Large-area wafer bonding of GaAs using hydrogen and ultrahigh vacuum atmospheres

Uniform direct or fusion wafer bonding of GaAs wafers up to 4 inch in diameter was achieved by means of two methods: (i) pre-heating, bonding at elevated temperatures and post-annealing in a H₂ atmosphere (gas environmental hot bonding) and (ii) bonding inside an UHV apparatus at temperatures as low as 150 °C after cleaning with atomic hydrogen. Both methods yield atomically abrupt interfaces as shown by cross-sectional TEM and by imaging the screw-dislocation network formed at low angles of twist between the wafers. At large twist angles, additional "step" dislocations arising from bonding across surface steps could be clearly imaged. The problem of occasionally occuring microvoids, probably arising due to insufficient pre-cleaning or at excessive post-annealing, is addressed. Both bonding procedures neither need mechanical loading of the wafers nor channel-patterning of the surfaces.