Thickness and refractive index measurements using multiple beam interference fringes (FECO)

Area of Science:

• Physics
• Materials Science
• Surface Science

Background:

• Accurate measurement of thin films and confined media is crucial for understanding material properties.
• Optical interferometry techniques are widely used but can face challenges with precision and applicability across different separations.
• The Surface Force Apparatus (SFA) provides a unique platform for in-situ measurements of interfacial phenomena.

Purpose of the Study:

• To report the application of optical interferometry with Fringes of Equal Chromatic Order (FECO) in an SFA.
• To demonstrate a method for calculating surface separation (film thickness) using FECO fringes, even when the contact position is not directly measured.
• To evaluate the measurement accuracy of this technique for confined media across a wide range of separations.

Main Methods:

• Utilizing optical interferometry with FECO within a Surface Force Apparatus (SFA).
• Developing and applying a novel calculation method for surface separation based on two FECO fringes.
• Performing measurements on confined media to determine film thicknesses and refractive indices.

Main Results:

• Successfully determined film thicknesses and refractive indices of confined media for a broad range of separations.
• Demonstrated a reliable method for calculating surface separation using FECO fringes without needing to measure the contact position.
• Observed that while theoretical accuracy is 1 Å, practical accuracy for large separations is challenging to achieve.

Conclusions:

• FECO interferometry in an SFA is a viable technique for characterizing confined media.
• The developed method offers flexibility in determining film thickness by not requiring direct contact measurement.
• Further refinement is needed to consistently achieve high accuracy, particularly at larger separations.