Dichroic Multiple Beam Interferometer Microscope M. J. Irland and E. B. Schermer

Scientific Laboratory, Ford Motor Company, Dearborn, Michigan. Received 12 June 1962.

For precise measurement of thicknesses of thin films in the range 0.1–0.01 μ or less, the advantages of multiple-beam inter-ferometry with the microscope over two-beam interferometry are well known.1 Gains in transparency and potential resolution result from the use of a nonabsorbing multilayer film stack in place of a metallic film.2,3 The small air space permissible with multiple-beam interferometry makes contact between the speci­men and the reference flat almost inevitable. When a micro­scope is provided with a special objective having a built-in reference flat, contact with specimens is likely to damage the coating on the flat, requiring renewal more frequently than is convenient. In an effort to avoid excessive frequency of film renewal, the authors replaced the glass reference flat with one of fused silica coated with a multilayer stack composed of titanium dioxide (rutile) and silicon monoxide, materials known for their good adhesion and abrasion resistance.4,5

Since multilayers can be made to yield widely different re­flectances at relatively nearby wavelengths, it was decided to design the multilayer for high reflectance at the green line of mercury and high transparency in blue light. Figure 1 shows the calculated and measured reflectances for a three-layer film combination, HLH on fused silica. Refractive indices at the mercury green line are taken as 2.66 for TiO2 and 1.87 for SiO in agreement with values measured for single films on glass. The films are one-quarter wavelength thick at 6250 Å. Dispersions of the films are taken into account, but that of silica was neglected as insignificant.

The reflectance curve was measured with an attachment for a Beckman DU Spectrophotometer built at this laboratory which permits measurement of absolute reflectance at angles of in­cidence from 5° to nearly 90°.

The TiO2 films were formed by evaporation of the metal in a vacuum of 10 - 5 Torr at a rate of ± 2 5 Å/sec to an optical trans­mission of 4 .8% at 5461 Å, followed by heating in air at 500°C for 8 hr. Silicon monoxide was vacuum evaporated at ±150 Å/ min to an optical thickness of λ/4 at 6250 Å. This optical thickness was substantially unchanged by subsequent heating after the third (titanium) layer had been deposited.

In use, the film has proved substantially more satisfactory than any metal or dielectric film previously used. I t is unmarred after approximately 100 applications, whereas other metal and dielectric films have shown significant damage after this much use. The reflection coefficient matches approximately that of polished steel, giving good fringe contrast on metallographic specimens. The fringe width when sharpened by photography permits measurement of the thickness of thin films with an ac­curacy of ± 1 0 Å (see Fig. 2). Further accuracy can be attained by adding more layers, at some sacrifice. In principle, each additional pair of nonabsorbing layers having the refractive

540 APPLIED OPTICS / Vol. 2, No. 5 / May 1963

Fig. 1. Calculated and observed reflectance curves for three-layer dielectric stack.

Fig. 2. Multiple-beam fringes between three-layer stack and Ge-coated surface.

indices of the materials used here will approximately halve the fringe width. In practice, the widths are greater because of absorption in the film and the difficulty of controlling the thick­ness of TiO2 layers with the necessary precison. Moreover, increasing the number of layers decreases the abrasion resistance.

References 1. S. Tolansky, Multiple Beam Interjerometry (Clarendon,

Oxford, 1948), p. 45 ff. 2. J. A. Belk, S. Tolansky, and D. Turnbull, J. Opt. Soc. Am.

44, 5 (1954). 3. O. S. Heavens, Optical Properties of Thin Solid Films (Butter-

worths, London, 1955), pp. 215, 224. 4. G. Hass, Vacuum II, 331 (1952). 5. G. Hass and N. W. Scott, J. Opt. Soc. Am. 39, 179 (1949).

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