The SurfaceSeer I
Providing surface analysis with imaging and chemical mapping.
Providing surface analysis with imaging and chemical mapping.
The SurfaceSeer I is a high sensitivity TOF-SIMS for imaging and chemical mapping of insulating and conducting surfaces. The SurfaceSeer I is ideal for investigating the chemistry of surfaces and is equally at home in R&D as well as industrial quality control applications.
The SurfaceSeer I uses the same TOF-MS technology as the SurfaceSeer S, but is fitted with a high brightness high spatial resolution 25 kV liquid metal ion gun (LMIG) as the primary ion source. Additional computer control allows the gun to be scanned during the mass spectral acquisition so that chemical images, or maps, may be collected. A secondary electron detector is also provided for tuning of the primary beam.
A high performance liquid metal ion beam system (LMIG) designed to provide a range of ion beams for SIMS applications. It offers a wide current range with fine probe capability and d.c. or pulsed operation. Digital control allows easy set-up of the gun and a provision for remote control is included.
The gun column consists of a liquid metal ion source and a high precision two-lens optics assembly, including:
For an imaging TOF-SIMS system it is essential to have a secondary electron imaging system. This has three main functions:
The SED system comprises a channeltron detector inside the analytical chamber, with the SED preamplifier mounted on the external flange. A power supply unit with SED controls is made available on a separate ‘sample viewing’ electronics unit.
In surface analysis, it is extremely helpful to view the sample optically to assist in navigation and to determine the correct location for subsequent analysis. Kore has developed a viewing system with the following capabilities:
X, Y, Z High Stability Stage. The stage has motions of ~ ±10mm in X and Y and 2mm in Z. There is a concept of an optimum z height at which all the beams are confocal. The sample surface is brought to this position. If the sample is relatively thick >1mm, then there are two possibilities:
Samples are pumped down within 2-10 minutes in a small volume load lock and are then entered into the analytical chamber (via a manual gate valve) with a simple forward motion and 90° twist action of a magnetically coupled sample introduction rod. Porous or ‘wet’ samples can take longer to pump down.
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 (60ns) on its own.
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.
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 pump controllers are integrated into the main instrument frame. Two ion pumps maintain vacuum in the analytical chamber and LMIG source. A turbomolecular pump is used for the sample load lock, backed by a 2-stage rotary pump. Load lock venting and pumping is achieved with a single manual button. 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.
The instrument will be provided with the Surface Spectra Static SIMS Library. This software has a mass spectral library of more than 1900 spectra covering data more than 1000 different material.
The software also has peak searching tools to allow the analyst to input mass peaks and search the library to identify unknown compounds and materials.
|High surface sensitivity||1x109 atoms/cm2 (ppm)|
|Conducting and insulating surfaces|
|Positive and Negative SIMS|
|Mass resolution||>3000 m/δm (FWHM) using time-of-flight Reflectron mass analyser|
|Mass accuracy||± 5 milli amu|
|Analytical spatial resolution||≤0.5µm|
|Elemental and molecular information|
|Separates common organics from elements|
|Sputter cleaning capability|
|5 minute sample pump down from atmosphere|
|1 minute analysis|
|Data libraries available|
The LMIG is not suitable for eroding craters, therefore, in order to perform SIMS depth profiling, we use a second ion gun dedicated to sputtering.
By having a second, dedicated ion gun, we can also vary the energy of the sputtering ion beam. For instance, for shallow depth profiles of a few hundred nanometers, it will be appropriate to use perhaps 1keV impact energy, whereas for deeper profiles 2-3keV will be appropriate.
This is rather similar to XPS and Auger depth profiling in that the analytical phase and the sputtering phase are separated. The technique works by repeatedly looping through the following steps:
Data points in the depth profile are plotted at the end of each analytical ‘phase’. A limited set of species is declared at the time of the ‘live’ acquisition, but after the run has finished the data may be replayed, and any combination of species may be re-plotted. This is made possible because information about every recorded ion is stored to disk, as the ion detection in a TOF-SIMS is ‘parallel’.
Positive secondary ion emission is greatly enhanced when the sample surface has a native oxide. If the experiment involves a high enough ion dose to strip away the native oxide, there is a dramatic fall in signal. A well-known solution, used in dedicated depth profiling SIMS instruments, is to use an oxygen primary ion beam, which results in an oxygen-rich ‘altered layer’ and enhanced signal. It is also the case that a large percentage of that enhancement is achieved through oxygen co-implantation, achieved by jetting oxygen gas onto the sample surface whilst using an alternative primary beam species such as Ga+ or Ar+.
Accordingly, we offer an oxygen leak option in which a precision leak valve supplies oxygen down a capillary line close to the sample to ‘jet’ oxygen to the sample surface.
Applications for the imaging version of the Surface Seer are similar to the ‘S’ model, but now the imaging facility extends the analysis to samples with heterogeneity on the micrometer scale:
Kore Technology is a centre of excellence in time-of-flight mass spectrometer technology and has a very strong R&D capability in terms of its personnel, all of whom have been heavily involved in a variety of analytical instrumentation development programmes.