From Micro-area Thin Films to Sub-nanometer Heterostructures: Polycapillary X-ray Optics Empowering XRF Analysis of Semiconductor Thin Films

Figure 2. Cross-sectional micrograph of a polycapillary X-ray lens

Based on this concept, this article will elaborate on several capabilities required for semiconductor thin film analysis: high-throughput micro-area excitation, depth-resolved analysis of layered structures, low-background particle detection, in-situ observation of solid/liquid interfaces, and ultimately the identification of sub-nanometer ultra-thin layers in semiconductor heterostructures. Through these references, it can be seen that the value of polycapillary X-ray lenses lies not only in "making the signal stronger," but also in advancing laboratory XRF toward smaller spot sizes, lower backgrounds, and more complex sample environments.

Figure 3. Confocal micro-XRF systems compared by Smolek et al. and depth profiling results of a multilayer automotive coated steel sheet. The left image shows the actual optical path of the Osaka City University system, and the right image shows the results of elements such as Ti/Sn, Mn/Ni/Zn, and Fe varying with scanning depth in the coated steel sheet sample.

Smolek et al. compared two confocal micro-XRF systems from Osaka City University and Vienna University of Technology^[1]. In the system characterization phase, the researchers used single-element thin film reference samples with a thickness of 500 nm to evaluate depth/spatial resolution, and used NIST multi-element glass standards to evaluate detection limits; in the application validation phase, non-destructive depth profiling was further performed on pigment layer samples and automotive coated steel sheet samples. The Osaka City University system employed XOS polycapillary optics at both the source and detector ends, with a nominal spot size of approximately 10 μm under Mo Kα conditions and an intensity gain of 7700 times. For the multilayer automotive coated steel sheet, the system was able to resolve the depth distribution of Ti/Sn in the upper layer, Mn/Ni/Zn in the intermediate layer, and Fe from the steel substrate at the bottom, demonstrating that the combination of polycapillary focusing and confocal detection volume can advance laboratory XRF toward micron-scale layered structure analysis. In the context of semiconductor thin films, this capability corresponds to non-destructive micro-area depth analysis of metallization layers, barrier layers, packaging coatings, and thicker functional film layers.

Figure 4. Optical path schematic and principle diagram of confocal μXRF

The research team at Osaka City University constructed a confocal micro-XRF optical path using dual polycapillary lenses^[2]. The lens at the source end focuses X-rays into the sample, while the lens at the detector end restricts the detector to receiving fluorescence signals emitted only from a specific spatial volume; when the two focal points coincide, a micron-sized "confocal detection volume" is formed. The actual samples in this study were forensic materials such as automotive paint chips and artificial leather, rather than semiconductor samples; however, it addresses a problem isomorphic to semiconductor thin films: how to obtain elemental depth distributions inside multilayer materials without sectioning. In the experiments, the system achieved a depth resolution of approximately 13.7–22.6 μm and performed non-destructive depth imaging of elements such as Cl, Ti, Fe, Ba, and Zn in multilayer coatings. This result can serve as a methodological reference for depth analysis of semiconductor packaging coatings, multilayer metallization structures, and thicker functional film layers.

Figure 5. Non-destructive elemental depth imaging of Cl, Ti, Fe, Ba, and Zn in a black paint chip using confocal μXRF.

Figure 6. Optical path layouts of μXRF and C-M-XRF and spectral comparison of small particles. The confocal configuration reduces substrate scattering background by limiting the detection volume.

The experimental results showed that in detecting small particles, the confocal configuration significantly improved the signal-to-noise ratio: for Si, the SNR improvement was approximately 14.6 times; for Fe and Cu, the improvements reached 21.9 times and 43.5 times, respectively. Meanwhile, the limit of detection (LLD) improved by approximately 10.8 to 24.7 times, with the detection limit for Cu reaching the order of 0.034 ng. For semiconductor thin film processes, this capability can correspond to rapid non-destructive identification of particles on film surfaces, metallic contamination, process residues, and micro-area impurities in packaging materials.

Figure 7. Confocal micro-XRF elemental distribution results of the corrosion process of a hot-dip galvanized steel sheet in NaCl solution.

This experiment is not a direct semiconductor thin film case, but it demonstrates an important capability: X-rays can penetrate the liquid layer, and the polycapillary confocal structure can confine the analysis volume to the vicinity of the target interface. The researchers recorded in real time the loss of the Zn coating and the distribution changes of corrosion-related elements such as Fe and Cl. For semiconductor thin film research, this type of method is more suitable as a capability expansion for wet processing, electroplating, interfacial reactions, and material reliability studies, rather than being simply equivalent to in-line production inspection.

Researchers from the University of Sheffield in the UK and Bruker Nano Analytics compared EDX, WDX, and a μXRF system equipped with a polycapillary focusing X-ray source in an SEM^[6]. Polycapillary optics focus X-rays from a high-voltage X-ray tube to a few microns in scale, enabling the SEM platform to not only rely on electron beam excitation for EDX/WDX, but also introduce an independent micro-area XRF excitation channel. It should be accurately stated that this study did not prove that μXRF outperforms WDX in all metrics; the paper notes that WDX has the best sensitivity due to its low background. However, the advantage of μXRF lies in providing a new X-ray excitation path within the SEM, which, combined with a high-voltage X-ray tube and polycapillary focusing, can reliably detect and quantify sub-nanometer semiconductor layers.

Figure 8. μXRF spectral data of the InAs-on-GaAs sample at 25 kV and 50 kV. The upper right inset shows an enlarged view of the 2.2–4.2 keV region around the Rh-L and In-L lines.

The研究对象 was a 0.8 nm InAs ultra-thin layer deposited on a GaAs(001) substrate. EDX, WDX, and μXRF were all able to detect and quantify the In-L line signal, with μXRF giving an InAs thickness of approximately 0.8 ± 0.2 nm, consistent within error with the results of the other methods. The authors noted that this was, to their knowledge, the first time μXRF was used in an SEM to reliably detect and analyze a sub-nanometer surface layer. In the progressive logic of this article, this case precisely brings the high-throughput micro-area excitation, low-background analysis, and thin film quantification capabilities mentioned in the previous sections back to the characterization of semiconductor ultra-thin heterostructures.

Figure 9. Schematic comparison of the excitable/detectable elemental range between SEM-EDS and an add-on μXRF module.

Furthermore, coupling a μXRF module with polycapillary optics in an SEM is not intended to replace EDX or WDX, but to add an X-ray excitation path to electron microscopy analysis. Electron beam analysis has advantages in light elements, surface sensitivity, and high spatial resolution; μXRF, on the other hand, can more effectively excite certain higher-energy spectral lines using a high-voltage X-ray tube and reduce the impact of the electron beam's continuous background on weak peak identification. For semiconductor thin films and microelectronic materials containing heavy elements, thick packaging layers, metallic interconnects, or buried structures, this complementarity has practical significance.

In this capability chain, polycapillary X-ray lenses play the key role of "turning a laboratory X-ray source into a usable micro-area probe." XOS, as an important manufacturer in the field of polycapillary optics, has its polycapillary optics widely used in commercial instruments and various customized X-ray analysis systems, serving fields such as microelectronics, semiconductor manufacturing, materials science, pharmaceuticals, and life sciences.

As a long-term strategic partner and general agent of the U.S. XOS company in the Chinese and Southeast Asian markets, Top Unistar aims not only to introduce a component, but to help users weigh "light source, lens, detector, sample environment, and analysis target" within the same system. For semiconductor thin films, advanced coatings, and microelectronic material testing, truly valuable solutions often come from the matching of these details.

WeBeijing Top Unistar Technology Co., Ltd.understand the working principles of capillary X-ray lenses, performance simulation, and their coupling requirements with X-ray sources, and can provide specification recommendations, customized solutions, engineering integration support, and testing and verification services for users across different application scenarios such as two-dimensional diffraction XRD, parallel beam XRD, XRF, full-field microscopy, extreme ultraviolet spectroscopy, and neutron collection. Putting every X-ray photon in a more appropriate place is also the most fundamental and important value of polycapillary X-ray lenses in advanced thin film characterization.