1. Thermo-compression Bonding
Mohammadreza Soleymaniha

2. Principles
Thermo-compression bonding can be used for wire bonding or Flip Chip bonding and wafer bonding as well.
Other names:
diffusion bonding, pressure joining, thermo-compression welding or solid-state welding
Two metals, e.g. gold (Au)-gold (Au), are brought into atomic contact applying force and heat simultaneously(above 400 c)

3. Used Material Al: gold : copper : high diffusion rates - soft metals
gold (Au)
copper (Cu)
aluminum (Al)
Al:
needs extensive deposition ,
requires a high applied force to crack the surface oxide.
gold :
Lower Temperature(around 300 °C)
does not form an oxide.
skip a surface cleaning procedure before bonding
copper :
disadvantage that the damascene process is very extensive.(patterning technique)
forms immediately a surface oxide that can be removed by formic acid vapor cleaning.
high diffusion rates
- soft metals
- good ductile properties
The oxide removal conduces also as surface passivation.

4. TC steps(Pre-conditioning)
Oxidation and impurities in the metal films affect the diffusion reactions by reducing the diffusion rates.
Methods: oxide etch chemistry methods
Dry etching processes (formic acid vapor cleaning)
Chemical Mechanical Polishing/Planarization(CMP) is a process of smoothing surfaces with the combination of chemical and mechanical forces.( For Cu and Al)
surface treatment for organic removal (UV-ozone exposure)
Plasma surface pretreatment, provide an accelerated diffusion rate based on an increased surface contact.
formic acid vapor cleaning, are preferred based on the minimization of the immersion in fluids and the resulting etching of the passivation or the adhesion layer.

5. TC steps(Deposition of metals)
Methods:
Evaporation
Sputtering
Electroplating
Evaporation and sputtering:
producing high quality films with limited impurities,
slow (used for micrometre and sub-micrometre layer thicknesses)
Electroplating :
commonly used for thicker films
needs careful monitoring and control of the film roughness and the layer purity
The gold film can also be deposited on a diffusion barrier film like oxide or nitride
Also, an additional nano crystalline metal film, e.g. Ta, Cr, W, or Ti, can enhance the adhesion strength of the diffusion bond at decreased applied pressure and bonding temperature
barrier(diffusion barrier is a thin layer (usually micrometres thick) of metal usually placed between two other metals.)

6. TC steps(Bonding) Diffusion rate defines the temperature and the force
The atoms migration based on crystal lattice vibration
diffusion process
surface diffusion(the most rapid diffusion process)
grain boundary diffusion
bulk diffusion
bonding temperature can be lowered using a higher applied pressure and vice versa
The bonding process itself takes place in a vacuum or forming gas environment, e.g. N2
Due to difficulty of control of thermal expansion differences between the two wafers, precision alignment and high quality fixtures are needed
- This atomic interaction sticks the interface together
-high pressure increases the chances of damage to the structural material or the films
The grain boundary diffusion terms the free migration of atoms in free atomic lattice spaces. This is based on polycrystalline layers and its boundaries of incomplete matching of the atomic lattice and grains.
The diffusion through bulk crystal is the exchange of atoms or vacancies within the lattice that enables the mixing. The bulk diffusion starts at 30 to 50% of the materials melting point increasing exponentially with the temperature

7. challenges Apply force and heat to the component simultaneously
Provide thermally and mechanically stable component heater.
Minimize substrate heater's compliance to absorb bonding force without deformation
Provide stable, fine resolution bonding force control to bond even the thinnest and most brittle materials as well as large components.
Therefore, the temperature of both heaters needs to be matched. This results in a synchronized wafer expansion.