Eutectic Bonding is a wafer-level bonding technique that uses a eutectic alloy system to join two surfaces at a temperature significantly below the melting point of either constituent metal — exploiting the eutectic phase diagram where two metals form a low-melting-point alloy at a specific composition ratio, enabling hermetic, electrically conductive bonds for MEMS packaging, LED die attach, and advanced semiconductor packaging.

What Is Eutectic Bonding?

• Definition: A bonding process where thin films of two metals (e.g., Au and Sn, or Al and Ge) deposited on opposing wafer surfaces are brought into contact and heated above the eutectic temperature, causing the metals to interdiffuse and form a liquid eutectic alloy that wets both surfaces and solidifies into a strong, hermetic bond upon cooling.
• Eutectic Point: The specific composition and temperature where two metals form a liquid alloy at the lowest possible melting point — Au-Sn eutectic (80/20 wt%) melts at 280°C, far below Au (1064°C) or Sn (232°C) individually.
• Isothermal Solidification: In some eutectic systems, the liquid phase solidifies isothermally as continued interdiffusion shifts the local composition away from the eutectic point, forming intermetallic compounds with higher melting points than the bonding temperature.
• Hermetic and Conductive: Unlike adhesive or oxide bonding, eutectic bonds are both hermetically sealed and electrically/thermally conductive, making them ideal for applications requiring both encapsulation and electrical interconnection.

Why Eutectic Bonding Matters

• MEMS Hermetic Packaging: Eutectic bonding provides vacuum-compatible hermetic seals for MEMS resonators, gyroscopes, and infrared detectors, with the added benefit of electrical feedthrough capability through the bond ring.
• LED Die Attach: Au-Sn eutectic is the standard die attach method for high-power LEDs, providing excellent thermal conductivity (57 W/m·K) to extract heat from the LED junction through the bond to the substrate.
• Moderate Temperature: Eutectic temperatures (280°C for Au-Sn, 363°C for Au-Si, 424°C for Al-Ge) are compatible with CMOS back-end processing and most MEMS devices.
• Self-Aligning: The liquid eutectic phase provides surface tension forces that can self-align bonded components, useful for flip-chip assembly of small die.

Common Eutectic Systems for Semiconductor Bonding

• Au-Sn (280°C): The gold standard for hermetic MEMS packaging and LED die attach — excellent wettability, high bond strength, and no flux required. Cost: high (gold content).
• Au-Si (363°C): Used for silicon-to-silicon bonding where gold is deposited on one surface and reacts with the silicon substrate — no separate solder layer needed on the silicon side.
• Al-Ge (424°C): CMOS-compatible alternative to gold-based eutectics — aluminum is standard in CMOS metallization, and germanium can be deposited by sputtering or CVD.
• Cu-Sn (227°C): Low-cost alternative using copper and tin — forms Cu₃Sn intermetallics with high re-melt temperature (>600°C) through transient liquid phase bonding.

Eutectic bonding is the hermetic, conductive bonding solution for semiconductor packaging — exploiting low-melting-point alloy formation between deposited metal films to create strong, gas-tight, electrically and thermally conductive interfaces at moderate temperatures, serving as the standard die attach and MEMS sealing technology across the semiconductor industry.