Original Motivation

1. An Initial Study of the Surface Contamination and Oxidation on SiliconLiz Strein, Amy Grigg, and Dr. David Allred

2. Original Motivation Background: New XPS machine. Need to better understand of one of the features: the Tilt Stage. Our Goal: Understand how deep into the thin film x-rays penetrate when the angle is varied with the Tilt Stage.

3. An Accident • A well-characterized sample was needed. We chose a basic silicon wafer. • At a glance, film’s surface appeared clean. • However after baking it under Excimer Lamp, the extent of contamination on the surface of the wafer became visible

4. Analysis of the contamination (in the form of rainbow-looking spots) with XPS revealed an unusual amount of carbon in the surface What XPS has to say

5. Defining Clean • The “accident” wafer had a layer of SiO2 with a lot of hydrocarbons. • Need to determine how clean the wafer is before we put it under the Excimer Lamp. • Spectroscopic Ellipsometry used to determine thickness. • A sufficiently clean wafer is 16-18 Ǻ thick. • The cleanest sample found had ~19.5 Ǻ.

6. Preliminary Cleaning Results • Data from the ellipsometer suggests that 90 seconds under the Excimer Lamp is the optimum cleaning time. • The thin film surface rapidly rebuilds the oxidation and hydrocarbon levels. The rate of surface contamination is unknown, however the surface is significantly dirty after only a couple of days from the original cleaning. (Samples are stored in clean containers. Exposure to the open air is kept at a minimum.)

7. Typical XPS results Cleaning the films has reduced the carbon on the surface of the films.

8. Resulting Questions • How fast and in what way is the surface of the silicon oxidizing? • Does the use of the Excimer Lamp effect the oxidation? If so, in what way? • Why is there still so much carbon? (Old data from the previous XPS machine suggests the surface should be composed of no more than 12% carbon and that the percentage of carbon should drop off VERY rapidly with depth.) • What can we do to make our films cleaner? Is there a better way for us to store them so they remain clean over time?

9. About the sample • We chose to use silicon because it’s very well characterized. • The silicon wafers we used were too big for the Tilt Stage. We used a tungsten knife to cut them to size. • Once cut, the wafers are stored in thin plastic containers in a relatively dust-free environment. • “The cleanest” wafer we’ve been able to find was one that had remained unopened in its original packaging. Even that was declared dirty after measuring the thickness with Ellipsometry.

10. About the Excimer Lamp • A laser cleaning technique that is good for removing sub-micron particles from substrates without generating the hazardous waste products intrinsic in solvent cleaning methods. • In the literature, some research suggests that UV laser irradiation can create a hydrophobic silicon surface—in other words, the surface becomes permanently clean.* *This is reported in laser cleaning studies at Macquarie University, Australia

11. How Spectroscopic Ellipsometry Works • Measures the change in polarization of light as it is reflected from the surface of the film. • The measured values can be used to determine Fresnel reflection coefficients and ultimately allows a thickness calculation Picture from http://www.uta.edu/optics/research/ellipsometry/ellipsometry.htm

12. How XPS works