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Mid-IR reflector for enhancing the light collection of FTIR spectrometer in gas concentration monitoring

An ellipsoid reflector was designed and machined to increase the light collection efficiency for Fourier Transform IR (FTIR) spectrometer in mid-IR region. The ellipsoid reflector is used to collect and focus the IR radiation from a Globar, then a parabolic mirror was used to generate the parallel beam as input beam for FTIR spectrometer. Experimental results showed that the ZPD value of the spectrometer with this Mid-IR reflector has increased by a factor of two comparing to typical one-mirror collecting method for FTIR. Due to the higher launching efficiency of the mid-IR reflector, the scans required to obtain a certain spectrum resolution for gas absorption spectrum with required SNR can be reduced. The best collection efficiency with ellipsoid reflectors has been calculated. The further improvement with different reflector shapes and polishing surface will be discussed. As a result, with higher SNR of FTIR, this research has gone one step further to realize real-time/on-line monitoring and control of gas concentration in a CVD chamber in reducing the monitoring time.