Low-temperature anodic bonding of silicon to silicon wafers by means of intermediate glass layers

Silicon wafers have been anodically bonded to sputtered lithium borosilicate glass layers (Itb 1060) at temperatures as low as 150–180 °C and to sputtered Corning 7740 glass layers at 400 °C. Dependent on the thickness of the glass layer and the sputtering rate, the sputtered glass layers incorporate compressive stresses which cause the wafer to bow. As a result of this bowing, no anodic bond can be established especially along the edges of the silicon wafer. Successful anodic bonding not only requires plane surfaces, but also is determined very much by the alkali concentration in the glass layer. The concentration of alkali ions as measured by EDX and SNMS depends on both the sputtering rate and the oxygen fraction in the argon process gas. In Itb 1060 layers produced at a sputtering rate of 0.2 nm/s, and in Corning 7740 layers produced at sputtering rates of 0.03 and 0.5 nm/s, respectively, the concentration of alkali ions in the glass layers was sufficiently high, at oxygen partial pressures below 10^-4 Pa, to achieve anodic bonding. High-frequency ultrasonic microanalysis allowed the bonding area to be examined non-destructively. Tensile strengths between 4 and 14 MPa were measured in subsequent destructive tensile tests of single-bonded specimens.