A Closer Look Inside Semiconductors

Research team is further developing a high-resolution imaging process with which materials can be examined non-destructively and with nanometer accuracy

Images provide insights. What we can observe with our own eyes makes us understand. Constantly expanding the field of vision, even in dimensions that are initially hidden to the naked eye, is what drives science: ever more powerful microscopes allow insights into cells and tissues of living beings, into the world of microorganisms as well as into inanimate nature. But even the best microscopes have their limits. " In order to be able to observe structures and processes down to the nanoscale level and below, we need new methods and technologies, " says Dr. Silvio Fuchs from the Institute for Optics and Quantum Electronics at the University of Jena. This applies in particular to technological areas such as materials research or data processing. "Electronic components, computer chips or circuits are getting smaller and smaller today, ”continued Fuchs. Together with his colleagues, he has now further developed a method that makes it possible to depict and examine such tiny, complex structures and even to be able to “look into” them non-destructively. In the current issue of the specialist magazine “Optica”, the researchers present their method - coherence tomography with extreme ultraviolet light (XCT for short) - and demonstrate its potential for research and application.

Light penetrates the sample and is reflected off internal structures
The starting point for the imaging process is so-called optical coherence tomography (OCT), as it has been established in ophthalmology for a number of years, explains doctoral student Felix Wiesner, the first author of the study presented. “ These devices have been developed in order to be able to examine the retina in the eye non-invasively layer by layer and thus to create 3-dimensional images .” In OCT at the ophthalmologist, the retina is illuminated with infrared light. The radiation is chosen so that it is not absorbed too much by the tissue to be examined and cannot be reflected on the internal structures. For their OCT, the Jena physicists use extremely short-wave UV light instead of long-wave infrared. “ This is due to the size of the structures that we want to map“, Says Felix Wiesner. In order to be able to look into semiconductor materials with structure sizes of a few nanometers, light with a wavelength of just a few nanometers is required.

Non-linear optical effect produces coherent extremely short-wave UV light
Generating such extremely short-wave UV light (XUV) has previously been a challenge and almost exclusively possible in large research facilities. However, the Jena physicists generate broadband XUV in an ordinary laboratory and use so-called high harmonics for this. This is radiation that is created by the interaction of laser light with a medium and has a multiple of the frequency of the original light. The higher the harmonic order, the shorter the resulting wavelength. “ We use infrared lasers to generate light with a wavelength between 10 and 80 nanometers, ” explains Prof. Dr. Gerhard Paulus. “ Like the incident laser light, the resulting broadband XUV light is also coherent, meaning it has laser-like properties“, Explains the professor for non-linear optics at the University of Jena.

In the work that has now been published, the physicists illuminated nanoscopic layer structures in silicon with the coherent XUV radiation and analyzed the reflected light. The silicon samples contained thin layers of other metals such as titanium or silver at different depths. Since these materials have different reflection properties than silicon, these can be detected in the reflected radiation. The method is so sensitive that not only can the deep structure of the tiny samples be mapped with nanometer accuracy, but also - via the different reflection behavior - the chemical composition of the samples can be determined precisely and, above all, non-destructively. "This makes the application of coherence tomography for the inspection of semiconductors, solar cells or multilayer optical components interesting, ”emphasizes Paulus. It could be used for quality control in the manufacturing process of such nanomaterials in order to detect internal defects or chemical impurities.