Electron Microscope: Scanning, Field Emission (LEO)

Quick Specs

Provides resolution up to 1nm
Equipped with below-lens and in-lens secondary electron detectors as well as a backscatter electron detector
Equipped with two energy dispersive X-ray detectors

The LEO 982 ultrahigh-performance field-emission scanning electron microscope is used for imaging and analyses of micro- or nanoscale objects. The instrument provides resolution of 1 nm at 30 kV and 4 nm at 1.0 kV. Its large specimen chamber is equipped with several detectors: a below-lens secondary electron detector, an in-lens secondary electron detector, a backscatter electron detector, two energy dispersive X-ray detectors, and a detector for electron backscatter diffraction.

This instrument may be applied to several areas of research, including materials science, nanotechnology, geology, and biology. For example, researchers have used this instrument for microscopic and elemental characterization of materials such as sodium aluminate alanate (Figure 1) and other similar materials as part of an investigation of these materials as hydrogen storage media. In addition, the instrument has been used to examine soil bacteria Shewanella oneidensis (Figure 2) membrane features and mineral association in response to various simulated soil conditions.

sodium aluminate alanate seen through the microscope

Figure 1. Sodium aluminum alanate crystals.
Full Image (jpg 10 kb)

Figure 2. Scanning electron microscopy image of soil bacteria Shewanella oneidensis strain MR-1. Full Image (jpg 11 kb)

Performance Specifications

Point-to-point resolution of 1 nm at an accelerating voltage of 30 kV and 4 nm at 1.0 kV is achieved with this instrument using a Schottky field-emission source, a beam booster that maintains high beam energy throughout the microscope column, an electromagnetic multihole beam aperture changer, and a magnetic field lens. The beam path is designed to prevent crossover of beam electrons. These features result in reduced chromatic aberration, improved beam brightness (even at low-electron probe energies), and little beam energy spread.

Sample Preparation and Handling

The laboratory where the field-emission scanning electron microscope is housed is fully equipped for sample preparation. It contains lab benches, a sink, a fume hood, flammable-liquid storage cabinets, nitrogen, compressed air, and deionized water. The laboratory is serviced by high-speed network connections for data transfer to other computers in EMSL or to the Internet.

In addition, stereoscopic microscopes are available to aid in mounting samples for examination. Support equipment includes a critical point drier and instruments for etching samples and coating them with carbon, platinum, or osmium.

Samples are placed in the sample chamber through a large front door. The sample chamber can accommodate specimens of up to 200 mm in diameter. It is equipped with a TV-camera mounted parallel to the sample stage tilt axis to facilitate operations at short working distances. The eucentric specimen stage is capable of an 80-mm × 80-mm × 30-mm range of travel, 360 degrees of rotation, and -15 degrees to +80 degrees of tilt. The stage is motorized in four axes with computer control. The stage position is displayed and recorded in the data field below the image. Up to 120 positions can be stored. The chamber is evacuated with a turbo pump for oil-free operation.

All work with the field-emission scanning electron microscope and in the associated laboratory must be performed in compliance with EMSL practices and permits.

Electron Source

The field-emission scanning electron microscope uses a Schottky field-emission source for electrons. The Schottky emitter combines the high brightness and low energy spread of the cold field emitter with the high stability and low beam noise of thermal emitters. The instrument is equipped with about 100 times the emitting area as a cold emitter, and as a result, has much higher probe currents that give it an advantage for analytical applications. The electron gun is directly linked to a beam booster. This linkage eliminates, among other things, crossover of electrons along the beam path, which aids in operating at very low beam energies.

Objective Lens

The instrument's objective lens is comprised of a high-performance magnetic lens with an inserted electrostatic lens. The high-angled lens body allows a 50-degree tilt of large specimens (e.g., 6-in. wafers) at a working distance as low as 6 mm, plus x-ray takeoff from a horizontal specimen at an angle of 35 degrees. The lens design minimizes transverse chromatic and other scanning aberrations.

LEO 982 Field-Emission Scanning Electron Microscope Resolution

The field-emission scanning electron microscope can operate at accelerating voltages from 0.2 kV to 30 kV. At 1.0 kV the resolution is 4.0 nm or better, and at 30 kV the resolution is 1.0 nm or better.

Electron Detectors

The field-emission scanning electron microscope is equipped with two secondary electron detectors: below-lens for low-resolution work and in-lens for high-resolution imaging. The backscattered electron detector is solid state and is optimized for short working distances. The four diodes of the detector are designed for both low- and high-accelerating voltage operations.

Image Storage and Processing

Digital images (TIF and other formats) can be stored in a random access memory (RAM) frame store or on a hard drive, zip drive, or jaz drive. These images can be transferred to CDs or to the high-speed network. Images are filed complete with labeling and can be transmitted to the integrated camera at maximum resolution. Images can be collected at a variety of resolutions up to 2048 × 2048 pixels. Depending on pixel density, up to 15 images can be filed in the RAM frame store. These images are immediately available for comparison and combination purposes. Different signals can be mixed in an image (e.g., secondary and backscattered signals). Image processing includes frame averaging, integration, contrasts and brightness adjustments, various filters, arithmetic functions, and other image enhancements. Screen annotation and feature measurement are available.

A Soft Imaging System ADDA II was added to the field-emission scanning electron microscope. With this addition, high-resolution images up to 4000 × 4000 pixels can be recorded directly to a networked computer. Modules added to the ADDA II in the future will allow image analysis and three-dimensional imaging.

Accessories

The field-emission scanning electron microscope is equipped with both Oxford ISIS and Rontec energy dispersive X-ray microanalysis systems. The Oxford system has a SiLi detector with 128-eV resolution that is capable of light element detection and can perform both qualitative and quantitative elemental analyses, elemental mapping, digital imaging, abd microscope automation. In addition, it can combine compositional information with a secondary electron image in a software package called CAMEO. The Rontec system has a high-throughput, silicon-drift detector designed for high-speed elemental mapping that can also couple X-ray information with secondary or backscatter images.

Also attached to the microscope is the Oxford OPAL system for visualization and analysis of electron backscattered diffraction patterns for the determination of crystal orientations.

Individuals may use this instrument independently for their research following the necessary training. The level of training depends on the complexity of the user's project, and may range from a few hours for simple micrography to days or weeks for full analytical requirements.

Arey, Bruce | bruce.arey@pnl.gov, 509-371-6485