A new chemical technique has been developed for obtaining mirror-finished surface of germanium without scratches and etch-pits by etching a germanium wafer in an etchant named CPD-2.
Such a surface is very useful in the fabrication of transistors. This method is simpler and easier than polishing-on-a-lap method for obtaining mirror-finished surface on diffused-transistors.
In this experiment, germanium wafers are etched after being polished with alundum powder of No. 2000 and No. 4000.
Etching in CP-4 etchant gives mirror surface to germanium specimens, but under a microscope etch-pits are seen on them. The reaction of CP-4 with germanium goes slow initially and becomes violent after about 30_??_60 seconds. The mild reaction in the early period brings about etch-pits. Therefore, to obtain a perfect mirror-finished surface it is necessary to find out an etchant which reacts violently with germanium in the instant of throwing-in.
Treatment in the new etchant, which is composed of fluoric acid 15 cc, nitric acid 15 cc and bromine 0.02 cc, gives always good mirror-finished surface without etch-pits even to germanium wafers with high dislocation density. Etching is completed in 1_??_2 minutes. Flatness of the etched surface is satisfactorily confirmed by interference fringe test.
The effect of varying the composition of CPD-2 is studied. Solution, in which the volume ratio of HF to HNO
3 is 1.5:1 or 1:1.5 and the volume of bromine is kept unchanged, roughens the etched surface and gives etch-pits on it. If more nitric acid is used, the surface becomes covered with yellow film. As the content of bromine is increased, more etch-pits are formed on the surface.
Thus it was found that the optimum composition of the etchant which gave a perfect mirrorfinished surface corresponded to the composition of CPD-2.
Diffused transistors produced from germanium wafers treated in CPD-2 solution showed an excellent high-frequency characteristics (ƒ
α=300_??_500 MC,
b=30_??_200).
抄録全体を表示