The TESCAN MIRA 4 is a high resolution field emission scanning electron microscope (SEM) that uses a Schottky source. The main parts of the system are:

(1) The column and specimen chamber, resiliently mounted on top of the column console (2), which houses a turbo pump and some electronics. Further electronics are housed in the electronics cabinet (3) just visible behind the screens. The MIRA 4 comes with a single 32” screen (4). I have added a second smaller screen to permit easy access to my stubs database and image library.

The microscope is controlled using the latest release of TESCAN ESSENCE software installed on a Windows®10 PC (5) and is operated using a keyboard, mouse and tracker ball. Optionally, a control panel may be supplied. The MIRA 4 is supplied with a scroll pump for evacuating the chamber, and an uninterruptible power supply unit (6), to preserve the electron source in the event of short term power failure.

 Also required are a small air compressor and a cylinder of dry, oxygen-free nitrogen gas - used during venting the chamber and also for the Univac (low vacuum) mode.

The picture on the right shows the area of the column in more detail. The cowling over the upper half of the structure conceals the column, which contains the electron source, magnetic lenses, and some vacuum pumps. The vertical rectangle in the lower half of the photograph is the door to the chamber, where the specimens to be examined are inserted. The sloping cylinder on the right is part of the secondary electron detector.

The Everhart-Thornley secondary electron detector (ETD) can just be seen within the chamber (left). My configuration also has a low energy four-quadrant Backscattered Electron Detector (BSED), a Gaseous Secondary Detector (GSD) for low vacuum mode and an in-lens secondary electron detector (in-lens SE) for ultimate resolution at high magnifications.

The picture on the left shows the two ion getter pumps, used to maintain the vacuum at the electron gun at <10-8 Pa, and that of the column at ~ 5 x 10-6 Pa. The vacuum in the chamber is maintained (in HighVac mode) at around 5 x 10-4 Pa by the turbo pump in the console.

Opening the chamber door reveals the stage (right), which has positions for up to seven 12 mm diameter sample stubs. The stage can me moved, using the ESSENCE software, in the X, Y and Z directions, and may also be rotated and tilted, again under software control. The software has provision to maintain the position of the stub in relation to the pole piece following rotation or tilting operations.

The scroll pump (far left) is used to “rough pump” the chamber down to the pressure at which the turbo pump can start operating. This pump continues to operate all the time the SEM is in use, and is only switched off when set to power-save mode.

The air compressor (near left) provides a reservoir of compressed air that is used to actuate a Column Isolation Valve which isolates the column from the chamber. It is used to maintain the high vacuum in the column and gun when the chamber door is opened, or to interrupt the electron beam when not imaging, to preserve the gun vacuum, avoid the build-up of charge on a specimen or to prevent damage to delicate samples. 

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A Tour of the SEM

Technical specification for the MIRA 4

All the pumps, valves and compressor mentioned are “oil free” so that there is no risk of specimen contamination, the enemy of quality imaging in an SEM.  The MIRA does not require cooling water, as its low power modern electronics are very economical and (importantly these days!) reduce the overall carbon footprint. In fact the whole system can operate off a single 13 Amp outlet.