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

Abstract

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 measurement results of the SiO2 thickness. This improved the accuracy of the temperature measurement from 5.3 to 3.5 °C.

Low-Coherence Interferometry-Based Non-Contact Temperature Monitoring of a Silicon Wafer and Chamber Parts during Plasma Etching

“…We demonstrated the temperature monitoring of a semiconductor wafer by optical fiber based interferometry using a low-coherence light source (LCI). 17,18) This technique enables the measurement of the temperatures of multiple layers. However, the temperature measurement had not been performed in-situ during plasma processing.…”

Simultaneous In situ Measurement of Silicon Substrate Temperature and Silicon Dioxide Film Thickness during Plasma Etching of Silicon Dioxide Using Low-Coherence Interferometry

“…When the electric fields of the reflected light from the sample and reference arm interfere at a detector, the interference term of the interference intensity is given by 25,29) IðÁlÞ…”

“…The SiO 2 thickness was deduced from the ratio of interference intensities of two light sources having different wavelengths. 25) Moreover, the measurement accuracy of the Si temperature was improved by compensating for the peak shift of interference at the SiO 2 /Si interface on the basis of SiO 2 thickness measurements. In the etching equipment, the ratio of the interference intensities of two wavelengths had an error relative to the theoretical ratio due to disturbances such as the vibration noise of the equipment and the change in ambient temperature.…”

Noncontact measurement of substrate temperature by optical low-coherence interferometry in high-power pulsed magnetron sputtering

“…To solve this problem, we have developed a noncontact temperature measurement method based on optical low-coherence interferometry (LCI) and reported the accurate substrate temperature measurement in the plasma etching process. [16][17][18][19][20][21] It is necessary to evaluate the repeatability of the temperature measurement by replacing the plasma-treated substrate with new one. In addition, there are no reports on the application of LCI to the measurement of the substrate temperature in deposition processes.…”