Bernhard G. Frost

An electron holographic study of electric charging and electric charge distributions

Abstract Some electron optical parts of a transmission electron microscope modify the object wave-function of an electron wave. As experimentally found by low-magnification holography, both the size of the condenser aperture and the excitation of the lenses between the electron gun and the biprism strongly modify the phase of the image wave. It appears that changing these parameters changes the charging–discharging equilibrium between sample, supporting foil, parts of the microscope, and the electron beam. By using a special procedure to process holograms which were acquired under different conditions of charge equilibrium it is possible to reveal the phase shifts caused by the charging effects only. These phase shifts observed for uniformly charged polymer particles are in excellent agreement with the phase shifts simulated using a simple model of charged spheres. A comparison of experimental and modeled phase images suggests that the charge is uniformly distributed inside the particles rather than on their surface. In addition, this comparison enables us to quantitatively correlate an increase in the positive charge to an increase in the mean inner potential.

An improved mode of operation of a transmission electron microscope for wide field off-axis holography

Abstract An improved mode of operation for low magnification off-axis electron holography is presented which permits a significant increase of the field of view over that obtained by the standard low magnification technique in a standard field emission transmission electron microscope. This improvement is achieved by a weak excitation of the objective lens. Ray diagrams for the different low magnification modes are discussed and verified on a Hitachi HF-2000 transmission electron microscope operated in free lens control mode.

On the reliability of quantitative phase measurements by low magnification off-axis image plane electron holography

We experimentally found that the object wave function of an electron wave in a transmission electron microscope can depend on the diameter of the condenser aperture and on the excitation of the objective lens. This can be seen by low magnification holograms utilizing as sample electrically charged latex spheres of different diameters, the electric field at a pn-junction and the magnetic leakage field of a magnetic memory cell.

Two-dimensional dopant profiling of ultrashallow junctions by electron holography

Electron holography using a transmission electron microscope equipped with a Moellenstedt biprism has emerged as a viable technique for creating two-dimensional voltage maps of semiconductor devices. We are presenting an introduction to this dopant profiling method. Practical details are given on sample preparation, instrumentational considerations, and data interpretation.

Holographic voltage profiling on 75 nm gate architecture CMOS devices

Voltage profiles of the source-drain region of a CMOS transistor with 75nm gate architecture taken from an off-the-shelf Intel PIII processor are presented. The sample preparation using a dual beam system is discussed as well as details of the electron optical setup of the microscope. Special attention is given to the analysis of the reconstructed holograms.

Simulation of electron phase shifts by electrostatic potential across electroceramic interfaces

An expression for the electron-wave phase shifts caused by the potential of an electric dipole is extended to a general equation representing the phase shifts caused by the electrostatic potential across a parallel-plate capacitor. The capacitor is then used to construct a simple model for the phase shifts caused by the potential barriers present across internal interfaces in solids. The phase simulations yields phase shifts for electroceramic interfaces in good agreement with the phase images reconstructed from off-axis electron holograms of acceptor-doped grain boundaries in SrTiO3.

The Side-Effects of a Non-Mechanical Shutter in the Gun Area of a Transmission Electron Microscope for Off-Axis Electron Holography

There are basically two shutter positions on a standard transmission electron microscope, one above the sample that shutters the illumination off the target, and one below the sample that shutters the image from the detector. The lower shutter is typically electro-mechanical, while the upper shutter could be either electro-mechanical, or employ electromagnetic or electrostatic beam deflection. Use of the lower shutter allows continuous observation of the target, as it closes only when acquiring an image either digitally or on film. Under those circumstances, the total radiation dose for the target is high, and consequently suitable primarily for use in imaging beam-insensitive materials. For radiation sensitive targets, e.g., biological materials, an upper shutter is preferred, as it allows minimizing the total electron dose for the target (the beam is deflected until an image is taken). In the most stringent case, only one image is taken from the target at exactly the dose required for a minimum signal/noise ratio in the image.

Low-voltage-point source microscope for interferometry

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.

Initial results with a point projection microscope

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.

Scanning electron microscopy: Present capability, future improvements and potential replacements

The continued dominance of electron-beam instruments as the tool of choice for metrology is threatened by the requirements demanded for sub-100nm device structures. The key factors required for effective metrology are identified, the limitations of existing instruments are considered, and a new type of microscope tool for metrology which avoids this limitation is proposed.