Secondary Electron Generation Research Articles

Nanometer structure fabrication attained by nanometer e-beam lithography system (NSF-1)

Exposure spreading and nanometer pattern delineation characteristics have been investigated to explore ultimate limit of e-beam lithography in pratical samples, i.e., thick resist on bulk Si. 8 nm wide, 100 nm spaced line patterns were delineated in 230 nm thick PMMA on Si, which may be close to the ultimate limit for practical exposure. Monte Carlo calculation which includes secondary electron generation can predict the experimental results with enough accuracy.

Theoretical study of the ultimate resolution in electron beam lithography by Monte Carlo simulation, including secondary electron generation: Energy dissipation profile in polymethylmethacrylate

A new Monte Carlo simulation including generation of secondary electrons has been performed to study ultimate resolution in electron beam lithography. With respect to the simulations of inelastic scattering process of the primary electrons we used Gryzinski’s excitation function for single electron excitation processes, generating secondary electrons, while the inelastic mean free path proposed by Seah and Dench for organic material and energy loss spectrum obtained by Ritsko were used for simulating the inelastic scattering processes of the secondaries. The calculations were made to obtain energy dissipation profile for 20-kV electrons, particularly, in the vicinity of the point of incidence on nanometer-scale in 3000-Å-thick films of polymethylmethacrylate (PMMA) on a silicon substrate. The result clearly indicates that the secondary electrons play a most important role in determining the energy dissipation profile in the vicinity of the point of incidence, being the significant source of broadening of the energy dissipation profile. The blurring due to secondary electrons at the surface of the PMMA resist is of the order of 10 nm.

Exposure and development simulations for nanometer electron beam lithography

A new Monte Carlo simulation including generation of secondary electrons combined with development process was applied to study ultimate resolution of electron beam lithography. With respect to the simulation of inelastic scattering processes of the primary electrons we used Gryzinski’s excitation function for single electron excitation processes, and for generating secondary electrons, while the inelastic mean free path proposed by Seah and Dench for organic materials, and the energy loss spectrum obtained by Ritsko were used for simulating the inelastic scattering processes of the secondaries. The etching process which we used is the time evolution development proposed by Neureuther et al., using the model of Hatzakis et al. for polymethylmethacrylate (PMMA). The calculations were made to assess the line‐profile resist development of 30 nm‐thick films of PMMA on 60 nm‐thick silicon substrate for line exposure by a parallel electron beam 6 nm wide at 50 keV. The result shows a developed profile given by the contour representing the shortest evolution time which intersects the substrate is ∼18 nm wide at the surface. The theoretical result was compared with the experimental study of ultimate resolution of electron beam lithography, done by Broers.

Photoemission and optical processes in multialkali photocathodes

Multialkali (${\mathrm{Na}}_{2}$KSb)Cs photocathodes have been studied in the range of photon energy of 1 to 6 eV by means of photoelectron spectroscopy, measurement of optical properties, and photoemissive quantum efficiency. The optical properties of these photocathodes are similar to the other alkali antimonides. The coincidence of the position of peaks and other similarities with the optical spectra of ${\mathrm{Na}}_{2}$KSb supports the surface heterojunction hypothesis proposed by us earlier. The pseudopotential band-structure calculation by Klimin et al. on these materials has been used to identify the possible transitions taking place in the Brillouin zone corresponding to the observed structures in the optical spectra. From photoelectron spectroscopy the optical transitions appear to be of direct type, which puts this material in sharp contrast with other alkali antimonides, all of which showed the features of nondirect transitions. The threshold energy of electron-electron scattering for the photoexcited electrons have been found to be 2.3 eV which is more than twice the band gap above the conduction-band edge. The mean free path for this scattering has been found to be decreasing with increase of energy of the photogenerated electrons and, at an excitation energy of 6 eV, it is 43 \AA{}. Apparently, the scattered electrons fall into a valley located 0.95 eV above the bottom of the conduction band. The increase of quantum efficiency with decrease of absorption after the onset of electron-electron scattering probably takes place due to generation of secondary electrons from the valence band due to scattering. Some of the peaks in the photoelectron spectra have been identified with the transitions predicted through the band-structure calculations.

Study of generation of secondary electrons and radiation defects in alkali halides irradiated with swift electrons

Abstract The delta-electron generation by primary electrons with energies from 1.0 to 10 MeV in various absorbers was simulated for one dimensional geometry by the use of the “segments model”. The depth distribution of moderated electrons and radiation damage in model alkali halide absorbers were calculated. In experiments the depth profiles of colour centre density in KCI and NaCl crystals, irradiated with 1.0 and 2.0 MeV electrons at the incidence angle θ = 0° 20° 45° were measured. The integral dose of incident beam was varied from 1.1014 to 6.1015 cm−2. The depth density of colour centres is found to be determined by the moderated delta-electron distribution for irradiation doses smaller than 1015 cm−2. Along the <100> direction of KCI and NaCl crystals a distinct anisotropy of depth colouration profiles as well as F- and M-centre density distributions is observed.

Application of Monte Carlo calculation to fundamentals of scanning Auger electron microscopy

Spatial distributions of Auger signals generated in an aluminum target in scanning Auger electron microscopy were obtained by Monte Carlo calculations including secondary electron generation. In the low-energy region, the cross sections calculated by the partial wave expansion method were used instead of the screened Rutherford cross section to describe the elastic scattering process. The result suggests that secondary electrons of high energy are a significant source of Auger signals, particularly LVV-Auger electrons, in scanning Auger electron microscopy.

Estimation of dose to the urinary bladder and to the gonads

The cellular microdosimetry parameters including the cellular S-value and the single-event specific energy distribution for alpha particles and electrons are important in radiation dosimetry and biology. These parameters may be used to determine the relative biological effectiveness of radiations in the boron neutron capture therapy. In the present work, such parameters were calculated for different source to target region combinations, i.e. cell surface, cytoplasm, nucleus and cell. Calculations were made using a semi-analytical model that simulated the emission of alpha particles or electrons by the Monte Carlo method and calculated the energy imparted to the target volume by the analytical method. Delta particle equilibrium and partial delta particle equilibrium were applied to alpha particles and electrons, respectively. Range–energy relations were employed to determine the incident and emerging energies of the primary particles. For electrons, the fraction in the energy loss resulting from the generation of bremsstrahlung and high-energy secondary electrons was estimated. The energy loss straggling of electrons entering and leaving a target volume was also estimated. Calculated cellular S-values were compared to corresponding data of the MIRD Committee. Calculated single-event specific energy distributions were also compared to results calculated using the Penelope code.

Generation of secondary electrons and radiation defects in matter during absorption of electrons with energy above 1 MeV

Using the model of sections with one-dimensional geometry, the output of δ electrons generated by electrons with energies from 1 to 10 MeV is considered as a function of the thickness and the atomic number of various absorbers. The depth distributions of thermalized electrons and radiation defect profiles are calculated for the model absorbers KCl and NaCl. Radiation defects due to irreversible displacement of lattice atoms upon elastic scattering of rapid electrons are considered. The microphotometry method is used to obtain depth distributions of enrichment centers in the ionic crystals KCl and NaCl for irradiation by electrons with energies E=1, 1.5, and 2 MeV for electron-beam incidence on the absorbers at angles ofΘ0=0, 20, 45°. The integral density of the incident beam was varied from 1·1014 to 6·1015 cm−2. It was found that in ionic crystals the enrichment center density is proportional to the density of thermalized δ electrons, if the concentration of radiation defects generated is less than 1% of the density of preradiation anion vacancies.

Multiple scattering of fast electrons and their secondary electron generation within semi-infinite targets

By means of a new film-bulk method, the absorption depth distribution of fast electrons within Be, Al, Cu and Au targets as well as their inner secondary electron SE generation was measured directly. The primary electrons (PE) had initial energies of EO=1-10 keV. The most probable and average absorption depths xm and x respectively were determined as functions of the maximum range R(Z, EO) in the various target materials of atomic number Z. The characteristic of the inner SE generation has a maximum near the surface and agrees qualitatively with the depth dependence of the energy transfer of scattered PE to the solid obtained by other authors from Monte Carlo calculations. By integration of the measured SE generation functions, the total number of SE within the bulk targets as well as the mean excitation energy E1 for a single SE were determined.

Current-gain characteristics of Schottky-barrier and p-n junction electron-beam semiconductor diodes

Measured and calculated current gains are presented for silicon, gallium-arsenide, and gallium-arsenide-phosphide electron-beam-semiconductor Schottky-barrier diodes. Silicon and gallium-arsenide-phosphide (GaAs 0.7 P 0.3 ) diodes, having comparable critical voltages, provided current gains of 1400 and 960, respectively, at the beam voltage of 9 kV. A gallium-arsenide diode had a gain of 2100 at 12.7 kV. It is shown that Schottky-barrier diodes provide a good diagnostic tool for measurement of energy per electron-hole pair in semiconductors. The average pair energies for silicon, gallium arsenide, and gallium arsenide phosphide, determined by use of these diodes, were 3.6, 4.6, and 5.2 eV, respectively. Expressions for static and time-dependent current gains are derived for abrupt p-n-junction EBS diodes based on measured and simplified (constant) lineal densities of secondary-electron generation. It is shown that the current gain of p-n junctions is generally lower than that of Schottky-barrier diodes and decreases with increasing density of the semiconductor. For shallow (<0.2 µmr) silicon junctions, the calculated current gain is only slightly smalle. (<8 percent) than that of comparable Schottky-barrier diodes Measurements performed on a relatively deep (0.8 µm) silicon p-n junction, which provided a current gain of 2500 at the beam voltage of 13.6 kV, are in good agreement with the theory. The gain of this junction was typically 40 percent lower than that of a comparable Schottky-barrier diode. This difference was significantly greater for gallium arsenide as a result of its higher density. Approximate expressions and calculations are presented for estimating frequency limitations of p-n-junction EBS targets.

Total secondary electron emission from polycrystalline Nickel

The importance of secondary electron emission in its relation to the excitation of soft X-rays has been pointed out in a recent paper by Prof. O. W. Richardson. He has shown that at every potential where there is an increased excitation of soft X-rays, there is correspondingly an increase in the emission of secondary electrons, and has discussed at some length the mechanism of the generation of secondary electrons. It was therefore felt that a much clearer idea of the phenomenon of soft X-ray excitation from metallic surfaces could be had by studying the secondary electron emission from polycrystalline and single crystal faces. As early as in 1908 Richardson showed that slowly moving electrons are reflected in considerable proportion from metallic plates. Davisson and Kunsman, in a series of papers commencing from 1921, showed that at low voltages up to about 9 volts most of the secondary electrons were purely reflected electrons with velocities the same as the incident electrons. The percentage of the reflected electrons fell rapidly as the applied potential was increased above 9 volts, while that of low velocity electrons increased steadily. Farnsworth, with improved apparatus, added much valuable information regarding the generation of secondary electrons and the conditions operating in such cases. These observers showed that the total emission of secondary electrons from a metal surface depended on the applied potential, the nature of the surface and the previous heat treatment of the metal. They also found that the ratio of the secondary beam to the primary increases with applied potential and becomes greater than 1 after a certain potential, depending on the nature of the bombarded metal, is reached.

The emission of secondary electrons and the excitation of soft X-rays

Experiments I have recently made in collaboration with Dr. F. C. Chalklin on the one hand and with Mr. F. S. Robertson on the other, together with some observations not yet published made in the Wheatstone Laboratory by Mr. E. Rudberg, taken in conjunction with Krefft’s results for the secondary electron emission from baked tungsten, throw a good deal of light on the mechanism of the generation of secondary electrons at the surfaces of solids, particularly in the range where the energy of the primary electrons is sufficient to generate soft X-rays. I shall first consider what are the chief essential facts from this point of view as to what happens when a beam of electrons falls on a conductor. In 1908 I found that a considerable proportion of slowly moving electrons was reflected by a metal plate; in the particular case of the electrons coming from a hot platinum strip under no applied electric force on to a brass plate, I estimated the proportion reflected at roughly 30 per cent. A similar result with electrons of 2, 4 and 8 volts, equivalent energy was obtained independently about the same time by von Baeyer. Since that time a number of investigations of electron reflection at conductors have been published. Speaking broadly, it appears that with increasing energy of the primary electrons the proportion reflected, increases to a maximum, at a value which is in the neighbourhood of 11 volts for a number of metals, falls to a minimum at a value which is comparable to 30 volts, rises to a second maximum at a value which is of the order of 200 volts, and then falls off slowly and continuously with further increase in the energy. These results vary to some extent with the nature and treatment of the metal surfaces, but it is important to observe that there is generally some range of voltage in which the number of secondary exceeds the number of primary electrons. This is usually in the region in which the soft X-ray emission becomes important. Farnsworth examined the electrons emitted from a nickel plate and found that with primary electrons having 9 volts energy or less, a large proportion of the secondary electrons had an amount of energy nearly equal to that of the primary electrons, and but a small proportion had a velocity of 1 volt or less. As the energy of the primary electrons was increased above 9 volts, the proportion of low velocity electrons steadily increased and the proportion of secondary electrons having energy close to that of the primary electrons steadily fell to a very small percentage at 110 volts, a result previously obtained by Davisson and Kunsman.