Measurement of the thickness of thin films

Abstract

An ultraviolet source (14) directs selected ultraviolet radiation onto a thin film (10) to excite it. The thin film fluoresces, either naturally or as a result of adding fluorescing material. The amount of light fluoresced is proportional to the film thickness. An optical filter (16) selectively transmits fluoresced wavelengths, excluding exciting wavelengths. A photodetector (18) converts the light to an electrical signal which is processed by signal processing circuits (20) and displayed on a readout (26). The output is compensated for variations in the intensity of the exciting ultraviolet radiation by generating a second electrical signal (13', 18') proportional to the intensity of the exciting radiation and dividing the first electrical signal by a function of said second electrical signal. The compensated output is then calibrated to assure precision and accurate measurements. Various materials may be selectively measured, individually in the presence of others by choosing appropriate excitation and emission wavelengths. The amount of fluoresced light is linear with respect to the amount of material present as long as the layer is quite thin. As the layer thickness increases, the amount of light to thickness relationship becomes non-linear.