Specimen (Stage) Drift/Instability in TEMs/STEMs
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Specimen drift in TEM observation is a common problem. The main reasons causing specimen drift are:
         i) mechanical instability of specimen holder,
         i) charging effects in the specimen,
         i) thermal instability of specimen holder.

Historically, mechanical instability of the specimen stage had been one of the major problems behind the resolution improvement. For instance, the specimen needs to be exchanged quickly and easily, and then needs to be moved smoothly in both x- and y-directions and be tilted at proper angles along the x- and y-axes. Once the position and tilt are selected, the specimen should stay still in atomic scale at high magnifications (e.g. x1,000,000) during the observation or exposure time. However, in many cases and labs, the operator needs to wait for ≥30 min (sometimes it takes several hours) after the specimen movement.

For TEM-related measurements, including EELS, EDS, STEM, TEM, and EFTEM measurements, the TEM condition should enable sufficient contrast and intensity to be recorded on a negative, image plate or CCD camera within a reasonable acquisition time to avoid specimen drift.

The interference pattern of the Ronchigram at the smooth center flashes randomly due to the small movements of the incident electron probe and specimen.

Note that the specimen drift also limits the phase contrast transfer function in TEMs.

Specimen cooling is one of the efficient common methods to reduce or even eliminate specimen contamination in TEM analysis. In this way, the mobility of the offending species is suppressed. However, specimen cooling causes some problems:
        i) Ice may form on the specimen. To avoid the ice formation, intermediate temperatures from -130 to 50 °C can be used.
        ii) Specimen drift.

Figure 4300 shows Thon rings with different image qualities.

Thon rings with different image qualities

Figure 4300. Thon rings with different image qualities: (A) good; (B) anisotropic due to astigmatic; (C) anisotropic due to specimen drift. [1]





[1] Electron Microscopy: Methods and Protocols, Second Edition, 2007, Edited by John Kuo.