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Performance of YAG:Ce Scintillators for Low-Energy Electron Detectors in S(T)EM
Lalinský, Ondřej ; Bok, Jan ; Schauer, Petr ; Frank, Luděk
Cerium activated single crystals of yttrium aluminium garnet (YAG:Ce) Y3-xCexAl5O12 are widely used as scintillators in electron detectors for S(T)EM. Nowadays, it is sometimes necessary to detect low-energy electrons without post-acceleration. In such cases, extremely sensitive detectors are required that are able to detect even electrons with energies of only hundreds of eV while avoiding charging of the scintillator surface. However, commonly used scintillators strongly lose their light yield with the decrease of the incident electron energy. Moreover, a thinner conductive layer on the scintillator surface has to be used to allow low-energy electrons to pass through. Possible charging of the surface negatively affects its cathodoluminescence (CL) light yield. The low-energy electron excitation takes place closer to the scintillator surface where damage can be expected owing to its preparation, which also reduces the CL light yield. The aim was to study the influence of the scintillator and its conductive layer on the low-energy electron detection efficiency.
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Innovation possibilities of scintillation electron detector for SEM
Schauer, Petr ; Bok, Jan
To evaluate performance of a scintillation detection system for SEM, it is necessary to consider many scintillator parameters. Various attributes of the scintillator for the SEM electron detector are listed in. The very important parameters are those affecting the detective quantum efficiency (DQE) which is primarily a measure of image noise. Not a less important indicator of image quality is the modulation transfer function (MTF) which describes the ability to show fine image details. Therefore, using a scanning imaging system, the detector bandwidth, which is given especially by the scintillator decay time, is the key to the good MTF. Currently, the YAG:Ce single crystal scintillator (introduced already in 1978 having somewhat limiting decay characteristic is the most frequently used scintillator in the SEM. The aim of this paper is to outline possibilities of scintillator innovation to get the improved MTF and DQE.
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Ultra-low-energy STEM in SEM
Frank, Luděk ; Nebesářová, J. ; Müllerová, Ilona
Examination of thin samples in TEM or STEM has been performed at hundreds of keV. This energy range offered high resolution but low contrasts which meant that tissue sections had to be contrasted with heavy metal salts. Recent TEM with aberration correctors preserve an acceptable resolution down to 20 keV and provide enhanced contrasts. The LVTEM device is operated at 5 keV on samples thinner than 20 nm. STEM attachments to SEMs have become widespread [3] profiting from an image contrast substantially increasing even for light elements at tens or units of keV. The methods for the preparation of ultrathin sections of various substances are capable of producing layers at and even below 10 nm which enables one to further decrease the energy of the electrons provided the image resolution is maintained. When using the STEM technique virtually all transmitted electrons can be utilised for imaging, while in TEM we are restricted to using electrons capable of forming the final image at acceptable quality. This forces us to narrow the ranges of the angular and energy spreads of electrons that enter the image-forming lenses. Consequently, the STEM technique promises higher contrasts at comparable resolutions. Unlimited reduction of the energy of the illuminating electrons is possible by employing the cathode lens principle. This consists of biasing the sample together with its holder (made flat on both sides) to a high negative potential that retards the incident electrons before they land on the sample surface and accelerates backscattered and transmitted electrons to their respective detectors above and below the sample. Calculations have shown a final spot size only moderately extended even at units of eV so that quality-consistent micrographs can be recorded over the full energy scale.
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Measurement of current density distribution in shaped e-beam writers
Horáček, Miroslav ; Bok, Jan ; Kolařík, Vladimír ; Urbánek, Michal ; Matějka, Milan ; Krátký, Stanislav
The ZrO W(100) Schottky cathode is used in our e-beam writing system working with a rectangular-shaped electron beam. The homogeneous angular current density distribution is crucial for quality of exposures of the shaped beam lithography systems. Two basic types of the angular emission distribution can be observed in dependence on the microscopic final end form shape of the emitter tip, with bright centre and more common dark centre. The stable operation of the cathode thus stable end form shape requires a delicate balance of parameters inside the gun which however can slightly change during cathode life time. This implies the necessity of analysing and periodical monitoring the current density distribution in e-beam. Four methods enabling this measurement are presented.
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Very low energy STEM for biology
Frank, Luděk ; Nebesářová, Jana ; Vancová, Marie ; Paták, Aleš ; Müllerová, Ilona
Examination of tissue sections with transmitted electrons has been performed at energies of hundreds and tens of eV with thicknesses of sections of 10 nm or less. This was possible by employing the cathode lens principle working without lowest energy limitations with the help of biasing the sample to a high negative potential. The reflected and transmitted electrons were attracted with the same electric field to earthed detectors situated above and below the sample. Very high image contrasts have been obtained even for samples free of any heavy metal salts for contrast enhancement.
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Electron vortex beam
Müllerová, Ilona ; Řiháček, Tomáš
Vortex electromagnetic waves contain a phase singularity along their propagation direction and are formed by the spiralling wave fronts that give rise to angular momentum in that direction. Vortex photon beams are widely used optical tweezers to manipulate micrometre-sized perticles, as optical micro motors etc.
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Nano modification of the W(100)/ZrO electron emitter tip using reactive ion etching
Horáček, Miroslav ; Matějka, František ; Kolařík, Vladimír ; Matějka, Milan ; Urbánek, Michal
The W(100)/ZrO electron emitter tip is typically prepared from a tungsten single-crystal shaft of a diameter of 125 μm using electrochemical anodic etching. In order to prepare an emitter for e-beam writer with a shaped beam it is desirable to etch the tip with a radius around 100 nm. Despite the anodic etching is precisely controlled using dedicated software, the desired final form shape of the emitter tip is not achieved in every case. The correcting anodic etching is not possible due to the technology principle of the etching itself. We present in this contribution the procedure that modifies/repairs the tungsten tip shape in a nanoscale region using a reactive ion etching (RIE) in CF4 + O2 gaseous mix in a barrel type reactor at the radio frequency of 13,56 MHz and the working pressure of 1000 Pa. The change of the geometry after the RIE process is checked using a high resolution scanning electron microscope. The influence of the tip modification of the activated thermal-field W(100)/ZrO electron emitter on its emission characteristics is also presented.
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Analysis of electron current instability in E-beam writer
Bok, Jan ; Horáček, Miroslav ; Král, Stanislav ; Kolařík, Vladimír ; Matějka, František
The electron beam writer Tesla BS600 works with a thermal-field electron emitter, fixed electron energy of 15 keV and a rectangular shaped variable-size electron beam. The size of the shaped beam (stamp) can be set from 50 to 6300 nm in standard mode and from 16 to 2100 nm in high-resolution mode. The basic increment of the stamp size is 50 nm, resp. 16 nm. Electron current density inhomogeneity and long-term instability in stamps can have negative impact on the exposure quality. Therefore, we focused on a study of the current time instability. The current density in variously sized stamps was measured by a picoammeter and a PIN diode video channel as a function of time. We analyzed short-term and long-term current instabilities using filtering techniques, as well as the Fourier analysis. Based on the results, we could be able to find reasons of the current instabilities and to propose improvements to achieve higher exposure quality.
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