In-SEM electrical probing

Electrical engineers rely on needle-like probes for electrical measurements. This seems to be true at all physical scales, from average circuit-testing using a digital multimeter, up to atomic-resolution scanning tunnel microscopy. Probes are used to measure or apply electrical voltages between points of a circuit. In a SEM, probes with diameters in the nanometre range are desirable to probe microstructures and highly miniaturized components in integrated circuits. Such probes are produced using electro-chemical etching of metallic wires. Usually tungsten wire is used, although other metals and various coatings are available too. Often, probe material is not crucial and specialty tips are reserved to measurements where contact potential difference (Volta potential) is of concern. 

Positioning the probe and landing it on the sample require extreme precision. This is achieved using piezoelectric mounts capable of both high-speed, fast stepping movements (stick-slip piezo motion), for displacements over millimetres, and very fine movements for landing the probe without damage. Using probes of the finest diameters, expert users can probe areas on the sample as small as 10 nanometres in diameter. 

Depending on the experiment performed, multiple robots can be used jointly. Typical measurements include 2- and 4-point resistivity measurements, transistor testing, as well as I-V curve measurements on diodes and solar cells and, of course, EBIC/EBAC imaging. In the constrained space offered by an SEM chamber, integrating more than a few robots is a real challenge. The most advanced systems offer up to 8 probes that can be used jointly. Figure 2 shows three commonly encountered nanoprobing configurations. The type and complexity of the measurement determines the number of probes needed. 

To summarize, nanoprobes are essential to perform electrical measurements in localized regions of various samples. Their use inside scanning electron microscopes removes any gap that could exist between the resolution of advanced lithography techniques, capable of creating devices of nanometric dimensions, and the resolution of common imaging techniques. Their in-SEM use also expands the possibilities of combining various characterization techniques and allows the user to specifically and dynamically probe regions of a sample that present interesting physical properties identified by optical spectroscopy. Nanoprobes are widely used in the development of microelectronic devices, but also find use in establishing the properties of novel materials, that often only exist as micro crystals.

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In-SEM electrical probing instrumentation pivots around two devices. First, the probes themselves. Common systems typically use 2, 4 or even 8 probes in parallel, with a tip radius that varies from <100 nm to microns depending on the application. Temperature control both for cryogenic and high temperatures are sometimes used. 

On the other hand, the electrical test equipment itself is important. Source measurement units (SMUs) are often preferred for their versatility. These instruments combine a high-stability constant voltage/current source with a precision digital multimeter. Any combination of voltage/current sourcing (sinking) and voltage/current measurement can be performed on two or four terminals. In some cases, such as transistor testing illustrated above, two SMUs can be coupled to control gate voltage and source-drain voltage while measuring source-drain current and gate leakage current simultaneously. As increasingly complex experiments are performed, requiring higher number of probes and more control signals, large systems combining many SMUs in a single Semiconductor Parameter Analyzer system can be used.