[Paper Review] Is the second harmonic method applicable for thin films mechanical properties characterization by nanoindentation? Is the second harmonic method applicable for thin films mechanical properties characterization by nanoindentation?

Gaylord Guillonneau, Guillaume Kermouche+4 more|arXiv (Cornell University)|Feb 2, 2015

Metal and Thin Film Mechanics4 citations

TL;DR

This study evaluates the second harmonic method (SHM) for characterizing mechanical properties of thin films via nanoindentation, demonstrating its accuracy at depths below 100 nm—even for films thinner than 500 nm—where classical methods like CSM fail. SHM avoids direct depth measurement, relying on dynamic response analysis, and shows strong agreement with AFM and FEM simulations at low depths, though discrepancies emerge at higher depths due to contact depth model limitations.

ABSTRACT

The second harmonic method is a dynamic indentation technique independent of the direct indentation depth measurement. It can be used to determine near-surface mechanical properties of bulk materials more precisely than classical dynamic nano-indentation. In this paper, the second harmonic method is extended to the measurement of the mechanical properties of thin PMMA layers deposited onto silicon wafers. It is shown that this new technique gives precise results at small depths (less than 100nm), even for films with a thickness lower than 500nm, which was not possible to achieve with the classical CSM method. However, experimental and numerical results obtained both with classical nanoindentation and second harmonic methods differ at high indentation depth. Using FE simulations and AFM measurements, it is shown that the contact depth calculation with classical models can explain this difference.

Motivation & Objective

• To assess the applicability of the second harmonic method (SHM) for measuring mechanical properties of thin polymer films.
• To compare SHM results with classical nanoindentation and CSM methods, especially at low indentation depths.
• To investigate discrepancies between classical models and experimental results at high indentation depths.
• To validate SHM performance using finite element method (FEM) simulations and atomic force microscopy (AFM).
• To determine whether SHM can reliably characterize near-surface mechanical properties in thin films where conventional methods fail.

Proposed method

• The second harmonic method (SHM) is applied to PMMA thin films deposited on silicon wafers, using dynamic nanoindentation with harmonic frequency excitation.
• SHM extracts mechanical properties from the second harmonic response of the load-displacement signal, avoiding direct depth measurement.
• Finite element method (FEM) simulations are used to model contact mechanics and validate contact depth calculations.
• Atomic force microscopy (AFM) is employed to measure film thickness and surface topography for accurate sample characterization.
• Classical nanoindentation and CSM methods are used for comparative analysis under identical conditions.
• Experimental data are analyzed using standard contact mechanics models to extract elastic modulus and hardness, with adjustments for film thickness effects.

Experimental results

Research questions

• RQ1 Can the second harmonic method accurately determine mechanical properties of thin films with thicknesses below 500 nm?
• RQ2 How does SHM performance compare to classical nanoindentation and CSM methods at low indentation depths (<100 nm)?
• RQ3 What causes discrepancies between classical models and experimental results at higher indentation depths?
• RQ4 To what extent do FEM simulations and AFM measurements support the validity of SHM results?
• RQ5 Is SHM a viable alternative for near-surface mechanical characterization in thin films where depth sensing is unreliable?

Key findings

• The second harmonic method provides precise mechanical property measurements for thin films at indentation depths below 100 nm, even when film thickness is less than 500 nm.
• SHM results show strong agreement with AFM and FEM simulations at low depths, confirming its reliability for near-surface characterization.
• Discrepancies between classical models and experimental results at high indentation depths are attributed to inaccuracies in contact depth calculation.
• Classical models overestimate the depth of penetration, leading to errors in modulus and hardness extraction when film thickness is small.
• The study confirms that SHM is independent of direct depth measurement, making it more robust for thin film applications than conventional methods.
• FEM simulations and AFM measurements validate the SHM results and support the conclusion that model-based depth estimation is the primary source of error in classical approaches.