Gas tightness of cavities sealed by silicon wafer bonding

The hermetic sealing of cavities formed by low temperature silicon direct bonding (SDB) and anodic bonding (AB) of plain and structured silicon surfaces is quantitatively investigated: Gas leakage rates along the bonding interface are determined for pressure sensor test structures by monitoring the pressure increase after a 700 h storage in a 6 bar hydrogen atmosphere. Corresponding leakage rates under atmospheric conditions as small as 10(-14) mbar l/s could be determined by this method. In the case of AB and of hydrophilic SDB with plain bonding surfaces, no measurable gas leakage is observed. For SDB however, only 6 nm deep grooves on one bonding surface do already cause a considerable gas leakage. For AB grooves with a depth of up to ca. 50 nm are still perfectly sealed. Here, the electrostatic pressure leads to a surface conformation through mainly elastic deformation of the bonding surfaces.