P. Kruit

Field emission energy distributions from individual multiwalled carbon nanotubes

Abstract We measured field emission energy distributions of electrons emitted from individual multiwalled carbon nanotubes mounted on tungsten tips. The shape of the energy distribution is strongly sample dependent. Some nanotube emitters exhibit an almost metallic behaviour, while others show sharply peaked energy distributions. The smallest half-width we measured was only 0.11 eV, without correction for the broadening of the energy analyzer. A common feature of both types of carbon nanotube energy spectra is that the position of the peaks in the spectrum depends linearly on the extraction voltage, unlike metallic emitters, where the position stays in the vicinity of the Fermi level. With a small modification to the field emission theory for metals we extract the distance between the highest filled energy level of the nanotube and the vacuum potential, the field on the emitter surface, the emitter radius and the emitting area, from the energy distribution and the Fowler–Nordheim plot. The last two parameters are in good agreement with transmission electron micrographs of such samples. The sharply-peaked energy distributions from other samples indicate that resonant states can exist at the top of the nanotube.

MAPPER: high-throughput maskless lithography

MAPPER Lithography is developing a maskless lithography technology. The technology combines massively-parallel electron-beam writing with high speed optical data transport used in the telecommunication industry. The electron optics generates 13,000 electron beams that are focused on the wafer by electrostatic lens arrays which are manufactured by using MEMS manufacturing techniques. Each beam has its own optical column to avoid a central cross-over. This secures high throughput (> 10 wafers per hour) at high resolution (< 45 nm half pitch). The 13,000 e-beams are generated by splitting up a single electron beam that originates from a single electron source and are finally accelerated to 5 kV to expose the resist on the wafer. The e-beams are arranged in such a way that they form a rectangular slit with a width of 26 mm, the same width of a field in an optical stepper. During exposure the e-beams are deflected over 2 μm perpendicular to the wafer stage movement. This means that with one scan of the wafer a full field of 26 mm x 33 mm can be exposed. During the simultaneous scanning of the wafer and deflection of the electron beams the beams are switched on and off by 13,000 light signals, one for each e-beam. The light beams are generated in a data system that contains the chip patterns in a bitmap format. This bitmap is divided over 13,000 data channels and streamed to the ebeams at 1-10 GHz. This paper will explain the design drivers behind the system and provide more detail on the current design. Finally, results of our technology Demonstrator are presented, showing the viability of MAPPERs concept.

Reduced brightness of the ZrO/W Schottky electron emitter

The reduced brightness for a 〈100〉 ZrO/W Schottky electron emitter with a tip radius of 0.8 μm has been measured. The maximum reduced brightness measured was 2×108 A/(m2u200asru200aV). The measurements of the reduced brightness are compared with the extended Schottky theory and the theory on stochastic Coulomb interactions. At high angular current densities the reduced brightness is limited by statistical Coulomb interactions in the gun lens region. The limits to the maximum reduced brightness in an ideal configuration are explored and found to be 2×109 A/(m2u200asru200aV) for a 0.2 μm tip and a current limiting aperture in the extractor electrode.

OPTICS AND DESIGN OF THE FRINGE FIELD MONOCHROMATOR FOR A SCHOTTKY FIELD EMISSION GUN

Abstract For the improvement of high-resolution electron energy loss spectroscopy a new electron source monochromator, based on the Wien filter principle, is presented. In the fringe field monochromator the electric and magnetic filter fields are tightly enclosed by field clamps to satisfy the Wien condition, E=vB. The whole monochromator including the 150xa0nm energy selection slits (Nanoslits) is positioned in the gun area. Its total length is only 42xa0mm. Using electron trajectory simulation through the filter fields the dispersion and aberrations are determined. The parasitic astigmatism of the gun lens needs to be corrected using an electrostatic quadrupole field incorporated in the filter. Estimations of the influence of filter electrode misalignment show that at least six filter electrodes must be used to loosen the alignment demands sufficiently. Using theoretical estimations of the Coulomb interaction the final energy resolution, beam brightness and current are predicted. For a Schottky field emission electron gun with typical brightness of 108xa0A/srxa0m2xa0V the monochromator is expected to produce a 50xa0meV 1xa0nA beam with a brightness of 107.

Source brightness and useful beam current of carbon nanotubes and other very small emitters

The potential application of carbon nanotubes as electron sources in electron microscopes is analyzed. The resolution and probe current that can be obtained from a carbon nanotube emitter in a low-voltage scanning electron microscope are calculated and compared to the state of the art using Schottky electron sources. Many analytical equations for probe-size versus probe-current relations in different parameter regimes are obtained. It is shown that for most carbon nanotube emitters, the gun lens aberrations are larger than the emitters’ virtual source size and thus restrict the microscope’s performance. The result is that the advantages of the higher brightness of nanotube emitters are limited unless the angular emission current is increased over present day values or the gun lens aberrations are decreased. For some nanotubes with a closed cap, it is known that the emitted electron beam is coherent over the full emission cone. We argue that for such emitters the parameter “brightness” becomes meaningless. The influence of phase variations in the electron wave front emitted from such a nanotube emitter on the focusing of the electron beam is analyzed.

Experimental evaluation of the extended Schottky model for ZrO/W electron emission

The energy distribution of electrons emitted from a ZrO/W electron source with a radius of curvature of 0.9 μm was determined for extraction voltages ranging from 3000 to 6000 V and emitter temperatures from 1200 to 1900 K. Full width at half maximum values of the energy spectra between 0.3 and 0.8 eV were found. The experimental data are analyzed according to the extended Schottky model for electron emission, yielding the electric field at the emitter surface. The tunneling probabilities are evaluated numerically by integrating the one-dimensional Schrodinger equation, and analytically by employing the commonly used Wentzel–Kramers–Brillouin approximation. Both approaches give good agreement with the experimental data, except for small differences probably due to Coulomb interactions (Boersch effect). The same analysis is also applied to the experimental results for a Schottky emitter with a radius of curvature of 0.3 μm, taken from literature.

Optimum dose for shot noise limited CD uniformity in electron-beam lithography

To maximize the performance of an electron-beam lithography system the resist sensitivity must be chosen carefully. Very sensitive resists require only a low illumination dose, thus increasing the throughput. However, shot noise effects may give rise to unacceptable line edge roughness and variations in critical dimension (CD). In this study, the physical parameters which influence the effect of shot noise statistics on CD uniformity (CD-u) and linewidth roughness (LWR) are determined and an analytical model for CD-u and LWR is derived. It is found that the CD-u and LWR depend on the dose, the Gaussian beam probe size, the diffusion length dr of secondary electrons and acids in resist. The influence of background dose and non-shot-noise dose variations must also be taken into account. Monte Carlo simulations are performed to obtain the statistical variation of the two-dimensional solubility distribution of illuminated resist in a developer. The results of this simulation are used to validate the model. Fo...

Brightness measurements of a ZrO/W Schottky electron emitter in a transmission electron microscope

Abstract We determined the reduced brightness of a ZrO/W Schottky electron emitter, with an apex radius of curvature of 0.9 μm, for extraction voltages ranging from 2.75 kV to 4 kV. The reduced brightness was found by measuring the diameter of a small focused probe in the specimen plane of a Transmission Electron Microscope, together with the current in the probe and the half opening angle. We find that the experimental brightness values, once corrected for lens aberrations and electron diffraction effects, approach the theoretical maximum axial brightness well. No effects of electron–electron interactions (trajectory displacement) were observed for the range of extraction voltages we studied. The highest corrected reduced brightness value we determined was 1.2×10 8 A/m 2 sr V, at an extraction voltage of 4 kV.

Integrated multi-electron-beam blanker array for sub-10-nm electron beam induced deposition

An integrated multi-electron-beam blanker array is proposed for the multi-electron-beam source reported by van Bruggen et al. [J. Vac. Sci. Technol. B 23, 2833 (2005)], which aims at the throughput improvement of sub-10‐nm electron beam induced deposition. The integrated blanker array consists of a current limiting aperture array, a blanker array, and a microaperture-lens array. The integrated blanker array generates 100 individually controlled beamlets, projecting the virtual source image in the principle plane of the field lens. The electrostatic cross-talk, charging, and contamination are reduced by the grounded current limiting aperture plate above the blankers. The blanker array and microaperture-lens array are fabricated on a first wafer, while the current limiting aperture array is fabricated on a second, 100μm thick wafer. The wafers will be bonded with an alignment accuracy of approximately 200nm. The first test chips, where the blankers are grouped and controlled by external circuitries, are und...

Development of a multi-electron-beam source for sub-10nm electron beam induced deposition

A multibeam electron beam induced deposition (EBID) system is presented that aims at the fabrication of sub-10nm structures with EBID. This system consists of a multibeam source (MBS) module, delivering 100 virtual sources and a standard scanning electron microscope column to image the 100 sources onto a wafer. In this paper, the concept for the MBS is presented: starting with a single Schottky field emission gun, its broad beam is divided into 100 individually focused sub-beams. This is accomplished with an aperture plate with small current-limiting apertures on the side irradiated by the beam and larger holes on the other side, acting as aperture lenses due to the presence of a macroelectrode in front of it. With this concept, in addition to the miniature aperture lenses, a negative macrolens effect is established that can serve to compensate for both the third-order geometric and first-order chromatic aberration of the collimator lens in front of the aperture plate. A simple aberration model is present...