The Hitachi S-4300 E/N (FESEM), with guaranteed 1.5 nm resolution at 15 kV (5.0 nm at 1.0 kV), combines both high resolution and variable pressure into a single instrument, providing the greatest flexibility for investigators across a broad spectrum of departments at TTU.
This instrument utilizes a thermal Schottky field emission cathode, a hybrid electron beam source that offers high brightness and excellent resolution over a wide range of accelerating voltages. This type of emitter combines the benefits of a true cold-cathode field emission source (low beam energies that are not well suited for analytical techniques, but achieves a small, extremely stable beam diameter leading to improved resolution and high brightness) with traditional thermionic emission (heating the electron emission source to produce a wider, but more energetic electron beam, but with decreased ultimate resolution).
In addition, this instrument was also chosen because of its ability to accommodate relatively large samples (specimens up to 160 mm diameter examined) on a five axis, motorized eucentric stage. The instrument is Web-based, and can be operated in real time from a computer with the appropriate software. This makes it invaluable for collaborative projects. The S-4300SE/N has extensive automated features, including auto-focus, auto-stigmator, auto-brightness and auto-contrast. Images are acquired digitally and image processing and image management systems are incorporated into the instrument. The ancillary analytic instruments are fully integrated into the software of the machine and can therefore be operated by the same system.
Detailed Description of Instrument Capability.
The S-4300 E/N field emission variable pressure scanning electron microscope (FEVPSEM) is a new generation instrument that combines the high resolution and image quality of a field emission source with the versatility of variable pressure (VP) operation, allowing it to handle difficult or ‘dirty' samples at much higher chamber pressures.
- This microscope has a pressure operating range of 10 - 1000 Pa, which means it can be used to examine a broader range of wet, oily or non-conducting samples than most other VPSEMs.
- The FE electron source is kept at ultra-high vacuum by differential pumping through apertures built into the column when in the VP mode. The high resolution, coupled with the VP chamber allows for minimal sample preparation and no conductive metal coating that may obscure the ultimate obtainable resolution.
- Hydrated, fresh samples can also be imaged in conjunction with a cooling stage that keeps delicate samples from desiccating under the vacuum.
- The VP mode is also important for routinely imaging samples from the material sciences such as non-conductors, photo-resist, carbon nano-tubes, wet fiber analysis, polymer chemistry, filters, combustion synthesis materials, etc.
- High resolution can be achieved in both high pressure and high vacuum modes, using a variety of detectors such as secondary electron, backscattered electron and an environmental secondary electron detector (ESED).
- All operations within and between the various use modes are controlled and safety-interlocked by the instrument, so the SEM is self-protected against most user error, a necessity in a multi-user facility.
The long-term beam stability of the Schottky emitter makes it ideally suited to elemental analysis. The absence of the thin, conductive metal coat generally applied to traditional SEM samples, allows for greater resolution of fine structure as well more accurate elemental analysis, particularly for light elements. The ability to place non-coated samples in the microscope is also of great interest to the museum science and criminal forensic groups on campus, since many of their specimens cannot be perturbed by traditional sample preparation techniques. The microscope is fully compatible with energy dispersive X-ray (EDS) spectrometers as well as electron backscattered diffraction analysis (EBSD) (described below).
Because of the hydrated nature of living organisms, traditional sample preparation for living specimens includes killing and fixing the organism, a chemical dehydration, followed by critical point drying (CPD) and coating with a thin conductive metal layer prior to placing it in the vacuum of the SEM. This is a necessary, albeit time-consuming process, that can either subtly or drastically change the true, fine structure of a sample. Within recent years, physical fixation protocols for the SEM (i.e. ultra-rapid freezing) have been developed to replace the chemical procedures for some samples. A cryo-stage system attached to an SEM, allows the direct imaging of rapidly frozen samples in their native state. These processes involve flash-freezing a sample, placing it in the vacuum on a cold stage, and using the energetic beam to sublime ice off the surfaces to reveal underlying structure. A knife is employed to fracture the sample to reveal internal details of cells and organisms that might otherwise be damaged or perturbed by the arduous chemical fixation process. The cryo-stage also allows the direct imaging of materials such as soft insulators and combustibles that might otherwise be ignited or damaged by the beam in by the electron beam.
EBSD is a relatively recently marketed method used to analyze sample surfaces and crystallographic orientations. As the energized electrons interact with the sample, some are reflected off of the surface. These reflections are collected and analyzed to give data on relationships between the physical properties of a sample, its structural morphology, chemistry, and crystallography. Alone, or in combination with the appropriate software, the simultaneous collection of EDS (chemistry) and EBSD (structure) allows for the direct correlation between the elemental content and micro-structural aspects of the material being studied. These tools allow for a detailed and rapid chemical and physical analysis of materials providing qualitative, quantitative, and spatial information about the sample. This instrumentation has relevance to a great many areas of research on this campus including mineralogy, metallurgy, combustible synthesis and nano-technology, criminal and environmental forensics, microelectromechanical systems (MEMS) and others.
Detailed Description of the Analysis System.
The ancillary analytical system is the EDAX Pegasus 4040 integrated EDS and EBSD. Energy dispersive X-ray microanalysis (EDS) and electron-backscattered diffraction (EBSD) are the most versatile and useful analytical tools that can be added to an electron microscope. As electrons strike and interact with the surface of a sample, X-rays are generated. These X-rays are indicative of the particular elements present in the sample, except for those lighter than Z = 4 (Be). This is a very powerful analytic technique that gives qualitative as well as quantitative information regarding the chemical composition of the sample. Analysis is fast, taking between 15-30 seconds, and data can be displayed either as a spectrum, or as an elemental dot map providing accurate spatial data. Combined with the VPSEM, this technique is nondestructive to the sample, and allows the user to analyze hydrated or living samples as well as forensic or museum samples that must be treated with particular care in regards to limiting damage to the specimen.