Satoru Suzuki

The response of a material to light is described by the complex dielectric function . Therefore, determining (E) at various photon energies E is crucial for the development of photosensitive and optical materials. Valence-band excitation electron energy loss spectroscopy (EELS) is a powerful analytical technique for determining  (E) from visible light to the vacuum ultraviolet. EELS is primarily used in conjunction with a transmission electron microscope and can achieve atomic-level spatial resolution. However, EELS generally requires thin-film processing of the sample, making it impossible to analyze a thin film supported on a substrate. In this study, we attempted EELS measurements using hard X-ray photoelectron spectroscopy (HAXPES). Here, we use core level photoelectrons as the electron source. First, the validity of the measurement was verified by analyzing Ar gas using Ar 1s photoelectrons as the electron source. Next, a thin film of ZEP520A resist on a Si substrate was analyzed using Si 1s photoelectrons from the Si substrate as the electron source. This enabled us to obtain EELS data for a thin film sample on a substrate without the thin-film processing.

The electrical charging of insulating samples is a common problem in photoelectron spectroscopy. Although heating has been suggested as a way to minimize surface charging, systematic studies of this are scarce. We have found that during 8 keV photoelectron spectroscopy using a synchrotron, the extent of charging of three highly insulating materials could be significantly reduced with increasing temperature. The charging of glass and LiNbO3 samples could be almost completely compensated at a moderate temperature of ∼400 °C. Based on the temperature dependence of charging-induced binding energy shift, the activation energies of the charge compensation were evaluated to be about 0.73, 0.65, and 0.21 eV for a glass slide, cover glass, and LiNbO3, respectively. Furthermore, charge compensation was achieved by surface conduction rather than bulk conduction.

Abstract Detection of a small amount of oxygen vacancies is often difficult. In this study, using near-ambient pressure hard X-ray photoelectron spectroscopy, oxygen vacancies were formed in situ in SnO_{2 − x} and WO_{3 − y} under a reducing gas atmosphere. The number of oxygen vacancies was so small that they could not be detected in the core-level photoelectron spectra. However, the effects of the vacancies were observed in the valence band spectra. This is because under the measurement conditions, the relative sensitivity of the metal outer orbitals (Sn 5s and W 5d) occupied when the oxygen vacancies are formed is significantly higher than that of the O 2p orbital predominantly forming the valence band. The x and y values were estimated to be ∼0.007–0.01 and ∼0.008–0.02, respectively, which correspond to vacancy ratios of sub-percent.

In recent years, enclosing liquid in a “liquid cell” (Fig. 1) has made it possible to analyze the chemical composition of the liquid sample and map samples in the liquid-solid interface using XPS, EDX, SEM in ultra-high vacuum (UHV). The most important part of the cell is the electron transmission window that allows electrons to input to or extract from the liquid sample. Silicon nitride (SiNx) is the most suitable material as the window, which has excellent mechanical properties. R. Endo et. al. has successfully measured the concentration of CsCl solution using a SiNx window with the film thickness of 5 nm¹. However, the transmittance of 1 keV photoelectron is about 10% with the thickness of SiNx membrane. When the membrane thickness is decreased to 2 nm, the transmittance can be improved to more than 40%, which means that even higher sensitivity can be achieved. However, the burst pressure must be 1 atm or more because the membrane needs to perform enclosing a liquid sample in UHV. In previous research, we demonstrated low damage etching of an SiNx film using gas cluster ion beam (GCIB) for thinning the window². Herein, GCIB is composed of aggregates of several thousands of atoms, and the energy of the individual atoms in the GCIB reduces to several eV/atom with an acceleration voltage of several kV. This enables irradiation on the target surface with little damage. Combining O₂-GCIB irradiation and exposure of acetylacetone (Hacac) gas enable reactive etching of SiNx². In this study, we utilized the above techniques to ultra-thin SiNx films and investigate whether the low-damage irradiation effect of GCIB is effective for SiNx films. In addition, Proof-of-principle of high sensitivity in liquid sample detection using ultra-thinning SiNx films is demonstrated using SEM/EDS. We have evaluated the burst pressure of ultra-thinned SiNx membranes of TEM window chips (SiMPore, Inc.) by reactive etching with O₂-GCIB and Hacac gas, where the thickness was originally 11 nm. The SiNx membrane was also etched in the same way using 400 eV Ar⁺ beam, and the burst pressure was evaluated. These irradiation doses were controlled so that the remaining SiNx membrane thickness was 4.5 nm. As a result, the burst pressure was 1 atm in the case of Ar⁺ beam, whereas it was 2.5 atm in the case of the GCIB etching. Therefore, it was found that low damage irradiation of GCIB was valid for thinning SiNx membrane. We sealed pure water in the liquid cell shown in Fig. 1 and performed EDS measurement. When electron beam with the energy of 5 keV was injected at the SiNx/water, generated characteristics X-ray of oxygen which photon energy is 0.52 keV could be observed. On the other hands, no O peak was observed in the SiNx/Si region. This result indicates that the O peak originated from the pure water under the SiNx membrane. Next, we carried out the measurements on a pristine SiNx membrane (t = 11 nm) and the GCIB-etched SiNx membrane (t = 4.5 nm), and compared their O peak characteristic X-ray intensities. Herein, the incident electron energies were 1.0 and 1.5 keV. The peak intensity of the GCIB-etched SiNx membrane was 1.6 times stronger than that of the pristine membrane at 1.5 keV. At 1.0 keV, the O peak was observed in GCIB-etched SiNx, but not in pristine SiNx. This is because the penetration depth of electrons in the SiNx film decreases with decreasing electron energy. From the above results, we have shown that the ultrathin SiNx membrane thinned by GCIB could be used for highly sensitive detection of pure water. Acknowledgement This work was supported by JSPS KAKENHI Grant Numbers 23K13236 and 22K04930. References [1] R. Endo, D. Watanabe, M. Shimamura et. al., Appl. Phys. Lett., 114, 173702 (2019). [2] M. Takeuchi R. Fujiwara, N. Toyoda, Jpn. J. Appl. Phys., 62, SG1051 (2023). Figure 1 <p></p>