Paper PS1-TuM4
Real-Time Plasma Deposition Thickness Control using In Situ Optical Emission Interferometry

Tuesday, October 30, 2012, 9:00 am, Room 24

Measurement of the thickness of a growing film as it is deposited can provide real advantages in a production environment. Consistent film thickness is achieved despite long term process drifts and machine to machine variations in deposition rate. Also the time lost due to system re-qualification after routine cleans and maintenance procedures can be reduced significantly.

In this work film thickness is measured in situ in a PECVD system by reflectance techniques, using the plasma emission as a light source (Optical Emission Interferometry, OEI). A parallel plate deposition system is described in which the plasma emission reflected from the substrate is monitored through one of the gas introduction holes in the upper electrode. During the deposition of silicon dioxide and silicon nitride films, the emission bands from molecular nitrogen in the 300 – 400nm region are used for measurement purposes. As the film thickness increases the reflected intensity undergoes a cyclical variation due to interference effects. The film thickness change for one complete cycle is known from the values of the wavelength and the refractive index of the film at that wavelength. The number of interference cycles, including fractional cycles is counted and the film thickness calculated in real time as the film is deposited. The process is terminated when the desired film thickness is reached. An example is shown where the long term variation in film thickness is significantly reduced as compared to running a process terminated by time alone.

For thin films (eg < 100nm) a series of interference cycles is not generated, so the “peak counting” approach cannot be used. Instead, multiple wavelengths (multiple nitrogen bands) are monitored, generating more data points which permits a more accurate determination of the film thickness. This approach is applied to the deposition of a 50nm silicon nitride film.

The above approaches strictly measure change in film thickness, not absolute film thickness. It is shown that by monitoring the complete spectrum and calculating the spectrum change with time, the result is equivalent to the differential of the film's reflectance spectrum. From this the absolute film thickness is calculated. This is particularly useful for the deposition of thicker films where a final film thickness is required, despite a variable or unknown starting film thickness. Although not discussed here, the same approach can be used when etching thick films down to a final required thickness.