Abstract: Back via-hole technology with different thickness GaAs is researched. Back via-hole process plays an important role in process of GaAs as the substrate, which has directly effected on the performance of the device. Firstly, the via-hole process of GaAs with thickness of 100 μm is described. Secondly, the deep via-hole etching of GaAs with a thickness of 150 μm is analyzed. Then, through the study of different pressure and different bias power in the etching process, the deep hole etching process with the depth of 200 μm of GaAs is mastered. Finally, according to the process experience of 200 μm, a deep via-hole etching process with 250 μm thickness is developed.