Semiconductors – Examples of services
- Determination of nature and distribution of dislocations, stacking faults, inclusions in semiconductor wafers
Example: White beam topography of 300 mm Si wafers
- Characterisation of organic semiconducting thin-films
Example: Initial growth of phthalocyanines on oxide substrates
The ANKA synchrotron light source is operated with a ring electron energy of 2.5 GeV and beam currents of 180–80 mA. It results in a characteristic wavelength of 2 A ° which is perfectly suited for topography. Consequently, a dedicated experimental set-up for synchrotron white beam X-ray topography is installed at the Topo–Tomo beamline.
For demonstration a commercial 300 mm Si wafer with a thickness of about 750 lm was used, which shows a low number of defects with a range of contrasts. To locate defects, such a mapping can be performed with drastically shortened exposure times of some tenths of seconds. Once the defects are found, the topography is repeated with longer exposure time for better signal to noise ratio.
In Figure 1 two topographs are compared which are taken with the camera for 1 and 5 min, respectively. The large contrasts near the edge and the notch of the wafer are clearly visible, even at short exposure time. Curved dislocation lines can be identified as well as large black contrasts corresponding to micro-cracks. At shorter exposure times the overexposed strong contrast regions are reduced and more details of the dislocations and microcracks become visible.
Figure 1: Large area topography near the notch of a Si-wafer with defects, taken with camera (dark corrected and image processed): (a) 1 min exposure time (b) 5 min exposure time (same as for high resolution film)
Digital synchrotron X-ray topography is well suited for the rapid metrology of large Si wafers in section and large area transmission mode. The exposure time can be reduced drastically without losing information. Pendello¨sung fringes as well as dislocations, micro-cracks and long range strain become visible by choosing one appropriate diffraction vector. For the detailed characterisation of dislocations and the Burgers-vector analysis additional exposures with an adequate choice of diffraction vectors are required. In future the resolution will be increased using camera optics with higher magnification. An experimental set up for analysis in the for back reflection mode is currently under construction.
Organic semiconducting thin-films have become increasingly important for the construction of lowcost electronic and optical devices, e.g. field effect transistors, light emitting diodes or displays, and are beginning to substitute inorganic semiconductors. It has been shown that improved crystal perfection enhances electronic and transport properties and thus the ordering and the orientation of organic molecules are crucial for the device efficiency. In devices it may affect the charge carrier injection and electronic properties and at the interface to dielectric materials it directly affects, e.g., the efficiency of organic field effect transistors. Due to the anisotropic electrical conductivity of small molecule semiconductors it is crucial to know the molecular orientation directly at the interface, which may be different with respect to the overall orientation in a thin film, and it is necessary to investigate the first layers for each substrate under consideration.
The family of phthalocyanines is a model system for the entire class of low molecular weight organic molecules and one of the most promising candidates for a variety of applications due to their electronic and optical properties, their tunability based on the flexibility in functional group substitution, and the very high degree of ordering even on relatively ill-defined substrates. Despite the remarkable progress in the understanding of the growth of organic molecules on metal substrates, this is not the case for organics/oxides systems. The growth of copper phthalocyanine (PcCu) and perfluoro copper phthylocyanine (F16PcCu) on silicon dioxide and titanium dioxide is the subject of the example. Silicon dioxide (SiO2) is a broadly used material and has a specific function as gate insulator material for the design of field effect transistors. Especially for mass production of organic electronics, low cost substrates like native silicon oxides with an easy preparation procedure are desirable. Titanium dioxide (TiO2) is apart from various other applications a very appealing material for gate insulators , and a fundamental understanding of the growth mode of organic semiconductors on this material is of basic interest for further developments.
For a detailed understanding of the interface a systematic study of the molecular orientation in the monolayer regime is necessary, for it has been shown, that even in thin-films evaporated onto polycrystalline gold, with an overall bulk-like structure, a buried interfacial layer of lying molecules is present.  Polarisation dependent near edge x-ray absorption fine structure (NEXAFS) spectroscopy is an ideal technique for such investigations. The final states of the N1s → * transitions are anisotropic in these molecules, as the molecular π-system consists of the 2pz orbitals of the participating atoms perpendicular to the molecular plane (see Figure 2). Due to their energetic positions the N1s → * and N1s → * transitions can be separated from each other.
Figure 2: NEXAFS polarization dependence: N1s → π* maximum for standing molecules at normal incidence and for lying molecules at grazing incidence.
NEXAFS spectra have been recorded at grazing (φ=10°) and normal (φ=90°) beam incidence. The molecular orientation can be deduced from the angular dependence of the spectra. Molecules lying on the substrate lead to strong N1s → π* transitions below 402 eV and weak N1s → σ* transitions above 402 eV at grazing beam incidence, and vice versa at normal beam incidence. (Fig. 2)
Native SiO2, which has not been cleaned by sputtering in ultra high vacuum, still contains a small carbon contamination in the submonolayer range, which is evident from XPS spectra (not shown). In Fig. 2 NEXAFS spectra of PcCu on this substrate are shown for a film thickness of ~0.5 nm and 1.5 nm. Both films show a N1s → π* emphasis at normal beam incidence, corresponding to predominantly standing molecules. The given angular dependence, in fact, approximately represents the same orientation as known from much thicker films. It is concluded that the first adsorbed molecules quickly nucleate the formation of bulk-like clusters. The initial growth of the F16PcCu is different, the missing angular dependence in the identical spectra for the different experimental geometries shows that the molecules are statistically disordered in the 0.5 nm film. At 1.5 nm, yet, the situation is similar to the bulklike PcCu film. Thus, the formation of crystallites takes place at a higher coverage, compared to PcCu. TiO2(110), as an atomically flat and clean single crystal is an completely different substrate. In these first experiments it could be shown that lying molecules are principally possible in TiO2(110), and that different preparation conditions might be able to increase the degree of ordering.