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The Anodic Bonding Process

Anodic bonding is a method of hermetically and permanently joining glass to silicon without the use of adhesives. The silicon and glass wafers are heated to a temperature (typically in the range 300-500oC depending on the glass type) at which the alkali-metal ions in the glass become mobile. The components are brought into contact and a high voltage applied across them. This causes the alkali cations to migrate from the interface resulting in a depletion layer with high electric field strength. The resulting electrostatic attraction brings the silicon and glass into intimate contact. Further current flow of the oxygen anions from the glass to the silicon results in an anodic reaction at the interface and the result is that the glass becomes bonded to the silicon with a permanent chemical bond.

The Benefits of Anodic Bonding

  • Low bonding temperature giving more design flexibility.
  • Thermally matched stress free bond; stable mechanical dimensions with temperature.
  • Flat assembly.
  • No measurable flow of the glass occurs, hence enabling sealing around previously machined grooves, cavities etc. without any loss of dimensional tolerances.
  • Since glass is an electrical insulator, parasitic capacitances are kept extremely small.
  • Hermetic seals. The sealing process can readily be performed in vacuum, allowing hermetically sealed reference cavities to be formed (or the sealing in of special gas mixtures).
  • Glass transparency at optical wavelengths enables simple, but highly accurate, alignment of pre-patterned glass and silicon wafers. This transparency can also be exploited via optical addressing, and to 'see' inside micro-fluidic devices.
  • High yield process. Tolerant to particle contamination and wafer warp (the electrostatic field generates a high clamping force which overcomes these surface irregularities).
  • Low cost wafer scale process for first order packaging can be done at chip level if required.
  • Multi-layer stacks allow an easy route to complex 3-D microstructures.
  • High strength bond - higher than the fracture strength of glass.

Applications

The technique of anodic bonding has found many applications in the field of MST, MEMS or microengineering. These include the fabrication of pressure sensors, accelerometers, micropumps and other fluid handling devices. The process is also used for first order packaging of silicon microstuctures to isolate package induced stresses. By having the sensitive microstructure bonded to a relatively thick (~1mm) glass base the device can be mounted on PCB's and other substrates having a thermal expansion mismatch with silicon. In this manner, the high stress regions, which would have occurred in the silicon microstructure, instead occur in the glass.

Sealed cavities/ leadthroughs

Sealed cavities are used in many MEMS devices including both capacitive and optically addressable pressure sensors. For electrical devices we have a process for forming hermetically sealed leadthroughs between internal electrodes and external bond pads.

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