Total Secondary Electron Yield Research Articles

New experimental methodology for determining the second crossover energy in insulators under stationary e-irradiation in a SEM

A new experimental methodology is proposed which uses the electrostatic influence method (EIM) in scanning electron microscope (SEM) in order to estimate the second crossover energy EC2 for uncharged insulators. This experimental methodology based on simultaneous time measurement of the displacement and leakage currents, is approached to the short pulse irradiation technique but under stationary e-irradiation and allows determining the intrinsic secondary electron emission yield, σ0 (σ0 is the value of the total secondary electron yield just at the beginning of the irradiation before significant charge accumulation or before the formation of a surface potential). The obtained value of EC2 for soda-lime glass is confirmed by two additional experiments based on secondary electron imaging. This value is in good agreement with those previously obtained by other studies based on the surface potential measurement or the pulsed irradiation technique.

Measurements of secondary electron emission yield in the linear plasma simulator BPD-PSI

The secondary emission properties of the surface play one of the key roles in its interaction with plasma. This paper presents the first study of electron-induced electron emission yield performed in the linear plasma simulator BPD-PSI. To verify the applicability of the measurement technique used, a series of experiments was conducted to investigate the total secondary electron yield as a function of the energy of primary electrons for several pure elements, such as carbon, copper, and tungsten. Particular attention was paid to the latter as one of the essential plasma-facing materials, due to its ability to withstand moderate plasma loads. The change in the total secondary electron yield of tungsten was studied in-vacuo after each subsequent change in its surface morphology as a result of: annealing at 2770 K, exposure to pure helium plasma at 1250 K and annealing of plasma-modified tungsten surface at 2770 K. The growth of nanoscale structures, known as fuzz, on its surface caused a reduction in the total yield of secondary electrons by 10–18% compared to the clean surface. A non-monotonic relationship between total secondary electron yield and primary electron energy was observed. The interpretation of this phenomenon is given in the present work.

Suppression of secondary electron emission of oxygen free copper by two-step electrodeposition of rGO/Cu composite coating

We prepared a reduced-graphene-oxide/Cu (rGO/Cu) composite coating to suppress the total secondary electron yield by two-step electrodeposition. We found that driven by the applied electric field, copper ions can enter the interior of graphene oxide (GO) film and nucleate, while the surface is still GO. The rGO/Cu composite coating was obtained by heat treatment with a combination of hydrogen and argon. After heat treatment, XPS analysis shows that the oxygen content of rGO on the surface of the composite coating is reduced from 31.29 at.% to 11.00 at.%, and the carbon content is 85.25 at.%. Raman analysis shows that rGO is still mainly sp2 hybrid structure. The deposited copper particles sintered in the composite coating and formed Cu-O-C chemical bond. The maximum secondary electron yield (δmax) is 1.10, which is close to the δmax of rGO coating (1.09). Compared with the rGO coating, the presence of copper in the rGO/Cu composite coating slightly increases the TEY, but significantly improves the bonding strength between the coating and the substrate at high temperatures.

Suppression of secondary electron emission on oxygen-free copper surface of reduced graphene oxide coatings prepared by electrophoretic deposition

We proposed a new carbon coating to inhibit secondary electron emission. Graphene oxide(GO) coatings were prepared on the surface of oxygen-free copper by DC electrophoretic deposition, and reduced graphene oxide(rGO) coatings were obtained by hydrogen thermal reduction. The X-ray photoelectron spectroscopy (XPS) analysis shows that the oxygen content of the coating reduced from 26.37 at.% to 9.89 at.% after thermal reduction, The XPS and Raman analysis shows that rGO still maintained the sp2 hybrid structure. The inhibitory effect of rGO coatings on secondary electron emission was tested. It was found that the maximum total secondary electron yield (δmax) of the oxygen-free copper surface reduced from 1.38 to 1.09 after the preparation of rGO coating. The total secondary electron yield did not decrease further with the film thickness increasing when the rGO coating reached a certain thickness. Our research provides a new perspective on the preparation of carbon coatings to reduce the secondary electron yield.

The Applications of Ultra-Thin Nanofilm for Aerospace Advanced Manufacturing Technology.

With the development of industrial civilization, advanced manufacturing technology has attracted widespread concern, including in the aerospace industry. In this paper, we report the applications of ultra-thin atomic layer deposition nanofilm in the advanced aerospace manufacturing industry, including aluminum anti-oxidation and secondary electron suppression, which are critical in high-power and miniaturization development. The compact and uniform aluminum oxide film, which is formed by thermal atomic layer deposition (ALD), can prevent the deep surface oxidation of aluminum during storage, avoiding the waste of material and energy in repetitive production. The total secondary electron yield of the C/TiN component nanofilm, deposited through plasma-enhanced atomic layer deposition, decreases 25% compared with an uncoated surface. The suppression of secondary electron emission is of great importance in solving the multipactor for high-power microwave components in space. Moreover, the controllable, ultra-thin uniform composite nanofilm can be deposited directly on the complex surface of devices without any transfer process, which is critical for many different applications. The ALD nanofilm shows potential for promoting system performance and resource consumption in the advanced aerospace manufacturing industry.

Secondary electron emission yield measurements of dielectrics based on a novel collector-only method

The total electron-induced Secondary Electron Yield (SEY) of alkali-free glass has been determined by a novel technique which is solely based on the use of a collector electrode and of two interchangeable dielectric samples. The current from a charge-saturated sample provides a direct measure of the primary beam current. The SEY from the (uncharged) sample under investigation can then be determined simply as the ratio between the collector currents measured at the same electron energy from both samples. We further apply a low-current, short-pulse procedure to limit sample charging, needed to mitigate distortion of the SEY by charging effects. The potential build-up during the experiment could be reconstructed based on the primary electron currents and the measured SEY. This strategy permitted us to measure a 6-point SEY curve, without the need for intermediate discharging of the sample, as the total induced surface potential rise could be kept below +2.55 V.

Monte Carlo simulation of secondary electron emission from an N2-adsorbed layer on a Cu surface

Surface adsorption of N2 molecules is a critical factor to limit the reduction of secondary electron (SE) yield (SEY) in many fields of applied physics. On the basis of the Polanyi potential theory, we describe a multi-layer N2 physical adsorption model to arrange the distribution of N2 molecules adsorbed on a Cu (110) surface. Considering six scattering types, we used a Monte Carlo method to develop a three-dimensional numerical model that simulated the scattering processes between electrons and adsorbed molecules. Thus, the SEY of a surface covered by an adsorbed layer could be obtained by statistical analysis of the final SE states. We found that the maximum of total SEY decreased exponentially with increasing N2 pressure between 0 and 0.1 Torr and decreased linearly with increased adsorbed layer thickness between 0 and 10 nm. The reduction in SEY was due mainly to the elastic scattering of large scattering angle and ionization of energy consumption in the scattering processes of electrons and adsorbed molecules. Accordingly, our model and results provide a powerful tool to fully understand the microcosmic mechanism of the SE emission of a metal surface with adsorbed layer.

Angular dependence of secondary electron yield from microporous gold surfaces

We report exhaustive measurements of the secondary electron yield (SEY) from a gold film containing an array of micropores as a function of the angle of incidence of the primary electrons. The SEY measurements are in good agreement with Monte-Carlo (MC) simulations. A highly accurate empirical fit to the SEY data as a function of the incident electron impact angle is also proposed. In this study, the micropores have aspect ratios (ratio of pore height over pore diameter) ranging from about 1.5 to 3.5. The effect of the pore array density (porosity) and pore aspect ratio is analyzed in greater detail. It is found that increasing the pore aspect ratio and porosity leads to a sharp reduction in the total SEY in agreement with MC simulations.

The influence of low energy titanium ion beam irradiation on secondary electron emission of metal materials by electron impact

Argon ion beams with energy from hundreds eV to several keV can suppress the secondary electron emission (SEE) on surface of metal materials by changing surface morphology and chemical composition of the materials through surface sputtering. In this work, it was found that the low-energy titanium ion irradiation with an average energy from 40 keV to 100 keV did not apparently change the surface morphology and chemical composition of the metal materials, but still significantly reduced the secondary electron yield (SEY) of the materials. Pulsed titanium ion beams generated by an ion implanter, which is based on metal evaporation vacuum arc (MEVVA) ion source, were adopted to irradiate oxygen-free copper and stainless-steel samples. The fluence of titanium ions was in the range of 1 × 1015 to 1 × 1017 ions/cm2. By comparing the effects of different titanium ion energies and fluence on the roughness, chemical composition, total SEY of the sample surface, and calculating the ion range, sputtering yield, nuclear stopping power and defect distribution of titanium ions in these materials, it is reasonable to believe that the irradiation damage caused by titanium ion implantation increases the probability of trapping of the low-energy secondary electrons (SEs) by defects, leading to better SEE suppressing.

Extreme UV secondary electron yield measurements of Ru, Sn, and Hf oxide thin films

Background: The secondary electron yield (SEY) of materials is important for topics as nanoparticle photoresists and extreme ultraviolet (EUV) optics contamination. Aim: Experimentally measure SEY and secondary electron energy distributions for Ru, Sn, and Hf oxide. Approach: The SEY and energy distribution resulting from 65 to 112 eV EUV radiation are measured for thin-film oxides or films with native oxide. Results: The total SEY can be explained by EUV absorption in the topmost nanometer of (native) oxide of the investigated materials. Conclusions: Although the relative SEY of Ru and Sn is well-explained by the difference in EUV absorption properties, the SEY of HfO2 is almost a factor 2 higher than expected. Based on the energy distribution of secondary electrons, this may be related to a lower barrier for secondary electron emission.

Dopant profiling in p-i-n GaN structures using secondary electrons

We show that secondary electrons in a scanning electron microscope can provide important information about spatial dopant distribution in p-i-n GaN structures, with the highest contrast observed for a primary electron beam accelerating voltage in the range of 1 to 2 kV. The current through the specimen is used to determine the total backscattered and secondary electron yield. We establish a correlation between the secondary electron emission intensity and the doping characteristics of the material. The secondary electron emission intensity was found to be highest for p-type GaN, intermediate for n-type GaN, and lowest for undoped GaN. Specimen currents are found to have a strong correlation with the Mg concentration in p-GaN films. The contrast associated with dopants is shown to depend on experimental parameters such as primary electron beam voltage, total electron beam exposure, and specimen surface history. This technique can serve as a powerful tool for the development and characterization of thin films for GaN power electronics.

Measurement of yield and spectrum of secondary electron emission and their characteristics under modification of conductive materials.

Surface modification technique of secondary electron emission (SEE) characteristics of materials, which is utilized to suppress or promote the SEE from material surface under electron bombardment, has extensive applications in a variety of fields. Measurement of the secondary electron yield (SEY) and the secondary electron spectrum (SES) before and after surface modification is essential for the evaluation of effectiveness and the investigation of mechanism of material modification. A SEY and SES measurement system is reported in this article. The comparative measurements of the total SEY, the true SEY, the backscattered electron yield, and the SES of nickel and free-standing vertical graphene, which was grown in situ on the surface of nickel substrate by plasma enhanced chemical vapor deposition, were performed using this system. The measurement results demonstrate that this system could facilitate the study of surface modification on the SEE characteristics of conductive materials.

Secondary electron yield suppression using millimeter-scale pillar array and explanation of the abnormal yield–energy curve**Project supported by the National Natural Science Foundation of China (Grant Nos. U1832190, 61501364, U1537211, and 11705142).

The phenomenon of secondary electron emission is of considerable interest in areas such as particle accelerators and on-board radio frequency (RF) components. Total secondary electron yield (TSEY) is a parameter that is frequently used to describe the secondary electron emission capability of a material. It has been widely recognized that the TSEY vs. primary electron energy curve has a single-hump shape. However, the TSEY–energy curve with a double-hump shape was also observed experimentally—this anomaly still lacks explanation. In this work, we explain this anomaly with the help of a millimetre-scale (mm-scale) silver pillar array fabricated by three-dimensional (3D) printing technology. The TSEY–energy curve of this pillar array as well as its flat counterpart is obtained using sample current method. The measurement results show that for the considered primary electron energy (40–1500 eV), the pillar array can obviously suppress TSEY, and its TSEY–energy curve has an obvious double-hump shape. Through Monte Carlo simulations and electron beam spot size measurements, we successfully attribute the double-hump effect to the dependence of electron beam spot size on the primary electron energy. The observations of this work may be of help in determining the TSEY of roughened surface with characteristic surface structures comparable to electron beam spot size. It also experimentally confirms the TSEY suppression effect of pillar arrays.

The total secondary electron yield of a conductive random rough surface

One problem with current 3D printed microwave components is that they usually have more surface roughness than components manufactured with traditional machining processes. To estimate the multipactor threshold of 3D printed microwave components, it is important to evaluate the total secondary electron yield (TSEY) of the random rough surface. In this work, taking copper as an example, we conduct TSEY simulations for random rough surfaces. First, we generate both isotropic and anisotropic Gaussian type random rough surfaces; then, the dependence of the TSEY on both the roughness and correlation length is studied using Monte-Carlo simulations. Results show that when the root-mean-square roughness increases from 0 to 30 μm, the maximum TSEY increases slowly at first and then decreases rapidly. In other words, roughness can also enhance and not just suppress the TSEY. The results can be interpreted as follows: for a relatively smooth/rough surface, the TSEY enhancement/suppression induced by oblique incidence/a shadowing effect plays a dominant role. Simulation results also show that correlation length has a notable effect on the TSEY. Furthermore, for anisotropic surfaces, the azimuthal angle also influences the TSEY. Compared with currently published work, we provide an optional evaluation method for the TSEY of random rough surfaces. The predictions in this work not only confirm existing observations (the TSEY suppression effect) but also present new predictions (the TSEY enhancement effect and the azimuthal angle effect).

Modeling micro-porous surfaces for secondary electron emission control to suppress multipactor

This work seeks to understand how the topography of a surface can be engineered to control secondary electron emission (SEE) for multipactor suppression. Two unique, semi-empirical models for the secondary electron yield (SEY) of a micro-porous surface are derived and compared. The first model is based on a two-dimensional (2D) pore geometry. The second model is based on a three-dimensional (3D) pore geometry. The SEY of both models is shown to depend on two categories of surface parameters: chemistry and topography. An important parameter in these models is the probability of electron emissions to escape the surface pores. This probability is shown by both models to depend exclusively on the aspect ratio of the pore (the ratio of the pore height to the pore diameter). The increased accuracy of the 3D model (compared to the 2D model) results in lower electron escape probabilities with the greatest reductions occurring for aspect ratios less than two. In order to validate these models, a variety of micro-porous gold surfaces were designed and fabricated using photolithography and electroplating processes. The use of an additive metal-deposition process (instead of the more commonly used subtractive metal-etch process) provided geometrically ideal pores which were necessary to accurately assess the 2D and 3D models. Comparison of the experimentally measured SEY data with model predictions from both the 2D and 3D models illustrates the improved accuracy of the 3D model. For a micro-porous gold surface consisting of pores with aspect ratios of two and a 50% pore density, the 3D model predicts that the maximum total SEY will be one. This provides optimal engineered surface design objectives to pursue for multipactor suppression using gold surfaces.

Mechanism of total electron emission yield reduction using a micro-porous surface

Suppression of the total secondary electron yield (TEY) of metal surfaces is important in many areas such as accelerator, satellite, and Hall thruster. Among TEY suppression techniques, micro-porous surfaces have been demonstrated as an effective method. In this work, we developed an analytical model that is able to obtain the contributions of TEY from both the 1st and 2nd generation secondary electrons (SEs). Calculation results show that the TEY contributed by the bottom of the hole dominates the TEY of the micro-porous surface with the aspect ratio we have chosen. Thus, we developed the following design guidance for the improvement of the TEY suppression efficiency of the micro-porous surface: either lower the TEY of the bottom or guide its SEs to the lateral side of the hole. To verify this idea, we performed the following numerical simulations: a micro-hole with its inner surfaces coated with a low TEY material and a micro-hole with nano-triangular grooves or nano-truncated cone pillars embedded at its bottom. Compared with a usual micro-hole, the proposed hybrid micro/nano structures show improved TEY suppression efficiency as expected from the analytical model. The percentage ratios of the 1st and 2nd generation SEs obtained from the simulation agree well with the predictions of the analytical model. What is more, we also present the results of the emitting angle distribution of SEs which represent remarkable deviation from the usual cosine distribution.

Positive bias and vacuum chamber wall effect on total electron yield measurement: A re-consideration of the sample current method

The measurement of the total secondary electron yield (TEY, δ) is of fundamental importance in areas such as accelerator, spacecraft, detector, and plasma system. Most of the running TEY facilities in the world are based on the kind of bias strategy. The applied bias can assist in the collection of the secondary/primary electrons. In the prevailing sample current method, the TEY is obtained by the measurement of the current from the sample to ground with a negative/positive bias applied to the sample. One of the basic assumptions in this method is that the positive bias can retain most of the electrons emitted by the sample. This assumption is generally recognized based on the seeming fact that the low energy secondary electrons dominate the emitted electrons. In this work, by considering the full electron energy spectrum including both the true secondary and backscattered electrons, we give a new insight in this TEY measurement method. Through the analytical derivation as well as the Particle-in-Cell numerical simulation, we show that it is due to the following two factors, other than the assumption mentioned above, which make the sample current method works satisfactorily: (a) the TEY relative error is related to the TEY itself in the form of |1−δ|/δ, which indicates a smallest error when measuring samples with TEY closest to 1; and (b) the compensation effect of the vacuum chamber wall. Analytical results agree well with numerical simulations and furthermore, we present a correction method for reducing the TEY relative error when measuring samples with TEY below 1. By sweeping the positive bias from 50 to 500 V, a flat silver sample in the as-received state with maximum TEY larger than 2 and a laser etched sample with maximum TEY close to 1 were measured for further verification. The obtained experimental results agree well with the theoretical analysis.

Theoretical analysis and simulation of the influence of self-bunching effects and longitudinal space charge effects on the propagation of keV electron bunch produced by a novel S-band Micro-Pulse electron Gun

As an important electron source, Micro-Pulse electron Gun (MPG) which is qualified for producing high average current, short pulse, low emittance electron bunches steadily holds promise to use as an electron source of Coherent Smith-Purcell Radiation (CSPR), Free Electron Laser (FEL). The stable output of S-band MPG has been achieved in many labs. To establish reliable foundation for the future application of it, the propagation of picosecond electron bunch produced by MPG should be studied in detail. In this article, the MPG which was working on the rising stage of total effective Secondary Electron Yield (SEY) curve was introduced. The self-bunching mechanism was discussed in depth both in the multipacting amplifying state and the steady working state. The bunch length broadening induced by the longitudinal space-charge (SC) effects was investigated by different theoretical models in different regions. The 2D PIC codes MAGIC and beam dynamic codes TraceWin simulations were also performed in the propagation. The result shows an excellent agreement between the simulation and the theoretical analysis for bunch length evolution.

Analysis of secondary electron emission for conducting materials using 4-grid LEED/AES optics

A facility utilizing 4-grid optics for LEED/AES (low energy electron diffraction/Auger electron spectroscopy) was developed to measure the total secondary electron yield and secondary electron energy distribution function for conducting materials. The facility and experimental procedure were validated with measurements of 50–500 eV primary electrons impacting graphite. The total yield was calculated from measurements of the secondary electron current (i) from the sample and (ii) from the collection assembly, by biasing each surface. Secondary electron yield results from both methods agreed well with each other and were within the spread of previous results for the total yield from graphite. Additionally, measurements of the energy distribution function of secondary electrons from graphite are provided for a wider range of incident electron energies. These results can be used in modeling plasma-wall interactions in plasmas bounded by graphite walls, such as are found in plasma thrusters, and divertors and limiters of magnetic fusion devices.

Measurement of backward secondary-electron yield under molecular ion impact coincident with emerging projectiles

We have measured the backward yield of secondary electrons induced by 1MeV C2+ coincident with the emerging projectiles. The total electron yield decreased by a few percent with the number of detected projectiles, which indicates the contribution of inner-shell electron excitation to the secondary-electron yield. The inner-shell contribution for the C2+ impact was approximately the same as that for the C+ impact, whereas the cluster effect on the total secondary electron yield remains.