A Universal Scanning Electron Microscope
Results of the Highly Successful EU-FP7 Project "UnivSEM"
- A Universal Scanning Electron Microscope - Results of the Highly Successful EU-FP7 Project "UnivSEM"
- Fig. 1: The Witec RISE microscope. The sample is moved inside the vacuum from the SEM column to the optical column. Measurements on graphene show the excellent resolution as well as the multimodal image overlap. The bottom images show graphene on GaN nanorod LEDs (sample courtesy of Michael Latzel, MPL).
- Fig. 2: (a) The TOF-SIMS add-on. (b) Integration into an SEM allows a simultaneous charge compensation with the electron beam. (c) In combination with an SPM the milled craters can be analyzed. (d) 3D-reconstruction of a complex layer stack.
- Fig. 3: Technical drawing of all prossible add-ons that can be attached at the same time. Every imaginable subconfiguration can be realized.
In the last years SEM & FIB equipment with analytical add-ons has followed a trend of complex integration. Instead of having several instruments each requiring space and environment, customers now desire highly integrated systems with the option to correlate several analysis techniques. To meet this desire and to enable new scientific possibilities, a European consortium of researchers and companies led by the FIB/SEM manufacturer Tescan has developed an innovative type of correlated microscope.
The EU-FP7 funded project “UnivSEM” (Universal Scanning Electron Microscope), which ended in March 2015, gathered together researchers from three academic institutes and four companies from Germany, Switzerland and the Czech Republic, and was coordinated by Tescan. The ambitious project included several novel developments: improvement of the performance and integration of a time-of-flight secondary ion mass spectrometer (TOF-SIMS) exploiting the focused ion beam (FIB), a high-speed vacuum compatible scanning probe microscope (SPM) with large scan range and, most importantly, integration of a fully capable confocal Raman microscope. In addition, new electron detectors, a color cathodoluminescence detector and a Xe-plasma FIB were developed and integrated and the overall SEM performance was improved. The unique feature of the project was that all mentioned add-ons, together with standard techniques like EDS or EBSD detectors, fit into one single system and can be used together.
The consortium consisted of industrial partners for each add-on: TOFWERK from Thun, Switzerland developed the TOF-SIMS, SPECS from Berlin, Germany, built the SPM and WITec from Ulm, Germany, customized one of their Raman microscopes. Furthermore, three academic partners helped with their expertise to generate benchmark experiments and to demonstrate the possibilities of the prototypes: The Max Planck Institute for the Science of Light in Erlangen, Germany, the Brno University of Technology from Brno, Czech Republic, and the Swiss Federal Laboratories for Materials Science and Technology from Thun, Switzerland.
Integration of Scanning Electron & Confocal Raman Microscopes
In the beginning, UnivSEM aimed for one commercial product, but during the course of the project it was realized that several products could be commercialized.
The first outcome was the award-winning WITec RISE microscope.
Correlative light & electron microscopy (CLEM) is an established and powerful analysis technique which is realized with two individual microscopes that are coupled by a certain type of position recognition system. The logical advancement was the integration of both microscopes into a single system, but our project goal went even further and integrated the light microscope with a state-of-the-art Raman spectrometer 1. Consequently, the SEM gained a huge improvement in analytical capabilities. Raman spectroscopy enables the analysis of chemical (composition, modification), structural (strain/stress) as well as electrical properties such as doping. Thus, it is an ideal instrument not only for life science, but also for materials science in which Raman spectroscopy is still not well-known. Since the RISE microscope is kept at best flexibility, it is no problem to integrate further complemental techniques like energy-dispersive x-ray spectroscopy (EDS) or electron backscatter diffraction (EBSD).
In figure 1 we present the RISE approach which is realized by integrating a scanned objective into the vacuum chamber. Our method allows for much better performance regarding resolution (360 nm for a green laser) and flexibility compared to typical realizations which utilize a parabolic mirror (2 - 5 µm resolution).
Finally, a correlative SEM & Raman analysis is just one mouse click away: After identifying the area of interest with the SEM, the sample automatically moves to the Raman objective and the exact same area can be mapped by hyperspectral Raman imaging. Figure 1 also shows two measurement examples: The first one is mono- and bilayer graphene on silicon on which Raman can visualize the amount of layers (blue/green monolayer, red bilayer). The second is a sample with graphene-covered GaN nanorods and a colored Raman map that shows the GaN in red and the graphene in green.
The SEM part was improved to reach ultra-high resolution of 1 nm at 15 kV and 1.4 nm at 1 kV of accelerating voltage. This may be combined with focused ion beam (FIB) of either Ga or Xe ions.
Integration of TOF-SIMS & SPM
TOF-SIMS is a powerful technique that boosts the analytical capabilities of FIB/SEMs. 3-dimensional chemical imaging with ultrahigh resolution in both depth and lateral dimensions and elemental or even isotope identification enables a completely new understanding of complex materials. Such a system works as follows: The FIB continuously mills an area of interest while the sputtered secondary ions and clusters are guided into an orthogonal time-of-flight mass analyzer. As the FIB beam scans across the sample at normal imaging speeds, each voxel is associated with a complete mass spectrum. A powerful and user-friendly software handles the massive amount of raw data and is capable of generating 3-dimensional visualizations of the dataset. A lateral resolution of 40 nm and a depth resolution of 5 nm enable a precise investigation of nanostructured complex materials.
Furthermore, the project partner SPECS developed a Scanning Probe Microscope which fits onto the microscope stage inside the vacuum chamber. This enables true topographic analyses of, for example, nanoscale features, deposited materials or milled crater depths. The latter possibility provides a useful correlation of SPM & TOF-SIMS: true depth values can be assigned to the SIMS data which is difficult without an SPM.
A most interesting option is to combine a xenon plasma FIB with a TOF-SIMS. A much higher sputter rate permits the 3-dimensional analysis of volumes-of-interest as large as 100x100x10 µm³. Furthermore, gallium-rich samples can be analyzed, such as industry relevant GaN LED materials. Since the Xe-FIB is of great importance for the semiconductor industry, it also became a product of its own, the Tescan XEIA, which can also be equipped with a TOF-SIMS. The modular SPM from SPECS, called Curlew, constitutes another commercial product that is based on the UnivSEM project.
Such a highly integrated system allows for correlating several analytical methods at the same position to learn the most possible about the material. We have already shown the combination of SEM & Raman and SPM & TOF-SIMS, but there are much more useful combinations: FIB & Raman enable depth-resolved spectroscopy of layered systems. EBSD & Raman is an interesting combination for multicrystalline semiconductors. SPM and a gas injection system (GIS) enable the calibration of deposition rates. Raman, EDX and CL are a promising combination for geological sciences. The microscope GAIA combines all presented techniques and is the final commercial product which is shown in figure 3.
The EU-funded project UnivSEM brought together a complementary group of industrial and academic partners and the resultant synergy led to several commercial products. New types of flexible and highly-capable microscopes offer great benefits for research in many different fields such as materials science, life science or industrial quality assurance. The integration of two new analytical techniques into common FIB/SEMs will most likely influence the way correlated microscopy will be performed in the future.
The research leading to these results has received funding from the European Union Seventh Framework Program [FP7/2007-2013] under grant agreement nr. 280566, project UnivSEM.
 Jiruše J. et al.: J. Vac. Sci. Technol. B, Nanotechnol. Microelectron. Mater. Process. Meas. Phenom. 32, 06FC03 (2014)
Dipl.-Phys. Björn Hoffmann
Max Planck Institute for the Science of Light
Christiansen Research Group, Erlangen, Germany