Synchrotron radiation collimation

Detection limits for various elements by TXRF on Si wafers are shown in Fig. 4.13. Synchrotron radiation (SR) enables bright and horizontally polarized X-ray excitation of narrow collimation that reduces the Compton scatter of silicon. Recent developments in the field of SR-TXRF and extreme ultra violet (EUV) lithography nurture our hope for improved sensitivity down to the range of less than 10 atoms cm ... 

The use of synchrotron radiation overcomes some of the limitations of the conventional technique. The high brilliance of up to 10 ° photons s mm mrad /0.1% bandwidth of energy, and the extremely collimated synchrotron beam lead to a large flux of photons through a very small cross section (0.1-1 mm ). This allows measurements with samples of small volume if isotopi-cally enriched (with the relevant Mossbauer isotope, e.g., Fe). Measurements that were described earlier [4] and that require a polarized Mossbauer source now become experimentally more feasible by making use of the polarization of the synchrotron radiation. Additionally, the energy can be tuned over a wide range. This facilitates measurements with those Mossbauer nuclei for which conventional sources are available but with life times that are too short for most experimental purposes, e.g., 99 min for Co —> Ni and 78 h for Ga —> Zn. 

Better conventional collimation will not do, except for the largest synchrotron radiation installations to obtain snb-arc-second collimation in the laboratory would require a collimator some 100m long with a sealed-tube source, and at this distance the intensity would be impracticably low. The problem is solved by the use of a beam conditioner, which is a further diffracting system before the specimen The measnred rocking cnrve is then the correlation of the plane wave rocking cnrves of the beam conditioner and the specimen crystals, from which most of the diffracting characteristics of the specimen crystal may be deduced. 

The very high intensities and good collimations available make synchrotron radiation very suitable for topography. Against this must be set the inconvenience and slow turn-round necessitated by transport to a central synchrotron radiation laboratory. Hence, the technique is most appropriately used in the following cases ... 

Since the electrons in storage rings are travelhng at relativistic speeds, the emission of electromagnetie radiation is foreshortened into a cone whose axis is tlie instantaneous direetion of motion of the eleetron. The radiation is therefore intrinsically collimated and is a good mateh to the subsequent beam conditioner. This contrasts favourably with a laboratory somce, in which very little of the more-or-less isotropie emission reaehes the speeimen. The principal characteristics of synchrotron radiation are ... 

Thus we calculate the reflectivity of a whole layered material from the bottom up, using the amplitude ratio of the thick crystal as the input to the first lamella, the output of the first as the input to the second, and so on. At the top of the material the amplitude ratio is converted into intensity ratio. This calculation is repeated for each point on the rocking curve, corresponding to different deviations from the Bragg condition. This results in the plane wave reflectivity, appropriate for synchrotron radiation experiments and others with a highly collimated beam from the beam conditioner. 

For SAXS investigations, the primary experimental requirement is a well-collimated X-ray beam with a small cross-section. Synchrotron radiation sources... 

An EXAFS experimental set-up has three primary components (i) a source of X-rays, (ii) a monochromator (and collimator) and (iii) a detector. Synchrotron radiation is being widely used for EXAFS, but where this facility is not available, a rotating anode source would be suitable. Progress in EXAFS instrumentation has been comprehensively reviewed in the AIP proceedings (1980). 

Once the the energy resolution requirements are established the vertical collimation of a synchrotron radiation beam is defined. This is not so with the horizontal divergence of the beam, where focusing elements in the beam path can increase the intensity of the sample considerably with nearly no deterioration of the wavelength... 

The total reflection of mirrors can be used to focus the radiation. Synchrotron radiation, while collimated in the vertical plane it spreads over the horizontal one. Wavelength resolution requirements normally restrict the vertical aperture to one mm or so, in any case. On the other hand it is desirable to condense the horizontal spread into a focal point. Double focussing with a mirror system is possible and an ideal mirror geometry has been worked out For a point source the mirror has to be shaped like an ellipsoid, and the source and the image have to be placed in the respective foci. The long distances involved in synchrotron work mean that a good approximation to shape is achieved by making use of bent cylindrical mirrors ... 

In the first of five steps (see Fig. 1) a pattern of an X-ray absorber mask is transferred onto a resist layer hundred or so micrometers in thickness by X-ray shadow projection. In practice often synchrotron radiation is used due to its very high degree of collimation and short wavelength (0.2-1 nm). 

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