Optimized Al₂O₃/MgO Emission Layers for Ultra‑High Gain Electron Multipliers

Abstract

Secondary electron emission (SEE) is the cornerstone of electron‑multiplier performance. The SEE coefficient depends strongly on the emission‑layer material and its thickness. Thin layers yield insufficient SEE, while excessively thick layers prevent timely charge replenishment by the conductive underlayer, limiting the avalanche gain. Al₂O₃ and MgO are the most common emission‑layer films because of their high intrinsic SEE. MgO, however, readily deliquesces to Mg(OH)₂, Mg₂(OH)₂CO₃ and MgCO₃, which degrades its SEE. Al₂O₃ is chemically stable but offers a lower intrinsic SEE. We engineered a spherical test system that uses low‑energy secondary electrons for charge neutralisation, eliminating the drawbacks of conventional low‑energy electron‑beam methods. By precisely controlling film thickness via atomic‑layer deposition (ALD) and analysing the depth‑dependent C‑concentration with XPS, we derived empirical SEE‑vs‑thickness relationships for Al₂O₃, MgO, Al₂O₃/MgO, and MgO/Al₂O₃. The resulting emission‑layer architecture – 2–3 nm Al₂O₃ buffer, 5–9 nm MgO main body, and 0.3–1 nm Al₂O₃ protective or enhancement – was integrated into microchannel plates (MCPs) and demonstrated a substantial gain improvement. This design is also applicable to channel electron multipliers (CEMs) and separate electron multipliers (SEMs).

Introduction

Secondary electron emission (SEE) is defined as the ratio of emitted secondary electrons to incident primary electrons. It underpins a broad spectrum of technologies, from electron multipliers (CEM, MCP, SEM, MPG, dielectric windows, atomic clocks) to surface‑analysis instruments (TEM, SEM, AES, electron diffractometer) and micro‑discharge suppression (electron‑cloud mitigation, high‑power microwave device reliability, spacecraft surface charging).

Electron multipliers consist of a substrate, a conductive underlayer, and an emission layer. Incident electrons strike the emission layer, liberating secondary electrons that are accelerated by a bias voltage, initiating an electron avalanche that culminates in a macroscopic electron cloud. The conductive layer continuously replenishes the charge lost from the emission layer during this avalanche.

The SEE coefficient is highly sensitive to the emission‑layer thickness. Experiments indicate that a thickness of 5–15 nm optimises gain, balancing sufficient SEE against efficient charge replenishment. Consequently, detailed study of both material properties and thickness is essential for high‑performance multipliers.

Al₂O₃ is prized for its high SEE and chemical stability, yet MgO offers a higher intrinsic SEE. MgO’s drawbacks—deliquescence, excessive film thickness (~35 nm) required for saturated SEE, instability, and more complex fabrication—have limited its adoption. ALD enables the deposition of continuous, pin‑hole‑free films with sub‑nanometre thickness control, making it ideal for constructing precisely engineered emission layers.

Experimental Methods

Atomic‑Layer Deposition of Al₂O₃ and MgO

ALD deposits material one atomic layer at a time through alternating precursor and reactant pulses. For Al₂O₃, trimethylaluminium (TMA) and water were pulsed at 200 °C, following the reactions:

Al–OH* + Al(CH₃)₃ → Al–O–Al(CH₃)₂* + CH₄↑
Al–O–Al(CH₃)₂* + 2 H₂O → Al–O–Al(OH)₂* + 2 CH₄↑

Typical pulse durations were 0.1–1 s for TMA, 5–45 s for H₂O. MgO deposition employed bis(cyclopentadienyl)magnesium (MgCp₂) and water at 200 °C:

Mg–OH* + MgCp₂ → Mg–O–MgCp* + C₅H₆↑
Mg–O–MgCp* + H₂O → Mg–O–Mg(OH)₂* + C₅H₆↑

Pulse times matched those used for Al₂O₃. The resulting films exhibited excellent coverage and thickness precision.

Emission‑Layer Fabrication

We fabricated four sample series on Si wafers: (1) Al₂O₃ of 1–50 nm, (2) MgO of 1–35 nm, (3) Al₂O₃ (0.6–30 nm) followed by 9 nm MgO, and (4) 35 nm MgO followed by Al₂O₃ (0.3–20 nm). All films were characterised by XPS (Ar ion sputter etching) and XRD.

Secondary‑Electron‑Emission Measurement

A custom spherical collector system ensured full capture of secondary electrons. The SEE coefficient was calculated as SEE = I_s / I_p, where I_p is the primary‑electron current (measured with the sample connected) and I_s is the secondary‑electron current (measured after disconnecting the sample). Low‑energy secondary electrons, rather than an auxiliary electron beam, neutralised surface charging, avoiding dose‑related artefacts.

XPS Transition‑Layer Analysis

Depth‑resolved XPS quantified the C, Al, and Si atomic concentrations during Ar sputter etching, revealing the deliquescence depth of MgO and the extent of Al₂O₃ intermixing at interfaces.

Results and Discussion

SEE Versus Incident‑Electron Energy

We defined a nearest‑neighbour ratio R_SEE = SEE(x + b) / SEE(x) to partition the energy range into low, medium, and high regions. Al₂O₃ exhibits a stable SEE for 7–30 nm thicknesses across the medium energy range (250–500 eV). MgO reaches saturated SEE beyond 20 nm, with a similar medium‑energy stability. Al₂O₃/MgO composites maintain stable SEE after 3 nm Al₂O₃, while MgO/Al₂O₃ requires 3 nm Al₂O₃ to stabilise SEE. Empirical fits for each system are:

• Al₂O₃: B_SEE = 3.99 – 2.5 e^{–t/1.73}
• MgO:  B_SEE = 9.56 – 8.64 e^{–t/7.39}
• Al₂O₃/MgO: B_SEE = 7.94 – 1.21 e^{–t/1.03}
• MgO/Al₂O₃: B_SEE = 4.69 + 3.64 e^{–t/2.11}

These relations quantify how thin Al₂O₃ buffers and MgO main layers enhance SEE, while ultrathin Al₂O₃ capping layers (0.3–1 nm) preserve stability and protect against deliquescence.

Effect of Deliquescence and Transition Layers

Air exposure transforms MgO into Mg(OH)₂, Mg₂(OH)₂CO₃, and MgCO₃, drastically reducing SEE. XPS depth profiling shows that a 1 nm Al₂O₃ layer can fully replace the surface lattice of MgO, forming a solid‑solution transition layer that blocks further deliquescence and preserves SEE. When the Al₂O₃ coverage falls below one atomic layer, C‑signal persists after etching, indicating incomplete protection and continued deliquescence.

Optimised Three‑Layer Emission Structure

Combining the empirical data and transition‑layer insights, we propose a sandwich architecture: 2–3 nm Al₂O₃ buffer, 5–9 nm MgO main body, and 0.3 nm Al₂O₃ enhancement (or 1 nm protective) on top. Integrating this stack into MCP channels increased the gain by > 30 % relative to conventional Al₂O₃‑only plates, as shown in the gain–voltage comparison.

Conclusions

We introduced a spherical SEE‑measurement system that employs low‑energy secondary electrons for charge neutralisation, improving accuracy for insulating materials. By analysing nearest‑neighbour SEE ratios, we derived empirical thickness‑SEE relations for Al₂O₃, MgO, and their bilayers. The transition‑layer concept explains the enhanced SEE of MgO when overlaid with thin Al₂O₃. The optimized 3‑layer Al₂O₃/MgO/Al₂O₃ emission structure delivers high SEE, chemical stability, and thin‑film advantages, offering significant performance gains for MCPs, CEMs, and SEMs.

Availability of Data and Materials

The authors reserve the data for future research to maintain competitive advantage.