X-Ray Photoelectron Spectroscopy

X-ray Photoelectron Spectroscopy

- A Report

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I. Overview

This report is a detailed study on X-ray Photoelectron Spectroscopy (XPS), a surface

characterization technique. The first question that probably comes in your mind is,

what exactly is surface characterization? Well, surface characterization is a method

which enables us to know everything about the surface in focus. Through this

method, we know the type of atoms present on the surface, the concentrations of the

atoms, the exact locations of the atoms on the surface, the bond lengths and angles of

the molecules on the surface, the strength of the bonding to the surface, and finally

the effect the surface bond will have on surface reactivity. Basically, upon

characterization, you will know the surface like the back of your hand.

The next question that might be asked would be, why do we need all this information

about a surface? The answer to this question is a lot more universal and general than

you would expect. All this information about a surface comes in handy when you are

working with or analyzing a specific surface. This method can analyze a surface as it

is received and we can use the information, or we can use it to compare after the

surface has undergone certain treatment. Some examples of this 'treatment' are

cutting, scraping, fracturing, Ultra High Vacuum (UHV), ion beam etching, exposure

to heat, exposure to reactive gases or solutions, exposure to ion beam implant, and

exposure to UV light amongst many others. The 'treatments' serve purposes like

exposure to bulk chemistry (UHV), cleaning surface contamination (ion beam

etching), and study of changes (due to exposure to heat, reactive gases, solutions, ion

beam implant and UV light) and surface characterization can help in analysis.

An overview on 'surface characterization' has been provided in this section. In the

coming sections, you will learn everything there is to know about X-ray

Photoelectron Spectroscopy (one of the surface characterization techniques) ranging

from the technical side with physical and chemical principles to the practical side

with applications of this technique.

II. Introduction

X-ray Photoelectron Spectroscopy (XPS) is one of the surface characterization

techniques and is used to measure the empirical formula for pure materials, elements

that contaminate a surface, elemental composition of the surface (1-10 nm), chemical

or electronic state of the elements that exist in the surface, uniformity of elemental

composition across the top of the surface (line profiling), and uniformity of elemental

composition as a function of ion beam etching (depth profiling).

Image 1: Shows the basic parts of the XPS system.

X-ray Photoelectron Spectroscopy

- A Report

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Looking at Image 1, you can figure out how the XPS functions. First, a focused beam

of X-rays is shot towards the surface. As this is happening, the 'electron detector' and

'electron energy analyzer' will measure the number of electrons and the kinetic

energy of the electrons that escape from the surface. When you irradiate a material

with X-ray beams, it is highly likely that the electrons that escape as a result are only

from the top 1-10 nm of the material being analyzed. The reason for this can be

understood in a much simpler way if you compare this with an analogy of sitting on

the beach. When you are on the beach and under the sun for too long, although your

entire body is there, only the surface gets sun-burned and thus substantially affected.

The XPS technique works in a similar fashion only affecting the molecules close to

the surface. Once the electrons are detected by the electron energy analyzer and the

electron detector, a signal is transmitted and we are able to see and analyze the

collected data.

Now that you know how the XPS system works, we will discuss some essential and

some interesting facts about this technique. They are as follows:

* The detection of the XPS technique is limited. The technique detects elements

with an atomic number between that of lithium (Z=3) and lawrencium

(Z=103). As obvious from this limitation, the XPS technique cannot detect

hydrogen (Z=1) and helium (Z=2). Also, the detection limits of this technique

for most of the elements are in the PPTh or parts per thousand range.

* The XPS technique can be performed using two different types of systems.

The first type is a commercially built XPS system which uses either a highly

focused 20 to 200 micrometer beam consisting of monochromatic aluminum

K-alpha X-rays or a broad 10-30 mm beam