Physics projects for Y3 and Y4 students

Project description

Variable Angle Spectroscopic Ellipsometry of thin oxide films

(supervisor: Matt Gunn)

Nature of project: experimental, instrumental

Available to students on full-time physics degree schemes or joint students.

Project description and methodology

Many materials readily form a thin surface oxide film when exposed to a source of oxygen. The thickness of the film depends on many factors but can range from a few nm to several um. Oxide films find many practical applications including dielectric (insulating) layers in electronic devices manufactured from semiconducting materials such as Silicon and passivation (corrosion protection) layers on reactive metals such as aluminium and titanium. These films can form naturally in the presence of oxygen (such as the native oxide on silicon and aluminium) or may be grown in controlled ways such as thermal oxidation or electrolytic processing (such as anodising).

Typically, oxide layers are too thin to be measured by conventional mechanical means, but their thickness, optical properties and other physical characteristics can be determined through optical measurements. Ellipsometry is a highly sensitive optical characterisation technique in which the change in polarisation of light reflected from a sample surface is used to determine the properties of the surface. Either the angle of incidence or wavelength of the light can be varied to carry out Variable Angle or Spectroscopic Ellipsometry respectively or both can be varied to conduct Variable Angle Spectroscopic Ellipsometry. The desired properties of the sample are retrieved from the measurements by constructing a numerical model of the sample system and adjusting the properties until the simulated polarisation change matches the measurements – this is generally performed by sophisticated data fitting software.

In this project the thickness and optical properties of oxide films will be investigated using Variable Angle Spectroscopic Ellipsometry. Initially the thickness and optical properties of existing samples of Silicon Dioxide (SiO2) on Silicon (Si) will be characterised. Once familiar with the experimental measurement and data analysis, investigations can be extended to characterise the growth of oxide films on various materials such as Silicon, Aluminium or Titanium.

A successful project will develop beyond the above in one/some of the following directions:
There are several mechanisms by which oxide films can be grown on different materials and these may be investigated as part of the project. Some of these options could include (but are not limited to):

Thermal oxidation of metals and semiconductors. The thickness of oxide films on Silicon and reactive metals such as aluminium and Titanium can be controlled by time, temperature and atmospheric composition. These may be investigated by growing oxide films on selected materials in a tube furnace.

Electrolytic oxidation of Metals. Thick oxide films on reactive metals such as Aluminium and Titanium can be grown by electrolytic processes. The properties of the film as a function of the growth conditions such as current density, electrolyte, potential etc can be investigated through spectroscopic ellipsometry.

When considering where to take your project, please bear in mind the time available. It is preferable to do fewer things well than to try many and not get conclusive results on any of them. However, sometimes it is useful to have a couple of strands of investigation in parallel to work on in case delays occur.

Additional scope or challenge if taken as a Year-4 project: Spectroscopic Ellipsometry is well suited to in-situ investigations. A sample environment may be designed and constructed to allow the oxide film growth to be monitored in-situ and in real time.

Please speak to Matt Gunn (mmg) if you consider doing this project.

Initial literature for students:

  1. Harland G. Tompkins, William A. McGahan, Spectroscopic Ellipsometry and Reflectometry: A User's Guide, 1999. ISBN: 0471181722, 9780471181729
  2. Jungk, G. and Grabolla, T., 1998. Spectroscopic ellipsometry on silicon-oxide films on silicon. Thin solid films, 335(1-2), pp.253-257.
  3. Ohtsuka, T. and Nomura, N., 1997. The dependence of the optical property of Ti anodic oxide film on its growth rate by ellipsometry. Corrosion Science, 39(7), pp.1253-1263.

Novelty, degree of difficulty and amount of assistance required

Training will be required in the use of the Woollam RC2 Spectroscopic Ellipsometer and CompleteEase analysis software. Training will also be required in the use of laboratory equipment such as the tube furnace.

Project milestones and deliverables (including timescale)

milestoneto be completed by
Project outlineend of November
Initial measurements and analysisend of February
Sample / sample environment preparationmid-March
Complete measurements and data analysisEaster

Students taking this project will have to submit a full risk assessment form