WITH INTERMEDIATE LAYERS

A quick review

29/06/2012 P. Maoddi 1 Intermediate Layer Bonding

General procedure: • Surface preparation • Deposition of intermediate layer • Patterning of intermediate layer (optional) • Bonding (heat, pressure) General concept: • Lower temperature  lower bond strength

29/06/2012 P. Maoddi 2 Intermediate Layer Types

• Metals – Thermocompression bonding – • – bonding • Polymers – bonding

29/06/2012 P. Maoddi 3 Thermocompression

• Bonding between two metal-coated wafers using heat and pressure • Interdiffusion of atoms between metal lattices  bonding occurs at metal/metal interface • Typical bondings – Al/Al (T>400°C, P>2.2 MPa) – Cu/Cu (T>380°C, P>1 MPa) – Au/Au (T>300°C, P>1.3 MPa)

29/06/2012 P. Maoddi 4 Eutectic Bonding: Concept

• Eutectic system

The melting temperature of an alloy at the eutectic Example: composition is much lower Au : 1064 °C than the melting points of the Si : 1414 °C single composants. Au 19% + Si 81% : 363 °C 29/06/2012 P. Maoddi 5 Eutectic Bonding: Procedure

• Metal deposition (e.g. Au) • Optional: metal patterning (e.g. lift-off) • Wafers are brought in contact and heated. Metal diffuses into . At a certain depth the eutectic composition is met and the bond is formed. • Can be done with glass/Si as well

29/06/2012 P. Maoddi 6 Eutectic bonding example

• MEMS capacitive microphone • Au/Sn eutectic alloy 80-20%  280 °C

29/06/2012 P. Maoddi 7 Ultrasonic Bonding

• Mechanism similar to thermocompression but energy given by ultrasounds

• Research only, to my knowledge never performed at wafer level

29/06/2012 P. Maoddi 8 Glass Frit bonding

• Glass paste deposition – Powdered leaded glass + organic binder, commercially available pastes – Can be screen printed • Thermal treatment – Decomposition of organic binder, low melting point glass layer (glass frit) is formed • Bonding – Relatively low temperature (T< 450 °C)

29/06/2012 P. Maoddi 9 Glass Frit Bonding Example

Vacuum packaged RF resonators

29/06/2012 P. Maoddi 10 Adhesive Bonding

• Deposition of a polymer – Spin coating + prebake or dry lamination • Optional: photolithography • Bonding by thermal crosslinking – BCB (200 – 250°C) – SU-8 (100 – 150 °C) – Ordyl DFR (100-150°C) – Cytop (~160 °C)

29/06/2012 P. Maoddi 11 Adhesive bonding example

Peltier-cell micropump

29/06/2012 P. Maoddi 12 Our experiments with SU-8 at EPFL • SU-8 photolithography on two wafers • Alignment and bonding (150°C, 1 MPa) using SUSS MA6 + SB6 • Release from sacrificial layers

29/06/2012 P. Maoddi 13 References

Thermocompression • http://en.wikipedia.org/wiki/Thermocompression_bonding • SUSS Microtec, Metal Based Wafer Bonding Techniques for Wafer Level Packaging (2009) • M. Spearing, Gold Thermocompression Wafer Bonding (2004)

Eutectic bonding • http://en.wikipedia.org/wiki/Eutectic_bonding • SUSS Microtec, Metal Based Wafer Bonding Techniques for Wafer Level Packaging (2009) • R.F. Wolffenbuttel, Low-temperature intermediate Au-Si wafer bonding; eutectic or silicide bond (1997)

Ultrasonic bonding • K. Jongbaeg et al, Ultrasonic bonding of In/Au and Al/Al for hermetic sealing of MEMS packaging (2002)

29/06/2012 P. Maoddi 14 References

Glass frit bonding • http://en.wikipedia.org/wiki/Glass_frit_bonding • R. Knechtel, Glass frit bonding: an universal technology for wafer level encapsulation and packaging (2005)

Adhesive bonding • http://en.wikipedia.org/wiki/Adhesive_bonding • BCB: F. Niklaus et al, Low-temperature full wafer adhesive bonding (2000) • Cytop: W. Kwang et al, A low-temperature bonding technique using spin-on fluorocarbon polymers to assemble microsystems (2002) • Ordyl: T. Huesgen et al, Optimization and characterization of wafer-level adhesive bonding with patterned dry-film photoresist for 3D MEMS integration (2010)

29/06/2012 P. Maoddi 15