A Versatile Tool for Surface Analysis on the Nanoscale
- UHV-Gemini SEM column with superb spatial resolution (achieved: SEM < 3 nm, SAM < 5 nm) and additional optimisation for Auger electron microscopy at low beam energies and high currents
- Wide range of analysis techniques, including Auger electron microscopy and spectroscopy (SAM/AES), depth profiling, SEM with Polarization Analysis (SEMPA), Focussed Ion Beam milling (FIB), and many others
- High-stability 4-axis sample stage for Scienta Omicron’s flag-style sample holders with a maximum tilt angle of ± 60°, radiative heating to > 750 K and optional LHe cooling to < 40 K
- Integration with other Scienta Omicron system modules allows customers to get a solution tailored to their needs
The NanoScan Lab is a versatile tool for surface analysis on the nanoscale. Based on a UHV-version of Zeiss’ Gemini SEM column, there is a wide range of options, such as Auger electron microscopy and spectroscopy (SAM, AES), depth profiling, SEM with Polarization Analysis (SEMPA), focussed ion beam milling (FIB), X-ray photoelectron spectroscopy (XPS), e-beam lithography (EBL), and energy-dispersive X-ray spectroscopy (EDX).
The NanoScan Lab is the ultimate tool for the analysis of small structures. Driven by the unique performance of the UHV Gemini electron column, it guarantees unrivalled resolution below 6 nm in Scanning Auger Microscopy (SAM) and better than 3 nm in SEM.
In contrast to other Auger tools, the extremely good resolution is not only available at standard 20 keV beam energy, but even at 5 keV the SAM resolution remains below 10 nm. This allows operation in a parameter range where the Auger cross sections are high and well documented for quantitative analysis.
The NanoScan Lab is tailored for the fast and efficient acquisition of data on routine samples, while maintaining the flexibility to operate in untypical parameter ranges on challenging materials. Furthermore, the NanoScan Lab may be extended with additional, complementary techniques, for example, depth profiling or SEM with Polarization Analysis (SEMPA) to get access to the magnetic domain structure. Other available methods for the NanoScan Lab include focussed ion beam milling (FIB), X-ray photoelectron spectroscopy (XPS), electron beam lithography (EBL), energy-dispersive X-ray spectroscopy (EDX), electron beam induced deposition (EBID) and cathodoluminescence (CL).
The patented UHV Gemini SEM column is the electron source with the highest resolution available for UHV operation. Designed for true UHV (< 1x10 -10 mbar), the column is fully bakeable up to 180 °C and operates without any measurable outgassing – a must for contamination-free research on many materials.
The design of the UHV Gemini is the result of a collaboration between Scienta Omicron, CEOS and Carl Zeiss NTS, employing the same electron optics as used in the Zeiss Gemini SEM product range. Both, the leading edge SEM software SmartSEM and the Gemini control electronics, are standard products from Carl Zeiss NTS, incorporated also in the UHV version.
Besides the integrated in lens secondary electron detector, which grants optimal detection efficiency, the UHV version offers special features that make it especially suitable for high-performance Auger electron microscopy.
The main factor for achieving a high resolution is the spot size of the electron beam. A small spot size at high voltages and low beam currents may be the solution for SEM. However, for best performance in Scanning Auger Microscopy, the beam energy should be in the range of 3-10 keV to achieve an optimum output of Auger electrons. The generation of X-rays becomes dominant for higher beam energies. Due to the low number of released Auger electrons, measurement times in SAM can be up to several hours. It is therefore of great interest to increase the beam current whilst maintaining a small spot size. This is a strong point of the UHV-Gemini column, as compared to other SAM offerings: a SAM resolution < 5 nm has been demonstrated with a beam energy of 10 keV and a beam current of 1 nA.
The UHV Gemini offers an ultimate SEM resolution below 3 nm (80-20 % criterion) at a working distance of 8 nm, as demonstrated here with a line profile of a Ag island on a Si(111) subtrate.
4-Axis Sample Stage
Ultimate SEM resolution can only be achieved with a highly stable sample stage. The sample stage used for the NanoScan Lab is mounted on an ex vacuo goniometer and an x/y/z stage with a travel range of 10 mm for all three axes. Due to the compact shape of the sample stage, the sample normal can be tilted by up to ± 60° with respect to the SEM axis. (Please note that the exact limits depend on the configuration with analytical components.)
The stage is designed for Scienta Omicron’s flag-style sample holders with a size of approximately 15 x 18 mm². A combination of a NanoScan module with any other Scienta Omicron module, e.g. MBE or ARPES, therefore offers the pathway to tailor a solution exactly to the customer’s needs.
The stage is equipped with a radiative heater element that allows sample heating from room temperature to 750 K. LHe cooling is optionally available and extends the accessible temperature range down to well below 40 K.
Electrical sample contacts and other customizations are available on request.
High magnification Auger measurements make great demands on the mechanical stability of the sample stage. Based on Scienta Omicron´s SPM expertise, the 4-axis sample stage represents an extremely stable and low drift platform with a non-magnetic UHV drive technology.
Measurement times in Scanning Auger microscopy can easily exceed several minutes - in the case of elemental maps even hours. A technically mature electronic drift correction is therefore indispensable to access a suitable performance level at small structures or at very low elemental concentrations. Based on the conventional approach employing image correlation of sequential SEM images and step-wise correction of the electron beam position, the new Dynamic Drift Correction minimises the effects of thermal and mechanical drift.
This combination of careful mechanical design and Dynamic Drift Correction provides outstanding drift values below 10 nm vectorial shift per 12 hours.
Dynamic Drift Correction in SAM
Despite the careful design of the 4-axis sample stage, thermal and mechanical drift cannot be avoided completely during the acquisition of an elemental Auger map, which can take several hours of continuous measurement. These effects can be minimised by the dynamic drift correction. The drift vector is determined from SEM images (right, top to bottom) that are acquired regularly every few minutes and serve as a basis for a correction of the position of the electron beam. The image on the left shows an elemental SAM image of a Ag island (a) that was accumulated over 12 hours.
NanoSAM Energy Analyser
The NanoSAM EA is a hemispherical energy analyser that has been optimised for the best performance in Scanning Auger Microscopy. Its large acceptance angle of about ± 10°, high transmission, and multi-channel detector (7 channeltrons) are the ingredients to achieve an excellent sensitivity and thus reduced measurement time.
The slim lens geometry makes it possible to bring the analyser lens close to the SEM column and leaves sufficient of the precious space around the SEM column for other detectors and excitation sources, while it also allows tilting of the sample within a large range to mitigate topography shadowing effects.
A lens-integrated octopole deflector is used to compensate the magnetic stray field of the SEM column. An interpolation algorithm choses the correct octopole potentials depending on the beam energy, yet leaving the possibility of manual optimisation for delicate measurements at low kinetic energies.
A set of variable apertures and magnification modes allows the user to chose the best balance between energy resolution and transmission to meet the needs of his application best. Two modes are available for the relation between kinetic and pass energy: the constant retard ration (CRR) mode, which is typically used for Auger electron spectroscopy and microscopy, and the constant analyser energy (CAE) mode, which is employed for XPS if an X-ray source is available on the system.
The NanoSAM EA is controlled through Scienta Omicron’s MATRIX software for electron spectroscopy, both for Auger and X-ray photoelectron spectroscopy. The MATRIX software is integrated with Zeiss’ SmartSEM software and includes features such as drift compensation for long-term measurements and a periodic table of the elements with typical electron peak energies. Data can be exported into standard formats such as VAMAS or ASCII for subsequent data processing with analysis software, e.g. Casa XPS. A software module for automated execution of batch jobs such as sputter depth profiling using Auger electron spectroscopy for the elemental composition analysis is also included.
Achievable SAM resolution < 5 nm, as determined by measurement of the 80-20 % edge resolution of Ag islands on Si(111)
Fine Focus Ion Source
Similar to depth profiling in XPS, repeated cycles of precisely controlled ion milling, followed by a composition analysis with Auger electron spectroscopy or microscopy extends the accessible volume for chemical analysis from the sample surface to a depth of up to several 100 nm.
The FDG 150 has been designed for sputter depth profiling with noble gas ions in the energy range from a few 100 eV for delicate samples up to 5 keV for rapid depth profile surveys. The sputter crater will be carved out with an ion spot size on the order of 150 µm on the sample, an adjustable raster field of up to 10 mm x 10 mm and a maximum beam current exceeding 10 µA (at 5 keV beam energy). For depth profiling measurements, the FDG will be controlled by software that also controls the NanoSAM EA and in turn the UHV Gemini, so that time-consuming measurements do not need the attention of the scientist at all times.
Besides sputter depth profiling, the FDG 150 can also be used to compensate negative charges induced by the electron beam with a flood of slow, positively charged noble gas ions. SEM and SAM analysis of insulating materials was otherwise not possibe under stable conditions. The ion source and control electronics offer additional features to improve its performance further, such as a keystone correction for the sputter crater and an intermittent acceleration to mitigate the effect of stray fields that would otherwise deflect slow ions.
When secondary electrons are emitted from the surface of a magnetic material, the electrons carry a spin polarisation that reflects the magnetic polarisation at the position where the secondary electrons were excited. A SEMPA detector (SEM with Polarisation Analysis, also called spin-SEM) gives access to the two transverse components of the spin polarisation and allows imaging of the sample’s magnetic domain structure with a spatial resolution below 50 nm.
The working principle of the SEMPA detector is based on the observation that LEED patterns of certain materials, in this case the 2nd order normal LEED spots of a W(001) crystal, show a strong asymmetry, which depends on the spin polarisation of the incoming electron beam. This asymmetry of the LEED spot intensities is most pronounced at a kinetic energy of 104.5 eV.
In the experimental setup, a transfer optics extracts, focuses and accelerates the slow secondaries to the desired energy before they hit the scattering target, the W(001) crystal. The four spot intensities are measured simultaneously by means of four channeltron detectors. The spin polarisation can then be calculated from the intensities C1, C2 measured with two opposite channeltrons, e.g. for the (2,0) and the (-2,0) spots, as P=(C1-C2)/(C1+C2) and accordingly for the other polarisation, while the sum of the four intensities can be used to generate a conventional SEM image. Compared to a Mott-type spin detector, the low scattering energy of only 104.5 eV has the advantage that no high voltages are needed.
To maintain the high spin asymmetry, the W crystal needs to be cleaned regularly by a combination of oxygen dosing and heating, both of which are already built into the SEMPA detector. The detector can be mounted on a differentially pumped rotary platform to achieve the best possible signal-to-noise ratio through proper alignment of the channeltron pairs to the prevalent magnetic domains on the sample.
SEMPA on Fe Whisker
SEM with Polarisation Analysis (SEMPA) applied to image the magnetic domain structure of a Fe whisker. C1, C2 and C3, C4 denote the countrates measured by the opposite channeltron pairs of the SEMPA detector. The sum C1+C2+C3+C4 gives a conventional SEM image, while the polarisation functions P1 and P2 give the polarisations in the two transverse spin polarisation directions. While the SEM image is largely free of structure, the false colour representation of the functions P1 and P2 shows a spatial distribution of magnetic domains.
The NanoScan Lab is a highly stable system platform for SEM-based applications with the UHV Gemini, the NanoSAM EA, the FDG 150 sputter gun, SEMPA, FIB and other sophisticated scientific components. Its proven design is based on Scienta Omicron’s installed base of several hundred UHV systems, providing an environment with an ultimate base pressure in the 10 -11 mbar range and thus much cleaner than typical SEM systems.
The UHV system is built on a solid bench frame, designed for highest stability and equipped with auto-leveling air damping legs. The vacuum chamber is made of non-magnetic stainless steel with a wall thickness of 30 mm. This gives the necessary mechanical stability and helps to minimise the detrimental effects of room temperature changes.
The chamber design is optimised for routine sample characterisation by high-resolution SEM and Auger electron spectroscopy with the UHV Gemini on top of the chamber and ports for a hemispherical energy analyser and many other components arranged around the SEM. However, a bespoke layout to meet a customer’s individual needs is possible, too.
The NanoScan Lab is equipped with a loadlock chamber to introduce samples from atmosphere into the UHV system without the need to vent the analysis chamber. A preparation chamber with ports for typical sample cleaning and preparation techniques is available as an option.
Since the system and sample stage are made for flag-style sample holders, the NanoScan Lab is compatible with the entire range of Scienta Omicron UHV systems and components, which offers a wealth of world-class nanotechnology tools for sample preparation and characterisation.
Guaranteed specification: < 1x10 -10 mbar,
acceptance criterion: < 3x10 -10 mbar
10 mm x 10 mm x 10 mm, ± 60°
Limitations by scientific components may apply.
> 750 K
Guaranteed specification: < 40 K
typically achieved: < 30 K
< 0.4 nA beam current: < 6.5 nm
1 nA beam current: < 12 nm
< 0.4 nA beam current: < 5 nm
1 nA beam current: < 7.5 nm
< 0.4 nA beam current: < 3 nm
1 nA beam current: < 4 nm
28 nA beam current: < 18 nm
50 nA beam current: < 12 nm
1 nA beam current: < 13 nm
1 nA beam currnet: < 3 nm
0.1 - 30 keV
Resolution < 6 nm
Resolution < 10 nm
> 12 kcps per nA beam current and channeltron for the 352 eV Ag peak at CRR 4 (0.5 % resolution),
i.e. > 420 kcps for typical configuration (5 nA, 7 channeltrons)
For full specifications and more information about product options, please do not hesitate to contact your local sales representative.
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