ABOUT THE TECHNIQUE

In a Scanning Electron Microscope (SEM), a beam is scanned over the sample surface in a raster pattern while a signal from secondary electrons (SE) or Back-scattered electrons (BSE) is recorded by specific electron detectors. The electron beam, which typically has an energy ranging from a few hundred eV up to 40 keV, is focused to a spot of about 0.4 nm to 5 nm in diameter. Latest generation SEMs indeed can achieve a resolution of 0.4 nm at 30 kV and 0.9 nm at 1 kV.

Beyond the ability to image a comparatively large area of the specimen, SEM can be equipped with a variety of analytical techniques for measuring the composition, crystallographic phase distribution and local texture of the specimen. Chemical composition analysis can be performed by Energy Dispersive X-ray Spectroscopy (EDS) which relies on the generation of an X-ray spectrum from the entire scan area of the SEM. An EDS detector mounted in the SEM chamber collects and separates the characteristic X-rays of different elements into an energy spectrum and EDS system software is used to analyze the energy spectrum in order to determine the abundance of specific elements. A typical EDS spectrum is portrayed as a plot of X-ray counts vs. energy (in keV). Energy peaks correspond to the various elements in the sample. EDS can be used to find the chemical composition of materials down to a spot size of a few microns and to create element composition maps over a much broader raster area. Together, these capabilities provide fundamental compositional information for a wide variety of materials.

 

A general purpose, ultra high-resolution SEM

Within the NFFA’s project activities, a ZEISS Field Emission (FE) SEM Supra 40, located in a dedicated lab at the CNR-IOM Institute, is made available to access. The microscope is equipped with a conventional Everhart-Thornley secondary electrons detector and with a last generation Gemini column featuring the innovative so-called beam booster and an objective lens that consists of a combined electrostatic/electromagnetic lens doublet. A Schottky field emitter serves as gun. Electrons are emitted from the heated filament while an electrical field is excited by applying the extractor (Uext) voltage. To suppress unwanted thermoionic emission from the shank of the Schottky field emitter, a suppressor voltage (Usup) is applied as well. The emitted electrons are accelerated by the acceleration voltage (Ueht). The beam booster (Ub, booster voltage), which is always at a high potential when the acceleration voltage is at most 20kV, is integrated directly after the anode. This guarantees that the energy of the electrons in the entire beam path is always much higher than the set acceleration voltage. This considerably reduces the sensitivity of the electron beam to magnetic stray fields and minimizes the beam broadening. The great advantages of such a design are superb resolution even at ultra low voltages with an increased signal to noise ratio. Thanks to the special Gemini’s lens shape minimizing the magnetic field at the specimen, high resolution imaging of dia-, para, or ferromagnetic materials is possible with very short working distances.

 

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A high efficiency In-lens detector for high resolution SE imaging is integrated in the GEMINI column above the objective lens. Low energy secondary electrons are intercepted at the point of impact by the weak magnetic field at the specimen surface, then accelerated in the booster column and focused on the In-lens above the objective lens thus yielding images with a signal to noise ratio increased by a factor of 2-3 compared to conventional Everhart-Thornley detector.

A second In-column detector will be also installed on the microscope (scheduled on July 2015) enabling energy- and angle-selective simultaneous detection of backscattered electrons (BSE) positioned directly above the In-lens SE detector allowing for channelling contrast (crystal orientation), as well as compositional contrast. 

 

An EDAX X-ray detection system is currently annexed to the microscope allowing to perform chemical analysis of the investigated samples which will be soon (commissioning scheduled on July 2015) replaced by a latest generation Oxford X-ray detection system – Aztec EDS, providing a high-degree accuracy at a fast data acquisition speed. The Oxford detector is a LN2-free X-Act Silicon Drift Detector with 10 mm2 active area and a Super Atmosphere Thin Window (SATW) Light Element Window allowing for:

  • Excellent low energy analysis with detection from Be to Pu
  • Resolution guaranteed in accordance with ISO15632:2002 for:
    •  Mn Kα (129 eV or better)
    • F Kα (75 eV or better)
    • C Kα (72 eV or better)
  • Stability guaranteed from 1,000 to 100,000 cps – peak shift and  resolution change <1 eV

 

Software is provided for performing quantitative analysis, digital imaging, line-scan x-ray profiles, multiple element x-ray maps, image analysis with:

  • high accuracy in light elements quantification in the low fluorescence energy range
  • standardless quant technology which approaches standards based quant accuracy
  • full software correction at very high count rates including pile-up correction

TECHNICAL SPECIFICATIONS

Resolution (optimal WD) 1.0 nm @ 15kV, 1.9 nm @ 1kV
Magnification 12 -1,000,000 x
Emitter Thermal field emission type
Acceration Voltage 0.02 – 30 kV
Probe Current Configuration 1: 4pA -20nA/Configuration 2:12 pA – 100nA
Detectors High efficiency in-lens detector
Everhart-Thornley Secondary Electron Detector
Cap mounted AsB detector
Chamber 330 mm (Ø) x 270 mm (h),
2 EDS ports 35° to optional axis,
CCD-camera with IR illumination,
Additional 3rd EDS port 35° to optical axis
Specimen stage 5-Axes Motorised Eucentric Specimen Stage
X = 130 mm, Y = 130 mm, Z = 50 mm,
T = -3 - +70°
R = 360° (continuous)
6-Axes Eucentric Stage
X = 100 mm, Y = 100 mm, Z = 42 mm, Z’ = 13 mm,
T = -4 to 70°, R = 360° (continuous)
Image processing Resolution: Up to 3072 x 2304 pixel,
Noise reduction: Seven integration and averaging modes

The map of techniques

at MM building at Q2 building at Elettra experimental hall at CNR-IOM cloud at Fermi-T-Rex laboratory Surface & Nano Science Lab, STM/STS PLD XPS & ambient pressure XAS ARPES & Spin ARPES MOKE & Masked deposition system XPS MBE Oxides SPRINT laboratory SEM XRD PVD data repository open data data analysis
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Scientists in charge