Vacuum modes
Some SEMs will allow you to control the level of vacuum that is being achieved in the sample chamber. This can be very useful when working with poorly-conducting or insulating samples, as the variation in chamber vacuum induces a self-neutralising effect at the sample surface, allowing imaging without the need for a sample coating. But it comes at a trade-off, the increased pressure impacts the quality of the imaging.
High vacuum (SEM) mode
It is always best, whenever possible, to operate an SEM under high vacuum. This removes as much of the atmospheric gas molecules from the source and column region and from around the sample surface. Under high vacuum conditions the maximum amount of signal can be detected without interference, leading to better quality images.
Additionally, some detectors, like most secondary electron detectors, require high vacuum conditions to function correctly. This is because they use a high voltage to pull in the SE signal would be damaged if operated under poor vacuum conditions.
Imaging non-conductive samples using the high-vacuum mode, for example to get the best resolution by collecting the SE signal, will usually require samples to be coated with a conductive layer of gold, platinum, or carbon.
Variable-pressure (VP-SEM) mode
When it is preferable to not apply a coating to a non-conductive sample, some SEM instruments can be instead operated in a variable pressure, or VP mode. Here the vacuum system allows a small amount of atmospheric gas into the sample chamber. These gas molecules are ionised by the electrons from the beam and the sample, and create positively charged ions. These ions can then neutralise some of the accumulated negative charge on the sample surface, reducing charging artefacts.
There are some drawbacks though: the SE detector cannot be used in the VP-SEM mode, leaving the BSE signal as the primary imaging signal. The increased pressure also interferes with the beam and the signal, leading to reduced image quality. However for many researchers the benefits of not coating samples greatly outweighs the slight decrease in performance.