Protection of SiO2from sodium contamination with phosphosilicate glass

Low and stable surface state densities can be achieved on silicon devices with thermally-grown SiO2. Such systems are however extremely sensitive to contaminants, such as sodium. The addition of a phosphosilicate glass layer to the top of thermally-grown SiO2provides comparable initial oxide properties and also excellent protection against sodium contamination. For this work MOS diodes were fabricated with phosphorus-glassed SiO2. Some were purposely contaminated with a dilute solution of NaOH before contact deposition ( Na ions/cm²). Others were processed routinely. Neither of these groups of MOS diodes showed the typical increase of surface states during thermal compression wire bonding. Approximately further states/cm²were introduced during temperature-field aging for uncontaminated diodes; contaminated diodes showed slightly more. As a control, ordinary thermally-grown SiO2was contaminated with the same NaOH solution. Na contaminated thermally-grown SiO2showed approximately states/cm²introduced during wire bonding and a further 10¹³states/cm²during temperature-field aging.