Contribution Of Backscattered Electrons Research Articles

Proximity effects in formation of photonic crystals by lithographic methods

Proximity effects in formation of photonic crystals in the form of ordered arrays of holes of similar radii close to 100 nm by methods of electron beam lithography are considered. The coefficients of the proximity function characterizing the contribution of back-scattered and secondary electrons to the exposure dose are experimentally determined. It is demonstrated that the minimum standard deviation from the mean radius of the elements in the array is provided by means of the proximity effect correction with the use of experimentally obtained coefficients and an iterative equation with an increased contribution of back-scattered electrons.

Nanoscale elemental quantification in heterostructured SiGe nanowires.

The nanoscale chemical characterization of axial heterostructured Si1-xGex nanowires (NWs) has been performed using scanning Auger microscopy (SAM) through local spectroscopy, line-scan and depth profile measurements. Local Auger profiles are realized with sufficient lateral resolution to resolve individual nanowires. Axial and radial composition heterogeneities are highlighted. Our results confirm the phenomenon of Ge radial growth forming a Ge shell around the nanowire. Moreover, quantification is performed after verifying the absence of preferential sputtering of Si or Ge on a bulk SiGe sample. Hence, reliable results are obtained for heterostructured NW diameters higher than 100 nm. However, for smaller sizes, we have noticed that the sensitivity factors evaluated from bulk samples cannot be used because of edge effects occurring for highly topographical features and a modified contribution of backscattered electrons.

Variation of backscatter electron intensity

The authors report experimental data on the dose contribution of backscattered electrons on a silicon substrate. The backscattered electron intensity, i.e., the relative dose contribution from backscattered electrons with respect to direct write dose, is not constant, but varies with the percentage of the total dose received by backscattered electrons. In order to measure the backscattered electron contribution, the position and number of electrons are controlled by electron beam lithography. The dose contribution is measured using a negative electron beam resist, which quantifies the electron interactions in a nanoscale volume on the surface of the substrate. The data presented here will lead to improvements in proximity effect correction algorithms and ultimately improve pattern creation using electron beam lithography.

Photoresist Shrinkage Caused by Single-Line Scan of Electron Beam

Shrinkage behavior caused by a single-line scan of an electron beam over a photoresist line was observed, including shrinkage distribution in the photoresist-line direction. A new method for evaluating the minute amount shrinkage and the shrinkage distribution caused by a single-line scan was developed. According to the results of evaluations with this method, the shrinkage of an about 50-nm-wide photoresist line caused by a single-line scan is less than 0.1 nm under landing energies of 200, 300, and 500 eV and probe current of 8 pA. This shrinkage is more than ten times smaller than the typical amount of shrinkage caused by a standard two-dimensional scan. This result indicates the possibility of a significant reduction of photoresist shrinkage during scanning-electron-microscope measurements. The evaluation method also yielded the first observation of the shrinkage distribution in the photoresist-line direction. The results show that the shrinkage caused by a single-line scan distributes more than 30 nm, which is wider than the calculated electron-scattering range. This result suggests that there likely to be an additional mechanism involved in photoresist shrinkage other than the microscopic interaction between incident electrons and photoresist molecules. An elastic-relaxation effect and a contribution of back-scattered electrons are plausible additional mechanisms for photoresist shrinkage.

Analysis of Resist Surface Deformation during Electron Beam Irradiation

It is known that polymers mostly shrink during observation by scanning electron microscope. However, resist surface deformation during electron beam irradiation has not been discussed in electron beam lithography. In the present study, poly(methyl methacrylate) (PMMA) film is taken as an example of polymers, and the surface deformation by electron beam irradiation is measured by atomic force microscope. Exposure dose varies in a range to obtain the exposure intensity distribution (EID) curve for point exposure, that is from 10-11 to 10-6 C at an acceleration voltage of 20 kV. Even at a dose of 5×10-10 C, where a contribution of backscattered electrons from the Si substrate becomes significant in the distribution, a dimple of around 1 µm radius and around 30 nm depth is produced. The EID curve derived from the experiment might be influenced by the deformation. Assuming that the development process starts from this deformed surface, it should be a serious problem in lithography, especially for low-sensitivity resists. The deformation is not only shrinkage, but also deposition along the beam axis for the resists. In the present study, the surface deformation is analyzed in terms of the electron-beam-induced shrinkage and deposition. A simulation model is presented to quantify the mechanism, and the range of its validity is discussed.

Contribution of backscattered electrons to the total electron yield produced in collisions of 8–28 keV electrons with tungsten

It is shown experimentally that under energetic electron bombardment the backscattered electrons from solid targets contribute significantly (∼80%) to the observed total electron yield, even for targets of high backscattering coefficients. It is further found that for tungsten (Z = 74) with a backscattering coefficient of about 0.50, about 20% of the total electron yield is contributed by the total secondary electrons for impact energies in the range of 8–28 keV. The yield of true backscattered electrons at normal incidence (η0), total secondary electrons (δ) and the total electron yield (δtot) produced in collisions of 8–28 keV electrons with W have been measured and compared with predictions of available theories. The present results indicate that the constant-loss of primary electrons in the target plays a significant role in producing the secondary electrons and that it yields a better fit to the experiment compared to the power-law.

Study of increased radiation when an x-ray tube is placed in a strong magnetic field

When a fixed anode x-ray tube is placed in a magnetic field (B) that is parallel to the anode-cathode axis, the x-ray exposure increases with increasing B. It was hypothesized that the increase was caused by backscattered electrons which were constrained by B and reaccelerated by the electric field onto the x-ray tube target. We performed computer simulations and physical experiments to study the behavior of the backscattered electrons in a magnetic field, and their effects on the radiation output, x-ray spectrum, and off-focal radiation. A Monte Carlo program (EGS4) was used to generate the combined energy and angular distribution of the backscattered electrons. The electron trajectories were traced and their landing locations back on the anode were calculated. Radiation emission from each point was modeled with published data (IPEM Report 78), and thus the exposure rate and x-ray spectrum with the contribution of backscattered electrons could be predicted. The point spread function for a pencil beam of electrons was generated and then convolved with the density map of primary electrons incident on the anode as simulated with a finite element program (Opera-3d, Vector Fields, UK). The total spatial distribution of x-ray emission could then be calculated. Simulations showed that for an x-ray tube working at 65 kV, about 54% of the electrons incident on the target were backscattered. In a magnetic field of 0.5 T, although the exposure would be increased by 33%, only a small fraction of the backscattered electrons landed within the focal spot area. The x-ray spectrum was slightly shifted to lower energies and the half value layer (HVL) was reduced by about 6%. Measurements of the exposure rate, half value layer and focal spot distribution were acquired as functions of B. Good agreement was observed between experimental data and simulation results. The wide spatial distribution of secondary x-ray emission can degrade the MTF of the x-ray system at low spatial frequencies for B < 0.5 T.

Monte Carlo calculation of the contribution of backscattered electrons to secondary electron emission from aluminium

A Monte Carlo model for the simulation of electron penetration into aluminium and for the generation and emission of secondary electrons is presented. Monte Carlo calculations for the penetration and backscattering of primary electrons show good agreement with experiment, while the dependence of the secondary electron yield on the primary electron energy show qualitative agreement with experimental measurements. beta , which is defined as the relative efficiency of backscattered electrons for producing secondary electrons which escape the surface over that of the primary electrons on their inward path, is found to be approximately 6-7 for primary electrons in the energy range 3-25 keV.

A model for calculating secondary and backscattered electron yields

SUMMARYImages in the scanning electron microscope (SEM) are formed from both low‐energy secondary, and high‐energy backscattered, electrons. The quantitative interpretation of SEM images therefore requires a model which can predict the magnitude of both of these signal components for a specimen whose geometry and chemistry is known. It is shown that the combination of a simple electron diffusion model with a Monte Carlo trajectory simulation allows both yields to be calculated, simultaneously, with good accuracy. Data, such as the magnitude and energy of the maximum secondary yield, the secondary variation with tilt, and the contribution of backscattered electrons to the secondary yield coefficient, computed from this model are in excellent agreement with experimental data.

Characterization and performance improvement of secondary electron analyzers

The detection of voltage contrast in scanning electron microscopes requires the use of secondary (SE) analyzers that allow the SE spectrum shift to be determined accurately and with high signal-to-noise ratio. In order to improve the performance of a given analyzer, a characterization scheme has been developed that allows one to pinpoint critical detector parts and provides guidelines for design changes. Performance determining factors, e.g., the operation point of the analyzer, the contribution of backscattered electrons (BSE) and SEs generated by BSEs (tertiary electrons) to the background signal, and changes of the S-curve shape, are determined separately by using specially developed characterization techniques. The optimum analyzer operation point, where the signal difference between two sample voltages is a maximum, is determined by simultaneously varying the retarding voltage and the voltage at the sample. Reflecting the primary electron beam with a negatively biased electrode simulates BSEs and reveals areas of high tertiary electron generation. Replacing the sample by a low energy electron gun with a small energy spread simulates SEs. This allows a detailed energy resolution determination of the analyzer to be performed.

Characteristics of a miniature dosimeter developed for measurement of electrons

Characteristics of the miniature dosimeter developed for the measurement of electrons are described in this report. This dosimeter is very easily used without applied bias voltages. This is a silicon diffused n-p + junction device. The silicon detecting element of this dosimeter has the form of a disk, and the front area is 1.72 × 10 −2 cm 2. The energy range of electrons used in this experiment is 0.6–1.8 MeV. The electron-induced currents were proportional to the number of incident electrons over the range of 1.5 × 10 4–1.5 × 10 9 electron/s, and the energy absorbed in the sensitive volume of this dosimeter was about 39 keV per incident electron and had little dependence on the electron energy range of 0.8–1.8 MeV. The depletion layer of this dosimeter and the contribution of backscattered electrons to the total energy absorbed in the sensitive volume were about 90 μm and about 15%, respectively.

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.

Auger and secondary electrons excited by backscattered electrons

The contribution of backscattered electrons to Auger and secondary electron yields was examined by in situ measurements while evaporating Be onto Cu substrate at constant rate of 1.2 Å/min. The ratios βA and βs of Auger (KVV) and secondary yields of Be due to backscattered electrons to those due to primary electrons were experimentally obtained from the Auger and true secondary electron yields versus backscattering coefficient η curves, and were evaluated to be 1.6 and 4.4 at a primary energy of 1.0 keV, respectively. The backscattering factor r for Be (KVV Auger) was experimentally found to be 1.1 at a primary energy of 1.0 keV.

Auger and Secondary Electrons Excited by Backscattered Electrons

The contribution of backscattered electrons to the Auger and secondary electron yields was examined by in-situ measurements while evaporating Be onto a Cu substrate at constant rate, 1.2 Å/min. The ratios, βA and βS, of the Auger (KVV) and secondary yields of Be due to backscattered electrons, to those due to primary electrons, were experimentally obtained from the Auger and true secondary electron yields versus backscattering coefficient η curves, and were evaluated to be ∼1.6 and ∼4.4 at a primary energy of 1.0 keV, respectively. The backscattering factor r for Be (KVV Auger) was experimentally found to be ∼1.1 at a primary energy of 1.0 keV.

Range of electrons and contribution of back-scattered electrons in secondary production in aluminium

The range of electrons as a function of primary energy below 2 keV, the escape depth of slow secondaries and the efficiency of back-scattered electrons in the production of slow secondaries have been determined in thin aluminium layers deposited on platinum, gold and silver substrates, using an automatic yield measuring device which enables the measurements to be made from contamination-free surfaces.

Contribution of Backscattered Electrons to Secondary Electron Formation

It is shown experimentally that backscattered electrons emitted from solids under electron bombardment contribute significantly to the observed secondary yield, even for the case of low backscattering coefficients. Thus, it was found that in Al with a backscattering coefficient of only 0.14, about 40% of all secondaries are produced by backscattered electrons for initial energies from several kev to several tens of kev. The large contribution of backscattered electrons to secondary formation even for materials of low atomic number agrees approximately with what one would expect from the larger rate of energy loss and the greater path lengths of the backscattered electrons in the secondary electron escape region compared to that of the incoming primaries.