c Transmission Electron Microscopy techniques 61 

TEM techniques make use of the transformation of the energy, direction or phase of electrons sent through a thin lamella of a given sample. Electrons, as theorised by De Broglie [33], can indeed be seen both as particles or waves and will therefore show both behaviours when interacting with the sample. On the same principle as for light microscopes, this feature can be used for magnifying and revealing properties of samples. Compared to most photons, electrons however have much smaller wavelengths. This gives electron microscopes much higher spatial resolutions than with any conventional light microscope, enabling examination of extremely fine details such as atomic columns in advanced instruments (maximum resolution of state-of-the-art TEMs being of ~0.05 nm against a few tens of nanometres for best light microscopes) [34]. For this reasons TEM is (in microelectronics) most used for direct visualisation of the cross-section of nanometric or deca-nanometric structures such as heterogeneous layer stacks of actual transistors. The information given by such simple analysis are sample conformity (i.e. thickness of the different layers/size of the different regions, interface widths) and crystalline state of each component (mono- or polycrystalline, amorphous, stacking faults) along with qualitative information on layer composition. In conventional imaging mode, TEM image contrast is indeed due to absorption of electrons in the material [35]. In this mode, called the bright field imaging mode, thicker regions of the sample, or regions with a higher atomic number will appear darker. The observed image can be explained as the two dimensional projection of the sample following the optic axis. En example of such bright field imaging is given in Figure II.14.A. However there are many other uses of TEM, yielding a variety of different information depending on the contrast method. We will now present a few of those, which found application in our study. At high magnifications, image contrast is no more dominated by absorption of electrons but by the phase difference between electron waves of different paths.

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Solutions explored to answer ToF-SIMS characterisation needs

62

Figure II.14.ABright field TEM image of a Si/SiGe:C superlattice. Ge containing zones appear darker. The brightest zone, in the top right corner, corresponds to a low density oxide deposited prior to lamella preparation for the purpose of measurement.

BHRTEM imaging of the oxide/silicon interface of the same sample than in A. Crystallographic organisation of Si is clearly visualised, in contrast with amorphous Si where atoms are disordered. The interface between both materials is seen as perfectly sharp (inferior to a monolayer).

C Strain mapping of the same sample obtained by Dark Field Electron Holography. Si zones appear as unstrained, while SiGe layers are compressively strained.

DSchematic representation of the instrumental setup for electron holography measurements.

The image is therefore greatly influenced by the complex modulus of the electron waves [36- 37]. Complex phase retrieval is the basis of phase contrast TEM, also known as High Resolution TEM (HRTEM), and allows investigation of crystal structure because of its lateral resolution, inferior to the lattice parameter. Depending on the orientation of the specimen relative to the electron beam, atomic columns (crystallographic arrangement), stacking faults and dislocations can therefore be imaged. An example of HRTEM imaging is given in Figure II.14.B. Another way of making use of the phase modification of electron waves passing through a sample is electron holography. TEM allows, by superposition of a coherent reference wave, and of waves which underwent phase modification by going through a specimen, to record holograms, from which the image wave can be completely reconstructed in amplitude and phase. The specimen is thus quantitatively described by two separate images: one representing the amplitude, the other the phase. From the amplitude image one can perform “conventional” TEM imaging; while from the phase image, electric and magnetic

CHAPTER II

Solutions explored to answer ToF-SIMS characterisation needs

fields, but also sample strain state can be quantitatively determined (provided that beams pass through a reference sample at the same time) [38-41]. An example of strain mapping obtained via electron holography technique is displayed in Figure II.14.C. Lastly, due to inelastic collisions between electrons from the beam and electron clouds of the specimen’s atoms, the original electron beam loses some of its energy by going through the sample. Different elements, having different electronic layer filling will therefore yield different energy losses in the beam after the sample. Although this usually results in chromatic aberration, it can also be used to obtain laterally resolved information on the elemental composition of a sample by recording the electron beam energy loss. This technique is thus called Electron Energy Loss Spectroscopy (EELS) [42]. However, due to its high detection limit (~1019 at.cm-3 for As in Si), the impossibility to monitor crucial elements such as boron or phosphorous (the energy loss provoked by B being too low and the ionisation level of P being too close to that of Si), this technique will only be used for monitoring of major matrix element in this study. The spatial resolution in this mode being reduced to ~2 nm, this technique couldn’t either be applied to the quantitative analysis of ultra-thin layers.

These techniques necessitate heavy sample preparation in order to obtain either thin parallel sided lamellae of between a few tens to a few hundreds of nanometres width, or ultra thin bevels. The analysis quality being strongly dependent of the quality of sample preparation, only a few of these analyses were performed during this thesis work. Within these, we focused on obtaining the following information:

- Absolute layer thickness (quantitative)

- Interfacial roughness (quantitative)

- Layer stress mappings (quantitative)

- Layer composition in matrix elements (qualitative, quantitative in EELS)

Similarly to techniques presented in the two previous subsections, determination of layer thickness is a matter of prime interest to ensure accurate evaluation of ToF-SIMS sputter rates, or to serve as cross characterisation for a depth scale established via other means. Direct determination of interfacial roughness by TEM also helps in the interpretation of depth profile broadening effects due to roughness pile-up phenomena. Otherwise, stress mapping gives complementary information and provides understanding on the structural properties of some samples (such as strained samples undergoing annealing) which ToF-SIMS alone could not.