Helical etch channels in synthetic quartz crystals

Helical etch channels were found in the synthetic quartz crystal etched by both aqueous hydrofluoric acid (HF) solution at 20°C and ammonium fluoride (NH₄F)+ HF mixture solution at 60°C. The quartz crystal used in this study contained many dislocations caused by the damaged layer of the seed surface. The etch channel density amounted to 2×10⁴ pcs/cm² and about 0.5% of them showed helical form. The distortion of the crystal was measured by X-ray reflection and the change of the parallelism of the lattice planes was observed: the crystal was curving. The direction of change of lattice planes suggests that a tension acts in the crystals. The cause of helical etch channel formation is explained as follows. Due to the tension, the growth direction of the dislocations are bent along weaker bonded lattice planes (for example R-planes) corresponding to the anisotropy of quartz. Thus, after etching, helical etch channels are formed