Bonding

Bonding techniques

• Wafer to-wafer Bonding

• Chip-to-wafer bonding

  Wafer Bonding can be done with or without intermediate layers to bond the wafer temporarily or permanently.

  Available bonding techniques are described below:
  

1.With the intermediate layer

a. Metal layer

1. Hybrid bonding

   • Combination of fusion bonding and metal diffusion bonding into a single process

   • Extension of fusion bonding with wafer surface having dielectric and embedded metal interconnects layer

   • Wafer pre-processing/ cleaning with CMP

   • Prebonding at room temperature under plasma and atmospheric pressure allows the bonding of the dielectric layers.

   • Annealing at 100-300C allows electrical contact via metal diffusion bonding.

   • Alignment accuracy ±1 μm 

   • Application 3D stacking

2. Metal Diffusion bonding

   • Also known as Thermo-compression bonding

   • Uses two metals Al-Al, Cu-Cu, and Au-Au, for bonding wafers together with heat and compression

   • Metals diffusion allows both mechanical and electrical bonding in one step.

3. Eutectic bonding/ Solder bonding/ Eutectic Soldering

   • Uses an intermediate metal layer (eutectic alloys) that melts at low temperature than individual metals

   • Eutectic alloy deposition, adhesion layers(Ti or Cr), diffusion barrier layers (Ni or Pt) by sputtering

   • Pretreatment: removing oxide by wet/dry etching/ ashing

   • AuSn low-temperature bonding at a temperature lower than 300C

   • AlGe

   • AuGe

   • AuSi (producing hermetic seal at low temperature (390C))

   b. Insulator layer

   1. Glass frit bonding

• Glass soldering/ seal glass bonding

• Bonding technique with specially developed intermediate glass layer (glass frit) to match CTE (coefficient of thermal expansion)

• Steps

• Frit layer deposition by spin coating/ screen printing

• Heating glass frit to form compact glass layers

• Thermo compression process (400C)

• Glass-metals, glass ceramic metal bonding

• Application MEMS packaging

  2. Adhesive bonding

• Low-temperature, low-cost bonding

• Permanent or temporary bonding

• thermally curable or UV-curable polymer adhesives

• Polyimide

• Epoxy

• BCB

2. Without the intermediate layer

a. Direct bonding

1. Fusion bonding

• Bonding at room temperature followed by annealing

• Wafer pre-processing/ cleaning with CMP

• Plasma activated bonding

• Low-temperature fusion bonding (100-300C)

1. 2. Room temperature bonding/ surface-activated bonding (SAB)

• Surface activation by FAB (Fast atomic beam) (remove oxide layers)

• High or ultra-high vacuum requirements

• Max. applied load 100kN

• Examples: LiNbO3 wafer to Si bonding

• b. Anodic bonding

• Electrostatic field 

• Low temp. Bonding

• Vacuum or atmospheric pressure bonding

• Ion migration due to electrical field (use of a sodium-containing glass)

• Need of the same coefficient of expansion as that of glass

• Glass layer deposition by sputtering is possible

• Bonding glass with silicon or metal, glass silicon glass bonding (triple stack bonding)

• Generation of hermetic bonds

• Applications: Microfluidics, MEMS

Alignment by IR transparent image

Size: from small chips to 12-inch wafers

Bonding Inspection by  Scanning Acoustic microscope images (C-SAM) 

AFM to check the surface roughness