Grain Size Refinement in Cu bottom lead in CPP GMR Stack

Abstract

GMR current-in-plane (CIP) spin valves are currently used in production of high density recording heads. For the next generation of heads, with recording densities higher than 100 billion bits per square inch (Gbpsi), confined-current-path (CCP), current- perpendicular-plane (CPP) GMR heads are the subject of intensive study. For the CPP stack, thick (150 nm) Cu leads are required to minimize the potential drop across the leads. This often leads to a rough film surface in the bottom lead which translates up into the GMR stack, thereby reducing the GMR effect because of roughening of the interfaces. Thick bottom leads also have large average grain sizes, leading to an increase in the coercivity of the ferromagnetic layers deposited on top of these leads. In order to decrease the grain size and roughness of the bottom Cu lead, lamination of the thick (150 nm) Cu with Ta interlayers was carried out, keeping the total Cu and Ta thicknesses constant. As the number of Ta-Cu bilayers was increased, the roughness decreased significantly. It was observed that the roughness decreased from over 1 nm for a single bilayer to about 0.3 nm for 10 bilayers. Atomic force microscopy (AFM) was used to determine the roughness of the samples and in-plane grain size. Scanning electron microscopy was used to confirm the in-plane grain size. X-ray diffraction was used to analyze the out-of-plane grain size of the Cu/Ta laminates. Cross-sectional transmission electron microscopy (TEM) was also utilized to analyze the interface quality. An increase in the resistance of the laminated lead was seen with increase in the number of laminations. This undesirable side effect can be minimized by improvement of the base vacuum of the sputter system, deposition at relatively high rates, and reduction of the latency time between successive film depositions. The in-plane film stress was also studied as a function of lamination frequency, and found constant for the same total thickness of Ta and Cu, as expected. The coercivity of Co90Fe10 layers deposited over laminated Cu/Ta leads was seen to decrease with increasing lamination frequency, also as expected. An even stronger effect on the CoFe coercivity was produced by using a 5nm Ta layer to cap the thick Cu lead, yielding significantly lower coercivity