A high accuracy surface inspection system for testing polished surfaces is based on a Fabry-Perot resonator. The
inspected surface serves as a relay mirror in a cat-eye retroreflector incorporated into the resonating cavity, which makes
the optical configuration insensitive to surface tilts. The laser wavelength is swept periodically over a given range, and
the local surface height is obtained by timing the resonance occurrence during each sweep. An additional highly stable
reference Fabry-Perot interferometer using the same laser is employed for obtaining differential measurements, yielding
absolute height values, distinguishing between up and down defects. Due to the finesse of the multi-beam Fabry Perot
interferometer relative to the two-beam Michelson interferometer response function, the height sensitivity is greatly
enhanced. In order to detect small contamination particles, the interferometer was supplemented by a scattering detection
channel integrated into the same compact optical head. The combination of the bright-field interferometric signal,
yielding both the phase (surface height) and amplitude (surface reflectivity), and the dark-field scattering channel, allows
one to build a sensitive and reliable defect detection and classification procedure. The interferometer was incorporated
into high-speed high-accuracy in-line machines for inspection of the surfaces in data storage applications. With a 0.2
Angstroms resolution, the height rms repeatability at a surface scanning speed of 40 m/s is 1.5 Angstroms.
In earlier publications, it was shown that scanning of surfaces by dark beams can be exploited for sub-wavelength feature analysis. In this work, we present vector simulations based in Rigorous Coupled-Wave Analysis with the purpose to estimate the expected resolution of the method, both lateral (feature size) and axial (height). The dark beam used in this study has a line singularity generated by a π-phase step positioned in a Gaussian beam. Various combinations of the illumination and detection nuFmerical apertures (from NA=0.2 to NA=0.8) and different surface features were studied. Polarization effects which become significant at high numerical apetures, were considered as an additional source of information for the analysis. In the case of a sub-wavelength feature on an ideal surface, the resolution of the method is limited only by the electronics noise. In particular, under a reasonable assumption of a 10<sup>5</sup> signal to noise ratio, it is possible to detect a 0.2 nm step.
Surface feature evaluation with resolution beyond the classical diffraction limit can be achieved by a combined space--frequency representation of the scattered field. This was demonstrated in a measuring procedure where the surface was consecutively illuminated by a collection of focused beams and the diffracted data was measured in the far field. Mathematically, if the focused beam has a Gaussian profile, the optical system implements a Gabor transform. Other transformations, such as wavelet transforms can be obtained by properly structuring the illuminating beam. This work presents an approach where structured beams at several wavelengths are used. This additional information gathered by this procedure allows an increased resolution and the reduction of ambiguities that may occur in the analysis of single wavelength measurements.
Reliable in-line and in-situ measurement of structure of highly polished surfaces remains a major challenge for the modern industry. Evaluation of the wavefront of a scanning laser beam reflected from a surface allows one to establish a direct correlation between the statistics of the optical signal and the surface roughness. Phase structuring of the laser beam greatly increases the height sensitivity down to the nanometer level. High sampling rate allows one to collect a very large number of sampled data and provide a complete analysis of the surface structure rather than a single parameter such as the rms roughness.
The method of Black Beam<SUP>R</SUP> Interferometry, which was developed for inspection of glass substrates in LCD manufacturing, is extended in order to achieve a detailed description and classification of defects and other features on the surface under inspection. With a properly arranged detection system, not only a high defect detectability is provided, but also the ability to discriminate between the up or down defects (bumps or pits) in addition to the possibility of measuring the defect geometry (for large defects or structures) and the equivalent defect volume. The method is described and the results of simulations and experiments are presented.
The reading signal of an optical disk relays on phase, amplitude and/or polarization changes induced on a
focused coherent light beam at the written (stored) domain boundaries (data) [1, 2, 3]. The interaction of light
with these storage media are usually treated under the assumption of plane wave illumination since an exact
mathematical analysis of focused beams is quite difficult . In this work we show that very useful results can
be obtained by solving the scalar diffraction integral in the vicinity of a complex step function.