Nanoindentation Experiment of Aluminum Nitride (AlN) Thin Film on Silicon –

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1. Abstract

This study conducted nanoindentation experiments on AlN thin films deposited on Si (111) substrates using Berkovich indenters, aiming to reveal the intrinsic correlation between their microstructure and deformation behavior. The AlN film was prepared by spiral sputtering technology, exhibiting a distinct columnar grain structure with grain diameters ranging from 20-40nm and a thickness of approximately 350nm. Microscopic analysis of the indentation area using cross-sectional transmission electron microscopy (XTEM) revealed the presence of cracks caused by inter column shear in the AlN film. In addition, significant shear steps were observed at the interface between the film and substrate, which is well correlated with the multiple pop in events that occur multiple times in the load displacement curve. XTEM analysis further indicates that not only slip bands along the {111} crystal plane appear inside the Si (111) substrate, but also nanoindentation induced phase transition regions are formed, including Si III and Si XII metastable phases, accompanied by a large number of amorphous regions. Based on the deformation characteristics dominated by shear cracks mentioned above, this study proposes an indentation energy model to evaluate the inter column shear stress in AlN thin films.

2. Experiment

The experiment is divided into three main stages: thin film deposition, structure and morphology characterization, and nanoindentation testing and microstructure analysis.

In the first stage, a self-made spiral sputtering system was used to deposit AlN thin films on Si (111) substrates. A high-purity aluminum target (99.999%) was used to complete the preparation under specific sputtering parameters, including a Ar: N2 mixture ratio of 1:3, a substrate temperature of 400 °C, and an RF source power of 150 W.

In the second stage, the crystal structure, preferred orientation, surface morphology, and roughness of the thin film were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively, to confirm its structural quality.

Fig. 1 (a) XRD of AlN thin film; (b) AFM of AlN thin film

The third stage involves nanoindentation testing using MTS NanoXP ® Obtain load displacement curves using a system and Berkovich indenter with a maximum load of 80mN, analyze mechanical response and deformation characteristics, and prepare cross-sectional samples of the indentation area using focused ion beam (FIB). Use XTEM and selected area electron diffraction (SAD) to observe the microstructure changes induced by indentation (such as dislocations, phase transitions, and amorphous regions) in depth. These three stages are closely connected, jointly revealing the mechanical behavior and deformation mechanism of AlN thin films during nanoindentation process.

3. Conclusion

This study systematically investigated the nanomechanical behavior and deformation mechanism of columnar AlN thin films deposited on Si (111) substrates through comprehensive nanoindentation experiments and microstructure analysis. The research results indicate that:

The multiple “pop-in” events that occur during the loading process are due to the combined effects of multiple complex deformation mechanisms, including delamination between the film and substrate, shear cracks and sliding between AlN columnar grains, formation of shear steps at the interface, and plastic deformation and pressure induced phase change of the Si substrate itself.

Fig. 2 (a) Typical P-h curve of AlN/Si (111) thin film under Berkovich indentation with an indentation load of 80mN, exhibiting multiple “pop-in” and “pop-out” features; (b) SEM image of the corresponding indentation

The “pop-out” events observed during the unloading phase are mainly attributed to the pressure induced phase transition occurring in the Si substrate, which generates metastable body centered cubic Si-III (BC8) and rhombohedral Si-XII (R8) phases and a large number of amorphous regions, accompanied by volume expansion, which is absorbed by the top AlN thin film portion.

Fig. 3 XTEM image of 80mN indentation (left) and diffraction pattern of selected area below Berkovich indentation (right), (I): AlN thin film indentation and (II): transition region in Si (111) substrate. Inset: Display of inter column shear cracking and transverse cracking events

Under an indentation load of 80mN, the AlN film bends and exhibits inter column shear cracks and steps (as shown in Fig. 3: inset); By simplifying the energy model, the shear stress between the friction columns was estimated to be about 3.5 GPa, which is much lower than the modulus of bulk AlN, confirming the compatible deformation mechanism of hard thin films on soft substrates dominated by shear. In addition, the study emphasizes the decisive influence of columnar microstructure on the deformation behavior of AlN thin films.

In summary, this work clearly reveals the complex mechanism of AlN/Si (111) system during indentation process from macroscopic mechanical response correlation to microscopic structural evolution, providing important basis for optimizing AlN thin film preparation and device performance.

Whether you need AlN on Si for research or for industrial applications, please contact us email at [email protected] and [email protected].

 

Reference：

Jian, S. R., & Juang, J. Y. (2025). Indentation-induced intercolumnar shearing in AlN thin films grown on Si (111) substrate. Journal of Physics and Chemistry of Solids, 112932.