Secondary electron yield of air-exposed ALD-Al_(2)O_(3) coating on Ag-plated aluminum alloy

Secondary electron yield(SEY)of air-exposed metals tends to be increased because of air-formed oxide,hydrocarbon,and other contaminants.This enhances the possibility of secondary electron multipacting in high-power microwave systems,resulting in undesirable occurrence of discharge damage.Al_(2)O_(3) coatings have been utilized as passive and protective layers on device packages to provide good environmental stability.We employed atomic layer deposition(ALD)to produce a series of uniform Al_(2)O_(3) coatings with appropriate thickness on Ag-plated aluminum alloy.The secondary electron emission characteristics and their variations during air exposure were observed.The escape depth of secondary electron needs to exceed the coating thickness to some extent in order to demonstrate SEY of metallic substrates.Based on experimental and calculated results,the maximum SEY of Ag-plated aluminum alloy had been maintained at 2.45 over 90 days of exposure without obvious degradation by applying 1 nm Al_(2)O_(3) coatings.In comparison,the peak SEY of untreated Ag-plated aluminum alloy grew from an initial 2.33 to 2.53,exceeding that of the 1 nm Al_(2)O_(3) sample.The ultra-thin ALDAl_(2)O_(3) coating substantially enhanced the SEY stability of metal materials,with good implications for the environmental dependability of spacecraft microwave components.

Effect of Cu doping on the secondary electron yield of carbon films on Ag-plated aluminum alloy

Reducing the secondary electron yield(SEY)of Ag-plated aluminum alloy is important for high-power microwave components.In this work,Cu doped carbon films are prepared and the secondary electron emission characteristics are studied systematically.The secondary electron coefficientδ_(max) of carbon films increases with the Cu contents increasing at first,and then decreases to 1.53 at a high doping ratio of 0.645.From the viewpoint of surface structure,the higher the content of Cu is,the rougher the surface is,since more cluster particles appear on the surface due to the small solid solubility of Cu in the amorphous carbon network.However,from viewpoint of the electronic structure,the reduction of the sp2 hybrid bonds will increase the SEY effect as the content of Cu increases,due to the decreasing probability of collision with free electrons.Thus,the two mechanisms would compete and coexist to affect the SEY characteristics in Cu doped carbon films.

A synthetic semi-empirical physical model of secondary electron yield of metals under E-beam irradiation

Calculations of secondary electron yield（SEY） by physical formula can hardly accord with experimental results precisely. Simplified descriptions of internal electron movements in the calculation and complex surface contamination states of real sample result in notable difference between simulations and experiments. In this paper, in order to calculate SEY of metal under complicated surface state accurately, we propose a synthetic semi-empirical physical model. The processes of excitation of internal secondary electron（SE） and movement toward surface can be simulated using this model.This model also takes into account the influences of incident angle and backscattering electrons as well as the surface gas contamination. In order to describe internal electronic states accurately, the penetration coefficient of incident electron is described as a function of material atom number. Directions of internal electrons are set to be uniform in each angle. The distribution of internal SEs is proposed by considering both the integration convergence and the cascade scattering process.In addition, according to the experiment data, relationship among desorption gas quantities, sample ultimate temperature and SEY is established. Comparing with experiment results, this synthetic semi-empirical physical model can describe the SEY of metal better than former formulas, especially in the aspect of surface contaminated states. The proposed synthetic semi-empirical physical model and presented results in this paper can be helpful for further studying SE emission, and offer an available method for estimating and taking advantage of SE emission accurately.

Secondary electron yield suppression using millimeter-scale pillar array and explanation of the abnormal yield–energy curve

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(3 D) 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.

Formulae for secondary electron yield from insulators and semiconductors

The processes and characteristics of secondary electron emission in insulators and semiconductors were studied, and the formulae for the maximum yield(δ_m) at W_(pOm)≤ 800 eV and the secondary electron yield from insulators and semiconductors δ at the primary incident energy of 2 keV≤ W_(pO) < 10 keV(δ_(2-10)) and10 keV ≤ W_(pO)≤100 keV(δ_(10-100)) were deduced. The calculation results were compared with their corresponding experimental data. It is concluded that the deduced formulae can be used to calculate δ_(2-100)at W_(pOm)≤ 800 eV.

Secondary Electron Yield Studies on Vacuum Materials

A secondary electron yield test device for vacuum material study is set up,and its detailed design described in this paper.The test results for a few common vacuum materials with and without TiN film coating are presented,and the influential factors on secondary electron yield are analyzed.All the work will be helpful to the surface pretreatment of vacuum materials.

New neutralization method for measuring the secondary electron yield of insulative material

Purpose The limitation of the traditional bias neutralization method is proved,and a new neutralization method is proposed to measure the secondary electron yield of insulating materials.Method While measuring the secondary electron yield of an insulating sample using the bias neutralization method,the region of an insulating sample irradiated by an electron beam may not be neutralized,because electrons enforced by the bias are not returned to the proper location.The above-mentioned phenomenon is verified by a simulation.To achieve proper neutralization,we propose a method of moving the electron beam to irradiate the metal sample stage without applying a bias voltage,which generates many low-energy electrons around the insulating sample.Those electrons are automatically attracted to the positively charged region of the insulating sample surface and rejected if enough electrons accumulated on the surface.Result and conclusion The limitation of neutralization of bias voltage was verified by simulation,and the new neutralization method was proved to be effective through experiments.

Secondary electron emission yield from vertical graphene nanosheets by helicon plasma deposition

The secondary electron emission yields of materials depend on the geometries of their surface structures.In this paper,a method of depositing vertical graphene nanosheet(VGN)on the surface of the material is proposed,and the secondary electron emission(SEE)characteristics for the VGN structure are studied.The COMSOL simulation and the scanning electron microscope(SEM)image analysis are carried out to study the secondary electron yield(SEY).The effect of aspect ratio and packing density of VGN on SEY under normal incident condition are studied.The results show that the VGN structure has a good effect on suppressing SEE.

Total electron yield mode for XANES measurements in the energy region of 2.1-6.0 keV

The total electron yield （TEY） mode has been developed successfully for XANES measurements at Beamline 4BTA of BSRF （Beijing Synchrotron Radiation Facility）. Its performance was studied by measuring sulphur K-edge XANES of three CdS samples （mixed with graphite powder as an electric conductor） with different concentration： 75%, 50~ and 25%. The data are collected in TEY mode and fluorescence yield （FY） mode respectively for comparison. The results demonstrate that the TEY spectra of three samples agree well with each other after the background is subtracted and normalized. The measured XANES spectra by TEY mode without bias and with 100V bias are almost identical to one another, but the signal-to-noise ratio of spectra measured without bias is better than that with 100V bias. The consistency of the self-absorption corrected FY spectra and TEY spectra are within 10% for the three samples.

The charging stability of different silica glasses studied by measuring the secondary electron emission yield

This paper reports that the charging properties of lead silica, Suprasil silica and Infrasil silica are investigated by measuring the secondary electron emission （SEE） yield. At a primary electron beam energy of 25 keV, the intrinsic SEE yields measured at very low injection dose are 0.54, 0.29 and 0.35, respectively for lead silica, Suprasil and Infrasil silica glass. During the first e-beam irradiation at a high injection current density, the SEE yields of lead silica and Suprasil increase continuously and slowly from their initial values to a steady state. At the steady state, the SEE yields of lead silica and Suprasil are 0.94 and 0.93, respectively. In Infrasil, several charging and discharging processes are observed during the experiment. This shows that Infrasil does not reach its steady state. Two hours later, all samples are irradiated again in the same place as the first irradiation at a low current density and low dose. The SEE yields of lead silica, Suprasil and Infrasil are 0.69, 0.76 and 0.55, respectively. Twenty hours later, the values are 0.62, 0.64 and 0.33, respectively, for lead silica, Suprasil and Infrasil. These results show that Infrasil has poor charging stability. Comparatively, the charging stability of lead silica is better, and Suprasil has the best characteristics.

Secondary electron emission and photoemission from a negative electron affinity semiconductor with large mean escape depth of excited electrons

The formulae for parameters of a negative electron affinity semiconductor(NEAS)with large mean escape depth of secondary electrons A(NEASLD)are deduced.The methods for obtaining parameters such asλ,B,E_(pom)and the maximumδandδat 100.0 keV≥E_(po)≥1.0 keV of a NEASLD with the deduced formulae are presented(B is the probability that an internal secondary electron escapes into the vacuum upon reaching the emission surface of the emitter,δis the secondary electron yield,E_(po)is the incident energy of primary electrons and E_(pom)is the E_(po)corresponding to the maximumδ).The parameters obtained here are analyzed,and it can be concluded that several parameters of NEASLDs obtained by the methods presented here agree with those obtained by other authors.The relation between the secondary electron emission and photoemission from a NEAS with large mean escape depth of excited electrons is investigated,and it is concluded that the presented method of obtaining A is more accurate than that of obtaining the corresponding parameter for a NEAS with largeλ_(ph)(λ_(ph)being the mean escape depth of photoelectrons),and that the presented method of calculating B at E_(po)>10.0 keV is more widely applicable for obtaining the corresponding parameters for a NEAS with largeλ_(ph).

Analysis of secondary electron emission using the fractal method

Based on the rough surface topography with fractal parameters and the Monte–Carlo simulation method for secondary electron emission properties, we analyze the secondary electron yield(SEY) of a metal with rough surface topography. The results show that when the characteristic length scale of the surface, G, is larger than 1 × 10^(-7), the surface roughness increases with the increasing fractal dimension D. When the surface roughness becomes larger, it is difficult for entered electrons to escape surface. As a result, more electrons are collected and then SEY decreases. When G is less than 1 × 10^(-7),the effect of the surface topography can be ignored, and the SEY almost has no change as the dimension D increases. Then,the multipactor thresholds of a C-band rectangular impedance transfer and an ultrahigh-frequency-band coaxial impedance transfer are predicted by the relationship between the SEY and the fractal parameters. It is verified that for practical microwave devices, the larger the parameter G is, the higher the multipactor threshold is. Also, the larger the value of D,the higher the multipactor threshold.

Comparison of the photoemission behaviour between negative electron affinity GaAs and GaN photocathodes

In view of the important application of GaAs and GaN photocathodes in electron sources, differences in photoe- mission behaviour, namely the activation process and quantum yield decay, between the two typical types of III-V compound photocathodes have been investigated using a multi-information measurement system. The activation exper- iment shows that a surface negative electron affinity state for the GaAs photocathode can be achieved by the necessary Cs-O two-step activation and by Cs activation alone for the GaN photocathode. In addition, a quantum yield decay experiment shows that the GaN photocathode exhibits better stability and a longer lifetime in a demountable vacuum system than the GaAs photocathode. The results mean that GaN photocathodes are more promising candidates for electron source emitter use in comparison with GaAs photocathodes.

表面镀膜调控二次电子发射的研究进展

二次电子发射现象属于材料表面的一种特殊物理效应,广泛存在于显微分析和信号探测领域、空间大功率微波部件、大型粒子加速器以及高压真空绝缘部件中。表面镀膜技术是二次电子发射调控的重要研究方向之一,通过引入镀层材料来改变衬底的表面状态,从而影响材料对电子的散射能力。从镀膜材料二次电子发射的物理机制和理论模型出发,分析了半经验物理模型的适用条件及优化方向。随后,根据二次电子发射特性应用场景的不同,对常用的镀层材料及其调控方法进行了文献调研和综合分析,从镀膜工艺和显微结构等方面进行了讨论。对电子倍增领域而言,镀覆Al_(2)O_(3)、MgO和Zn O等氧化物薄膜作为二次电子倍增层材料,可以提高器件的探测效率。对空间大功率微波部件而言,镀覆碳化物、氮化物等低SEY的材料,可以降低材料的二次电子发射能力,避免由电子倍增所引起微放电效应。最后,总结了镀膜结构在调控二次电子发射特性的研究进展,并对其未来发展进行了合理展望。

多层球形二次电子测量装置的栅网电子透过率仿真研究

精确测量材料的二次电子发射系数(SEY)对研究航天器表面材料充放电具有重要意义。在二次电子测量装置中,栅网电子透过率是影响材料SEY测量精度的重要参数。文章通过仿真计算对一种多层球形SEY测量装置中影响栅网电子透过率的因素进行研究,并对栅网网格参数进行优化。结果表明:栅网偏压对栅网电子透过率的影响较小;栅网网格参数是影响栅网电子透过率的主要因素,栅网线径为0.025 mm、格线间距为1 mm时栅网电子透过率达到最大。研究可为优化SEY测量装置设计提供参考。

金属二次电子发射系数表达式

根据二次电子发射的主要物理过程,推导出二次电子发射系数和单位背散射电子产生的内二次电子数与原电子产生的内二次电子数之比、原电子入射能量、参数、最大二次电子发射系数、背散射系数之间的关系式。根据实验结果,给出了2～10keV比率的表达式。根据推导的关系式,用实验数据分别计算出6种金属的平均参数。发现6种金属的平均参数都近似为一个常数11.89(eV)0.5。根据推导的关系式和计算的参数,推导出以背散射系数、原电子入射能量和最大二次电子发射系数为变量的二次电子发射系数通式。用该通式计算出二次电子发射系数,并与相应的实验值进行了比较,最后成功地推导出金属2～10keV的二次电子发射系数通式。

电子入射角度对聚酰亚胺二次电子发射系数的影响

采用具有负偏压收集极的二次电子发射系数测试系统,对聚酰亚胺样品的二次电子发射系数与入射电子角度和入射电子能量的关系进行了测量.测量结果表明,在电子小角度入射样品的情况下,随着入射角度的增加,二次电子发射系数单调增加,并符合传统的规律,但是在电子大角度入射时,却与此不符合.测量显示,出现偏差时对应的临界电子入射角度随着入射电子能量的降低而减小.采用简化的电子弹性散射过程和卢瑟福弹性散射截面公式对这种偏差的出现进行了分析,并推导出修正后的二次电子发射系数的计算公式.修正后的二次电子发射系数的计算结果更加符合实验结果.

10～30keV二次电子发射系数的表达式

根据二次电子发射的主要物理过程和特性，推导出最大二次电子发射系数（δm）的表达式。还推导出平均每个高能原电子发射的二次电子数（δPE）的表达式。根据δPE、δm和高能二次电子发射系数（δ）之间的关系，推导出以δm、原子序数、原子质量数、物质密度、背散射系数、高能背散射系数、参数A、能量幂次（n）和原电子入射能量为变量δ的通式。用ESTAR程序计算出一些材料的10—30keY能量范围内的参数A和n。用该通式计算出δ并与相应的实验值进行了比较，结果表明，成功地推导出金属、半金属和元素半导体10～30kev的艿通式。

金属规则表面形貌影响二次电子产额的解析模型

表面形貌是影响二次电子发射特性的重要因素,但目前仍缺乏刻画这一影响规律的解析模型.本文通过分析发现表面结构的遮挡作用是影响二次电子发射特性的主要因素.基于二次电子以余弦角分布出射的规律,提出了建立表面形貌参数与二次电子产额之间定量关系的方法,并以矩形槽和三角槽为例,建立了电子正入射和斜入射时的一代二次电子产额的解析模型.将推导的解析模型与Monte Carlo模拟结果和实验结果进行了比较,结果表明本文建立的模型能够正确反映规则表面形貌的二次电子产额.本文的模型对于反映常用规则结构影响二次电子出射的规律以及指导通过表面结构调控二次电子发射特性都具有参考价值.

磁控溅射铂抑制镀银表面的二次电子发射

降低表面的二次电子产额是抑制微波部件二次电子倍增效应和提升功率阈值的有效途径之一,目前主要采用在表面构造陷阱结构和沉积非金属薄膜的方法降低二次电子产额,其缺点是会改变部件的电性能.针对此问题,采用在表面沉积高功函数且化学惰性的金属薄膜来降低二次电子产额.首先,采用磁控溅射方法在铝合金镀银样片表面沉积100 nm铂,测量结果显示沉积铂后样片的二次电子产额最大值由2.40降至1.77,降幅达26%.其次,用相关唯象模型对二次电子发射特性测量数据进行了拟合,获得了在40-1500 eV能量范围内能够准确描述样片二次电子产额特性的Vaughan模型参数,以及在0-50 eV能量范围内能够很好地拟合二次电子能谱曲线的Chung-Everhart模型参数.最后,将获得的实验数据和相关拟合参数用于Ku频段阻抗变换器的二次电子倍增效应功率阈值仿真研究,结果表明通过沉积铂可将部件的功率阈值由7500 W提升至36000 W,证实了所提方法的有效性.研究结果为金属材料二次电子发射特性的研究提供实验数据参考,对抑制大功率微波部件二次电子倍增效应具有参考价值.