Still going strong at 40

How SEM technology has succeeded in its quest for world dominance

In 1965, cosmonaut Aleksei Leonov took the first space walk, astronauts Charles Conrad and Gordon Cooper set a space endurance record of eight days in their Gemini space craft and the first batch of 5 Scanning Electron Microscopes was manufactured. Aleksei Leonov, Charles Conrad and Gordon Cooper are all still going strong. And so is the SEM.                                                            
                                                                 
That first commercial SEM was assembled in the UK by the Cambridge Instrument Company. Four decades later and the ‘industrial genepool’ of that UK success is apparent in a line of instruments that runs unbroken to today’s Zeiss electron microscopes, still manufactured in Cambridge.

40 years old, this British development has become an indispensable tool for at least two generations of technologists. From its early home in materials science it has cut a swathe through the disciplines of electronics, forensics, paper and archaeology. More recently, it has evolved to stake a claim to a place on the lab benches of the pharmaceutical researchers, food technologists and biologists, adapting subtly to fulfil the discrete requirements of these individual disciplines. It is, in fact, the ultimate nanotechnology tool.

Just how has the SEM come to dominate the Nano world? And, where is it finding new relevance today?

Flexibility, form and function
In scanning electron microscopy a highly focussed electron beam is scanned across the surface of a specimen within an evacuated chamber to unleash an array of signals. The atoms at the very surface produce secondary electrons and, if a small diameter electron beam is used, analysing the secondary electrons can produce a high resolution, high contrast image of the surface. This is imaging at the nanoscale with magnification up to more than 500,000X and resolution better than 1nm. Backscattered electrons are primary beam electrons that are 'reflected' from atoms in the solid and show the distribution of different chemical phases in the specimen. When the primary electron beam interacts with atoms within the specimen, X-rays are emitted with an energy characteristic of the parent element. Detection and measurement of the X-rays permits elemental analysis, usually referred to as EDS, either of the surface or the substrate to a depth of two microns.

Given suitable detectors, and the room to deploy them, these three primary signals may be used to form a finely detailed image of the surface or to measure the elemental composition of the substrate. In other words, we can see what our specimen looks like and what it is made of. It is this combined capability that is the basis of the SEM’s power and universal acceptance.


Brineshrimp

Applying pressure for a win-win situation
If that were the sum total of the SEM’s capabilities, it would still be a valuable research tool. However, most modern SEM systems can now operate in a Variable Pressure (VP) mode as well as the traditional high vacuum mode. In VP mode, a small amount of gas is introduced to the chamber, up to approx. 400 Pa, which compensates for the charge that accumulates on the surface of non conductive specimens at high vacuum. This allows naturally insulating materials, such as paper and plastic, to be analysed without the need for surface coating.

By eliminating the need for specimens to be coated, time consuming specimen preparation is reduced, the overall operation of the microscope is simplified, the range of application areas where the SEM can play a role widened, and specimen throughput increased. This increased flexibility is one of the key reasons behind the SEM’s adoption in the examination of ceramics, plastics, forensic specimens and art objects.

However, introducing even tiny amount of gas atoms into the path of the primary electron beam means that collisions will occur in which the massively larger gas atoms deflect the electrons away from the main axis of the primary beam. If high energy primary electrons are ‘loose’ within the specimen chamber they are capable of generating secondary and backscattered electrons plus X-rays from areas of the specimen outside the focus area.

In this trade off, an acceptable degree of loss in resolution is accepted in exchange for a huge increase in flexibility. Too much gas, though, and the loss of resolution cannot be worked through.

Water, water everywhere
A new generation of SEMs is now emerging that challenges this status quo, exemplified by the Zeiss EVO. A direct descendant of those first five microscopes produced in Cambridge, this new design allows much higher chamber pressures to be used and will even allow for the introduction of water vapour. It’s eXtended Variable Pressure (XVP) and Extended Pressure (EP) modes operate at up to 750 Pa and 3000 Pa respectively and open up a new realm of life science, healthcare, food and pharmaceutical research as well as creating a bridgehead into the new science of bioelectronics. 
                                          E-coli, total field veiw measures 7x5 microns

But, if a tiny amount of gas atoms causes a depreciable loss in resolution, how can meaningful results be obtained from a chamber full of collision-ready atoms and water molecules?

The answer is to be found in a four-stage vacuum system coupled with a new design of narrow objective lens, a high pressure deflector shield and a choice of two secondary electron detection systems (VPSE and EPSE) specially developed to work at high pressures. The new lens and chamber design also tackle the perennial problem for the flexible SEM – finding room to put the detectors in.

The XVP Vacuum system divides the specimen chamber and the electron gun column into four, discrete pressure zones separated by pressure limiting apertures. The gun area and upper part of the column is always maintained at high vacuum but, as soon as XVP mode is selected, the system automatically sets up the pressure gradient through the instrument and selects the VPSE or EPSE detector. This is necessary as the standard Everhart-Thornley and in-lens SE detectors are both designed solely for use in high vacuum.

The new Zeiss EVO allows much higher chamber pressures to be used

Pump it up
The narrow objective lens is shown in cutaway form in Figure 1. Tapering at just 80O, it opens up valuable working space to install multiple secondary electron detectors, EDS detectors and EBSD cameras. It also features an analytical working distance less than half its nearest competitor, at just 8.5mm, and goes on to introduce a series of breakthroughs to objective lens design.

The first is Through the Lens (TTL) pumping, which creates an intermediate pressure boundary within the lens between the high vacuum of the primary electron gun and the high pressure around the specimen. This reduces the number of collisions between the primary electrons and the chamber gas and contributes to the class leading XVP and EP high pressure imaging modes.

Isolation of the electron probe from the high pressure gas at the specimen can be boosted by fitting the BeamSleeve. This tubular extension shield is added to the objective lens and, in combination with any of the EVO detectors, allows lower voltages to be used to give brilliant images and accurate X-ray analysis. The addition of the BeamSleeve also creates a Beam Gas Path Length (BGPL) of just 2mm, compared to the more normal 10-15mm. Since BGPL is the distance over which the electron beam and the chamber gas can interact, designing it to be the shortest length possible is a key contributor to high quality imaging and X-ray analysis.
Figure 1: Impact of shortened beam gas path
length on resolution

High pressure on the detectors
To unite the new design, SE detectors capable of working in high pressure and in the presence of water were developed. Unlike the traditional Everhart-Thornley detector, where secondary electrons cause scintillation on a screen in front of a light guide, in the new detectors secondary electrons cause scintillation of gas molecules in front of a light guide and the photons are detected using a photomultiplier.

The result is excellent secondary electron imaging, even at high pressures, for true surface images of insulating materials

What’s more, the Everhart-Thornley SE detector and the enhanced VPSE and EPSE detectors are identically inclined in the same plane as the objective lens. Also in this plane are the EBSD camera, the EDS detector and the specimen tilt stage. This coplanar geometry means that users can overlay all the different types of images knowing that they all relate to the same portion of the specimen.

Life science imaging – a unique experience
The optimised geometry is important, ensuring that the EVO retains the core attributes of a high vacuum SEM – fine detail, high contrast, large depth of field and elemental analysis. All the characteristics that make it ideally suitable for materials research, failure analysis, quality control and forensics.

However, for the life scientist, it delivers a unique imaging experience. From butterfly to bacteria, flora and fauna, it offers a unique  appreciation of the way our natural world is    
put together. Insight that is unclouded by artefacts induced by the preparation needed to get the specimen ready for imaging.                             
 

Until now, observation of life forms in the SEM has always involved a guessing game. Is the conformation of a particular feature the way it existed when the plant or animal was alive or is it a mere by-product of the coating process, a relic of the desiccation brought about by the high vacuum? With extended pressure imaging we can answer many of those questions.

They say that you should not judge a book by its cover. In the same way, you need to get inside and understand the subtleties of design of advanced laboratory instrumentation to appreciate their full capabilities. Ultimately, though, a microscope should be judged by the images it produces.

By Jack Vermeulen, Carl Zeiss SMT, Oberkochen, Germany
enquiry number 10002