THE HEBREW UNIVERSITY CENTER FOR NANOSCIENCE AND NANOTECHNOLOGY

High Resolution Scanning Electron Microscope Sirion


Overview of Technique

High resolution scanning electron microscope (HR SEM) Sirion (FEI company) is a versatile tool for advanced studies in the areas of materials science, semi-conductor and the electronics industries and life-science. The microscope uses Shottky type Field Emission Source and allows wide range of accelerating voltages from 200 V to 30 kV. It is able to achieve resolutions of 1.5 nm at > 10 kV and 2.5 nm at 1 kV. The microscope has complete set of detectors providing imagining in secondary and back-scattered electrons including Through-the-Lens detector with variable bias for ultra high resolution observation. The system is optimised for operation at low kV, allowing un-coated and isolating materials to be examined with minimum charging. Sirion offers ultra-high spatial resolution both for structural research and high resolution analytical work (energy dispersive X-ray spectroscopy (EDS), cathodoluminiscence (CL), electron back-scattered diffraction (EBSD) and electron beam induced current (EBIC)).

Basics and Tutorials

The scanning electron microscope (SEM) generates a beam of electrons in a vacuum. That beam is collimated and focused by electromagnetic lenses and scanned across the surface of the sample by electromagnetic deflection coils. Interaction of primary electron beam with the material of the sample in SEM causes excitation of secondary, backscattered, Auger electrons, characteristic X-ray radiation and photons of light. The primary imaging method is by collecting electrons that are released by the sample. Depending on their energy, angular distribution and the excitation energy of the primary beam, the electrons emitted by a sample are detected by different electron detectors mounted in the microscope chamber up to a sample surface. Detection of the electron signals is done either through solid state silicon-based detectors, or via photomultiplier-type detector involving double-conversion of a signal through light photons. By correlating the sample scan position with the detected signal, an image of a sample is formed. This image could be strikingly similar to what would be seen through an optical microscope and is, therefore, more or less intuitively understood by a human brain. Generally, each detected signal provides specific type of imaging in SEM. Imaging in secondary electrons (former samples' electrons leaving its surface with up to 250 eV excessive energy) provides mainly topographic information. Imaging in back-scattered electrons uses high energy electrons that emerge nearly 180 degrees from the illuminating beam direction. The backscatter electron yield is a function of the average atomic number of each point on the sample, and thus can give compositional information. Specimen current imaging using the intensity of the electrical current induced in the specimen by the illuminating electron beam is used to produce an image (electron beam induced current (EBIC)). It can often be used to show subsurface defects. Scanning electron microscopes are often coupled with X-ray analyzers. The energetic electron beam - sample interactions generate X-rays that are characteristic of the elements presenting in the sample and are used to identify local chemical composition of the samples (energy dispersive X-ray spectroscopy (EDS)). Light luminescence from the sample scanned by electron beam carries information about its electron and defect structure. Detection of this light by photomultiplier and imaging with this signal is called cathodoluminiscence (CL). Diffraction of backscattered electrons emitted from the surface irradiated by primary beam in SEM is used to investigate crystallographic properties of samples. Such diffraction patterns are recorded by CCD camera and are called electron back scattered diffraction (EBSD).

For basic education and principle understanding of Scanning Electron Microscopy, please, use following web resources:
Electron Beam Induced Current (EBIC)

Excess carriers are generated with electron beam striking a semiconductor. These carriers can be collected with an electric field inside the specimen. The electric field can be produced by external voltage or be due to a built in potential in, e.g., pn-junction. EBIC current is typically in nano- or microampere range. EBIC is used to determine the location of a pn-junction. The method is based on the fact that EBIC current has a maximum value when excess carriers are generated inside the depletion region of pn-junction. Current reaches the maximum value because the electric field separates holes and electrons inside depletion region. EBIC is also used for measuring diffusion length of minority carriers from Schottky- or pn-junctions. The measured current decreases as distance of the generation spot to the junction increases. Measured current is proportional to diffusion length. Electron Beam Induced Current system of GATAN is installed on the high resolution SEM Sirion (FEI company).

For more information on the Electron Beam Induced Current Technique, please, refer to:
1. Richards, B. P. and Footner, P.K., The Role of Microscopy in Semiconductor Failure Analysis, Oxford University Press, New York, 1992
2. Berz, F. and Kuiken, H K., Theory of life time measurements with the scanning electron microscope: steady state, Solid-St. Electron., Vol. 19, 1976, 437-445

EBIC includes following main components:
  • Advanced sample holder with 4 electrical contact points, electrically isolated sample mount, Faraday cup (for beam current measurements) and 2 adjustable spring-mounted tungsten probes.
  • 4 BNC high vacuum compatible feed-throughs.
  • Low noise battery powered current amplifier.
  • Configurable second stage amplifier with offset/gain for user-friendly imaging control.
Cathodoluminiscence Spectroscopy and Imaging (CL)

Cathodoluminiscence (CL) is the process by which electron bombardment o materials results in a characteristic emission of light in the UV, visible, or infra red parts of the spectrum. The flexible injection source of the focused beam together with high vacuum, and options for specimen cooling makes CL in the electron microscope a powerful characterization technique for a wide range of (non metallic) materials of scientific and technological interest. Main applications of CL technique are opto-electronics (semiconductor materials, characterization of growth, microstructure, defects, epitaxy, quantum wells, strain, device failure, physics of recombination), geological materials (quartz stratigraphy, diagentic studies, minerals, rare earth impurities), diamond (CVD, DLC, impurities, natural gems), wide band gap AlN and SiC semiconductor research.
For basic education and principle understanding of the Cathodoluminiscence Spectroscopy, we please, use following resources:
1. B. G. Yacobi and D. B. Holt, Cathodoluminescence Microscopy of Inorganic Solids (Plenum, 1990).
2. S. Myhajlenko: Book chapter entitled Cathodoluminescence in Luminescence of Solids: Plenum Publishing (1998).

Cathodoluminiscence (CL) system for high resolution imaging and spectroscopy MonoCl3 of GATAN is installed on high resolution scanning electron microscope Sirion. This microscope is also equipped by alternative cooling stage, which allows cooling down to 5 K.

Electron Back Scattered Diffraction

When the beam of a Scanning Electron Microscope (SEM) strikes a crystalline material mounted at an incline around 70°, the electrons disperse beneath the surface, subsequently diffracting among the crystallographic planes. The diffracted beam produces a pattern composed of intersecting bands, termed electron backscatter patterns, or EBSPs. The patterns can be imaged by placing a suitable film or phosphor screen in close proximity to the sample in the SEM sample chamber.
For basic education and principle understanding EBSD technique, please, use following web resources:
TSL-EDAX system for Electron Back Scattered Diffraction (EBSD) is mounted on our HR SEM Sirion. The system includes acquisition hardware (DigiView1612 CCD Camera), data collection software and analytical software for Orientation Imaging (OIM) and Phase Identification (Delphi).

Equipment Specifications

Resolution: * 1.5 nm at > 10 kV
* 2.5 nm at 1 kV
Accelerating Voltage: 200 V to 30 kV
Filament High stable Schottky Field Emission Source
5-axis motorized eucentric stage: * x ,y = 50 mm
* Z = 25 mm (+25mm)
* Tilt -15 to +750 at 10 mm FWD
* rotation continuous
Detectors * Conventional SE
* Through-the-lens Detector with changeable bias from -250 to +250 V for detection of SE and BSE in UHR mode
* Solid State BSE with TV rate imaging and high sensitivity at low kV
* CCD camera for chamber observation
Operation modes:

* High Resolution Mode for general survey and low magnification viewing
* Ultra High Resolution Mode for high magnification and high resolution viewing
* EDX Mode for EDX microanalysis and magnetic samples observation

Vacuum system: Oil-free vacuum
Microanalysis: * X-MAX20 SDD Inca 450 EDS LN2 free detector (Oxford Instruments, UK) with spectral resolution of 129 eV
Cathodoluminescence spectroscopy set: * GATAN system MonoCl3 for high resolution imaging (pan- and monochromatic) and spectroscopy within extended spectral range (up to 1700 nm)
* GATAN helium cooled replacement SEM stage (temperature range 5 - 300 K), 3-axis (manual)
Electron beam induced current module: GATAN EBIC system for non-destructive failure analysis of fine-scale semiconductor electronic devices
Electron back scattered diffraction module: TSL-EDAX stand-along system for fast high resolution crystallographic acquisition of bulk samples