Sample Mounting and Preparation Considerations
Surface Analysis Facility (SAF)
Sample Size.
In general, if you have the luxury of cutting or preparing your sample prior to
analysis, a sample size of 1 cm by 1 cm by 1 to 2 mm thick is ideal. Thinner or
thicker samples can also be accommodated, although samples much thicker than about
1 cm are difficult to mount and insert into the instruments. A circular sample
disk of 1 cm diameter is also ideal, with the same 1 to 2 mm thickness as for
a square sample. Samples do not need to be square or circular. Smaller sample
sizes can also be accommodated, as long as it is possible to physically hold and
manipulate the sample with forceps.
Conducting versus Non-conducting.
It used to be the case that non-conducting samples could not be analyzed routinely by
XPS or TOF-SIMS. Now any sample can be analyzed, and non-conducting samples are
possible to analyze because both instruments use a low-energy electron beam to
"bathe" the sample and thereby establish a uniform and hopefully steady charge
state at the surface. The electrons act to replenish photoelectrons that leave
the sample in XPS and ions and ion-induced secondary electrons that leave the
sample in TOF-SIMS. It would help if you knew ahead of time whether your sample
was insulating or conducting. Non-conducting samples will be mounted beneath a
grounded metallic mask with a small-diameter aperture exposing the sample area
to be analyzed. This will help ground the sample and expose only a small area
of the sample to the ionizing radiation.
Sample Cleanliness.
Remember that your fingerprints on the sample will cause both a serious contamination effect in your analysis as well as sample outgassing problems in the vacuum chamber. Please always
handle samples with clean latex or other gloves and avoid touching the face of
the sample to be analyzed, even with gloves. Prior to arriving in the SAF, make
sure that your samples have been cleaned appropriately and are carried to the
SAF in a clean, closed container. If the sample surface is reactive and prone
to contamination, it may be necessary for you to load your samples in a glove
box or vacuum chamber and transport them to the SAF in a specially designed "vacuum
suitcase". This allows the samples to be transferred directly into the vacuum
chamber, never being exposed to the ambient atmosphere. Remember, whether your
reactive sample is exposed to the atmosphere for a minute or a year, both situations
represent more than a billion collisions per site, and so a minute is just as
bad as a year, since it is all over in a nanosecond anyway!
Sample Outgassing.
Some types of samples will tend to give off gases in the vacuum chamber, preventing
the system from attaining its ultimate low pressure, and possibly never even allowing
it to pump down to a sufficiently low pressure to begin the analysis. If you anticipate
that your sample may require special handling with respect to this problem, please
let us know ahead of time. We may be able to place it into the sample introduction
chamber and allow it to pump down overnight prior to a morning analysis.
Being Present for the Analysis.
We strongly encourage you to be present for the analysis. Your chances to learn the maximum amount of information about your sample, with the least amount of wasted time, are greatest when you are present. So save time and money, and learn more while you are at it!
A. If your sample is a solid such as a wafer or other flat, rigid material
- Examples include silicon wafers, glass cover slips and microscope slides, polymer films, monolayers grown or supported on solid surfaces, etc.
- Bring samples to the SAF pre-cut to the size specified above.
- If a surface treatment has been applied to only one side, be sure to
identify the proper side for the analysis!
- These are the easiest and most routinely analyzed types of samples in the SAF.
B. If your sample is a powder
- Most powders are non-conducting and can be somewhat tricky to mount and analyze if not done properly.
- Your powder sample may be pressed into a pellet ahead of time, or
pressed by us at the time of analysis. Make sure that all surfaces of the press
are scrupulously cleaned, otherwise you will be analyzing the contamination layers
that have been transferred from the press to your pellet surface.
- Your powder sample may also be smeared as a quasi-continuous film onto an adhesive surface. We have both conducting and non-conducting adhesive tape. In this sample-mounting
method it is important to make sure that the underlying adhesive tape does not
"shine through" in the analysis. Or if it does, it will be important to obtain
a control spectrum of the adhesive tape in the absence of your sample.
- We avoid placing the powder onto a flat surface with nothing holding it there because it
goes all over the place during pump-down, and static charge tends to have the
same negative effect.
- Some powder samples are very "gassy" because of high surface
areas or internal surfaces. See the previous section entitled Sample Outgassing.
C. If your sample is a liquid containing dissolved solids that are the object of the analysis
- These types of samples need to be applied as a droplet to an appropriate solid substrate and allowed to dry under clean conditions.
- If the solvent is highly volatile, the evaporation can be done as the sample is being
pumped down in the sample introduction chamber.
- If the solvent is of low volatility, or if a large volume of high-volatility solvent must be removed prior to analysis, this could present a serious problem for the analysis as described in the previous section under Sample Outgassing.
D. If your sample is a liquid and the liquid is the object of the analysis
- This type of analysis is quite rare, and we generally do not do it. In addition to the possibilities for ruining the sample vacuum, the analysis of a liquid surface is something that requires very special considerations.