Method and apparatus for treating an edge region of a wafer. The angle adjusting device includes a rod unit, a first gear unit, a second gear unit, and a positioning unit. The diameter of the array may be greater than one-half the wavelength of the signal, and the receiver elements each have an individual surface area whose height or diameter corresponds at most to one-half the wavelength of the signal, and the emitter has a height or a diameter which is greater than one-half the wavelength of the signal.