Diving into thin-film interfaces using Low Energy Ion Scattering
Adele Valpreda is a PhD student in the department XUV Optics. (Co)Promotors are prof.dr. M.D. Ackermann; dr. A. Yakshin and dr.ir. J.M. Sturm from the faculty Science & Technology.
Interfaces play a key role in a variety of applications, including multilayers for EUV and X-ray optics and semiconductor devices. When the size of a device is reduced, the relevance of the interface quality increases. Considering the need for smaller feature sizes in the semiconductor industry, being able to characterize interfaces with sub-nanometer resolution becomes more and more important. The work presented in this thesis aims to answer the research question of whether the sub-surface signal of LEIS spectra can be used to measure the width of buried interfaces, with the ultimate goal of adding complementary (non-destructive, depth-resolved, and quickly accessible) information to the commonly used techniques to further improve the analysis and understanding of thin film growth in multilayer systems.
The first study presented in this thesis answers the need for proper simulations of LEIS spectra to enable quantitative depth-resolved measurements with LEIS. For this study, we used Si-on-W and Si-on-Mo model structures. The results are a proof of concept that the method of comparing the experimental and simulated LEIS spectra is sensitive to the interface width in the case of ultrathin films (<2 nm), where the broadening of the signal (the straggling) is minimal.
In the second study, we tested the accuracy and precision of the developed method for interface quantification. We extracted depth profiles of the Si-on-W interface using three different techniques, LEIS, XRR, and TEM, and on two different structures, having 4 and 20 nm of W below the Si. We fitted each depth profile with an error function (erf), and by comparing the value of the parameter σ (in nm) of the erf, we were able to quantitatively compare the effective interface width of the Si-on-W interface as measured by the different techniques. All techniques found that the structure with 20 nm of W has a sharper Si-on-W interface (σ = 0.3 ± 0.1 nm) compared to the structure with 4 nm of W (σ = 0.5 ± 0.1 nm), which we interpret as the effect of both the correlated roughness from the substrate present in the case of 4 nm W, and the larger crystals present in the case of 20 nm W resulting in less intermixing. More importantly, the effective interface width measured by all techniques agreed within a 0.1 nm error margin. Considering the extensive experimental effort required by TEM, these results exemplify the value of LEIS for resolving thin film interfaces buried in the first few nm of the sample.
As a continuation, we investigated the opposite scenario, namely W-on-Si structures. Specifically, we used a film of 2.2 nm of W deposited by magnetron sputtering on an amorphous Si film. Following the approach developed by Brüner et al. [1], and with the correction described in the first study of this thesis, TRBS spectra of backscattered particles were multiplied by the W reionization function, obtaining simulated LEIS spectra. These results prove that the method of comparing experimental and simulated LEIS spectra can also be applied to probe interfaces formed by a heavy atom deposited on a lighter atom.
Next, we made use of the developed method to investigate an interface that is interesting for its potential use in X-ray optics. Specifically, we measured the interface width in three structures having different amounts of B4C at the W-on-Si interface. The LEIS analysis revealed that the W distribution at the interface does not get sharper when B4C atoms are deposited at the interface, and the reconstruction of the XRR curves confirmed that adding a few B4C monolayers at the W-on-Si interface does not lead to a significantly sharper W-on-Si interface. Possible reasons for the discrepancy between the values of the effective interface width extracted by LEIS and XRR were discussed. These results showed the value of obtaining depth-resolved information on the interface of interest from LEIS spectra.
In the fifth (and final) study presented in this thesis, we used calcium niobate nanosheets (CNO for short) as a 2D model system for further investigating the shape of LEIS spectra. We confirmed the evolution of the LEIS sub-signal as a function of depth that we previously characterized. Furthermore, the comparison of the LEIS spectra of CNO and amorphous Nb oxide samples indicated that the sample’s crystalline structure may affect the ion scattering via channelling, meaning that LEIS has the potential to be both depth-resolved and sensitive to the microstructure of the film.
Throughout this thesis, we discuss how the depth-dependent straggling of the signal and the surface roughness limit the resolution of the LEIS buried interface profiling. We also show that a small difference (<0.2 nm) in effective interface width can still be measured from the shape of LEIS tails in suitable conditions (e.g., when the atomic mass difference between the elements is high enough, and the interface is close to the surface of the sample). Compared to a layer growth approach, buried interface profiling has the advantage of needing a single sample and a single LEIS measurement, albeit supported by simulations. Furthermore, understanding the depth-resolved information contained in the tails of LEIS spectra expands the use of LEIS to the characterization of those materials that are not quantifiable by the surface peaks (e.g., because of matrix effects).
Coming back to the applications of thin films and multilayer systems, the results presented in this thesis showed that LEIS is a valuable technique to obtain non-destructive, depth-resolved, and quickly accessible information on the composition of the first few nanometers of a sample. We showed that this information can be used to measure the width of an interface of interest, in cases where the mass difference between the thin film is sufficiently large.
[1] P. Brüner, T. Grehl, H. Brongersma, B. Detlefs, E. Nolot, H. Grampeix, E. Steinbauer, P. Bauer, Thin film analysis by low-energy ion scattering by use of TRBS simulations, Journal of Vacuum Science & Technology A 33(1) (2015) 01A122.
