photo of ion beam column housing

Ion Beams and their Applications

Ion beams come in many shapes and sizes, with multiple source options and applications. A minefield of options awaits if you are unfamiliar with them. This application note will shed some light on Ionoptika’s range of ion beams to help you choose the right one for your application.

Contents

  1. Sputter vs Analytical Ion Beams
  2. C60 Beams
  3. Gas Cluster Ion Beams
  4. Liquid Metal Ion Beams
  5. Plasma Ion Beams
  6. Conclusions

Sputter vs Analytical Ion Beams

We split our range of ion beams into two groups based on their applications or purpose – sputter beams and analytical beams.

Sputter Beams

While all ion beams will sputter a surface, we make this distinction based on the area and speed with which this occurs. Sputter beams have three characteristic features: high current, large spot size, and wide field of view. They deliver a large dose of ions over a wide area as quickly as possible to optimise etch rates.

Sputter beams remove material before analysis, either for cleaning purposes or for depth profiling through the sample. Techniques employing sputter beams include SIMS, XPS, SEM, TEM, and Auger.

Analytical Beams

Rather than being used to facilitate analysis using a different technique, analytical beams perform the analysis themselves. They also have three characteristic features; wide energy range, small spot size, and variable current control. These features give the user excellent control over the beam characteristics, enabling them to optimise their experiment.

Analytical beams are primarily used for secondary ion mass spectrometry (SIMS) and work well in traditional focused ion beam (FIB) applications such as secondary electron imaging and FIB milling.

C60 Beams

C60 molecule

Carbon-60, or just C60, is a fullerene molecule consisting of sixty carbon atoms formed into a hollow sphere, with a shape very similar to a soccer ball. The first commercial C60 ion beam was produced in 2002 by Ionoptika in collaboration with the University of Manchester, and since then, we have sold more than 150 units worldwide.

Compared to monatomic ion beams, C60 beams result in a much “gentler” sputtering action, reducing molecular fragmentation and damage to sub-surface layers. When employed as an analytical beam, this gentle sputtering action significantly increases sensitivity to intact molecular ions.

As the C60 molecule is larger (~ 7 Å) than the lattice constant for most materials, it also does not channel through the lattice the way monatomic ions do, reducing preferential sputtering. C60 beams exhibit incredibly uniform sputter rates across a wide range of materials, including challenging poly-crystalline materials.

The properties of C60 make it suitable for both sputtering and analysis. Ionoptika offers three C60 ion beam systems: a broad-beam sputtering system, the C60-20S, and two analytical beams, the C60-20 and C60-40.

See our application note all about C60 beams for more information.

Gas Cluster Ion Beams

Illustration of a GCIB sputtering material from a surface

Gas cluster ion beams (GCIB) are high-energy beams of cluster ions, ideal for sputtering and analysing organic matter. GCIBs are an incredibly versatile ion source, as both the ion species and the beam properties can be varied, allowing the user to tune the beam to the needs of their experiment.

The source operates through the adiabatic expansion of gas in a vacuum, causing rapid cooling and cluster formation. The clusters are then ionised through electron bombardment and accelerated towards the target. The size of the cluster is a vital parameter, and users can adjust this over a wide range.

Organic Analysis

GCIBs are the ideal choice for sputtering organic matter. Etch rates of organic matter are orders of magnitude higher than for metals or semiconductors, making cluster beams such as the GCIB 10S an excellent tool for surface cleaning. The cluster distributes the ion’s energy across all constituent atoms/molecules, resulting in a very gentle sputtering effect and almost no damage to layers underneath—GCIBs perform much better than C60 on both fronts.

GCIBs must be operated at high energy to maximise their benefits for SIMS, as the secondary ion yield increases as a function of beam energy. We currently offer a 40kV variant, the GCIB 40, and a 70kV variant, the GCIB SM.

The J105 SIMS utilises the benefits of gas cluster beams for organic analysis. Combining the gentle sputter action of large cluster ions with increased secondary ion yield has extended the usable mass range to > m/z 2500.

Choice of gas

The versatility of GCIBs comes from having a choice of input gas. Argon is the most common as it is an inert gas that forms clusters easily, but Ar/CO2 mixtures and pure CO2 gas are also becoming standard for SIMS applications.

The stronger van der Waals forces between CO2 molecules result in much larger clusters than would be available for Ar – up to 20,000 in some cases. A wider range provides greater control of the all-important E/n value (energy per nucleon). Research has shown that optimising E/n results in an enhancement of the secondary ion signal. The presence of O ions at the surface may also improve the ionisation probability – further enhancing ion yield.

We have recently developed a GCIB source that runs on water vapour, which is currently available as an optional add-on for the J105 SIMS. Water molecules have even greater binding energy and can form enormous clusters of up to 60,000 molecules. Water clusters provide secondary ion yields up to 500 times greater than argon and are the best choice for state-of-the-art biological SIMS.

See our application note on choosing the best GCIB for your application for more detailed information.

Liquid Metal Ion Beams

Liquid metal ion beams, also known as LMIS, or LMIG, are a well-established source technology. The source operates by a liquid metal reservoir feeding a blunt tungsten tip, from which a strong electric field extracts ions. Due to their elegant and reliable design, FIB systems have been using LMIS for decades. Ionoptika offers a 25 kV LMIG system in two variants; the IOG 25AU gold-cluster system and the IOG 25GA gallium system.

Liquid metal beams produce monatomic or small-cluster ion beams, such as Au+, Ga+, and Au3+. They feature high currents and small spot sizes (< 100 nm), making them ideal for high-resolution analysis applications.

Small, high-energy ions can penetrate far beneath the surface before dissipating their energy. Known as channelling, this causes significant sub-surface damage, making depth profiling unreliable. It also results in considerable fragmentation, making LMIS more suited to analysing hard materials.

Plasma Ion Beams

Plasma ion beam

Plasma ion sources are characterised by incredibly high brightness, making them ideal for high throughput applications. A single plasma source can run on various gases without changing parts, providing flexibility. Gases available for our plasma ion beams include hydrogen, helium, oxygen, nitrogen, argon, and xenon.

Plasma sources are monatomic and do not form clusters, resulting in lower energy distributions and smaller spot sizes. Combined with their high brightness, this leads to a very high current density beam.

Plasma beams are an excellent choice where the primary goal is high-volume etching or milling. For analysis purposes, plasma beams best suit harder materials such as metals, semiconductors, and inorganics.

FLIG – Floating Low Energy Ion Beam

The FLIG 5 is a unique ion beam system based on a floating column design. The design enables ultra-low energy operation to 200 eV while still delivering a high current. Operating at such low impact energies significantly reduces the beam’s penetration depth, improving the depth resolution. Due to its high performance at ultra-low energies, the FLIG 5 has been the industry standard for shallow junction depth profiling for almost two decades.

Conclusion

The table below compares Ionoptika’s ion beam products under several categories discussed in this article (best viewed on desktop).

ION BEAMSPECIESENERGY RANGEMIN SPOT SIZEBEAM CURRENTAPPLICATIONBEST FOR
C60 Ion Beams
C60-20SC60+, C60++, C60+++5 – 20 kV100 μm50 nASPUTTEROrganic, biological, inorganic, metals
C60-20C60+, C60++, C60+++5 –20 kV2 μm2 nAANALYTICALOrganic, biological, inorganic, metals
C60-40C60+, C60++, C60+++10 – 40 kV300 nm1 nAANALYTICALOrganic, biological, inorganic, metals
Gas Cluster Ion Beams
GCIB 10SArn+, (CO2)n+, or (Ar/CO2)n+1 – 10 kV250 μm60 nASPUTTEROrganic & biological, polymers
GCIB 40Arn+, (CO2)n+, (Ar/CO2)n+, or (H2O)n+5 – 40 kV3 μm200 pAANALYTICALOrganic & biological, polymers
GCIB 70/SMArn+, (CO2)n+, (Ar/CO2)n+, or (H2O)n+20 – 70 kV1.5 μm300 pAANALYTICALInorganic, organic & biological, polymers
Liquid Metal Ion Beams
IOG 25AUAu+, Au++, Au2+, Au3+, Au3++5 – 25 kV100 nm10 nAANALYTICALInorganics, metals, semiconductors
IOG 25GaGa+, 69Ga+5 – 25 kV50 nm20 nAANALYTICALInorganics, metals, semiconductors
Plasma Ion Beams
IOG 30ECRN2+, O2+, Ar+, & Xe+5 – 30 kV500 nm500 nAANALYTICALSemiconductors, metals, inorganics
IOG 30DH2+, He+, N2+, O2+, & Ar+5 – 30 kV500 nm500 nAANALYTICALSemiconductors, metals, inorganics
FLIG 5H2+, He+, N2+, O2+, & Ar+0.2 – 5 kV15 μm500 nAANALYTICALSemiconductors, depth profiling

ToF SIMS – Time of Flight Secondary Ion Mass Spectrometry

What is ToF SIMS? What is it used for, and what sort of information can it provide? Which samples are suitable (and which are not)? In this series, we will answer all these questions and more.

Time-of-Flight Secondary Ion Mass Spectrometry (ToF SIMS) is a surface analysis technique used to study the chemical composition of solid surfaces and thin films in three dimensions.

Illustration describing ToF SIMS

A focused beam of primary ions bombards a target surface, creating a plume of neutral atoms/molecules, secondary ions, and electrons. The secondary ions are collected and analysed using a time-of-flight mass spectrometer. The mass spectrometer measures an ion’s mass-to-charge ratio (m/z) by precisely timing how long it takes to reach the detector – the “time of flight”.

By scanning the primary ion beam across an area of the sample, a chemical map of the surface is formed pixel by pixel. Scientists and technicians use ToF SIMS daily for fundamental research, routine analysis, and quality control in academic and industrial settings.

For many years, the limitations of the primary ion beam confined the analysis to looking at atomic species and small molecules. With advances in instrument and ion beam design, modern instruments such as the J105 SIMS are now routinely imaging large intact molecules. These new capabilities have caused an explosion in new applications, and more papers are published each year in bio and bio-related fields using ToF SIMS.

Anatomy of a ToF SIMS instrument

ToF SIMS instruments are often larger and more expensive than most other analytical instruments found in a lab. High-vacuum conditions (< 1×10-6 mbar) are required to prevent ions from colliding with gas molecules in the air, requiring bigger vacuum pumps, more robust seals, and additional precautions to prevent leaks.

Graphic showing the operation of the J105 SIMS instrument from Ionoptika.
Operation of the J105 SIMS ToF SIMS instrument. 1. The ion beam bombards the sample releasing secondary ions, electrons, and neutrals. 2. The secondary ions are collected. 3. Secondary ions are cooled and focused into the mass spectrometer. 4. The mass spectrometer records the flight time of the ions and converts this to a mass spectrum.
Primary componentsSecondary components
Sample analysis chamber (SAC)Sample introduction System
Primary ion beamCryogenic cooling for low-temperature analysis
Secondary ion extraction opticsCharge compensation, e.g., electron beam
Mass spectrometerSecondary electron imaging

Key Benefits of ToF SIMS

  • Spatial resolution. ToF SIMS achieves significantly higher spatial resolutions than other imaging methods, thanks to beam sizes as small as a few hundred nanometres.
  • Speed. The time-of-flight mass spectrometer operates at much higher rates than other MS techniques. ToF SIMS instruments can run at speeds up to 1000 pixels per second.
  • 3D imaging. The primary ion beam removes a small amount of material each time it scans across the surface. By making multiple passes over the same area, a 3D map of the material builds up layer by layer.
  • Sensitivity. Small spot sizes and shallow impact craters result in tiny analysis volumes, which require great care to prevent signal loss. As a result, SIMS is generally more sensitive than other forms of mass spectrometry.
  • Dynamic range. The ions in a ToF SIMS spectrum can range from a single hydrogen ion to intact protein molecules several thousand daltons in size.
  • Applications. The breadth of applications for ToF SIMS is enormous, ranging from metallurgy to fundamental biology and most things in-between.

Applications of ToF SIMS

ToF SIMS provides a detailed three-dimensional chemical map of a sample. Information about the atoms and molecules that make up the sample, their distribution, and any contamination present are all revealed. This type of information is beneficial for many applications.

Academic research labs, industrial quality control, and research organisations use ToF SIMS daily. Disciplines as diverse as materials science, analytical chemistry, biology, geology, pharmaceutical science, and many others benefit from the detailed chemical information ToF SIMS provides.

2D Imaging

2D images are the most common mode of operation for ToF SIMS applications, whereby the ion beam scans the surface, acquiring a mass spectrum at each pixel. The image resolution can vary from a few hundred pixels to over four million.

Images of individual mass channels show the precise distribution of ions across the field of view. Overlaying multiple mass channels can show the distribution of different ions and how they relate to each other.

The image below shows three individual ion images and an overlay image representing different components of a biological tissue sample.

ToF SIMS image of a rat cerebellum

Spectrometry

Analysis of a ToF SIMS spectrum provides information on the atomic or molecular makeup of the sample and can inform about the general abundance of various compounds. It is also possible to determine atomic ratios in some cases, but this requires well-controlled samples and careful use of reference materials.

Tandem mass spectrometry is a feature on most major ToF SIMS instruments and is extremely useful for confidently identifying ions. Tandem MS, also known as MS/MS, or MS2, involves isolating a secondary ion of interest, fragmenting it, and collecting the resulting fragments in a mass spectrum. By analysing the daughter peaks, it is possible to determine the parent ion with a high level of precision.

MS/MS spectrum of the phospholipid PC34:1+K acquired on the J105 SIMS instrument.
A Tandem MS spectrum of a phospholipid species in a tissue sample acquired on the J105 SIMS instrument. Analysing the fragment pattern confirms the identity of the parent ion as PC34:1+K.

Depth profiling

A powerful analysis mode, depth profiling involves etching vertically through the sample and acquiring a mass spectrum at every layer. The result is a profile of all atoms/molecules through the sampled volume. Large cluster ions reduce damage to sub-surface layers, minimising interlayer mixing and maximising depth resolution. With the right ion beam and sample combination, depth resolution as low as a few nanometres is possible.

Depth profile through the NIST Ni/Cr standard reference material using a C60 beam.

Depth profile through the NIST Ni/Cr standard reference material using a C60 beam, showing 5 nm depth resolution.

3D Imaging

The feature that sets ToF SIMS apart from other mass spectrometry and analytical techniques is the ability to acquire 3D data sets. Like a depth profile, a 3D analysis involves acquiring many 2D layers repeatedly over the same area, etching material with each pass, and building up a three-dimensional view of the sample. Large cluster ions are ideal for 3D analysis as they produce very little damage and can therefore be used to etch and analyse the sample simultaneously.

Unlike techniques like AFM, which capture the 3D topography of the sample, SIMS cannot distinguish 3D objects from flat objects. The technique works best for flat samples with layers of interest below the surface, as in the OLED example below. It is possible to reconstruct the topography of a non-flat sample; however, this requires prior knowledge of the material structure. 

3D ToF SIMS image of an OLED screen, showing the different components of each sub-pixel unit.
This 3D ToF SIMS image of an OLED screen is acquired on the J105 SIMS using a 70kV water cluster primary ion beam. The RGB subpixel units appear at different distances from the surface, depending on their colour.

Read more about the applications of ToF SIMS in our Application Notes section. Or, to dive deeper into more advanced topics, check out the list of publications using our equipment here. You might also like to learn more about how the J105 SIMS operates, which you can read here.

How to choose the best GCIB for every application

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How to choose the best GCIB
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Gas Cluster Ion Beams (GCIB) are an incredibly versatile resource for surface scientists. But it can be tricky to know what set of parameters to choose to get the most out of your experiment. This application note breaks down each parameter and provides a simple set of guidelines to help you choose the best GCIB for your next experiment.

This application note is essential for anyone using SIMS, XPS, SEM, or any technique that uses cluster beams. Fill out the form below to get your free copy.