Plasma-surface interactions for poly-Si gate etching have been studied in Cl2 and Cl2/O2 electron cyclotron resonance (ECR) plasmas, through in situ diagnostics of reaction products during etching and numerical simulation for the etched profile evolution. The in situ observation of product species was conducted both in the gas phase and on the substrate surface, by employing Fourier transform infrared (FTIR) absorption spectroscopy. For both Cl2 and Cl2/O2 plasmas, silicon tetrachloride SiCl4 was the only IR-absorbing etch product species detected in the gas phase, while unsaturated SiClx (x= 1-3) as well as SiCl4 were observed on the surface; moreover, a broad absorption feature due to Si-O vibrations or silicon oxides was found to occur both in the gas phase and on the surface, implying competitive chlorination and oxidation of the surface, and also competitive etching and surface inhibitor deposition. Based on these observations, the surface chemistry was modeled for Cl2 and Cl2/O2 plasma etching of poly-Si, including ion-assisted reaction processes, physical sputtering or desorption of adsorbates by energetic ion bombardment, and passivation layer formation consisting of surface oxidation and inhibitor deposition. Moreover, numerical calculations were performed for the profile evolution during etching, by employing the surface chemistry model proposed and taking into account a variety of neutral transport mechanisms in microstructures that are known from the literature. The etched profiles numerically and experimentally obtained were then compared to unravel competitive mechanisms responsible for the anisotropy and microscopic uniformity in poly-Si gate etching in low-pressure, high-density chlorine-containing plasmas.