Paper PS-TuP12
Dry Etching of CoFeB Films using BCl₃-based Inductively Coupled Plasma for MRAM Application

Tuesday, October 21, 2008, 6:30 pm, Room Hall D

We have been in the personal computing age, but person and computer are harmonized uneasily with each other. Now, however, it is within the range of possibility. Recently many researchers are studying about ubiquitious that can provide users to access computers at anytime and everywhere. If we could make devices of smaller size, higher speed and lower power consumption, it can be realized. Magnetic RAM(MRAM) using tunnel junction is the device which can meet this requirement. Tunnel junction is consisted of two ferromagnetic layers separated by an insulator. One of the ferromagnetic layers is pinned-layer that fixed magnetization, whereas the other ferromagnetic layer is free-layer unfixed magnetization. Due to spin dependent electron tunneling one can thus have two distinct resistance states, associated with the magnetizations of the pinned and free layers parallel or anti-parallel. To improve device performance, one continuously aims to achieve higher tunnel magnetoresistance (TMR), better thermal stability and low ferromagnetic coupling between pinned and free layers. The use of amorphous CoFeB films in the free and pinned layers of optimized tunnel junctions enabled us to obtain a higher TMR coefficient, good transport properties upon annealing and lower coupling fields. Up to now, there are few papers on the plasma etching of CoFeB films using high density plasmas. However, those papers did not show the changes of component on the etched surface of CoFeB. In this study, CoFeB films were etched with using the inductively coupled plasma system and BCl₃-based gas chemistries. Etch rate and selectivity of the CoFeB was systemically studied by the process parameters including BCl₃/Ar gas mixing ratio, RF power, DC-bias power, substrate temperature. The changes of electron temperature, ion energy and radical volume densities were characterized by optical emission spectroscopy analysis, Langmuir prove and quadrupole mass spectrometer. The etch rate of CoFeB showed highly dependency on the DC bias voltage and pressure due to effective removal of etch byproducts from the exposed CoFeB surface in plasma by sputtering de-sorption. The changes of components on the surface of CoFeB were investigated with X-ray photoelectron spectroscopy. The variation of surface was also investigated with atomic force microscopy and scanning electron microscopy.