BackgroundIntroduction SIMS Feasibility of
SIMSVacuum – Efficiency and FlexibilitySample HandlingAnalysis ComponentsIG-20 Gas Ion GunIG-5C Caesium Ion GunIon Gun ControlMAXIM SpectrometerSNMS – Crossing the Boundary of QuantificationInstrument Control – Taking out the GuessworkSample ViewingOxygen FloodingConclusion
Analytical was founded in 1981 and is presently situated in a
2,130m2 manufacturing plant in Warrington, England with a staff of
over 50. As a privately owned company our reputation is built on creating close
and positive relationships with our clients. Many of these customers are working
at the forefront of new technology - in the fields of plasma research, surface
science, vacuum processing and gas analysis. To maintain this reputation Hiden
Analytical have, over the years, established exceptional levels of technical
expertise in these areas within our company.
ion mass spectrometry is the most sensitive of the generally available
surface analysis techniques. It is able to measure low level concentrations down
to parts per billion, provide imaging of chemical distributions and determine
material chemistry as a function of depth – depth profiling. SIMS finds
applications in a wide range of industrial and research settings including
semiconductors, coatings technology, fuel cell development, photovoltaics,
metallurgy and pharmaceuticals. Being a technique based on mass spectrometry it
has the ability to detect specific isotopes, making it ideally suited to nuclear
and isotopic tracer work. In use, a focussed beam of ions (the primary ions)
bombards the solid specimen in vacuum and ionised material characteristic of the
specimen (the secondary ions), is analysed by mass spectrometry.
Feasibility of SIMS
Despite the undoubted benefits of this extremely powerful technique, SIMS is
often perceived as difficult and expensive, frequently regulating it to be a
technique of last resort. This attitude has arisen because early SIMS
instrumentation was highly complex and expensive both in terms of capital and
running costs and also required PhD level staff to operate and interpret it.
However, just as the complexity of early electron microscopes has given way to
user friendly semiautomated instruments, so the essence of SIMS
analysis has become more refined. In this article we will take a tour around the
Workstation, a complete, highly flexible general purpose SIMS
instrument, carefully designed for ease of operation and low cost of
Vacuum – Efficiency and Flexibility
The SIMS process relies on the ability of the primary ions to
interact with the specimen and generate secondary ions that reach the detector.
Interactions with the residual gas must be avoided, in a worse case they may
cause contamination of the spectrum – for example oxidation of the sputtered
The SIMS Workstation runs at UHV pressures (typically 1E-9 torr)
and is based around an 18 port, spherical, turbomolecular pumped chamber. The
ion guns and loadlock are each equipped with their own turbomolecular pumps and
vacuum gauges, ensuring that the main chamber is kept as clean as possible.
Vacuum interlocks ensure safe operation and power supply protection.
The entire system is fully bakeable and uses industry standard Conflat® type
seals. There are a number of unused ports designed to permit the system to be
easily reconfigured or for customers to add their own devices specific to their
requirement, such as lasers, heating, cooling or mechanical attachments, X-ray
and electron guns etc. The SIMS workstation is designed to be flexible and upgradeable,
an instrument for tomorrow as much as today.
analysis was (and still is) heavily used by the semiconductor industry and this
has always been noticeable in the sample stages provided for commercial
instruments. However, most other customers do not have thin, perfectly planar,
easily cut samples. In the real world it would be a great advantage to analyse
entire components or industry specific test coupons. The sample stage and
handling in the Workstation is designed around this philosophy. Firstly, the
entire analytical system (ion guns and spectrometer) are above the plane of
analysis, meaning that large and awkwardly shapes test pieces can be
accommodated (Hiden can also design a customer specific chamber if required
to take very much larger than normal specimens). The standard specimen holders
are based on a flat plate with wire springs and these are easily modified to
take a wide variety of samples, from small flat plates to standard embedded
metallurgical samples. Entire small components, such as the fuel injector below,
can be attached without modification. This is especially important where cutting
may cause contamination or be very time consuming.
Fuel injection component mounted for analysis
Samples are introduced to the main chamber via a turbomolecular pumped
loadlock and UHV sealed magnetically coupled arm. This ensures that the main
vacuum is not compromised and provides rapid transit from air to UHV. The
standard holder provides for 5 or 10 small samples mounted behind 6mm diameter
windows. Samples mount from the rear so that the front face is always correctly
positioned for analysis.
Having transferred the sample to the main chamber under UHV, SIMS
analysis can now proceed. The standard complete Workstation is equipped with two
ion guns, both introduced through 35 mm ports. The IG20 gas ion
gun is used for bombardment with oxygen or inert gases, and the IG-5C caesium
ion gun produces a focussed beam of caesium ions. At the heart of the whole
system lies the MAXIM quadrupole SIMS/SNMS analyser that is responsible for the
excellent sensitivity of the instrument.
IG-20 Gas Ion Gun
On the right hand side of the picture of the whole instrument the IG20 gas ion
gun can be seen, mounted at 45°. The IG20 is a highly versatile gun
producing an intense focussed ion beam. Ions are produced by electron
bombardment of gas admitted via a precision leak valve. For most SIMS
analysis the gun is used with oxygen gas, providing the highest possible
sensitivity for electropositive elements.
For sputtered neutral mass spectrometry, where enhancement of the ion yield
is not required, the IG20 is run with argon. Gas consumption is low and a
standard high pressure pure gas bottles will last many months, supplying gas to
the leak valve via a regulator set at approx 0.5 bar.
The IG20 ion optical column comprises two threeelement Einzel lenses, beam
alignment stages, a bend to remove neutral particles and beam steering stages
for scanning during depth profiling and imaging.
The beam steering electronics fit directly onto the ion gun feedthrough,
removing the need for large multiway cables – a frequent source of unreliability
on some systems.
IG-5C Caesium Ion Gun
The IG-5C (launched in 2008) opens up sensitive analysis of the
electronegative elements as well as MCs+ cluster detection (where M is the
element of interest). Ions are generated by a miniature low power thermal
contact ionisation source, manufactured by Hiden, which
uses safe caesium salts. The source may be vented to air when still warm (a few
minutes after operation ceases) and is user changeable in a few minutes (source
lifetime is estimated to be 500 hours of operation).
The ion gun is of a two lens design with two sets of independent alignment
stages. The upper stage is positioned immediately after the source to ensure
that the emerging beam is optimised as soon as possible onto the optical axis.
The beam passes through a defining aperture that is easily serviceable by the
end user, either for replacement or to modify the beam characteristics. The
lower alignment stage incorporates a double bend to remove neutrals and
approximates two small-angle electrostatic sectors. The ions are finally
focussed by a low spherical aberration lens and scanned by a group of four short
plates. The thermal management of the IG-5C is provided automatically via the PC
controlled ion gun interface unit.
Ion Gun Control
2008 also saw a radical updating of the ion gun control system to make it
easier to use and add functionality. The main picture (on page 1) shows the
manual ion gun controllers (still available for IG20 if required), however, the
new generation of ion gun controller is microprocessor based and is accessed via
a PC interface.
Using this controller it is possible to store and recall ion gun settings.
This makes life for the analyst very easy as settings for high current depth
profiling, low current fine focus imaging, low energy high depth resolution
profiling and other useful tasks may simply be called from a pull down menu.
PC Ion gun control interface
Diagnostic modes are instantly available with beam parameter measurements
(current and beam shape) via an electron suppressed faraday collector.
A useful feature is the automatic switching of the target bias between modes
which ensures that the correct bias is applied when running analyses or
SIMS/SNMS spectrometer is mounted on the back of the chamber in an off axis
position and has been specifically designed to give sensitive, reproducible,
analyses. Ions are collected from the sample by a shaped extraction field and
energy filtered using a parallel plate system, with the energy resolution
matched to that of the quadrupole analyser. The 9 mm triple filter has
molybdenum rods for thermal stability and the pulse counting detector has a 4
keV post acceleration potential to increase efficiency at high mass. The
spectrometer is available in mass ranges of 300u, 510u and 1000u for different
Immediately behind the extractor is a high efficiency electron impact ion
source which can be used for either sputtered neutral mass spectrometry (SNMS)
or residual gas analysis.
SNMS – Crossing the Boundary of Quantification
analysis is highly sensitive and ideal for the analysis of dopants, diffusion
studies and contamination. Quantification schemes for SIMS
generally assume that the impurity being measured is dilute (less than a few
percent). Above this dilute limit the probability of ionisation becomes
dependent upon the impurity concentration itself, as well as the chemistry of
the matrix. Sputtered neutral mass spectrometry (SNMS) neatly overcomes this
MAXIM running SNMS analysis
Firstly, most ions from the specimen (the SIMS
signal) are rejected by suitable target and deflection potentials. Next, the
neutral particles drifting from the sample are ionised in the electron impact
source; they are separated from residual gas ions, also forming, by their
kinetic energy which is significantly higher. As ionisation occurs away from the
sample surface the ionisation probability is always the same, thus the SNMS
signal is easy to quantify.
Instrument Control – Taking out the Guesswork
The instrument is controlled using the Hiden MASsoft
software suite. This provides automatic efficient and reproducible optimisation
of the secondary ion column and spectrometer – taking out the guesswork. If
required all of the parameters may also be manually tuned. MASsoft provides a
very powerful set of spectrometer, rastering, gating and data handling controls,
with a simple clear process flow tree. A facility is provided so that the amount
of control may be limited, with a few options for the inexperienced user right
up to full control for those wishing to experiment.
Usefully, commonly undertaken tasks (like column tuning and spectrum
collection) may be set as simple user programmable quickstart buttons, enabling
an analysis to be initiated by a single click of the mouse.
It is often overlooked but a clear view of the sample before and during
analysis makes the analyst’s job significantly easier. It allows areas to be
accurately and confidently targeted. In some cases colour changes during
analysis of thin films provide added information about the erosion rate and
consistency. The workstation is equipped with a normal incidence colour CCD
camera and LED cold light source, giving a clear view of the sample surface.
Sample view of analysis craters.
When making high depth resolution depth profiles, especially at low energy
(below 2 keV) with oxygen, the surface is known to roughen and degrade the very
resolution being sought. However, if the surface is made to fully oxidise during
analysis the depth resolution is maintained. This can be achieved by either
mounting the gun at normal incidence (very possible on the SIMS
Workstation) or by flooding the sample area with oxygen. In the Workstation, a
fine capillary brings oxygen gas to exactly where it is required, bringing about
a drastic improvement in resolution.
Effect of oxygen flood on analysis of 3.6nm Fe/Si neutron mirror
SIMS Workstation has been developed with strong interaction with those used
to performing SIMS analyses for both contract service work and research in
many fields of analysis. The philosophy of design has been simple, to produce a
proper SIMS analysis instrument which combines high sensitivity,
flexibility and ease of operation, but also to make it cost effective both in
terms of capital and ongoing costs.
SIMS Workstation has achieved these aims, being the only mid-priced
instrument to incorporate both oxygen and caesium ion guns – a necessity for
general analysis. It provides a true UHV environment, flexible loadlock and
sample handling system. The embedded SNMS facility ensures that the tool covers
the full analytical concentration range, from ppm through to 100% bulk and the
data system means that valuable results are easily stored and exported to other
applications. It is easy enough to configure for production line analysis, yet
will provide the dedicated research professional with a tool limited only by
Source: Hiden Analytical
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