NOx detection is an innovative approach in CMP (Chemical Mechanical Polishing) process control. CMP smoothens the wafer surface with a combined chemical and mechanical polishing procedure. During production, wafers are coated by a thin layer. To avoid damage to the wafer surface, for instance by stress cracks, the NOx emitted as soon as the wafer is plane and blank must be measured. Therefore, a high-resolution NOx-detector, capable of determining exactly when the polishing process must be stopped, is essential. This approach is used successfully in manufacturing and development lines worldwide, since 1996.