The Dynamic Etching Behavior And Mechanism Of Sapphire

A-{11(2|-)0}, C-{0001} and M-{1(1|-)00} surfaces of sapphire crystal were etchedfor 5 to 39 minutes in fused KOH at 280℃, 290℃, 300℃and 310℃, respectively.The morphologies of surfaces, the dislocation etch pits and the cavities on thesurfaces were observed by means of atomic force microscope (AFM), opticalmicroscope (OM) and laser scanning confocal microscope (LSCM). The evolutionof etching morphology, surface roughness and etching rate with etching time andetching temperature was analyzed.It was found that the surface morphology of A and M surface evolved fromrough to smooth gradually with an increase of etching time, the surface roughnesscorrespondingly increased first, and then decreased, tended to be stable finally.Therefore, the etching of A and M surfaces can be considered as surface polishingprocess. The etching of C plane was accompanied with the nucleation of twodimentional etch pits and the expandsion of etching steps. The etching steps wereparallel to [11(2|-)0] direction. A large number of triangular cone cavities arranging inrows appeared on the etching steps. The surface roughness of C plane initiallyincreased, then decreased, then became stable, finally increased rapidly and tendedto be stable at last. The evolution of surface roughness conforms to the evolution ofsurface morphology.The results show that the morphology of dislocation etch pits exhibittingrhombic pyramidal shape on the A plane and trapeziform pyramidal shape on the Mplane underwent lightly change, but the size increased gradually and the bottoms ofthe pits became smooth. In the case of C plane, because the etching rates on thethree longer edges of the hexagon is larger than the three shorter edges, thedislocation etch pits evolved from hexagon to triangle. These special morphologiesare believed to be corresponding to the atomic structures of the surfaces.It was revealed that the edge of cavities was etched faster than their walls,leading the cavity edge etched into willow leaf shape on A plane, and themorphology of the cavity wall transformed from hexagon to rhombus, some specialplanes and etching steps were formed on the wall. A rectangular of A plane formedat the bottom of the cavity. On C plane, the morphology of etch pits and cavitymatched together, both of the top and the bottom of the cavity turned from circularinto hexagon. Meanwhile, etching steps and dislocation etch pits appeared at thebottom of the cavities. In a word,the evolution of the cavity on M plane was thesame as the cavity on A plane, but the etch pits of cavities exhibited heart-shaped,the wulff shape of the cavity was octagon, and the plane at the bottom of the cavity was a rectangular M plane.At the same etching temperature, the order of the etching rates of dislocationetch pits and cavity etch pits on A, C and M plane is V_t(c)<V_t(a)<V_t(m). With anincrease of etching temperature, the magnitude of V_t(a),V_t(c) and V_t(m) increases.