ion beams and their applications

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 are often used to remove material before analysis using a separate analytical technique, e.g. SIMS, XPS, SEM, TEM, Auger etc. This can be for cleaning purposes or used as a means of depth profiling through the sample.

Analytical Beams

Rather than being used to facilitate analysis using a separate technique, analytical beams are designed to perform the analysis themselves. They also have three characteristic features; wide energy range, small spot size, and variable current control. This gives the user incredibly fine control over the beam characteristics, which enables them to optimise their experiment.

Analytical beams are primarily used for secondary ion mass spectrometry (SIMS) but can also be used 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, more than 150 units have been sold worldwide.

Compared to monatomic ion beams, C60 beams result in a much “gentler” sputtering action, greatly reducing the damage caused to sub-surface layers. Preferential sputtering – normally an issue for monatomic beams – is largely non-existent for C60, while the etch rate is also relatively consistent across different material types. This makes C60 a very powerful, very consistent sputter source.

As an analytical beam, the gentle sputtering action of C60 also reduces the fragmentation of larger molecules, resulting in enhanced molecular signal intensity. For techniques such as SIMS, this can be incredibly important, as it significantly increases the sensitivity of the technique to intact molecular ions.

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 the sputtering and analysis of organic matter. They are an incredibly versatile ion source, as both the ion species and the properties of the beam can be varied as needed. This allows 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 resulting in cluster formation. The clusters are then ionised through electron bombardment and accelerated through the column. The size of the cluster is a vital parameter and may be tuned over a wide range by adjusting the source conditions.

GCIB for Organic Analysis

GCIBs are the ideal choice for sputtering organic matter. Etch rates of organic matter are several orders of magnitude higher than metals or semiconductors. This makes cluster beams such as the GCIB 10S an excellent tool for cleaning surfaces prior to analysis. The large cluster species also produce very little fragmentation or sub-surface damage – performing better than C60 on both fronts.

In order to maximise the benefits of GCIBs for SIMS, the beam must be operated at high energy. This is because the secondary ion yield shows a non-linear increase 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. The combination of the gentle sputter action of large cluster ions with increased secondary ion yield has extended the usable mass range to > m/z 2500.

For more detailed information, see our application note on how to choose the best GCIB for your application.

Choice of gas

The versatility of GCIBs comes from having a choice of input gas. Argon has historically been a favourite, as it is an inert gas that forms clusters easily. Ar/CO2 mixtures are also common – the CO2 acts as a cluster formation agent. However, pure CO2 gas is now favoured for many of our customers using GCIBs for SIMS.

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. This gives users much greater control over 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. It is also thought that the presence of O ions at the surface improves the probability of ionisation – further enhancing ion yield.

We have recently developed a water vapour (H2O) source for GCIBs. This is currently available as an optional add-on for the J105 SIMS.

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. The source design is elegant and reliable and has been used in FIB systems 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 have very 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. Also known as channelling, this causes significant sub-surface damage, and as such, depth profiling can be unreliable. It also results in considerable fragmentation, making LMIS much 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. This type of ion source allows for many different source gases, such as oxygen, nitrogen, helium, argon, neon, xenon, and hydrogen.

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

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
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