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A sensing system for monitoring the thickness of thin films by spectrum analysis of white-light interference

In order to determine the properties of thin films with the required performance and reliability, a sensing system for dynamic monitoring is proposed to measure the thickness of thin films. The system is based on the principle of white-light interference, and is combination of spectrum analysis and optical fiber techniques. When two reflected lights interfere within white-light coherent length range, relationship of between interference intensity and the wavelength of incident light is achieved according to the equation of interference light's intensity. According to different thickness of film, the relevant method is selected to calculate the thickness of thin film. So the interference can be analyzed in spectral domain. The scheme of the system is set up including white-light source, multi-mode optical fiber, beam splitter, spectrometer. With the help of optical fiber, the interference pattern is captured by spectrometer. When thickness of thin film is varying, spectra curve will shift. The original spectra curve is processed in the computer. In order to determine accurate extreme points, many methods of curve processing are used to decrease the noise. The spectra curve is smoothed by signal processing method called empirical mode decomposition (EMD). This method is suitable for non-linear and non-stationary data processing. The experimental data is contrast to the calibrated value. The results show that the relative error of this method is lower 1%. This method has advantages over other measuring methods, such as higher accuracy, low-cost instrument, extensive measurement range, simple structure and non-destructive.