In situ monitoring of film deposition with a four detector ellipsometer.
Authors: Masetti E. , Montecchi M. , Larciprete R. , Cozzi S. 
Published: February 2023

The application of ellipsometry to the monitoring of the film deposition is quite recent in comparison to other optical techniques. Two aspects could be considered as responsible of that: the intricate nature of the ellipsometric curves, especially when compared to the spectrophotometric ones, and the cost of the experimental set up. On the other side, ellipsometry is the only optical technique for film characterization with a sensitivity in the nanometer range that is also able to implement standard monitoring methods as, for example, the turning point. In order to make these attractive aspects compatible with the cost of the available photometric devices, we built up an ellipsometer based on a four detector photopolarimeter (E-FDP). The FDP, firstly introduced by Azzam [ref.1], allows the measurements of the polarization of the incident light without moving part or optical modulations. In the easier scheme, it is constructed by arranging four detectors in a way to reflect the incident beam from one surface to the successive. Different constraints must be taken into account in order to make a properly working device. All constraints are well satisfied by using Silicon detectors at an incidence angle close to 60°, with a tilt between the successive planes of incidence close to 45°. By the operative point of view, the FDP directly measures the light polarization in terms of the Stokes vector. Therefore it fits very well the need of measuring the change of the polarization due to the reflection on the sample surface once the incident beam has a well known state of polarization. In a previous work [ref. 2] the ability of the E-FDP, that is composed by a linear polarizer oriented at 45° and a FDP, was reported together with its main drawbacks: the low signal level due to the division of the optical amplitude among the four detectors and the strong influence of the instrument alignment on measurement accuracy. To properly work, in fact, the FDP needs a calibration performed by measuring four known states of polarization of a monochromatic beam arriving exactly from the same direction. A well calibrated FDP is able to measure the polarization only when the incoming light beam has the same direction as during the calibration. As a consequence the instrument misalignment gives the main contribution to the measurement errors. In practical applications a laser source is needed like the HeNe laser (632.8 nm) of the present work, even if the E-FDP has intrinsic spectroscopic capability. For the application an adequate alignment procedure was developed and firstly tested by simulating the in-situ difficulties on a standard ellipsometric set up. Different samples were also measured and the results compared with the ones obtained by a very accurate four zone null ellipsometer. The monitoring of the process was performed by arranging the E-FDP outside the windows of the vacuum chamber of a HV-CVD deposition system in the way shown in Fig.1. During the process a Germanium film was deposited by thermal dissociation of GeH4 on heated Silicon substrate. The film refractive index and its behavior with the film thickness d was estimated by fitting the measured ellipsometric angles (ψ, Δ)d with the values obtained by a proper fílm/substrate model. Additional information on the film were obtained by AFM, XPS and LEED analysis.