High Sensitivity of Energy Dispersive X-Ray Spectroscopy Measurement Using Liquid Cell with Electron Transmittance Window Ultra-Thinned by a Gas Cluster Ion Beam
Authors: Takeuchi Masaya , Suzuki Satoru , Toyoda Noriaki 
Published: December 2024

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