Diamond Detectors for Timing Measurements in High Energy Physics

At a Glance

Metadata	Details
Publication Date	2020-07-14
Journal	Frontiers in Physics
Authors	E. Bossini, N. Minafra
Institutions	European Organization for Nuclear Research, University of Kansas
Citations	29
Analysis	Full AI Review Included

Diamond Detectors for Ultra-Fast Timing in High Energy Physics

Technical Analysis and Material Solutions from 6CCVD

This document analyzes the requirements and achievements detailed in the research paper “Diamond Detectors for Timing Measurements in High Energy Physics” (Bossini & Minafra, 2020) and correlates them with the advanced MPCVD diamond solutions offered by 6CCVD.

Executive Summary

This paper confirms that Single Crystal Diamond (SCD) is the superior material for ultra-fast timing detectors in extreme radiation environments, such as those found in High Energy Physics (HEP) experiments (CERN, GSI).

Core Achievement: SCD detectors achieved time precision as low as $\sim$ 50 ps for Minimum Ionizing Particles (MIPs), significantly surpassing traditional silicon detectors.
Material Preference: Ultra-pure Single Crystal CVD (scCVD) diamond is preferred over Polycrystalline CVD (pcCVD) due to higher carrier mobility ($\mu_{e}$ up to 4,551 cm²/Vs) and near-unity Charge Collection Efficiency (CCE).
Radiation Hardness: Diamond exhibits superior radiation tolerance, maintaining performance up to fluences of $\sim$ 10¹⁶ protons/cm².
Design Optimization: Achieving sub-100 ps timing relies on minimizing parasitic capacitance by placing high input impedance front-end amplifiers (k$\Omega$ range) within millimeters of the sensor.
Advanced Architecture: The Double Diamond (DD) architecture, utilizing two SCD crystals glued back-to-back, demonstrated enhanced performance, achieving 50 ps precision without increasing the channel count.
6CCVD Value Proposition: 6CCVD specializes in the high-purity SCD material required for these applications, offering custom dimensions, precise thickness control (300 µm to 500 µm), and advanced metalization schemes (Cr/Au, Ti/W) critical for ohmic contact formation.

Technical Specifications

The following hard data points highlight the performance characteristics of scCVD diamond for timing applications:

Parameter	Value	Unit	Context
Time Precision (DD)	$\sim$ 50	ps	TOTEM/CMS Double Diamond (DD) architecture
Time Precision (SD)	$\le$ 100	ps	HADES START Detector (Single Diamond)
Radiation Tolerance	$\sim$ 10¹⁶	protons/cm²	scCVD/pcCVD (800 MeV & 24 GeV protons)
Electron Mobility ($\mu_{e}$)	4,551	cm²/Vs	Ultra-pure scCVD (Room Temperature)
Hole Mobility ($\mu_{h}$)	2,750	cm²/Vs	Ultra-pure scCVD (Room Temperature)
Electron Saturation Velocity (v_e)	2.6 $\times$ 10⁷	cm/s	scCVD
Band Gap (E_g)	5.47	eV	Diamond (Indirect)
Relative Permittivity ($\epsilon_{r}$)	5.7	N/A	Diamond
Typical SCD Thickness	300 to 500	µm	Used in HADES and TOTEM/CMS detectors
Electrode Resistivity (3D pcCVD)	10^-12	$\Omega$m	Graphitization technique

Key Methodologies

The successful deployment of diamond timing detectors relies on precise material synthesis and optimized front-end design:

MPCVD Synthesis:
- Diamond crystals are grown using Chemical Vapor Deposition (CVD), specifically plasma-assisted processes (Microwave Plasma CVD, MPCVD), which yield the highest quality material.
- Growth occurs at low temperature (< 1,000 °C) and low pressure ($\sim$ 0.1 bar) using methane (CH₄) and molecular hydrogen (H₂) mixtures.
- Ultra-pure SCD substrates are used to promote single-crystal growth, maximizing carrier mobility and CCE.
Metalization and Segmentation:
- Ohmic contacts are essential for uniform electric fields and stable operation under high bias (up to 500 V).
- Multi-layer metalization schemes (e.g., Cr/Au or Ti/W alloy) are deposited and subsequently annealed to form carbides at the interface, ensuring ohmic contact.
- Segmentation into pads or strips (e.g., $4.3 \times 4.3$ mm² quadrants, or $4.5 \times 4.5$ mm² pads separated by 100 µm clearance) is achieved using masking or lithographic processes.
Front-End Electronics Optimization:
- The detector capacitance (C) must be minimized (typically 0.29 pF to 2 pF for small pads).
- High input impedance amplifiers (k$\Omega$ range) are placed within a few millimeters of the sensor to minimize parasitic capacitance (C_w, L_w) introduced by bonding wires.
- The RC time constant is optimized to be fast ($\sim$ 1 ns) to maximize the signal rise time while maintaining sufficient Signal-to-Noise Ratio (SNR $\ge$ 6).
Signal Digitization and Time Walk Correction:
- Signals are digitized using fast electronics like the SAMPIC chip (up to 10 GSa/s) or, more commonly, a combination of a fast discriminator (NINO) and a High Performance Time to Digital Converter (HPTDC).
- Time Over Threshold (TOT) or Constant Fraction Discriminator (CFD) algorithms are applied offline to correct for “time walk,” the dependence of the measured time on the statistical fluctuations of the deposited energy (Landau distribution).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-specification MPCVD diamond materials necessary to replicate and advance the timing detector research described in this paper.

Applicable Materials

To achieve the high mobility, low defect concentration, and superior radiation hardness required for sub-100 ps timing, the following 6CCVD material is recommended:

Optical Grade Single Crystal Diamond (SCD): This material offers the highest purity and lowest defect density, ensuring maximum Charge Collection Efficiency (CCE $\approx$ 1) and optimal carrier drift velocity, which are critical for fast signal rise times and high timing precision.
Polycrystalline Diamond (PCD): While not ideal for MIP timing due to lower CCE ($\sim$ 50%), 6CCVD’s large-area PCD (up to 125mm wafers) is suitable for high-energy heavy ion detection or applications where large sensitive area and lower cost are prioritized (e.g., the GSI detector achieving < 50 ps precision with heavy ions).

Customization Potential

The success of HEP timing detectors hinges on precise dimensions, specific metalization, and optimized surface quality—all core capabilities of 6CCVD:

Research Requirement	6CCVD Capability & Solution	Technical Advantage
Custom Dimensions	Plates/wafers up to 125mm (PCD). Custom laser cutting for SCD pads (e.g., $4.5 \times 4.5$ mm²).	Enables rapid prototyping and production scaling for segmented arrays (e.g., TOTEM/CMS).
Thickness Control	SCD thickness range: 0.1 µm to 500 µm.	Provides the exact 300 µm or 500 µm thickness used in HADES and TOTEM/CMS detectors, optimizing capacitance and signal amplitude.
Metalization Schemes	Internal capability for Au, Pt, Pd, Ti, W, Cu.	We provide the specific ohmic contact stacks required (e.g., Ti/W alloy or Cr/Au) and perform controlled annealing to ensure stable, low-resistance contacts.
Double Diamond (DD) Architecture	Supply of matched, highly uniform SCD pairs with identical metalization geometry.	Facilitates the 50 ps precision achieved by the DD design by ensuring minimal performance mismatch between layers.
Surface Quality	SCD polishing to Ra < 1 nm. Inch-size PCD polishing to Ra < 5 nm.	Ultra-smooth surfaces minimize surface leakage current and prevent discharges, crucial for operating detectors under high bias (up to 500 V/500 µm).

Engineering Support

6CCVD’s in-house PhD team offers comprehensive engineering consultation, specializing in the material science of diamond detectors. We can assist researchers in:

Material Selection: Determining the optimal SCD grade and thickness for specific particle types (MIPs vs. heavy ions) and required timing precision.
Metalization Recipe Development: Customizing metal stacks (e.g., Cr/Au vs. Ti/W) and annealing protocols to guarantee robust, ohmic contacts for high-bias operation.
Radiation Hardness Planning: Providing materials certified for extreme radiation environments, suitable for future High Luminosity LHC (HL-LHC) upgrades.

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

View Original Abstract

Timing detectors are a well established part of High Energy Physics experimental instrumentation. The choice of sensors with fast (less than 10 ns) and precise (better than 100 ps) signals is an essential part of the design of a timing detector, together with radiation resistance considerations. Single crystal diamond sensors are one of the most promising technologies in this field. In this paper, the main characteristics that make single diamond crystal sensors ideal for timing applications will be described and an introduction to the design of fast front-end electronics will be given. Finally, two examples of diamond timing detectors used in High Energy Physics, the START detector of HADES and the TOTEM/CMS timing detector, will be discussed.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphy.2020.00248

References

2019 - The proton timing system of the TOTEM experiment at LHC [Crossref]
2011 - A review of diamond synthesis by CVD processes [Crossref]
2010 - Diamond growth by chemical vapour deposition [Crossref]
1990 - Nucleation of diamond crystals [Crossref]
2009 - Diamond-metal contacts: interface barriers and real-time characterization [Crossref]
2001 - The interface diffusion and reaction between Cr layer and diamond particle during metallization [Crossref]
2011 - Laser graphitization for polarization of diamond sensors [Crossref]
2017 - Diamond graphitization by laser-writing for all-carbondetector applications [Crossref]