page_title: Adhesive bonding
section_title: Procedural steps
hierarchical_section_title: Adhesive bonding / Benzocyclobutene (BCB) / Procedural steps
caption_attribution_description: English: Cross-sectional view of bonded wafer pair with an intermediate BCB layer width of 100&#160;µm
context_page_description: Adhesive bonding describes a wafer bonding technique with applying an intermediate layer to connect substrates of different types of materials. Those connections produced can be soluble or insoluble. The commercially available adhesive can be organic or inorganic and is deposited on one or both substrate surfaces. Adhesives, especially the well-established SU-8, and benzocyclobutene, are specialized for MEMS or electronic component production.
  The procedure enables bonding temperatures from 1000 °C down to room temperature. The most important process parameters for achieving a high bonding strength are:
  adhesive material
  coating thickness
  bonding temperature
  processing time
  chamber pressure
  tool pressure
  Adhesive bonding has the advantage of relatively low bonding temperature as well as the absence of electric voltage and current. Based on the fact that the wafers are not in direct contact, this procedure enables the use of different substrates, e.g. silicon, glass, metals and other semiconductor materials. A drawback is that small structures become wider during patterning which hampers the production of an accurate intermediate layer with tight dimension control.
context_section_description: The procedural steps for dry etch BCB are:
  Cleaning
  Supplying the adhesion promoter
  Drying of the primer
  BCB deposition
  Photosensitive BCB
  Exposure and development
  Dry etch BCB
  Pre-bake/soft-cure
  Patterning of the BCB layer by lithography and dry-etching
  Bonding at specific temperature, ambient pressure for specific amount of time
  Post-bake/hard-cure to form solid BCB monomer layer
  The wafers can be cleaned using H₂O₂ + H₂SO₄ or oxygen plasma. The cleaned wafers are rinsed with DI water and dried at elevated temperature, e.g. 100 to 200 °C for 120 min.
  The adhesion promoter with a specific thickness is deposited, i.e. spin-coated or contact printed on the wafer to improve the bonding strength. Spray coating is preferable when the adhesive is deposited on free standing structures.
  Subsequently, the BCB layer is spin or spray coated, usually 1 to 50 µm thick, to the same wafer. To prevent that the patterned layer has a lower bond strength than the unpatterned layer, due to the cross-linking of the polymer, a soft-curing step is applied before bonding. The pre-curing of the BCB takes place for several minutes on the hot plate at a specific temperature ≤ 300 °C. The soft cure prevents bubble formation and unbonded areas as well as the distortion of the adhesive layer during compression to improve the alignment accuracy. The degree of polymerization should not be over 50%, so it is robust enough to be patterned and still sufficiently adhesive to be bonded.
  If the BCB is hard-baked (far over 50%), it loses its adhesives properties and results in an increased amount of void formation. But also if the soft-curing is above 210 °C the adhesive cures too much, so that the material is not soft and sticky enough to achieve a high bonding strength.
  The substrates with the intermediate layer are pressed together with subsequent curing results in a bond. The post-bake process is applied at 180 to 320 °C for 30 to 240 min usually in a specific atmosphere or vacuum in the bond chamber. This is necessary to hard-cure the BCB. The vacuum prevents air trapped in the bond interface and pumps out the gases of the out-gassing residual solvents during annealing. The temperature and the curing time are variable, so with a higher temperature curing time can be reduced based on a quicker cross-linking. The final bonding layer thickness depends on the thickness of the cured BCB, the spinning speed and the shrink rate.
caption: Cross-sectional view of bonded wafer pair with an intermediate layer width of 100 µm.[3]
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