Optimization of the secondary electron yield of laser-structured copper surfaces at room and cryogenic temperature

Electron cloud ($e$-cloud) mitigation is an essential requirement for proton circular accelerators in order to guarantee beam stability at a high intensity and limit the heat load on cryogenic sections. Laser-engineered surface structuring is considered a credible process to reduce the secondary electron yield (SEY) of the surfaces facing the beam, thus suppressing the $e$-cloud phenomenon within the high luminosity upgrade of the LHC collider at CERN (HL-LHC). In this study, the SEY of Cu samples with different oxidation states, obtained either through laser treatment in air or in different gas atmospheres or via thermal annealing, has been measured at room and cryogenic temperatures and correlated with the surface composition measured by x-ray photoelectron spectroscopy. It was observed that samples treated in nitrogen display the lowest and more stable SEY values, correlated with the lower surface oxidation. In addition, the surface oxide layer of air-treated samples charges upon electron exposure at a low temperature, leading to fluctuations in the SEY.

Figure 1

Schematics of the laser treatment setup. The chamber was filled with gas of 99.998% purity at a flow rate of approximately $5\mathrm{L}/\min$. The system was then allowed to reach equilibrium for several minutes before processing commenced.

Figure 2

(a) Variation of the energy dependence of the SEY for a CH-patterned air-treated sample, for three consecutive measurements at each temperature (RT denotes the measurements at room temperature). (b) Variation of the energy dependence of the SEY for a CH-patterned air-treated sample after thermal treatment and short air exposure, for three consecutive measurements at each temperature. The peak at 450 eV is an artifact of the measurement.

Figure 3

Room-temperature SEY curves measured on the CH-patterned air-treated sample before (blue line) and after (red line) thermal treatment at $300°\mathrm{C}$ in a vacuum (no air exposure after heat treatment). The energy for which the SEY increases above unity is indicated.

Figure 4

X-ray photoelectron spectra of the Cu 2p state (left) and the Cu LMM Auger transition (right) of sputter-cleaned copper (red line) and CH-patterned air-treated copper before (black line) and after (green line) the thermal treatment in a vacuum.

Figure 5

SEM images of the sample after heat treatment compared to a similar reference sample before heat treatment.

Figure 6

Room-temperature SEY results of LH-patterned Cu samples treated in nitrogen (red line), argon (blue line), and air (gray line). The results are the average of three measurements at different locations at the sample. The energy for which the SEY increases above unity is indicated.

Figure 7

XPS survey (left) and Cu 2p (right) spectra for samples treated under different atmospheres: nitrogen (red line), argon (blue line), air (gray line), and reference sputtered copper (black line).

Figure 8

SEY measurements of a LH-patterned sample treated in nitrogen atmosphere: at room temperature (black line), at $\approx 10\mathrm{K}$ (green line), and after exposure to an electron dose of $6.1\times {10}^{-6}$ (red line) and $1.8\times {10}^{-5}\mathrm{C}{\mathrm{mm}}^{-2}$ (blue line).

Figure 9

SEY measurements of a LH-patterned sample treated in argon atmosphere: at room temperature (black line), at $\approx 10\mathrm{K}$ (green line), and after exposure to an electron dose of $6.1\times {10}^{-6}$ (red line) and $1.8\times {10}^{-5}\mathrm{C}{\mathrm{mm}}^{-2}$ (blue line).

Figure 10

SEY measurements of a LH-patterned sample treated in air: at room temperature (black line), at $\approx 10\mathrm{K}$ (green line), and after exposure to an electron dose of $6.1\times {10}^{-6}$ (red line) and $1.8\times {10}^{-5}\mathrm{C}{\mathrm{mm}}^{-2}$ (blue line). The energy for which the SEY increases above unity is indicated.

Figure 11

Scanning electron micrographs of LH-patterned surfaces treated in nitrogen (top), air (middle), and argon (bottom) at two different magnifications.