The construction of compact semiconductors with reduced thickness and size is desirable for their application in microelectronic devices and transistors. However, measuring the parameters such as contamination, surface composition, and thickness of a thin film semiconductor material is challenging.
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It is therefore necessary to address the above issues via an analytical technique without deconstructing the material. To this end, Rutherford backscattering spectroscopy (RBS) can serve the purpose of parameters analysis. The backscattering of helium (4He+) charged ions, called alpha (α) particles, allows the determination and depth profiling with a depth resolution of 100-300 Å in the solid region up to 1µm thickness.
RBS and its Advantages in Thin Films
RBS is an analytical technique based on an ion beam that provides accurate information on the elemental composition near the surface of a material. This technique helps obtain the composition profile of the material within the depth of a few micrometers from the surface. In addition to thin films, this method is also used in bulk samples.
The basic principle behind RBS consists of placing a target in a monoenergetic 4He+ ion beam generated from an ion accelerator. These α particles interact with the target sample wherein a few ions penetrate the target while the other ions are elastically scattered by surface atoms.
Particles backscattered by the target are detected and analyzed and, using electronic techniques, a backscattering energy spectrum that contains the information on the nature of elements present in the target and their depth distribution can be obtained.
Analysis of Depth Profile
The maximum depth analysis in RBS is defined by the depth from which the scattered particles emerge with zero energy. RBS can be widely applied for atom localization in the form of impurity, implanted species, or markers in thin films by converting the energy scale into a depth scale.
The Rutherford scattering cross-section highly influences the interpretation of the RBS spectrum. The energy dependence leads to an increase in backscattering yield when the energy of the incident He ions decreases.
In thin film analysis, the total energy loss directly correlates to the depth, and the relation between the signal’s energy width (∆E) of a thin film of thickness ∆t is given by:
In the above equation, k is the kinematic factor and the subscript “in” and “out” refer to the average incoming and outgoing energy for the α-particles.
The quantitative analysis of various constituents in a target is possible if all the parameters in the scattering cross-section are known. Another method to analyze the composition of the target is a direct comparison with reference standard thin film of gold (Au) or tantalum (Ta) deposited on silicon (Si) or carbon substrates.
Thus, RBS is mainly applied to determine the stoichiometry of compounds and thickness of thin films, which are essential factors in compound formation and alloy composition.
Recent Applications of RBS in Thin Films
In a recent article published in the journal Vacuum, researchers fabricated a self-supporting titanium (48Ti) film of approximately 500 µg/cm2 thickness, using 100 mg of the material via electron-beam evaporation technique under a high vacuum.
Compared to the complex setup modifications utilized in earlier studies, this was a simple and novel method of slowly evaporating the enriched material. Further, the team applied the RBS technique and confirmed that the thickness of the fabricated thin films was between 319–548 cm2.
In another article published in The Journal of Physical Chemistry, researchers used the RBS analytical technique and polarized neutron reflectometry as a function of annealing temperature to measure the thin films of Haynes 230.
They observed the migration of chromium to the surface of the thin films, annealed at 400 and 600 °C. The combination of the two techniques indicated that more than 60% of chromium consisting of the as-prepared Haynes 230 layer moved to the surface on annealing at 600 °C and formed an oxide layer.
Another study published in the journal Physica B: Condensed Matter investigated the role of oxygen pressure on the optical, structural, and photoluminescence properties of germanium oxide (GeO2) grown by pulsed laser deposition. They employed RBS to determine the elemental composition and thickness of films.
In summary, RBS is a robust technique to determine the thickness of thin films as it allows the mass separation of various elements in a target. The energy spectrum is detected from the reflected α-particles and is unique to a sample target. This energy spectrum determines the nature of a target sample, and the depth determines the concentration of detected atoms in the target. Thus, RBS provides efficient mass-sensitive depth microscopy.
The experimental observations on thin films indicated that the RBS spectra detect the nature of surface atoms, which gives valuable insights into the depth distributions.
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References and Further Reading
Pavan M. V. Raja and Andrew R. Barron. Rutherford Backscattering of Thin Films. [online] Rice University. Available at: https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Physical_
Umapathy, G. R., Ojha, S., Rani, K., Thakur, M., Mahajan, R., Chopra, N. K. S et al. (2016). Composition profile of thin film target by Rutherford backscattering Spectrometry. In Proceedings of the 61st DAE-BRNS Symposium on Nuclear Physics. http://www.sympnp.org/proceedings/61/G47.pdf
Arora, H., Abhilash, S. R., Umapathy, G. R., Kapil, A., & Behera, B. R. (2022). Fabrication and characterization of self-supporting 48Ti thin films. Vacuum, 201, 111052. https://doi.org/10.1016/j.vacuum.2022.111052
Doucet, M., Browning, J. F., Doyle, B. L., Charlton, T. R., Ambaye, H., Seo, J et al. (2022). Study of Chromium Migration in a Nickel-Based Alloy Using Polarized Neutron Reflectometry and Rutherford Backscattering Spectrometry. The Journal of Physical Chemistry C, 126(1), 605-610. https://doi.org/10.1021/acs.jpcc.1c08216
Rathore, M. S., Vinod, A., Angalakurthi, R., Pathak, A. P., Thatikonda, S. K., Nelamarri, S. R. (2022). Role of oxygen pressure on the structural and photoluminescence properties of pulsed laser deposited GeO2 thin films. Physica B: Condensed Matter, 625, 413466. https://doi.org/10.1016/j.physb.2021.413466