Work Package 1: Solid-State Photodetectors

The Silicon Photomultiplier (SiPM) is a Solid State Photodetector (SSPD) that has revolutionised photodetection - in HEP and other fields. Its properties have paved the way to new applications and will continue to do so in the coming decade. The following WPs focus on the further improvement of the SiPM technology. The participating groups and an overview of the resources can be found at the end of this chapter.

Task 1.1 - SSPD with new configurations and modes

Long-term goals: In this work package new SiPM technologies, ultra-high granularity, and integration with readout electronics will be studied. The main goal will be higher efficiency and the study of properties of solid-state photo-sensors for future high-energy physics experiments. The sensors will be further optimized in the subsequent work packages to allow for operation in extreme working conditions ranging from operation at very low temperatures down to liquid nitrogen to extreme photon densities where high time resolution is required together with ultrafine granularity

Objectives:
1. In the first phase, the work will be focused on the backside illuminated (BSI) technology of SiPMs. A carefully designed device’s electric field and dopant layer configuration, together
with the uniform light entrance window on the backside of the detector, enable advanced processing, such as plasma doping molecular beam epitaxy, which could eventually provide both an enhanced PDE and a better radiation tolerance. The sensitivity of SiPMs typically covers the entire visible range, but in some applications, sensitivity in the vacuum ultraviolet (VUV) or near-infrared (NIR) range is required, posing significant challenges in technology development and device design.
The main objective is designing, fabricating, and characterising new types of photon detectors, which will follow the first demonstration of BSI SiPMs obtained by thinning SiPM wafers to about 10 µm, with an anti-reflective coating on the backside and bonded to a glass substrate for mechanical stability. If successful, the development would be useful for different applications, from RICH and TOF detectors in HEP experiments to imaging readout in liquid argon and space instrumentation, with impact on future projects such as the upgrades at LHCb, DUNE, EIC, Belle II, and the ALICE 3 outer TOF detector.
2. A successful BSI SiPM technology paves the way towards the second objective: developing ultra-granular SiPM that closely integrates with the readout electronics by using 2.5D or
3D interconnection techniques. Such deep integration with the Front-end electronics is extremely important for achieving improved timing addressed in Task 1.3
3. In this objective a demonstrator of a digital SiPM will be built and characterised.
4. Implementation and characterization of CMOS-SPAD sensors for light detection in high energy physics, mainly for RICH and calorimetry application. Standard CMOS processes
provide a mature and reliable technology, which allows the co-integration of SPADs and electronics at low costs. Combining SPAD and CMOS readout electronics on a single chip with a custom read-out can provide digitized output signals with low power consumption and fast read-out.
5. Studies of potential new materials for light detection, e.g., SiC, GeC, and investigation of InGaAs, GaAs technology for photon detectors: Colour centers in SiC have recently
emerged as one of the most promising emitters for bright single-photon emitting diodes. The precise properties of GeC, such as its band gap and electrical conductivity, would depend on its crystal structure and specific composition. As it could be integrated into semiconductor devices, its properties may offer advantages in SSPDs.

Description of work:
In collaboration with the SiPM producers, the requirements and possibilities for custom BSI SiPMs will be reviewed. Early BSI samples will be evaluated and characterized, and further design steps will be defined. We will characterize the sensor timing properties, cross talk, after pulsing, and determine sensors’ optimal operating conditions.
The goal of the CMOS-SPAD activity is the development of a technology platform for the design, production, and commissioning of digital silicon photomultipliers (SiPMs) based on a planar monolithic sensor in a standard CMOS technology with the processing electronics and the sensitive element in the same substrate. The monolithic structure of the sensor simplifies the assembly of large-area detectors.

Milestones:

M1.1.1 Tested Back-side illuminated SiPM samples (M24)
M1.1.2 Tested CMOS-SPAD samples (M24)

Deliverables:

D1.1.1 Demonstrator of Back-side illuminated SiPM array (M36)
D1.1.2 Demonstrator of CMOS-SPAD monolithic sensor (M36)

Task 1.2 - Fast radiation hard SiPMs

Long term goals:
In Cherenkov Ring Imaging detectors (RICH), detecting single photons with high efficiency and good position resolution is crucial. SiPMs are competitive sensors compared to complex-tooperate vacuum devices with a limited magnetic field tolerance. However, their use is constrained due to their sensitivity to bulk damage, particularly due to neutron irradiation. The long-term goal is the development of radiation-hardened SiPMs and radiation mitigation techniques that would result in an acceptable complexity of the operation for detecting single photons in extreme radiation conditions. As the definition of neutron resistance depends on the experimental requirements, we will first focus on the current and, in the next stage, on future experiments.
An important goal will be standardizing characterization procedures and standards to measure SiPM performance after irradiation.

Objectives:
1. Establish procedures for quantification of radiation effects.
2. Characterize the irradiated SiPMs and determine the working conditions when used in extreme environments. Test operation of SiPMs with associated electronics in a wide range of temperatures down to -200◦C.
3. Development of procedures for the annealing of SiPMs after irradiation. The annealing has great potential to improve SiPM performance after irradiation however a widely accepted standard doesn’t exist yet.
4. Study and quantify other measures enabling the use of SiPM in highly irradiated areas, e.g., using smaller SiPMs to reduce the radiation-sensitive volume and macro- and microlight collectors to maximize the yield and extend operation at low temperatures.

Description of work: After establishing the protocols, the samples developed in Task 1.1 will be characterized and compared to existing technologies. Mitigation techniques will be studied, and results will be extrapolated to different use cases.

Milestone:

M1.2 Standardization of characterization procedures and standards to measure SiPM performance after irradiation (M18)

Deliverable:

D1.2 Report on achieved state-of-the-art fast and radiation hard SiPMs and analysis of prospects (M36)

Task 1.3 - Timing of SSPD – including the appropriate readout electronics

Long term goals:
Particle identification algorithms of the future particle identification systems will rely on measuring the arrival times of single or few photons. To achieve the ultimate timing performance of silicon photomultipliers, fine segmentation of the sensor active area and local interconnection to the ASIC is needed. In this work package, we will focus on optimizing the
timing of photon sensors, the front-end electronics, and their integration.

Objectives:
1. Study and improve the timing of Silicon Photomultipliers.
2. Enable the exploitation and co-design of a suitable, multi-channel readout ASIC capable of exploiting all the detection’s timing potential.
3. Develop an optimized, reliable, cost-effective integration scheme and packaging solution with integrated cooling.
4. Study the vertical integration of dedicated SiPM arrays to the readout electronics to optimize timing resolution by reducing the interconnections’ parasitic inductances and capacitances.

Description of work:
Initially we will study the timing properties of single channel SiPMs by using high power amplifiers. The effect of packaging and interconnections will be explored.
Based on knowledge gained, an optimal electronics, that can be closely integrated with the SiPM will be designed and tested. A demonstration multichannel sensor will be build and its
properties characterized.