Dr. Bernhard G. Frost Profile

Dr. Bernhard G. Frost

Research Associate at Oak Ridge National Lab

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Publications (3)

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Proceedings Article | 16 July 2002 Paper

Low-voltage-point source microscope for interferometry

Bernhard Frost, David Joy, Alexander Thesen

Proceedings Volume 4689, (2002) https://doi.org/10.1117/12.473498

KEYWORDS: Microscopes, Reflection, Carbon, Interferometry, Lenses, Microchannel plates, Sensors, Electron microscopes, Electron beams, Holograms

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Conventional scanning electron microscopes are now close to the limit of their performance for tasks such as the metrology of sub-micron design rule devices. In order to overcome these limits we have designed, and are presently testing, a low voltage point source microscope operated with a nanotip field emitter and without any electron optical lenses. The microscope is designed such that can be operated in the transmission mode as well as in a reflection mode. The ultra-sharp field emitter delivers emission currents of several nanoamps at energies less than 100 eV. The magnification of the object wave is achieved by placing the specimen in the divergent electron beam from the nanotip and observing the object wave using a microchannel plate (MCP) at a great distance from the sample. Images obtained that way are out of focus images. As no lenses are present a special procedure for scaling the magnification has been developed. Since electrons from a point source are highly coherent the out of focus images of the sample are interferograms. Electrons diffracted at an edge of the specimen cause Fresnel fringes in the image plane. An electrically charged holey carbon foil acts in the same way on the electrons as the Youngs double slit experiment and results in an interference pattern consisting of parallel fringes. A comparison between the transmission mode and the reflection mode shows great similarities with respect to the magnification and the interference pattern. An electron gun needed in the transmission mode is the most important difference between the two modes of operation. The experimental results at a reflection of 45 degrees are in good agreement with our simulation. Following our simulations a reflection angle of 90 degrees is most promising for easiest image interpretation.

Proceedings Article | 22 August 2001 Paper

Initial results with a point projection microscope

Bernhard Frost, David Joy

Proceedings Volume 4344, (2001) https://doi.org/10.1117/12.436765

KEYWORDS: Holograms, Microscopes, Holography, Sensors, Electron microscopes, Reflection, Projection systems, 3D image reconstruction, Fourier transforms, Distance measurement

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Conventional scanning electron microscopes are now close to the limit of their performance for tasks such as the metrology of sub-micron design rule devices. In order to overcome these limits we are investigating the use of in-line electron holography for device metrology. The in-line holograms are formed in a point projection microscope using ultra-low energy electrons (50-250eV) emitted from a nano-tip electron source. Holograms in the transmission mode and in the reflection mode of the microscope as well are possible. Since these in-line holograms are equivalent to out of focus micrographs acquired in a transmission electron microscope with a field emission gun we can reconstruct the original wave front by means of Fourier optics. The resolution of the point projection microscope is given by the sharpness of the emitter. We investigate the electric potential of the emitter using off-axis electron holography in a transmission electron microscope and compare the results to simulations obtained by solving the appropriate Laplace equation.

Proceedings Article | 25 June 1999 Paper

In-line holography using a point source

Bernhard Frost, David Joy

Proceedings Volume 3676, (1999) https://doi.org/10.1117/12.351149

KEYWORDS: Holograms, Microscopes, Sensors, Optical simulations, Holography, Chemical species, Ray tracing, 3D image reconstruction, Reflection, Electron microscopes

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Conventional scanning electron microscopes are now close to the limit of their performance for tasks such as the metrology of sub-micron design rule devices. In order to overcome these limits we are investigating the use of in- line electron holography for device metrology. This device will use ultra-low energy electrons emitted from a nano-tip electron source. Out of focus holograms of a mesh were simulated and reconstructed in the transmission mode of the microscope whereas in the reflection mode a sample consisting of only two points was used. In both operation modes of the microscope it is possible to change the distance from the point source to the sample and the distance from the sample to the detector plane such that the magnification is kept constant. Series of simulated holograms consisting of only a few points reveal the distances resulting in the easiest interpretable images. When in-line holography is performed using electrons, the beams are deflected by the electric field between point source and sample. Ray tracing of the electrons performed by solving the appropriate Laplace equation can help to determine an optimum geometry for the microscope.

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