SPM is a powerful technique for determining the topography and nanoscale characteristics of a surface, whereas ToF-SIMS is equally powerful for determining a surfaces local chemical information. The two techniques both require vacuum conditions to operate and are localised to tiny regions of a surface, meaning using them alongside one-another is difficult. Now, a new combined SPM/ ToF-SIMS system has been developed that significantly increases the throughput and accuracy of combined SPM/ ToF-SIMS experiments.
AZoNano spoke to Raphaëlle Dianoux, CEO of Nanoscan, about their role in the collaborative effort to produce the new system, the novel research it will allow its users to undertake and the latest developments NanoScan are delivering in the field of SPM.
Please could you give our readers a brief overview of SPM and ToF-SIMS?
SPM and ToF-SIMS are two, very complementary, methods for surface and sub-surface analysis at the nanoscale.
Scanning Probe Microscopy (SPM) detects forces to determine the physical characteristics of a surface. This involves using a cantilever with a very sharp tip that senses a surface as it is scanned over it. This interaction is recorded and used to determine the spatial characteristics of the surface such as its topography or other physical properties at the nanoscale.
Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) maps the local chemical information of a surface. It achieves this by bombarding the surface with ions which in turn give rise to the emission of secondary ions which are ejected from the surface.
The mass of these surface ions is very accurately measured by time-of-flight mass spectrometry, which allows the elemental, isotopic and even molecular structure of the sample surface to be determined.
A simplified schematic of a scanning probe microscope. The tip of the cantilever traces over the sample surface, responding to the localised spacial or physical properites of the sample, and its motion is detected using a laser.
Principle of Time-of-Flight Secondary Ion Mass Spectrometry. Primary ions bombard the sample surface and eject ions from the sample itself. These secondary ions are then analysed according to their mass using a time-of-flight technique. Image Credit: ION-ToF GmBH
How do the two techniques complement each other?
SPM is the only tool for measuring the topography of any surface. It can even map individual atoms! And this is not all: it can measure other physical properties such nano-friction or nanoscale mechanical, electrical or magnetic properties.
Today, SPM is used in practically every surface analysis lab around the world, as it is a fast and non-destructive technique. However, it is limited to surface analysis. There is no way of getting information about sub-surface properties and there is little or no information on the samples’ chemical make-up.
On the other hand, ToF-SIMS provides chemical information from the surface and can identify anything from atoms to complex molecules, weighing many thousands of atomic unit masses. It's an extremely sensitive technique that can resolve isotopes and even molecules that are almost identical in composition.
ToF-SIMS is inherently destructive, because it involves bombarding the surface with energetic ions and thereby removing atoms and molecules from the top layers of the surface.
When tuned adequately, controlled layer removal – called sputtering – is an advantage: by alternating layer removal and surface analysis researchers can access sub-surface chemical information and hence map the chemical structure of the sample in 3 dimensions.
However, the 3D reconstruction of the analysed sample volume is distorted by the fact that the vertical axis represents sputter time and not real distance in nanometers. In complex samples with different chemical species, the sputter rate can differ widely so that layer removal becomes inhomogeneous and can impact negatively on the 3D reconstruction.
SPM compensates for this shortfall by providing depth and topography information after sputtering and thereby enabling accurate 3D data reconstruction to be made. Without ToF-SIMS, the SPM technique would be unable to access sub-surface information. So, combining the two analytical methods is a win-win situation.
Please could you introduce our readers to the combined SPM/ ToF-SIMS system that NanoScan have worked on? What role did NanoScan play in this?
The combined instrument overcomes the shortcomings of both analytical methods by bringing together SPM and ToF-SIMS in a single UHV chamber, and – most importantly – by providing a highly accurate and yet fast transfer system between them.
The transfer systems makes it possible to drive back and forth between the two analysis positions and perform complementary measurements, taking only seconds to transfer the sample. Only with these capabilities is it possible to reliably combine chemical and physical information at the nanoscale and in-situ.
Combining the two techniques in one instrument was not a simple straightforward setting up of two established techniques next to one another, though.
The narrow collaboration of teams of experts was the key to this challenge. This was made possible thanks to a FP7 EU-project, which began in 2008. The consortium consisted of two industrial partners and six partners from research organisations such as the EMPA in Switzerland who contributed with expertise in AFM and piezo drive technology.
The first industrial partner was ION-TOF in Germany, the European-market leader in TOF-SIMS. They have more than 300 instruments installed worldwide.
The second industrial partner was NanoScan. We were selected for our knowledge and experience in vacuum SPM instrumentation. We were responsible for developing the SPM software and providing expertise with our existing high-vacuum instrument, our first generation hr-MFM.
An image of the combined SPM ToF-SIMS system.
What type of materials was the integrated SPM/ ToF-SIMS system made to analyse?
The combined system is not limited to one specific kind of material, whether it be conducting or insulating, soft or hard, organic or inorganic. Over the years, as the techniques of SPM and ToF-SIMS have evolved, analysis modes for all types of material have been developed.
The integrated system allows in-situ 3D reconstruction and characterisation, at the nanoscale, of almost any sample, and this on an unequalled time frame.
The only limitation of the system is the requirement of ultra-high vacuum (UHV). To ensure a high sensitivity, the integrated system must be operated in a UHV which means liquid samples cannot be used.
What makes the combined system unique?
Firstly, using SPM to measure the depth of the crater produced by a ToF-SIMS measurement is one of the unique features offered by our integrated system. Indeed, we have developed a profiler mode which measures surface profiles over long distances, and by that I mean on the millimetre (rather than the nano) scale. And while the scan is a millimetre long, the resolution is still on the nanometer level: that’s six orders of magnitude!
This feature is the solution to convert the pseudo vertical axis into a physical distance axis when sputtering in depth. This is real 3D-reconstruction.
Secondly, the accurate transfer system saves researchers a tremendous amount of time. There is no instrument on the market that can perform an SPM measurement followed by a ToF SIMS measurement on the same area, and this within one minute.
Thirdly, we also recognise that both of these techniques are already elaborate and combining them may create a challenge for researchers who are only used to using one (or neither) of the techniques. To alleviate this issue we have simplified our software and automated some processes, such as SPM calibration, to allow researchers easier access to the information they need.
Please could you give us an example of an experiment where the combined technique has been used for novel research?
One interesting and nice experiment that we have done in the past is the investigation of complex multi-layers of a ferromagnet underlying an anti-ferromagnet. We observed how the magnetic domains, which have definite locations at the beginning of the experiments, start moving around as we gently remove the first 10 nanometers of anti-ferromagnetic layer.
This allowed us to show that the localisation of the domains is strongly influenced by the properties of the upper surface layers.
This is an excellent example of how we can determine highly interesting properties relating to nano-magnetism using this combined tool.
Accessing in-depth magnetic properties of an exchange-bias multilayer.
a) - c): MFM data on the virgin surface (a), after removal of the top anti-ferromagnetic layer by gentle sputtering (b), and after partial removal of the underlying ferromagnetic structure. The contrast in Hz differentiates magnetic domains pointing upward from those pointing downward. With the removal of the anti-ferromagnetic layer, the domains are un-pinned and can be moved around by the scanning tip until they eventually disappear.
d): A long-distance profile taken across the sputter crater in contact mode after completion of the MFM imaging. The profile of more than a millimeter shows that the sputter crater is only 16 nm deep, in accordance with the structure of the multilayer. The spikes are reproducible topographic features. (Sample courtesy of Dr. J. Chen, TC Dublin)
What were the challenges of adapting your existing SPM system so it worked as part of a combined instrument?
The main challenge was to design a positioning system that would allow a fast and reliable transfer between the two analysis sites over tens of centimetres. This involved tremendous mechanical, as well as software development, work. The requirements in positioning accuracy were very high but we made it: the area of interest of a sample can be positioned within one micron between the two analysis sites.
Next, the handshake of our SPM software with the ToF-SIMS software required a large amount of work. The result is for the user a single interface to work with.
Collectively, this took a lot of work, with the first prototype ready in 2013. Our prototype performed so well, and gave such promising results, that our first customer, Imec, purchased an instrument before it had even been launched.
How are researchers at the IMEC Center in Belgium using the first instrument?
Imec is a research institute at the cutting edge of nanoelectronics that works closely alongside industry.
Nanoelectronics devices have growingly complex 3D structures which must be characterised electrically at the nanoscale. For example, the mapping of defects is of great importance to understand the functionality of the chips.
Our new SPM/ToF-SIMS system is the perfect tool for this high-end, investment-driven industry.
Are you doing anything to fulfil the needs of existing ToF-SIMS users?
There are already more than 300 ToF-SIMS systems installed around the world and for existing TOF-SIMS users it is not possible to retrofit their instrument.
To fulfil the needs of these users, we have designed a stand-alone AFM unit, called the VLS-80 which is fully compatible with the ToF-SIMS system and which can be operated under atmospheric as well as vacuum conditions.
The sample holder from the TOF-SIMS system is compatible with our stand-alone instrument.
The unique profiler mode is available too and since the VLS-80 is compatible with the ION-TOF Navigator system, finding and analyzing a sputter crater is completed within a matter of minutes.
The VLS-80 for AFM and SPM from Nanoscan
What about your other customers? Do they benefit from your recent improvements?
Of course! All of the new software features such as Navigator, automated calibration and assisted measurements are available for VLS-80 only users.
Additionally, the VLS-80 has an improved top-view access, meaning that if you want to navigate by eye, you can do so using a camera that allows the surface to be observed at micrometer resolution. This can be used to position the tip of the AFM very precisely on the surface.
The VLS-80 can be operated at high sensitivity in a high vacuum environment, rather than an ultra high vacuum. This allows researchers to conduct accurate experiments without the cost, in both time and money, associated with the extended pumping time required to create a UHV environment. The VLS-80's high vacuum can be achieved in 20 minutes.
As a company we have traditionally focused on equipment that measures magnetism. For our users who are interested in magnetism we have still retained the unique features of our hr-MFM system such as the perpendicular and planar magnetic field options.
What industries do you expect to benefit most from this development?
Any industry that is interested in 3D analysis and in controlling the properties on a nanoscale will be interested in an SPM/ToF-SIMS system.
I expect the electronics and display industries to benefit a lot from this new technology and I’m excited to see who decides to start using SPM in conjunction with ToF-SIMS as part of their research and development process.
Where can our readers find out more about Nanoscan, the VLS-80 and the new combined system you helped to develop?
You can read more about Nanoscan on our website and if you are interested in finding out about ToF-SIMS please visit the ION-TOF website.
Of course, interested readers are welcome to contact us directly too!
We are also going to be exhibiting at the SIMS Europe conference that is taking place in Muenster, Germany, in mid-September. Both Nanoscan and ION-TOF will be exhibiting. Following the conference, there will be a user’s meeting where both the combined instrument and the stand-alone system will be demonstrated. It would be great to see anybody who is interested there.
About Raphaëlle Dianoux
Raphaëlle Dianoux completed her physics studies between Grenoble, France and Karlsruhe, Germany before obtaining her PhD from the Université Joseph Fourier in Grenoble.
Her thesis was already focussed on Scannning Force Microscopy, back then in air and investigating electrical properties of semiconductor nano dots. During her post-doc at Empa, Switzerland, she built a low-temperature UHV-SFM before joining the NanoScan team in 2009.
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