THE HEBREW UNIVERSITY CENTER FOR NANOSCIENCE AND NANOTECHNOLOGY

Environmental Scanning Electron Microscope Quanta 200


Overview of Technique

An analytical Quanta 200 Environmental Scanning Electron Microscope (ESEM) is produced by the FEI company. The microscope combines high vacuum, low vacuum and wet-mode to support a variety of material characterization applications. The instrument allows for examination of non-conducting, contaminated, hydrated and even living samples without significant sample preparation, in addition to those samples that have always been viewable under conventional scanning electron microscopes. It allows for user selection of accelerating voltage, magnification, gas type, gas pressure and detector type. This flexibility promotes cross-disciplinary research for all who utilize the ESEM.

The microscope uses Tungsten electron source and allows wide range of accelerating voltages from 200 V to 30 kV. The microscope has complete set of detectors providing imagining in secondary and back-scattered electrons in all the operating modes (high and low vacuum and ESEM mode) at the resolution of 3.5 nm.

The system also includes energy dispersive spectroscopy (EDS) (qualitative and quantitative) with Ultra-Thin Window for light element detection down and including Carbon with spectral resolution better than 132 eV.

Chamber pressure could be changed from 6x10^-6 to 26 mbar by using various types of gases (currently water vapor is available). A Peltier cooling stage could be used for moist samples observation.

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. Scanning electron microscopes are often coupled with X-ray analysers. 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)).

An electron microscope requires a good vacuum for the generation and propagation of the electron beam, which in the past meant that the specimen under examination had to be placed also in vacuum. Scanning of the non-conductive sample with a beam of negatively charged electrons causes its surface charging, which seriously complicate or exclude at all observation of such specimens without conductive coatings. However, it is now possible to view specimens inside a gaseous environment so that wet samples in a water vapor environment can also be examined. The conductive coating of specimens according to conventional practice is no more necessary because the gaseous layer around the specimen becomes ionized and suppresses charge accumulation. The gas itself can also be used as detection medium giving rise to novel detection and imaging techniques. These innovations have resulted in the Environmental Scanning Electron Microscope (ESEM).
In our ESEM Quanta 200 we employ water vapors to generate either low vacuum (LV) mode with pressure < 1 Torr, or ESEMTM mode with pressure as high as 20 Torr.

For basic education and principle understanding of Scanning Electron Microscopy, please, use following web resources:

Equipment Specifications

Resolution: * 3.5 nm at 30 kV High vacuum mode
* 3.5 nm at 30 kV Low vacuum mode
* 3.5 nm at 30 kV ESEMTM mode
* 15 nm at 3 kV Low vacuum mode
Magnification: 7x - 1.000.000x High Vacuum Mode
7x - 1.000.000x Low Vacuum Mode
Accelerating Voltage: 200 V to 30 kV
Filament Tungsten Hairpin
4-axis motorized eucentric stage: x = y = 50 mm (motorized)
Z = 50 mm (25mm motorized)
Tilt -15 to +750
R=3600
Tilt eucentric at analytical working distance of 10 mm
Image Processor: up to 3584 x 3094 pixelsFile type TIFF (8 or 16 bit), BMP or JPEG
System Control: 32 bit graphical user interface with Windows 2000
Detectors * 3rd generation, large-field gaseous SE detector with enhanced gain preamplifier suitable also for use at low voltages
* Standard Gaseous SE detector for operation up to 13 mbar (= 1300 Pa = 10 Torr)
* A CCD camera for chamber observation
* 2-segment solid-state backscattered detector optimized for low kV operation (down to 3 kV)
* 2-segment solid-state electron backscattered detector (Gaseous Analytical BSED) for microanalysis data collection in low vacuum mode
Microanalysis Fully embedded EDAX EDS detector (spectral resolution better than 132 eV)
In situ freeze-drying observation: Microprocessor controlled Peltier stage for cooling down to -200C
Features of the vacuum system include: * Patented through-the-lens (ESEMtm) differential pumping technology;
*250 l/s turbomolecular drag pump;
* 2 x 8 l/s rotary pumps;
* Seamless transition between the vacuum modes.