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Unno, Y. et al, Garcia, C., Jimenez, J., Lacasta, C., Marti-Garcia, S., & Soldevila, U. (2014). Development of n(+) -in-p large-area silicon microstrip sensors for very high radiation environments-ATLAS12 design and initial results. Nucl. Instrum. Methods Phys. Res. A, 765, 80–90.
Abstract: We have been developing a novel radiation tolerant n(+)-in-p silicon microstrip sensor for very high radiation environments, aiming for application in the high luminosity large hadron collider. The sensors are fabricated in 6 in., p-type, float zone wafers, where large area strip sensor designs are laid out together with a number of miniature sensors. Radiation tolerance has been studied with ATLAS07 sensors and with independent structures. The ATLAS07 design was developed into new ATLAS12 designs. The ATLAS12A large-area sensor is made towards an axial strip sensor and the ATLAS12M towards a stereo strip sensor. New features to the ATLAS12 sensors are two dicing lines: standard edge space of 910 pm and slim edge space of 450 pm, a gated punch-through protection structure, and connection of orphan strips in a triangular corner of stereo strips. We report the design of the ATLAS12 layouts and initial measurements of the leakage current after dicing and the resistivity of the wafers.
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Unno, Y. et al, Bernabeu, J., Lacasta, C., Solaz, C., & Soldevila, U. (2023). Specifications and pre-production of n plus -in-p large-format strip sensors fabricated in 6-inch silicon wafers, ATLAS18, for the Inner Tracker of the ATLAS Detector for High-Luminosity Large Hadron Collider. J. Instrum., 18(3), T03008–29pp.
Abstract: The ATLAS experiment is constructing new all-silicon inner tracking system for HL-LHC. The strip detectors cover the radial extent of 40 to 100 cm. A new approach is adopted to use p-type silicon material, making the readout in n+-strips, so-called n+-in-p sensors. This allows for enhanced radiation tolerance against an order of magnitude higher particle fluence compared to the LHC. To cope with varying hit rates and occupancies as a function of radial distance, there are two barrel sensor types, the short strips (SS) for the inner 2 and the long strips (LS) for the outer 2 barrel cylinders, respectively. The barrel sensors exhibit a square, 9.8 x 9.8 cm2, geometry, the largest possible sensor area from a 6-inch wafer. The strips are laid out in parallel with a strip pitch of 75.5 μm and 4 or 2 rows of strip segments. The strips are AC-coupled and biased via polysilicon resistors. The endcap sensors employ a “stereo-annulus” geometry exhibiting a skewed-trapezoid shapes with circular edges. They are designed in 6 unique shapes, R0 to R5, corresponding to progressively increasing radial extents and which allows them to fit within the petal geometry and the 6-inch wafer maximally. The strips are in fan-out geometry with an in-built rotation angle, with a mean pitch of approximately 75 μm and 4 or 2 rows of strip segments. The eight sensor types are labeled as ATLAS18xx where xx stands for SS, LS, and R0 to R5. According to the mechanical and electrical specifications, CAD files for wafer processing were laid out, following the successful designs of prototype barrel and endcap sensors, together with a number of optimizations. A pre-production was carried out prior to the full production of the wafers. The quality of the sensors is reviewed and judged excellent through the test results carried out by vendor. These sensors are used for establishing acceptance procedures and to evaluate their performance in the ATLAS collaboration, and subsequently for pre-production of strip modules and stave and petal structures.
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Ullan, M., Benitez, V., Quirion, D., Zabala, M., Pellegrini, G., Lozano, M., et al. (2014). Low-resistance strip sensors for beam-loss event protection. Nucl. Instrum. Methods Phys. Res. A, 765, 252–257.
Abstract: AC coupled silicon strip sensors can be damaged in case of a beam loss due to the possibility of a large charge accumulation in the bulk, developing very high voltages across the coupling capacitors which can destroy them. Punch-through structures are currently used to avoid this problem helping to evacuate the accumulated charge as large voltages are developing. Nevertheless, previous experiments, performed with laser pulses, have shown that these structures can become ineffective in relatively long strips. The large value of the implant resistance can effectively isolate the “far” end of the strip from the punchthrough structure leading to large voltages. We present here our developments to fabricate lowresistance strip sensors to avoid this problem. The deposition of a conducting material in contact with the implants drastically reduces the strip resistance, assuring the effectiveness of the punch-through structures. First devices have been fabricated with this new technology. Initial results with laser tests show the expected reduction in peak voltages on the low resistivity implants. Other aspects of the sensor performance, including the signal formation, are not affected by the new technology.
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Studen, A., Chesi, E., Cindro, V., Clinthorne, N. H., Cochran, E., Grosicar, B., et al. (2011). A silicon PET probe. Nucl. Instrum. Methods Phys. Res. A, 648, S255–S258.
Abstract: PET scanners with high spatial resolution offer a great potential in improving diagnosis, therapy monitoring and treatment validation for several severe diseases. One way to improve resolution of a PET scanner is to extend a conventional PET ring with a small probe with excellent spatial resolution. The probe is intended to be placed close to the area of interest. The coincidences of interactions within the probe and the external ring provide a subset of data which combined with data from external ring, greatly improve resolution in the area viewed by the probe. Our collaboration is developing a prototype of a PET probe, composed of high-resolution silicon pad detectors. The detectors are 1 mm thick, measuring 40 by 26 mm(2), and several such sensors are envisaged to either compensate for low stopping power of silicon or increase the area covered by the probe. The sensors are segmented into 1 mm(3) cubic voxels, giving 1040 readout pads per sensor. A module is composed of two sensors placed in a back-to-back configuration, allowing for stacking fraction of up to 70% within a module. The pads are coupled to a set of 16 ASICs (VaTaGP7.1 by IDEAS) per module and read out through a custom designed data acquisition board, allowing for trigger and data interfacing with the external ring. This paper presents an overview of probe requirements and expected performance parameters. It will focus on the characteristics of the silicon modules and their impact on overall probe performance, including spatial resolution, energy resolution and timing resolution. We will show that 1 mm(3) voxels will significantly extend the spatial resolution of conventional PET rings, and that broadening of timing resolution related to varying depth of photon interactions can be compensated to match the timing resolution of the external ring. The initial test results of the probe will also be presented.
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Studen, A., Burdette, D., Chesi, E., Cindro, V., Clinthorne, N. H., Cochran, E., et al. (2010). Timing performance of the silicon PET insert probe. Radiat. Prot. Dosim., 139(1-3), 199–203.
Abstract: Simulation indicates that PET image could be improved by upgrading a conventional ring with a probe placed close to the imaged object. In this paper, timing issues related to a PET probe using high-resistivity silicon as a detector material are addressed. The final probe will consist of several (four to eight) 1-mm thick layers of silicon detectors, segmented into 1 x 1 mm(2) pads, each pad equivalent to an independent p + nn+ diode. A proper matching of events in silicon with events of the external ring can be achieved with a good timing resolution. To estimate the timing performance, measurements were performed on a simplified model probe, consisting of a single 1-mm thick detector with 256 square pads (1.4 mm side), coupled with two VATAGP7s, application-specific integrated circuits. The detector material and electronics are the same that will be used for the final probe. The model was exposed to 511 keV annihilation photons from an Na-22 source, and a scintillator (LYSO)-PMT assembly was used as a timing reference. Results were compared with the simulation, consisting of four parts: (i) GEANT4 implemented realistic tracking of electrons excited by annihilation photon interactions in silicon, (ii) calculation of propagation of secondary ionisation (electron-hole pairs) in the sensor, (iii) estimation of the shape of the current pulse induced on surface electrodes and (iv) simulation of the first electronics stage. A very good agreement between the simulation and the measurements were found. Both indicate reliable performance of the final probe at timing windows down to 20 ns.
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