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We have designed a low voltage point source microscope operated with a nanotip field emitter and without any electron optical lenses such that it can be operated in the transmission mode as well as in a reflection mode. An electron gun needed in the reflection mode is the most important difference between the two modes of operation. The magnification of the object wave is achieved by placing the specimen in the divergent electron beam of a nanotip field emitter and observing the object wave using a micro channel plate (MCP) at a great distance from the sample. As no lenses are present a 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 and contain true three dimensional information of the sample. The transmission mode gives us the following results: (1) Electrons diffracted at an edge of the specimen cause Fresnel fringes in the image plane. (2) An electrically charged holey carbon foil can act in the same way on the electrons as the Young's double slit experiment and results in an interference pattern consisting of parallel fringes. The reflection mode shows us: (1) The image depends on the angle of reflection. (2) An arrangement similar to a field ion microscope resolves single atoms of the specimen. [DOI: 10.1380/ejssnt.2004.81]

We briefly discuss how to set-up our Hitachi HF-2000 transmission electron microscope for medium magnification holography. Then we apply this technique to examine the activation of an as-doped wafer by annealing. We also present voltage profiles of the source-drain region of a CMOS transistor with 75 nm gate architecture taken from an off-the-shelf Intel PIII processor. Special attention is given to the analysis of the reconstructed holograms. [DOI: 10.1380/ejssnt.2004.119]

Digital recording has become a basic requirement for electron holography for many reasons. The fact that it allows live-time evaluation of the phase information and easy recording of a reference hologram are two very important reasons that are widely appreciated. Here we will discuss requirements for recording electron holograms under the special conditions imposed by the Nyquist limit and the modulation transfer function (MTF) of the charge-coupled device (CCD) camera. As electron holography provides complex images carrying both the amplitude and phase of the image wave, the question of how to best display the information will be investigated. This is not an easy question, because special aspects of different applications require different solutions. Methods for display and evaluation of holographic data are described. Published by Elsevier Science Inc., 1999.

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.

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.

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.

This study describes the use of transmission electron holography to determine the mean inner potential of polystyrene. Spherical nanoparticles of amorphous polystyrene are studied so that the effect of specimen thickness on the phase shift of an incident electron wave can be separated from the intrinsic refractive properties of the specimen. A recursive four-parameter χ-squared minimization routine is developed to determine the sphere center, radius, and mean inner potential (Φ0) at each pixel in the phase image. Because of the large number of pixels involved, the statistics associated with determining a single Φ0 value characteristic of a given sphere are quite good. Simulated holograms show that the holographic reconstruction procedure and the χ-squared analysis method are robust. Averaging the Φ0 data derived from ten phase images from ten different polystyrene spheres gives a value of Φ0PS = 8.5 V (σ) = 0.7 V). Specimen charging and electron-beam damage, if present, affect the measurement at a level below the current precision of the experiment.

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.

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.

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 perfection 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.

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.

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.

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 ultralow energy electrons (50-250eV) 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 (crossover of a lens) 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.

We simulated in-line holograms as obtained from a point projection microscope (PPM) [1]. The principal of the PPM is shown in Fig.1. A coherent electron wave emitted from a point source illuminates a sample transparent to the wave and travels to an electron detector. Since the wave from a point source is divergent the distance d of two points in the object plane results in a magnified distance D in the detector plane. The magnification of this arrangement is given by M=D/d=(z1+z2)/z1, where z1 and z2 are the tip to sample distance and the sample to detector distance, respectively. As no lenses are used we find a highly defocused shadow image in the detector plane. This arrangement is equivalent to Gabor's in-line holography with a physical source in plane of a lens-focused crossover, and the out-of-focus images are in-line electron holograms.

We briefly describe the basics of electron holography, the hologram formation, as well as its acquisition and reconstruction. Our test sample requires a holographic field of view in the medium magnification range of about 20–30 k×. Two of the microscopes (Philips) utilized for this Round Robin experiment feature a Lorentz lens for medium magnification whereas the third one (Hitachi) requires a special lens current setting in its free lens control mode. The phase images from electron holograms reveal potential differences caused by active dopants. The comparison between the phase images reconstructed from our test holograms taken with the different microscopes show excellent agreement.

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.