Anodic bonding
The substrates are bonded at elevated temperature (~400 degC) by placing and clamping the substrates between two metal electrodes. A high DC potential (up to >1kV) is applied between the electrodes creating an electrical field, which penetrates the substrates. One substrate is a glass that contains sodium ions, which at the elevated temperature are displaced from the bonding surface of the glass by the applied electrical field. The depletion of sodium ions near the surface of the glass makes the surface highly reactive with the silicon surface of the other substrate forming a solid chemical bond.