page_title: Adhesive bonding
section_title: Overview
hierarchical_section_title: Adhesive bonding / Benzocyclobutene (BCB) / Overview
caption_attribution_description: Schematic process flow of dry etch and photosensitive BCB wafer bonding technique.
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: Benzocyclobutene (BCB) is a hydrocarbon that is widely used in electronics. BCB exists in a dry etch and a photosensitive version, each requiring different procedural steps for structuring (compare BCB process flow).
  It does release only small amounts of by-products during curing which enables a void-free bond. This polymer ensures very strong bonds and excellent chemical resistance to numerous acids, alkalines and solvents. The BCB is over 90% transparent to visible light that enables the use for optical MEMS applications.
  Compared to other polymers the BCB has a low dielectric constant and dielectric loss. The polymerization of BCB is taking place at a temperature around 250 to 300 °C and it is stable up to 350 °C. Using BCB does not ensure a sufficient hermeticity of sealed cavities for MEMS.
caption: Schematic process flow of dry etch and photosensitive BCB wafer bonding technique.[14]
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