We studied the improvement in the polycrystalline silicon (poly-Si) etching rate in the downflow etching process using microwave-excited CF4/O-2 plasma by enhancing the dissociation reaction of the etching gas and the etching reaction on the poly-Si film surface through the use of a catalyst. A piece of platinum (Pt), gold (Au) or silver (Ag) was placed in a quartz tube as a potential catalyst for the downflow etching of poly-Si films. The results revealed that the etching rate using Au was up to 3.6 times higher than that without any catalyst. The mechanism for the improvement in the etching rate using a Au catalyst was analyzed by evaluating the plasma and etching species in transportation paths using optical emission spectral analysis and mass spectrometry, and by examining the poly-Si film with thermal desorption spectrometry, scanning electron microscopy and x-ray photoelectron spectroscopy. The Ali placed between the plasma and the sample is oxidized by the active gas dissociated from CF4/O-2 gas, and Au oxides and their compounds including F and CFx are transported and deposited onto the surface of the poly-Si film. Although, the precise mechanism of these reactions is not clear, it was presumed that the gold oxides and their reaction compounds acted as catalysts in the etching reaction of the poly-Si film and significantly accelerated the etching rate.