Investigation of process window during dry etching of ZnO thin films by CH4–H2–Ar inductively coupled plasma

The etching characteristics of ZnO thin films, etched with a positive photoresist mask and a process window in inductively coupled CH4–H2–Ar plasmas were investigated by varying the various process parameters. It was found that the process window for ZnO etching is closely related to the balance between the deposition and removal processes of the a-C:H (amorphous hydrogenated carbon) layer on the ZnO surface. Under certain conditions, the etch rate selectivity of ZnO to PR is infinite, because the ZnO films continue to be etched, but a net deposition of the a-C:H polymer occurs on the top of the photoresist. The measurements of the radical species in the plasma and the surface binding states by optical emission spectroscopy (OES) and x-ray photoelectron spectroscopy (XPS), respectively, revealed that the chemical reaction of the CH radicals with the Zn atoms in ZnO and the ion-enhanced removal mechanism of the a-C:H layer play an important role in determining the ZnO etch rate, as well as the etch by-products.