Q14Micro and Smart System Technology

Wafer Bonding and Metallization in Silicon Wafer Processing

Wafer bonding is the process of joining two (or more) wafers together to form a single, mechanically and often hermetically sealed composite structure, essential in MEMS fabrication for capping delicate microstructures, creating sealed reference cavities (as in pressure sensors), or building multi-layer 3D device structures. Anodic bonding, one of the most widely used techniques, joins a silicon wafer to a glass (typically borosilicate/Pyrex) wafer by heating the assembly to 300-400 degrees Celsius and applying a high DC voltage (several hundred volts) across the stack, with the silicon at positive potential and the glass at negative potential.

Under this applied field, mobile sodium ions within the glass migrate away from the silicon-glass interface toward the negative electrode, leaving behind a thin, negatively-charged, immobile depletion layer at the glass surface adjacent to the silicon; this creates a very strong electrostatic attraction (essentially a large parallel-plate-capacitor-like force) that pulls the two surfaces into intimate contact, while the elevated temperature promotes a chemical bonding reaction (formation of Si-O-Si bonds) at the interface, producing a permanent, hermetic, mechanically robust bond without requiring any intermediate adhesive layer that could otherwise introduce contamination, outgassing, or unwanted mechanical compliance.

An alternative technique, silicon fusion (direct) bonding, joins two silicon wafers directly by first activating their surfaces (through careful cleaning and hydration to form a thin native oxide with surface hydroxyl groups), bringing the wafers into contact at room temperature (where weak hydrogen bonding initially holds them together), and then annealing at high temperature (800-1100 degrees Celsius) to convert these weak bonds into strong, permanent covalent Si-O-Si bonds throughout the interface, giving exceptionally strong, void-free bonding suitable for structural and hermetic sealing applications, though at a substantially higher required process temperature than anodic bonding.

Metallization is the subsequent (or sometimes preceding) process of depositing thin conductive metal layers (commonly aluminum, gold, or platinum) onto the processed wafer to form electrical interconnects, bond pads, and sometimes functional structural or electrode layers for the MEMS device itself. Metallization is typically performed by physical vapor deposition techniques - thermal evaporation (heating the metal source in vacuum until it vaporizes and condenses on the cooler wafer surface) or sputtering (bombarding a metal target with energetic ions to eject metal atoms that then deposit onto the wafer) - followed by photolithographic patterning and etching (or a lift-off process) to define the specific interconnect and pad geometry required by the circuit or device layout. Together, wafer bonding and metallization are essential back-end processing steps that transform a set of individually fabricated micromachined wafer structures into a complete, electrically connected, and often hermetically packaged functional MEMS device ready for dicing, packaging, and final testing.