As a result of the extensive and demanding applications for GaAs wafers, there are strict industry requirements for flatness, which typically needs to be within ±2 μm.
Another vital parameter that must be controlled is TTV (total thickness variation) – the difference between the maximum and minimum thickness values of a wafer. Typically this should be within 1 μm per 25 mm for GaAs wafers.
In all cases, a fundamental understanding of the manufacturing process is required to ensure a high quality outcome. Different types of slurries and pads, along with plate speed and jig load all impact the resulting surface. To improve the surface roughness when manufacturing GaAs wafers, a dynamic lapping process can be performed.
Dynamic lapping is when the load starts off high at, say 100 g/cm² and, as the thickness of the GaAs wafer is reduced, the load is also reduced in phases to 75 g/cm², then 50 g/cm², and finally 25 g/cm² before achieving the final lapping target thickness.
The lighter load during the final lapping stages produces a better surface roughness at the end of the lap and therefore reduces the polishing time needed to achieve the target surface roughness, which is typically Ra