Thickness Measurement of Built-Up Monolayers by Multiple Beam Interferometry V. K. Srivastava

Physics Department, University of Roorkee, Roorkee, India. Received 20 October 1972. Shirahata and Scott1 have reported measurements of the

thickness of built-up Blodgett-Langmuir monolayers of barium salts of some fatty acids by multiple beam inter-ferometry using Tolansky's fringes of equal chromatic order (Feco). In the above work the essentially identical measurements of the thickness, first reported by Srivasta­va and Verma,2 ,3 have not been cited. Shirahata and Scott have introduced some modifications in the measure­ment technique and have investigated a larger number of fatty acid salts. The main modifications are the use of the intermediate protective layer of collodion to avoid damage of the monolayers in vacuum and the evaluation of the phase change corrections. So far as the use of the intermediate protective layer is concerned, the author be­lieves that this is not at all necessary for the films investi­gated. Even without the use of such a layer, very good unique and expected correlation between the interferome-tric and x-ray measurements of the monolayer thickness was established by Srivastava and Verma.3 The accurate micro-focus x-ray measurements, of course, did not in­volve exposure of the films to vacuutn. It can thus be safely concluded that films of barium salts of fatty acids are not significantly altered in vacuum. Only stearic acid monolayers on quartz have been shown4,5 to be removed considerably on pumping under vacuum. Even if this is also true for glass substrates, it will not matter so far as the measurement of the metrical thickness is concerned

936 APPLIED OPTICS / Vol. 12. No. 5 / May 1973

because the soap film will at most undergo vacuum skele-tonization^ owing to the removal of the free fatty acid from the film. The skeletonization, as is well known,6

does not affect the metrical thickness although it changes the optical thickness because of the expected change of refractive index.

The evaluation of the phase change corrections in film thickness measurements is, undoubtedly, desirable; and this may explain the slightly better accuracy obtained by Shirahata and Scott in their work.

References: 1. R. Shirahata and G. D. Scott, Appl. Opt. 10, 2192 (1971). 2. V. K. Srivastava and A. R. Verma, Proc. Phys. Soc. (London)

80,222(1962). 3. V. K. Srivastava and A. R. Verma, Solid State Communica­

tions, 4, 367 (1966). 4. R. W. Roberts and G. L. Gaines, Jr., Trans. Ninth Natl. Vacu­

um Symp. Am. Vacuum. Soc, 515 (1962). 5. G. L. Gaines Jr., and R. W. Roberts, Nature 197, 787 (1963). 6. I. Langmuiг, Proc. Roy. Soc. (London) A170, 1 (1939).