Absolute quantification of boron in nanometric thin films

A multitude of new materials are being developed and investigated to enhance the performance of the future generations of logic, interconnect, and memory devices. To characterize the nanometer scale components, imec uses various advanced characterization methods. Amongst these are ion beam analysis methods, like for example Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERD), particle-induced X-ray emission spectrometry (PIXE), nuclear reaction analysis (NRA) etc.

In semiconductor technology, boron-doped thin films have attracted significant attention due to their application in advanced CMOS technology. One of the current challenges in this context is the accurate quantification of the concentration of boron. Recently, we conducted a study about the quantification the absolute amount of boron in thin films by combining substrate-less Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA). First, a substrate-less boron pellicle was realized for which the absolute amount of boron was quantified with RBS. Then, the obtained absolute standard was used as a reference for the analysis with the 11B(p,α1)2α nuclear reaction at a proton energy of 640 keV. The strength of the RBS-NRA hybrid approach was illustrated with the analysis of a boron doped SiGe epilayer, demonstrating the traceable uncertainty and an accuracy of 1.5%. See refs. [1] and [2].

It should be noted that a naturally-boron-containing sample has an assumed isotopic ratio of 11B/ 10B = 4.0 ± 0.1. Our previous studies only focused on the quantification of 11B. However, exploratory studies have indicated that either the isotopic ratio of the industrially supplied boron deviates from this ratio, or that the used analysis methods (atom probe - AP, secondary ion mass spectrometry - SIMS) have mass fractionation effects. Clarifying this ambiguity will be the subject of the Master thesis.

As a main technique, you will employ Rutherford backscattering spectrometry (RBS) on substrate-less boron thin films at the Van de Graaff accelerator. RBS has excellent mass resolving power and thanks to the removal of the substrate it can differentiate the two boron isotopes. Thanks to our recent improvements to the set-up, it is possible to detect the signals of the two isotopes with a much-improved signal-to-noise ratio. The RBS spectra will be collected at various energies, which will allow you to infer non-Rutherford behavior if it is present. As a complement you will compare the RBS results with results from time-of-flight/energy elastic recoil detection (ToF-E ERD) analysis on boron-containing films on a thick substrate.

The study is key to understand the role and impact of the isotopic ration in relating various analysis techniques like secondary ion mass spectrometry and atom-probe analysis.

[1] Q. Bai, M. Dialameh, R.J.H. Morris, I. Vickridge, A. Vantomme, J. Meersschaut, Elastic backscattering during boron implantation in Si1-xGex, Vacuum 219 (2024) 112740

[2] Q. Bai, M. Dialameh, A.H. Chakkunny, R.J.H. Morris, C. Porret, A. Vantomme and J. Meersschaut, “Quantification of boron in a SiGe epilayer using ion beam analysis”, January 2026, accepted for publication in Applied Surface Science

Master's degree: Master of Science, Master of Engineering Science

Required educational background: Physics, Nanoscience & Nanotechnology

University promotor: Stefan De Gendt (Chemistry, Nano)

For more information or application, please contact the supervising scientist Johan Meersschaut (johan.meersschaut@imec.be).