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With modern instrumentation, ellipsometry makes it possible to develop a number of noncontact, nondestructive, sensitive techniques for studies of the structure and composition of thin surface films. Even a minute accuracy in the measurements of polarization angles (psi) and (Delta) characterizing the variations in the light beam polarization resulting from reflection from the object under study, allows the surface film thickness to be determined accurate to plus or minus 0.1 nm, and the index of refraction to the third decimal place. At the same time, the minute accuracy is obviously insufficient to study superthin (under 10 nm) films and to determine numerous unknown parameters of multilayered surface structures. The main obstacle in the way leading to higher accuracy is the lack of the well-developed theory of the instrumentation and reliable metrology basis. The metrology now in use in ellipsometry employs standard samples and averaging of the measured data over four measurement zones. The measured data show a considerable spread over different zones of the instrument which can be attributed primarily to the limited technique adopted for determining the phase compensator parameters as well as to the noticeable anisotropy of the standard sample which is ignored in the 'isotropic' relations involved in the experimental determination of the polarization angles. As a result, the averaged values of the angles (psi) and (Delta) , though slightly improved, are still essentially limited in their accuracy, failing to represent the true properties of the sample adequately. This tendency in metrology does not encourage the improvement in the precision of instrumentation, disguises the anisotropy properties of reflecting systems, eventually limiting the capabilities of the ellipsometry technique. Mostly for this reason, a precision ellipsometer for metrology purposes has not been constructed so far which could be applied in both academic and industrial environments. However, the spread in the experimental angles (psi) and (Delta) over different measurement zones can be regarded as a metrology criterion of the measuring accuracy for the polarization angles, and hence, the accuracy of determining the physicochemical properties of the surface layers. The magnitude of the spread depends on the precision of angle measuring instruments, photodetector sensitivity and measuring accuracy and invariability of all the parameters of the optical elements (mainly, of the phase compensator). This approach underlies the one-zone technique of ellipsometric measurements intended to reduce the spread in the experimental polarization angles (psi) and (Delta) over the measurement zones down to the minimum dictated by the required measurement accuracy. Thus, the one-zone technique is a theoretical basis for the development of reliable metrological control in ellipsometry and the designing of new types of precision instruments.
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