Diamond-Like Carbon for the Fast Timing MPGD

At a Glance

Metadata	Details
Publication Date	2020-04-01
Journal	Journal of Physics Conference Series
Authors	A. Colaleo, G. De Robertis, F. Licciulli, M. Maggi, A. Ranieri
Institutions	University of Pavia, Istituto Nazionale di Fisica Nucleare, Sezione di Lecce
Citations	7
Analysis	Full AI Review Included

Executive Summary

The research focuses on developing high-quality Diamond-Like Carbon (DLC) films for use as resistive electrodes in Fast Timing Micro-Pattern Gaseous Detectors (FTM MPGDs), aiming for sub-nanosecond time resolution.

Core Challenge: Standard MPGDs are limited to 5-10 ns time resolution. The FTM concept requires splitting the drift gap into multiple layers using resistive DLC electrodes that must withstand subsequent wet etching processes.
Material Requirement: DLC films must exhibit high adhesion to polyimide substrates and achieve a target surface resistivity of 100 MΩ/square.
Failure Analysis: Initial attempts using magnetron sputtering resulted in poor DLC adhesion, leading to delamination and large hole diameters during the critical wet etching step.
Alternative Deposition Methods: Two advanced Physical Vapor Deposition (PVD) techniques were investigated: Dual Ion Beam Sputtering (IBD) and Pulsed Laser Deposition (PLD).
Key Achievement (Resistivity): Both IBD and PLD successfully produced DLC films achieving the target 100 MΩ/square surface resistivity on polyimide.
Key Achievement (Stability): PLD-deposited films demonstrated exceptional stability in surface resistivity over time, even after annealing at 150 °C.
Next Steps: The high-quality films produced via IBD and PLD will now be tested using the wet-etch procedure to perforate the foils with a GEM-like mask, validating the improved adhesion.

Technical Specifications

Parameter	Value	Unit	Context
Target Surface Resistivity	100	MΩ/square	Required for FTM operation
Standard MPGD Time Resolution	5-10	ns	Limited by primary ionization fluctuations
FTM Time Resolution (Target)	~1	ns	Achieved with N=8 layers of fixed 250 µm width
FTM Total Gas Volume (Simulated)	4	mm	Split into 1 to 32 layers
Gas Mixture (Simulation)	Ar:CO₂ (70:30)	Ratio	Used for time resolution modeling
Electron Drift Velocity (v_d)	70	µm/ns	At E = 3 kV/cm
DLC Electrode Thickness (Target)	~100	nm	Acting as resistive electrode
FTM Base Substrate	50	µm	Polyimide foil thickness
FTM Hole Pitch (Hexagonal)	140	µm	GEM-like pattern
FTM Gain (Simulated)	10⁴	N/A	At 500 V Anode Potential (Ar:CO₂)
IBD Sample 1819 Resistivity	~400	MΩ/square	High resistivity, stable over time
IBD Sample 1821 Resistivity	~30	MΩ/square	Nitrogen doped, low resistivity
PLD Laser Fluence (100 MΩ/square)	5	J/cm²	Fixed substrate deposition condition
PLD Annealing Temperature	150	°C	Used to stabilize resistivity (shifted 100 MΩ/square to ~300 MΩ/square)
PLD Laser Pulse Duration	20	ns	Used for graphite ablation

Key Methodologies

The study employed two distinct Physical Vapor Deposition (PVD) techniques to deposit DLC films onto polyimide substrates, focusing on adhesion and resistivity control.

Dual Ion Beam Deposition (IBD):
- Setup: A home-made dual ion-beam system utilizing Kaufman ion sources (Main and Assistance).
- Sputtering Source (Main): Bombarded a 10 cm pyrolitic graphite target at a 45° angle to deposit carbon ions. Main ion energy was typically 1200 eV.
- Assistance Source: Focused directly on the polyimide substrate to compactify the deposited carbon film, enhancing uniformity and quality.
- Gas Control: Argon (Ar) was used for sputtering and assistance. Nitrogen (N₂) was introduced in the assistance beam for specific samples (e.g., 1821) to achieve lower surface resistivity (~30 MΩ/square).
- Pre-treatment: Organic compounds were removed from the substrate surface using the assistance Ar ion source before DLC deposition commenced.
Pulsed Laser Deposition (PLD):
- Setup: Used a multi-gas excimer laser (248-193 nm) operating at 10 Hz with 20 ns pulses in a vacuum chamber.
- Target: Rotating pyrolitic graphite target, ablated by the laser to create a plasma plume containing carbon species (C, C⁺, C₂, etc.).
- Substrate Control: Substrates (polyimide or Si/SiO₂) were placed in front of the target. Substrate rotation was implemented to mitigate non-uniformity caused by the V-shaped plasma plume.
- Resistivity Tuning: Laser fluence (J/cm²) was identified as the critical parameter for controlling the DLC sp³/sp² ratio and thus the sheet resistance.
- Characterization: Sheet resistance was measured using a four-point probe station applying the Van Der Pauw method, following annealing at 150 °C for stabilization.

Commercial Applications

The development of high-quality, resistive DLC films on flexible polyimide substrates is critical for several high-tech sectors:

High Energy Physics (HEP):
- Next-generation particle detectors (MPGDs) requiring time resolution < 1 ns for precise bunch crossing identification (e.g., FCC-hh).
- Radiation-hard detectors for high-luminosity upgrades (HL-LHC, CMS, ATLAS).
Advanced Sensor Technology:
- Development of compact, spark-protected single amplification-stage MPGDs (like µ-RWELL) using resistive DLC electrodes.
- High-rate gaseous detectors for industrial or medical imaging applications.
Flexible Electronics and Coatings:
- DLC films are amorphous carbon with significant sp³ hybridization, offering excellent mechanical and resistive properties.
- Applications requiring highly uniform, resistive coatings on flexible polymer substrates.
Material Science and PVD:
- Demonstration of highly controlled deposition of functional films using Ion Beam Sputtering (IBD) and Pulsed Laser Deposition (PLD), techniques valuable for producing specialized coatings (e.g., optical, protective, or semiconductor films) where precise energy control is necessary.

View Original Abstract

Abstract The present generation of Micro-Pattern Gaseous Detectors (MPGDs) are radiation hard detectors, capable of detecting effciently particle rates of several MHz/cm 2 , while exhibiting good spatial resolution (≤ 50 µm) and modest time resolution of 5-10 ns, which satisfies the current generation of experiments (High Luminosity LHC upgrades of CMS and ATLAS) but it is not sufficient for bunch crossing identification of fast timing systems at FCC-hh. Thanks to the application of thin resistive films such as Diamond-Like Carbon (DLC) a new detector concept was conceived: Fast Timing MPGD (FTM). In the FTM the drift volume of the detector has been divided in several layers each with their own amplification structure. The use of resistive electrodes makes the entire structure transparent for electrical signals. After some first initial encouraging results, progress has been slowed down due to problems with the wet-etching of DLC-coated polyimide foils. To solve these problems a more in-depth knowledge of the internal stress of the DLC together with the DLC-polyimide adhesion is required. We will report on the production of DLC films produced in Italy with Ion Beam Sputtering and Pulsed Laser Deposition, where we are searching to improve the adhesion of the thin DLC films, combined with a very high uniformity of the resistivity values.

Tech Support

Original Source

DOI: https://doi.org/10.1088/1742-6596/1498/1/012015

References

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