Scanning electron microscopy (SEM) – in-situ techniques

About this technique

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There are many different techniques that involve the modification or manipulation on samples inside the scanning electron microscopy (SEM), usually accompanied by simultaneous imaging or analysis. These in-situ techniques are typically used to investigate the properties of samples on the micrometre or nanometre scale, although some in-situ techniques can be used to modify or create particular sample characteristics. Most in-situ techniques involve the use of specially designed sub-stages or probes that fit inside the SEM chamber but are controlled from software or control panels outside the chamber. A wide range of conventional SEM imaging and analytical techniques can be used with in-situ SEM, including secondary electron (SE) and backscatter (BSE) imaging and energy dispersive X-ray spectroscopy (EDS), cathodoluminescence (CL) and electron backscatter diffraction (EBSD) analyses. Many SEMs also provide the facility to record videos of dynamic changes in the sample: these videos can even be linked to other measurements such as sample temperature or the force of loading.

In-situ heating

In-situ heating in the SEM is achieved using a special high temperature sub-stage in which the sample is heated to a predetermined temperature at a specified rate. Temperatures up to 1500°C can be achieved, although temperatures in the range of room temperature to 600°C are more commonplace. The infrared radiation at higher temperatures can make conventional SEM imaging problematic, so dedicated high temperature detectors are sometimes used to provide real time imaging. In-situ heating is often used to study phase transformations as well as recrystallisation and annealing processes.

The sample size for in-situ heating is generally limited (<10 mm diameter), especially when very high temperatures are required, although lower temperature experiments can use significantly larger samples, even in combination with tensile or compression stages.

In-situ cooling

In-situ cooling in the SEM involves the use of cryogenic sub-stages that can be attached to the standard SEM stage. There exist several ways to cool the substages, but the most common involve cooling nitrogen gas with liquid nitrogen (achieving temperatures as low as about -180°C) or using peltier cooling technology (for temperatures down to about -50°C). More advanced stages using liquid helium cooling can achieve temperatures as low as +5° K, ideal for some semiconductor and thin film applications. Most cooling stages also have a limited heating capability, typically to about +100°C.

The most common applications for in-situ cooling are in the biological sciences, as samples with high water content can be observed without dehydration (although specialised cooling techniques may be needed to avoid damage to the sample structure). In-situ cooling can also be used to observe beam sensitive samples as well as ice.Dedicated cryo-transfer systems allow better preparation of samples for cryo-electron microscopy, including fracturing and coating of the samples whilst under vacuum and cryogenic conditions.

In-situ testing

In-situ testing techniques are increasingly common in the SEM (and TEM) and typically involve mechanical testing of samples using tensile stages that fit onto (or replace) the SEM stage. These can be configured for tension or compression, and involve forces ranging from <50N to >5000 N. It is common for in-situ testing to be carried out at high temperatures, especially during the testing of metallographic, geological or ceramic samples. Other applications include the testing of polymers, fibres or wires.

Most in-situ testing experiments involve recording of videos or timed sequences of images, correlated with the force measurements, in order to show the failure or deformation mechanisms of the sample in question.

Micromanipulation

There exists a whole range of micro- and nano-manipulation techniques for the SEM. Early micromanipulators were developed for use inside a focused ion beam (FIB)-SEM for lifting out FIB-prepared TEM samples, but there are now many novel techniques that include (but are not limited to):

• Electrical probing
• Microinjection
• Force measurement
• Nano/micro-indentation
• Nanoforging
• In-situ liftout

These micromanipulation techniques can be applied to almost any type of sample in an SEM.