The effect of substrate temperature on the etching properties and the etched surfaces of magnetic tunnel junction materials in a CH3OH inductively coupled plasma system

The etching characteristics of the magnetic films (PtMn, CoFe) and hard mask materials (W, Ta) forming a magnetic tunnel junction (MTJ) stack in a CH3OH inductively coupled plasma (ICP) system were investigated. We examined the etch rates of the metal films as a function of substrate temperature, and assessed the microstructures of the etched surfaces using high resolution transmission electron microscopy (HR-TEM). We also analyzed the surface states using X-ray photoelectron spectroscopy (XPS) and TEM electron energy loss spectroscopy (TEM-EELS). The PtMn and CoFe etch rates increased as the temperature increased, whereas the etch rates of W and Ta decreased slightly. Therefore the etch selectivity increased linearly with increasing substrate temperature. The CH3OH plasma formed nonvolatile etching byproducts with the magnetic films and hard mask metals. In the case of PtMn and CoFe, the surface composition of the etching byproducts changed with increasing temperature; the relative concentration of pure metal compared with metal oxide or carbide increased as the substrate temperature rose. The etch rate was determined by the sputtering yield of the materials formed on the etched surface; accordingly the etch rates of those magnetic films would increase due to the higher sputtering yield of pure metal.