kV Beam Voltage Research Articles

Fabrication and characterization of gold plated edges on thin substrates for measurement of e-beam diameter

Gold plated edges were fabricated on various substrates and were used as targets for precise electron beam diameter measurement and for system calibration in a commercial Cambridge Instruments e-beam lithography machine. Use of 4 μm square gold plated marker (0.85 μm thick), fabricated on 2 μm thick silicon nitride membrane, was found to enhance significantly the marker-to-substrate signal ratio, in comparison to commonly used lifted-off marker, and thus has improved the reliability of beam diameter measurement accuracy at low beam current for a 40 kV beam voltage. In a statistical sampling of about 2000 measurements, an accuracy of 5 nm (1 sigma value) has been obtained for 38 nm beam diameter by using this gold plated edge on silicon nitride membrane. Further, a Monte Carlo program, which models the effects of electron scattering taking place in target and substrate was used to increase the understanding of the experimental results. Simulated contrast behaviour was found to agree well with the experimental data for all three types of marker used in this study. Finally, the influence of the geometrical shape of the marker edge on beam diameter measurement was studied. By using an appropriate analytical function it was possible to take into account the effect of not perfectly vertical edge on small beam diameter measurements.

Primary considerations for image enhancement in high‐pressure scanning electron microscopy

SUMMARYThe mechanisms of electron beam scattering are examined to evaluate its effect on contrast and resolution in high‐pressure scanning electron microscopy (SEM) techniques reported in the literature, such as moist‐environment ambient‐temperature SEM (MEATSEM) or environmental SEM (ESEM). The elastic and inelastic scattering cross‐sections for nitrogen are calculated in the energy range 5–25 keV. The results for nitrogen are verified by measuring the ionization efficiency, and measurements are also made for water vapour. The effect of the scattered beam on the image contrast was assessed and checked experimentally for a step contrast function at 20 kV beam voltage. A considerable degree of beam scattering can be tolerated in high‐pressure SEM operation without a significant degradation in resolution. The image formation and detection techniques in high‐pressure SEM are considered in detail in the accompanying paper.

FOCUSED DROPLET BEAM FROM A GOLD LIQUID METAL ION SOURCE

A gold LMIS has been used in the charged droplet emission mode with a single lens focusing column operating at 20 kV beam voltage to achieve a deposit size at the target of 2.3 to 6.0 depending on the exposure time. The net mass deposit rate maximized at 1.5 µm3/s. From the focusing characteristics an energy spread of 50 V and an energy deficit of 350 to 500 V could be inferred for the charged droplet beam.

Improvements in electron microscopy by application of superconductivity

Resolution tests on amorphous carbon foils were carried out in an electron microscope with a superconducting system containing 4 lenses including a shielding lens at 200 kV beam voltage. Due to the mechanical and electrical stability of the system and the absence of contamination of the specimen the highest space frequencies transferred at vertically incident beam were 6 nm-1 corresponding to a resolution of 0.17 nm, a value which approaches the theoretical resolving power of the electron optical system. It should also be feasible to apply such a lens system for microprobe analysis without strongly reducing the theoretical resolution limit, if the construction of the shielding lens is slightly changed.

Klystrons for the Stanford linear accelerator center

A 2-mile-long linear accelerator has been built at Stanford University that uses 245 klystrons to produce an electron beam with an energy up to 20 GeV. Although most of the tubes used are being procured from industry, a major portion of the development work has been and is still being done at Stanford. We will review the important results of this work. In general, the klystrons are designed to produce a minimum of 21 MW peak power with extremely stringent phase and amplitude stability performance requirements. They are focused by permanent magnets and operate up to 250 kV beam voltage with a microperveance of 2. During the past few years, gradual improvements in performance have been achieved by optimizing the bunching parameter and the output cavity coupling, and we have measured efficiencies approaching 45 percent in a permanent magnet. Permanent magnet structure limitations, field shaping, and changes in the klystron design to reduce the magnetic field requirements will be discussed. The output window life has been improved by coating techniques developed to decrease single surface multipactor and reduce window operating temperature. Questions of coating stability will also be discussed. Finally, the paper will touch on the initial results of statistical analysis of tube life as a function of operating experience.