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A novel Ultra Fast Multi-Wavelength Ellipsometric (UFMWE) unit capable to monitor fast processes

The development of UFMWE has been approached to meet the specific requirements and demands (in-line monitoring and control of very fast vacuum web coating processes). The reasons for designing a new monochromator FUV200:UT 300 and using a new light source (150 Watts Xenon) include one of the main UFMWE unit requirements, that is high data acquisition speed for following processes in real time, yielding exploitable results in a total time of less than 100ms and over a 32 wavelength range, and the spectral range of 190-830 nm. The new approach made use of a new ultra fast acquisition board (DSP inside) which can simultaneously convert up to 32 channels at a rate of 800kHz per channel combined with a hybrid multi-channel detector including a photodiode coupled with a photo cathode. The new monochromator FUV200 has a focal length 200 nm single grating, the calibration is automatic with opto-switch, and the spectral range is: 1.5 – 6.5eV. In order to cover the 190-830 nm spectra range, two PMT for the ViS and UV energy ranges are used. The need for the FUV range led to the design of a dedicated light source. In order to overcome the problem of the strong absorption of the atmosphere, a Nitrogen purge system was incorporated with the safety requirement for ozone evacuation. Since the life time of the Xenon source is around 2000h in the FUV range, a time counter was inserted in the block source. For an accurate measurement, any perturbations from the external environment need to be taken into account. A microshutter is incorporated to this end.

The spectrograph is based on a specific holographic fixed grating specifically designed and manufactured by JY for this project. A flat field spectrum of 150mm width is obtained with a dispersion of around 2 nm/mm. In this spectrum field, 16 wavelengths were selected as defined by AUTh. This corresponds to the optimised wavelength of the project. The selection is done by optical fibre, using the optical demultilplexing technology already developed by JY and then coupled to 32 photomultiplier tubes (provided by the Hammatsu Company). The detectors and the spectrograph were designed to achieve a high level of integration in an industrial standard rack (height: 3U, width: 19").

The Integrated Spectroscopic Detector is fully integrated into a PC based platform and is based on the use of the CAUTH40 spectrograph and a linear CCD detector (2048 pixels). The software is based on Windows 2000. The work on firmware included the communication of the ellipsometric unit with sensors through ultrafast acquisition boards to perform acquisition, modelling, local database support, data management and statistics. In addition to the selected technical requirements concerning the proper spectral range and the number of wavelengths, a new ultra fast acquisition board (DSP inside) has been optimised in order to meet the basic demand for high data acquisition. The board can simultaneously convert up to 32 channels at a rate of 800 kHz per channel combined with a hybrid multi-channel detector including a photodiode coupled with a photo cathode.

Fibre optic was not used to connect the Xe-lamp with the Analyser housing in order to increase the intensity & accuracy and to reduce the cost. The software delivered with the prototype UFMWE was basic software (DP1 version), which was optimised during the project. Aspects of the software, such as: i) the industrial communication, ii) the full parallelism and iii) its total integration, were finalised in the second version of the software (DP2).

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Aristotle University of Thessaloniki
54124 Thessaloniki