Simultaneous measurement of substrate temperature and thin-film thickness on SiO₂/Si wafer using optical-fiber-type low-coherence interferometry

This paper proposes a technique for simultaneously monitoring the thickness of a SiO₂ thin film and the temperature of a Si substrate. This technique uses low-coherence interferometry and has the potential to be used for online monitoring of semiconductor manufacturing processes. In low-coherence interferometry, when the optical path length of a layer is shorter than the coherence length of the light source, the two interference at the top and bottom interfaces of the layer overlap each other. In this case the detected peak position of the interference is shifted from the actual interface, resulting in an error in the temperature measurement, since the temperature is derived from the optical path length of the layer. To improve the accuracy of the temperature measurement, the effect of the overlapping interference was compensated by measuring the SiO₂ thickness. The thickness of the Si substrate was 750 μm and the thickness of the SiO₂ film was varied between 0 and 2 μm. The SiO₂ thickness, which is shorter than the coherence length of the light source, was measured from the ratio of interference intensities of two superluminescent diodes (wavelengths: 1.55 and 1.31 μm). The measured ratio corresponded well with the theoretical one for SiO₂ film thicknesses between 0 and 2 μm, and the error was less than 25 nm. The Si temperature was measured from the optical path length. In order to compensate for the overlapping interference, the shift in the peak position of the interference at the SiO₂/Si interface was estimated from the measuremen- t results of the SiO₂ thickness. This improved the accuracy of the temperature measurement from 5.3 to 3.5 °C.