An etching end point of a plasma etch is determined by defining an etch-stop condition. A layer formed on a substrate is etched using a plasma. A luminous intensity of the plasma is measured to determine a first luminous intensity. The luminous intensity is measured again after a predetermined time to determine a second luminous intensity. A determination is made whether a disturbance occurs. Compensation is applied to the measured luminous intensity if the disturbance occurs. A determination is made whether the measured luminous intensity or the compensated luminous intensity satisfies the etch stop condition.