Cleanable anti-reflection coating
on a display window.
(Courtesy
of Applied Multilayers Ltd)

NPL is developing a traceable thin film thickness measurement capability.

Overview

Transparent thin films are used in many industrial sectors, most notably optics. A thin film (in this context ‘thin’ refers to films that are less than 5 μm in thickness) is a layer of a particular material that is deposited on a substrate surface to change the manner in which the surface physically behaves. For example, films may be added to a surface to change its optical properties and make the surface less reflecting to light in a certain bandwidth. It is very common to deposit more than one thin film to impart the desired function on the substrate surface. Keeping with the optics theme, a very common thin film ‘stack’ is that which produces an anti-glare coating on windows. Other uses of thin films include: a hard layer to physically protect the substrate, hydrophobic layers to make glass essentially self cleaning and layers that promote growth of certain molecules. In order to control the physics of a thin film is important to be able to predict the film’s physical and chemical makeup, and its physical thickness. There are many methods for measuring the thickness of a film but there is often little agreement between the methods when compared. For this reason NPL has embarked on a research project that will culminate in a traceable measurement service for transparent thin film thickness.

A scanning white light interferometer
that can be used for surface topography
and thin film thickness measurement.
(Courtesy of Taylor Hobson Ltd)

Research

The current research aims at setting up a traceable measurement service for transparent thin film thickness measurement. The research is broken down into the tasks below:

1. Development and characterisation of single and multi-layer thin film thickness transfer artefacts. These artefacts will be traceable to the definition of the metre by having a physical step that is measured using traceable surface texture measuring instrumentation
2. Development of methods using scanning white light interferometry (SWLI) and ellipsometry to measure film thickness.
3. A UK comparison of thin film measurement capabilities.

Publications

• Yacoot A, Leach R 2007 Review of x-ray and optical thin measurement methods and transfer artefacts NPL Report DEPC EM-13
• Mansfield D 2006The distorted helix: thin film extraction from scanning white light interferometry Proc. SPIE 6186
• Conroy M 2007 Advances in thick and thin films using interferometry Proc/ 11th Int. Conf. Metrology & Properties of Engineering Surfaces, Huddersfield
• Tompkins GG, Irene EA 2005 Handbook of Ellipsometry (Springer-Verlag)

Collaboration

We are collaborating with Taylor Hobson Ltd (www.taylor-hobson.com) to develop methods for thin film measurement using SWLI. NPL is also collaborating with Taylor Hobson, AppliedMultilayers, IQE (www.iqep.com) and the University of Huddersfield (www.hudd.ac.uk) to develop methods for using SWLI to measure the surface texture of ultra-smooth thin films (see www.soladim.co.uk).

The UK comparison will involve NPL, Taylor Hobson, University of Southampton (www.soton.ac.uk), Applied Multilayers (www.applied-multilayers.com), Intense Limited (www.intenseco.com), Aquila Instruments Ltd. (www.aquila-instruments.com), PANalytical Research Centre (http://www.panalytical.com/)

MNT Measurement Club  |  EMAN