X-Ray Photoemission Spectroscopy (XPS/ESCA) Laboratory

Uses for XPS/ESCA

This easy-to-use instrument provides elemental and chemical information about the surface region (first 1- 30 monolayers) of nearly any solid material. Over the last 5 years, about 140 students from 8 academic departments have been trained to use the facility for diverse fields of research, such as medical implants, polymer monolayers, semiconductor quantum wells, or mining slag deposits. XPS is useful for determining the elemental composition on the surface of all non-volatile materials semiquantitatively. It is sensitive to all elements except H and also provides some chemical information about the valence state of elements. XPS is sensitive to the top 5 nm of a sample, but it can be used to provide elemental composition as a function of depth by analyzing a sample while removing surface layers by ion etching. XPS is useful for chemically sensitive materials since damage from the x-ray beam is minimal. It is particularly useful for insulating materials such as polymers, oxides, and powders were charging effects limit other surface techniques.

The instrument

We have an SSI S-Probe XPS Spectrometer and a PHI VersaProbe Scanning XPS Microprobe. These systems allow for high sensitivity elemental surface composition measurements and high resolution binding energy chemical shift measurements of solid samples under high vacuum. Both use Al(ka) radiation (1486 eV) and are equipped with argon ion sputter guns to clean the sample surface or to do depth profile analysis. The SSI system is capable of is situ sample heating for evaluation of evolution of thermal surface modification of a sample. In addition to argon ion sputtering, the PHI system is equipped with a C60 cluster ion sputtering gun for depth profiling of polymer and organic films with minimum ion impact modification to the surface.

Basic theory of operation

In XPS, soft x-rays (about 1500 eV for our instrument) illuminate a region of the sample being analyzed, and photolelectrons emanating from it are energy analyzed. From the energies detected, the elements present and sometimes their chemical state (valence) can be determined. Because only atoms near the surface of the sample emit electrons which escape without losing energy, the technique is very surface sensitive.

Some Specific Capabilities.

Compositional Analysis:

Atomic abundance of all elements (except H) on insulators, powders, metals, etc.

Chemical Analysis:

Identifies bonds between specific atoms in the surface region. Polymers, oxides, etc. characterized.

Depth Profiles:

Characterizes buried regions as they are exposed by ion etching.

Specifications

• Detection limits (all elements except H): ~ 0.01 monolayer, or ~ 0.1% bulk

• Measurement depth: 10 - 50 Å; area: 0.2 - 1 mm dia.

• Sample size: 0.1mm to 3" dia. x 1" thick

• Spectrometer: Monochromatized Al(ka) Source; Vacuum ~ 5 x 10E(-10) torr

Restrictions on samples

Samples must be solid and vacuum compatible. A sample can range in size from 0.1 mm in diameter and very thin to 8 cm in diameter and 2 cm thick. Powdered and other rough surfaced samples can also be analyzed. Samples are generally mounted using clips or adhesives onto standard fixtures. It is useful to know in advance if the sample is conducting.

Other sources of information on XPS/ESCA

There are numerous texts discussing the use of XPS. Some useful examples are:
Electron Spectroscopy: Theory, Techniques and Applications. C. R. Brundle and A. D. Baker, eds. (a 5 volume series)
Practical Surface Analysis, D. Briggs and M. P. Seah eds.
Handbook of XPS, C. D. Wagner (published by Physical Electronics, Inc.)

Or try http://www.york.ac.uk/org/esca/tech/xps.html.

For futher information about XPS at Stanford, please contact Chuck Hitzman (chitzman@stanford.edu)