Escape Depth Of Secondary Electrons Research Articles

Experimental and theoretical studies on X-ray induced secondary electron yields in Ti and TiO2

Abstract Generation of X-ray induced secondary electrons in Ti and TiO2 was studied from both experimental and theoretical approaches, using X-ray photoelectron spectroscopy (XPS) attached to a synchrotron radiation facility and Monte Carlo simulation, respectively. The experiment revealed that the yields of secondary electrons induced by X-rays (electrons/photon) at photon energies to 4950 and 5000 eV for Ti and TiO2 are δTi(4950 eV) = 0.002 and δTi(5000 eV) = 0.014 while those for TiO2 are δ Ti O 2 ( 4950 eV ) = 0.003 and δ Ti O 2 ( 5000 eV ) = 0.018 . A novel approach to obtain the escape depth of secondary electrons has been proposed and applied to Ti and TiO2. The approach agreed very well with the experimental data reported so far. The Monte Carlo simulation predicted; δ Ti * ( 4950 eV ) = 0.002 and δ Ti * ( 5000 eV ) = 0.011 while δ Ti O 2 * ( 4950 eV ) = 0.003 and δ Ti O 2 * ( 5000 eV ) = 0.015 . An experimental examination on the contribution of X-ray induced secondary electrons to photocatalysis in TiO2 has also been proposed.

Electron-induced secondary electron emission yield from condensed rare gases: Ne, Ar, Kr, and Xe

The secondary electron emission (SEE) yield $\ensuremath{\delta}$ has been measured for condensed layers of Ne, Ar, Kr, and Xe excited by primary electrons of energy ${E}_{0}$ ranging from 0.05 up to $3\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. The deposited thickness $D$ ranges from 0 up to a few thousands of monolayers. For very thick specimens, the evolution of the $\ensuremath{\delta}({E}_{0})$ curves is very similar for all the four gases and it may be described by a single analytical expression. At $3\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$, the maximum yield is not attained for the thickest layers despite the fact that the measured yields exceed several tens. These observed high yields are analyzed in terms of escape depth for secondary electrons (\ensuremath{\sim} the micrometer range) and of escape probability $(>75%)$. Such results are consistent with those previously obtained by proton bombardment and by x-ray photon irradiation. More surprising is the observed increase of $\ensuremath{\delta}$ with $D$ when all the primary electrons remain confined in the film, $D>R$ (range of incident electrons), and several hypotheses are suggested to explain this effect: increase of the crystallite grain size, charging mechanism, SE reflection effects at the film∕solid interface. For very thin films where the constant thickness $D$ is $D⪡R$, the observed decrease of $\ensuremath{\delta}$ as a function of ${E}_{0}$ is qualitatively explained but, when $D$ is changed, additional calculations are needed.

Escape depth of secondary electrons induced by ion irradiation of submicron diamond membranes

The emission of secondary electrons from any material is governed by electron excitation in the bulk, their transport to the surface, and their escape through the surface into the vacuum. Here, we address the question of the transport of electrons in polycrystalline diamond and amorphous carbon membranes and discuss the factors that limit it. The results of the measurements of the escape depth of the secondary electrons from the membranes of submicron polycrystalline diamond and amorphous carbon films induced by the hydrogen ion impact are reported here. It is found that the escape depth for the secondary electrons emitted from diamond scales with the grain size of the crystallites in the polycrystalline diamond films and it can be very large. In contrast, for the case of the amorphous carbon membranes, we find this depth to be much shorter. The extremely high electron emission yield, which have been measured following the slowing down of the electrons or ions in diamond, can be explained by the fact that secondary electrons can move rather freely in diamond, hence, can reach the surface from large distances inside the diamond sample.

Characterization of CsI photocathodes at grazing incidence for use in a unit quantum efficiency x-ray streak camera

The performance of CsI photocathodes has been characterized for use with grazing incidence soft x rays. The total electron yield and pulsed quantum efficiency of a CsI photocathode has been measured in a reflection geometry as a function of photon energy (100 eV to 1 keV), angle of incidence, and the electric field between the anode and photocathode. The total electron yield and pulsed quantum efficiency increase as the x-ray penetration depth approaches the secondary electron escape depth. Unit quantum efficiency in a grazing incidence geometry is demonstrated. A weak electric-field dependence is observed for the total yield measurements; while no significant dependence is found for the pulsed quantum efficiency. The effect of the pulse height distribution on the detective quantum efficiency is discussed. Theoretical predictions agree accurately with experiment.

Objective comparison of scanning ion and scanning electron microscope images

AbstractCommon and different aspects of scanning electron microscope (SEM) and scanning ion microscope (SIM) images are discussed from a viewpoint of interaction between ion or electron beams and specimens. The SIM images [mostly using 30 keV Ga focused ion beam (FIB)] are sensitive to the sample surface as well as to low‐voltage SEM images. Reasons for the SIM images as follows: (1) no backscattered‐electron excitation; (2) low yields of backscattered ions; and (3) short ion ranges of 20–40nm, being of the same order of escape depth of secondary electrons (SE) [=(3–5) times the SE mean free path]. Beam charging, channeling, contamination, and surface sputtering are also commented upon.

Sample preparation for electron beam testing: An electrical characterization method to obtain an optimized anti-charge-up carbon layer

Abstract In order to limit positive charge build up in the passivation layer of integrated circuit scanned by low energy electron beam tester (E-BEAM tester), we deposit an ultra thin carbon layer on the device. The thickness of this layer must be weaker than the secondary electron escape depth (voltage contrast) but large enough to ensure sufficient conductivity to evacuate the charge that builds up. Unfortunately, the good thickness depends on circuits, evaporating systems and so on. We present an electrical characterization method based on the measure of the standby current. At the begining of the carbon layer growing, this current slowly rises. After, it sharply increases and finally it proportionally augments as the thickness. From an anti-charge up point of view, the best possible thickness is the end of the standby current skyrocketting. With this method, we obtain an optimized thickness for each circuit.

Escape depth of secondary electrons from electron-irradiated polymers

Measurements on polymers (Teflon, FEP and Mylar) have shown that the secondary electron emission from uncharged surfaces exceeds that from surfaces containing a positive surface charge. The reduced emission of charged surfaces is due to recombination between electrons undergoing emission and trapped holes within the charged layer. During the experiments the surface of the material was kept at a negative potential to assure that all secondary electrons reaching the surface from within the material are actually emitted. An analysis of the results yielded the maximum escape depth of the secondary electrons, and showed that the ratio of the maximum escape depth of the secondaries from Mylar to the maximum escape depth from Teflon is almost the same as the ratio of the corresponding second crossover energies of these polymers. >

Secondary Electron Imaging of Crystal Surface Steps

High resolution secondary electron (SE) images can be obtained in a scanning transmission electron microscope (STEM) by collecting the type I secondary electrons generated directly by the incident electron beam. The high resolution SE signal is, in general, localized to about 1 nm in the generation process. The escape depth of the secondary electrons, however, varies from about 1 nm for most metals to about 10 nm for some insulating materials. Surface steps can be imaged by collecting high resolution secondary electrons Among other possible contrast mechanisms for SE imaging of surface steps, topographic contrast plays an important role. The topographic contrast of surface steps depends on: (1) the incident electron probe size; (2) step height to SE escape depth ratio; (3) localization of the generated SE signals and (4) the incident beam angle relative to the specimen surface.In our UHV STEM (MIDAS) instrument, both the entrance and exit surfaces of the sample can be imaged by collecting secondary electrons. For thin samples, the resolution is < lnm for SE images of entrance and exit surfaces but for thicker samples the resolution of the SE image obtained from the exit surface will be degraded by the beam broadening effects. For studying steps on thick or bulk crystal surfaces, secondary electrons emitted from the entrance surfaces are, in general, collected to form images with high contrast and high resolution. In this report, all the SE images are obtained, in MIDAS, by collecting secondary electrons emitted from the entrance surfaces of the samples. Figure 1 is a SE image of a MgO smoke crystal revealing the surface growth steps with high contrast. The parallel straight steps, along <100> directions, constitute the edges of unfinished atomic planes on the MgO {100} crystal surfaces. This image indicates that the {100} surfaces of MgO smoke crystals are flat surfaces. Figure 2 shows a SE image of a MoO3 crystal with surface growth steps spiraling from an end-on screw dislocation (the black dots in the SE image represent electron beam induced reduction products). The dark lines (indicated by D) represent up-steps and the bright lines (indicated by B) represent down-steps. It is interesting to note that the two-dimensional crystal growth around the dislocation is anisotropic, forming terraces with long edges parallel to [010] direction.

Monte Carlo calculation of secondary electron emission from carbon-surface by obliquely incident particles.

Incidence angle dependences of secondary electron emission from a carbon surface by low energy electron and hydrogen atom are calculated using Monte Carlo simulations on the kinetic emission model. The calculation shows very small increase or rather decrease of the secondary electron yield with oblique incidence. It is explained in terms of not only multiple elastic collisions of incident particles with the carbon atoms but also small penetration depth of the particles comparable with the escape depth of secondary electrons. In addition, the two types of secondary electron emission are distinguished by using the secondary electron yield statistics; one is the emission due to trapped particles in the carbon, and the other is that due to backscattered particles. The high-yield component of the statistics on oblique incidence is more suppressed than those on normal incidence.

X-ray-induced secondary-electron emission from solid xenon.

The secondary-electron yield has been measured for solid Xe films excited by (1--10)-keV x rays. High yields are observed due to the long escape depth for secondary electrons (\ensuremath{\sim}1 \ensuremath{\mu}m). The measurements are discussed in terms of a one-dimensional diffusion model. The electron escape depth is found to be sensitive to the temperature at which the film is grown. The electron yield was found to exhibit a time dependence due to the combined effects of charging and radiation damage. A large increase in the yield was observed when an electric field was applied. Finally, the effects of added impurities, ${\mathrm{CO}}_{2}$ and propane, were investigated. It is demonstrated that solid Xe will be useful as a very sensitive x-ray photocathode.

Electron yields and escape depths from Kapton and Teflon

AbstractSecondary electron emission (SEE) characteristics, i.e., SEE yield maximum, the electron energy corresponding to the maximum yield, Emax, and crossover energies at which the SEE yield is unity, as a function of angle of incidence were determined for Kapton and Teflon. Photoelectron yield as a function of angle of incidence from Kapton were also determined. Secondary electron escape depth and photoelectron escape depth from Kapton were calcuated from the experimental data.Pulse beam techniques were used to reduce surface charging problems. Three μsec pulses of electrons were used in SEE experiments, and 100 msec to 1 sec pulses were used in photoemission experiments. The maximum yield of Kapton increases from 1.7 at normal incidence to 3.0 at 80° angle of incidence, Emax increases from 280 eV to 620 eV. The maximum yield of Teflon increases from 2.4 at normal incidence to 3.1 at 80° angle of incidence. The secondary electron escape depth in Kapton was calculated t be 55 ± 5 Å. Photoelectrons excited by 21 eV photons have a 87 ± 30 Å escape depth in Kapton.

Effect of Cs and Li atom adsorption on MgO: Secondary emission and work function

Adsorption of Cs and Li atoms on the surface of single crystal magnesium oxide films has been investigated using Auger, LEED and contact difference techniques. A decreased work function for a single crystal MgO film grown on the Mo (100) face was observed to be accompanied by an increased secondary electron emission yield shown to be due to a larger escape depth for secondary electrons. LEED showed well ordered layers of adsorbed Cs on the MgO film surface. A model to explain the behaviour of Cs atoms on the film surface is proposed. It is shown that the stability of the Cs coating is not dependent on a prolonged bombardment of the film by incident electron beams of high current density. Depositing and implanting of thin single crystal MgO films with Li were found to result in an increased secondary electron emission yield, with Li adsorption on the MgO film surface being disordered.

Interaction between solid nitrogen and 1–3-keV electrons

Experimental studies were made of the interaction between solid nitrogen and beams of 1–3-keV electrons. The projected range for the electrons was measured by means of the mirror-substrate method (gold substrate), giving the result 9.02×1016 E1.75 molecules/cm2 with the energy given in keV. The escape depth for secondary electrons was studied by means of the equivalent-substrate method (carbon substrate). The results varied from 280 Å at 1 keV to 400 Å at 3 keV. Measurements were also made of the secondary-electron-emission coefficient, which varied from 2.3 el/el at 1 keV to 1.2 el/el at 3 keV. At 3 keV, the SEE coefficient is 12 times that for solid deuterium. This is attributed partly to the larger production rate for low-energy electrons in nitrogen and partly to the larger escape probability for these electrons. Moreover, measurements were made of the electron-reflection coefficient, both for solid nitrogen and for the carbon substrate. For nitrogen, it varied from 0.17 el/el at 1 keV to 0.13 el/el at 3 keV, and for carbon it varied from 0.13 to 0.12. The observations are discussed and comparisons made with other theoretical and experimental results. The agreement ranges from good to fair.

Secondary electron emission from thin carbon films

For the purpose of investigating how secondary electrons are produced in carbon, the correlation between energy-loss events and secondary electrons was studied experimentally by using the coincidence method. If a secondary electron is detected in coincidence with an electron transmitted through a thin film which has lost an amount of energy E, then the process causing this energy loss results in the production of secondary electrons. We established the existence of these coincidences and have taken inelastic and coincidence spectra for films of different thickness. We found that in carbon secondary electrons are predominantly produced as a result of energy losses of about 20 eV, with an efficiency of about 5%. The escape depth of secondary electrons was also estimated to be approximately 30 Å.

Auger and secondary electrons excited by backscattered electrons; An approach to quantitative analysis

The contribution of backscattered electrons (BE) to Auger electrons (AE) and secondary electrons (SE) was studied by depositing Be onto a polycrystalline or deposited Cu substrate. The effects of backscattering on SE and maximum escape depth of SE were obtained by using the so called δ-η method. This method was also applied to the AE and the effects of BE on AE were experimentally evaluated. The AE yield versus primary energy curve which was corrected for BE was compared with other experiments and theories and considerably good agreement was obtained. From this analysis, the excitation efficiencies of AE by primary electrons and by BE could also be obtained. The absolute AE yield of Be (KVV) was estimated by “area” measurements. The changes of plasma losses, the elastic peaks, the energy distribution of BE, and the true SE were also observed as a function of deposited film thickness, and the results are discussed.

Thickness Dependence in Secondary Electron Emission from Carbon

Secondary electrons are used in making topographical pictures of specimens in the scanning electron microscope. A better understanding of the secondary emission process will contribute in improving the resolution in this mode of operation.Recent experiments have indicated first that the escape depth of secondary electrons is a few atomic layers at the surface of the solid and second that the backscattered electrons are much more efficient in producing secondaries than the incoming ones. The results vary considerably. However, any model that one makes, for example similar to that of Jonker, consistent with these recent experimental results, will have the thickness as an important parameter.

The effective cross section for ionization by electrons of molecules in hydrogen clusters

The effective cross section, σ eff, for ionization by electrons of molecules in hydrogen cluster beams is measured in the energy range from 30 to 500 eV. The mean cluster size is varied from N ≈ 340 up to N = 85000 molecules per cluster. For N ⩾ 8000 the energy dependence of σ eff shows two humps at ≈ 180 eV and ≈ 260 eV respectively which have not been observed in previous measurements of the ionization cross section of clusters. The result is explained in terms of a simple model developed previously. From the dependence of the ionization cross section of clusters on their size we conclude that the average escape depth of secondary electrons is 5.5 molecular diameter.

Zur Lage des Maximums der Ausbeutekurve bei der Sekundärelektronenemission

There exists a quantitative connection between penetration depth of primary electrons, the maximum escape depth of secondary electrons and the position of the maximum of the yield curve if one assumes: (1) the maximum of the yield curve is determined by an energyE p max of the primary electrons where the maximum penetration depth of primary electrons equals the maximum escape depth of the secondary electrons; and (2) the maximum escape depth of secondary electrons is given by 10 atomic layers for all metals investigated as it has been found earlier by Mayer and Holzl for potassium. These assumptions are confirmed experimentally by measurements of yield curves and energy distributions at defined conditions and by variation of several parameters as temperature, contamination of the surface, surface roughness, evaporation conditions, and angle of incidence of the primary electrons.