the present method provides for the detection and assessment of the net charge in a pecvd oxide layer deposited on a surface of a semiconductor substrate . electrical potential differences across pecvd oxide layers on as - produced semiconductor substrates are measured . resultant pecvd oxide charge derivative values are plotted on an control chart and compared to calculated control parameters . all measurement techniques are non - contact and non - destructive , allowing them to be performed on as - processed semiconductor substrates at any time during or following a wafer fabrication process . in a first embodiment , a contact potential difference v cpd between a vibrating electrode and the semiconductor substrate is measured while the semiconductor substrate beneath the vibrating electrode is subjected to a constant beam of high intensity illumination . the resultant value of v cpd is equal to the electrical potential difference across the pecvd oxide layer v ox . in a second embodiment , the semiconductor substrate is not illuminated during the measurement of v cpd . a conventional spv apparatus is used to measure the surface barrier potential v sp of the semiconductor substrate . subtracting the measured value of v sp from the measured value of v cpd yields the value of v ox .