Anodic bonding

Anodic bonding is commonly used to seal glass to silicon wafers in electronics and microfluidics.

The coefficient of thermal expansion (CTE) of the processed glass needs to be similar to those of the bonding partner.

[6] Hermetic seals of silicon to glass using an aluminum layer with thickness of 50 to 100 nm can reach strengths of 18.0 MPa.

[8] Even though anodic bonding is relatively tolerant to contaminations, a widely established cleaning procedure RCA takes place to remove any surface impurities.

The glass wafer can also be chemically etched or powder blasted for creating small cavities, where MEMS devices can be accommodated.

If using a needle electrode, the bond spreads radially to the outside which makes it impossible to trap air between the surfaces.

The radius of the bonded area is approximately proportional to the square root of time elapsed during the procedure.

[13] The use of a full area cathode electrode shows bond reactions over the whole interface after powering up the potential.

[8] This causes a diffusion of sodium ions (Na+) out of the bond interface to the back side of the glass to the cathode.

The high voltage helps to support the drifting of the positive ions in glass to the cathode.

This affects[clarification needed] a positive volume charge in the silicon wafer on the opposite side.

[12] As a result, a high-impedance depletion region a few micrometres (μm) thick develops at the bond barrier in the glass wafer.

The electrical field intensity in the depletion region is so high that the oxygen ions drift to the bond interface and pass out to react with the silicon to form SiO2 (compare to figure "ion drifting in bond glass" (2)).