page_title: Anodic bonding
section_title: Heating and bonding by application of electrostatic field
hierarchical_section_title: Anodic bonding / Procedural steps of anodic bonding / Heating and bonding by application of electrostatic field
caption_attribution_description: English: Scheme of anodic bonding procedure. The top tool works as a cathode and the chunk as anode. The process parameter are bond voltage UB, current limitation IB and bond temperature TB. Deutsch: Schematische Darstellung des anodischen Bondprozesses. Die Prozessparameter sind die Bondspannung UB, die Strombegrenzung IB und die Bondtemperatur TB.
context_page_description: Anodic bonding is a wafer bonding process to seal glass to either silicon or metal without introducing an intermediate layer; it is commonly used to seal glass to silicon wafers in electronics and microfluidics. This bonding technique, also known as field assisted bonding or electrostatic sealing, is mostly used for connecting silicon/glass and metal/glass through electric fields. The requirements for anodic bonding are clean and even wafer surfaces and atomic contact between the bonding substrates through a sufficiently powerful electrostatic field. Also necessary is the use of borosilicate glass containing a high concentration of alkali ions. The coefficient of thermal expansion of the processed glass needs to be similar to those of the bonding partner.
  Anodic bonding can be applied with glass wafers at temperatures of 250 to 400 °C or with sputtered glass at 400 °C. Structured borosilicate glass layers may also be deposited by plasma-assisted e-beam evaporation.
  This procedure is mostly used for hermetic encapsulation of micro-mechanical silicon elements. The glass substrate encapsulation protects from environmental influences, e.g. humidity or contamination.
context_section_description: The wafers are placed between the chuck and the top tool used as bond electrode at temperatures between 200 and 500 °C (compare to image "scheme of anodic bonding procedure") but below the softening point of glass (glass transition temperature). The higher the temperature the better is the mobility of positive ions in glass.
  The applied electrical potential between is set to a voltage of several 100 V. This causes a diffusion of sodium ions (Na⁺) out of the bond interface to the backside of the glass to the cathode. That results, combined with humidity in formation of NaOH. High voltage helps to support the drifting of the positive ions in glass to the cathode. The diffusion is according to the Boltzmann distribution exponentially related to the temperature. The glass (NaO₂) with its remaining oxygen ions (O²⁻) is negatively volume charged at the bonding surface compared to the silicon (compare to figure "ion drifting in bond glass" (1)). This is based on the depletion of Na⁺ ions.
  Silicon is unlike, e.g. aluminium, an inert anode. In result no ions drift out of the silicon into the glass during the bond process. This affects a positive volume charge in the silicon wafer on the opposite side. As a result a few micrometer thick high-impedance depletion region is developed at the bond barrier in the glass wafer. In the gap between silicon and glass the bond voltage drops. The bond process as a combination of electrostatic and electrochemical process starts.
  The electrical field intensity in the depletion region is so high that the oxygen ions drift to the bond interface and pass out to react with the silicon to form SiO₂ (compare to figure "ion drifting in bond glass" (2)). Based on the high field intensity in the depletion region or in the gap at the interface, both wafer surfaces are pressed together at a specific bond voltage and bond temperature. The process is realized at temperatures from 200 - 500 °C for about 5 to 20 min. Typically, the bonding or sealing time becomes longer when temperature and voltage are reduced. The pressure is applied to create intimate contact between the surfaces to ensure good electrical conduction across the wafer pair. This ensures intimate contact for the surfaces of the bonding partners. The thin formed oxide layer between the bond surfaces, siloxane (Si-O-Si), ensures the irreversible connection between the bonding partners.
  If using thermally oxidized wafers without a glass layer, the diffusion of OH⁻ and H⁺ ions instead of Na⁺ ions leads to the bonding.
caption: Scheme of anodic bonding procedure. The top tool works as a cathode and the chuck as anode.
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