You couldn’t live without semiconductors these days. It’s the engine that drives LED lights, computer displays and other technologies. Most electronics employ semiconductors, most notably computer chips.
Photoluminescence (PL) is a powerful tool for semiconductor characterization in the various stages in its life cycle. That includes development, testing, quality control, and failure analysis.
Most modern semiconductor devices are engineered materials made from multilayered structures fabricated on wafers. These are then diced up into individual devices. The process of engineering the base material, fabricating the wafers and characterizing the devices made from these wafers all depend on techniques like PL.
Photoluminescence phenomena result from materials absorbing excitation light photons and being raised into an excited state. In the case of semiconductors, these levels are typically above the bandgap of the material. When the excited species relax, it releases this excess energy in the form of luminescence or emission of photons. The emitted light is often characteristic of either the material or its surrounding environment, and can even provide information about local dynamics around emitting species.
PL is entirely a photon mediated process, so it is a non-contact and non-destructive method of probing materials. Therefore, manufacturers can integrate PL into the production process without destroying or contaminating the tested sample.
Modern semiconductors are highly engineered materials designed to exhibit certain defined behaviors. These materials are usually made on various substrates by epitaxial processes that stack different material layers one over another. Engineers build semiconductors from stacks of various atoms. This process has very tight tolerances, and PL is a tool that can be used to verify what is being made will perform as expected. The way you stack up the atoms, and which atoms you stack up, determines the function of what you ultimately get, from solar cells to LEDs.
For example, the LED lights in a home supply store have different colors. Some are bright white, and others have a yellowish tint. That is all materials engineering, and PL helps determine which parts of the wafer are fit for the various light sources.
PL offers unique signatures, providing operators with information on quality and other characteristics. It allows device designers and manufacturers to determine, prior to the manufacturing process, which parts of the wafer meet the functional requirements of the intended device before they are actually made. Once made, failure of a device to meet performance expectations can be very costly – so it is a lot cheaper to determine and weed out non-performing devices early on in the process. PL is a key technique for doing this.
Photoluminescence is also widely used for defect analysis in semiconductor analysis. Defects are usually foreign dopants that are embedded into a host material matrix – by design or accident, or, they can be structural deformations of the material itself. In either case, these defects affect the band structure of the material in which they occur. Since PL is really a measure of the band structure of the material, it therefore serves as a useful tool for defect analysis both in material engineering and device fabrication and quality control.
“Many of our industrial and research customers use our PL mappers such as (HORIBA’s) MicOS mainly in the process of determining wafer homogeneity and in other cases for defect analysis,” said HORIBA Scientific Optical Spectroscopy Division Product Line Manager Francis Ndi, Ph.D.
A typical PL mapper, such as the HORIBA MicOS PL mapper, works by scanning a focused excitation laser beam over a wafer or device and collecting the full PL spectrometer at thousands to millions of points across the structure. Various parameters of the PL emission can be displayed, as shown in figure 1
Foundational to any semiconductor device endeavor is the material engineering effort, to ensure that the materials used in the device exhibit the right properties that support the expected device performance. This work often boils down to band structure engineering, so PL is obviously a key analytical technique at this stage.
At the pre-production level, manufacturers use PL to fine tune the actual wafer fabrication process by characterizing such considerations as the homogeneity of the deposition process or presence and location of defects introduced intentionally or otherwise.
Technicians use PL at the end stage to do quality control to ensure that device performance is consistent across different fabrication batches. PL is also used to check and maintain the stability and robustness of the fabrication process itself – a tool to monitor and ensure the often tight tolerances required for correct device fabrication is maintained.
Finally, PL is also key in the analysis of failed devices, still part of quality control. A device may fail in the field. A section of your monitor might not appear correctly. It could happen on an industrial level, or even for an agency like NASA, where device failure can be very expensive. Researchers use PL to understand why the component failed, so they can correct it in the development or manufacturing stages.
HORIBA offers the MicOS Microscope Optical Spectrometer, part of its Standard Microscope Spectroscopy Solutions (SMS), to carry out these and other analyses. It is a modular, versatile and cost effective microspectrometer platform for steady state and lifetime photoluminescence.
The MicOS combines a custom high throughput microscope head with a high-performance, triple grating, imaging spectrometer that can accommodate up to three different detectors. HORIBA can customize the MicOS with various light sources and detectors to meet the requirements of the research at hand. The MicOS merges microscopy and photoluminescence spectroscopy, to provide optimal coupling from the sample, all the way to the detector.
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