James F. Biegen

Determination of the phase change on reflection from two-beam interference

Light reflected by a nondielectric material experiences a phase change on reflection that differs from light reflected by a dielectric material and other nondielectric materials. The complex degree of coherence for small optical path differences is derived for two-beam interference when the illumination source is extended, incoherent, and quasi-monochromatic. An analysis of the two-beam interference pattern reveals a simple relationship between the phase of the interference pattern at the point of maximum fringe visibility and the material-dependent phase change on reflection.

Calibration requirements for Mirau and Linnik microscope interferometers.

In interferencem icroscopesr equiringo blique incidence illumination (i.e., Michelson, Mirau, and Linnik), the fringe spacing varies with illumination numerical aperture so that calibration is needed for accurate measurements.

Polarization interferometer for measuring the flying height of magnetic read–write heads

Traditional optical flying-height testers use only the normal-incidence reflectivity of the interface between the read-write slider and a glass disk surrogate. We propose a tester that fully analyzes the complex amplitude ref lectivity of the interface, including the polarization-dependent complex phase. The new approach is more accurate and repeatable and has no loss of precision at zero flying height. Further, the same instrument directly measures the complex index of refraction for the slider material in situ, obviating the need for a separate metrology step with an ellipsometer.

Optical flying-height testing of magnetic read-write heads

Quality control in the production of rigid disk drives depends in part on accurate metrology of the read-write head. We review the established technologies for measuring the distance or flying height between the head and the rotating disk, and propose a new instrument based on polarization interferometry. The new instrument has excellent repeatability, high sensitivity at low flying height and in situ determination of the phase change on reflection.

A new class of wide-field objectives for 3D interference microscopy

We propose a new type of interference objective that makes use of two partially-reflective beamsplitter plates arranged coaxially with the objective lens system, in an assembly that is better suited to large fields of view than the traditional Michelson design. The coaxial plates are slightly tilted to direct unwanted reflections outside of the imaging pupil aperture, providing high fringe contrast with spectrally-broadband, spatially extended white light illumination. Examples include a turret-mountable 1.4× magnification objective parfocal with high-magnification objectives up to 100×, and a dovetail mount 0.5× objective with a 34×34mm field for wide-field measurements of surface form.

Interference microscope objectives for wide-field areal surface topography measurements

Abstract. We propose a type of interference objective that extends the range of application for flexible microscope platforms to larger fields of view. The objective comprises a beamsplitter plate and a partially transparent reference mirror arranged coaxially with the objective lens system. The coaxial plates are slightly tilted to direct unwanted reflections outside of the imaging pupil aperture, providing high fringe contrast with spatially extended white-light illumination. Examples include a turret-mountable 1.4× magnification objective parfocal with high-magnification objectives up to 100× and a dovetail mount 0.5× objective with a 34×34  mm field. This design is a practical alternative to the classical Michelson and Mirau type objectives for low magnifications.

Characterization of a geometrically desensitized interferometer for flatness testing

We describe the detailed design of a geometrically desensitized interferometer using two transmission diffraction gratings. A number of models of the instrument are used to eliminate object ghosts and stray light contributions. We then investigate analytically the influence of object slope variations on the instrument precision. We show that the part can be located at a measurement location where the metrology is optimized. Analytical and raytracing models demonstrate excellent agreement with experiment.

Step Height Measurement Range Extended For An Interference Microscope Utilizing The Obliquity Effect

Step heights measured with an interference microscope in monochromatic light are generally limited to steps not exceeding 1/4 of the illuminating wavelength due to the difficulty in following the fringe order across the step. There are several ways to overcome this ambiguity. One method tracks the fringe contrast across a step to help determine the absolute phase across the step height.1 Another method uses two measurements at differing wavelengths (i.e., two-wavelength interferometry) to create a synthetic wavelength larger than either single wavelength.2 The quarter-wave limit of the synthetic wavelength then being significantly larger than either single wavelength measurement. The method I propose utilizes the fact that the fringe spacing in interference microscopes with extended source illumination is greater than half a wavelength3 (i.e., 1 fringe width = λ/2•obliquity factor). The obliquity factor varies as a function of the illumination numerical aperture and the distribution of the intensity of the illumination across the pupil of the microscope objective. Changing the aperture stop setting in the illumination arm of the interference microscope between successive measurements changes the magnitude of the obliquity factor, thereby creating two single effective wavelengths. Two-wavelength interferometry can be accomplished by the creation of a larger equivalent wavelength from the two single effective wavelengths; that is, from one wavelength multiplied by two different obliquity factors.