Angstrom resolution optical profilometry for microscopic objects

Microscopy of non-birefringent transmissive phase samples using Sagnac laser interferometer

A cyclic interferometer, appropriately combined with a long working distance microscope objective, is adapted for quantitative phase microscopy. In such an arrangement, the sample information, in terms of the diffracted orders emerging from the sample, is carried by both the counter propagating beams within the cyclic interferometer. However, positioning the sample close to the input/output cube beam splitter and use of a suitably converging laser beam of light as the input to the interferometer ensure that only one of the counter propagating beams carries the object information to the objective while the other beam, which serves as the reference, allows only the undiffracted component to contribute to the process of image formation. Use of suitable polarization optics renders the interferometer its polarization phase shifting property. Using the proposed arrangement, the experimental results showing the quantitative 3D phase rendering of polystyrene microspheres and micro-wells etched in glass are presented.

Effects of phase change on reflection in phase-measuring interference microscopy

We show by analytical and numerical calculations that the phase change on reflection that occurs in interference microscopy is almost independent of the numerical aperture of the objective. The shift of the microscope interferogram response due to the phase change on reflection, however, increases with the numerical aperture. Measurements of the interferogram shift are made with a Linnik interference microscope equipped with various numerical-aperture objectives and are reported and compared with theory.

Phase measurements with wide-aperture interferometers

An interferogram produced by wide-aperture interferometers is studied both theoretically and experimentally. The fringe spacing is shown to increase nonlinearly with the numerical aperture and the fringe envelope to become narrower as the numerical aperture is increased. Phase measurements with wide-aperture interferometers therefore require calibration, and the phase can be measured only over a limited range. A calibration is given for accurate phase measurements, and the range over which the phase can be measured is calculated. Experimental measurements are presented and compared with theory.

Absolute measurement of surface roughness

In an interferometer which uses a reference surface, the measured surface heights correspond to the difference between the test and reference surfaces. To accurately determine the rms roughness of supersmooth surfaces, the effects of the reference surface roughness need to be removed. One technique for doing this involves averaging a number of uncorrelated measurements of a mirror to generate a reference surface profile which can then be subtracted from subsequent measurements so that they do not contain errors due to the reference surface. The other technique provides an accurate rms roughness of the surface by taking two uncorrelated measurements of the surface. These two techniques for measurement of supersmooth surfaces are described in detail, and results of the measurement of a 0.7-A rms roughness mirror are presented. The expected error in the rms roughness measurement of a supersmooth mirror due to instrument noise is 0.02 A.

Differential interference contrast imaging on a real time confocal scanning optical microscope

The first uses of Nomarski differential interference contrast imaging with the real time confocal scanning optical microscope are described. The advantage of differential interference contrast imaging in a confocal scanning microscope is that both the height and width of an edge can be measured without ambiguity, even if the edge is taller than half of a wavelength. Imaging modes are described in which (1) average reflectivity changes of the sample have been eliminated but the resulting edge enhanced images are nonlinear in phase, or (2) linear phase images are superimposed on an average reflectivity image of the sample. A third method has been developed which both eliminates the reflectivity image of the sample and generates edge enhanced images which are linear in phase for small phase changes. Applications to box-in-box overlay targets, imaging the passivation layer of an integrated circuit, and the measurement of sidewall slope are described.

Photoresist thickness measurement using laser-induced fluorescence

A technique is proposed to measure the thickness of a small area of photoresist film on a substrate. This technique uses the fluorescence (peak wavelength, ~600 nm) induced in the film by a focused laser beam (wavelength, 497 nm). An initial calibration line is obtained by comparing several thicknesses and their related fluorescence. The film's thickness is then determined from its fluorescence signal. The laser spot size and the accuracy of this measurement have also been estimated.