Method of measuring thickness of thin layer in semiconductor device and apparatus for performing method

An apparatus for measuring a thickness of a target thin layer formed on a substrate, the apparatus comprising:

• a storing unit adapted to store recipe data including a reference index and a putative thickness of the target thin layer, wherein the reference index is derived from a regression fitting process performed on data obtained with respect to a reference thin layer formed on a reference wafer;
• a measured signal generating unit adapted to generate a measured signal spectrum by irradiating a first light on the target thin layer to produce a first reflected light and analyzing the first reflected light;
• a theoretical signal generating unit adapted to generate a theoretical signal spectrum using a theoretical calculation whose inputs include a putative thickness of the target thin layer;
• a skew generating unit adapted to generate a target skew signal spectrum as a difference between the measured signal spectrum and the theoretical signal spectrum;
• and,
• a processing unit comprising a processing element adapted to calculate a reliability index from the target skew signal spectrum and the reference index, and a selector adapted to select the putative thickness of the target thin layer as an actual thickness of the target thin layer when the reliability index is within an predetermined desired range.