Cathodoluminescence For High-volume Manufacturing
Our industry prides itself on the production of countless high-quality, reliable devices, which can be degraded at the nanoscale by threading dislocations, stacking faults, inclusions and point defects. Often these imperfections arise during the epitaxial process, due to differences in the lattice or thermal expansion coefficients between the substrate and subsequent layers. However, they can go unnoticed until the end of the line, or even appear in shipped product, where they pose a serious threat to the device's capability and reliability.
If unnoticed, a killer defect can turn out to be incredibly costly. It is not just the expense associated with undertaking unnecessary production steps on a chip that is destined for failure. The cause of the defect may also need tracing "“ and when it is exposed, it can reveal a drift in the manufacturing process, and a need to scrap much material. There is also the worst case scenario: chips fail in the field, resulting in angry customers that take their business elsewhere; and the need for expensive failure analysis.
What's needed is a reproducible, non-destructive defect inspection method that is fast enough for use in production. And at Attolight of Lausanne, Switzerland, we have a cathodoluminescence tool that does just that, the Säntis 300.
Cathodoluminescence is a characterisation technique that involves directing a beam of electrons at a sample, and recording the light that is emitted from it. Much can be resolved, because the electron beam can be focused down to a few nanometres. This allows a sample to be mapped with nanometre resolution, beating light's diffraction limit by several orders of magnitude. What's more, cathodoluminescence can be spectrally resolved, offering tremendous insight into material properties (see Figure 1).
Cathodoluminescence is even capable of detecting threading dislocations, a defect responsible for short circuits in power transistors. This may raise a few eyebrows, given that a threading dislocation has a diameter of an atom, and normally it cannot be resolved by an electron probe that is one or two orders of magnitude larger. But cathodoluminescence offers unparalleled sensitivity to defects, due to the light emission process.
In the cathodoluminescence process, electrons that hit the surface subsequently impinge on the semiconductor, slow down and undergo multiple scattering events. Every time there is scattering, electrons within the semiconductor are excited from the valence band to the conduction band to form electron-hole pairs. These pairs eventually recombine, emitting photons in the visible range.
As the cathodoluminescence emission spectrum is directly related to the difference in energy between the valence and conduction bands, it is extremely sensitive to the electronic band structure "“ and ultimately extremely sensitive to any atomic change or atomic defect. A mere vacancy or dopant inclusion can disrupt the material's band structure over length scales up to tens of nanometres. Consequently, even though a defect may be far smaller than the electron probe, it affects cathodoluminescence over tens of nanometres, and its presence can easily be detected with a state-of-the-art cathodoluminescence scanning electron microscope.
The emission energy of the cathodoluminescence depends on the nature of stacking faults, local strain variations and point defects, such as dopant inclusions or vacancies. Threading dislocations act as non-radiative recombination centres, quenching cathodoluminescence. Defect sensitivity can be as good as 1016 atoms cm-3, making this, to our knowledge, the most sensitive non-destructive method for uncovering defects.
It is easy to see why cathodoluminescence's high sensitivity to defects makes it a great technique for performing failure analysis and R&D characterization. However, in a high-volume manufacturing environment, how can it be practical to sample a full wafer every ten nanometres? The good news is that it doesn't have to be this way, thanks to cathodoluminescence's greatest weakness becoming its advantage: carrier diffusion.
Figure 1. Acquisition of a cathodoluminescence map. The electron beam scans the surface of a wafer. At each point, light is emitted, spectrally resolved so that a hyperspectral map (a map made of multiple colours) is measured.
Carrier diffusion: a curse to a blessing
When electron-hole pairs recombine at the place of generation, cathodoluminescence stems from the precise location of the focused probe. But in a defect-free semiconductor that's not always the case, as electron-hole pairs can diffuse over hundreds of nanometres before recombining.
At first glance, that suggests that electron-hole pair diffusion reduces the image resolution. But that's not the case.
Consider a killer defect, such as a threading dislocation. The defect disrupts the local electric field, and traps electron-hole pairs that are nearby. As these pairs cannot diffuse or recombine radiatively, local cathodoluminescence is quenched. For electron-hole pairs generated further from the defect, the chances of diffusing towards it are far less, making radiative recombination more likely. Due to these factors, defects appear in cathodoluminescence images as very small dark spots, limited by the probe size and interaction volume, that fade away over a few hundreds of nanometres.
Sometimes defects may radiate. However, they will always modify the carrier wave function in their vicinity, and provide a distinctive cathodoluminescence spectrum. This may be used to identify a defect or a population of defects, even if they have an atomic size.
Note that a map can even be acquired by spacing out measurement points by hundreds of nanometres. So long as the spacing between measurement points is smaller than the diffusion length, the mapping technique will uncover defects. It is this feature that makes cathodoluminescence the ideal low-sampling technique for spotting loosely distributed defects without having to destroy the sample.
Figure 2. (left) Spectra from point 1 (on a dislocation) and point 2 (stacking fault). (centre) Secondary electron microscope map acquired simultanously to (right) hyperspectral map, having a colour for each defect band. The blue band shows threading dislocations, the green and red band show stacking faults on a GaN template.
A poor reputation
Cathodoluminescence is not a new technique. The phenomenon was first reported as far back as 1879, and cathodoluminescence microscopy has been known since the 1960s. However, use of this technique has been limited to the laboratory, where it has a poor reputation, due to the lack of dedicated instruments.
While collecting cathodoluminescence emitted by the sample is simple in principle, it is not in practice. Difficulties occur, because in the electron microscope that provides the well-defined electron beam, there is competition for the same space by the objective lens of the light and that of the electron microscope.
Up until now, the solution has been to insert a conical mirror, in either a parabolic or elliptical form, into an existing electron microscope.
Figure 3. Wafer map showing a wafer with the user defined measurement locations.
One of the many downsides of this approach is that the conical mirror exhibits significant off-axis aberrations, impairing imaging when the wafer is probed anywhere except at the exact focal point of the mirror. Aberrations are so significant that light emitted from the edges of the cathodoluminescence map hits the aperture stop, so it is clipped before reaching the detector "“ a measurement artefact called vignetting.
Additional drawbacks are that: secondary electron detection is affected by the mirror, making it hard to reach the microscope's ultimate resolution; and electron microscopes are not designed to work at the optimal conditions for cathodoluminescence, which are a low beam energy and a high current. Instead, a high electron beam current is often traded for a very high resolution at low current.
All these issues have hampered cathodoluminescence, which has a reputation for a lack reproducibility and stability, and lengthy alignment times. These drawbacks motivated us to build the first dedicated cathodoluminescence scanning electron microscope, the Allalin, and its fullwafer counterpart, the Säntis 300.
The latter breaks new ground by bringing cathodoluminescence to high-volume semiconductor production. It has been constructed to produce the highest resolution cathodoluminescence maps possible, and deliver an unprecedented signal-tonoise ratio. Measurement physics, not technology, is the tool's ultimate limit.
One of the key features of the Säntis 300 is its entirely new cathodoluminescence collection objective, which has zero off-axis aberration and zero photon loss "“ allowing for perfectly uniform, reproducible, and quantitative measurements with the highest possible collection efficiency. This approach eliminates alignment time and maximises the signal-to-noise ratio to its physical limit "“ no matter where the electron probe is placed.
Another attribute of our tool is its state-of-the-art field emission gun technology. The advanced gun trims aberrations by a factor of three and produces higher brightness. A higher probe current is used without compromising resolution. This helps us to dramatically improve the signal-to-noise ratio, speeding cathodoluminescence measurements of weakly emitting specimen, and in some cases making it possible to image a sample.