The thermo-compressive bonding is based on the plastic deformation of two metallic or metal-oxidic surfaces. The metallic layers are applied by thin film coating (sputter deposition, evaporation) or electroplating. The film thicknesses range from 100nm to several micrometres.
The advantages compared to eutectic bonding or soldering are:
- higher structural precision, since there is no liquefaction
- much higher thermal stability relative to the bonding temperature, and
- therefore no limitations due to hierarchies in sequential bonding steps
Unlike anodic bonding there is no need for an electrical field in thermo-compressive bonding.
Common layer systems include platinum-to-platinum (Fig. 1) in the temperature regime between 450 °C and 550 °C and gold-to-gold between 250 °C and 350 °C. Platinum-platinum thermo-compressive bonding is used in the high thermal applications and where chemical and mechanical stability are required.
Gold-to-gold bonding is particularly suitable for the hermetic packaging of MEMS where temperatures above 300 °C are not allowed.
Thermo-compressive bonding of glass-to-glass is suitable for the assembly of structured glass substrates (Fig. 2). The necessary temperatures for thin glasses is between 450 °C and 500 °C, comparable with those of anodic bonding. It offers extended freedom of design in the fabrication and encapsulation of microsystems (Fig. 3)
Thermo-compressive bonding is very sensitive to the surface quality of the substrates in terms of thickness variation, roughness cleanliness.