(Digital Presentation) Investigation of the Damage of Focus Ion Beam (FIB) on Nanocrystals Formed in Terbium-Doped Silicon Oxide Thin Films
Authors: Badkoubeh Hezaveh Parnia , Mascher Peter , Khatami Zahra 
Published: August 2024

Terbium-doped oxygen-rich silicon oxide (ORSO:Tb) thin films were fabricated using Integrated magnetron sputtering and electron cyclotron plasma-enhanced chemical vapor deposition (IMS ECR-PECVD) method. To investigate damage from focus ion beam (a comparison is made between mechanical and FIB methods measurements, which did not reveal crystal damage induced by FIB.
Silicon-based thin films are fundamental materials in electronics and photonics, and doping is a common method to enhance silicon-based thin film’s luminescence. Terbium (Tb), characterized by well-defined emission lines, ranging from 400 to 620 nm (green emission), provides compelling reasons for using Tb as a dopant in silicon-based thin films (1).
TEM is a method employed for studying. Preparation methods are required to make a TEM sample that is transparent to the electron beam (with a thickness of approximately 100 to 200 nm) (2). While conventional techniques, like electrolytic polishing and mechanical milling, are commonly used for preparing TEM samples, they fall short of achieving the precise preparation of a TEM sample in a specific area of interest with high spatial resolution.
FIB has been used for preparing TEM specimens, which is fast and reliable. By scanning the beam across the cross-section, it becomes possible to reduce the thickness of the sample to prepare it for TEM (4)(5). The primary issue is due to the impact of ion beams accelerated to high energy therefore, examination effect of FIB on structure is important. Damages resulting from using FIB have mainly been studied on Silicon-based materials (6)(7)(8).
In this paper, Tb-doped silicon oxide (oxygen-rich) thin film deposited by IMS ECR-PECVD offers a more uniform distribution and well-controlled rare-earth doping (9). To investigate the damages induced in the preparation process, ORSO:Tb annealed at 1200 °C was divided into two parts, and then a comparison between the mechanical and FIB preparations for TEM samples was made.
ORSO:Tb were fabricated using an IMS ECR-PECVD system, employing 2 sccm SiH4 at 30% Ar and 28 sccm O2 at 10% as silicon (Si) and oxygen (O) precursors. These gas sources were delivered to the ECR chamber, pure Tb target with a sputtering power of 30 W and the substrate temperature was set at 350 °C. Following deposition, the thin film underwent annealing at 1200 °C for 1 hour.
The TEM samples prepared by mechanical method and annealed at 1200 ⁰C, Fig. 1(a), showed a well-defined crystal structure. The dark points on the sample represent the nanocrystals of terbium disilicate. The average thickness of the sample is approximately 168.88 nm. Fig. 1(b) displays the ORSO:Tb annealed at 1200 ⁰C, showing the cross-sectional morphology of well-ordered nanostructures on the specimen with Tb doping, prepared by FIB. The cross-section of the thin film is well-ordered, and the FIB process did not disrupt the crystal structure of the sample annealed at high temperature, nor did it lead to the formation of a new structure. Furthermore, a comparative analysis between the TEM-prepared sample employing the mechanical method and the one prepared using FIB makes it evident that the FIB-prepared sample possesses superior resolution. The size distribution of particles is approximately the same, while due to the higher resolution of the TEM sample prepared through FIB, exhibits a particle distribution with a greater range with more details. In contrast, particle size determination in the mechanical method is more challenging due to its lower resolution, determining details and size characterization more difficult.

Conclusion

The ORSO:Tb sample was fabricated using the IMS ECR-PECVD system at 1200 degrees. Based on the findings presented in the paper, the effect of FIB, particularly at high annealing temperatures, does not induce significant damage to the crystal structure of ORSO:Tb thin film. The particle size distribution in the TEM sample prepared with FIB is broader and more detailed than in the mechanical due to its higher resolution. Particle size determination using the mechanical method is more challenging due to its lower resolution, making it more difficult to distinguish details and characterize sizes. Additionally, atomic composition analysis indicated the accurate abundance of oxygen in oxygen-rich silicon oxide.

Figure 1