We have employed attenuated total reflection Fourier transforms infrared spectroscopy (ATR-FTIRS) to monitor the profile evolution of patterned mesoporous, low-dielectric-constant SiO2 films in situ and in real time during plasma etching. A stack of patterned photoresist, anti-reflective coating, and mesoporous SiO2 is etched in an inductively coupled plasma reactor, using CHF3 and Ar. During etching, the IR absorbance of Si-O-Si stretching modes near 1080 cm(-1) decreases, and the rate of decrease in Si-O-Si absorbance translates to the SiO2 removal rate. When corrected for the exponentially decaying evanescent electric field, the removal rate helps monitor the profile evolution and predict the final etch profile. The predicted profiles are in excellent agreement with the cross-sectional images taken by scanning electron microscopy. In a similar approach, we calculate the absolute total number of C-F bonds in the sidewall passivation and observe its formation rate as a function of time. Assuming that the thickness of the sidewall passivation tapers down towards the trench bottom, we deduce that C-F formation occurs mostly in the final stage of etching when the trench bottom meets the Ge ATR crystal and that a critical amount of C-F buildup is necessary to maintain the anisotropic etch profile. © 2005 American Vacuum Society.