An Evaluation of the Prism Coupler for Measuring the Thickness and Refractive Index of Dielectric Films on Silicon Substrates

Abstract

Four different film evaluation techniques, prism coupling, reflectance spec-troscopy, ellipsometry, and mechanical measurement of a step height, have been compared by measuring the thickness and refractive index of thermally grown silicon dioxide and plasma-deposited silicon nitride films. The four methods have similar accuracies and precisions for measuring the film thickness ; the agreement among the four methods is generally within _+0.01 ;~m for silicon dioxide films less than 0.6 ~m thick and within +-1.5% for thicker films. Prism coupling and ellipsometry also have similar accuracies for measuring the film refractive index; however, the prism coupling measurement is much more precise. In addition, the accuracy of the prism coupling measurement is only slightly affected by variations in the film refractive index or by optical absorption in the film, effects which cause complications in the reflectance spectroscopy and ellipsometry measurements. The most important limitation on the prism coupling technique is the restriction on the film thickness. Films thinner than 0.2 ~m cannot be measured. When the film thickness is between 0.2 and 0.5 ~m, either the thickness or the refractive index can be measured if the other quantity is known. Both quantities, the film thickness and the refractive index, are measured when the film thickness is greater than 0.5 #m. A prism coupling technique has recently been described for measuring the refractive index and the thickness of transparent films. Briefly, the technique consists of measuring the angles at which a prism will couple light from a laser beam into the sample film. The thickness and refractive index of the film are calculated from the measured angles. Several papers describing the prism coupler (1-4), the necessary optical theory (5-8), and the accuracy of the technique (9, 10) have been published. The technique has been experimentally demonstrated by measuring the thickness and the refractive index of aluminum oxide * Electrochemical Society Active Member.