A new method to measure spectral reflectance and film thickness using a modified chromatic confocal sensor

Abstract

A chromatic confocal sensor is modified to measure the spectral reflectance of nontransparent surfaces or transparent thin films. According to the Fresnel formula, specular reflectance depends on the wavelength, refractive indexes of the media around the reflecting interface, angle of incidence, and also the thickness of (thin) films. To provide a constant angle of incidence, a suitably designed annular aperture is installed in the dispersion probe. In this way, it is possible to generate a conic incident beam onto the reflecting interface. Furthermore, a self-reference method is used to acquire the reflected spectrum using a continuous scan across the dispersion range. Then, the spectral reflectance is calculated based on a comparison with a standard mirror with known spectral reflectance. The experimental results show a reflectance measurement error of about ±0.05 within the spectral range from 445 nm to 645 nm. Furthermore, the thicknesses of several silicon-dioxide films on silicon substrates were determined by fitting the measured spectral reflectance with the theoretical spectral reflectance. The repeatability of the measurement is within about 0.6 nm, and the results agree with the ellipsometry and commercial film thickness gauge. When the scan speed is set to 0.05 mm/s, 0.1 mm/s, 0.2 mm/s, and 0.4 mm/s, the measured thicknesses change slightly (<0.22 nm), which means there is still a significant potential to enhance the measurement efficiency. Finally, the film thickness topography is obtained to determine the uniformity of the film. Overall, the proposed method has a big potential for the measurement of spectral reflectance and film thickness.