Novel research in nanomechanics and nanostructuring requires specialised instrumentation, not all of which is available commercially. The microanalysis group within the Laboratory for Mechanics of Materials and Nanostructures has

• glow discharge instruments which are used for (quantitative) chemical depth profiling with nanometer depth resolution and part per million level detection limits. We have both GD-OES and GD-TOFMS instruments.
• a Raman microscope, capable of also using the surface-enhanced and tip-enhanced Raman effects (SERS and TERS respectively). This can be used for stress mapping on a sub-micron spatial scale.
• a high spatial resolution Secondary Ion Mass Spectrometer (SIMS), integrated within a dual beam SEM-FIB instrument. This is used for chemical imaging (including depth profiles) on a smaller spatial scale than can be accessed by the glow discharge instruments.

We maintain research interests in the use of radiofrequency pulsed glow discharges, in the application of glow discharge analysis to thin films and to molecular materials, in the coupling of time-resolved mass spectrometric detection to pulsed discharges and in the use of plasmonic effects to enhance Raman signals. We have custom made instruments which improve the lateral spatial resolution of a glow discharge one hundred times (from several millimeters to tens of microns), which permit the time-resolved mass spectral analysis of molecular species in afterglowing plasmas and which allow SIMS measurements with spatial resolutions down to 10 nm.

Recent developments in instrumentation and in the range of applications expose Glow Discharge Optical Emission Spectroscopy techniques to new challenges in the physical and chemical basics of GD and in quantification. The dominant physical processes occurring in continuous glow discharges, such as ionisation and excitation are generally well understood, particularly for discharges in the noble gases (mostly argon). However, the instrumental development and application of pulsed direct current (dc)- and rf-GD has just started and research for instrumental optimisation and data evaluation is needed. Furthermore, many samples introduce molecular gases such as hydrogen and nitrogen into the discharge, and it is essential to know how these gases affect the ionisation and excitation processes, and hence the accuracy of the results. State of the art is therefore often qualitative and comparative analysis only, in particular in new application domains like ultra-thin films or biomaterials where accurate quantitative analysis would be most urgently needed. We study in particular pulsed radiofrequency glow discharges in view of their possible advantages for optical emission spectroscopy (GD-OES). The effects of a pulsed power supply on the sputtering are investigated and the effect on the emission yield for resonant and non-resonant emission lines are under investigation. The possibility of analysing heatsensitive non-conductive materials and layers through pulsed rf-GDOES is demonstrated. The work opens several subjects and questions for further research on understanding the plasma processes linked to analytical rf-GD-OES. The applications cover common industrial tasks such as heat treatments, studies of diffusion processes at interfaces, and electrochemical treatments of medical implant surfaces.

GDOES Elemental Mapping.
Glow discharge optical emission spectroscopy (GDOES) has gained increased interest for allowing elemental depth profiling with very high depth resolution, in the order of nanometers. On the other hand, the lateral resolution in GDOES is typically limited to the diameter of the sample surface ex-posed to the plasma, which is in the order of millimeters. A few studies have shown the possibility of obtaining laterally resolved information from within the sample surface exposed to the plasma, thus showing the possibility of obtaining three dimensional elemental maps from solid surfaces. To accomplish this one needs to obtain images of the sample GDOE at characteristic wavelengths of interest. At EMPA Thun, we have developed a push-broom hyperspectral imaging system for this purpose. This instrument allows obtaining one spatial dimension and the spectral dimension simul-taneously, while the remaining spatial dimension has to be scanned to gather the whole image. Ap-plications where the lateral heterogeneity is in the order of 50-100um should be within reach, in-cluding, among many others, analysis of composite materials or the simultaneous characterization of homogeneity of thin film deposition over large areas. This technique has already been applied successfully to the analysis of blotted protein maps.

GDOES Simultaneous Determination of Optical Properties and Elemental Composition of Trans-parent Thin Films.
Several novel thin film (TF) solar cell concepts have transparent conductive oxide (TCO) films serving as a front contact. The electrical and optical properties of such films can be tuned by varying their composition. Zinc, as opposed to other typical elements used for conductive oxides, is a relatively abundant element thus Aluminium dopped Zinc Oxide (AZO) has great potential for widespread use in future cost effective TF PV concepts. We have shown that GDOES can be ap-plied as a fast and reliable measurement technique to simultaneously determine thickness, mean chemical composition and refractive index of homogeneous transparent AZO thin films with the use of a set of matrix matched standards. This is achieved by analysing the modulated optical emission signals due to optical interferometric effects taking place at the transparent TF and the highly reflec-tive substrate Si wafer. The accuracy of the determined refractive indices is just as accurate as ellip-sometry but not dependent on the correctness of complicated modelling assumptions related to the analysed mater. The technique has been applied successfully on AZO layers of different Aluminium content which were prepared by atomic layer deposition.

Contact: Gerardo Gamez or Damian Frey

Glow discharge (GD) is a highly specialised source that especially meets the requirements for accuracy, simplicity and speed for content depth profiling and bulk analysis in both optical emission (OES) and mass spectrometry (MS). The pulsed radio frequency GD source has the potential for both elemental and molecular analysis of conductive and non-conductive materials. To exploit the information delivered by pulsed radio frequency (r.f.)-GD sources, fast sampling is required, and is available only through time-of- flight mass spectrometry (ToF-MS). Compared to optical glow discharge (GD-OES) instrumentation, a GD-ToF-MS system shows much simpler spectra, lower background signals and lower detection limits. The recently developed r.f.-GD-ToF-MS system is a successful combination of a commercial high-end glowdischarge instrument and an extremely fast and high-resolution time-of-flight mass spectrometer. This new instrument was applied to analyse conductive and non-conductive materials like anodic thin films. We could resolve 2-nm Cr makers in aluminium oxide layers and measure trace elements in ultra thin titanium oxide films. Furthermore, we show the potential of the pulsed mode to separate analyte species from elements originating from residual gas.

Contact: Johann Michler

(SIMS) is an analytical technique that uses an energetic beam of ions to sputter a sample. A small fraction of the sputtered material is electrically charged, and these secondary ions can be collected and mass analysed. The strength of SIMS is not so much in quantitative analysis as for depth profiling and chemical imaging. The SIMS instrument at EMPA Thun is integrated into a dual beam SEM-FIB (Secondary Electron Microscope – Focused Ion beam) which allows excellent imaging of the sample, and very high spatial resolution SIMS measurements using the liquid metal gallium ion source primary beam (at some penalty to sensitivity compared to alternative primary ion beams). The limiting spatial resolution of the instrument is the spot-size of the primary ion beam which is about 5 nm. The use of a time-of-flight mass analyser allows all mass to charge ratios to be measured simultaneously.
The development work for this instrument was supported by the EC (part of the FIBLYS project, http://www.fiblys.eu/) with support from Tofwerk AG and Tescan a.s. 

Contact: JamesWhitby or Johann Michler

Raman spectroscopy is a powerful non-destructive optical technique which provides detailed information on the structure of materials, such as chemical composition, cristallinity, lattice strains, defects, and crystal size, by probing their vibrational modes. When combined with an optical microscope, Raman spectroscopy can map the intensity of a definite Raman peak over a portion of the surface of a sample with micrometer lateral precision. For instance, thin films of polycrystalline silicon are being considered for low-cost production of solar cells, but high residual stresses influence both their mechanical and electrical integrity. We have shown that Raman spectroscopy can map these residual stresses, which are found mainly close to grain boundaries with amplitudes reaching GPa values.

Contact: Victor le Nader

Raman spectroscopy is a powerful technique for identification of molecules by probing their vibrational modes, but the efficiency of the Raman scattering process is very low, with approximately one emitted photon for 10¹² incident photons, thus reducing its applicability to highly concentrated solutions only. However, Raman signals can be tremendously enhanced when molecules are adsorbed on specific metallic nanoparticles. This surface-enhanced Raman spectroscopy (SERS) effect, which can increase the signal by many orders of magnitude, allows us to detect Raman spectra of very low concentrations of molecules. This high sensitivity associated with the spectroscopic wealth of information that can be recorded by SERS presents a huge potential for chemical/biomolecular trace analysis. However, SERS can be envisaged as an analytical tool only if substrates with reproducible enhancement can be produced. Recently, we have tailored new substrates with well-defined nanostructures in accordance with a detailed comprehension of the SERS mechanism. For example, we have fabricated large ordered areas of silver nanoparticles on a Si(111) surface using electroless metal deposition and the nanospheres lithography method. The possibility to control the size, the interdistance and the crystallinity of these nanoparticles allowed us to systematically investigate their influence on SERS, demonstrating the existence of an ideal size because too large particles allow the excitation of nonradiative multipoles whereas too small particles lose their electrical conductance and cannot enhance the field.

Contact: Victor le Nader

Surface-enhanced Raman spectroscopy (SERS) is based on the increase of the electromagnetic field of light by nanometer-sized metal particles, resulting in a large enhancement of the Raman signal. By replacing the metal particles by the metallic nanotip of an atomic force microscope (AFM), the enhancement can be localized at will. The resulting tip-enhanced Raman spectroscopy (TERS) is capable of measuring Raman spectra with nanoscale resolution, effectively overcoming the diffraction limit. A successful TERS experiment depends heavily on the ability to fabricate tips of a definite metal with the appropriate shape and size, which is still a challenging process. For instance, we have shown that hemispherical gold droplets on top of silicon nanowires grown by the vapor-liquid-solid mechanism produce a significant enhancement of Raman scattered signals. In more recent results, we have shown that silver nanowires attached to AFM cantilevers produce a strong, localized enhancement of the Raman spectroscopy intensity. The nanowires are synthesized by electrochemical deposition inside the pores of an alumina membrane. By subsequently dissolving the alumina membrane, freestanding nanowires are obtained, with an approximate diameter of 200 nm. In the next step, the silver nanowires are attached onto AFM tips by using the electron beam of a scanning electron microscope (SEM) to perform localized electron impact-induced dissociation and deposition. Applications of this novel nanowire-based TERS technique are widespread in the field of solid state research, e.g. in silicon technology where the material composition, doping, crystal orientation and lattice strain can be probed by Raman spectroscopy at the nanoscale.

Contact: Victor le Nader

Glow-discharge optical emission spectrometry (GD-OES) allows fast compositional depth-profiling from the nm range up to several 100 um in depths. The depth resolution of this technique is of a few nanometres for thin layers, but increases for thicker layers to reach a few percent of the total sputtered depth. The major advantages of the technique are its speed and ease of use. The spatial separation of the sputtering processes and the excitation/ionisation processes leading to the signal detection result in reliable quantification procedures, compared to surface analytical tools. A further advantage of GD-OES is its ability of qualitatively determining the hydrogen content in the sample. Though GDOES offers an excellent depth resolution it does virtually offer no lateral resolution, because the analysis area is at the best of a few mm2. 

Contact: Gerardo Gamez 

In Differential Thermal Analysis or DTA the difference in the amount of heat required for increasing the temperature of a sample and reference is measured as a function of temperature. The temperatures of both, sample and references are increased at a constant rate simultaneously maintaining them at nearly the same value. The reference sample must have a well-defined heat capacity over the range of temperatures to be scanned. The underlying basic principle of DTA is that, the transfer of thermal energy necessary for a temperature change in the sample will depend on the on-going physical transformation in the sample, such as a phase transition. Depending on the transformation processes more (or less) thermal energy will have to be transferred to the sample compared to the reference in order to maintain both at the same temperature. Whether more or less heat must flow to the sample depends on whether the process is exothermic or endothermic. For example, as a solid sample melts to form a liquid it will require more heat flowing to the sample to increase its temperature at the same rate as the reference. The reason is the additional energy that is needed for melting a solid. Likewise, as the sample undergoes exothermic processes (such as crystallisation) less heat is required to raise the sample temperature. Recording the difference in energy transfer between the sample and reference, DTA instruments measure the energy absorbed or released during such transitions. DTA is also being used for the observation of more subtle phase changes, such as glass transitions. DTA is widely used in industrial settings as a quality control instrument due to its applicability in evaluating sample purity and for studying polymer curing.

Contact: Johann Michler or Damian Frey