Surface roughness control of the Al and Al₂O₃ thin films deposited by using pulsed DC magnetron sputtering

The thickness of the Al₂O₃ layer used in the magnetic tunneling junctions FM₁/Al₂O₃/FM₂ is less than 2 nm, here FM₁ is for the ferromagnetic layer 1 and FM₂ is for ferromagnetic layer 2. In order to obtain ultra-thin Al₂O₃ layer with higher breakdown voltage and pinhole free, extremely smooth surface roughness of this layer is required. The influence of the sputtering gas pressure, DC pulsed frequency, DC pulsed power, substrate bias and buffer layer on surface roughness and properties of Al thin films were studied. The single layer Al films are usually amorphous, texture (111) Al films can be obtained while using thin Ta 5 nm or Ta5/NiFe2 as underlayer. Very smooth Al thin film can be sputtered on Si/SiO₂ (100) wafer with Ta/NiFe buffer layer at f=15 kHz (DC pulsed frequency) and with RF substrate biasing (Vpp is about 21 V). High quality MTJs with high MR ratio up to 44.6% and high field sensitivity up to 19.3%/Oe were finally demonstrated after optimization of thin film deposition process.