Thin layer thickness measurements by zero group velocity Lamb mode resonances

Area of Science:

• Materials Science
• Physics
• Engineering

Background:

• Accurate measurement of thin film thickness is crucial for material characterization and quality control.
• Non-contact and local measurement techniques are highly desirable for in-situ monitoring and analysis of delicate layers.

Purpose of the Study:

• To demonstrate a non-contact method for measuring the thickness of thin layers deposited on thick plates.
• To utilize zero group velocity (ZGV) Lamb modes for precise layer thickness determination.

Main Methods:

• Generation of ZGV Lamb modes using a pulsed Nd:YAG laser.
• Measurement of normal displacement via an optical interferometer.
• Analysis of resonance frequency shifts proportional to mass loading.

Main Results:

• Successfully detected thin gold layers (hundreds of nanometers) on a 1.5-mm Duralumin plate.
• Established a proportionality between resonance frequency shift and layer thickness (10 kHz/μm).
• Achieved a ±4% uncertainty for a 240-nm gold layer measurement, accounting for temperature effects.

Conclusions:

• ZGV Lamb modes offer a viable and accurate method for non-contact, local thickness measurement of thin films.
• The technique's sensitivity allows for nanoscale layer detection through a substrate.
• Resonance frequency shift is a reliable indicator of mass loading and layer thickness.