Charge storage in double layers of thermally grown silicon dioxide and APCVD silicon nitride

Experimental results on charge storage and discharge in double layers of silicon dioxide and silicon nitride will be reported and discussed. SiO/sub 2/ with a thickness of 300 nm was thermally grown on silicon wafers, while cover layers of Si/sub 3/N/sub 4/ with thicknesses of 50, 100, and 150 nm were deposited chemically at atmospheric pressure. The samples were charged by the point-to-grid corona method. At room temperature, the measured surface potential V was stable during a period of almost three years. Isothermal measurements under different environmental conditions showed an improved charge retention compared to a single layer grown silicon dioxide. After /spl sim/3 h at 300/spl deg/C, the observed voltage drop was <10% for the double layers and /spl sim/60% for bare SiO/sub 2/. Similar results were obtained under a humid condition of 95%RH and 60/spl deg/C. Besides, thermally stimulated current (TSC) was measured in setup with a temperature ramp of 200/spl deg/C/h. For the double layers, a current peak with a maximum temperature at /spl sim/500/spl deg/C was observed. The measured current in the range of 300 to 400/spl deg/C, the location of current maxima observed in thermally grown silicon dioxide or APCVD silicon nitride, was negligible. In addition to improved electret properties the internal stress in the investigated double layers can be adjusted by a proper thickness ratio of oxide layer to nitride layer. Therefore double layers of silicon dioxide and nitride seem to be promising materials for integrated sensors and actuators based on the electret effect.