The SurfaceSeer I
TOF-SIMS

Primary Ion Gun

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:

• Stigmation and alignment units
• Aperture selection, to allow a wide choice of output current (normally manual, but there is a motorised option)
• Optional Mass filter, used with alloy sources
• Deflection plates for blanking the beam
• Optional Pulse bunching
• Raster plates for imaging

Secondary Electron Detection

For an imaging TOF-SIMS system it is essential to have a secondary electron imaging system. This has three main functions:

• To focus the primary ion beam
• To permit the beam to be set for ‘motionless blanking’ – this is a tuning that permits the ion beam to be blanked at high speed with minimal distortion of the beam (necessary for TOF-SIMS imaging)
• To obtain ion-induced secondary electron images. This latter function can only be achieved with a continuous primary ion beam, and is normally reserved for when all analytical work is complete on a sample

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.

Optical Viewing

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:

• Zoom from ~3mm to ~400µm field of view.
• High lateral optical resolution at high magnification (<5µm).
• A long working distance of 175 mm ideally suited to ultra­ high­ vacuum chambers where it is not possible to locate a camera near to the sample.
• Mounting onto a 70mm OD CF window.
• A colour camera mounted on the microscope.
• Dedicated colour monitor display.
• Cold, dichroic halogen illuminator mounted on a 70mm OD CF window.
• Power supplies for the illuminator and camera

Sample Handling

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:

• The sample can be ‘back-mounted’, meaning that the sample is located behind a mask that is at the correct height. The maximum thickness of sample that can be mounted in this way is ~5mm (5 high x 8 wide x 20mm)
• The sample is ‘top-mounted’ onto a sample holder with a cutaway of 1mm (deeper on request). For top-mounted thicker samples, it is possible to use the z height adjustment of the stage to lower the sample holder so that the sample surface is positioned at the correct height.

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.

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 (60ns) 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

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.

Static SIMS Library

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.

Depth Profiling with a second ion source

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:

• Analysis is performed in pulsed beam mode using the analytical gun for x seconds in a small area, where x is a user-definable time, to acquire data with significant statistics (experiment dependent).
• Analysis is stopped.
• The sputtering beam is turned on and rastered over a larger area. The sample is sputtered in continuous beam mode for y seconds to remove material, where y is a user-definable time, depending upon current and raster size.
• The sputtering gun is turned off, ready for the next repeat of step 1.

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’.

Oxygen Leak (Oxygen Jet)

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.

Application Areas

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:

• Surface chemistry
• Microstructural surfaces
• Patterned devices (semiconductors etc)
• Failure analysis at micrometer scale
• Powders
• Fibres
• Multilayer films
• Adhesion
• Delamination
• Printability
• Surface modification
• Plasma treatment
• Surface contamination
• Trace analysis (ppm in surfaces)
• Catalysis
• Isotopic analysis

Application Notes

• Materials Research

Product Note

• SurfaceSeer I