NEW: SurfaceSeer demo facility

The SurfaceSeer: Surface Analysis by TOF-SIMS

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Kore has designed and delivered a new compact TOF-SIMS instrument that provides the power of surface analysis to industrial clients at approximately one fifth the cost of high-end research TOF-SIMS instruments. The instrument is currently designed to be a 'work horse' for spectroscopy in both positive and negative TOF-SIMS modes, and has a mass resolution of 1000 and a mass accuracy of ± 10mamu.

Overview

The driving force behind the design of this instrument is definitely ease of use and speed of data acquisition. The client for whom it was initially designed is representative of a class of engineers/scientists who are not catered for with existing SIMS instrumentation. They definitely understand and have experienced the benefits of surface analysis for their products, but are not in academia, and thus do not have either access to grant money or the luxury of unlimited research time. Their principal goal is to run samples quickly and efficiently and to answer specific questions concerning the surface chemistry of their samples/products.

It was for this type of client that the SurfaceSeer was designed.

Kore has long had an interest in making a 'low-cost' TOF-SIMS, and developed a prototype instrument known as the MS-1000 some years ago to address this market. The SurfaceSeer is really a development on from that earlier instrument, with more analytical capability.

Samples are pumped down within 5 minutes in a small volume load lock and are then entered into the analytical chamber with a simple forward motion and 90° twist action.

Typical analysis times on polymers are 1-2 minutes, after which a mass spectrum with typically half a million counts is recorded (polymer dependent: signal rates are higher from higher yielding samples such as metals or semiconductors).

The 5kV gas ion gun has a scanning capability but is used in a static (non- scanning) mode at the moment. Addition of a secondary electron detector and imaging software/hardware modules would permit imaging capability. The pulsing probe is variable between (typically) 400µm and 1mm for fast data acquisition (high current), although smaller probes are obtainable with lower primary currents. The sample is mounted on a sample holder such that the area to be analysed is located in the centre of a cover plate with a 6mm aperture.

Unlike a research TOF-SIMS instrument, all the spectrometer tuning has been pre-set in the electronics, and from sample to sample 'tuning' is reduced to a single potentiometer adjustment with real time feedback to maximise count rate.

Instrument details

Primary Ion Gun

The instrument uses a 5keV inert gas (Ar+) ion beam that is pulsed to minimise the surface damage that would otherwise be caused by a continuous ion beam. The gun pulses on for only 140ns in every 128µs TOF cycle, and thus the primary beam delivers almost 10,000 times less current than if it were on continuously. A typical experiment time of 1 minute delivers a dose of 3 x 10 11 to 1 x 1012 ions/cm2 (focus and beam-current dependent). Despite the low ion doses, the TOF analyser is very efficient, and this accounts for the high secondary ion data rates (typically 5000 c/s even on relatively low ion yield polymers).

Where required, the primary ion current can be increased and the ion gun turned on continuously and rastered for sputter cleaning of suitable samples.

Delayed Extraction

The instrument also employs a technique known as 'delayed extraction' for the secondary ions produced. In this technique the primary ions bombard the surface and produce the analytically important secondary ions. A short time after the primary beam pulse has finished bombarding the sample, the ion extraction field is pulsed on. This results not only in secondary ion extraction, but also secondary ion compression as the ions travel through the analyser to the detector. In some TOF-SIMS instruments the primary beam is compressed or 'bunched', but in this instrument it is the secondary ions that are bunched. This delayed extraction is set so that the secondary ions of the same m/z are temporally focused to produce better mass resolution than would otherwise be obtained with the long primary pulse (140ns) on its own.

Charge Neutralisation

One of the advantages of using a pulsing ion beam/delayed extraction combination is that there are relatively long periods in each TOF cycle when there is no ion extraction field applied. In that period a pulse of low energy electrons (30eV) is directed at the analytical area. By doing this it is possible to neutralise the effect of positive charge that would otherwise build up on the surface as the primary ion beam bombards an insulating sample.

TOF Analyser

The instrument has a 150mm diameter reflectron analyser, with a total effective flight-length (including the flight tube) of 2 metres. It is a dual-slope reflectron with in-vacuo high precision resistors, and has an adjustable 'retard' potential within the reflectron that has been set for optimum spectral performance.

Vacuum Pumping

The instrument has a single rack-mounted vacuum controller that controls the vacuum system. Turbomolecular pumps are used to maintain vacuum in the analytical chamber and the sample load lock, each of which is backed by a 2-stage rotary pump. Load lock venting and pumping is achieved with a single manual button on the vacuum controller. A high vacuum gauge (inverted magnetron) monitors the pressure in the analytical chamber at all times, and is used to provide vacuum interlock protection, shutting down high voltages if the pressure rises beyond a set point.

Expansion

The current instrument has been designed to permit the following additions:

  1. Linear motion stage with multiple sample holder to permit analysis of more than one sample per sample load
  2. Optical microscope/camera with external light illumination
  3. Imaging SIMS version through addition of SED detector/pc card and upgrade software
  4. Sample treatment possibilities in load lock

Examples of Data

Mass Resolution and Mass Accuracy

High resolution spectral plot
Silicon wafer contaminated with copper, iron and chromium

The above mass spectrum shows a narrow mass range from mass 50 to mass 71 for a silicon wafer with surface contamination of copper and iron and chromium at levels of ~2 x 1012 atoms/cm2 (one thousandth of a monolayer coverage). The mass resolution (M/ΔM) is >1000, permitting separation of metals from hydrocarbons at the same nominal mass. The mass accuracy permits extra levels of confidence when assigning peaks. For instance, the peak at mass 62.94 is a single peak, and corresponds to 63Cu, whose exact mass is indeed 62.94. The peak at mass 65 is a doublet, with the main contribution from a mass at 64.95. This is the 65Cu isotope, exact mass 64.93. Typically the mass accuracy is ≥20 milli mass units. At mass 55, there is a single peak with a mass 'excess' of 0.06 mass units, and this is C4H7, exact mass 55.055. One mass unit higher, the main peak is at 55.94, corresponding to 56Fe, exact mass 55.935. At mass 52 there is another doublet, one peak at mass 51.94 and another at 52.04. These correspond to 52Cr (mass 51.94) and C4H4 (mass 52.03).

Sensitivity

The instrument is recording >26,000 counts at Cu in a five minute acquisition for a known surface concentration of 2 x 1012 atoms/cm2, with a detection limit of ~2 x 109 atoms/cm2.

Mass Range

Although the system does not have a post-accelerated detector, the system is capable of measuring out past 1000 m/z (provided ions are created by the SIMS process). Examples are shown below:

Wide mass range spectral plot
Caesium iodide clusters in positive SIMS

Resolution at high mass plot
Molybdenum oxide clusters in negative SIMS (MoO3)3

Wide mass range spectral plot
Full mass range spectrum of crystal violet, with the M-Cl+ peak at mass 373

Insulator Analysis

SIMS analysis of insulating samples could not be easier. All types of insulating samples can be analysed. Low energy electrons are pulsed onto the sample every TOF cycle, preventing charge build-up.

Spectrum
Positive SIMS spectrum from double-sided Scotch tape

This tape is particularly clean and free from siloxane contaminants. Note the presence of Lithium on the surface (mass 6 and 7 in the correct isotopic ratio)

Spectrum
Positive SIMS spectrum from generic double-sided tape

By contrast, this generic double-sided tape shows the classic signs of siloxane surface contamination: higher than usual peaks at 28, 43, 73 and 147.

Spectrum
Positive SIMS spectrum from generic double-sided tape

If we zoom in to mass 28, we see that it is a split peak; the lower mass peak is silicon 28, and the higher mass peak is C2H4. Detection of atomic silicon, along with the other characteristic peaks from PDMS, confirms the siloxane identification. The message is to avoid low-cost generic double-sided tape products, and use Scotch brand products that are very clean and suitable for mounting samples in SIMS.

Spectrum
Negative SIMS spectrum from generic double-sided tape

The negative SIMS spectrum also shows characteristic peaks due to siloxane at 28 (Si), 59 (CH3SiO), 60 (SiO2), 149 (CH3)3Si-O-SiO2 and 165.

In the next example we see mass spectral data taken from an unprinted paper (blue trace), and the same paper with an ink print (red trace). The unprinted paper has a characteristic peak at mass 39.96 due to calcium, which on paper surfaces is normally due to kaolin loading of the paper (extremely white paper has a high kaolin loading). Once the paper has been printed, the paper is covered over, and so the Ca peak disappears. By contrast the hydrocarbon peaks increase due to presence of the organic-based ink.

Spectrum
Positive SIMS spectrum from paper

Finally a couple of relatively pure polymer examples

Spectrum
Positive SIMS spectrum from PET

Positive SIMS spectrum from PET (polyethylene terephthalate). Characteristic peaks observed at 104/105, 149 and 191/193.

Spectrum
Positive SIMS spectrum from PTFE (logarithmic scale)

Positive SIMS spectrum from PTFE tape, showing characteristic ions through to mass 531. The peaks are all assignable to various CxFy combinations. Note that to accommodate the large dynamic range, a logarithmic scale has been used

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New SurfaceSeer Demonstration Facility

Our client at Innovia has kindly agreed to run demo samples for us on their SurfaceSeer instrument. Because this is an industrial company using the instrument regularly for their own work, we have limited access and therefore we must limit the number of samples we can run free-of-charge. However, even running two samples should allow you to see the type of data that the instrument produces. A short report will be issued by Kore, who have years of experience in interpreting TOF-SIMS data.

If you are interested to have a sample run, please contact Fraser Reich first ( or via the enquiry form) who will discuss the matter with you

TOF-SIMS is very surface sensitive! Poorly prepared samples produce poor results!

If you are interested to supply a sample, please follow these simple guidelines:

  1. Never touch a sample with your bare hands, always use fresh laboratory rubber gloves (nitrile) and clean tweezers
  2. Cut sheet material with clean scissors and mark the side to be analysed with a permanent marker pen: put a cross in the corner
  3. Never place samples inside plastic bags! The internal surfaces of almost all plastic bags are covered in siloxane release agent. This is very mobile and will contaminate your sample
  4. Use fresh aluminium foil (standard kitchen foil is very clean) and place the sample onto the foil and fold the foil to make an envelope around the sample.
  5. Remember that the SurfaceSeer sample holder is currently suited to sheet material
  6. Please provide what useful information you can about the sample, particularly if you want to send us a 'good' sample and a 'bad' sample for comparison

Using a sheet of aluminium foil creates a very clean sample preparation area

Always use fresh gloves, clean tweezers and clean scissors


Now the sample is prepared. You do not need to cut the material into suitable-sized samples - you can supply a larger sheet, and we will prepare the sample.

Photographed against a paper sheet for clarity - but always use a fresh sheet of aluminium foil as your worktop


Place the sample (ringed in red) onto clean foil and fold the foil to make a clean envelope


The finished folded foil envelope with sample inside

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Monday, June 25, 2007, 14:19

© Kore Technology Limited 2007