Anodic bonding

Anodic bonding is a connection process that is used especially in the manufacture of sensors and micromechanical devices in the semiconductor and microsystems technology.

Procedure

The procedure is as follows: A silicon wafer with a polished surface and a very flat wafer of glass, the alkali ions - so similar to window glass, are placed on a hot plate and this is heated to about 300 to 400 ° C. The silicon wafer is located directly on the hot plate and above the glass wafer. Now, a pixel electrode is easily pushed onto the glass wafer at a point. The heater plate is the counter electrode. The silicon wafer is sufficient intrinsic at the temperatures. The heating causes the ions present in the glass, such as potassium and sodium can move more freely. Now, an external voltage in such a way that the glass wafer is negative and the heating plate is positively charged, the ions move in the glass slowly to the tip electrode. By this charge transfer is reduced on the surface of the glass to the silicon wafer, the amount of ions, that is, this area has a negative load on. This produces a space charge zone.

Simultaneously, the silicon wafer is positively charged by the external voltage. Thus, the two surfaces will. Due to the very smooth and flat surfaces of silicon and glass is only a very small distance between the two. The attraction of opposite electrodes increases the distance is reduced to ( Coulomb's Law ). Thus both surfaces will always be pulled closer to each other until the point is reached at which the distance is so small that the surface atoms of the glass, may react chemically with that of the silicon. They form chemical bonds between the silicon of the silicon wafer and the oxygen from the silicon oxide of the glass, although it the actual reaction temperature is not yet reached.

Externally, this is visually traceable. Of the tip electrode on the basis, a dark circular front spreads over the entire surface. The further the distance from the tip electrode, the slower migrating this front. Small disturbances of the surface or particles lead to gas inclusions. After turning off the power and cooling, both wafers are no longer separable. If you try it anyway, the fracture is usually in the glass, but not at the interface between glass and silicon.

Applications

The method is used in the manufacture of pressure sensors, which are used in automotive electronics, for example.