How ESEM Works
Environmental scanning electron microscopy (ESEM) was developed about 15 years ago.
Although it would likely be very expensive to modify a normal scanning electron microscope
(SEM) to perform as an ESEM, a microscope designed from the beginning with a dual purpose
(ESEM/SEM) can work quite well either way. A company called ElectroScan made the first
ESEMs, and they were built on a chassis provided by Philips Electron Optics. Philips Electron
Optics then bought ElectroScan, and Philips Electron Optics in turn was bought by FEI
Company. Philips NV, in Holland, at one time owned controlling interest in FEI, but that may not
presently be the case.
One advantage to using the environmental scanning electron microscope (ESEM) as an ESEMoperating it in 'wet' mode-is that it is not necessary to make nonconductive samples conductive.
Materials samples do not need to be desiccated and coated with gold-palladium, for example, and
thus their original characteristics may be preserved for further testing or manipulation. We may
image the sample, modify the sample, and image the sample again, ad infinitum, without
destroying its usefulness by having coated it to make it conductive. We can also perform dynamic
experiments with the ESEM in wet mode; one of the hot stages may be used to heat a small
sample to as much as 1500 C and image it during every step of the heating/cooling process (once
we pass a certain temperature, above 1100 C, we actually need to adjust the bias to reject thermal
electrons, but this can be done easily). The Peltier heating/cooling stage lets us work within 20Celsius degrees above or below ambient temperature, and the combination of low temperature
(e.g., 4 C) and high water vapor pressure (e.g., 6.1 Torr) permits us to achieve 100% relative
humidity (RH) at the sample surface. At 100% RH we are not dehydrating the sample during the
imaging process (at less than 100% RH, a moist sample will be constantly losing water as the
vacuum in the chamber pumps on it; in the 'scope this appears as constant movement of the
sample). One of our users who images the growing parts of corn plants can simply expose the
areas he wants to view and put them right onto the Peltier stage for imaging.
When we use our 'scope (FEI XL30 ESEM-FEG) as an ESEM, we must isolate the specimen
chamber from the upper and lower portions of the vacuum column. This must be done because
we are going to introduce water vapor, as an imaging gas, to the specimen chamber (other
imaging gases will work, but water is used primarily), but we do not want water vapor in other
parts of the vacuum column. Water vapor, or any ions, particles, molecules, or atoms in a
vacuum, normally interferes badly with the imaging process. For years we explained how SEM
works by insisting that it would not work without a good vacuum. And we were correct, but now
we can modify the vacuum in one important area and get away with it. With our 'scope we can
add as much as 10 Torr of water vapor to the specimen chamber, so our ESEM is 'environmental'
only inasmuch as the chamber can reach one seventy-sixth of an atmosphere. Shutting the main
valve closes off the bottom of the specimen chamber, and a large-bore pipe allows the oil
diffusion pump that would normally pump on the chamber from below to bypass the chamber and
pump off the upper portion of the column instead. The top of the specimen chamber cannot be
completely closed off because the electron beam must be able to enter it. So the pole piece insert
(a bullet-shaped device that contains the final aperture and is located where the electron beam
enters the specimen chamber) is replaced with one specifically designed for ESEM. This 'wet
bullet' insert has four pressure-limiting apertures (PLAs) in it. The apertures are simply discs with
small holes bored through the center. The principle at work here is that if there is a small enough
pinhole between two different vacuum levels, and the difference between levels of vacuum is not
that great, vacuum will not 'diffuse' from one level to another through the pinhole. So we can
have a very good vacuum at the electron gun, at the top of the column where we need a very good
vacuum, and at the mid-portion of the column, in the specimen chamber, we can have a relatively
poor vacuum, without endangering the electron gun. At the bottom of the column, the oil
diffusion pump is, as noted, bypassing the mid-portion and contributing to the better vacuum in
the upper column; this arrangement also helps scavenge any water vapor that rises through the
pressure-limiting apertures. The hole in the center of the gaseous secondary electron detector
(GSED) functions as the final aperture through which the primary electron beam passes, and its
bore size determines how poor the vacuum can be in the specimen chamber. The GSED in most
cases forms a fitted seal over the pole piece insert (the wet bullet). If the GSED has a 500-micron
aperture in it, we can increase the pressure in the chamber to as high as 10 Torr; if the GSED has
a 1-millimeter aperture in it, we can take the pressure in the chamber only as high as 5 Torr. And
if we use the large-field detector (LFD) version of the GSED, we do not actually fit it over the
pole piece insert, so the wet bullet itself provides the final aperture, and we can take the pressure
in the chamber only as high as 1 Torr.
The GSED has as much as a 600-Volt positive bias on it to attract secondary electrons. Modifying
the contrast control controls the bias; if the contrast is set at 100% we have 600 Volts on the
detector. This may be compared to the Everhart-Thornley secondary electron detector (ET SED)
on a normal SEM. The ET SED ordinarily has only as much as a 300-Volt positive bias on it, and
in addition it is relatively far from the sample. Thus the GSED is set up to collect secondary
electrons very efficiently. And now, this is how it works: Water vapor is introduced to the
specimen chamber via a separate dedicated vacuum pump that can control the chamber pressure
with great accuracy. The primary electron beam is very energetic, and it penetrates the water
vapor with little apparent scatter, scanning across the surface of the sample. Secondary electrons
are released from the surface of the sample, as they are in normal SEM, but they encounter water
vapor molecules once they exit the surface. The water vapor molecules, when they are struck by
the secondary electrons, produce secondary electrons themselves, which in turn produce
secondary electrons from adjacent water vapor molecules. Thus the water vapor functions as a
cascade amplifier, amplifying the original secondary electron signal from the sample. The
amplified secondary electron signal is collected at the GSED (very efficiently, as noted above),
with its strong and local positive charge. So we get a very good signal from the sample, and the
intensity of that signal is converted into a brighter or darker portion of the image at a given point
(x, y) on the sample as the electron beam moves across it.
Why don't we get charging from all of the secondary electrons impinging on the nonconductive
sample? Charging, which occurs in normal SEM when the energy from the primary electrons is
retained by the sample instead of being shed to an electrical ground, can produce terrible images.
The reason we don't get charging in ESEM-in wet mode-is that the strong positive bias on the
GSED drives the water vapor molecules, which are now nice plump positive ions, having lost
their secondary electrons, toward the sample. The sample has a net negative charge from the
primary beam electrons that have been bombarding it, and the positive ions that are driven toward
it effectively neutralize that charge. And everything looks great-we get great images from the
sample without having made it conductive or doing any other destructive things to it.
Of course wet mode imaging does not always work as perfectly as described above. One needs to
find the right combination of accelerating voltage, spot size, vapor pressure, and working distance
(very important), as well as working with a sample that doesn't have a huge nonconductive
surface. There are little tricks that may be performed to make it work right. Often, for example,
the image will look best if it is averaged a few times. And the parts of a large nonconductive
sample that are not to be imaged can be made conductive by painting them with carbon or silver
paint, helping the local process work that much better. ESEM is like SEM with two added
degrees of difficulty. But it has distinct advantages.