Resistive diamond-like carbon coatings for micro-pattern gaseous detectors: a review from USTC

Abstract

The integration of resistive materials has fundamentally advanced Micro-Pattern Gaseous Detector (MPGD) technology, enabling robust, high-gain operation essential for modern particle physics experiments. Diamond-Like Carbon (DLC) has emerged as a superior material for this purpose due to its uniquely tunable electrical, mechanical, and chemical properties. This article provides a comprehensive review of the development, fabrication, and application of DLC-based resistive coatings for MPGDs, with a focus on work conducted by the University of Science and Technology of China MPGD group. We detail the material fundamentals of DLC, systematically introduce magnetron sputtering as the core deposition methodology, and analyze the critical relationship between coating properties—such as resistivity, uniformity, adhesion, and stability—and final detector performance. A thorough examination of specific detector architectures (μ-RWELL, μRGroove, THGEMs, RWELL, Picosecond Micromegas) demonstrates the transformative impact of DLC. Finally, we summarized the outstanding technical challenges and present a perspective on future research directions aimed at scaling this technology for next-generation experiments.

1 Introduction

Micro-Pattern Gaseous Detectors (MPGDs) are gas avalanche devices with feature sizes (anodes, cathodes, holes) on the order of micrometers to hundreds of micrometers enabled by the advent of modern photolithographic techniques (Sauli, 2020). MPGDs offer several key advantages: high-rate capability due to the small amplification cell size enabling fast evacuation of positive ions, excellent spatial resolution (down to <100 µm) enabling very precise particle tracking, good time resolution benefitted by fast signal collection structure, as well as radiation hardness against harsh radiation environments and cost-effectiveness for large-area production. And those advantages make them promising sensitive elements in modern high-energy and nuclear physics experiments for detection and tracking of ionizing radiation. Since the invention of the first MPGD, Micro-Strip Gas Chambers (MSGCs) (Oed, 1988), in 1988, a wide variety of MPGD systems have been developed. Two typical examples are GEM (Sauli, 1997; Sauli, 2016) (Gas Electron Multiplier) and Micromegas (Barouch et al., 1999; Attié et al., 2021) (Micro-Mesh Gaseous Structure). GEM features a thin polymer foil coated with metal on both sides and perforated with a high density of microscopic holes, in which a strong electric field acts as an electron multiplication stage. For Micromegas, a very fine metallic mesh is stretched extremely close (50–100 µm) above a pixelated anode plane. A very high electric field exists in the gap between the mesh and the anode, providing the conditions for rapid avalanche multiplication.

However, a fundamental challenge for early MPGDs was their inherent susceptibility to discharge-induced damage (Sauli, 2020; Bressan et al., 1999; Bachmann et al., 2002; Gasik et al., 2017). High-gain operation in the presence of highly ionizing particles could lead to sparks and catastrophic breakdown, destroying the delicate metallic micro-patterns. The mitigation of discharges in MPGDs relies on two main approaches: cascaded multi-stage detector structures and the integration of resistive materials, see the recent review (Gasik, 2024) and references therein. In a cascaded design, multiple gain stages operate at reduced individual gains, lowering the probability of discharge while maintaining a high total gain. The implementation of resistive layers fundamentally improves detector robustness by providing intrinsic discharge quenching capabilities.

The early MPGDs used only conductive electrodes (like copper). When a heavily ionizing particle passes through the gas and triggers a streamer, then it creates a short-circuit between the anode and cathode. In a detector with pure metallic electrodes, this short-circuit has very low resistance. A massive amount of energy is dumped from the power supply into a microscopic point, vaporizing the metal electrodes and creating a permanent, dead spot on the detector. By replacing the purely conductive electrodes with materials of precisely controlled resistivity, these “resistive” MPGDs gained inherent discharge protection (Gasik, 2024; Peskov et al., 2012). The resistive layer limits the current flow during a discharge, enabling rapid quenching and thereby preventing damage to the detector. This innovation not only enhanced detector robustness but also enabled new functionalities, such as superior spark-tolerant operation.

Numerous resistive materials have been studied by researchers from different research institutions in an effort to find the optimal solution for high-performance and robust MPGDs. In the 1990s, high-resistivity diamond-like carbon (DLC) layer was used in MSGC to achieve stable running at high gain and high count-rate (Barr et al., 1998). And this marked the first use of DLC in MPGD. In the 2000s, resistive graphite paint and resistive Kapton were applied in GEM like detectors to prevent discharge and surface streamers (Di Mauro et al., 2006; Oliveira et al., 2007). In recent 10 years, resistive coatings and bulk resistive plate were fully utilized either to overcome the shortcomings of the existing MPGDs or to develop novel detector designs. Examples include (but not limited to) resistive THGEM detectors (Song et al., 2020a; Bressler et al., 2023), Micromegas with resistive readout anode (Chefdeville et al., 2021; Feng et al., 2022), RPC (resistive plate chamber) and MRPC (multi-gap resistive plate chamber) with resistive coatings for higher rate (Wang et al., 2021; Bencivenni et al., 2022), μRWELL (micro-resistive well detector) (Bencivenn et al., 2015), RPWELL (resistive plate well detector, single faced THGEM coupled to bulk resistive plate) (Rubin et al., 2013), RWELL (Resistive WELL detector, single faced THGEM coupled to anode with resistive coatings) (Arazi et al., 2014); μPIC (micro pixel chamber) with resistive electrodes for spark reduction (Ochi et al., 2014); and the new type of detector derived from resistive electrodes: μRGroove (micro-resistive groove detector) (He et al., 2024), PCa (proportional counter array) (Tian et al., 2023) and μRTube (micro resistive tube) (Farinelli, 2025).

Among many resistive materials, diamond-like carbon (DLC) (Robertson, 2002), is of the most promising one which has been applied to many detectors described above. DLC is an amorphous carbon layer with tunable high resistivity, good chemical inertness and good mechanical strength, making it very suitable for MPGDs. The USTC (University of Science and Technology of China) MPGD group is committed to the R&D of MPGDs with resistive DLC coatings since 2017, and accumulated a great deal of experience on the production of DLC coatings. This review details the USTC group’s comprehensive work on DLC-based MPGDs. We cover the material science of DLC, its deposition via magnetron sputtering, the optimization and characterization of coatings, and its integration into novel and enhanced detector architectures.

2 Fundamentals of diamond-like carbon

2.1 Material structure and bonding

In 1971, Aisenberg and Chabot first prepared amorphous carbon films with diamond-like characteristics using carbon ion beam deposition technology (Aisenberg et al., 1971). These films are non-crystalline metastable materials composed of three bonding modes of sp¹, sp², and sp³ carbon atoms, known as diamond-like carbon due to their numerous diamond-like properties. DLC can be classified on ternary phase diagram of bonding in amorphous carbon-hydrogen alloys, see Figure 1. DLC is primarily composed of sp³ (diamond-structure) and sp² (graphite-structure) hybridized states of carbon, see Figure 2. Depending on the bonding modes of atoms in the film (including C-H, C-C, sp², sp³, etc.) and the proportions of various bonding modes, DLC films are referred to by different names: amorphous carbon (a-C) films with high sp² content, hydrogenated amorphous carbon (a-C:H) films containing significant amounts of hydrogen, and tetrahedral amorphous carbon (ta-C) films with sp³ components exceeding 80%, also known as amorphous diamond films. The specific properties of a DLC film are largely determined by the ratio of diamond-like (sp³) to graphite-like (sp²) carbon bonds and its hydrogen content (Robertson, 2002; Ohtake et al., 2021).

FIGURE 1

FIGURE 2

2.2 Key properties relevant to MPGDs

2.2.1 Tunable resistivity

The surface resistivity of DLC films is a critical parameter for MPGD electrodes. The electrical behavior of DLC films is primarily governed by the sp³/sp² ratio, hydrogen content, and the incorporation of dopant atoms (Choumad Ould et al., 2024). DLC films can exhibit a wide range of surface resistivity, typically from 10⁴ to 10¹¹ Ω/□. The bulk resistivity can be adjusted by choosing the appropriate deposition technique and process parameters. Following this, surface resistivity is controlled by adjusting the film thickness. The resistivity of DLC films prepared by different methods varies significantly, with hydrogenated DLC films generally exhibiting higher resistivity than non-hydrogenated films (Lv et al., 2020). Doping nitrogen into DLC films can decrease resistivity (Son et al., 2017), while doping boron increases it (Park et al., 2013). When metals are doped into the film, resistivity decreases (Khan et al., 2019). Doping with different impurities can yield DLC films with varying resistivities.

2.2.2 Mechanical and chemical robustness

Beyond its electrical tunability, DLC offers outstanding mechanical and chemical robustness, ensuring durability in harsh environments. Its mechanical properties include high hardness [10–50 GPa for hydrogenated DLC films (Robertson, 2002)], low friction coefficient [0.01–0.2 (Robertson, 2002)] and superior wear resistance, which are essential for withstanding handling, photolithography, and abrasion during MPGD manufacturing (Shang, 2019). Chemically, DLC is inert and highly resistant to oxidation and corrosion from reactive gases or processing chemicals. Additionally, hydrogenated DLC surfaces can display negative electron affinity and a low effective work function, enabling stable, uniform electron emission at low threshold fields. This makes specific DLC formulations promising alternatives for robust photocathodes in fast-timing detectors, surpassing more fragile materials like cesium iodide.

2.2.3 Synthesis of properties for MPGD applications

The synthesis of DLC’s properties directly addresses several core requirements of modern MPGDs (Serra et al., 2024). Firstly, a DLC layer with appropriately high resistivity integrated into the detector anode enables the safe dissipation of charge from occasional discharges, providing spark quenching and discharge protection. Significantly, this tunable resistivity can be engineered to remain functional under cryogenic conditions, as demonstrated by the stable operation of an RWELL detector with a DLC electrode at liquid argon temperature (∼87 K) (Leardini et al., 2023; Tesi et al., 2023), opening avenues for use in noble liquid time-projection chambers. Secondly, the intrinsic radiation hardness of carbon-based materials grants DLC films resistance to ionizing radiation damage (Serra et al., 2024), ensuring long-term stability, while their mechanical hardness and chemical inertness allow them to survive the etching, cleaning, and patterning steps of MPGD microfabrication. Furthermore, DLC’s low work function and negative electron affinity properties enable its use as a robust photocathode in hybrid MPGDs, such as in picosecond Micromegas detectors (Bortfeldt et al., 2018) for fast Cherenkov light detection, where traditional photocathodes fail due to humidity sensitivity or ion bombardment. In summary, Diamond-Like Carbon is not a single material but a versatile platform; its sp³/sp²/hydrogen matrix can be engineered to deliver a specific combination of tunable resistivity, exceptional mechanical durability, and superior chemical stability. This unique property suite makes DLC indispensable for advancing MPGD technology by solving critical challenges related to robustness, rate capability, and long-term performance in high-radiation environments.

3 Deposition and optimization of DLC coatings at USTC

3.1 Deposition methods

The preparation of DLC films has advanced significantly in recent years, yielding a diverse array of deposition techniques (Robertson, 2002). These methods are fundamentally classified into two categories: Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD). The following outlines several prevalent techniques, each imparting distinct structural and compositional characteristics to the resulting films.

Plasma-Enhanced Chemical Vapor Deposition (PECVD) is a widely adopted CVD method where hydrocarbon precursor gases (e.g., methane, toluene) are decomposed within a glow discharge plasma, facilitating film growth on the substrate. This process typically yields hydrogenated amorphous carbon (a-C:H) films. In contrast, Ion Beam Deposition (Robertson, 2002) represents a PVD approach wherein a beam of ionized carbon is generated and directed onto a substrate, a technique pioneered by Aisenberg and Chabot for room-temperature DLC synthesis. A particularly advanced PVD variant is Filtered Cathodic Vacuum Arc (FCVA) Deposition. This method initiates a vacuum arc discharge at a graphite cathode to produce a highly ionized carbon plasma. A magnetic filter effectively removes macroscopic particles and neutral atoms, permitting only energetic carbon ions to reach the substrate. FCVA enables high-rate deposition of hydrogen-free tetrahedral amorphous carbon (ta-C) films with a very high fraction (up to 90%) of sp³ hybridized bonds, conferring properties that approach those of diamond.

For industrial-scale production, sputtering techniques are commonplace. In this PVD process, argon ions bombard a graphite target, ejecting carbon atoms which then condense on a substrate. While direct-current or radio-frequency sputtering can be used, magnetron sputtering is frequently employed to enhance the deposition rate. A noted limitation of standard sputtering is the generally moderate sp³ bond content achievable in the films.

The composition, structure, and functional properties of DLC films are profoundly influenced by the chosen deposition methodology. Consequently, the selection of an appropriate technique is paramount to obtaining films with tailored characteristics. From the perspective of applications in MPGDs, an ideal deposition method should offer: ease and precision in tuning electrical resistivity, excellent adhesion to substrate, uniform resistivity over the coated area, low substrate temperature compatibility, scalability for large-area coverage, high deposition rates, and the ability to produce smooth surfaces. Given this set of requirements, the Magnetron Sputtering method is currently considered a superior and practical choice for such applications, balancing controllability, industrial viability, and film performance.

Magnetron sputtering (Kelly and Arnell, 2000) is a PVD process, Figure 3 shows a diagram of this process. The process begins in a vacuum chamber filled with a low-pressure inert gas, typically argon (Ar). A target (the material to be deposited, like graphite for DLC) is mounted on a cathode, and a substrate (the object to be coated) faces it. An electric field is generated by negative high-voltage applied to the cathode. The electric field ionizes the argon gas into a plasma of Ar⁺ ions and electrons. A magnetic field (created by permanent magnets behind the target) perpendicular to electric field traps electrons near the target surface. This dramatically increases the probability of electron-atom collisions, sustaining a dense plasma, consequently increases the sputtering rate. The electric field accelerates positively charged ions from the plasma toward a negatively biased target. The positive ions strike the target with sufficient force to dislodge and eject atoms from the target. These atoms ejected from the face of the target will condense on substrate surfaces that are placed in proximity to the target.

FIGURE 3

3.2 Deposition devices and process

At USTC we have tried many different devices, mainly two magnetron sputtering devices are used to produce the resistive DLC coatings, TEER UDP-650 (manufactured by TEER Coatings Ltd.) and HAUZER Flexicoat® 850 (manufactured by HAUZER Techno Coating). The TEER and HAUZER are versatile magnetron sputtering coating machines designed for depositing high-performance films for industrial applications.

The TEER is a PVD system based on non-balanced magnetron sputtering technology commonly employed for depositing carbon-based films and metal films. The HAUZER is a highly flexible coating platform that integrates multiple deposition technologies including sputtering, arc evaporation, and PECVD—within a single machine. This flexibility allows it to deposit a wide range of coatings. This flexibility enables it to deposit a wide range of coatings. A key feature of the HAUZER system is its automatic shutter, which allows for sequential deposition of different coatings, such as DLC and Cr/Cu, in a single batch process. These two devices are suitable for R&D and production of resistive DLC coatings, especially for large-area and uniform DLC coating production thanks to their large chamber work size, TEER Φ650mm × 700 mm and HAUZER Φ800mm × 900 mm. Figure 4 shows the coating chamber of TEER 650 and its structural and schematic diagram.

FIGURE 4

Figure 5 shows a general process of DLC coating. Prior to DLC deposition, substrates must undergo pretreatment to ensure they are flat, smooth, clean, and dry. Inadequate preparation compromises coating adhesion and quality. For instance, FR4 substrates for DLC coating are cleaned ultrasonically in ethanol to remove contaminants like dust and fingerprints, followed by oven-drying at 70 °C for several hours to eliminate residual moisture. Following pretreatment, substrates are mounted on fixtures within the deposition chamber, positioning them at a standard distance of 10–30 cm from the target. The chamber is sealed and evacuated to a specified vacuum degree (typical range: 10^-6 to 10^-8 mbar). Upon reaching this vacuum level, the magnetron sputtering program initiates to deposit the DLC coating. After deposition, the system undergoes a controlled cool-down period under vacuum. Finally, the chamber is vented to atmospheric pressure to retrieve the coated samples.

FIGURE 5

3.3 Performance optimization of DLC coatings

3.3.1 Controlling thickness

Achieving a uniform DLC coating with specific mechanical and electrical properties necessitates precise control over its thickness, which is governed by several interdependent deposition parameters. The primary factors for thickness modulation in magnetron sputtering processes are deposition time, sputtering power, and target-to-substrate geometry (Shang, 2019; Münz and Zufraß, 2020).

Deposition time serves as the most direct control variable. For a stabilized process with a constant deposition rate, film thickness exhibits a linear proportionality to the duration of the sputtering process. The sputtering power (defined by target current) is a fundamental lever influencing this rate. Higher power increases plasma density and the energy of ions bombarding the graphite target, thereby enhancing the sputtering yield and resulting in a greater film thickness per unit time. Concurrently, the target-to-substrate distance is a critical geometrical parameter governing film uniformity. A shorter distance typically yields a higher deposition rate but can lead to significant thickness gradients across the substrate. Conversely, a greater distance promotes superior thickness uniformity at the expense of a reduced overall deposition rate. In practice, a robust process is first optimized by calibrating power, gas pressure, and geometry to establish a stable, uniform deposition rate. For instance, the TEER under conditions of a 2A target current and a 15 cm target-to-substrate distance, a coating time of 120 min yields a DLC film thickness of approximately 800 nm, corresponding to a calibrated deposition rate of about 6.7 nm/min (Song et al., 2020b). Once this rate is established, the final coating thickness can be precisely controlled by simply setting the appropriate deposition time (Lv et al., 2020).

Using magnetron sputtering, DLC coatings can be deposited across a broad thickness range, from just a few nanometers up to several tens of micrometers. The primary factor limiting the thickness of a DLC coating is high internal compressive stress. As the film grows thicker, this accumulated stress can cause the coating to crack, buckle, and delaminate from the substrate. Figure 6 illustrates the effects of internal stress in the DLC coating: bending of soft substrates (APICAL films) and cracking of the coating on a rigid substrate (PCB).

FIGURE 6

Accurate thickness measurement is essential for process calibration and quality control. A common method involves mechanical profilometry. This technique requires the creation of a deliberate step edge between coated and uncoated regions on a smooth, flat substrate (e.g., a glass plate or silicon wafer). A practical method to create this feature is to apply a small dot of removable ink or a similar temporary mask to a clean substrate prior to deposition. The DLC coating forms everywhere except on the masked area, as Figure 7a shows a DLC coated glass plate with two dots masked and cleaned after coating process. After deposition and removal of the mask, a profilometer stylus scans across the resultant boundary. The vertical displacement of the stylus as it traverses the step directly provides a measurement of the coating thickness, see Figure 7b the scanned result. This method offers a straightforward and reliable means for thickness verification.

FIGURE 7

3.3.2 Surface resistivity control

The surface resistivity of DLC coatings can be precisely engineered through the careful modulation of deposition parameters and the strategic incorporation of dopant elements during magnetron sputtering.

3.3.2.1 Modulation via deposition parameters

The intrinsic resistivity (volume resistivity) of a DLC film is governed by its atomic bonding structure, specifically the ratio of insulating sp³ to conductive sp² carbon bonds, which can be directly modulated through key deposition parameters. Elevated substrate temperature and applied bias voltage promote the formation of conductive sp² clusters, thereby reducing resistivity. Furthermore, the deposition pressure (vacuum degree) is a critical parameter. A lower base pressure produces denser, more graphitic DLC films, directly leading to lower electrical resistivity. This is exemplified by coatings deposited using a HAUZER system: DLC films with a thickness of 85 nm produced at 5 × 10⁻⁵ mbar exhibit a surface resistivity of 800 MΩ/□, while those deposited at 3.4 × 10⁻⁶ mbar—without changes to other coating parameters and thickness—show a significantly reduced resistivity of 5 MΩ/□ (Zhou, 2019).

For thin DLC coatings, surface resistivity (ρ_s, in MΩ/□), is a function of volume resistivity (ρ, in MΩ·cm), and coating thickness (t) by ρ_s = ρ/t. Consequently, increasing the coating thickness generally reduces surface resistivity, as shown in Figure 8. Thickness is directly controlled by deposition time at a given coating rate (Lv et al., 2020).

FIGURE 8

3.3.2.2 Post-deposition thermal treatment

Thermal annealing is an effective method for permanently altering resistivity (Guo et al., 2025). Heating induces structural relaxation and graphitization, converting sp³ bonds to sp². For instance, heating an as-deposited film (∼80 MΩ/□) to 200 °C can lower its resistivity to approximately 300 kΩ/□. While post-deposition annealing is effective (e.g., reducing resistivity from 80 MΩ/□ to 23 MΩ/□), pre-deposition substrate heating typically produces films with lower initial resistivity (Zhou, 2019). The annealing temperature must be selected based on the substrate’s thermal stability. Equipment such as the HAUZER system, with a chamber capable of reaching 500 °C, offers a broad operational range for this process.

3.3.2.3 Strategic elemental doping

Strategic elemental doping serves as a precise method for tuning the resistivity of DLC coatings, achieved through the deliberate incorporation of foreign atoms during deposition. The introduction of hydrogen or nitrogen significantly increases resistivity, with hydrogen exhibiting a more pronounced effect (Song et al., 2020a; Lv et al., 2020; Zhou, 2018; Li et al., 2020), see Figure 9. This is typically accomplished by introducing nitrogen-containing process gases—such as nitrogen (N₂) or hydrogen-containing process gases such as acetylene (C₂H₂) or isobutane (iC₄H₁₀) into the sputtering chamber during deposition. Conversely, doping with boron introduces charge carriers, effectively lowering resistivity, which is achieved using a composite boron-carbon target. Similarly, silicon doping increases resistivity and is implemented using a silicon-containing target.

FIGURE 9

3.3.3 Stability of DLC surface resistivity

The surface resistivity of DLC films exhibits a characteristic time evolution when exposed to air, primarily attributed to a surface passivation mechanism. This process involves the gradual neutralization of dangling bonds and defects within the carbon matrix by atmospheric gases, leading to an attenuation of electronic conduction pathways and a consequent increase in measured resistivity. Experimental data on DLC-coated APICAL foils, as shown in Figure 10, reveal distinct stabilization behaviors dependent on dopant (Zhou, 2018; Zhou et al., 2019). Hydrogen-doped films produced by TEER typically undergo a moderate resistivity increase of approximately 30%–40% before stabilizing within a few days. In contrast, nitrogen-doped films demonstrate significantly greater instability, with resistivity increasing more than tenfold over a similar period and failing to reach a stable plateau within the observed timeframe. Possible reason is that nitrogen promotes a more graphitic, sp²-bond-rich structure in DLC films, which is less dense and more chemically reactive. When exposed to moisture, nitrogen in the film can trigger a self-accelerating electrochemical cycle that causes severe localized corrosion and property drift over time (Zhao et al., 2023).

FIGURE 10

This comparative instability demonstrates that for applications requiring predictable long-term electrical performance, hydrogen is a superior dopant to nitrogen. Experimental data show that the resistivity of hydrogen-doped DLC films stabilized after an initial increase of 30%–40% for TEER-deposited films and 40%–120% for HAUZER-deposited films (Zhou, 2019), as shown in Figure 11. The difference between TEER- and HAUZER-deposited films rooted in their different deposition technologies, which affect how the coating is built and its resulting properties. A clear example is the gas for hydrogen doping: TEER systems commonly use iC₄H₁₀, whereas HAUZER systems typically utilize C₂H₂.

FIGURE 11

Furthermore, while resistivity is known to decrease with increasing temperature due to enhanced charge carrier mobility, studies indicate that hydrogen-doped DLC films possess good long-term stability in air, with resistivity largely insensitive to variations in atmospheric pressure and humidity once the initial post-deposition stabilization is complete (Shang, 2019; Guo et al., 2025).

3.3.4 Uniformity of DLC surface resistivity

The uniformity of DLC surface resistivity is a critical parameter that directly governs the performance homogeneity of MPGDs. Achieving this uniformity requires optimization of the deposition geometry and process parameters. In magnetron sputtering, coating material is ejected linearly from the target, meaning the incident angle and flux density of arriving atoms vary across a stationary substrate. To average out these inherent gradients, substrates are mounted on rotating bracket within the chamber. The design of bracket is tailored to the substrate geometry, ranging from cylindrical holders for flexible foils and quadrangular brackets for rigid panels to multi-axis systems for complex three-dimensional components, as illustrated in Figure 12. This continuous rotation ensures all surfaces receive a time-averaged, uniform exposure to the sputtering flux, promoting consistent film microstructure—and thus resistivity—across the sample.

FIGURE 12

The scalability of large-area DLC coatings is fundamentally limited by the “uniform zone” of the deposition chamber, defined by the plasma distribution and target size. Systems like the HAUZER, with its large chamber (Φ80 cm × 90 cm) and tall targets (65 cm), can coat flexible substrates up to 65 cm × 125 cm. Here, the vertical limit is set by the target length (65 cm), and the lateral limit by the perimeter of the cylindrical holders (40 cm). Figure 13. Shows the largest APICAL film (120 cm × 60 cm) and FR4 film (125 cm × 65 cm) clamped in the HAUZER coating chamber (Zhou, 2019). However, even with substrate rotation, macroscopic gradients can persist. Analysis of a large-area DLC film on an FR4 substrate (Figure 14a) reveals a vertical resistivity gradient—higher at the top and bottom edges—which correlates directly with a thickness gradient (Figure 14b). This non-uniform thickness stems from a lower sputtering rate at the target edges (Ozimek et al., 2016; Rogov and Kapustin, 2017). Quantitatively, excluding the non-uniform edge regions, the resistivity non-uniformity factor (standard deviation/mean) is approximately 45% across a central 50 cm × 125 cm area. This improves significantly to about 15% within a more confined central zone of 25 cm × 125 cm, corresponding to the target center, demonstrating that high uniformity is achievable over substantial areas with optimized process design and defined operational boundaries.

FIGURE 13

FIGURE 14

3.3.5 Adhesion of DLC coatings

The adhesion of a magnetron-sputtered DLC coating to its substrate is a critical determinant of its functional performance and operational longevity, governed by a complex interplay of interfacial chemistry, mechanical compatibility, and deposition physics. Adhesive strength is quantitatively assessed via scratch testing, where a stylus is drawn across the coated surface under a progressively increasing normal load; the critical load at which coating failure—through cracking, delamination, or spallation—occurs serves as a standard metric. To ensure robust adhesion across diverse substrates such as polyimides (e.g., Kapton, APICAL), FR4, glass, and metals, several key strategies are employed. In-situ plasma etching prior to deposition is a fundamental pre-treatment (Tanoue et al., 2009), wherein argon ion bombardment under high negative bias cleans the surface of contaminants, activates it by creating dangling bonds for stronger chemical bonding, and induces controlled micro-roughening to enhance mechanical interlock. Mechanical pre-treatments like sandblasting can further increase roughness for anchoring, Figure 15 shows APICAL film with rough surface produced by sandblasting to improve DLC adhesion.

FIGURE 15

For substrates with inherent chemical incompatibility, the application of an adhesive interlayer that bonds well to both the substrate and carbon is an essential method to bridge the interfacial gap and ensure overall coating integrity (Ma et al., 2023). A common example is depositing a copper (Cu) layer on top of DLC. This typically requires a chromium (Cr) interlayer between the Cu and the DLC. To optimize adhesion at both interfaces, co-deposition techniques are employed: a Cr-DLC co-deposited layer enhances bonding to the DLC below, while a Cr-Cu co-deposited layer improves bonding to the Cu above (Zhou, 2019; Zhou, 2023; Achasov et al., 2024). Co-deposition refers to the simultaneous deposition of two different elements within the same coating cycle.

4 Integration of DLC coatings in MPGDs

We have established comprehensive technological expertise in depositing high-quality DLC films across a diverse range of substrates, achieving coatings with uniform, stable, and precisely controllable electrical resistivity. Concurrently, our deposition systems are capable of coating metallic films to serve as conductive electrodes within detector assemblies. The integration of these resistive DLC layers enhances detector robustness, streamlines mechanical construction, and facilitates scaling for large-area applications. Leveraging a foundational magnetron sputtering platform in conjunction with lithographic patterning, chemical etching, and standard printed circuit board (PCB) manufacturing processes, we have successfully developed a series of advanced detector prototypes. A concise review of these detectors is presented in this section.

4.1 The micro-resistive WELL (μRWELL) detector

The μRWELL is a single-stage, spark-protected gaseous detector implemented as a monolithic structure on a PCB (Bencivenn et al., 2015; Bencivenni et al., 2023a). Its architecture, shown schematically in Figure 16a, integrates a perforated polyimide foil for electron avalanche multiplication with a DLC layer. This resistive DLC coating provides essential spark protection through its controlled resistivity and is positioned above a standard readout PCB for signal collection.

FIGURE 16

Prototypes fabricated at USTC (Lv et al., 2020; Zhou et al., 2019) employed a multi-step process: an APICAL foil was coated with DLC layer by magnetron sputtering and bonded to a readout PCB using a prepreg adhesive layer, with the opposite side of the foil laminated with copper. The well-type amplification holes were subsequently formed by etching through the copper and the APICAL foil from one side, terminating at the DLC layer. The readout PCB is designed to have two orthogonal layers of readout strips. A photograph of the completed μRWELL PCB is presented in Figure 16b. Detector characterization using DLC with a surface resistivity of approximately 40 MΩ/□ demonstrated excellent performance, achieving a gain greater than 10⁴, a spatial resolution better than 70 μm, and a muon detection efficiency exceeding 90%. The detector showed strong rate capability, with only a ∼10% gain drop under a high flux of 100 kHz/cm² (8 keV X-rays over the full 10cm×10 cm active area). Since rate capability depends on detector size and the DLC-to-ground distance, implementing optimized grounding designs will enhance the μRWELL’s performance at high rates (Bencivenni et al., 2023b; Bencivenni et al., 2024; Tian et al., 2024). These results confirm that magnetron-sputtered DLC is a highly suitable and promising material for fabricating the resistive electrodes in advanced MPGDs.

4.2 The micro-resistive groove (μRGroove) detector

The μRGroove integrates a DLC-based resistive electrode at the base of micro-groove amplification structures (Tian et al., 2024). While sharing a compact, laminated stack geometry similar to the μRWELL, the μRGroove is distinguished by its use of continuous groove channels for charge amplification rather than discrete wells. This design provides a natural one-dimensional (1D) readout via the top metal layer of the grooves, which forms an inherent strip array. The straightforward incorporation of an orthogonal, second 1D readout strip layer beneath the DLC electrode enables an efficient and native two-dimensional (2D) strip-readout scheme. Figure 17 depicts the layout structure of μRGroove.

FIGURE 17

Prototypes with active area of 10 cm × 10 cm have demonstrated excellent performance, achieving gains up to 2 × 10⁴, detection efficiencies of approximately 98%, and spatial resolutions finer than 80 μm (He et al., 2024). The design exhibits excellent scalability, with a large-area variant (50 cm × 50 cm) maintaining an efficiency greater than 96%. Furthermore, a cylindrical prototype achieved a spatial resolution better than 100 μm with an ultra-low material budget of approximately 0.23% of a radiation length (X₀) (He et al., 2026). The cylindrical μRGroove has been proposed as an option for the inner tracker of STCF (Super Tau-Charm Facility, a next-generation electron–positron collider proposed in China) (Achasov et al., 2024).

The μRGroove detector presents a promising MPGD architecture, characterized by high gain, excellent efficiency and resolution, inherent 2D readout capability, and favorable scalability. Its cylindrical variant shows particular promise for future particle physics experiments requiring lightweight, high-precision tracking.

4.3 Thick-GEM like detectors

4.3.1 Mitigation of the charging-up effect in THGEMs

The charging-up effect is a well-documented phenomenon in MPGDs characterized by the accumulation of charge on insulating surfaces, leading to temporal instabilities in detector gain (Shaked Renous et al., 2017; Hauer et al., 2020; Chernyshova et al., 2020). This effect is critical for the long-term operational stability of such detectors. The underlying mechanism involves electron avalanches within microscopic channels under high electric fields, during which a fraction of the charge carriers (electrons and ions) become embedded in the insulating substrate materials, such as polyimide or glass. This accumulated space charge progressively distorts the local electric field within the amplification regions, resulting in a drift of the effective gain following the initiation of detector operation.

A prominent and effective mitigation strategy involves the application of a thin resistive layer, such as DLC, onto the insulator surfaces. This coating establishes a controlled conductive pathway, facilitating the gradual dissipation of accumulated charge and thereby significantly accelerating the stabilization process. Alternative approaches focus on the use of detector substrates with intrinsic bulk resistivity, which similarly aids in charge dissipation.

Significant temporal gain variations, particularly during the initial hours of operation, have been observed in Thick Gas Electron Multipliers (THGEMs) (Bressler et al., 2023; Shaked Renous et al., 2017), especially in designs featuring large rims. This time-dependent gain evolution is a common feature in detectors where insulator surfaces are exposed to the working gas, as documented in GEMs (Hauer et al., 2020) and MSGCs (Barr et al., 1998). Both experimental studies and numerical simulations consistently attribute this gain instability to the charging-up effect (Song et al., 2020c; Correia et al., 2018; Lippmann and Ropelewski, 2012). In USTC, thin DLC films with high surface resistivity were coated on the open FR4 insulator surfaces (rims and hole walls) of standard THGEMs (Lv et al., 2020; Song et al., 2020b), see Figure 18. DLC-THGEMs with DLC coating show good gain stability over time, effectively eliminating observable signs of the charging-up effect, see Figure 19.

FIGURE 18

FIGURE 19

4.3.2 Resistive THGEM

The traditional THGEM production process involves mechanical drilling of hole arrays into a PCB substrate (like FR4) clad with copper electrodes on both sides, followed by chemical etching to create the crucial insulating rims around each hole (Bressler et al., 2023; Breskin et al., 2009). Achieving good concentricity between the hole and the rim is a key technical challenge. Otherwise, it will cause non-uniform detector gain over sensitive area or sparking discharge phenomena.

By coating thin DLC film on both sides, replacing the traditional copper electrodes, of a PCB substrate, followed by mechanical drilling of holes, Resistive-THGEMs (RTGEM) were produced (Song et al., 2020a). This coating helps mitigate the effects of discharges, making the detector more robust. Single-stage RTGEM detector achieved gain over 1,000 in argon-based gas mixture and under soft x-ray irradiation. And the RTGEM shows no long-term gain variation without insulating rims caused charging-up effect. However, the RTGEM detectors’ gain significantly decreased at high irradiation rates, and the gain declined rapidly in the RTGEM with higher DLC resistivity. To improve the detector rate capability for high-rate applications, we fabricated modified RTGEMs with DLC layers segmented by S-shaped copper lines (S-RTGEM) as fast-grounding paths, as shown in Figure 20. The fraction of gain decrease in the S-RTGEM detectors was significantly lower than the RTGEMs.

FIGURE 20

Compared with conventional THGEMs, no chemical etching of the rims, lithography, screen-printing, or gluing process is necessary for RTGEMs with DLC coatings. The RTGEM technology is easier to manufacture and scalable to large sensitive areas. A large area S-RTGEM with 20cm×100 cm sensitive area was produced, and achieved good gain uniformity.

4.3.3 Resistive WELL detector

THGEM detectors usually run in a cascaded mode, where two or more THGEM foils are stacked, to achieve the high total gain required for efficiently detecting single particles while maintaining stable, discharge-free operation. Keeping the gain per stage moderate (typically ≤100) drastically reduces the charge density in any single avalanche, minimizing the probability of initiating a damaging spark or streamer discharge (Gasik, 2024; Bressler et al., 2023). The main trade-off is increased complexity: more power supplies, more careful alignment of foils, and the need to optimize multiple electric fields (drift, transfer, induction) simultaneously.

There are several WELL-type detector configurations derived from THGEM and incorporated with resistive materials that can run in single-stage amplification mode and achieve moderate gain, including Resistive Plate WELL (RPWELL) and Resistive WELL (RWELL).

The RPWELL detector addresses spark quenching by using a high bulk-resistivity plate as the resistive element (Rubin et al., 2013; Arazi et al., 2014; Jash et al., 2024). It consists of a single-sided THGEM electrode (copper-clad on one side only) directly coupled to a resistive plate with a typical bulk resistivity of 10⁸ – 10¹⁰ Ω·cm. This plate is then coupled to a segmented metallic readout anode (pads or strips). The resistive plate effectively quenches sparks, enabling discharge-free stable, high-gain operation over a broad range of ionization densities.

The RWELL detector is constructed by coupling a single-sided THGEM (or RTGEM, S-RTGEM) electrode onto a readout PCB with resistive DLC coating on its surface, Figure 21 show the schematic diagram of the RWELL detector (Song et al., 2020a). The DLC resistivity ranging from 20 MΩ/□ to 2 GΩ/□. Small prototypes with 10cm×10 cm active area achieved maximum gas gain of 10⁴. A larger RWELL prototype with 25cm×25 cm active area achieved maximum gas gain of 8,000 with a 20.4% uniformity, and the gain can stay stable with 8 keV X-ray at a rate up to ∼ 300 kHz/cm² (Hong et al., 2020). The RWELL detector is considered as an option of the hadron calorimeter for the Circular Electron Positron Collider (CEPC) (CEPC Study Group, 2018). This RWELL provides a very compact, robust, and easily assembled detector choice for large-area high-energy physics applications.

FIGURE 21

4.4 The picosecond micromegas with DLC photocathode

The Picosecond Micromegas detector (Leardini et al., 2023; Aune et al., 2021), developed by the PICOSEC collaboration, achieves an exceptional timing resolution of approximately 24 ps by integrating a Cherenkov radiator with a photocathode with a Micromegas amplification structure, as illustrated in Figure 22a. This hybrid design utilizes prompt ultraviolet photons generated in a radiator crystal (e.g., MgF₂) by a traversing charged particle to produce photoelectrons at a semi-transparent photocathode, thereby circumventing the slower drift of ionization electrons.

FIGURE 22

The conventional use of cesium iodide (CsI) photocathodes, however, presents limitations due to material hygroscopicity and susceptibility to damage from ion bombardment (Wang, 2018; Xie et al., 2012; Razin et al., 1998). To enhance detector robustness, DLC films have been investigated as an alternative, robust photocathode material. A batch of DLC photocathodes with varying thicknesses was produced on MgF₂ substrates for evaluation [Figure 22b] by USTC MPGD group (Wang et al., 2024). Characterization under ultraviolet light identified an optimal DLC thickness of approximately 3 nm for quantum efficiency. Furthermore, aging tests under ion bombardment demonstrated the superior robustness of DLC photocathodes compared to CsI. DLC photocathodes maintained stable quantum efficiency (QE) performance after a slight initial decrease of less than 20%, following bombardment with feedback ions at an accumulated charge of approximately 100 mC/cm². In contrast, CsI photocathodes exhibited rapid damage and significant QE degradation when subjected to a much lower accumulated charge of only 10 mC/cm². The performance of a PICOSEC Micromegas prototype equipped with an optimized 3 nm DLC photocathode was validated in a muon beam. The detector achieved a time resolution of about 42 ps with a detection efficiency of 97% for 150 GeV/c muons. The time resolution obtained with the DLC photocathode does not reach the level of the CsI photocathode, which is primarily limited by the lower photoelectron yield. Nonetheless, these results confirm the potential of DLC as a durable material for photocathodes in high-precision timing detectors such as the PICOSEC Micromegas.

4.5 Broader applications and enabling role

The development of DLC resistive coating technology at USTC has extended its impact beyond the specific architectures of μRWELL, μRGroove, THGEM variants, and the Picosecond Micromegas. This versatile and controllable material platform has proven instrumental in enabling and advancing a diverse spectrum of MPGD concepts, addressing critical challenges in modern high-energy physics experiments. The application of DLC coatings has facilitated novel detector designs and enhanced the performance of existing ones. For instance, the technology has been successfully applied to coat glass electrodes for high-rate Resistive Plate Chambers (RPCs) (Wang et al., 2021), improving their rate capability and robustness. Furthermore, it has enabled the development of innovative structures such as the Proportional Counter Array (PCa) (Tian et al., 2023) and ACHINOS (Giomataris et al., 2020), demonstrating the adaptability of DLC to different readout and amplification geometries. Critically, this material platform has also been validated for operation in cryogenic environments. DLC coatings produced by the group with resistivities tunable across the 10^-1–10⁵ MΩ/□ range at 77 K have been successfully integrated into a RWELL detector, achieving stable operation at liquid argon temperature (∼87 K) (Leardini et al., 2023; Tesi et al., 2023), thereby enabling its potential use in future noble liquid time-projection chambers.

Beyond specific detector construction, USTC has established itself as a central coating facility for the international MPGD research community (Leardini et al., 2023; Zhou, 2020a; Zhou, 2020b; Scharenberg et al., 2024). This collaborative role involves providing expert DLC deposition services that empower external research and development efforts. By serving as this enabling technological hub, the DLC coating expertise contributes significantly to the broader ecosystem of MPGD innovation, accelerating progress across multiple experimental fronts.

5 Technical challenges and future outlook

The implementation of DLC coatings in MPGDs, while enabling significant advancements, is accompanied by a set of technical challenges that must be addressed to ensure reliable, large-scale deployment.

5.1 Key technical challenges in DLC implementation

The successful integration of DLC into MPGDs involves navigating several critical technical hurdles. Paramount among these is the achievement of precise and uniform electrical resistivity, typically targeting ranges between 10 and 100 MΩ/□. This must be coupled with ensuring robust adhesion to diverse substrate materials and scaling the deposition process cost-effectively for large-area applications. Furthermore, the coatings must demonstrate long-term stability under sustained irradiation and maintain performance under ultra-high particle fluxes exceeding 10 MHz/cm².

A central design trade-off in resistive MPGDs involves the resistivity of the coating: higher values enhance spark protection but simultaneously reduce the detector’s rate capability by impeding charge drainage. Under high particle flux, if the charge from successive hits cannot drain quickly enough, it accumulates on the resistive layer. This accumulation creates a local voltage drop, which in turn reduces the detector’s gain and counting efficiency. This fundamental compromise is well-documented in the literature on detector R&D (Song et al., 2020a; Song et al., 2020b; Zhou, 2019; Cesaria et al., 2022). Advanced hybrid design solutions are being developed to resolve this fundamental conflict. One promising approach involves embedding a network of conductive dots or a grid beneath a uniform, high-resistivity DLC layer. This architecture provides efficient, localized pathways for charge evacuation while preserving the DLC’s excellent spark protection properties across the entire surface (Bencivenni et al., 2023b; Achasov et al., 2024; Hong et al., 2020).

5.2 Challenges in achieving process standardization

Achieving process standardization for DLC coatings presents a significant challenge due to the high sensitivity of final film properties—such as resistivity and adhesion—to variables beyond core deposition parameters. Performance variability arises from substrate-dependent factors, including geometry and surface roughness, which alter local plasma dynamics and growth kinetics, and from fixturing methods that can cause shadowing and thermal gradients. Consequently, ensuring reproducible coatings requires a holistic approach to process control, mandating strict standardization of the substrate’s material, surface finish, and geometrical presentation, coupled with comprehensive calibration where functional properties like thickness and resistivity are jointly tuned for each specific application.

5.3 Patterning and structuring of resistive DLC layers

Integrating DLC coatings into sophisticated MPGD architectures necessitates precise patterning and structuring of the continuous resistive film into defined geometries, such as strips or pads, to facilitate segmented readout and manage charge flow. This is primarily achieved through two methodologies: subtractive patterning via standard photolithography and etching, whose success is critically contingent upon the DLC film’s adhesion and intrinsic stress to prevent delamination during wet chemical processes, and direct patterning via maskless laser ablation, which offers high precision and flexibility for prototyping or custom designs. The selection and efficacy of either patterning technique are fundamentally governed by the properties of the DLC layer—including its microstructure, stress state, and adhesion strength—which are, in turn, directly determined by the initial deposition method and its parameters.

5.4 Future research directions and prospects

Future research on DLC for next-generation MPGDs will progress along three interconnected fronts: advanced material engineering, innovative patterning, and system-level standardization. Material engineering will focus on sophisticated deposition techniques such as Pulsed Laser Deposition (PLD) (Robertson, 2002; Hastie et al., 2017), which enables the precise tuning of sheet resistance and ensures robust adhesion (Cesaria et al., 2022; Cho et al., 2012; Colaleo et al., 2020). The development of novel composite materials, including graphene-DLC hybrids (Orlandini et al., 2023; Stock et al., 2017), aims to achieve unparalleled control over properties like resistivity and interfacial adhesion. Concurrently, patterning innovations will leverage advanced methods like holographic lithography to create high-resolution features. This technique offers a viable path for the large-area, uniform patterning essential for transitioning MPGDs from lab-scale prototypes to industrial-scale production. It could facilitate the next leap in fast-grounding design, enabling even more efficient and finer patterns to enhance rate capability. Finally, to support reliable large-scale deployment, parallel efforts must establish rigorous production and quality control protocols. This must be underpinned by systematic studies on long-term radiation aging and modular design optimization. Mastering DLC properties through this integrated, multifaceted approach is fundamental to building the sophisticated detectors required for future scientific frontiers.

6 Conclusion

Diamond-Like Carbon coatings have evolved from a specialized research topic into a foundational technology for modern Micro-Pattern Gaseous Detectors. Their uniquely tunable electrical properties, specifically their controllable surface resistivity, provide an elegant and robust solution to the long-standing challenge of spark-induced damage, enabling stable, high-gain operation essential for contemporary particle physics experiments.

The comprehensive work from the USTC MPGD group, as detailed in this review, demonstrates a mastery of the full technological chain—from fundamental material science to industrial-scale implementation. By optimizing magnetron sputtering processes, the group has achieved precise control over critical coating parameters such as resistivity, uniformity, thickness, and adhesion on diverse substrates. This capability has been successfully translated into the development and validation of advanced detector architectures, including the μRWELL, μRGroove, and DLC-enhanced THGEMs and RWELLs, which exhibit excellent performance in gain, spatial resolution, rate capability, and timing.

Despite these successes, the path to widespread deployment in next-generation colliders like the High-Luminosity LHC presents defined challenges. These include ensuring ultra-high uniformity over large areas, achieving perfect long-term stability under intense irradiation, and developing cost-effective, standardized patterning and integration processes. Future progress hinges on a synergistic approach, combining advances in next-generation deposition techniques (e.g., PLD), novel material composites (e.g., graphene-DLC hybrids), and innovative system-level engineering for modular assembly.

In summary, DLC is more than an incremental improvement; it is a transformative enabler. The material’s versatility and the proven fabrication expertise position DLC-based resistive coatings as a cornerstone for building the robust, high-precision MPGD systems required for future scientific discovery at the intensity and energy frontiers.

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