3D silicon pixel detectors for the ATLAS Forward Physics experiment

At a Glance

Metadata	Details
Publication Date	2015-03-20
Journal	Journal of Instrumentation
Authors	J. Lange, E. Cavallaro, S. Grinstein, I. Lopez Paz, J. Lange
Institutions	Institute for High Energy Physics, Institució Catalana de Recerca i Estudis Avançats
Citations	18
Analysis	Full AI Review Included

Technical Analysis & Documentation: MPCVD Diamond for Extreme Radiation Environments

Executive Summary

This document analyzes the research on 3D silicon pixel detectors for the ATLAS Forward Physics (AFP) experiment, focusing on the critical challenges of achieving minimal dead area (slim edges) and extreme radiation hardness under non-uniform fluence. The findings validate key architectural and fabrication requirements that are perfectly suited for enhancement through 6CCVD’s high-purity MPCVD diamond materials.

Radiation Validation: 3D pixel detectors demonstrated high efficiency (up to 99%) even after non-uniform irradiation up to 4 × 10¹⁵ n_eq/cm², matching expected high-luminosity LHC scenarios.
Slim Edge Success: A simple diamond-saw cut successfully reduced the inactive edge area down to 87-215 µm. The FBK design achieved a remaining insensitive edge of less than 15 µm, vital for acceptance close to the beamline (2-3 mm).
Fabrication Technique: The use of diamond-saw cutting confirms the feasibility of mechanical processing for producing specialized detector geometries with minimized dead zones.
Material Limitation (Si): The Si detectors required operation at cryogenic temperatures (-15 °C to -50 °C) to mitigate leakage current induced by high radiation damage.
6CCVD Value Proposition: MPCVD Diamond (SCD/PCD) offers orders of magnitude superior radiation hardness and thermal stability compared to silicon, eliminating the need for extreme cryogenic cooling and ensuring signal integrity in the most aggressive fluence environments, such as those projected for future high-luminosity LHC upgrades.
Application Relevance: The demonstrated techniques and radiation requirements are directly applicable to future diamond-based tracking detectors, high-resolution dosimetry, and beam monitoring systems in High Energy Physics (HEP).

Technical Specifications

Parameter	Value	Unit	Context
Detector Placement Distance	2-3	mm	Closest approach to the beamline (AFP requirement)
Si Sensitive Layer Thickness	230	µm	Thickness of the p-type Si substrate used
Slim Edge Width Requirement	100-200	µm	Target width for minimized insensitive area
Achieved Edge Extension (Range)	87-215	µm	Resulting edge width after diamond-saw cut
Minimum Insensitive Edge Width	< 15	µm	Achieved by FBK design (without 3D guard ring)
Maximum Equivalent Fluence Tested	4 × 10¹⁵	n_eq/cm²	High-luminosity irradiation scenario
Test Beam Energies	4-5	GeV	Electrons (DESY)
Test Beam Energies (Irradiation)	23	GeV/MeV	Protons (CERN-PS/KIT)
Typical Bias Voltage (Si, pre-rad)	20-30	V	Low voltage operation for Si IBL spares
Bias Voltage (Si, post-rad max)	130	V	Applied for high efficiency in irradiated samples
Required Operating Temperature	-15 to -50	°C	Required to reduce post-irradiation leakage current
Measured Hit Efficiency (Irradiated Center)	93-97	%	Performance in the high-fluence center region

Key Methodologies

The study relied on specialized fabrication and testing protocols to validate the 3D detectors for the AFP application:

3D Sensor Fabrication: Devices were based on FE-I4 3D pixel detectors fabricated by CNM and FBK, utilizing a double-sided process with columnar n⁺ junction and p⁺ ohmic electrodes penetrating a 230 µm thick p-type Si substrate.
Slim Edge Processing: AFP-compatible slim edges were created by removing a wide inactive region (1.5 mm bias tab area) using a simple diamond-saw cut. This demonstrated a fast and effective method for custom geometric modification.
Non-uniform Irradiation: Two scenarios were tested to simulate real-world fluence profiles:
- Focussed 23 GeV proton beam (CERN-PS), achieving 4 × 10¹⁵ n_eq/cm² maximum fluence across a large, non-uniform region.
- Highly localized 23 MeV proton beam (KIT) using 5 mm thick Aluminum masks with circular or slit-like holes (3-4 mm width) to achieve abrupt transitions between irradiated and unirradiated areas (fluence up to 3.6 × 10¹⁵ n_eq/cm²).
Performance Characterization: Electrical tests included Current-Voltage (I-V) measurements to track leakage current. Hit efficiency and noise were assessed using test beams (electrons and 120 GeV pions) under cold conditions (down to -50 °C) to manage radiation-induced leakage current.

6CCVD Solutions & Capabilities

6CCVD provides the high-performance MPCVD diamond materials necessary to replicate, extend, and ultimately surpass the radiation and thermal constraints observed in this silicon-based research. Diamond, with its superior bandgap and displacement damage threshold, is the optimal material for detectors operating in next-generation, high-luminosity HEP experiments.

Applicable Materials

Material Specification	Requirement from Paper	6CCVD Advantage & Application
High Purity Single Crystal Diamond (SCD)	Ultimate radiation hardness (4×10¹⁵ n_eq/cm²) and low noise.	Ideal for high-resolution tracking detectors and precision dosimetry. SCD offers the highest charge mobility and lowest intrinsic noise, ensuring excellent signal quality even after extreme fluence exposure.
Polycrystalline Diamond (PCD)	Large active area (>2 cm² for Si used) required for tracking coverage.	Available in large formats (up to 125mm diameter wafers). Perfect for tiling large-area detector planes or high-stability radiation monitoring where large coverage is essential.
Boron-Doped Diamond (BDD)	Need for conductive materials for electrodes, guard rings, or termination structures.	6CCVD provides BDD layers for conductive surfaces, ohmic contacts, or fabricating the diamond equivalent of the 3D guard rings used in the CNM design.

Customization Potential

The success of the 3D Si detectors hinged on precise mechanical processing to achieve minimal dead zones. 6CCVD specializes in the post-processing and modification of CVD diamond wafers, offering superior accuracy and flexibility:

Precision Geometric Processing: The paper used a diamond-saw cut to achieve slim edges. 6CCVD offers advanced laser cutting and high-precision grinding/polishing services to replicate the required slim-edge geometries (e.g., 87-215 µm edges) with greater accuracy and cleaner edge termination, ensuring minimal edge leakage and dead area.
Custom Dimensions: 6CCVD can supply SCD plates and PCD wafers in custom dimensions needed for AFP-like module assembly, including wafers up to 125mm (PCD) and specialized substrates up to 10mm thick.
High-Quality Polishing: For applications requiring precise charge collection uniformity (essential for the non-uniform irradiation challenge), 6CCVD guarantees surface roughness down to R_a < 1nm for SCD and R_a < 5nm for inch-size PCD.
Integrated Metalization: 6CCVD provides in-house deposition of custom metal contact stacks (e.g., Ti/Pt/Au, W/Cu) necessary for high-energy physics sensor integration and bump-bonding to readout electronics (like the FE-I4 chips mentioned).

Engineering Support

The challenges identified in the research—specifically managing high leakage current under non-uniform irradiation and optimizing edge termination—are core problems that diamond materials solve intrinsically.

Radiation Damage Mitigation: 6CCVD’s in-house PhD team can assist engineers in designing diamond detectors that remain operational and stable at room temperature, eliminating the need for the complex -50 °C dry-ice cooling necessitated by radiation damage in the silicon devices.
High-Fluence Applications: We offer consultation on material selection (SCD vs. PCD) and 3D device design to ensure optimal charge collection efficiency and noise performance in environments exceeding the 10¹⁶ n_eq/cm² fluence expected in future HEP upgrades.
Global Logistics: All custom materials and devices are available for global shipping, DDU default with DDP available, ensuring prompt delivery for international research collaborations.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

The ATLAS Forward Physics (AFP) project plans to install 3D silicon pixel\ndetectors about 210 m away from the interaction point and very close to the\nbeamline (2-3 mm). This implies the need of slim edges of about 100-200 $\mu$m\nwidth for the sensor side facing the beam to minimise the dead area. Another\nchallenge is an expected non-uniform irradiation of the pixel sensors. It is\nstudied if these requirements can be met using slightly-modified FE-I4 3D pixel\nsensors from the ATLAS Insertable B-Layer production. AFP-compatible slim edges\nare obtained with a simple diamond-saw cut. Electrical characterisations and\nbeam tests are carried out and no detrimental impact on the leakage current and\nhit efficiency is observed. For devices without a 3D guard ring a remaining\ninsensitive edge of less than 15 $\mu$m width is found. Moreover, 3D detectors\nare non-uniformly irradiated up to fluences of several 10$^{15}$\nn$_{eq}$/cm$^2$ with either a focussed 23 GeV proton beam or a 23 MeV proton\nbeam through holes in Al masks. The efficiency in the irradiated region is\nfound to be similar to the one in the non-irradiated region and exceeds 97% in\ncase of favourable chip-parameter settings. Only in a narrow transition area at\nthe edge of the hole in the Al mask, a significantly lower efficiency is seen.\nA follow-up study of this effect using arrays of small pad diodes for\nposition-resolved dosimetry via the leakage current is carried out.\n

Tech Support

Original Source

DOI: https://doi.org/10.1088/1748-0221/10/03/c03031