Leakage current of high-fluence neutron-irradiated 8" silicon sensors for the CMS Endcap Calorimeter Upgrade

TL;DR

This work quantifies radiation-induced bulk damage in CMS HGCAL silicon pad sensors by examining leakage current across full and partial sensors after neutron irradiation up to $1.4 \times 10^{16}~n_{eq}/cm^{2}$. It combines sensor design variants (300, 200, 120 µm, HD/LD, partials) with irradiation campaigns at RINSC, implementing strategies to control in-reactor annealing (e.g., irradiation-round splitting) and employing multiple fluence estimation methods. The study finds mostly diode-like IV behavior, with occasional exponential leakage growth at high fluence and long annealing, which is mitigated by splitting irradiations; activation energies from temperature scans align with SRH bulk recombination, and current-related damage factors are consistent within uncertainties. Overall, results support the feasibility of CE silicon sensors operating within design limits at planned HL-LHC conditions, provided effective cooling and annealing control are maintained, while highlighting areas for improved fluence measurement and temperature monitoring in future irradiations.

Abstract

The HL-LHC will challenge the detectors with a nearly 10-fold increase in integrated luminosity compared to the previous LHC runs combined, thus the CMS detector will be upgraded to face the higher levels of radiation and the larger amounts of collision data to be collected. The High-Granularity Calorimeter will replace the current endcap calorimeters of the CMS detector. It will facilitate the use of particle-flow calorimetry with its unprecedented transverse and longitudinal readout/trigger segmentation, with more than 6M readout channels. The electromagnetic section as well as the high-radiation regions of the hadronic section of the HGCAL (fluences above $10^{14}~n_{eq.}/cm^{2}$) will be equipped with silicon pad sensors, covering a total area of 620 m$^2$. Fluences up to $10^{16}~n_{eq.}/cm^{2}$ and doses up to 1.5 MGy are expected. The sensors are processed on novel 8" p-type wafers with an active thickness of 300 $μm$, 200 $μm$ and 120 $μm$ and cut into hexagonal shapes for optimal use of the wafer area and tiling. Each sensor contains several hundred individually read out cells of two sizes (around 0.6 cm$^2$ or 1.2 cm$^2$). To investigate the radiation-induced bulk damage, the sensors have been irradiated with neutrons at RINSC to fluences between $6.5 \times 10^{14}~n_{eq.}/cm^{2}$ and $1.4 \times 10^{16}~n_{eq.}/cm^{2}$. Electrical characterization results are presented for full sensors, as well as for partial sensors cut from multi-geometry wafers with internal dicing lines on the HV potential within the active sensor area. Leakage current behaviour is investigated for various sensor types and fluence levels, including its temperature dependence. Finally, methods to limit the annealing time of the sensors during irradiation are investigated by analysing the impact of splitting high-fluence irradiations.

Figures (12)

• Figure 1: a) Sensor cross-sections for different manufacturing processes Paulitsch:2020hci. b) CMS simulation showing an optimized lateral layout for CE layer 11. c) Overview of LD and HD partial sensor cut types. d) Schematic (not to scale) of the HV protection structures for a full sensor. The outer edge of the sensor is at HV potential (HV ring shown in red). The first protection ring (blue) is a floating guard ring (GR), meaning it is not connected to any fixed potential. The innermost ring (green) is a grounded guard ring, which is connected to ground potential to shield the inner sensor cells from the high voltage at the sensor's edge. e) Layouts of LD and HD partial sensor design. f) Schematic (not to scale) of the HV protection structures for a partial sensor (LD Five), which are also present within the sensor's interior due to the internal dicing lines.
• Figure 2: a) Photograph of an LD Five partial sensor placed in the logistics tray. b) Microscope image showing the internal dicing lines, HV rings, and internal HV protection structures (including the floating GR and grounded GR), on a partial sensor.
• Figure 3: Microscope images of various cell types on an HD Top sensor: a) standard full cell, b) calibration cell, c) edge cells near internal dicing lines, and d) edge cell at outer perimeter. The visible contact marks from previous measurements are highlighted with green circles. They lie within the openings of the sensor passivation layer (visible as brighter regions on the sensor surface), ensuring proper electrical contact. Their positions also enable an estimate of the precision and repeatability of the contacting method.
• Figure 4: Schematics (not to scale) of the sensor placement within the aluminium transport cylinder at RINSC, as described in ref. Acar_2023. The positions of the PT1000 RTDs and dosimetry sensors are presented with the approximate dimensions within the puck container.
• Figure 5: An example of a temperature profile obtained from an irradiation round at RINSC, showing averaged measurements at the front, back, and the average of both sides. The distinct phases of irradiation can be inferred from the shape of the profile.