Conventional plasma etching process depends on the external parameters such as gas pressure and input power, and hereby the optimal process condition must be obtained by investigating etching characteristics for each process. On the other hand, smart nanoprocess, where both etching rate and pattern profile are controlled with the assistance of feedback system based on the information of species monitored in situ, will enable us to realize the high performance in nano-scale fabrication. In this study, we have focused on monitoring the behavior of radical densities in the plasma to understand gas phase and subsurface reactions of radicals. By choosing internal parameters of densities (radicals and electrons) and a substrate temperature, both etching rate and fine pattern profile were controlled precisely. The behavior of species in the gas phase and their effects on the etching characteristics of organic low-k film were investigated in 500 MHz ultrahigh frequency (UHF) plasma and 13.56 MHz inductively coupled plasma using N@sub2@/H@sub2@ and N@sub2@/NH@sub3@ gas chemistry. The absolute H and N radical densities were measured by vacuum ultraviolet absorption spectroscopy employing a very compact light source. The vertical profile with a high etch rate of about 400 nm/min was successfully obtained in both plasmas simply by choosing a radical density ratio of H/H+N=0.8 and a substrate temperature of 20°C. The time evolution of plasma-induced subsurface reaction for the vertical profile was observed by Fourier transform infrared attenuated-total-reflection (FT-IR ATR) and in-situ XPS. Furthermore, the fundamental etching mechanism was clarified using multi-beams of ions and radicals. It is indicated that the smart nanoprocess is promising for the precise etching of organic low-k films.