Abstract: A method for forming semiconductor features, e.g., gates, line widths, thicknesses and spaces, produced by a photoresist trim procedure, in a closed loop process is presented. The methodology enables the use of optical emission spectroscopy and/or optical interferometry techniques for endpoint monitoring during resist trim etching of photoresist structures. Various types of material layers underlying photoresist structures are employed in order to provide an endpoint signal to enable closed loop control, with resultant improved targeting of photoresist mask and reproducibility. In addition, the method provides for in situ etch rate monitoring, and is not adversely affected by etch rate variances within an etching chamber during an etch process.