Wafer bonding

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The wafer bonding is a step in the semiconductor and micro-systems technology , in which two wafers or slices (silicon, quartz, glass and others) are connected to each other. In microsystem technology, wafer bonding is used to produce the cavities required for the sensors , e.g. B. the reference pressure chamber for an absolute pressure sensor or the negative pressure chamber of some rotation rate sensors.

Wafer bonding process

Process without intermediate layers

  • Direct bonding: Silicon direct bonding (SFB) was first introduced in 1986 by JB Lasky.
    In this process, hydrophilic and hydrophobic surfaces are brought into contact at high temperatures. One wafer is pressed in the middle against the other wafer, creating the first contact point. Hydrogen bridges and van der Waals interactions in the area of ​​the contact zone form the basis of the mechanical connection . The remaining area is first separated from one another by means of spacers. The spacers are then removed and the silicon junction spreads from the center. Usual process temperatures are in the range between 1000 ° C and 1200 ° C. The pressure with which the wafers are pressed together is approx. 18 MPa.
  • Anodic bonding: When anodic bonding is glass with increased Na + used ion concentration. This glass is brought into contact with the silicon wafer and a voltage is applied so that the negative polarity is applied to the glass. As a result of this and the increased temperature, the sodium ions (Na + ) diffuse to the electrode. As a result, a space charge zone forms at the interface, which leads to a high field and thus to the formation of Si – O – Si bonds. The bond front now behaves like the SFB, only more slowly.
  • Anodic bonding at low temperature: Anodic bonding at low temperature is particularly suitable for temperature-sensitive materials. Certain processes for surface activation on the process wafers can reduce the usual bonding temperature from 400 ° C to below 180 ° C. The use of the anodic bonding process at low temperatures allows the material to be protected and material stresses to be reduced.

Process with intermediate layers

  • Eutectic bonding: The principle here is based on the formation of a connection through a eutectic alloy such as Si-Au or Ge-Al.
  • Glass-frit bonding: Formation of connections by melting glass solders / glass frits
  • Adhesive bonding: formation of a connection through adhesive as an intermediate layer
Table: Comparison of the different bonding methods
method material Intermediate layers Temperature
in ° C 		Surface treatment Selective bonding achieved through
Anodic bonding Glass-Si
Si-Si
Si-metal / glass 		Al, W, Ti, Cr sputtered on Pyrex > 250
> 300
300… 500 		Voltage 50 ... 1000 V Photolithography , etching, lift-off
Silicon direct bonding Si-Si
SiO 2 -SiO 2 		700 ... 1000 Standard cleaning Photolithography, etching
Anodic bonding at low temperature Glass si > 180 Voltage 10 ... 30 V
Glass frit bonding Si-Si
SiO 2 -SiO 2 		Na 2 O – SiO 2 and other sol-gel materials, boron glass 400 ... 600

> 450 		Spin coating

chemical vapor deposition , doping , 		screen printing
Low temperature silicon direct bonding Si-Si
SiO 2 -SiO 2 		200 ... 400 Plasma treatment, wet surface activation (diving) Photolithography, etching, lift-off
Eutectic bonding Si-Si Au, Al 379,580 Sputtering , electroplating Lift-off, etching
Weld bonding Si-Si Au, Pb-Sn 300 thermal evaporation, sputtering Lift-off, etching
Adhesive bonding Glass-Si
Si-Si
SiO 2 -Si 2
Si 3 N 4 -Si 3 N 4 		Glue, photoresist 25… 200 Rotation coating (engl. Spin coating ) spray coating
(engl. Spray coating ) 		Photolithography

literature

  • Jean-Pierre Colinge: Silicon-on-Insulator Technology. Materials to VLSI . 3. Edition. Springer Netherlands, 2004, ISBN 1-4020-7773-4 .
  • AR Mirza, AA Ayon: Silicon wafer bonding: Key to MEMS high-volume manufacturing . In: Sensors . tape 15 , no. 12 , 1998, pp. 24-33 ( sensorsmag.com ).
  • Peter Ramm, James Jian-Qiang Lu, Maaike MV Taklo (eds.): Handbook of Wafer Bonding . Wiley-VCH, 2012, ISBN 978-3-527-32646-4 .

Individual evidence

  1. ^ JB Lasky: Wafer bonding for silicon-on-insulator technologies . In: Applied Physics Letters . tape 48 , no. 1 , 1986, pp. 78-80 , doi : 10.1063 / 1.96768 .
  2. ^ Walter Lang: Lecture notes: Integrated Systems I. University of Bremen, winter semester 2006/2007.
  3. ↑ iX-factory GmbH (2014): iX-factory achieves success in the development of an anodic low-temperature bonding process. http://ix-factory.de/nachrichten.html?lang=de