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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 BEAM SPECIES ENERGY RANGE MIN SPOT SIZE BEAM CURRENT APPLICATION BEST FOR
C60 Ion Beams
C60-20S C60+, C60++, C60+++ 5 – 20 kV 100 μm 50 nA SPUTTER Organic, biological, inorganic, metals
C60-20 C60+, C60++, C60+++ 5 –20 kV 2 μm 2 nA ANALYTICAL Organic, biological, inorganic, metals
C60-40 C60+, C60++, C60+++ 10 – 40 kV 300 nm 1 nA ANALYTICAL Organic, biological, inorganic, metals
Gas Cluster Ion Beams
GCIB 10S Arn+, (CO2)n+, or (Ar/CO2)n+ 1 – 10 kV 250 μm 60 nA SPUTTER Organic & biological, polymers
GCIB 40 Arn+, (CO2)n+, (Ar/CO2)n+, or (H2O)n+ 5 – 40 kV 3 μm 200 pA ANALYTICAL Organic & biological, polymers
GCIB 70/SM Arn+, (CO2)n+, (Ar/CO2)n+, or (H2O)n+ 20 – 70 kV 1.5 μm 300 pA ANALYTICAL Inorganic, organic & biological, polymers
Liquid Metal Ion Beams
IOG 25AU Au+, Au++, Au2+, Au3+, Au3++ 5 – 25 kV 100 nm 10 nA ANALYTICAL Inorganics, metals, semiconductors
IOG 25Ga Ga+, 69Ga+ 5 – 25 kV 50 nm 20 nA ANALYTICAL Inorganics, metals, semiconductors
Plasma Ion Beams
IOG 30ECR N2+, O2+, Ar+, & Xe+ 5 – 30 kV 500 nm 500 nA ANALYTICAL Semiconductors, metals, inorganics
IOG 30D H2+, He+, N2+, O2+, & Ar+ 5 – 30 kV 500 nm 500 nA ANALYTICAL Semiconductors, metals, inorganics
FLIG 5 H2+, He+, N2+, O2+, & Ar+ 0.2 – 5 kV 15 μm 500 nA ANALYTICAL Semiconductors, depth profiling

GCIB-SEM: 3D electron microscopy with < 10nm isotropic resolution

GCIB-SEM is a new technique that combines high resolution electron microscopy with the damage free sputtering of gas cluster ions to produce incredible 3D tomography with less than 10 nm isotropic resolution.

Over the last two decades, gas cluster ion beams (GCIB) have become increasingly popular as add-on components for ultra-high vacuum techniques such as XPS, SPM, and SIMS. Due to their excellent combination of fast yet low-damage sputtering, GCIBs have been widely adopted as depth profiling ion beams, or as a means of cleaning samples in situ.

Very low impact energies, as little as 1 eV per atom, means cluster ions sputter material without modifying the surface chemistry, i.e. without breaking bonds. This makes GCIBs particularly effective for high-resolution depth profiling of soft materials such as polymers and organic matter.

GCIB 10S cluster schematic, and PET C 1s XPS spectrum comparing Ar1 and Ar2000.
The GCIB 10S is a powerful tool for damage-free depth profiling of polymers, organics, and other soft materials, delivering consistently superior results over monatomic beams.

Traditional sputter beams such as Ar1 typically have impact energies in the kilovolt range, resulting in not only large amounts of fragmentation to surface molecules, but also penetration of the ions beneath the surface causing further damage. This damage shows up in XPS and SIMS spectra, and limits the depth resolution of the technique.

Cluster beams also sputter soft, organic material much faster than hard, inorganic materials, making them extremely useful for removing adventitious carbon and other surface contamination without damaging the substrate — ideal for cleaning surfaces prior to analysis.

It is no surprise then that GCIBs have become so popular as add-on components for surface analysis instrumentation.

The GCIB 10S

  • 10 kV argon cluster ion source
  • Selectable clusters from Ar1 to > Ar3000
  • Real-time cluster measurement & adjustment
  • Sample current imaging
  • Gate valve for quick & easy servicing
  • Large spot size and wide scan field for even removal of material

The versatile nature of the GCIB makes it a useful tool in a variety of other techniques as well, beyond strictly surface science. In particular, the GCIB has recently been shown to be powerful tool in electron microscopy. A new technique pioneered by researchers at HHMI Janelia Research Campus combines high resolution electron microscopy with the damage free sputtering of gas cluster ions to produce incredible 3D tomography with less than 10 nm isotropic resolution.

Published in Nature Methods in 2019 the GCIB-SEM system developed by Hayworth et al. consists of a GCIB 10S from Ionoptika mounted on a Zeiss Ultra SEM. Using 1 µm thick serial sections of brain tissue, high-resolution electron imaging was interleaved with wide-area ion milling until the entire section was consumed. Full experimental details can be found in the paper linked above.

Figure detailing results achieved using GCIB SEM, by Hayworth et al
GCIB-SEM is a powerful technique for acquiring extremely detailed 3D maps on an unprecedented scale. Images from a GCIB-SEM run performed on three sequential 500-nm-thick sections of mouse cortex. bioRxiv: http://dx.doi.org/10.1101/563239.

The result is a 3D data set hundreds of microns in area by tens of microns deep, with less than 10 nm isotropic resolution throughout. Such a high resolution data set then allows researchers to map the brain structure in incredible detail. The figure above shows a 15 x 15 x 10 µm section of mouse brain, the detail of which is truly remarkable. Panel e shows a single spiney dentritic process with axons synapsing on it, while panel f shows various high-resolution 2D and 3D views of a single spiney synapse.

Other technologies used to perform similar experiments include FIB-SEM and diamond knife based sectioning, however both have their drawbacks. FIB provides the necessary resolution, but is thus far incompatible with the high-throughput needed for larger volumes, while diamond knife techniques are highly compatible with larger volumes, but lack the consistency needed at such thin cuts.

In contrast, the GCIB 10S mills away just the top few nanometres of the surface resulting in an improvement in depth resolution of a factor of 3 or more over other techniques, whilst simultaneously improving sectioning reliability. The rapid, wide area milling afforded by the GCIB 10S is also compatible with the new multi-beam SEM systems now on the market, which will enable even larger volumes to be analysed with no loss of resolution.

GCIB-SEM

  • Large-area and fast (up to 450 µm3 s-1).
  • Can be automated and is highly scalable.
  • Consistent performance over large volumes.
  • Simple, easy to maintain, and reliable.
  • Improves z resolution by a factor of 3 or more.

GCIB-SEM is a powerful technique for exploring complex materials and structures in three dimensions with extraordinary detail. For this application, control of the cluster size and current is critical to the result. Unlike other gas cluster beams, the GCIB 10S lets the user take complete control of the experiment. With real-time cluster measurement, cluster size can be tuned to the users’ needs and the settings saved for later use.

Real-time cluster measurement on the GCIB 10S

The GCIB 10S is easily installed on a range of instrumentation, from XPS and SIMS, to electron microscopes, Auger, and more. To speak with us and find out how the GCIB 10S might be right for your application, or to request a brochure, please get in touch via our Contact Page.

GCIB 10S Webinar in association with UCVAC

On Thursday, in collaboration with UCVAC, we held a webinar on the GCIB 10S Gas Cluster Ion Beam for potential customers in China. The webinar was a great success, and we will certainly look to use this format again to connect with potential customers around the world, particularly while travel restrictions remain in place.

IONOPTIKA recently authorized UCVAC as its sole agent in mainland China and Hong Kong. With extensive experience in the surface science markets, UCVAC are well placed to assist business development and provide technical support in the region. We look forward to working together, and this webinar was a fantastic way to kick things off.

The GCIB 10S is a high-performance gas cluster ion beam that delivers rapid, low-damage sputtering for superior quality surface analysis. An ideal upgrade for a variety of instruments, such as XPS, SEM, SPM, and SIMS, the GCIB 10S brings many powerful advantages in an economical, low-maintenance package.

Ultra-low-energy sputtering by argon cluster ions helps to efficiently remove material while producing very low damage and minimal loss of chemical information, leaving a pristine surface for analysis. Removing just a few nanometres per cycle, the GCIB 10S is the ideal tool for achieving ultra-high-resolution depth profile analysis.

If you missed it live, you can watch the full webinar below.