Gatan, Inc. 5 Cathodoluminescence Pointers

Five Pointers for Cathodoluminescence

Pointer 1: Specimen Preparation

Cathodoluminescence (CL) is a competitive process between relaxation events that do and do not involve the emission of a photon. To increase the CL signal and the quality of results, the key is often to reduce the probability of non-radiative recombination events. Often, the most practical step is to increase the quality of specimen preparation.

CL does not fall into the traditional category of surface science techniques, but the quality of the near surface (top few hundred nm) strongly impacts the CL emitted. For example, traditional polishing techniques employed for thin geological sections create a “dead layer” that prevents work at low kV and higher spatial resolution. Furthermore, remnants of polishing compound can be very much brighter than the area of interest, and this can saturate the detectors at the gain required to image the region of interest.

Using ion beam or colloidal silica as the last polishing step provides the best surface quality. These also combat the problems of luminescent polishing compounds, and leave a hydrocarbon free surface. Alternatives include fracturing specimens to reveal a fresh surface. For semiconductor devices where plan view information is required, better results can be achieved by restricting the thickness of capping layers, or removal of surface metallization layers. For devices where carbon contamination has occurred and this needs removing, then a plasma cleaner will help.

Precision Etching Coating System (PECS)

SOLARUS: Advanced Plasma System

Pointer 2: Low Vacuum Work

Carbon coating of insulating specimens cannot be high on any microscopist’s list of favourite tasks, especially when there is uncertainty over the thickness being deposited. For CL applications, the carbon coat reduces spatial resolution because the generation volume is spread between the carbon film and the specimen. Furthermore the lower transparency of the film reduces the CL efficiency of the specimen. Many modern SEMs allow work in low vacuum conditions, in which case ionisation events from the residual gas prevents electrostatic charge build up on insulating specimens. The CL mirror does not normally suffer from condensation or contamination associated with low vacuum conditions. Furthermore, the “skirt effect” which reduces spatial resolution is usually negligible compared with the effect of a carbon coat. The “skirt effect” is naturally a function of the pressure and working distance, but a short working distance CL mirror can be used to negate this effect. Luminescence from gases does not normally occur with the mirror inserted above the specimen.

Pointer 3: Working Distance and Alignment

Gatan’s high efficiency CL mirrors are based on a diamond turned paraboloid. The focal point of this mirror needs to be at the zoom axis of the SEM. This is usually close, but rarely dead centric to the hole aperture as viewed at very low magnification. It is more important for the focal point (or “hot spot” of maximum collection) to be at the zoom axis, than for the mirror to appear centric at low magnification. Adjustment of the mirror and CL system with respect to the pole piece is relatively simple. “X” adjustment is the retraction mechanism. “Y” adjustment is achieved using lateral screws in the “clam shells” which hold the unit to the microscope flange on a sliding O-ring seal. This should be correctly adjusted at installation, but may need occasional attention, e.g. as mirrors are swapped.

A correct working distance to a tolerance of about 0.2mm ensures that light is collimated and therefore coupled most efficiently to the chamber mounted spectrometer and thence onto the light detectors. The light path is most critical when trying to achieve the maximum signal to noise using the MonoCL3 and XiCLone products in spectroscopy mode. Optimising the collection efficiency does impact the field of view however. If there is plenty of signal, and field of view is more important, then a longer working distance can be tried.

For panchromatic imaging modes, or using the ChromaCL system, the light path is less critical and hence the tolerance on the working distance less strict. However, if the working distance is much increased, then the signal reduces significantly as the light is not efficiently collected.

This principle means that if the SEM does not have continuous and fine variation in the Z height, then specimens of known thickness on known height specimen holders need to be employed.

Low magnification overlay of SE image and MonoCL image from uniform specimen. Specimen is at working distance for highest intensity coupling of light through monochromator.

Drawing and photograph (retracted) of the standard Short Working Distance MonoCL3 mirror inserted beneath pole piece.

Pointer 4: Injection Conditions

Gatan’s CL equipment is designed for maximum collection and detection efficiency in order to negate the need for high beam currents and accelerating voltages. Working at high kV reduces the spatial resolution, and may over sample the depth. Working at high beam currents may saturate traps, alter equilibrium conditions, cause heating and local charge build up, and accelerate the beam -inducing changes to the specimen. With a focused probe, a low kV introduces a higher injection density for a given beam current, than a higher kV. This means that if low injection conditions are required at low kV, but the signal level is small, a useful compromise for a spectroscopy measurement is to defocus the beam. This is obviously only applicable for spectroscopy experiments where high spatial resolution imaging is not important.

Low injection conditions tend to produce results which are easier to interpret, and bear a closer analogue to photoluminescence spectroscopy measurements. For some specimens, the simplest method of achieving results at low injection conditions is to work at cryogenic temperatures. This is because for many recombination events, cryo temperatures greatly reduce the probability of non radiative recombination paths, and hence the quantum efficiency of CL is much higher.

For geological specimens, the opposite may be true, in that non-equilibrium conditions may be required in order to stimulate the full range of light emission colours. Under high injection conditions, the scanning speed of the electron beam is noticeable as a variable. CL can be stronger at TV rate than at slower scanning speeds. This is because kinetics of the filling or emptying of traps is commensurate with the movement of the scanned electron beam.

If a specimen has extreme topography, then the argument for a low kV to limit the dimensions of the generation volume becomes irrelevant. In the case of nano rods or particles, the spatial resolution will be closer to the spot size and will be determined by how the topography restricts the generation volume. In a thin film specimen in a STEM, then high spatial resolution is achieved using a standard STEM high tension, e.g. 100-200kV. The most appropriate kV will only be found with experience as this is determined by the ratio between the radiative properties of the bulk compared to the surfaces of the thin film specimen.

The best CL results can be achieved with a judicious choice of injection conditions, and this is often learnt for a given specimen type by empirical evidence. For some specimens types, a cold gun FESEM with limited beam current may be appropriate, but perhaps only when used with a cold stage. In contrast, when studying minerals, cryogenic temperatures may not help, and a microscope with a greater range of beam currents may be appropriate.

ChromaCL image. Weak luminescencing archaeological pottery specimen, recorded at room temperature benefit from relatively high beam currents.

Pointer 5: Multi-Signal Detection

Gatan’s standard CL mirrors are designed to collect the maximum amount of light from the specimen. As such they are “greedy” with the available solid angle. In some instances, simultaneous detection of other signals may be more advantageous than maximizing the CL signal. Sharing the solid angle using a CL mirror custom designed for a multi-signal approach avoids the need for sequential acquisitions.

Such mirrors are designed to be interchangeable on venting the chamber. Gatan’s DigiScan II Digital Beam control system provides a flexible foundation for a multi-signal approach as up to 4 analogue and 4 pulse signals can be recorded simultaneously.

Furthermore, for microscopes equipped with a modern EDS system, Gatan’s DigitalMicrograph software can interrogate the EDS system computer for every pixel position, and hence record a simultaneous CL and EDS spectrum image. This has the advantage of true correlation, and provides a powerful step forward in characterization techniques.

Example multi signal mirror for EDS detector mounted opposite to CL system. This design allows ~50% of the collection efficiency of the standard mirror, a similar proportion for a pole piece mounted BS detector, enhanced SE detection, and unimpeded EDS detection.

Schematic of multi signal (CL +EDS) spectrum imaging system based on DigiScan beam mapping. DigitalMicrographTM provides powerful solutions for interrogating CL and EDS spectrum images.

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