as possible with the readout chain of the CMS pixel detector. The tracking system is currently under construction and will be installed by the end of 2016. In this contribution the final design and the expected performance of the CT-PPS tracking system is presented. A summary of the studies performed, before and after irradiation, on the 3D detectors produced for CT-PPS is given.

3D silicon pixel detectors for the ATLAS Forward Physics experiment

The ATLAS Forward Physics (AFP) project plans to install 3D silicon pixel detectors about 210 m away from the interaction point and very close to the beamline (2–3 mm). This implies the need of slim edges of about 100–200 μm width for the sensor side facing the beam to minimise the dead area. Another challenge is an expected non-uniform irradiation of the pixel sensors. It is studied if these requirements can be met using slightly-modified FE-I4 3D pixel sensors from the ATLAS Insertable B-Layer production. AFP-compatible slim edges are obtained with a simple diamond-saw cut. Electrical characterisations and beam tests are carried out and no detrimental impact on the leakage current and hit efficiency is observed. For devices without a 3D guard ring a remaining insensitive edge of less than 15 μm width is found. Moreover, 3D detectors are non-uniformly irradiated up to fluences of several 1015 neq/cm2 with either a focussed 23 GeV proton beam or a 23 MeV proton beam through holes in Al masks. The efficiency in the irradiated region is found to be similar to the one in the non-irradiated region and exceeds 97% in case of favourable chip-parameter settings. Only in a narrow transition area at the edge of the hole in the Al mask, a significantly lower efficiency is seen. A follow-up study of this effect using arrays of small pad diodes for position-resolved dosimetry via the leakage current is carried out.

Simulation and laboratory test results of 3D CMS pixel detectors for HL-LHC

The CMS pixel detector is the innermost tracking device at the LHC, reconstructing interaction vertices and charged particle trajectories. The current planar sensors located in the innermost layer of the pixel detector will be exposed to very high fluences which will degrade their performances. As a possible replacement for planar pixel sensors in the High Luminosity-LHC (HL-LHC), 3D silicon technology is under consideration due to its expected good performance in harsh radiation environments. Studies are also in progress for using 3D silicon pixel detectors in near-beam proton spectrometers at the LHC. Deep Reactive Ion Etching (DRIE) plays a key role in fabricating 3D silicon detectors in which readout and ohmic electrodes are processed through the silicon substrate instead of being implanted on the silicon surface. 3D pixel devices considered in this study were processed at FBK (Trento, Italy), bump bonded to the CMS pixel readout chip, and characterized in the laboratory. Numerical simulations were also carried out. We report on selected results from laboratory measurements and TCAD simulations.

Precision scans of the Pixel cell response of double sided 3D Pixel detectors to pion and X-ray beams

Three-dimensional (3D) silicon sensors offer potential advantages over standard planar sensors for radiation hardness in future high energy physics experiments and reduced charge-sharing for X-ray applications, but may introduce inefficiencies due to the columnar electrodes. These inefficiencies are probed by studying variations in response across a unit pixel cell in a 55μm pitch double-sided 3D pixel sensor bump bonded to TimePix and Medipix2 readout ASICs. Two complementary characterisation techniques are discussed: the first uses a custom built telescope and a 120GeV pion beam from the Super Proton Synchrotron (SPS) at CERN; the second employs a novel technique to illuminate the sensor with a micro-focused synchrotron X-ray beam at the Diamond Light Source, UK. For a pion beam incident perpendicular to the sensor plane an overall pixel efficiency of 93.0±0.5% is measured. After a 10o rotation of the device the effect of the columnar region becomes negligible and the overall efficiency rises to 99.8±0.5%. The double-sided 3D sensor shows significantly reduced charge sharing to neighbouring pixels compared to the planar device. The charge sharing results obtained from the X-ray beam study of the 3D sensor are shown to agree with a simple simulation in which charge diffusion is neglected. The devices tested are found to be compatible with having a region in which no charge is collected centred on the electrode columns and of radius 7.6±0.6μm. Charge collection above and below the columnar electrodes in the double-sided 3D sensor is observed.

Design, simulation, production and initial characterisation of 3D silicon detectors

3D detectors are photodiode radiation detectors with n- and p-type electrode columns passing through a silicon substrate. This structure makes it possible to achieve a very small electrode spacing without reducing the sensitive thickness. This greatly reduces the detector's depletion voltage and collection time, and hence improves its radiation hardness. This could make 3D detectors useful as pixel detectors for future high-luminosity colliders, such as the Super-LHC. The research institute IMB-CNM (Centro Nacional de Microelectronica, Barcelona) have produced 3D pad, pixel and strip detectors with a “double-sided 3D” structure. This fabrication has been done alongside design and simulation work at the University of Glasgow. The first devices produced by CNM have been successfully IV and CV tested, and source tests are ongoing. Additionally, this conference record discusses work done by other 3D detector collaborations: Stanford, Manchester University and Sintef; FBK (Trento); and Glasgow, Diamond Light Source and IceMOS Ltd.

Monte Carlo simulation of the response of a pixellated 3D photo-detector in silicon

The charge transport and X-ray photon absorption in three-dimensional (3D) X-ray pixel detectors have been studied using numerical simulations. The charge transport has been modelled using the drift-diffusion simulator MEDICI, while photon absorption has been studied using MCNP. The response of the entire pixel detector system in terms of charge sharing, line spread function and modulation transfer function, has been simulated using a system level Monte Carlo simulation approach. A major part of the study is devoted to the effect of charge sharing on the energy resolution in 3D-pixel detectors. The 3D configuration was found to suppress charge sharing much better than conventional planar detectors.