Novel 3D Pixel Sensors for the Upgrade of the ATLAS Inner Tracker

The ATLAS experiment will undergo a full replacement of its inner detector to face the challenges posed by the High Luminosity upgrade of the Large Hadron Collider (HL-LHC). The new Inner Tracker (ITk) will have to deal with extreme particle fluences. Due to its superior radiation hardness the 3D silicon sensor technology has been chosen to instrument the innermost pixel layer of ITk, which is the most exposed to radiation damage. Three foundries (CNM, FBK, and SINTEF), have developed and fabricated novel 3D pixel sensors to meet the specifications of the new ITk pixel detector. These are produced in a single-side technology on either Silicon On Insulator (SOI) or Silicon on Silicon (Si-on-Si) bonded wafers by etching both n- and p-type columns from the same side. With respect to previous generations of 3D sensors they feature thinner active substrates and smaller pixel cells of 50 × 50 and 25 × 100 µm2. This paper reviews the main design and technological issues of these novel 3D sensors, and presents their characterization before and after exposure to large radiation doses close to the one expected for the innermost layer of ITk. The performance of pixel modules, where the sensors are interconnected to the recently developed RD53A chip prototype for HL-LHC, has been investigated in the laboratory and at beam tests. The results of these measurements demonstrate the excellent radiation hardness of this new generation of 3D pixel sensors that enabled the project to proceed with the pre-production for the ITk tracker.


INTRODUCTION
The Large Hadron Collider (LHC), located at CERN, is the world's largest and most powerful particle accelerator. ATLAS is one of two general-purpose detectors at the LHC. In order to advance our understanding of elementary particles and their interactions, the LHC accelerator will be improved to be able to reach about seven times its current nominal instantaneous luminosity. The High Luminosity LHC (HL-LHC) is currently foreseen to start operations in 2027. To cope with the higher particle rate, hit occupancy and radiation damage associated with the HL-LHC period, and maintain the overall detector performance, several ATLAS sub-systems will have to be upgraded. In particular, the current ATLAS tracking system will be replaced by a full silicon detector, called ITk, composed of an inner pixel detector surrounded by a microstrip system. Located in the immediate proximity of the protonproton beam collision region, the silicon pixel system is critical for the precise determination of particle tracks and vertices, allowing the identification of b-jets (b-tagging). The HL-LHC presents an unprecedented challenge to the silicon pixel sensor technologies: the detector has to provide excellent position resolution while sustaining radiation levels exceeding 1 × 10 16 n eq /cm 2 during its lifetime.
A new generation of 3D sensors has been designed to fulfill the requirements in terms of occupancy and radiation hardness for the innermost layers of the ITk pixel detector at the HL-LHC. 3D sensors are an established technology that has been already employed in experiments at the LHC such as in the ATLAS Insertable B-Layer (IBL) [1] and for the tracker of the AFP experiment [2]. With respect to these designs the new ITk 3D sensors feature a reduced pixel cell size of 25 × 100 and 50 × 50 µm 2 with one collecting electrode. In the following these pixel cell geometries will be abbreviated as 25 × 100-1E and 50 × 50−1E. A 25 × 100 µm 2 pixel cell design with two electrodes (25 × 100−2E) was also investigated, but eventually dropped because of technical difficulties in achieving a satisfactory production yield. All these sensor designs are compatible with the future ATLAS readout chip, the ITkPix. Moreover, in order to lower the occupancy, the thickness of the active substrate of these new sensors is reduced to 150 µm in comparison to the previous generation of 230 µm thick 3D sensors. To achieve such thickness the use of a single-side technology was necessary. This paper will describe the design and the technological challanges of the recent 3D sensor productions for ITk as well as the results of the characterization performed on assembled module prototypes to validate the performance of these novel 3D sensor designs.

THE ITK PIXEL DETECTOR
The ATLAS detector [3] at the CERN LHC accelerator complex is the largest volume detector ever constructed for a particle collider. ATLAS has the dimensions of a cylinder, 46 m long, 25 m in diameter, and sits in a cavern 100 m below ground. The ATLAS detector weighs 7,000 tonnes and consists of six different detecting subsystems wrapped concentrically in layers around the collision point to record the trajectory, momentum, and energy of outgoing particles, allowing them to be individually identified and measured. A huge magnet system bends the paths of the charged particles so that their momenta can be measured. As mentioned above, the ATLAS subsystems have to be upgraded in view of the challenges of the HL-LHC. The current tracking system will be completely replaced by the new ITk tracking detector.

Layout
The ITk tracker is composed of an inner Pixel Detector, followed by the Strip Detector ( Figure 1). The Strip Detector covers a pseudo-rapidity of η < 2.7 and a radial region of 40-100 cm, while the five-layered Pixel Detector extends the coverage to η < 4.0 and has an inner radius of r 3.4 cm. The Pixel and Strip Detector volumes are separated by a Pixel Support Tube (PST). In addition, the inner two pixel layers are separated from the outer three layers by an Inner Support Tube (IST), that facilitates a replacement of the two inner layers. The combined Strip plus Pixel Detectors provide a total of nine space points in the η < 4.0 volume. The new detector has been designed with less inactive material in the tracking volume and aims at maintaining and possibly improving the performance of the existing detector, but in a much more hostile tracking environment with an average of up to 200 proton-proton collisions per beam crossing. The innermost pixel layer will include sensors with a 25 × 100 µm 2 pixel geometry in the flat barrel region, and a 50 × 50 µm 2 geometry in the rings.

The HL-LHC Challenges
The HL-LHC presents two key challenges to the ATLAS experiment: while the detector has to cope with unprecedented radiation levels, it also has to be able to disentangle the huge amount of simultaneous events that will be generated in the proton-proton beam collisions. This is especially critical for the innermost silicon pixel layer, since it is the one that suffers the higher radiation levels and has to operate in the most dense particle environment. Thus, the inner layers of the ITk tracker demand the most radiation hard technology, but at the same time small pixels and thin active regions to mitigate the effect of the high particle multiplicity. These two requirements could be seen as competing, since, in standard silicon devices, thinner sensors usually collect less charge and thus are more sensitive to charge trapping effects introduced by radiation damage.
The HL-LHC period is expected to deliver a total integrated luminosity of 4,000 fb −1 , however, the ITkPix front-end is not expected to survive the large radiation doses associated with this scenario. The current strategy foresees replacement of the two inner layers. From the sensor point of view, the expected maximum particle fluence in the innermost layer (r 3.4 cm) after 2,000 fb −1 is 1.2 × 10 16 n eq /cm 2 (with a specification of 1.7 × 10 16 n eq /cm 2 including safety factors).

The ITk Pixel Front-End Chip
The front-end chip integrates the charge generated in the sensor by crossing particles, amplifies and digitizes the signal, and sends the hit information downstream to the DAQ system. The ITk pixel readout chip, called ITkPix, will present a matrix of 400 × 384 pixels of 50 × 50 µm 2 , which determines the active area of the chip. The ITkPix is based on the prototype chip developed by the RD53 Collaboration, the RD53A [4]. The RD53A readout chip presents a matrix of 400 × 192 pixels of 50 × 50 µm 2 with a total size of 20.0 × 11.8 mm 2 . The ASIC, also fabricated in the 65 nm CMOS technology, includes three different analog front-end designs to allow performance comparisons. These are the synchronous, linear and differential front-ends. The latter has been selected by the ATLAS Collaboration for the development of the final ITkPix chip for ITk.
Measurements of bare RD53A chips and modules (the chip interconnected to a pixel sensor) have shown that the ASIC achieves the desired functionality and can operate at a low threshold, which is a fundamental parameter to ensure high hit reconstruction efficiency after irradiation as it will be discussed below.

Sensor Requirements
Sensors shall be interconnected to the ATLAS ITk pixel frontend chips which impose a maximum capacitance per pixel of 100 fF. In the innermost layer the 3D modules are attached in groups of three to a single flex readout board (triplet) sharing the same bias voltage line connecting them in series. To ensure the proper operation of the sensors in this configuration, before irradiation the depletion voltage (V dep ) has been specified to be lower than 10 V and the breakdown voltage larger than the foreseen operational voltage of V op V dep + 20 V. A limit of 2.5 μA/cm 2 has been imposed for the leakage current at V op .
As mentioned above, the innermost layer imposes strict requirements in terms of performance after irradiation. With the replacement strategy, the fluence for the innermost pixel layer will exceed 1 × 10 16 n eq /cm 2 . In these conditions the hit efficiency is required to be larger than 97% (96%) before (after) irradiation at normal beam incidence and larger than 98% (97%) at 14-15°incidence (the final orientation of the modules in the ITk detector). During the whole lifetime of the detector the power dissipation performance of the 3D sensors shall be better than 40 mW/cm 2 at the benchmark sensor temperature of −25°C. The maximum operational bias voltage after irradiation should not exceed 250 V.

Single-Side Approach
Double-sided technologies, which proved to offer several advantages in terms of process complexity and throughput for the IBL production [5], are not favored for the fabrication of thin 3D pixels, due to mechanical integrity considerations. As a result, a single-side approach with a support wafer has finally been adopted by all the processing foundries. As compared to the technology developed in Stanford in the late 1990's, the process has been modified to be compatible with Si-on-Si wafers, which consist of a high-resistivity float zone active layer of the desired thickness directly bonded to a low-resistivity Czochralski handle wafer of adequate thickness, that might be different depending on the wafer diameter. All columnar electrodes are etched from the front-side and are (at least partially) filled with poly-Si: the junction (readout) columns stop at a safety distance of ∼ 25 µm from the handle wafer, so as to avoid early breakdown, whereas the ohmic (bias) columns are etched deeper and penetrate into the handle wafer. By doing so, the sensor bias can be applied from the back-side, which is a definite advantage for the assembly of the sensors in a real detector system. In case its thickness is large, so as to represent a possible issue for multiple scattering, the handle wafer shall be  partially removed as a post-processing step, but a thin portion of it could be left that would still allow a back-side metal to be deposited to apply the bias voltage. A schematic cross section of a device in this latter configuration is shown in Figure 2.
Note that surface insulation layers (p-stop or p-spray) are not shown.

Small Pixel Cells
On one hand, small pixel cells involve small inter-electrode distances, which inherently improves the radiation hardness, since they can be made comparable to the maximum drift length of charge carriers [6]. On the other hand, a high density of columnar electrodes could also result in a loss of geometrical efficiency, since the electrodes themselves are not efficient, as well as in an increase of the sensor capacitance (and noise). However, both these problems are attenuated by the use of thin active layers: 1) the electrode capacitance scales with its depth to a large extent, so thinner substrates naturally lead to smaller capacitance; 2) assuming the aspect ratio (depth to diameter) attainable with Deep Reactive Ion Etching (DRIE) to be constant (typical values range from ∼ 20:1 to ∼ 30:1), thinner substrates allow for narrower electrode diameters (typical values range from ∼ 5 to ∼ 8 µm), that also contribute to decrease the capacitance and improve the geometrical efficiency.
The layout of small 3D pixel cells is quite straightforward, provided the device geometries are all down-scaled. As an example, Figure 3 shows the pixel layouts, specific to the FBK technology, for the three considered geometries.
It can be seen that the main constraint is due to the fixed size (∼ 20 µm diameter at the metal level) of the bump pads, which is relatively large with respect to the pixel sizes. In addition, it should be noted that also the pad footprint is fixed (50 × 50 µm 2 ) due to the layout of the readout chip. While this is not an issue with the 25 × 100−1E and 50 × 50−1E pixels, it represents a critical problem in 25 × 100−2E pixels, where the bump pads are very close to both the readout and the ohmic columns. This makes the layout very sensitive to lithography misalignments of just a few micrometers, that are typically achieved with mask aligners, making it difficult to obtain a good fabrication yield.

FOUNDRIES AND PRODUCTIONS
Productions of 3D sensors compatible with RD53A chips have been carried out at CNM, FBK, and SINTEF. These productions all employ a single-side approach either with Silicon-On-Insulator wafers (SOI) or Silicon-On-Silicon wafers (Si-on-Si). Even if in some productions the thickness is still less with respect to the design specifications of the final sensor (130 µm in FBK, and 50 µm or 100 µm in SINTEF sensors), most of the results can be considered as worst-case scenarios due to the reduced number of initial charge carriers generated by Minimum Ionising Particles (MIPs). Indeed, a thicker active substrate is expected to perform better in terms of charge collection and hit efficiency before and after irradiation. Additional significant differences between the foundry prototype designs are: the edge termination, obtained with an edge doping (active edge) at SINTEF and instead with p-type column fences in CNM and FBK (slim edge design [5]); and the inter-pixel surface insulation where FBK and SINTEF use p-spray, while CNM uses p-stop rings around the n-type columns.  To reduce the thickness of the active volume, CNM developed ultra-thin 3D single-sided detectors for Neutral Particle Analysers (NPA) and thermal neutron detection in 2009 [9]. This technology was the basis for the development of the upgrade of the ATLAS Inner Tracker. Initially, the detectors were fabricated on SOI wafers with a p-type backside implant and a total thickness of 400 µm [10]. The active thickness is composed of a 150 µm (or 100 µm) thick p-type wafer with a nominal resistivity in the range 10-50 kΩcm; it is separated from a low resistivity handle wafer by a 1 µm layer of Buried OXide (BOX).
The fabrication procedure is performed on four-inch wafers and requires eight photo-lithography mask levels in single-sided processing; and a total of 140 steps are carried out in the controlled environment of the clean room facility. At the end of the process a temporary metal layer is deposited on the surface of the detectors to short the pixels together and test the electrical characteristics of the devices. This layer is then removed.
Run 9761 was the first run including pixel designs compatible with the RD53A chip. The majority of the wafer layout of this run is devoted to RD53A sensors, named as the corresponding readout chip, with 50 × 50−1E pixel geometry. Each of these sensors holds a matrix of 400 × 192 pixels with a total size of 20 × 11.8 mm 2 including 150 µm per side of p-type column fences filling the space between the last pixel cell and the dicing edge. Each wafer also includes two RD53A sensors with 25 × 100−1E pixel and two with 25 × 100−2E pixel geometry. These are both matrices of 200 × 384 pixels with the same total size of the 50 × 50−1E design including the p-type column fences at the edge.
Typical electrical characteristics of the different sensor geometries in this run, as measured on dedicated 3D diodes, are shown in Figure 4. The C-V curves show a depletion voltage lower than 5 V, when the capacitance saturates, with small differences depending on the specific pixel geometry in agreement with the simulation reported in [11]. The I-V characteristics show that a rectifying p-n junction has been formed between the electrodes. The breakdown voltage can be larger than 100 V for the 50 × 50−1E and the 25 × 100−1E geometries and it is in general lower for the 25 × 100−2E, but it should be considered that the full depletion voltage foreseen for these devices is very low. Therefore, the detectors can be operated in over-depleted mode without any constraint. A larger reverse leakage current density and a lower breakdown voltage are measured for sensors with 25 × 100−2E pixel geometry which have a smaller inter-electrode distance. This may be due to the increase of the electric field among the electrodes for the same bias applied.
In order to simplify the fabrication process and avoid the etching of the handle wafer, Si-on-Si bonded wafers were chosen for the production of 3D detectors of the ATLAS Inner Tracker. The first run with these Si-on-Si wafers (11119) was produced using a total thickness of 350 µm where 150 µm is the active layer and 200 µm the handle wafer. Around 70% of the RD53A 50 × 50−1E sensors and more than 50% of the RD53A 25 × 100−1E sensors produced in this run have a breakdown voltage higher than 25 V, whereas the 25 × 100−2E configuration suffers from a very low yield and could be very problematic for a large production.

FBK: 2nd and 3rd Batches
FBK started to develop 3D sensor technologies on four-inch wafers in 2004, in collaboration with INFN. For a few years the focus was on double-side fabrication processes, and led to the development of two device versions: one with columnar electrodes partially passing through the substrate [12], and another one with full-through electrodes [13]. The latter was the technology of choice for the ATLAS IBL 3D pixel production: for this application FBK introduced the slim-edge design based on p-type column fences and the temporary metal for the onwafer electrical characterization of pixel sensors before bump bonding [5]. , of less than 10% has been estimated for both current and capacitance measurements, due to temperature variations and the LCR circuit, respectively. The systematic uncertainties due to the test instrument for the considered voltage and current range are smaller than 1%.
Frontiers in Physics | www.frontiersin.org April 2021 | Volume 9 | Article 624668 5 After the completion of the IBL production, FBK upgraded its clean room to allow production on six-inch wafers, that require a minimum thickness of 300 µm to ensure a sufficient mechanical robustness. Therefore, given the requirement of a thin active thickness for the HL-LHC upgrades, FBK deemed more appropriate to develop a new generation of small-pitch, thin 3D sensors with a single-side process, pioneering the use of Si-on-Si substrates [14].
After producing a first R&D batch in 2015 [15], mainly aimed at the technology development, FBK produced two batches oriented to the ITk project. All these batches were made on six-inch diameter wafers. The second batch was made in 2017 using mask aligner lithography [16]. The wafer layout included 18 pixel sensors of different geometries compatible with the RD53A chip: 25 × 100−1E (three samples), 25 × 100−2E (seven samples), and 50 × 50−1E (eight samples). Many other sensors compatible with the ATLAS FE-I4 and CMS PSI46dig readout chips were also present, as well as test structures (mainly 3D diodes and strips). This second batch consisted of ten wafers (five SOI and five Si-on-Si) with 130 µm active thickness. The third batch was made in 2019 using stepper lithography. The wafer layout was arranged to host 47 RD53A-compatible pixel sensors of different geometries: 25 × 100−1E (ten samples), 25 × 100−2E (24 samples), and 50 × 50−1E (13 samples), as well as test structures. The third batch consisted of eight wafers (all Si-on-Si) with 150 µm active thickness.
Although the very large variety of different layout options and process splits implemented in both the second and third batches prevent accurate calculation of the fabrication yield, some preliminary conclusions can be drawn from the electrical characterization of the RD53A-compatible pixel sensors. In the second batch, about 60% of the 50 × 50−1E and 25 × 100−1E meet the specifications described in Section 2.4, although with large non-uniformities from wafer to wafer. On the contrary, these percentages fall to less than 20% for the more critical 25 × 100−2E sensors. This motivated the use of stepper lithography in the third batch, in order to improve the pattern definition and layer alignment which are essential for such a dense layout.
Despite the choice to accommodate many design variants, which caused the stepper to be operated in a non-optimal way, in the third batch the percentage of 50 × 50−1E and 25 × 100−1E sensors meeting the specifications was still about 60%, as in the second batch. On the contrary, for the 25 × 100−2E sensors, the percentage doubled with respect to the second batch, reaching almost 40%. While this result is still worse than for the other geometries, it confirms that by using stepper lithography in an optimized way, FBK would be able to produce also this critical sensor layout.

Electrical Characteristics
Besides measurement of the I-V characteristics of pixel sensors using the temporary metal, the quality of 3D sensors from different batches is monitored at FBK by measuring test diodes. These devices reproduce the electrode configurations and layout details (including the edge region) of their parent pixel sensors, but, due to their small size ( ∼ 2 mm 2 ), they are often free from process-related defects, making it possible to investigate the intrinsic properties of the different structures.
As an example, the I-V and C-V curves of diodes from the third batch are shown in Figure 5. The leakage currents of Figure 5 Left are very small: after normalization to the number of columns present in each diode, their values at 25 V reverse bias (much larger than the depletion voltage) are ∼ 1 pA/ column, regardless of the geometry. Also the breakdown voltage ( ∼ 130 V) is roughly the same for all devices, whereas the slopes of the curves increase as the inter-electrode distance decreases, in agreement with the expectation based on the increasing electric fields.
In the C-V curves of Figure 5 Right, the knee at low voltage ( ∼ 2 V) corresponds to the full depletion of the active volume between the columns. However, the curves do not show a perfectly flat saturation, mainly due to the increasing depletion

SINTEF: Run 4 and Run 5
The SINTEF 3D sensor technology was developed as single-sided from the beginning, initially on four-inch SOI wafers and, more recently, on six-inch Si-on-Si wafers. The technology features columnar electrodes completely filled with polysilicon and an active edge, a doped trench around the sensor that allows for termination of the active volume with minimal inactive material along the edge of the detectors.
SINTEF has currently produced a total of five batches of 3D silicon sensors. The early production runs delivered good electrical yield but they showed some weakness from the point of view of the mechanical yield. This issue was solved in more recent iterations, which also delivered improved electrical yield.
The first 3D sensor batch fabricated at SINTEF including small-pitch 3D pixel detectors was completed in early 2018. This sensor production, known as "run 4", was carried out on Si-on-Si wafers with active thicknesses of 50 and 100 µm. The layout of run 4 was mostly focused on FE-I4 pixel detectors but also included two detectors compatible with the RD53A chip, in 50 × 50−1E configuration. In addition, other flavors of pixel detectors were available, compatible with the MediPix, FE-I3 and CMS PSI46 readout chips.
The latest 3D sensor batch fabricated at SINTEF, "run 5", was completed in January 2020. Produced on Si-on-Si wafers with active thickness of 150 µm, run 5 featured two different layouts (12 wafers per layout) including both RD53A and ITkPix compatible sensors. The first layout was fully tailored to the 50 × 50−1E pixel configuration, while the second layout was split into two parts, the top part of the wafers for 25 × 100−1E sensors and the bottom part of the wafer for 25 × 100−2E sensors. Each layout had 3D diodes and strip detectors with the same electrode pitch as the main pixel detectors in addition to standard planar test structures for process monitoring. Both layouts still featured the active edge. The fabrication procedure was similar to the one used in run 4, with some minor modifications that also included extra quality assurance procedures at critical steps in the process. Particular care was taken in monitoring oxide thicknesses and evolution of the wafer curvature during the process. After deposition of the temporary metal layer, the electrical characterization of the sensors was carried out on wafer using an automatic probe station. After completion of the measurements, the temporary metal layer was removed and the final metal layer deposited. The final passivation was then deposited and patterned and a final inspection of the wafers was completed.
The overall electrical yield of run 4 was excellent, with over 70% of sensors on average across the different pixel configurations showing properties compatible with the requirements described in Section 2.4. Unfortunately, due to the tight layout of the active edge, the pads on the temporary metal for the small-pitch RD53A pixel configuration were too small to be contacted manually and a full on-wafer estimate of the yield was not possible for these devices.
A reliable estimate of the yield for small-pitch 3D detectors produced at SINTEF could only be carried out in run 5. For 50 × 50−1E devices, an average yield of 47% was achieved for RD53A sensors, while ITkPix sensors showed a considerably lower yield of around 21%. Similar figures were achieved for 25 × 100 µm 2 devices, with the 1E implementation showing considerably higher yield due to much more relaxed distances between electrodes of opposite type. In fact, the 25 × 100−1E design showed yield as high as 65% for the RD53A compatible devices and 35% for ITkPix compatible devices. The 25 × 100−2E configuration showed very low yield, below 20% for all sensor implementations. The outcome of run 5 was satisfactory, but a large variation in yield was found from wafer to wafer with some wafers exhibiting average yields above 60%, and other wafers well below 50%. The reason for the large variation in yield is related to the short distance between the active edge and the n + columns, in combination with the many challenges posed by the lithographic step creating the trench. The issues encountered in the formation of the active edge also explain why the ITkPix implementation generally shows lower yield (longer trenches means higher chance of photolithographic defects). For the next fabrication run, SINTEF will implement the common layout used by all foundries, which will not feature the active edge, hopefully solving the issues identified in run 5.

Electrical Characteristics
The measurements performed on the temporary metal can introduce spurious effects due to the positioning of the PADs and to the geometry of the metal grid necessary for the measurement. In order to isolate these effects, further electrical tests can be performed on 3D test diodes.
The SINTEF wafer layout includes 3D diodes in all flavors placed around the main detectors. The devices have an active area of roughly 1 × 1 mm 2 . The layout has active-edge termination on three sides and a slim-edge termination (column fence) on the fourth side, in order to fit the PADs. I-V and C-V measurements were performed on 3D diodes of each configuration at the end of the process, on the final metal layer. The results are reported in Figure 6. The I-V curves (Figure 6 Left) saturate at very low voltage and remain almost flat (slope equal to 1) up to the breakdown which typically occurs between 120 and 140 V. Current levels are higher for devices with a larger number of columns (e.g., 25 × 100−2E) and scale correctly for the other configuration. The C-V curves (Figure 6 Right) show initial saturation at around 5 V, but continue to decrease as the p-spray layer is progressively depleted. At 30 V a step is present in the C-V curves which indicates that the depletion region managed to completely deplete the slim-edge termination. In this layout the slim edge only featured three rows of ohmic columns; these measurements suggest that an additional row should be added to further prevent the depletion region from extending beyond the active area. The capacitance value correctly scales with the number of columns with the 25 × 100−2E diode, showing the highest capacitance.

BUMP BONDING AND ASSEMBLY
As mentioned above, the final 3D sensors will be read out by the ITkPix front end. Since this chip is not yet available, the results of RD53A prototypes will be presented. Each pixel of the 3D sensors is connected to the corresponding readout channel of the RD53A chip through a process called bumpbonding. The process consists in depositing a metal layer on top of ASIC and sensor pads (Under Bump Metallization or UBM) followed by solder balls (SnAg, SnPb, or Indium). Then the devices are interconnected through a thermal compression cycle called flip-chip.
The wafers of FBK second and third batches were sent for hybridization to the Leonardo Company (Rome, Italy) which performed electroplating UBM and bump-bonding to RD53A chips with Indium bumps. Wafers from the SINTEF run 4 were instead processed at IZM (Berlin, Germany) and flip-chipped to RD53A chips with solder bumps (SnAg). Also in the case of CNM sensors, RD53A chips with solder bumps were used, but the run 9761 was processed with electroless UBM at CNM, and flip-chipped at IFAE, while the run 11119 was sent to IZM, processed with standard electroplated UBM and flip-chipped partially at IZM and partially at IFAE. For module testing, the assemblies are then attached and wirebonded to a dedicated PCB designed by the University of Bonn.

IRRADIATION
The spectrum of particles expected in the ITk pixel detector consists mostly of charged hadrons (more than 80%) and, in particular, it is dominated by pions. Other contributions coming from neutral hadrons, electrons, positrons, muons and photons are minor. Presently none of the existing irradiation facilities is able to offer a pion beam with sufficient flux to reach the required high doses in a reasonable time. Nevertheless, many facilities offer proton beams with energies ranging from tens of MeV up to GeV which enable to study the radiation hardness of sensors in terms of damage caused by both Non-Ionizing Energy Loss (NIEL) and Total Ionizing Dose (TID) effects.
Proton beams with relatively low energies have been employed for RD53A module irradiation in three different facilities: at the cyclotron of the Karlsruher Institut für Technologie (KIT) in Germany where 23 MeV protons are provided; at the Medical Physics cyclotron of the University of Birmingham in the UK with proton beams of approximately 27 MeV; and at the Cyclotron Radio Isotope Center (CYRIC) of the Tohoku University in Japan using 70 MeV protons. In all these facilities, during irradiation, the modules are kept at temperatures below 0°C and the beam is moved over the sensor surface to obtain a uniform dose. Irradiations to reach the final target fluence of 1 × 10 16 n eq /cm 2 were mostly performed in sequential steps. This allowed mitigation of the effect of the TID damage on the chip which in most of the cases was not able to sustain a direct irradiation up to such high dose. Since the larger the energy of the protons, the lower is the TID and the damage to the chip, facilities offering higher energy beam have been preferred when available.
More RD53A modules have been irradiated with a 23 GeV proton beam from the PS accelerator at the IRRAD facility of CERN. The high energy of the particles provided by this facility allows irradiation of several modules at the same time and reaches directly the target fluence of 1 × 10 16 n eq /cm 2 without affecting the performance of the chip. In contrast to the other facilities, at IRRAD the temperature of the modules is not controlled during irradiation and significant annealing is expected due to the heat generated by the large particle flux. In addition, the irradiation dose over an RD53A module is not uniform due to the limited size of the beam, which is Gaussian with a standard deviation of about 12 mm, and due to the impossibility of scanning over the sensor surface. On one hand, these conditions add uncertainties to the effective fluence and do not allow extraction of a reliable leakage current or power dissipation measurements for the FIGURE 6 | SINTEF 3D diode measurements performed on wafer at the end of the process on the final metal. I-V (Left) and C-V (Right) curves are shown. A systematic uncertainty, not shown in the figures, of less than 10% has been estimated for both current and capacitance measurements, due to temperature variations and the LCR circuit, respectively. The systematic uncertainties due to the test instrument for the considered voltage and current range are smaller than 1%.
Frontiers in Physics | www.frontiersin.org April 2021 | Volume 9 | Article 624668 8 sensors. On the other hand, they allow probing of the effects of non-uniform irradiation on the module performance.
Irradiations have also been performed with neutrons in the TRIGA Mark II research reactor of the Jožef Stefan Institute (JSI) in Ljubljana, Slovenia. The neutron reactor allows investigation of only the NIEL damage since the TID due to gamma emission is very low, about 0.1 Mrad per 10 14 n eq /cm 2 [17]. Even if such irradiation is not as expected for ITk, it has the advantage of negligible radiation damage in the chip which allows disentanglement of its performance from the one of the sensor. Unfortunately, the tantalum contained in the RD53A chip gets activated by the interaction with neutrons resulting in several inconveniences such as long times for cooling down, expensive radiation transports and difficult handling. For this reason only a few RD53A modules have been irradiated at JSI, while this facility has been mostly used for diodes.
A summary of the RD53A modules employed for the following studies and their corresponding irradiation history can be found in Table 1. For proton irradiation, the dose received and the corresponding equivalent neutron fluence are measured from the activation of an aluminum foil placed in front of the sensors during irradiation. In the case of neutron irradiation, it is instead extrapolated from the predicted neutron flux and the immersion time in the reactor. In both cases a systematic uncertainty of 10% is associated to the fluence values.

Electrical Properties
The leakage current is a basic property of silicon detectors that indicates their electrical quality. This has been measured as a function of the bias voltage for 3D diodes and 3D sensors bumpbonded to RD53A chips before and after irradiation. The 3D modules are kept at constant temperature and relative humidity of less than 50% within a climate chamber. Before irradiation the measurements are carried out at 20°C, while after irradiation the sensors are cooled down to −25°C. Diodes are measured in the same conditions either on a probe station with a temperature controlled chuck or on dedicated Printed Circuit Boards (PCBs) inside a climate chamber. Systematic uncertainties for the measurements of the leakage current are dominated by the variation of the temperature which is measured with a precision better than 1°C corresponding to an uncertainty on the measured leakage current of less than 10%.
The leakage current as a function of the bias voltage for CNM and FBK RD53A sensors with different pixel geometries is compared in the plots in Figure 7. The properties before irradiation are mostly consistent with the measurements at wafer level with the exception of a few sensors which show a degradation of the I-V curves both in terms of leakage current level and breakdown voltage.
After irradiation the leakage current increases with the fluence and the breakdown is usually shifted towards larger voltages. Most RD53A sensors measured after irradiation show a breakdown larger than 120 V, while for two sensors (one from CNM and one from FBK) the steep increase of the leakage current is observed between 50 and 100 V. Conversely, the other sensors show very similar electrical behaviors after irradiation despite their different I-V curves observed before irradiation. No significant difference is observed in the I-V of these modules, for different pixel cell geometries after similar irradiation levels.
To increase the statistical data set and understand the variability of the I-V characteristics after irradiation, several When no irradiation facility or beam test facility is specified the module was not irradiated or was not measured at beam test, but only in the laboratories. An uncertainty of 10% is associated to the irradiation fluence (Φ) measured at the different irradiation facilities.
Frontiers in Physics | www.frontiersin.org April 2021 | Volume 9 | Article 624668 diodes from the FBK and CNM productions have been irradiated at JSI with neutrons. Only diodes that before irradiation met the specification set in Section 2.4 have been selected for this study. Their leakage current as a function of the bias voltage after irradiation up to 1.5 × 10 16 n eq /cm 2 is shown in Figure 8.
The different leakage current observed is consistent with the separation of the electrodes of the different pixel cell geometries that determines the strength of the electric field inside the active substrate. The diodes with the 25 × 100−2E pixel geometry show the highest leakage current levels and in many cases have a breakdown with a sudden increase of the current (hard breakdown) before reaching 250 V; the diodes with the 50 × 50−1E pixel geometry have lower currents, and in a few cases of the CNM production on Si-on-Si wafer (run 11119) a hard breakdown is observed at less than 250 V. Most of the CNM diodes have a smooth increase of the current (soft breakdown) around 150 V, which is consistent with the measurements on RD53A modules, while FBK diodes have a soft breakdown starting between 100 and 150 V; the 25 × 100−1E pixel geometry is the one showing the lowest current levels and better performance in terms of breakdown voltage both for FBK and CNM diodes. For this design no hard breakdown is observed in the measured diodes up to 250 V.
Only a small increase of the current can be seen between the diodes irradiated to 1 × 10 16 n eq /cm 2 and the ones irradiated to 1.5 × 10 16 n eq /cm 2 . This is more evident in the CNM diodes, since they have a more uniform behavior after irradiation, while the FBK diodes show a larger spread of the leakage current levels.
The larger current measured in the FBK diodes with respect to the CNM ones is consistent with the smaller dimensions of the former and the consequently larger fraction of the current coming from the edges with respect to that generated in the active area. A similar difference is observed on average between the two sizes of

Beam Test Measurements
Pixel modules have been measured before and after irradiation in two different beam test facilities: at the Super Proton Synchrotron (SPS) of CERN using pions of 120 GeV and at the Deutches Electronen-Synchrotronen (DESY) in Hamburg with electrons of about 5 GeV [18]. In both cases a EUDET-type telescope [19] has been used to reconstruct the trajectories of the particles in the beam and determine the impact point on the studied devices (Detectors Under Test or DUT). The telescope has two arms, each one consisting of three planes made of MIMOSA26 [20] monolithic sensors with an active area of 2 × 1 cm 2 , and can provide a pointing resolution of up to 2 µm depending on the beam energy, the amount of material between the telescope planes and the consequent multiple scattering. The coincidences of up to four scintillators placed at the two ends of the telescope and covering the MIMOSA26 sensor area are used to generate the trigger signal for the readout. The DUTs are placed between the two arms of the telescope inside a cooling box. At SPS a custom designed cooling box based on a commercial chiller allows the DUT to reach temperatures as low as -50°C to study irradiated modules, as well as to keep a constant temperature of 20°C for the operation of non-irradiated modules. The same solution could not be used at DESY due to the lower particle energy and the large mass of the chiller-based box which would lead to a significant amount of multiple scattering. In this facility a lightweight Styrofoam box is used instead. Here cooling for operations with irradiated modules is provided by dry ice which allows them to reach temperatures as low as the chiller-based box, but without the flexibility of a precise temperature control. Two different data acquisition systems (DAQ) have been used to equalize the threshold and operate the RD53A chips: the BDAQ53 [21] developed by the Bonn Silicon Laboratory and the YARR system developed at LBNL [22]. Due to the large integration time of the MIMOSA26 sensors FIGURE 9 | Hit efficiency as a function of the bias voltage for RD53A modules with CNM 3D sensors from run 9761 before and after irradiation. On the left (right) results of sensors with 50 × 50−1E (25 × 100−1E) pixel geometry obtained with the Differential (Linear) front-end are shown. The BDAQ readout system has been used for tuning and data-taking. The modules are tuned to a mean threshold of 1 ke. A systematic uncertainty of 0.3% is associated to all efficiency measurements. Partially adapted from Reference [25,26]. (about 300 µs) with respect to the DUTs (of the order of 25 ns), an additional reference plane made of a hybrid module composed of an FE-I4 chip and a planar sensor is used to provide timing information for track selection with 25 ns precision. In Table 1 are listed the sensors that have been investigated with particle beams at beam tests.

Hit Efficiency
The hit efficiency is defined as the fraction of events in which a particle passing through the active part of the DUT causes a recorded hit. Hence, it is highly influenced by the number of electron-hole pairs generated (proportional to the active thickness) and the chip threshold settings. The minimum hit efficiency target for ITk is 97% throughout the whole lifetime since lower values would cause problems for track pattern recognition [23]. Experimentally, the efficiency is determined in beam tests by extrapolating the reference track position to the DUT and searching for a hit cluster in the surroundings within a matching distance of up to two times the DUT pixel size. To disentangle the performance of the sensor from the one of the RD53A chip and from the hybridization process, noisy as well as disconnected channels are excluded from the efficiency calculation.
Results of hit efficiency measured as a function of the bias voltage are shown in Figures 9, 10 and 11 for RD53A chip modules with sensors produced by CNM, FBK and SINTEF, respectively.  [25]. From top to bottom the maps show modules measured before irradiation at perpendicular beam incidence (Φ 0, ϕ 0), after irradiation to 5 × 10 15 n eq /cm 2 at perpendicular beam incidence (Φ 5, ϕ 0) and at 15°beam incidence angle (Φ 5, ϕ 15). The hit efficiency maps of the pixel cells are obtained displaying the reconstructed track impact point expressed in pixel coordinates and projecting the data for all identical structures onto the same image. Inefficiencies mainly correspond to particles crossing the fully passing p-type columns which are inactive as can be appreciated looking at the efficiency distribution over one pixel cell shown in Figure 12. Indeed, the efficiency increases to over 99% when the sensors are tilted by 15°as it is possible to see from the results of RD53A modules using CNM 3D sensors in Figure 9. In this case the efficiency distribution over the pixel cell is uniform since the particles are not passing all the way through the p-columns.

After Irradiation
Results of RD53A modules with 50 × 50−1E sensors from all three production sites have demonstrated the possibility of reaching a hit efficiency larger than 96% for perpendicularly incident tracks after irradiation to 5 × 10 15 n eq /cm 2 with about 40 V and after irradiation to 1 × 10 16 n eq /cm 2 in the voltage range of 80-150 V with thresholds around 1 ke or lower. The 25 × 100−1E sensor from CNM after irradiation to a particle fluence of 5 × 10 15 n eq /cm 2 and operated at a threshold of 1 ke shows compatible results to the 50 × 50−1E design. The RD53A module with 25 × 100−1E sensor from FBK has been instead operated with a threshold of 1.5 ke and after irradiation to a fluence of 5 × 10 15 n eq /cm 2 reaches the 96% hit efficiency at 80 V. After irradiation to 1 × 10 16 n eq /cm 2 the 25 × 100-1E design has been measured only on one CNM sensor both at perpendicular track incidence and by tilting the device by 15°a round the long pixel side. In the first case a hit efficiency over 98% is obtained with a bias voltage of 140 V or larger, while tilting the devices an efficiency of more than 97% with just 100 V is achieved. In both cases the device has been tuned to a threshold of 1 ke.
These results are also compatible with FBK modules coming from the second batch production and measured by the CMS Collaboration [24], where an efficiency close to 97% was obtained for both 50 × 50−1E and 25 × 100−1E modules operated at 150 and 120 V, respectively.
The hit efficiency results of the different modules and productions can be considered to be consistent within the tuning uncertainties ( ∼ 200 e), taking into account the different active thicknesses of the prototypes and therefore meeting the requirements for the innermost pixel layer of the ATLAS ITk.

Charge Sharing and Cross-Talk
Cross-talk and in general larger charge sharing has been observed in 25 × 100−1E sensor prototypes before irradiation. The cross-talk threshold, defined as the average charge injected in a pixel for which a hit is observed in a neighboring pixel, has been measured on  FBK RD53A modules and is shown in Figure 13. In the 25 × 100−1E design cross-talk thresholds between 12 and 25 ke have been measured by injecting one pixel and reading out the pixels adjacent to the 100 µm sides. In the 50 × 50−1E design a cross-talk threshold of about 150 ke has been estimated by injecting the four neighboring pixels at the same time and scaling the result. A larger charge sharing in the 25 × 100−1E design with respect to the 50 × 50−1E design has been observed also in beam test measurements with CNM sensors. Before irradiation about 50% of the perpendicular incident tracks passing through the 25 × 100−1E sensor resulted in a cluster size larger than 1, while only 20% have this characteristic in the 50 × 50−1E sensor. After irradiation the charge sharing is mostly suppressed and for both designs the number of clusters with a size larger than 1 is lower than 20% at perpendicular track incidence. This level of cross-talk and charge sharing is not considered critical for the readout and occupancy of the ITk innermost layer. Figure 14 shows the power dissipation of RD53A modules and diodes after irradiation. The curves are calculated from the I-V characteristics presented in Section 7.1. The annealing times of the modules before the I-V was measured are of the order of 3-4 days. The diodes have been instead irradiated with neutrons up to 1.5 × 10 16 n eq /cm 2 and measured on a probe station with cold chuck set to −25°C after 7 days of annealing at room temperature. As expected a higher current is measured for pixel cell designs with larger inter-electrode distances, especially for the 25 × 100−2E design. Nevertheless, no significant difference is observed in the power dissipation between 50 × 50−1E and 25 × 100−1E sensors at this irradiation level for the operational voltage ranges between 80 and 150 V where more than 96% efficiency has been measured. Results of the RD53A irradiated with protons can be considered compatible with the measurements of diodes irradiated with neutrons given the uncertainties in the annealing times and irradiation levels. A few diodes and RD53A sensors show a larger power dissipation before 100 V due to an early breakdown. Nevertheless, results previously presented showed the possibility to operate RD53A modules irradiated to 1 × 10 16 n eq /cm 2 at even 60 V with more than 90% efficiency.

CONCLUSIONS
Novel 3D pixel sensor prototypes featuring small pixel cells and thin active substrates have been produced at FBK, CNM, and SINTEF using a single-side approach. These sensors have been assembled with the new RD53A readout chip and evaluated to instrument the innermost pixel layer of ITk. Results showed that these sensors meet the ITk specifications before irradiation in terms of capacitance, depletion voltage and leakage current. After irradiation up to 1 × 10 16 n eq /cm 2 , both the 50 × 50−1E and 25 × 100−1E designs were demonstrated to be able to reach a hit efficiency of 97% with a bias voltage lower than 150 V and a corresponding power dissipation within 40 mW/cm 2 . Thanks to the excellent radiation hardness of this novel technology, both of the 50 × 50−1E and 25 × 100−1E 3D sensor designs have been chosen to instrument the innermost pixel layer and rings of the ATLAS ITk. Due to the manufacturing complexity and the consequent low yield, the 25 × 100-2E design has instead been discarded at least before the replacement of the inner layers. The ITk pixel project is now advancing to the production phase of the fullsize ITkPix 3D sensor pre-series aiming at establishing the reliability of the manufacturing process for the final production. Further studies of the performance of these 3D sensors are foreseen after their irradiation up to 2 × 10 16 n eq /cm 2 to consider the safety factors required by the ITk specifications.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation. great support and discussions at the beam test as well as A. Dierlamm and F. Bögelspacher (KIT), K. Nakamura (CYRIC), L. Gonella and A. Hunter (University of Brimingham), V. Cindro (JSI) for the excellent support with the irradiation at the different facilities. The measurements leading to these results have been partially performed at the Test Beam Facility at DESY Hamburg (Germany), a member of the Helmholtz Association (HGF).