As in previous years, beam time allocation for applied or industrial projects has represented about 20% of the total beam time during 1998. Most of these projects were non-proprietary ones, which means that they have been peer-reviewed by scientific committees and must be published. Some of them were directly carried out by industrialists, but most often the work was done by public institutes contracted by companies. Proprietary research access which guarantees confidentiality represented less than 1% of the total allocated beam time. The analyses dealt with nearly all types of materials - proteins, drugs, cosmetics, cements, polymers, metals, wood, catalysts, fine chemicals, semiconductors etc.- and were led by companies or institutes from all member countries of the ESRF. The most popular techniques are bio-crystallography, powder diffraction, strain/stress analysis, micro-diffraction, EXAFS, XANES, micro-tomography and micro-fluorescence. Several typical examples of industry-related works are described in this section but many others can also be found in the various sections of this issue.

During year 1999 three important projects for industry should be completed: in order to permit a rapid access on bio-crystallography beamlines and to satisfy the increasing demand, slots of time will be reserved weekly for pharmaceutical companies on station BM14 starting in January, the dedicated station ID27 for contaminant-trace analysis of silicon wafers by TXRF will receive its first users from the microelectronics industry at the end of spring, the new diffractometer from the Riso Institute will be operational also at the end of spring on ID11 for strain-stress analysis of metals. New developments for improving the capacity of industrial micro-tomography at the ESRF are underway.

Micro-architecture of solid polymer foams using 3D computed microtomography

The specifications of industrial polymer foams are demanding and diverse. For example foams for floor-and wall-covering have to combine high sound-proofing with punch resistance. These properties mainly depend on the micro-architecture of the foam: the size, orientation and shape of the cells, their wall thickness, ... Quantitative three-dimensional characterization, involving, in particular, high-resolution imaging, is therefore a key point in the foam optimization process.

X-ray computed microtomography (CMT) has been performed on the ID19 beamline as a starting point of a joint research program between the Elf group and the ESRF on the relation between the structure morphology and the mechanical and acoustic properties of solid polymer foams.

Figure 99 shows a reconstructed slice (a) and volume (b) from a sample of PVC foam for wall covering, obtained by absorption CMT at a photon energy of 10 keV. This approach immediately provides quantitative two- and three-dimensional density representations of the internal structure of solid polymer foams with a spatial resolution of a few micrometers. Conventional microscopy techniques (optical, confocal, scanning electron microscopy, ...) require time-consuming sample preparation or stereology simulations.

When dealing with foams which, unlike PVC, contain no heavy element, the first attempts using phase CMT, made possible by the high spatial coherence of the X-ray beam, were successful.

CMT is thus shown to be a powerful tool for the non-destructive inspection of such complex porous systems and hence for research and the development of new solid foam formulations and processes.

G. Fuchs (a), W. Ludwig (b), to be published.

(b) ESRF



Defect detection on 300 mm diameter silicon wafers by X-ray diffraction topography using the Frelon camera

X-ray diffraction topography is a non-destructive technique based on Bragg diffraction from single crystals. It is sensitive to distortions (d/d and q from the order of about 10-3 to 10-7) related to defects, domains and other deviations from the perfect crystalline structure. It was, therefore, employed for the detection and analysis of defects on several silicon wafers of 300 mm diameter, the plane of each wafer being the (001) plane. Due to the large size of the crystals, it was necessary to acquire a series of topographs.

On ID19, one wafer was scanned in 1000 topographs (size about 10 x 10 mm2) exposed 5 seconds each and recorded by the Frelon camera (Fast readout, low noise), a 14-bit dynamic range CCD camera developed at the ESRF [1]. Although the usual way of image recording in X-ray topography is to use X-ray films and plates (resolution not better than about 1 µm), this was the first time that this good resolution, fast acquisition and high dynamic range CCD camera detector was applied for the purpose. The use of the camera gives a substantial reduction of the total experiment time compared to the use of X-ray films (each wafer scan lasted only 2 hours) and allows instant access to digitised images. The chosen 10 µm spatial resolution of the camera (the resolution limit is currently about 1 µm, but is achieved at the cost of a smaller field of view) was sufficient to identify the kind of defects present in the crystals and consequently to locate them in the wafers.

The scanning method is illustrated in Figure 100. The distribution of the different defects is shown in the typical mappings of Figure 101. Three types of defects were detected: stripes, dislocation lines nearly parallel to the [110] and [110] directions (some wafers), and defects close to the wafer edges (all wafers), see Figure 102. The other wafers which contain many defects display a more or less pronounced circular distribution of their defects, related to the wafer treatments.

We have thus established a fast and systematic method to characterise defects in large wafers used in microelectronics, this is an important contribution by the ESRF to the solution of industrial problems.

[1] J.C. Labiche, J. Segura-Puchades, D. van Brussel and J.P. Moy, ESRF Newsletter n°25

E. Boller (a,b), J. Baruchel (a), E. Pernot (a) P. Cloetens (a,c), W. Ludwig (a) to be published.

(a) ESRF
(b) INSA, Rennes (France)
(c) EMAT-RUCA, Antwerp (Belgium).



Residual strain in buried and non-buried semiconductor nano-structures

Patterning of periodic gratings at the surface of strained III-V-semiconductor layers is frequently required for the engineering of opto-electronic devices for telecommunication applications. It is based on holographic lithography with subsequent etching followed by a burying growth step [1]. The investigation of lattice strain relaxation phenomena and their evolution caused by the different technological treatments is crucial since the strain status influences the opto-electronic and optical properties of the devices via the modification of the electronic band structure.

X-ray diffraction techniques are non-destructive and does not need special sample preparation. They are at present the only methods for the non-destructive structure characterization of buried gratings, where surface methods like AFM and SEM are no longer applicable. In addition, X-ray diffraction allows the quantitative investigation of the morphological order and the lattice strain in the grating.

The idea was to combine high-resolution X-ray diffraction using synchrotron radiation and elasticity theory in order to determine experimentally for the first time the spatial strain distribution and relaxation phenomena in buried and non-buried gratings etched into strained periodic multilayers. The samples consisted of an InP substrate, on which alloy layers (InGaAsP) were deposited by MOVPE (see Figure 103). The upper third of the superlattice was patterned as a grating and subsequently buried in InP. Such structures are involved in the latest generation of lasers for optical telecommunication (gain coupled distributed feedback laser) where the grating is realized in multilayers with alternate and opposite strains. Moreover, the multilayer corresponds to the active part of the laser, particularly sensitive to strain relaxation.

Two- and three-dimensionally resolved high-resolution reciprocal space maps were obtained in complementary diffraction geometries, employing the unique source characteristics at ID19 and ID32. Figure 104 shows a detailed diffraction map of the asymmetric 668- diffraction of the buried structure. One can clearly observe the central (crystal-) and various grating truncation rods, indicating the high lateral periodicity of the structure. Each rod contains vertical superlattice satellites, arising from either the tensile or the compressively strained layers. The strain relaxation is responsible for the asymmetry between the left and right side truncation rods in the map. Moreover, grazing incidence diffraction also permits depth-selective measurements and enables the separate investigation of the surface shape and the strain. Figure 105 shows a map of the shape-sensitive 6-60-reflection for the free-standing (non-buried) grating, measured under grazing incidence, from which the side-wall inclination angle (among other things) can be determined. Finally, in Figure 106 calculated strain field is plotted for a free-standing grating of trapezoidal shape. Besides the net strain relaxation of the grating as a whole, an alternating strain modulation within the antisymmetrically strained layers is clearly visible.

[1] D. Lübbert (a), T. Baumbach (a), L. Leprince (b), J. Schneck (b), A. Talneau (b), R. Felici (c), to be published
[2] A. Mazuelas (c) et al., proceedings of "Meeting of the French-Spanish Group on new materials", 27-29 Nov. 1997; Ed. A.R. Yavari.

(a) Fraunhofer-Institut für Zerstörungsfreie Prüfverfahren, Saarbrücken and Dresden, (Germany) present address: ESRF
(b) France Télécom, CNET, LA CNRS 250, Bagneux (France)
(c) ESRF.



Strain induced patterning in superlattices ­ comparison of morphological ordering and strain ordering

Strained semiconductor hetero-structures are of increasing interest in fundamental physics and in device applications. One reason is the possibility of designing the valence band structure of III/V semiconductor structures as a function of the incorporated strain. For the successful fabrication of such devices, the structural stability is of crucial importance. Recent studies have shown the fundamental role of strain in determining the growth morphology. In the presence of stress, steps of a vicinal surface can bunch during the growth of a strained layer. Moreover, the achieved lateral order can serve as a base for &laqno;self-assembled» quantum wire nano-structures.

Promising morphological regularity has been obtained under certain conditions in growing symmetrically strained superlattices. Grown on off-oriented substrates, interface macro-steps form stair-like interfaces driven by the residual surface stress. However the detailed experimental investigation of the process of strain relaxation is still pending.

X-ray scattering methods provide information about the morphological ordering at the interface, indicating the lateral correlation properties of the interface steps established during growth and their vertical replication, both influenced by the lattice mismatch and the off-orientation. However, the simultaneous influence of strain and morphology makes the detailed interpretation of the X-ray data difficult. Since the self-organization process develops during the growth, it produces morphological properties which vary strongly as a function of depth below the surface.

High-resolution X-ray grazing incidence diffraction, applied for the first time to this type of structure, combines high strain sensitivity and interface sensitivity. Moreover it allows depth dependent investigation of the ordering, since the penetration depth of the X-ray beam can be controlled via the angle of incidence.

Measurements of reciprocal space maps around complementary reflections, carried out on ID32, have allowed the study of the compositional ordering separately from the related lattice strain correlation. Narrow grating rods in strain-insensitive 2-20 maps (Figure 107) have confirmed a remarkably periodic lateral morphological ordering at the interfaces, the satellites along the rods giving evidence for the vertical replication of the interface steps. Significant differences have been observed in the complementary strain sensitive 220 reflection, where the grating rod pattern is strongly perturbed by strain fluctuations (Figure 108). Moreover, comparing Figures 107 and 108 one can detect a qualitatively different behavior of the vertical interface step replication and of the interface strain replication, which varies also in the highly strain-modulated superlattice with respect to the low strained capping layers. From the X-ray data the models of strain-driven morphological ordering can be tested.

T. Baumbach (a), C. Giannini (b), D. Lübbert (a), R. Felici (c), L. Tapfer (b), T. Marschner (d) and W. Stolz (d), to be published.

(a) Fraunhofer Institut Zerstörungsfreie Prüfverfahren, Saarbrücken and Dresden (Germany) present address: ESRF
(b) Centro Nazionale Ricerca e Sviluppo Materiali (P.A.S.T.I.S.-C.N.R.S.M), Brindisi (Italy)
(c) ESRF
(d) Wiss. Zentrum für Materialwissenschaften und Fachbereich Physik, Philipps-Universität, Marburg (Germany).